Profile and Research Interests

My research interests are focused on two main areas: co-simulation and the engineering of digital twins.

Download my CV(pdf).

See my publications below.

For events and news, consult my Aarhus University page.

Biography

Cláudio Gomes earned his B.Sc. and M.Sc. degrees in Computer Science from the New University of Lisbon in 2013, graduating with honors. In 2019, he completed his Ph.D. in Computer Science at the University of Antwerp, with a focus on property preservation in co-simulation. Since 2022, he has served as an Assistant Professor in the Department of Electrical and Computer Engineering at Aarhus University, Denmark. Prior to this, from 2020 to 2022, he was a Postdoctoral Researcher at Aarhus University, following a brief tenure as a Visiting Researcher at Carnegie Mellon University in 2019.

Prof. Gomes’s research interests include co-simulation, digital twin engineering, and machine learning for digital twins. He has played key roles in several high-profile European and national projects, such as ROBOSAPIENS, DLTE, and DIGIT-BENCH, where he has served as co-coordinator and principal investigator. His work has been recognized with several accolades, including the Runner-Up Best Paper Award at the ANNSIM Conference in 2023 and the Best Paper Award at the SIMULTECH conference in 2019.

An active contributor to the academic community, Prof. Gomes has co-authored numerous journal articles and conference papers. He has supervised several Ph.D., M.Sc., and B.Sc. students and has been involved in organizing workshops, serving on program committees, and guest-editing special issues in leading journals. His ongoing contributions include editorial roles and active participation in conferences like RIVF, ANNSIM, ISoLA, and EDTConf. His current research focuses on advancing digital twin applications across engineering domains and developing methodologies for co-simulation.

Co-simulation

Modeling and simulation techniques enable companies to identify and address issues before expensive prototypes are built. However, as cyber-physical systems grow increasingly complex, companies must adopt a divide-and-conquer approach, developing different components concurrently and working with specialized suppliers. This creates challenges in achieving a “first time right” system development, as various teams use different tools for their models, and suppliers may be reluctant to share detailed models to protect their intellectual property.

Co-simulation offers a solution by providing a common interface that allows multiple simulation tools to communicate without exposing the specifics of the simulated models. This is one of my primary research interests. From an academic perspective, co-simulation presents unique challenges, particularly when connecting simulators of different formalisms, which can result in the creation of an ad-hoc hybrid modeling language. The simulation of dynamic systems can only approximate the exact behavior described by differential equations. When coupling dynamic system simulators, questions of reliability and accuracy arise. How can we trust the results, and how accurate are they?

Digital Twin Engineering

Modern Cyber-Physical Systems (CPS) are often deployed in environments that cannot be fully replicated in laboratory conditions, and they frequently operate for longer durations than are feasible for testing. When encountering unforeseen conditions, CPS must prevent failure or harm to their surroundings. During CPS development, a wealth of knowledge is generated through models and experimental results, yet much of this knowledge goes unused after deployment.

I envision that every CPS will eventually have its own Digital Twin (DT)—a virtual representation of its Physical Twin (PT). The DT continuously gathers data about the PT and its environment, correlates this data with PT models, and provides services like anomaly detection and self-adaptation. The DT will play a crucial role in mitigating the impact of unexpected environments.

DTs will be open-ended, allowing new services to be added by various stakeholders throughout the PT’s lifecycle. These services will undergo formal verification to ensure reliability and will cater to stakeholders with different interests, such as CPS operators focused on preventing downtime or manufacturers aiming to improve future products. For example, manufacturers should not have access to the CPS’s detailed control actions, as this constitutes the operator’s Intellectual Property (IP). DTs will, therefore, accommodate various stakeholder perspectives while protecting IP by design.

Self-adaptations will be tested and formally verified by the DT before being implemented in real-world scenarios. As systems of systems, DTs will be composed of other DTs, enabling more complex and interconnected systems.

Publications

2026

• [PDF] Gil, Santiago, Alvaro Miyazawa, Ana Cavalcanti, and Cláudio Gomes. 2026. “Application of FMI 3.0 Synchronous Clocks to Co-Simulation of Robotic Systems.” 2026 IEEE/SICE International Symposium on System Integration (SII), January 11, 1548–53. https://doi.org/10.1109/SII64115.2026.11404501.
  Abstract

  Given recent technological trends, such as the use of Digital Twins, modeling and simulation have become essential in both the design and the operation of robotic systems. However, developing large simulations that feature several aspects of a robotic system is not trivial. Co-simulation can be used to address this challenge by the hierarchical aggregation of heterogeneous simulators. The recent industry standard Functional Mock-up Interface version 3 (FMI3) adopts the concept of super-dense time to enable both continuous-time and discrete-event simulation. In this work, we extend an existing framework for robotics that supports the creation of modular co-simulation scenarios with simulated or real robots in the loop by upgrading its artifacts to support hybrid co-simulation with FMI3. With these upgraded artifacts, the extended framework accounts for both continuous-time and discrete-event simulations, providing a more comprehensive coverage of scenarios and behaviors of robotic systems. We use a UR5e robotic arm for the demonstration of the FMI3-enabled framework and provide our findings and recommendations from the experiments executed.

• [PDF] Biglari, Raheleh, Joachim Denil, and Cláudio Gomes. 2026. “Validity Frames for Self-Adaptive Systems.” Proceedings of the 14th International Conference on Model-Based Software and Systems Engineering, 434-41. https://doi.org/10.5220/0014461300004058.
  Abstract

  Validity Frames are a key concept in modelling and simulation, encoding the contexts in which models provide valid results. This position paper explores how Validity Frames propagate through optimisation processes and affect self-adaptive systems. We demonstrate that optimisers inherit constraints from model Validity Frames and generate their own validity conditions for the configurations they produce. These propagated Validity Frames can be formalised as software contracts with explicit assumptions and guarantees. Using a semi-active suspension system as a running example, we show how Validity Frames constrain design space exploration, enable runtime monitoring of validity boundaries, and support informed adaptation strategies. Our approach reveals a fundamental tradeoff between optimality and applicability: configurations optimised for narrow scenarios may achieve peak performance but fail when conditions change, while configurations evaluated across broader Validity Frames sacrifice peak performance for robustness. By explicitly tracking Validity Frames throughout the optimisation process, self-adaptive systems can reason about when adaptation is needed and select configurations appropriate to their current operational context. The full formalisation of validity frames into contracts and their composition, which allows for the precise integration of assumptions and guarantees across models, optimisers, and adaptive components, will be our main future work path.

• [PDF] Kristensen, Morten Haahr, Thomas Wright, Cláudio Gomes, Lukas Esterle, and Peter Gorm Larsen. 2026. “DynSRV: Dynamically Updated Properties for Stream Runtime Verification.” In Runtime Verification, edited by Bettina Könighofer and Hazem Torfah. Springer Nature Switzerland. https://doi.org/10.1007/978-3-032-05435-7_7.
  Abstract

  Systems that adapt to their environment or change based on new requirements pose challenges for runtime verification. Complexity is increased when the system needs to retain its internal state and continue monitoring while also updating properties or adding new ones during runtime. In this work, we propose DynSRV, a Stream Runtime Verification language that allows for dynamic updates of properties. A core benefit of this language is its capability to update properties at runtime without requiring a restart of the monitor, maintaining the internal state of the remaining properties. We formalise the semantics of our core primitives and demonstrate design patterns for allowing adaptations under certain constraints. Finally, we present an implementation of DynSRV and describe three memory strategies that balance memory usage and the ability to resolve dynamically added properties depending on historical data.

• [PDF] Isasa, Carlos, Noah Abou El Wafa, Claudio Gomes, Peter Gorm Larsen, and André Platzer. 2026. “Safe Temperature Regulation: Formally Verified and Real-World Validated.” In Integrated Formal Methods, edited by Ferruccio Damiani and Marie Farrell, vol. 16194. Lecture Notes in Computer Science. Springer Nature Switzerland. https://doi.org/10.1007/978-3-032-10794-7_8.
  Abstract

  This paper presents a case study in the design and formal verification of a safe controller for the generic two-element lumped-capacitance model of temperature regulation, using formal cyber-physical system (CPS) theorem proving. The coupled dynamics and the absence of a fallback state reflect the complexities of real-world control systems and make it a representative challenge for theorem proving. A controller is developed by a design-by-invariant methodology. Verification using the axiomatic theorem prover KeYmaera X revealed critical assumptions for safety and pushes the frontier of CPS theorem proving. The parametric, general and provably safe controller can be applied to a wide range of temperature regulation tasks and is validated by deploying an instance of the verified controller on a physical system, demonstrating its robustness under model inaccuracies and confirming its real-world usability.

• [PDF] Frasheri, Mirgita, Cláudio Gomes, Lukas Esterle, and Peter Gorm Larsen. 2026. “Obtaining Services from Digital Twins in Industrial Settings.” In Handbook of the Metaverse, edited by Pan Hui, Peng Yuan Zhou, Lik-Hang Lee, and Tristan Braud. Springer Nature Switzerland. https://doi.org/10.1007/978-3-032-03296-6_9.
  Abstract

  Digital Twin (DT) technologies are envisioned to bring substantial benefits to industry by optimising production processes, thus reducing the costs and time-to-market of products. A DT is a digital representation of a real-world entity which we call the Physical Twin (PT). The DT and PT are connected by a communications infrastructure which allows the DT to maintain a known level of fidelity to the PT it represents. A DT leverages the models at its core to offer its stakeholders a range of services that add value to the PT without unduly compromising the PT’s operation. These services could include visualisation and monitoring, predictive maintenance, fault diagnosis, what-if analysis and runtime reconfiguration of the PT. The key concepts behind DTs, as well as ways to obtain services from them in industrial settings, coupled with reports on existing case-studies, are the main topics covered in this Chapter.

2025

• [PDF] Andersen, Mikkel Schmidt, Cláudio Gomes, Sophia Thompson, and Peter Gorm Larsen. 2025. “Probabilistic Update Scheduling for Digital Twins: A Semi-Markov Approach.” 2025 ACM/IEEE 28th International Conference on Model Driven Engineering Languages and Systems Companion (MODELS-C), October, 250–56. https://doi.org/10.1109/MODELS-C68889.2025.00043.
  Abstract

  Digital Twins (DTs) often require maintenance throughout their life cycles, as their Physical Twin (PT) counterparts undergo maintenance and evolution. This necessitates software updates to the DT, but when should these updates be done? Updating the DT at the wrong time can lead to inconsistencies between the DT and PT, as well as failures and increased downtime. This study investigates the practicality and usability of applying Semi-Markov Processes (SMPs) to represent the connections and state transitions of the DT-enabled system. SMPs can be used to calculate the probability that all components in the system are collectively in a safe state, that is, in a state where updating the DT would result in minimal disruption to the DT-enabled system’s operation. We also discuss the limitations of the approach and the future work required to make it robust. Lastly, we present how SMPs can be used for an industrial concrete mixer as our case study, to remove the need for fixed maintenance intervals, which are costly.

• [PDF] Evans, Valdemar Tang, Claudio Gomes, and Daniel Lucani. 2025. “When Every Transmission Counts: Event-Trigger Threshold Regulation for STL Properties.” IEEE Internet of Things Journal, 1–1. https://doi.org/10.1109/JIOT.2025.3637893.
  Abstract

  We propose a novel event-trigger threshold (ETT) regulation mechanism ETT^ρ based on Signal Temporal Logic (STL) properties significantly extending recent work on ETT regulation for Propositional Logic properties. We utilize the quantitative semantics of STL to construct a method for computing and merging suitable ETTs for different requirements in complex STL specifications. Contrary to related work on event-triggered control for STL properties, we apply the event-triggering logic to the measured signals individually rather than the control output and assume that an existing periodic controller is defined. By exploiting the early satisfaction detection capabilities of STL and analyzing the property structure, our method aims to reduce the number of triggered events, while maintaining satisfaction of system properties. To evaluate ETT^ρ, we consider a simulated adaptive cruise control case-study where STL is used to encode complex safety and performance properties and the ETTs of measured signals are regulated accordingly. We test three different properties in two different scenarios to showcase how STL and ETT^ρ can identify intricate circumstances where it is possible to significantly reduce the number of triggered events relative to a constant ETT.

• [PDF] Biglari, Raheleh, Cláudio Gomes, and Joachim Denil. 2025. “Towards a Validity Frame of Multi-Modal Surrogate Models for Traffic Simulation.” 2025 Annual Modeling and Simulation Conference (ANNSIM), May, 1-13. https://ieeexplore.ieee.org/abstract/document/11214513.
  Abstract

  Multi-modal systems, such as urban traffic networks, require considering specific modeling considerations for varying operational and environmental conditions. Surrogate modeling offers a practical solution to represent these modes through different surrogate models. However, switching between modes needs careful management of the models’ experimental frames. In multi-model setups, overlap between these frames introduces unnecessary complexity and ambiguity in model selection. The concept of validity frame of multi-modal surrogate models is the model’s experimental frame plus the process allowing to change this frame. This paper introduces an approach to define such a validity frame ensuring that the union of all experimental frames covers the full system operating domain without redundancy and focusing on clean separation between models and interpretable mode switching. The proposed ensemble of surrogate models, coupled with adaptive techniques, represents a significant step forward in the performance modeling of dynamic and computationally expensive models.

• [PDF] Vanommeslaeghe, Yon, Cláudio Gomes, Bert Van Acker, Joachim Denil, and Paul De Meulenaere. 2025. “Adapting Parallel DEVS Semantics to FMI 3.0 Co-Simulation Using Synchronous Clocks.” SIMULATION, December 26, 00375497251393255. https://doi.org/10.1177/00375497251393255.
  Abstract

  Simulating cyber-physical systems (CPS) often requires integrating continuous dynamics, typically described by differential equations, with discrete-event dynamics that represent the behavior of embedded controllers. Version 3.0 of the Functional Mock-up Interface (FMI) standard introduces synchronous clocks and a discrete-event mode, creating new opportunities for the co-simulation of hybrid systems. So far, it has not been demonstrated that these features can faithfully capture the semantics of discrete-event formalisms such as the Discrete EVent System specification (DEVS) and its extension, Parallel DEVS (PDEVS). This leaves open the question of whether FMI 3.0 can effectively support complex hybrid dynamics. This paper addresses this gap by showing how PDEVS models can be exported and coupled as FMI 3.0 Functional Mock-up Units (FMUs) while preserving PDEVS semantics. We first derive requirements that any simulator must satisfy to correctly preserve PDEVS semantics. Based on these requirements, we present a systematic mapping of PDEVS abstract simulator functions to the FMI 3.0 interface with synchronous clocks and present an orchestration algorithm to co-simulate multiple PDEVS FMUs accordingly. The approach is validated using an adaptive cruise control case study, where our FMI-based co-simulation reproduces the results of an established PDEVS simulator. To support reproducibility and further research, we provide an open-source prototype together with the complete validation setup.

• [PDF] Baxter, James, Bert Van Acker, Morten Kristensen, Thomas Wright, Ana Cavalcanti, and Cláudio Gomes. 2025. “Formal Architectural Patterns for Adaptive Robotic Software.” International Conference on Fundamental Approaches to Software Engineering, 145-65.
  Abstract

  It is often the case that a robot must adapt to unexpected changes in its environment. It is, however, important that these changes can be demonstrated to maintain the safe operation of the robot. The adaptive systems community has developed the MAPE-K pattern as a widely recognised conceptual architecture. We propose extending MAPE-K to incorporate runtime verification, resulting in an architecture we call MAPLE-K. In this paper, we capture and formalise both the MAPE-K and MAPLE-K architectures using a domain-specific language. Additionally, we provide support for translation from architectural models to software models and code to facilitate the deployment of verified applications. MAPE-K is rarely maintained at the implementation level, but our work ensures traceability between the code and its design, enabling the use of architectural information to verify the correctness of the software.

• [PDF] Kakavandi, Fatemeh, Peihua Han, Tim Brix Nerenst, Roger de Reus, Cláudio Gomes, and Peter Gorm Larsen. 2025. “Comparative Study in Fault Detection under Data Scarcity for Pharmaceutical Device Manufacturing.” 2025 International Conference on Control, Automation and Diagnosis (ICCAD), July, 1-6. https://doi.org/10.1109/ICCAD64771.2025.11099417.
  Abstract

  Developing a data-driven fault detection model for an actual manufacturing process poses significant challenges, particularly during the design phase, when accessing a large volume of data is not feasible. One approach to address data scarcity is the generation of synthetic datasets to aid in the training process. This paper focuses on conducting a comparative study between deep generative models and a simulation model for generating synthetic samples. Various deep generative models such as Variational Autoencoder (VAE) are explored and applied in this study. Additionally, the Finite Element Analysis (FEA) technique is employed to develop the simulation model, leveraging physics-based approaches. Furthermore, the performance of these generative techniques is evaluated and compared using qualitative and quantitative evaluation methods. The results demonstrate that different methods exhibit varying levels of performance based on qualitative and quantitative criteria. Eventually, the impact of synthetic samples generated by different approaches on the fault detection model is evaluated and reported. Synthetic normal and anomalous samples are added to the training set, and the model is tested solely on real data. The results indicate that majority of generative techniques enhance the performance of the fault detection model, requiring fewer training iterations and improving the model’s predictive capabilities. Notably, synthetic data generated by VAE significantly improve the fault detection model’s F1 score when only synthetic normal samples added to the training set by 0.185 in our case.

• [PDF] Deantoni, Julien, Paula Muñoz, Cláudio Gomes, Clark Verbrugge, Rakshit Mittal, Robert Heinrich, Stijn Bellis, and Antonio Vallecillo. “Quantifying and Combining Uncertainty for Improving the Behavior of Digital Twin Systems.” At - Automatisierungstechnik 73, no. 2 (February 25, 2025): 81-99. https://doi.org/10.1515/auto-2024-0036.
  Abstract

  Uncertainty is an inherent property of any complex system, especially those that integrate physical parts or operate in real environments. In this paper, we focus on the Digital Twins of adaptive systems, which are particularly complex to design, verify, and optimize. One of the problems of having two systems (the physical one and its digital replica) is that their behavior may not always be consistent. In addition, both twins are normally subject to different types of uncertainties, which complicates their comparison. In this paper we propose the explicit representation and treatment of the uncertainty of both twins, and show how this enables a more accurate comparison of their behaviors. Furthermore, this allows us to reduce the overall system uncertainty and improve its behavior by properly averaging the individual uncertainties of the two twins. An exemplary incubator system is used to illustrate and validate our proposal.

• [PDF] “Gomes, Cláudio, Morten Haahr Kristensen, Mikkel Schmidt Andersen, et al. 2025. “Digital Twin Tutorial: The Incubator Case Study.” Lecture Notes in Computer Science (Singapore), 68-101. https://doi.org/10.1007/978-981-96-4656-2_3.
  Abstract

  This paper presents a comprehensive Digital Twin (DT) tutorial using the case study of an incubator system designed for tempeh fermentation (a traditional Indonesian soy product). The study explores the development, calibration, and validation of models based on thermodynamics, offering insight into how DTs can enhance control and monitoring. Key DT services such as monitoring, decision support, and reconfiguration are implemented and evaluated. The tutorial demonstrates the potential of DTs in optimizing real-time systems while ensuring operational consistency.

• [PDF] Gil, Santiago, Elif E. Baş, Christian D. Jensen, Sebastian Engelsgaard, Giuseppe Abbiati, and Cláudio Gomes. “FMI-Based Distributed Co-Simulation with Enhanced Security and Intellectual Property Safeguards.” In Proceedings of the 2025 Annual Modeling and Simulation Conference (ANNSIM'25). Madrid, Spain: Society for Modeling & Simulation International (SCS), 2025.
  Abstract

  Distributed co-simulation plays a key role in enabling collaborative modeling and simulation by different stakeholders while protecting their Intellectual Property (IP). Although IP protection is provided implicitly by co-simulation, there is no consensus in the guidelines to conduct distributed co-simulation of continuous-time or hybrid systems with no exposure to potential hacking attacks. We propose an approach for distributed co-simulation on top of UniFMU with enhanced cybersecurity and IP protection mechanisms, ensuring that the connection is initiated by the client and the models and binaries live on trusted platforms. We showcase the functionality of this approach using two co-simulation demos in four different network settings and analyze the trade-off between IP-protected distribution and performance efficiency in these settings.

2024

• [PDF] Baş, Elif Ecem, Giuseppe Abbiati, Cláudio Gomes, Uwe Jassmann, and Peter Gorm Larsen. “Digitalization of Large-Scale Testing Facilities for the Wind Industry: DIGIT-BENCH Digital Twin.” Journal of Physics: Conference Series 2767, no. 4 (June 1, 2024): 042033. https://doi.org/10.1088/1742-6596/2767/4/042033.
  Abstract

  Testing of large wind turbine components plays a central role in delivering reliable yet cost-effective technology. However, these experiments are often lengthy and costly. Fatigue testing of a wind turbine blade might take up to 12-14 months, whereas highly accelerated lifetime testing of a nacelle demands 6-8 months. The exchange of simulation models and data between OEMs and test facilities is recognized as a critical factor in the planning of an experimental campaign. In fact, OEMs are typically very protective of their industrial secrets, and sharing such sensitive information may constitute a threat. It follows that the use of simulation models to enable more effective experimentation is not pursued efficiently. Digital twins are emerging as a key enabling technology to improve the operation & maintenance of test benches for the wind industry. A digital twin combines physical systems and their digital models into a cyber-physical system to provide functionalities that cannot be attained by either physical or digital assets independently. The seamless integration of the test bench with digital models offered by a digital twin is expected to enhance the interaction between OEM and test bench operators. This proceeding illustrates the status of the development of the DIGIT-BENCH digital twin developed by R&D Test Systems (R&D) and Aarhus University to serve large-scale test facilities for the wind industry. The digital twin utilizes FMI-based co-simulation to enable the coupling of physical/digital components in an industrial-secret-friendly environment. The digital twin concept is demonstrated on a 2-degrees-of-freedom test bench installed at Aarhus University.

• [PDF] Kristensen, Morten Haahr, Alberto Bonizzi, Cláudio Gomes, Simon Thrane Hansen, Carlos Isasa, Hannes Iven, Eduard Kamburjan, et al. “Runtime Verification of Autonomous Systems Utilizing Digital Twins as a Service.” In 2024 IEEE International Conference on Autonomic Computing and Self-Organizing Systems Companion (ACSOS-C), 121-27, 2024. https://doi.org/10.1109/ACSOS-C63493.2024.00042.
  Abstract

  Autonomous Systems (AS) enable systems to adapt to drastic and unprecedented environmental changes, a capability that can be enhanced through the utilization of Digital Twins (DTs). However, the additional capabilities of AS come at the cost of explainability, as the expanding adaptation space complicates the reasoning about the system’s behavior. For certain types of systems, it is crucial to ensure that specific properties are upheld despite the system’s autonomous behavior. To facilitate the monitoring of these properties, we propose the use of Runtime Verification (RV). This tutorial demonstrates the integration of RV tools into the Digital Twins as a Service (DTaaS) platform to monitor and verify the behavior of AS in real-time. By exploring various methods to incorporate RV tools within a DT context, the tutorial aims to advance the application of RV technologies in autonomic computing and self-adaptive system design. Specifically, we demonstrate how the behavior of a self-configuring DT can be verified utilizing RV. This is accomplished through the DTaaS platform, which supports seamless deployment of DT-based AS.

• [PDF] Larsen, Peter G., Shaukat Ali, Roland Behrens, Ana Cavalcanti, Cláudio Gomes, Guoyuan Li, Paul De Meulenaere, et al. “Robotic Safe Adaptation in Unprecedented Situations: The RoboSAPIENS Project.” Research Directions: Cyber-Physical Systems 2 (2024): e4. https://doi.org/10.1017/cbp.2024.4.
  Abstract

  The robots of tomorrow should be endowed with the ability to adapt to drastic and unpredicted changes in their environment and interactions with humans. Such adaptations, however, cannot be boundless: the robot must stay trustworthy. So, the adaptations should not be just a recovery into a degraded functionality. Instead, they must be true adaptations: the robot must change its behaviour while maintaining or even increasing its expected performance, and staying at least as safe and robust as before. The RoboSAPIENS project will focus on autonomous robotic software adaptations, and will lay the foundations for ensuring that they are carried out in an intrinsically trustworthy, safe, and efficient manner, thereby reconciling open-ended self-adaptation with safety by design. RoboSAPIENS will transform these foundations into ‘first time right’-design tools and platforms, and will validate and demonstrate them.

• [PDF] Talasila, Prasad, Cláudio Gomes, Lars B Vosteen, Hannes Iven, Martin Leucker, Santiago Gil, Peter H Mikkelsen, Eduard Kamburjan, and Peter G Larsen. “Composable Digital Twins on Digital Twin as a Service Platform.” SIMULATION, December 1, 2024, 00375497241298653. https://doi.org/10.1177/00375497241298653.
  Abstract

  Establishing digital twins is a non-trivial endeavour especially when users face significant challenges in creating them from scratch. Ready availability of re-usable models, data, functions, and tool assets, can help with creation and use of digital twins. A number of frameworks/platforms exist to facilitate creation and use of digital twins. In this paper we propose such a platform to manage digital twin assets, create composable digital twins from re-usable assets and make the digital twins available as a service to other users. The proposed platform supports the management of re-usable assets, storage, provision of compute infrastructure, communication, monitoring and execution tasks. Two case studies are used to demonstrate the capabilities of this platform.

• [PDF] Tang, Valdemar, Cláudio Gomes, and Daniel E. Lucani. “Precision on Demand: Propositional Logic for Event-Trigger Threshold Regulation.” IEEE Internet of Things Journal, 2024, 1-1. https://doi.org/10.1109/JIOT.2024.3476922.
  Abstract

  We introduce a novel event-trigger threshold (ETT) regulation mechanism based on the quantitative semantics of propositional logic (PL). We exploit the expressiveness of the PL vocabulary to deliver a precise and flexible specification of ETT regulation based on system requirements and properties. Additionally, we present a modified ETT regulation mechanism that provides formal guarantees for satisfaction/violation detection of arbitrary PL properties. To validate our proposed method, we consider a convoy of vehicles in an adaptive cruise control scenario. In this scenario, the PL operators are used to encode safety properties and the ETTs are regulated accordingly, e.g., if our safety metric is high there can be a higher ETT threshold, while a smaller threshold is used when the system is approaching unsafe conditions. Under ideal ETT regulation conditions in this safety scenario, we show that reductions between 41.8 - 96.3% in the number of triggered events is possible compared to using a constant ETT while maintaining similar safety conditions.

• [PDF] Nordtorp, F., E. E. Baş, Cláudio Gomes, and G. Abbiati. “A Hybrid Testing Framework for Wind Turbine Mechanical Components.” Journal of Physics: Conference Series 2767, no. 5 (June 1, 2024): 052046. https://doi.org/10.1088/1742-6596/2767/5/052046.
  Abstract

  As wind turbines continue to increase in size, hybrid testing is emerging as a key-enabling solution for experimental assessment of their large mechanical components. Hybrid testing is conducted using a hybrid model which combines physically tested and numerically simulated components. In this work, the Kane method is proposed to formulate the equation of motion for the hybrid model of a wind turbine rotor system where one single blade is physically tested. The Kane method allows for formulating the equation of motion of multi-body-dynamic models efficiently and, therefore, it is widely used in state-of-art simulation software. The hybrid model of the rotor is successfully implemented on a single-degree-of-freedom test bench with a cantilever steel beam serving as the physical substructure. The performance of the implemented hybrid model is assessed through a comparison with a pure numerical simulation of the same system. The main finding of the study emphasizes the efficiency of incorporating physically measured restoring force as model parameters while formulating the equation of motion of a hybrid model.

• [PDF] David, Istvan, Guodong Shao, Cláudio Gomes, Dawn M. Tilbury, and Bassam Zarkout. “Interoperability of Digital Twins: Challenges, Success Factors, and Future Research Directions.” In Leveraging Applications of Formal Methods, Verification and Validation: Tools and Trends, 15223:27-46. Crete, Greece: Springer Nature Switzerland, 2024. https://doi.org/10.1007/978-3-031-75390-9_3.
  Abstract

  The widespread adoption of digital twins gave rise to emerging systems of interconnected digital twins, often dubbed aggregated or hierarchical digital twins. In such emerging systems, interoperability of digital twins is key in determining the capabilities and qualitative properties of the emerging system. In this paper, we report on a panel discussion that took place at the 2023 Annual Simulation Conference with four esteemed experts representing four distinct perspectives on the topic: strategic (why aggregated digital twins matter?), technical (how co-simulation supports a distributed set of concerns over multiple digital twins?), standardization (how standards enable interoperability?), and organizational (how organizations deal with digital twinning scenarios?). We report the panelists’ main arguments and synthesize them into a discussion. The main takeaway of the panel is that contrary to the state of affairs in digital twinning that limits interoperability to low levels, there is a clear need to reach higher levels of interoperability in digital twinning scenarios that necessitate a distributed approach. Moreover, there are emerging solutions to achieve these higher levels. To provide researchers with tangible leads, we distill challenges and success factors, and recommend future research directions in digital twin interoperability.

• [PDF] Gil, Santiago, Bentley J Oakes, Cláudio Gomes, Mirgita Frasheri, and Peter G Larsen. “Toward a Systematic Reporting Framework for Digital Twins: A Cooperative Robotics Case Study.” SIMULATION, August 2, 2024, 00375497241261406. https://doi.org/10.1177/00375497241261406.
  Abstract

  Digital Twins (DTs) can be constructed for many different applications, leading to substantial differences between different case studies. To be able to learn from the challenges and lessons learned by other DT practitioners, it is important that experience reports be consistent to facilitate comparisons. In this paper we merge three reference description frameworks for DTs, one generated from a systematic mapping study, one generated from an analysis of experience reports, and one from a systematic literature review, to come up with a unified characterization of DT applications. This analysis has identified six non-overlapping and three cross-cutting characteristics in the reference frameworks. This paper showcases the unified characterization with 21 characteristics to report on a DT case study called the Flex-cell, a manufacturing cell with two robotic arms used for cooperative assembly. The generalizability of this unified characterization is validated using a multi-case approach with another case study in robotics and another in the food industry. We call on the DT community to integrate these systematic reporting principles in their future DT experience reports such that other practitioners can learn from each other more effectively.

• [PDF] Oakes, Bentley, Cláudio Gomes, Eduard Kamburjan, Giuseppe Abbiati, Elif Ecem Bas, and Sebastian Engelsgaard. “Towards Ontological Service-Driven Engineering of Digital Twins.” In Proceedings of the ACM/IEEE 27th International Conference on Model Driven Engineering Languages and Systems, 464-69. MODELS Companion ’24. New York, NY, USA: Association for Computing Machinery, 2024. https://doi.org/10.1145/3652620.3688261.
  Abstract

  The systematic engineering of Digital Twins (DTs) requires the establishment of clear methodologies supported by intelligent tooling. We propose an approach to guide the user in the creation and deployment of services for DTs utilizing ontologies and workflows. In our approach, the user selects a desired DT service from an array of options. This selection is then used to suggest a) enablers and models to place in the DT, and b) development and deployment workflows for the DT service. The aim is to provide DT engineering guidance to assist non-software engineering experts to develop DT services more rapidly with less effort. We describe our initial work on applying this approach to a derived version of an industrial wind turbine generator case study, utilizing openCAESAR for ontology definition and enacting the workflows with Jupyter notebooks.

• [PDF] Fitzgerald, John, Cláudio Gomes, and Peter Gorm Larsen, eds. The Engineering of Digital Twins. Cham: Springer International Publishing, 2024. https://doi.org/10.1007/978-3-031-66719-0.
• [PDF] Hansen, Simon Thrane, Casper Thule, Cláudio Gomes, Kenneth Guldbrandt Lausdahl, Frederik Palludan Madsen, Giuseppe Abbiati, and Peter Gorm Larsen. ‘Co-Simulation at Different Levels of Expertise with Maestro2’. Journal of Systems and Software 209 (March 2024): 111905. https://doi.org/10.1016/j.jss.2023.111905.
  Abstract

  When different simulation units are coupled together there are different choices to take, in particular regarding the granularity of such a co-simulation. When prototyping systems, it is typically favourable to get an initial idea of how a collection of simulation units work together without spending too much time setting up the orchestration. However, the granularity of such a simulation may be far away from what is needed in relation to the purpose of the simulation. In order to enable more flexibility and control over the co-simulation it is necessary to be able to steer the orchestration in a more detailed manner. This paper presents an open source co-simulation orchestration engine based on the Functional Mockup Interface standard but with a Domain Specific Language (DSL) enabling detailed control between the individual simulation units. The same tool can thus be used right out of the box for low-granularity co-simulation, and for high-granularity simulation the DSL enable a significant flexibility.

• [PDF] Gil, Santiago, Peter H. Mikkelsen, Cláudio Gomes, and Peter G. Larsen. ‘Survey on Open‐source Digital Twin Frameworks-A Case Study Approach’. Software: Practice and Experience, 6 January 2024, spe.3305. https://doi.org/10.1002/spe.3305.
  Abstract

  Digital twin (DT) technology has been a topic with academic and industrial coverage in recent years. DTs are intended to be a virtual high-fidelity representation of a physical counterpart. Its complex nature requires several components to create and run a DT, and that is why many DT frameworks have been proposed in the literature. There are also many surveys of DTs, but none that is bottom-up with concrete examples and focused on open-source software. This survey analyzes 14 open-source DT frameworks in 10 different dimensions, which are then categorized in six different groups according to their modeling and technological domain, to present the reader different options for creating and managing DT applications, and to understand potential combinations, uses, and limitations of the tools. It also presents a case study with five of the explored DT frameworks, describing the process on how the DT is set up and comparing their capabilities based on the services to be provided by the DT. Finally, it discusses advantages and limitations of the tools according to domain, requirements, and scope, relevant aspects regarding built-in simulations and data analytics, theory-to-practice transition, and advantages/disadvantages of using open-source software instead of commercial. Main limitations of the study due to its narrow niche, conclusions, and opportunities for future research regarding the potential room for improvement in terms of out-of-the-box features and services for DTs, are also shown.

2023

• [PDF] Paredis, Randy, Cláudio Gomes, and Hans Vangheluwe. ‘A Family of Digital T Workflows and Architectures: Exploring Two Cases’. Innovative Intelligent Industrial Production and Logistics, Communications in Computer and Information Science, 1855 (2023): 93-109. https://doi.org/10.1007/978-3-031-37228-5_6.
  Abstract

  Digital Models/Shadows/Twins/. . . have been given numerous definitions and descriptions in the literature. There is no consensus on terminology, nor a comprehensive description of workflows nor architectures. In this paper, we use the catch-all “Digital T” (pronounced “Digital Twinning”) to refer to all concepts, techniques, architectures, . . . related to the “twinning” paradigm. In this paradigm, virtual instances, known as twins, of a System under Study (SuS) are continually updated with the SuS’s health, performance, and maintenance status, over its entire life-cycle. Digital T can be used for monitoring, analysis, optimization, and adaptation of complex engineered systems, in particular after these systems have been deployed. Digital T makes full use of both historical knowledge and of streaming data from sensors. Following Multi-Paradigm Modelling (MPM) principles, this paper proposes to explicitly model construction/use workflows as well as architectures and deployment of Digital T. Applying product family modelling allows for the de-/re-construction of the different Digital T variants in a principled, reproducible and partially automatable manner. Two small illustrative cases are discussed: a Line-Following Robot and an Incubator. These are representative for respectively an Automated Guided Vehicle and an Industrial Convection Oven, both important in an industrial context.

• [PDF] Kazemi, Zahra, Jonas Kjaer Rask, Cláudio Gomes, Emre Yildiz, and Peter Gorm Larsen. ‘Movable Factory—A Systematic Literature Review of Concepts, Requirements, Applications, and Gaps’. Journal of Manufacturing Systems 69 (August 2023): 189-207. https://doi.org/10.1016/j.jmsy.2023.06.008.
  Abstract

  The ability to move production between different geographical locations has recently become more important, due to the increasing need for sustainability and faster response times, complying with local regulations, and dealing with brittle international supply chains. To this end, the movable factory concept has been put forward, which is defined as mobile production units that can be installed near the customer’s location. Unfortunately, industrial paradigms such as Industry 4.0 make little if any notice of movable factories but instead focus on the digitalization of their fixed counterparts. In the form of a systematic literature review, this paper breaks down the concept of the movable factory, relates it to the state-of-the-art, and summarizes its main use cases, requirements, research gaps, and opportunities. The review covers over 100 relevant articles published in the past two decades. Compared to existing surveys, we have not only focused on the motivations for movable factories, but also identified research gaps and discuss the impact of modern technologies such as Internet-of-Things, digital twins, and modeling and simulation in fulfilling these gaps. The result is a survey of the state-of-the-art and a list of domains for potential future research on different aspects of movable factories.

• [PDF] Talasila, Prasad, Cláudio Gomes, Peter Høgh Mikkelsen, Santiago Gil Arboleda, Eduard Kamburjan, and Peter Gorm Larsen. “Digital Twin as a Service (DTaaS): A Platform for Digital Twin Developers and Users.” In 2023 IEEE Smart World Congress (SWC), 1-8. Portsmouth, United Kingdom: IEEE, 2023. https://doi.org/10.1109/SWC57546.2023.10448890.
  Abstract

  Establishing digital twins is a non-trivial endeavour especially when users face significant challenges in creating them from scratch. Ready availability of reusable models, data and tool assets, can help with creation and use of digital twins. A number of digital twin frameworks exist to facilitate creation and use of digital twins. In this paper we propose a digital twin framework to author digital twin assets, create digital twins from reusable assets and make the digital twins available as a service to other users. The proposed framework automates the management of reusable assets, storage, provision of compute infrastructure, communication and monitoring tasks. The users operate at the level of digital twins and delegate the rest of the work to the digital twin as a service framework.

• [PDF] Loper, Margaret L., Tuncer Ören, Cláudio Gomes, Valdemar Vicente Graciano Neto, and Ernest H. Page. “Infrastructure.” In Body of Knowledge for Modeling and Simulation, edited by Tuncer Ören, Bernard P. Zeigler, and Andreas Tolk, 149-65. Simulation Foundations, Methods and Applications. Cham: Springer International Publishing, 2023. https://doi.org/10.1007/978-3-031-11085-6_6.
  Abstract

  Providing the appropriate infrastructure for simulation is the topic of this chapter of the SCS M&S Body of Knowledge. It provides sections on various simulation standards, standard organizations, and compliance certificates. Publications of some applicable codes of best practices and lessons learned are provided as well as a section on resource repositories. The currently dominant standard Distributed Interactive Simulation (DIS) and High-Level Architecture (HLA) conclude the chapter.

• [PDF] Böttjer, Till, Daniella Tola, Fatemeh Kakavandi, Christian R. Wewer, Devarajan Ramanujan, Cláudio Gomes, Peter G. Larsen, and Alexandros Iosifidis. “A Review of Unit Level Digital Twin Applications in the Manufacturing Industry.” CIRP Journal of Manufacturing Science and Technology 45 (October 2023): 162-89. https://doi.org/10.1016/j.cirpj.2023.06.011.
  Abstract

  In recent years, the hype around Digital Twins (DTs) has been exponentially increasing in both industry and academia. DTs are a potential solution to increase automation and advance towards Smart Manufacturing. Manufacturing DTs have been implemented at different hierarchical levels, ranging from system of systems to unit level. Increasing computational capacity and data exchange rates can enable DT implementations for real-time applications. Several literature reviews on manufacturing DTs have been published. However, no previous paper focuses on manufacturing DTs at the unit level for which real-time control is most applicable. Simultaneously, the challenges to engineer DTs with real-time capabilities are enormous, both from a scientific and technological perspective. Therefore, we focus on DTs of single production units such as traditional machine tools, additive manufacturing machines and advanced robotic applications. In this systematic literature review, 96 papers about practical unit level DT applications found in the Scopus database using a combination of the keywords “Digital Twin”, “Production” and “Manufacturing” are reviewed. We summarize how DTs are currently implemented and operated, and what potential benefits DTs offer at the unit process level in four categories: generic reference models, services, DT content (models and data) and DT deployment (hardware and software). Following the thematic analysis, an overall discussion, summary of key contributions and identified research gaps, and outlook into future research avenues is given. Key findings of this review can be summarized as: focus on DT components versus being holistic; need to share data and models across multiple stakeholders; lack of physical fidelity of the models; stark contrast of lab scale developments and real world testing, e.g., historical data and storage related challenges; lack of clear definition of DT in industry, and missing semantic interoperability between a wide variety of domains.

• [PDF] Abbiati, Giuseppe, Ecem E. Baş, Cláudio Gomes, and Peter Gorm Larsen. “Hybrid Fire Testing Using FMI-Based Co-Simulation.” Fire Safety Journal 139 (August 2023): 103832. https://doi.org/10.1016/j.firesaf.2023.103832.
  Abstract

  The failure mode of a structural component subjected to fire loading is sensitive to its mechanical boundary conditions. It follows that controlling boundary conditions is crucial to obtain a realistic loading scenario with a fire experiment. Hybrid fire testing has been developed for this purpose. Specifically, a structural component under test is loaded using actuators and exposed to fire loading. A coordination algorithm updates actuator setpoints on the fly so that the tested structural component experiences the sought mechanical boundary conditions, e.g., associated with a virtual assembly that is simulated numerically. To enable interoperability of simulation tools and control systems utilized in hybrid testing, a number of middlewares have been proposed, some of which have been adapted to hybrid fire testing. Middlewares facilitate the implementation of hybrid fire testing within the same laboratory. However, the portability of hybrid models from one laboratory to another is critical when different middleware and simulation tools are adopted. A consequence of that is that round-robin verification cannot be easily deployed for hybrid fire testing. In response to this limitation, this paper proposes to craft hybrid fire testing experiments using the Co-Simulation paradigm supported by the Functional Mock-up Interface standard. The methodology is demonstrated experimentally.

• [PDF] Alfalouji, Qamar, Thomas Schranz, Basak Falay, Sandra Wilfling, Johannes Exenberger, Thorsten Mattausch, Cláudio Gomes, and Gerald Schweiger. “Co-Simulation for Buildings and Smart Energy Systems — A Taxonomic Review.” Simulation Modelling Practice and Theory 126 (July 2023): 102770. https://doi.org/10.1016/j.simpat.2023.102770.
  Abstract

  Modeling buildings and smart energy systems requires coupling a wide range of components into one unified simulation process, which can be difficult given the complexity of these systems. Hence, a large number of researchers integrate separate simulations for each of the individual components in a co-simulation instead. To systematically analyze techniques, standards, tools and applications of co-simulation in the field of buildings and smart energy systems, the publications on co-simulations in this field are reviewed by means of taxonomic analysis. Furthermore, the reproducibility as well as the validation approach of the respective papers are evaluated. Results show that Functional Mock-up Interface (FMI) is the most prominent standard for co-simulation in modeling buildings and smart energy systems. Co-simulation is mostly used in Heating, Ventilation, and Air Conditioning (HVAC) and occupancy analysis applications. Since nearly 70% of publications are not reproducible, we advocate that journals and funding agencies adopt stricter data and code-sharing policies.

• [PDF] Kakavandi, Fatemeh, Cláudio Gomes, Roger De Reus, Jeppe Badstue, Jakob Langdal Jensen, Peter Gorm Larsen, and Alexandros Iosifidis. “Towards Developing a Digital Twin for a Manufacturing Pilot Line: An Industrial Case Study.” In Digital Twin Driven Intelligent Systems and Emerging Metaverse, 39-64. Singapore: Springer Nature Singapore, 2023. https://doi.org/10.1007/978-981-99-0252-1_2.
• [PDF] Naseri, Farshid, Santiago Gil, Corneliu Barbu, Erdal Cetkin, Gulsah Yarimca, Anders Jensen, Peter Gorm Larsen, and Cláudio Gomes. “Digital Twin of Electric Vehicle Battery Systems: Comprehensive Review of the Use Cases, Requirements, and Platforms.” Renewable and Sustainable Energy Reviews 179 (2023): 113280. https://doi.org/10.1016/j.rser.2023.113280.
  Abstract

  Transportation electrification has been fueled by recent advancements in the technology and manufacturing of battery systems, but the industry yet is facing serious challenges that could be addressed using cutting-edge digital technologies. One such novel technology is based on the digital twining of battery systems. Digital twins (DTs) of batteries utilize advanced multi-layer models, artificial intelligence, advanced sensing units, Internet-of-Things technologies, and cloud computing techniques to provide a virtual live representation of the real battery system (the physical twin) to improve the performance, safety, and cost-effectiveness. Furthermore, they orchestrate the operation of the entire battery value chain offering great advantages, such as improving the economy of manufacturing, re-purposing, and recycling processes. In this context, various studies have been carried out discussing the DT applications and use cases from cloud-enabled battery management systems to the digitalization of battery testing. This work provides a comprehensive review of different possible use cases, key enabling technologies, and requirements for battery DTs. The review inclusively discusses the use cases, development/integration platforms, as well as hardware and software requirements for implementation of the battery DTs, including electrical topics related to the modeling and algorithmic approaches, software architectures, and digital platforms for DT development and integration. The existing challenges are identified and circumstances that will create enough value to justify these challenges, such as the added costs, are discussed.

• [PDF] Bogomolov, Sergiy, Cláudio Gomes, Carlos Isasa, Sadegh Soudjani, Paulius Stankaitis, and Thomas Wright. “Reachability Analysis of FMI Models Using Data-Driven Dynamic Sensitivity.” SIMULATION, Special Issue: Engineering of Dependable Digital Twins, 2023, 00375497241261409. https://doi.org/10.1177/00375497241261409.
  Abstract

  Digital Twin is a technology that facilitates a real-time coupling of a cyber-physical system and its virtual representation. The technology is applicable to a variety of domains and facilitates more intelligent and dependable system design and operation, but it relies heavily on the existence of digital models that can be depended upon. In realistic systems, there is no single monolithic digital model of the system. Instead, the system is broken into subsystems, with models exported from different tools corresponding to each subsystem. In this paper, we focus on techniques that can be used for a black box model, such as the ones implementing the Functional Mock-up Interface (FMI) standard, formal analysis, and verification. We propose two techniques for simulation-based reachability analysis of models. The first one is based on system dynamics, while the second one utilises dynamic sensitivity analysis to improve the quality of the results. Our techniques employ simulations to obtain the model’s sensitivity with respect to the initial state (or model’s Lipschitz constant) which is then used to compute reachable states of the system. The approaches also provide probabilistic guarantees on the accuracy of the computed reachable sets that are based on simulations. Each technique requires different levels of information about the black box system, allowing the readers to select the best technique according to the capabilities of the models. The validation experiments have demonstrated that our proposed algorithms compute accurate reachable sets of stable and unstable linear systems. The approach based on dynamic sensitivity provides an accurate and, with respect to system dimensions, more scalable approach, while the sampling-based method allows a flexible trade-off between accuracy and runtime cost. The validation results also show that our approaches are promising even when applied to non-linear systems, especially, when applied to larger and more complex systems. The reproducibility package with code and data can be found at https://github.com/twright/SIM-Black-Box-Reachability.

• [PDF] Inci, Emin Oguz, Cláudio Gomes, Jan Croes, and Wim Desmet. “Error Estimators for Adaptive Scheduling Algorithm for Serial Co-Simulation.” In Annual Modelling and Simulation Conference, 73-83. Ontario, Canada, 2023.
  Abstract

  In serial co-simulation, the subsystems are solved and exchange their variables subsequently. For this reason, the solution sequence of the subsystems plays an important role. Prior work has demonstrated the effect of solution sequence on the accuracy of serial co-simulation and proposed an adaptive scheduling algorithm that predicts when to change the solution sequence using global input estimation error. This paper introduces another mechanism that considers physical causality to determine the computational causality in the co-simulation. We benchmark this method against input estimation error and static solution sequences in adaptive scheduling algorithm in serial co-simulations of a linear problem. The resulting method generally avoids the worst solution sequence and mitigates the risk of co-simulation being unstable if one of the sequences produces a greater error.

• [PDF] Oakes, Bentley, Cláudio Gomes, Peter Gorm Larsen, Joachim Denil, Julien DeAntoni, João Cambeiro, and John Fitzgerald. “Examining Model Qualities and Their Impact on Digital Twins.” In Annual Modelling and Simulation Conference, 220-32. Ontario, Canada, 2023.
  Abstract

  Digital Twins (DTs) are built using modelling and simulation techniques in complex domains such as cyber-physical systems. However, further formal investigation is required for how a DT and the services it provides relate to the qualities of the models used by a service. Specifically, this article examines when a DT service can be said to have the qualities of relevant, verifiable, substitutable, and faithful based on the results of checking properties in comparison to the actual system. Using an incubator system as our running example, we show how a DT service relies on multiple models, present the consequences when these qualities are violated, and discuss strategies for adapting models to ensure these qualities.

• [PDF] Hansen, Simon Thrane, Cláudio Gomes, and Zahra Kazemi. “Synthesizing Orchestration Algorithms for FMI 3.0.” In 2023 Annual Modeling and Simulation Conference, 184-95. Ontario, Canada, 2023.
  Abstract

  An essential part of building reliable cyber-physical systems is to be able to predict the behavior of such systems using accurate simulations. Standards for describing the behavior of such systems and simulations are evolving to facilitate a broader range of applications. The Functional Mockup Interface (FMI) standard is no exception. FMI 3.0 introduces synchronous clocks to facilitate efficient and repeatable simulation of event-driven systems. Nevertheless, the standard does not specify how to implement the synchronization of models, and it is up to the tool vendors to implement the orchestration algorithm responsible for this task. This paper presents the first approach to synthesizing the orchestration algorithm for FMI 3.0 supporting synchronous clocks. A prototype implementation of the algorithm is presented.

• [PDF] Frasheri, Mirgita, Henrik Ejersbo, Casper Thule, Cláudio Gomes, Jakob Levisen Kvistgaard, Peter Gorm Larsen, and Lukas Esterle. “Addressing Time Discrepancy between Digital and Physical Twins.” Robotics and Autonomous Systems, Software Engineering for Robotics, 161, no. March 2023, 104347 (2023): 104347. https://doi.org/10.1016/j.robot.2022.104347.
  Abstract

  Digital twins (DTs) represent a key technology in the development, real-time monitoring and optimisation of cyber-physical systems (CPSs). Such potential emerges as a result of the real-time coupling between DTs and their physical counterparts, where it is possible to make use of operational data as it is being generated in order to aid decision-making. Harnessing this potential presents several design challenges, such as the parallel operation of the DT and its physical twin (PT), and the necessary synchronisation thereof, to ensure coherent execution of the system in ensemble. In this paper we present an approach that handles situations where a DT and its PT get out of sync as a result of disturbances in the normal operational conditions of the DT-PT system, e.g., due to network degradation or temporary network drop. The purpose is to provide a best-effort functionality covering: user notification, degradation of DT to digital shadow (DS), with recovery mechanisms to re-establish the synchronisation between DT and PT.

2022

• [PDF] Esterle, Lukas, Henrik Ejersbo, Mirgita Frasheri, Cláudio Gomes, Hugo Daniel Macedo, and Peter Gorm Larsen. “Digital Twins for Autonomous Intelligent Systems: From Development to Deployment.” In 2022 IEEE International Conference on Autonomic Computing and Self-Organizing Systems Companion (ACSOS-C), 53-54. CA, USA: IEEE, 2022. https://doi.org/10.1109/ACSOSC56246.2022.00028.
• [PDF] Feng, Hao, Cláudio Gomes, Santiago Gil, Peter H. Mikkelsen, Daniella Tola, Peter Gorm Larsen, and Michael Sandberg. “Integration Of The Mape-K Loop In Digital Twins.” In 2022 Annual Modeling and Simulation Conference (ANNSIM), 102-13. San Diego, CA, USA: IEEE, 2022. https://doi.org/10.23919/ANNSIM55834.2022.9859489.
  Abstract

  In recent years the concept of Digital Twins (DTs) has gained significant attention, in particular in the manufacturing industry. In proper DTs there is a bidirectional connection to/from their Physical Twins such that it is possible to leave humans out of the loop. However, the underlying ideas of DTs have been inspired from a number of different scientific communities, in particular in relation to autonomy including the “Monitor-Analyze-Plan-Execute over a shared Knowledge” (MAPE-K). In this paper, we bridge the two concepts by demonstrating an implementation of a MAPE-K loop for an incubator case study within the context of its DT. We also discuss some of the limitations of the MAPE-K concept when applied to Cyber-Physical Systems (CPS).

• [PDF] Feng, Hao, Cláudio Gomes, Michael Sandberg, Hugo Daniel Macedo, and Peter Gorm Larsen. “Under What Conditions Does a Digital Shadow Track a Periodic Linear Physical System?” In Software Engineering and Formal Methods. SEFM 2021 Collocated Workshops, 13230:143-55. Cham: Springer International Publishing, 2022. https://doi.org/10.1007/978-3-031-12429-7_11.
  Abstract

  The synchronization between a Digital Shadow (DS) and a Cyber-Physical System (CPS) is paramount to enable anomaly detection, predictive maintenance, what-if analysis, etc. Such synchronization means that a simulation reflects, as closely as possible, the states of the CPS. The simulation however, requires the complete initial state of the system to be known, which is often infeasible in real applications. In our work, we study the conditions under which knowing the initial state of the system is irrelevant for a simulation to eventually synchronize with the CPS. We apply traditional stability analysis to answer this question for linear periodic systems. We demonstrate the method using a simple but representative system, an incubator with a periodic control signal.

• [PDF] Hansen, Simon Thrane, Cláudio Gomes, Masoud Najafi, Torsten Sommer, Matthias Blesken, Irina Zacharias, Oliver Kotte, et al. “The FMI 3.0 Standard Interface for Clocked and Scheduled Simulations.” Electronics 11, no. 21 (November 7, 2022): 3635. https://doi.org/10.3390/electronics11213635.
  Abstract

  This paper presents an overview and formalization of the Functional Mock-up Interface (FMI) 3.0. The formalization captures the new FMI 3.0 standard and is intended to be used as an introduction for conceptualizing the use of clocks in the FMI standard to support the simulation of event-based cyber-physical systems. The FMI 3.0 standard supports two kinds of clock-based simulations: Synchronous Clocked Simulation to ensure predictable systems scheduling with multiple simultaneous events and scheduled execution to facilitate real-time simulations comprising multiple black-box models by allowing fine-grained control over the computation time of submodels. The formalization is a basis for developing tools for orchestrating, verifying and validating the composition of multiple FMUs. The formalization is provided as an accessible VDM-SL specification.

• [PDF] Hansen, Simon Thrane, Casper Thule, Cláudio Gomes, Jaco van de Pol, Maurizio Palmieri, Emin Oguz Inci, Frederik Madsen, Jesús Alfonso, José Ángel Castellanos, and José Manuel Rodriguez. “Verification and Synthesis of Co-Simulation Algorithms Subject to Algebraic Loops and Adaptive Steps.” International Journal on Software Tools for Technology Transfer 24, no. 6 (December 2022): 999-1024. https://doi.org/10.1007/s10009-022-00686-8.
  Abstract

  Simulation-based analyses are becoming increasingly vital for the development of cyber-physical systems. Co-simulation is one such technique, enabling the coupling of specialized simulation tools through an orchestration algorithm. The orchestrator describes how to coordinate the simulation of multiple simulation tools. The simulation result depends on the orchestration algorithm that must stabilize algebraic loops, choose the simulation resolution, and adhere to each simulation tool’s implementation. This paper describes how to verify that an orchestration algorithm respects all contracts related to the simulation tool’s implementation and how to synthesize such tailored orchestration algorithms. The approaches work for complex and adaptive co-simulation scenarios and have been applied to several real-world case studies.

• [PDF] Kakavandi, Fatemeh, Roger De Reus, Cláudio Gomes, Negar Heidari, Alexandros Iosifidis, and Peter Gorm Larsen. “Product Quality Control in Assembly Machine under Data Restricted Settings.” In 2022 IEEE 20th International Conference on Industrial Informatics (INDIN), 735-41. Perth, Australia: IEEE, 2022. https://doi.org/10.1109/INDIN51773.2022.9976173.
  Abstract

  Evaluating the product quality in an assembly machine is critical yet time-consuming since, in product assessment in batch manufacturing, a certain amount of products should be investigated in an invasive manner. However, continuous manufacturing ensures product quality assessment during assembly with high efficiency and traceability. This paper proposes a quality assessment method for an industrial use case. First, the data is prepared based on two indicators and expert knowledge. Then two data classification approaches (one-class classification and binary classification) are applied to evaluate the products’ quality by analysing the related data. Finally, the most efficient model is selected to predict the product labels and deviate anomalies from normal products. For the studied use case and the limited number of products, the binary classifier guarantees to detect 100% of defective products. The proposed approach can provide the engineers and operators with understandable extracted process knowledge, and can therefore be adapted to a high-speed manufacturing line where large data volume and process complexity can be problematic.

• [PDF] Kulik, Tomas, Cláudio Gomes, Hugo Daniel Macedo, Stefan Hallerstede, and Peter Gorm Larsen. “Towards Secure Digital Twins.” In Leveraging Applications of Formal Methods, Verification and Validation. Practice, edited by Tiziana Margaria and Bernhard Steffen, 13704:159-76. Cham: Springer Nature Switzerland, 2022. https://doi.org/10.1007/978-3-031-19762-8_11.
  Abstract

  Advanced digital technology is finding its way into industrial production and control systems. This led to development of further concepts such as digital shadow and digital twin. In the former an accurate model of the cyber-physical system (CPS) is used to monitor it virtually, while the latter provides a possibility to adapt the CPS’s behavior. These developments are often welcome from the operators perspective, however they also pose new challenges in terms of cyber security: an operator could be led to believe the system is operating correctly due to the represented digital image while the CPS is under a cyber attack. In this paper we investigate several cyber security challenges of the digital twin technology and discuss potential mitigations for these challenges based on well established practices within the area of industrial control systems. We further describe the potential cyber attacks and mitigations using a semi-formal notation based on problem frames, we suggest in order to simplify the communication about cyber security challenges of digital twins between different stakeholders. This is shown within a context of a small case study. Finally we outline areas of research for the development of secure digital twin technology.

• [PDF] Madsen, Emil, Daniella Tola, Carlos Hansen, Cláudio Gomes, and Peter Gorm Larsen. “AURT: A Tool for Dynamics Calibration of Robot Manipulators.” In 2022 IEEE/SICE International Symposium on System Integration (SII), 190-95. Narvik, Norway: IEEE, 2022. https://doi.org/10.1109/SII52469.2022.9708769.
  Abstract

  We introduce AURT, an open-source software for modeling and calibration of robot manipulator dynamics. AURT includes rigid-body dynamics and a selection of joint dynamics models. The separation of the robot dynamics into rigid-body dynamics and joint dynamics allows for fast recalibration of the robot dynamics using different joint dynamics models. We evaluate the calibration performance using a Universal Robots UR5e robot manipulator. We also demonstrate how the joint dynamics models embedded in AURT outperforms the standard approach of the state of the art.

• [PDF] Tola, Daniella, Emil Madsen, Cláudio Gomes, Lukas Esterle, Christian Schlette, Casper Hansen, and Peter Gorm Larsen. “Towards Easy Robot System Integration: Challenges and Future Directions.” In 2022 IEEE/SICE International Symposium on System Integration (SII), 77-82. Narvik, Norway: IEEE, 2022. https://doi.org/10.1109/SII52469.2022.9708846.
  Abstract

  The amount of robots in industry is increasing, pushing the need for easier configuration and integration of robots. The cost of robot integration is often larger than the cost of the robots themselves. With the increasing complexity of robotic systems and the tasks they perform, the future of reducing the expenses of robot system integration is significant. In this position paper, we report on some of the main challenges in robot system configuration, and give an overview of the current state-of-the-art of robot system configuration. We briefly look into the configuration of personal computers (PCs) and study how the robot system configuration can be improved inspired by the standards used in the PC area. We conclude with concrete proposals as to what can be done in the future, to make the process of robot system configuration and integration easier and cheaper.

• [PDF] Wright, Thomas, Cláudio Gomes, and Jim Woodcock. “Formally Verified Self-Adaptation of an Incubator Digital Twin.” In Leveraging Applications of Formal Methods, Verification and Validation. Practice, 13704:89-109. Cham: Springer Nature Switzerland, 2022. https://doi.org/10.1007/978-3-031-19762-8_7.
  Abstract

  The performance and reliability of Cyber-Physical Systems are increasingly aided through the use of digital twins, which mirror the static and dynamic behaviour of a Cyber-Physical System (CPS) in software. Digital twins enable the development of self-adaptive CPSs which reconfigure their behaviour in response to novel environments. It is crucial that these self-adaptations are formally verified at runtime, to avoid expensive re-certification of the reconfigured CPS. In this paper, we demonstrate formally verified self-adaptation in a digital twinning system, by constructing a non-deterministic model which captures the uncertainties in the system behaviour after a self-adaptation. We use Signal Temporal Logic to specify the safety requirements the system must satisfy after reconfiguration and employ formal methods based on verified monitoring over Flow* flowpipes to check these properties at runtime. This gives us a framework to predictively detect and mitigate unsafe self-adaptations before they can lead to unsafe states in the physical system.

2021

• [PDF] Böttjer, Till, Georg Ørnskov Rønsch, Cláudio Gomes, Devarajan Ramanujan, Alexandros Iosifidis, and Peter Gorm Larsen. “Data-Driven Identification of Remaining Useful Life for Plastic Injection Moulds.” In Towards Sustainable Customization: Bridging Smart Products and Manufacturing Systems, 431-39. Cham: Springer International Publishing, 2021. https://doi.org/10.1007/978-3-030-90700-6_49.
  Abstract

  Throughout their useful life, plastic injection moulds operate in rapidly varying cyclic environments, and are prone to continual degradation. Quantifying the remaining useful life of moulds is a necessary step for minimizing unplanned downtime and part scrap, as well as scheduling preventive mould maintenance tasks such as cleaning and refurbishment. This paper presents a data-driven approach for identifying degradation progression and remaining useful life of moulds, using real-world production data. An industrial data set containing metrology measurements of a solidified plastic part, along with corresponding lifecycle data of 13 high production volume injection moulds, was analyzed. Multivariate Statistical Process Control techniques and XGBoost classification models were used for constructing data-driven models of mould degradation progression, and classifying mould state (early run-in, production, worn-out). Results show the XGBoost model developed using element metrology & relevant mould lifecycle data classifies worn-out moulds with an in-class accuracy of 88%. Lower in-class accuracy of 73% and 61% were achieved for the compared to mould-worn out less critical early run-in and production states respectively.

• [PDF] Esterle, Lukas, Cláudio Gomes, Mirgita Frasheri, Henrik Ejersbo, Sven Tomforde, and Peter G. Larsen. “Digital Twins for Collaboration and Self-Integration.” In 2021 IEEE International Conference on Autonomic Computing and Self-Organizing Systems Companion (ACSOS-C), 172-77. DC, USA: IEEE, 2021. https://doi.org/10.1109/ACSOS-C52956.2021.00040.
  Abstract

  Digital twins are digital representations of cyber-physical systems, allowing bi-directional data and control flow between them. In this paper, we introduce the digital twin concept in self-integrating and self-improving systems. Specifically, we highlight the usage of digital twin models for autonomous systems spontaneously interacting with each other and aiming to improve collaboration and mutual safety. We present the main challenges and an initial architecture to enable autonomous cyber-physical systems to collaborate, self-integrate, and self-improve during run-time.

• [PDF] Feng, Hao, Cláudio Gomes, Michael Sandberg, Casper Thule, Kenneth Lausdahl, and Peter Gorm Larsen. “Developing a Physical and Digital Twin: A Process Model.” In 2021 ACM/IEEE International Conference on Model Driven Engineering Languages and Systems Companion (MODELS-C). Fukuoka, Japan: IEEE, 2021. https://doi.org/10.1109/MODELS-C53483.2021.00050.
  Abstract

  As a result of the fast development of technologies and diversity of requirements, complexity exists not only in a system itself but also in the development process and environment. The complexity in the development process comes from specialization and concurrency, and a Digital Twin (DT) has the potential to mitigate these issues. We hope, during the process of building a DT, the accidental complexity can be minimized by application of model-based systems engineering and the Multi-Paradigm Modeling (MPM). In this paper, we give an example of the process of building a DT combined with a process model. Our long term goal is to model the development of various DTs to identify the commonalities and variations across multiple DTs.

• [PDF] Feng, Hao, Cláudio Gomes, Casper Thule, Kenneth Lausdahl, Alexandros Iosifidis, and Peter Gorm Larsen. “Introduction to Digital Twin Engineering.” In 2021 Annual Modeling and Simulation Conference (ANNSIM), 1-12. Fairfax, VA, USA: IEEE, 2021. https://doi.org/10.23919/ANNSIM52504.2021.9552135.
  Abstract

  Cyber-Physical Systems (CPSs) are getting increasingly complex and generate large amounts of data. Analyzing such data provides us an insight into a given system. The digital twin concept emerges as an attempt to seamlessly integrate the data and insight in order to improve system performances. It enables applications such as visualization, monitoring, state estimation, and self-adaptation. In this paper, we demonstrate the construction of a digital twin exemplified by an incubator system, including the benefits and challenges of each application. The result is a description of the building blocks of a digital twin.

• [PDF] Feng, Hao, Cláudio Gomes, Casper Thule, Kenneth Lausdahl, Michael Sandberg, and Peter Gorm Larsen. “The Incubator Case Study for Digital Twin Engineering.” arXiv:2102.10390 [Cs, Eess], February 20, 2021. http://arxiv.org/abs/2102.10390.
  Abstract

  To demystify the Digital Twin concept, we built a simple yet representative thermal incubator system. The incubator is an insulated box fitted with a heatbed, and complete with a software system for communication, a controller, and simulation models. We developed two simulation models to predict the temperature inside the incubator, one with two free parameters and one with four free parameters. Our experiments showed that the latter model was better at predicting the thermal inertia of the heatbed itself, which makes it more appropriate for further development of the digital twin. The hardware and software used in this case study are available open source, providing an accessible platform for those who want to develop and verify their own techniques for digital twins.

• [PDF] Gomes, Cláudio, Giuseppe Abbiati, and Peter Gorm Larsen. “Seismic Hybrid Testing Using FMI-Based Co-Simulation.” In Proceedings of the 14th International Modelica Conference. online: Linköping University Electronic Press, Linköpings Universitet, 2021. https://doi.org/10.3384/ecp21181287.
  Abstract

  Hybrid testing is an experimental technique extensively utilized in earthquake engineering to study the seismic response of structures. It requires coupling physical and numerical models in a closed feedback loop. Although this methodology is mature, a commonly accepted standard for orchestrating simulations and experiments is still missing. As a result, setting up a hybrid testing campaign still requires substantial system integration effort, which is often not affordable. In this paper, we propose the Functional Mockup Interface as a possible standard for orchestrating hybrid testing and discuss the limitations in enabling such support.

• [PDF] Gomes, Cláudio, Masoud Najafi, Torsten Sommer, Matthias Blesken, Irina Zacharias, Oliver Kotte, Pierre Mai, et al. “The FMI 3.0 Standard Interface for Clocked and Scheduled Simulations.” In Proceedings of the 14th International Modelica Conference. online: Linköping University Electronic Press, Linköpings Universitet, 2021. https://doi.org/10.3384/ecp2118127.
  Abstract

  This paper gives an overview of the FMI 3.0 support for two kinds of clock-based simulations: Synchronous Clocked Simulation, and Scheduled Execution. The former is used when the information about multiple simultaneous events (cause and exact time of occurrence) can be unambiguously conveyed. The later facilitates real-time simulations comprising multiple black-box models, by allowing fine grained control over the computation time of sub-models. A formalization is presented along with example application scenarios, meant as an introduction to the conceptualization of clocks in the FMI Standard.

• [PDF] Hansen, Simon Thrane, Cláudio Gomes, Peter Gorm Larsen, and Jaco Van de Pol. “Synthesizing Co-Simulation Algorithms with Step Negotiation and Algebraic Loop Handling.” In 2021 Annual Modeling and Simulation Conference (ANNSIM), 1-12. Fairfax, VA, USA: IEEE, 2021. https://doi.org/10.23919/ANNSIM52504.2021.9552073.
  Abstract

  Simulation-based analyses of cyber-physical systems are becoming increasingly vital. Co-simulation is one such technique that enables the coupling of specialized simulation tools through an orchestration algorithm that dictates when, with which inputs, and how far each simulation tool should advance its corresponding subsystem. The result of a co-simulation is often sensitive to the orchestration algorithm. This algorithm should be designed based on the simulation tool’s implementation to minimize the co-simulation error. The paper extends current graph-based approaches for generating implementation-aware simulation algorithms to cover complex co-simulation scenarios where step negotiation and algebraic loops are part of the simulation. We show how the generation takes place for different complex co-simulation scenarios. An implementation of the approach is available online.

• [PDF] Hansen, Simon Thrane, Cláudio Gomes, Maurizio Palmieri, Casper Thule, Jaco van de Pol, and Jim Woodcock. “Verification of Co-Simulation Algorithms Subject to Algebraic Loops and Adaptive Steps.” In Formal Methods for Industrial Critical Systems, edited by Alberto Lluch Lafuente and Anastasia Mavridou, 12863:3-20. Lecture Notes in Computer Science. Cham: Springer International Publishing, 2021. https://doi.org/10.1007/978-3-030-85248-1_1.
  Abstract

  Simulation-based analyses of cyber-physical systems are increasingly vital. Co-simulation is one such technique that enables the coupling of specialized simulation tools through an orchestration algorithm. The orchestrator dictates how each simulation tool should simulate its corresponding subsystem. Obtaining correct simulation results requires an implementation-aware orchestration algorithm tailored to the specific scenario, without the orchestrator knowing each simulation tool’s implementation. Such an algorithm should stabilize algebraic loops, perform time step negotiation, and adhere to each simulation tool’s implementation. This paper describes an approach and implementation to prove that a given orchestration algorithm respects all contracts related to the simulation units’ implementation. The approach has been applied to an industrial case study and other complex scenarios. The tool and results are available online.

• [PDF] Hansen, Simon Thrane, Casper Thule, and Cláudio Gomes. “An FMI-Based Initialization Plugin for INTO-CPS Maestro 2.” In Software Engineering and Formal Methods. SEFM 2020 Collocated Workshops, edited by Loek Cleophas and Mieke Massink, 295-310. Virtual event: Springer International Publishing, 2021. https://doi.org/10.1007/978-3-030-67220-1_22.
  Abstract

  The accuracy of the result of a co-simulation is dependent on the correct initialization of all the simulation units. In this work, we consider co-simulation where the simulation units are described as Functional Mock-up Units (FMU). The Functional Mock-up Interface (FMI) specification specifies constraints to the initialization of variables in the scope of a single FMU. However, it does not consider the initialization of interconnected variables between instances of FMUs. Such interconnected variables place particular constraints on the initialization order of the FMUs. The approach taken to calculate a correct initialization order is based on predicates from the FMI specification and the topological ordering of both internal connections and interconnected variables. The approach supports the initialization of co-simulation scenarios containing algebraic loops using fixed point iteration. The approach has been realized as a plugin for the open-source INTO-CPS Maestro 2 Co-simulation framework. It has been tested for various scenarios and compared to an existing Initializer that has been validated through academic and industrial application.

• [PDF] Inci, Emin Oguz, Jan Croes, Wim Desmet, Cláudio Gomes, Casper Thule, Kenneth Lausdahl, and Peter Gorm Larsen. “The Effect and Selection of Solution Sequence in Co-Simulation.” In 2021 Annual Modeling and Simulation Conference (ANNSIM), 1-12. Fairfax, VA, USA: IEEE, 2021. https://doi.org/10.23919/ANNSIM52504.2021.9552130.
  Abstract

  In non-iterative serial co-simulation, the solution sequence of the systems is ordered either ad-hoc or based on the solution priorities. The selection of the priorities determines which systems approximate the upcoming inputs from connected systems. This input estimation causes an error on the time integration of the states. This article discusses indicators to estimate the extent of input estimation contribution on the overall state calculation error and addresses to the question if the solution sequence has an influence on the state evolution. Moreover, a temporal analysis of this effect is studied to predict when to flip the sequence at an interface between two systems on a linear numerical example. In conclusion, an adaptive algorithm which switches the co-simulation sequence run-time based on the input estimation accuracy is proposed and validated with a parameter study.

• [PDF] Junghanns, Andreas, Torsten Blochwitz, Christian Bertsch, Torsten Sommer, Karl Wernersson, Andreas Pillekeit, Irina Zacharias, et al. “The Functional Mock-up Interface 3.0 - New Features Enabling New Applications.” In Proceedings of the 14th International Modelica Conference. online: Linköping University Electronic Press, Linköpings Universitet, 2021. https://doi.org/10.3384/ecp2118117.
  Abstract

  The Functional Mock-up Interface (FMI) (Modelica Association 2021b) is a tool independent standard for the exchange of dynamic models and for co-simulation. FMI 2.0, released in 2014, is recognized as the de-facto standard in industry for exchanging models and tool coupling, and is currently supported by more than 160 simulation tools. Version 3.0 of the standard brings many new features that allow for advanced co-simulation algorithms and new use cases such as packaging and simulation of highly accurate virtual Electronic Control Units (vECUs). Besides Model-Exchange and Co-Simulation, a third interface type, Scheduled Execution, is defined for purely discrete, RTOS-like, simulation and supports preemption. Clocks allow the synchronization of events between Functional Mock-up Units (FMUs) and the importer. There is better support for data types including binary data and arrays. Advanced co-simulation approaches are enabled by intermediate variable access between communication points and allowing event handling. The composition of systems from FMUs is simplified by terminals that can bundle multiple signals. The concept of layered standards allows the extension of the FMI standard.

• [PDF] Legaard, Christian Møldrup, Thomas Schranz, Gerald Schweiger, Ján Drgoňa, Basak Falay, Cláudio Gomes, Alexandros Iosifidis, Mahdi Abkar, and Peter Gorm Larsen. “Constructing Neural Network-Based Models for Simulating Dynamical Systems.” ACM Computing Surveys, 2021, 3567591. https://doi.org/10.1145/3567591.
  Abstract

  Dynamical systems see widespread use in natural sciences like physics, biology, chemistry, as well as engineering disciplines such as circuit analysis, computational fluid dynamics, and control. For simple systems, the differential equations governing the dynamics can be derived by applying fundamental physical laws. However, for more complex systems, this approach becomes exceedingly difficult. Data-driven modeling is an alternative paradigm that seeks to learn an approximation of the dynamics of a system using observations of the true system. In recent years, there has been an increased interest in applying data-driven modeling techniques to solve a wide range of problems in physics and engineering. This paper provides a survey of the different ways to construct models of dynamical systems using neural networks. In addition to the basic overview, we review the related literature and outline the most significant challenges from numerical simulations that this modeling paradigm must overcome. Based on the reviewed literature and identified challenges, we provide a discussion on promising research areas.

• [PDF] Oakes, Bentley James, Cláudio Gomes, Franz Rudolf Holzinger, Martin Benedikt, Joachim Denil, and Hans Vangheluwe. “Hint-Based Configuration of Co-Simulations with Algebraic Loops.” Simulation and Modeling Methodologies, Technologies and Applications, Advances in Intelligent Systems and Computing, 1260 (2021): 1-28. https://doi.org/10.1007/978-3-030-55867-3_1.
  Abstract

  Co-simulation is a powerful technique for performing full-system simulation. Multiple black-box models and their simulators are combined together to provide the behaviour for a full system. However, the black-box nature of co-simulation and potentially infinite configuration space means that configuration of co-simulations is a challenging problem for today’s practitioners. Our previous work on co-simulation configuration operated on the notion of hints, which allow system engineers to encode their knowledge and insights about the system. These hints, combined with state-of-the-art best practices, can then be used to semi-automatically configure the co-simulation. We summarize our previous hint-based configuration work here, and explore the challenging problem of scheduling co-simulations which contain algebraic loops. Solving or “breaking” these loops is required for scheduling, yet this breaking process can induce errors in the co-simulation. This work formalizes this scheduling problem, presents our insights gained about the problem, and details an optimal search algorithm as well as greedy scheduling algorithms. These heuristic algorithms are evaluated on (synthetic) co-simulation scenarios to determine their relative speedup and optimality.

• [PDF] Paredis, Randy, Cláudio Gomes, and Hans Vangheluwe. “Towards a Family of Digital Model/Shadow/Twin Workflows and Architectures.” In Proceedings of the 2nd International Conference on Innovative Intelligent Industrial Production and Logistics, 174-82. Online Streaming: SCITEPRESS - Science and Technology Publications, 2021. https://doi.org/10.5220/0010717600003062.
  Abstract

  Digital Twins (DTs) can be used for optimization, analysis and adaptation of complex engineered systems, in particular after these systems have been deployed. DTs make full use of both historical knowledge and of streaming data from sensors. DTs have been given numerous (distinct) definitions and descriptions in the literature. There is no consensus on terminology, nor a comprehensive description of workflows nor architectures. Following Multi-Paradigm Modelling principles, this paper proposes to explicitly model construction and use workflows of DTs as well as their architectures. We apply the concepts of variability (also known as product family) modeling, in particular to DT workflow and architecture. This allows for the de-/re-construction of the different DT variants in a principled, reproducible and partially automatable manner. To illustrate our ideas, two small use cases are discussed: a line-following robot (representative for an Automated Guided Vehicle) and an incubator (representative for an Industrial Convection Oven). The use cases focus on important systems in an industrial context.

• [PDF] Tola, Daniella, Cláudio Gomes, Carl Schultz, Christian Schlette, Casper Hansen, and Lukas Esterle. “RoboCIM: Towards a Domain Model for Industrial Robot System Configurators despite Tribal Knowledge.” In 5th International Joint Conference on Rules and Reasoning. Leuven, Belgium, 2021. http://ceur-ws.org/Vol-2956/paper12.pdf.
  Abstract

  Determining which components are required for a system configuration, and whether they are compatible, can be a difficult task, especially in an industry with significant amounts of information that resides within a group of experts. In this paper we illustrate some of the main challenges we and our industrial partner (Technicon) face when configuring a robot system (typically consisting of a robotic arm, end effector, coupling device, and a base) and present our domain model, Robot System Configurator Information Model (RoboCIM). We formalise the model within a defeasible reasoning framework, in order to explicitly capture cases where information is missing or is obtained from the system integrators’ experience. We provide a prototype implementation of the framework in ASP and evaluate it on a subset of components from Technicon’s component catalogue, illustrating the feasibility of the configurator.

• [PDF] Woodcock, Jim, Cláudio Gomes, Hugo Daniel Macedo, and Peter Gorm Larsen. “Uncertainty Quantification and Runtime Monitoring Using Environment-Aware Digital Twins.” In Leveraging Applications of Formal Methods, Verification and Validation: Tools and Trends, 12479:72-87. Lecture Notes in Computer Science. Springer International Publishing, 2021. https://doi.org/10.1007/978-3-030-83723-5_6.
  Abstract

  A digital twin for a Cyber-Physical System includes a simulation model that predicts how a physical system should behave. We show how to quantify and characterise violation events for a given safety property for the physical system. The analysis uses the digital twin to inform a runtime monitor that checks whether the noise and violations observed fall within expected statistical distributions. The results allow engineers to determine the best system configuration through what-if analysis. We illustrate our approach with a case study of an agricultural vehicle.

2020

• [PDF] Gomes, Cláudio, Joachim Denil, and Hans Vangheluwe. “Causal-Block Diagrams: A Family of Languages for Causal Modelling of Cyber-Physical Systems.” In Foundations of Multi-Paradigm Modelling for Cyber-Physical Systems, edited by Paulo Carreira, Vasco Amaral, and Hans Vangheluwe, 97-125. Cham: Springer International Publishing, 2020. https://doi.org/10.1007/978-3-030-43946-0_4.
  Abstract

  The description of a complex system in terms of constituent components and their interaction is one of the most natural and intuitive ways of decomposition. Causal Block Diagram (CBD) models combine subsystem blocks in a network of relationships between input signals and output signals. Popular modelling and simulation tools such as Matlab/Simulink® implement different variants from the family of Causal Block Diagram formalisms. This chapter gives an overview of modelling and simulation of systems with software and physical components using Causal Block Diagrams. It describes the syntax and - both declarative and operational - semantics of CBDs incrementally. Starting from simple algebraic models (no notion of time), we introduce, first a discrete notion of time (leading to discrete-time CBDs) and subsequently, a continuous notion of time (leading to continuous-time CBDs). Each new variant builds on the previous ones. Because of the heavy dependency of CBDs on numerical techniques, we give an intuitive introduction to this important field, pointing out main solutions as well as pitfalls.

• [PDF] Gomes, Cláudio, Romain Franceschini, Nick Battle, Casper Thule, Kenneth Lausdahl, Hans Vangheluwe, and Peter Gorm Larsen. “Application of Model-Based Testing to Dynamic Evaluation of Functional Mockup Units.” In Proceedings of the American Modelica Conference, 149-58. Boulder, Colorado, USA: Linköping University Electronic Press, Linköpings Universitet, 2020. https://doi.org/10.3384/ecp20169149.
  Abstract

  A successful co-simulation standard is crucial in applying co-simulation in large scale distributed development processes. A factor that affects the success of a standard is how easily a vendor can implement it. In this paper, we describe an approach to facilitate the implementation of the Functional Mock-up Interface standard. In particular, we propose the use of model-based testing for evaluating the tools that export Functional Mock-up Units (FMUs). This has the benefit that the model used as documentation to describe the possible behaviors of an FMU, can also be used to test it. These principles are embodied in a tool, which is open source, and available online. We then use this tool to evaluate the FMUs available in the FMI Crosscheck repository.

• [PDF] Gomes, Cláudio, Casper Thule, Levi Lúcio, Hans Vangheluwe, and Peter Gorm Larsen. “Generation of Co-Simulation Algorithms Subject to Simulator Contracts.” In Software Engineering and Formal Methods, edited by Javier Camara and Martin Steffen, 12226:34-49. Lecture Notes in Computer Science. Oslo, Norway: Springer International Publishing, 2020. https://doi.org/10.1007/978-3-030-57506-9_4.
  Abstract

  Correct co-simulation results require a careful consideration of how the interacting simulators are implemented. In version 2.0 of the FMI Standard, input handling implementation is left implicit, which leads to the situation where a simulator can be interacted with in a manner that its implementation does not expect, yielding incorrect results. In this paper, we build on prior work to make information about each simulator implementation explicit, in order to derive correct interactions with it. The formalization we use is specific to two kinds of contracts, but could serve as a basis to a general approach to black box co-simulation. The algorithm we propose generates a co-simulation execution plan in linear time. It has been successfully applied to an industrial case study, and the results are available online.

• [PDF] Larsen, Peter Gorm, Hugo Daniel Macedo, Cláudio Gomes, Lukas Esterle, Casper Thule, John Fitzgerald, and Kenneth Pierce. “Collaborative Modelling and Co-Simulation in Engineering and Computing Curricula.” In Frontiers in Software Engineering Education, 12271:196-213. Lecture Notes in Computer Science. Cham: Springer International Publishing, 2020. https://doi.org/10.1007/978-3-030-57663-9_13.
  Abstract

  The successful development of Cyber-Physical Systems (CPSs) requires collaborative working across diverse engineering disciplines, notations and tools. However, classical computing curricula rarely provide opportunities for students to look beyond the confines of one set of methods. In this paper, we report approaches to raising students’ awareness of the integrative role of digital technology in future systems development. Building on research in open but integrated tool chains for CPS engineering, we consider how this has been realised in two degree programmes in Denmark and the UK, and give preliminary findings. These include the need for ensuring stability of research-quality tools, and observations on how this material is presented in Computing versus Engineering curricula.

• [PDF] Legaard, Christian Møldrup, Cláudio Gomes, Peter Gorm Larsen, and Frederik F. Foldager. “Rapid Prototyping of Self-Adaptive-Systems Using Python Functional Mockup Units.” In Proceedings of the 2020 Summer Simulation Conference, 1-12. SummerSim ’20. Virtual Event, Spain: Society for Computer Simulation Internationa, San Diego, CA, United States, 2020. https://doi.org/10.5555/3427510.3427532.
  Abstract

  During the development of Cyber-Physical Systems (CPSs), it is crucial to enable efficient collaboration between different disciplines. Co-simulation plays a key role in this by allowing the system as a whole to be simulated by composing simulations of its parts. The ability to do this coupling relies on the models adhering to a well-defined interface. The Functional Mockup Interface (FMI) defines this interface and the models that implemented it are called Functional Mockup Units (FMUs). While a wealth of specialized simulation tools can generate FMUs, they are often commercial and do not support of complex software prototypes. Rather than implement these as FMUs from scratch (FMI requires expertise in C), losing valuable time, the contribution presented in this paper is a tool that allows FMUs to be implemented rapidly in Python. The advantages of this approach are demonstrated in an industrial use case, where a tracking simulator is implemented as an FMU.

• [PDF] Legat, Benoit, Cláudio Gomes, Paschalis Karalis, Raphael M. Jungers, Eva M. Navarro-Lopez, and Hans Vangheluwe. “Stability of Planar Switched Systems under Delayed Event Detection.” In 2020 59th IEEE Conference on Decision and Control (CDC), 5792-97. Jeju, Korea (South): IEEE, 2020. https://doi.org/10.1109/CDC42340.2020.9304152.
  Abstract

  In this paper, we analyse the impact of delayed event detection on the stability of a 2-mode planar hybrid automata. We consider hybrid automata with a unique equilibrium point for all the modes, and we find the maximum delay that preserves stability of that equilibrium point. We also show for the class of hybrid automata treated that the instability of the equilibrium point for the equivalent hybrid automaton with delay in the transitions is equivalent to the existence of a closed orbit in the hybrid state space, a result that is inspired by the Joint Spectral Radius theorem. This leads to an algorithm for computing the maximum stable delay exactly. Other potential applications of our technique include co-simulation, networked control systems and delayed controlled switching with a state feedback control.

• [PDF] Legat, Benoît, Cláudio Gomes, Paschalis Karalis, Raphaël M. Jungers, Eva M. Navarro-López, and Hans Vangheluwe. “Stability of Planar Switched Systems under Delayed Event Detection.” arXiv:2102.10390 [Cs, Eess], September 9, 2020. http://arxiv.org/abs/2009.04505.
  Abstract

  In this paper, we analyse the impact of delayed event detection on the stability of a 2-mode planar hybrid automata. We consider hybrid automata with a unique equilibrium point for all the modes, and we find the maximum delay that preserves stability of that equilibrium point. We also show for the class of hybrid automata treated that the instability of the equilibrium point for the equivalent hybrid automaton with delay in the transitions is equivalent to the existence of a closed orbit in the hybrid state space, a result that is inspired by the Joint Spectral Radius theorem. This leads to an algorithm for computing the maximum stable delay exactly. Other potential applications of our technique include co-simulation, networked control systems and delayed controlled switching with a state feedback control.

• [PDF] Thule, Casper, Cláudio Gomes, and Kenneth Lausdahl. “Formally Verified FMI Enabled Data Broker: RabbitMQ FMU.” In Proceedings of the 2020 Summer Simulation Conference, Pages 1-12. SummerSim ’20. Virtual event: Society for Computer Simulation International, 2020. https://doi.org/10.5555/3427510.3427533.
  Abstract

  Automated monitoring of deployed Cyber-physical Systems during operation can assist in detecting misbehavior. One way to achieve such monitoring is via co-simulation of a modelled system corresponding to the deployed system. Our work addresses the topic of brokering external data into an Functional Mock-up Interface 2.0 enabled co-simulation via a Functional Mock-up Unit (a simulation unit) called RabbitMQ FMU and an external message queue. Our contribution includes a formal specification that captures the semantics of RabbitMQ FMU and ensures a maximum delay on the messages relayed to the co-simulation even through an unreliable network. This formal specification is used to generate test cases that exercise all states and transitions. The implementation is subjected to these test cases to verify that it is a refinement of the specification. The verification is ongoing and the implementation has successfully been subjected to 1,539,246 tests.

• [PDF] Thule, Casper, Maurizio Palmieri, Cláudio Gomes, Kenneth Lausdahl, Hugo Daniel Macedo, Nick Battle, and Peter Gorm Larsen. “Towards Reuse of Synchronization Algorithms in Co-Simulation Frameworks.” In Software Engineering and Formal Methods, 12226:50-66. Lecture Notes in Computer Science. Oslo, Norway: Springer International Publishing, 2020. https://doi.org/10.1007/978-3-030-57506-9_5.
  Abstract

  An immediate industry challenge is to fashion a co-simulation that replicates real systems behaviour with high fidelity. To achieve this goal, developers rely on frameworks to enhance the creation and analysis of the co-simulation. The problem is that new co-simulation frameworks require extensive development, most of which in non-essential functionalities, before they can be used in practice. Additionally, existing co-simulations demand a thorough understanding before they can be extended. Our vision is a modular co-simulation framework architecture, that is easily extensible by researchers, and can integrate existing and legacy co-simulation approaches. The architecture we propose permits extension at three levels, each providing different degrees of flexibility. The most flexible integration level involves the specification of a Domain Specific Language (DSL) for Master Algorithm (MA) and this paper sketches such DSL, and discusses how it is expressive enough to describe well-known MAs.

2019

• [PDF] Gomes, Cláudio. “Property Preservation in Co-Simulation.” University of Antwerp, 2019. https://repository.uantwerpen.be/docman/irua/57f437/163840.pdf.
  Abstract

  Modeling and Simulation (M&S) techniques are today extensively used both in industry and science, to develop and understand complex systems. As development processes morph to respond to economic pressures, they impose new demands on these techniques: • frequent full system evaluation is required to prevent late integration problems among specialized teams who worked in parallel on different, but interconnected, parts of the system; and • seamless integration of externally supplied component models into the M&S workflow of Original Equipment Manufacturers is needed to enable high fidelity simulations. Traditional M&S techniques, where a single model of the whole system is built and simulated, are insufficient to address these demands, because: • teams use mature M&S tools, each tailored to a particular domain, and not capable of exporting models that are compatible with any other M&S tool; and • external suppliers are not willing to share high fidelity models without expensive contracts protecting their Intellectual Property (IP). Co-simulation is a way to tackle these challenges. It consists of the theory and techniques to enable global simulation of a coupled system via the composition of simulators. Each simulator is broadly defined as a black box capable of exhibiting behavior, consuming inputs and producing outputs. This nature is also what aggravates the fundamental challenge in co-simulation: deciding whether the results can be trusted. This thesis is comprised of two parts. The first part explores the challenge, and tries to understand what makes co-simulation different than traditional simulation techniques. One of the conclusions of this part is the need to ensure that co-simulations preserve properties of the system being developed (e.g., stability, smoothness, etc. . . ). The second part represents a collection of work, each targeting an aspect of property preservation, including giving the users of co-simulation, the ability to control the implementation of participating simulators, and then providing a framework to help them do so correctly. The reported results provide a deeper understanding of the fundamental challenges that need to be addressed before co-simulation can become a seamless technique in the development of complex systems, including: 1. how to configure adaptive co-simulation algorithms that preserve stability; 2. how to configure state event location for co-simulation of hybrid systems; 3. a tool that allows the configuration of simulators participating in a co-simulation; and 4. a framework that guides the configuration of the co-simulation, so as to preserve domain specific description of system properties.

• [PDF] Gomes, Cláudio, Levi Lucio, and Hans Vangheluwe. “Semantics of Co-Simulation Algorithms with Simulator Contracts.” In 2019 ACM/IEEE 22nd International Conference on Model Driven Engineering Languages and Systems Companion (MODELS-C), 784-89. Munich, Germany: IEEE, 2019. https://doi.org/10.1109/MODELS-C.2019.00124.
  Abstract

  The rapid adoption of co-simulation techniques allows for holistic complex system development. However, ensuring trustworthy results when combining simulators requires a careful consideration of their implementation and capabilities. Especially in black box integration, these are frequently left implicit. In this paper, we explore a way to account for simulator capabilities, by formalizing the execution of a co-simulation that respects such contracts. This formalization is specific to two kinds of contracts, but could serve as a basis to a general approach to black box co-simulation. An example application of the semantics to generate master algorithms is presented.

• [PDF] Gomes, Cláudio, Bentley James Oakes, Mehrdad Moradi, Alejandro Torres Gamiz, Juan Carlos Mendo, Stefan Dutre, Joachim Denil, and Hans Vangheluwe. “HintCO - Hint-Based Configuration of Co-Simulations.” In International Conference on Simulation and Modeling Methodologies, Technologies and Applications, 57-68. Prague, Czech Republic, 2019. https://doi.org/10.5220/0007830000570068.
  Abstract

  Simulation-based analyses of Cyber-Physical Systems are fundamental in industrial design and testing approaches. The utility of analyses relies on the correct configuration of the simulation tools, which can be highly complicated. System engineers can normally judge the results, and either evaluate multiple simulation algorithms, or change the models. However, this is not possible in a co-simulation approach. Co-simulation is a technique to perform full-system simulation, by combining multiple black-box simulators, each responsible for a part of the system. In this paper, we demonstrate the difficulty of correctly configuring a co-simulation scenario using an industrial case study. We propose an approach to tackle this challenge by allowing multiple engineers, specialized in different domains, to encode some of their experience in the form of hints. These hints, together with state-of-the-art best practices, are then used to semi-automatically guide the configuration process of the co-simulation. We report the application of this approach to a use case proposed by our industrial partners, and discuss some of the lessons learned.

• [PDF] Gomes, Cláudio, and Hans Vangheluwe. “Co-Simulation of Continuous Systems: A Hands-on Approach.” In 2019 Winter Simulation Conference (WSC), 1469-81. National Harbor, MD, USA: IEEE, 2019. https://doi.org/10.1109/WSC40007.2019.9004704.
  Abstract

  Co-simulation consists of the theory and techniques to enable global simulation of a coupled system via the composition of simulators for each of the components of the model. Despite the large number of applications and growing interest in the challenges, practitioners still experience difficulties in configuring their co-simulations. This tutorial introduces co-simulation of continuous systems, targeted at researchers that want to develop their own co-simulation units and master algorithms, using the Functional Mock-up Interface (FMI) Standard. This document is complemented by online materials (Gomes, Cláudio 2019) that allow the reader to experiment with different co-simulation algorithms, applied to the examples introduced here.

• [PDF] Moradi, Mehrdad, Cláudio Gomes, Bentley James Oakes, and Joachim Denil. “Optimizing Fault Injection in FMI Co-Simulation.” In Proceedings of the 2019 Summer Simulation Conference, 12. Berlin, Germany: Society for Computer Simulation International, 2019. https://doi.org/10.5555/3374138.3374170.
  Abstract

  As society and industry relies more on Cyber-Physical Systems (CPS), any malfunctions can have unforeseen catastrophic failures. Fault Injection (FI) techniques perturbed a model of a CPS with the intention of causing a failure and measuring the robustness of the CPS. Naturally, the success of a FI simulation depends on three factors: (i) realism of the faults injected; (ii) how quickly the faults cause catastrophic failure; and (iii) fidelity of the model used. In this paper, we propose to improve the success rate of FI studies by addressing each one of these factors: we develop an algorithm that leverages traditional Sensitivity Analysis (SA) in hybrid systems to reduce an infinite fault search space to a optimal finite set (factors i,ii), and we use co-simulation as the model integration technique (factor iii). We evaluate our contribution on the power window system developed by MathWorks® .

• [PDF] An empirical survey on co-simulation: Promising standards, challenges and research needs
  Abstract

  Co-simulation is a promising approach for the modelling and simulation of complex systems, that makes use of mature simulation tools in the respective domains. It has been applied in various different domains, oftentimes without a comprehensive study of the impact to the simulation results. As a consequence, over the recent years, researchers have set out to understand the essential challenges arising from the application of this technique. This paper complements the existing surveys in that the social and empirical aspects were addressed. More than 50 experts participated in a two-stage Delphi study to determine current challenges, research needs and promising standards and tools. Furthermore, an analysis of the strengths, weakness, opportunities and threats of co-simulation utilizing the analytic hierarchy process resulting in a SWOT-AHP analysis is presented. The empirical results of this study show that experts consider the FMI standard to be the most promising standard for continuous time, discrete event and hybrid co-simulation. The results of the SWOT-AHP analysis indicate that factors related to strengths and opportunities predominate.

• [PDF] Thule, Casper, Kenneth Lausdahl, Cláudio Gomes, Gerd Meisl, and Peter Gorm Larsen. “Maestro: The INTO-CPS Co-Simulation Framework.” Simulation Modelling Practice and Theory 92, no. April (2019): 45-61. https://doi.org/10.1016/j.simpat.2018.12.005.
  Abstract

  Cyber-Physical Systems (CPSs) often operate in a critical context where it is crucial that they behave as intended. However, the heterogeneous nature of CPSs makes them inherently challenging to develop. To assist in the development process, one can perform co-simulation, where models of constituents of a CPS are coupled to jointly simulate the full system. The challenge herein is to combine heterogeneous formalisms in a sound fashion and address practical needs such as stability, performance, platform compatibility and so forth. To address this, Maestro is a tool for co-simulation using models adhering to the Functional Mock-up Interface standard for Co-Simulation. Its development was driven by needs from different industry domains such as railways, agriculture, building automation and automotive. It supports both a fixed and variable constraint-based iteration scheme along with platform distribution capabilities. The tool is open-source as an attempt to increase adoption of co-simulation and encourage researchers to collaborate. Maestro has been validated by industry through application in the aforementioned domains. It is a step in the direction of the two-folded long-term goals: ensure trustworthy co-simulation results and make co-simulation a technology taken for granted.

2018

• [PDF] Cambeiro, João, Cláudio Gomes, Vasco Amaral, Armanda Rodrigues, and Jácome Cunha. “A Building Automation Case Study - Setup and Challenges.” In 4th International Workshop on Software Engineering for Smart Cyber-Physical Systems, 41-44. Gothenburg, Sweden: ACM New York, NY, USA, 2018.
  Abstract

  Occupant behavior can wreck havoc in the performance indicators of smart building controllers, and this is usually caused by lack of knowledge about the operation of the system. However, there is evidence that the informed and motivated user will actually cooperate with the system. In this paper, we describe a system representative of the usual complexity found in cyber-physical systems, whose purpose is to address the chronic lack of real experiments involving gamification and control systems, in the context of building automation. Designed with pragmatic concerns, this system presents a unique set of challenges and opportunities to research a new generation of software control systems, and supporting interfaces, that leverage the occupants’ behavior.

• [PDF] Gomes, Cláudio, Raphaël Jungers, Benoît Legat, and Hans Vangheluwe. “Minimally Constrained Stable Switched Systems and Application to Co-Simulation.” Belgium: University of Antwerp, 2018. http://arxiv.org/abs/1809.02648.
  Abstract

  We propose an algorithm to restrict the switching signals of a constrained switched system in order to guarantee its stability, while at the same time attempting to keep the largest possible set of allowed switching signals. Our work is motivated by applications to (co-)simulation where numerical stability is a hard constraint, but should be attained by restricting as little as possible the allowed behaviours of the simulators. We apply our results to certify the stability of an adaptive co-simulation orchestration algorithm, which selects the optimal switching signal at run-time, as a function of (varying) performance and accuracy requirements.

• [PDF] Gomes, Cláudio, Benoît Legat, Raphaël Jungers, and Hans Vangheluwe. “Minimally Constrained Stable Switched Systems and Application to Co-Simulation.” In IEEE Conference on Decision and Control, 5676-81. Miami Beach, FL, USA, 2018. https://doi.org/10.1109/CDC.2018.8619223.
  Abstract

  We propose an algorithm to restrict the switching signals of a constrained switched system in order to guarantee its stability, while at the same time attempting to keep the largest possible set of allowed switching signals. Our work is motivated by applications to (co)simulation where numerical stability is a hard constraint, but should be attained by restricting as little as possible the allowed behaviours of the simulators. We apply our results to certify the stability of an adaptive co-simulation orchestration algorithm, which selects the optimal switching signal at run-time, as a function of (varying) performance and accuracy requirements.

• [PDF] Gomes, Cláudio, Bart Meyers, Joachim Denil, Casper Thule, Kenneth Lausdahl, Hans Vangheluwe, and Paul De Meulenaere. “Semantic Adaptation for FMI Co-Simulation with Hierarchical Simulators.” SIMULATION 95, no. 3 (2018): 1--29. https://doi.org/10.1177/0037549718759775.
  Abstract

  Model-based design can shorten the development time of complex systems by the use of simulation techniques. However, it can be hard to simulate the system as a whole if it is developed in a concurrent fashion, by multiple and specialized teams. Co-simulation, with the support of the Functional Mock-up Interface (FMI) Standard, is proposed as a way to promote tool interoperability while protecting intellectual property of subsystems. The standard allows a uniform communication between subsystem simulators, but does not state how the inputs and outputs should be interpreted, nor how the subsystems should interact correctly. Semantic adaptations can be quickly made to correct the interactions with sub-system simulators that were produced with different assumptions, and avoid changing those sub-systems, their simulators, or the orchestration algorithm that computes the co-simulation. In this work, we explore how to describe common adaptations and what their meaning is in the context of FMI co-simulation. The result is a sound language that enables the implementation of adaptations with minimal effort. A distinct feature is that it describes adaptations for groups of interconnected subsystem simulators in the same way as for a single simulator, and the implementation is itself a simulator, thanks to a sound definition of hierarchical co-simulation. This work paves the way for research into the correct combination and interfacing of different adaptations.

• [PDF] Gomes, Cláudio, Casper Thule, David Broman, Peter Gorm Larsen, and Hans Vangheluwe. “Co-Simulation: A Survey.” ACM Computing Surveys 51, no. 3 (2018): 49:1--49:33. https://doi.org/10.1145/3179993.
  Abstract

  Modeling and simulation techniques are today extensively used both in industry and science. Parts of larger systems are, however, typically modeled and simulated by different techniques, tools, and algorithms. In addition, experts from different disciplines use various modeling and simulation techniques. Both these facts make it difficult to study coupled heterogeneous systems. Co-simulation is an emerging enabling technique, where global simulation of a coupled system can be achieved by composing the simulations of its parts. Due to its potential and interdisciplinary nature, co-simulation is being studied in different disciplines but with limited sharing of findings. In this survey, we study and survey the state-of-the-art techniques for co-simulation, with the goal of enhancing future research and highlighting the main challenges. To study this broad topic, we start by focusing on discrete-event-based co-simulation, followed by continuous-time-based co-simulation. Finally, we explore the interactions be these two paradigms, in hybrid co-simulation. To survey the current techniques, tools, and research challenges, we systematically classify recently published research literature on co-simulation, and summarize it into a taxonomy. As a result, we identify the need for finding generic approaches for modular, stable, and accurate, coupling of simulation units, as well as expressing the adaptations required to ensure that the coupling is correct.

• [PDF] Gomes, Cláudio, Casper Thule, Julien DeAntoni, Peter Gorm Larsen, and Hans Vangheluwe. “Co-Simulation: The Past, Future, and Open Challenges.” In Symposium On Leveraging Applications of Formal Methods, Verification and Validation, Vol. 11246. Lecture Notes in Computer Science. Limassol, Cyprus: Springer Verlag, 2018. https://doi.org/10.1007/978-3-030-03424-5_34.
  Abstract

  In the engineering of heterogeneous systems, there have always been challenges related to ensuring a common understanding of the interfaces between the constituent systems. In these systems, the systematic analysis of the relevant artefacts is governed by different kinds of models based on different kinds of formalisms (e.g., state machine models for software-based controllers, and differential equations for physical sub-systems). In such a hybrid setting, it makes sense to examine how to combine different kinds of models in ways that enable a well-founded analysis of the interaction between these. Co-simulation has been proposed as a way forward by different stakeholders in different disciplines. It is a technique to couple multiple simulation tools, so that the interactions with, and within, a coupled system can be simulated through the cooperation of these tools. In this paper, we: provide an historical overview of the different facets of co-simulation; describe examples of industrial applications; identify the emerging trend and the challenges (both theoretical and practical) for the future use of this technology.

• [PDF] Gomes, Cláudio, Casper Thule, Peter Gorm Larsen, Joachim Denil, and Hans Vangheluwe. “Co-Simulation of Continuous Systems: A Tutorial.” Belgium: University of Antwerp, 2018. http://arxiv.org/abs/1809.08463.
• [PDF] Gomes, Cláudio, Casper Thule, Kenneth Lausdahl, Peter Gorm Larsen, and Hans Vangheluwe. “Stabilization Technique in INTO-CPS.” In 2nd Workshop on Formal Co-Simulation of Cyber-Physical Systems, Vol. 11176. Toulouse, France: Springer, Cham, 2018. https://doi.org/10.1007/978-3-030-04771-9_4.
  Abstract

  Despite the large number of applications and growing interest in the challenges that co-simulation poses, the field is fragmented into multiple application domains, with limited sharing of knowledge. This demo promotes a deeper understanding of a well known stabilization feature in co-simulation, which is used in the INTO-CPS tool chain. We develop the techniques that explain the empirical results of instability of the double mass-spring-damper system, and how to the stabilization feature improves the results. Moreover, we show how the restrictions of the Functional Mock-up Interface Standard impacts stability.

• [PDF] Schweiger, Gerald, Georg Engel, Josef Schoeggl, Irene Hafner, Cláudio Gomes, and Thierry Nouidui. “Co-Simulation - an Empirical Survey: Applications, Recent Developments and Future Challenges.” In Proceedings of the MATHMOD 2018, 125-26. Vienna, Austria: ARGESIM Publisher Vienna, 2018. https://doi.org/10.11128/arep.55.a55286.
• [PDF] Schweiger, Gerald, Cláudio Gomes, Georg Engel, Irene Hafner, Josef Schoeggl, Alfred Posch, and Thierry Nouidui. “Functional Mock-up Interface: An Empirical Survey Identifies Research Challenges and Current Barriers.” In Proceedings of the American Modelica Conference, 138-46. Cambridge, MA, USA: Linköping University Electronic Press, Linköpings Universitet, 2018. https://doi.org/10.3384/ecp18154138.
  Abstract

  Co-simulation is a promising approach for the analysis of complex, multi-domain systems, that leverages mature simulation tools of the respective domains. It has been applied in many different disciplines in academia and industry, with limited sharing of findings. With the increasing adoption of the FMI standard, researchers have set to work on surveying the scattered knowledge on co-simulation in academia. This paper complements the existing surveys by taking on the social and empirical aspect, corroborating, and prioritizing, previous findings. We focus on understanding the perceived research challenges, and the current barriers, based on expert assessment. One of the main barriers pointed out is the limited support for discrete event and hybrid co-simulation.

• [PDF] Schweiger, Gerald, Cláudio Gomes, Irene Hafner, George Engel, Thierry Stephane Nouidui, Niki Popper, and Josef-Peter Schoggl. “Co-Simulation: Leveraging the Potential of Urban Energy System Simulation.” EuroHeat&Power, 2018.
  Abstract

  Simulation-driven assessments and developments are regarded as key methods in designing and operating urban energy systems. Co-simulation enables the flexible coupling of established tools for the respective subsystems such as buildings, district heating and cooling systems or power distribution systems. The authors present an overview of the general concept behind co-simulation, promising standards, tools and discuss new possibilities for simulation of urban energy systems.

• [PDF] Thule, Casper, Cláudio Gomes, Julien Deantoni, Peter Gorm Larsen, Jörg Brauer, and Hans Vangheluwe. “Towards Verification of Hybrid Co-Simulation Algorithms.” In Workshop on Formal Co-Simulation of Cyber-Physical Systems. Toulouse, France: Springer, Cham, 2018. https://doi.org/10.1007/978-3-030-04771-9_1.
  Abstract

  Engineering modern, hybrid systems is becoming increasingly difficult due to the heterogeneity between different subsystems. Modelling and simulation techniques have traditionally been used to tackle complexity, but with increasing heterogeneity of the subsystems, it becomes impossible to find appropriate modelling languages and tools to specify and analyse the system as a whole. Co-simulation is a technique to combine multiple models and their simulators in order to analyse the behaviour of the whole system over time. Past research, however, has shown that the naïve combination of simulators can easily lead to incorrect simulation results, especially when co-simulating hybrid systems. This paper shows (i) how co-simulation of a family of hybrid systems can fail to reproduce the order of events that should have occurred (event ordering); (ii) how to prove that a co-simulation algorithm is correct (w.r.t. event ordering), and if it is incorrect, how to obtain a counterexample showing how the co-simulation fails; and (iii) how to correct an incorrect co-simulation algorithm. We apply the above method to two well known co-simulation algorithms used with the FMI Standard, and we show that one of them is incorrect for the family of hybrid systems under study, under the restrictions of the standard. The conclusion is that either the standard needs to be revised, or one of the algorithms should be avoided.

2017

• [PDF] Van Mierlo, Simon, Cláudio Gomes, and Hans Vangheluwe. “Explicit Modelling and Synthesis of Debuggers for Hybrid Simulation Languages.” In Symposium on Theory of Modeling & Simulation: DEVS Integrative M&S Symposium, Article No. 4. DEVS ’17. Virginia Beach, Virginia, USA: Society for Computer Simulation International, 2017.
  Abstract

  Any sufficiently complex system is best described (or specified) with a combination of models in multiple formalisms. To support the creation of such “hybrid models”, recent research focuses on the (syntactic and semantic) combination of formalism fragments. To implement the hybrid language’s operational semantics, the simulators of each of the formalisms are combined. Inspired by this same principle, we study how hybrid simulators can be instrumented with debugging capabilities. Previous work has shown that an explicit model of any simulator’s behaviour can be instrumented with common code debugging operations (e.g., stepwise execution, breakpoints, pause/play) and simulation-specific operations (such as (scaled) real-time simulation, or event injection). We extend this work by combining two debugging-enhanced simulators to create a hybrid simulator in a modular way, and instrument it with debugging support at the hybrid level. To demonstrate feasibility, we create a debugging-enhanced simulator of the Hybrid Automata formalism, by embedding a Causal Block Diagrams simulator in a Timed Finite State Automata simulator.

• [PDF] Gomes, Cláudio, Yentl Van Tendeloo, Joachim Denil, Paul De Meulenaere, and Hans Vangheluwe. “Hybrid System Modelling and Simulation with Dirac Deltas.” In Proceedings of the Symposium on Theory of Modeling & Simulation: DEVS Integrative M&S Symposium, Article No. 7. DEVS ’17. Virginia Beach, Virginia, USA: Society for Computer Simulation International, 2017.
  Abstract

  For a wide variety of problems, creating detailed continuous models of physical systems is impractical. Hybrid models can abstract away short transient behaviour in order to simplify the study of such systems. For example, when modelling a bouncing ball, the bounce can be abstracted as a discontinuous change of the velocity. Impulsive differential equations can be used to model and simulate hybrid systems such as the bouncing ball. In this approach, the force acted on the ball by the floor is abstracted as an impulse. Current simulators cannot handle such approximations well due to the limitations of machine precision. In this paper, we present two approaches for the simulation of impulses: symbolic and numerical. Our contribution is a theoretically founded description of both approaches in a Causal Block Diagram simulator, and an analysis of the conditions for which a symbolic approach is better than a numerical one.

• [PDF] Gomes, Cláudio, Yentl Van Tendeloo, Joachim Denil, Paul De Meulenaere, and Hans Vangheluwe. “Hybrid System Modelling and Simulation with Dirac Deltas.” Antwerp: University of Antwerp, February 14, 2017. http://arxiv.org/abs/1702.04274.
  Abstract

  For a wide variety of problems, creating detailed continuous models of (continuous) physical systems is, at the very least, impractical. Hybrid models can abstract away short transient behaviour (thus introducing discontinuities) in order to simplify the study of such systems. For example, when modelling a bouncing ball, the bounce can be abstracted as a discontinuous change of the velocity, instead of resorting to the physics of the ball (de-)compression to keep the velocity signal continuous. Impulsive differential equations can be used to model and simulate hybrid systems such as the bouncing ball. In this approach, the force acted on the ball by the floor is abstracted as an infinitely large function in an infinitely small interval of time, that is, an impulse. Current simulators cannot handle such approximations well due to the limitations of machine precision. In this paper, we explore the simulation of impulsive differential equations, where impulses are first class citizens. We present two approaches for the simulation of impulses: symbolic and numerical. Our contribution is a theoretically founded description of the implementation of both approaches in a Causal Block Diagram modelling and simulation tool. Furthermore, we investigate the conditions for which one approach is better than the other.

• [PDF] Gomes, Cláudio, Casper Thule, David Broman, Peter Gorm Larsen, and Hans Vangheluwe. “Co-Simulation: State of the Art.” University of Antwerp, February 1, 2017. 1702.00686. http://arxiv.org/abs/1702.00686.
  Abstract

  It is essential to find new ways of enabling experts in different disciplines to collaborate more efficient in the development of ever more complex systems, under increasing market pressures. One possible solution for this challenge is to use a heterogeneous model-based approach where different teams can produce their conventional models and carry out their usual mono-disciplinary analysis, but in addition, the different models can be coupled for simulation (co-simulation), allowing the study of the global behavior of the system. Due to its potential, co-simulation is being studied in many different disciplines but with limited sharing of findings. Our aim with this work is to summarize, bridge, and enhance future research in this multidisciplinary area. We provide an overview of co-simulation approaches, research challenges, and research opportunities, together with a detailed taxonomy with different aspects of the state of the art of co-simulation and classification for the past five years. The main research needs identified are: finding generic approaches for modular, stable and accurate coupling of simulation units; and expressing the adaptations required to ensure that the coupling is correct.

• [PDF] Gomes, Cláudio, Benoît Legat, Raphaël M. Jungers, and Hans Vangheluwe. “Stable Adaptive Co-Simulation: A Switched Systems Approach.” In IUTAM Symposium on Co-Simulation and Solver Coupling, 35:81-97. Darmstadt, Germany: Springer, Cham, 2017. https://doi.org/10.1007/978-3-030-14883-6_5.
  Abstract

  Co-simulation promotes the idea that domain specific simulation tools should cooperate in order to simulate the inter-domain interactions that are often observed in complex systems. To get trustworthy results, it is important that this technique preserves the stability properties of the original system. In this paper, we show how to preserve stability for adaptive co-simulation schemes, which offer fine grained control over the performance/accuracy of the co-simulation. To this end, we apply the joint spectral radius theory to certify that an adaptive co-simulation scheme is stable, and, if that is not possible, we use recent results in this field to create a trace of decisions that lead to instability. With this trace, it is possible to adjust the adaptive co-simulation in order to make it stable. Our approach is limited by the fact that computing the joint spectral radius is NP-Hard and undecidable in general. In practice, we successfully applied our results to the co-simulation of a double mass-spring-damper system.

• [PDF] Gomes, Cláudio, Paschalis Karalis, Eva M. Navarro-López, and Hans Vangheluwe. “Approximated Stability Analysis of Bi-Modal Hybrid Co-Simulation Scenarios.” In 1st Workshop on Formal Co-Simulation of Cyber-Physical Systems, 345-60. Trento, Italy: Springer, Cham, 2017. https://doi.org/10.1007/978-3-319-74781-1_24.
  Abstract

  Co-simulation is a technique to orchestrate multiple simulators in order to approximate the behavior of a coupled system as a whole. Simulators execute in a lockstep fashion, each exchanging inputs and output data points with the other simulators at pre-accorded times. In the context of systems with a physical and a cyber part, the communication frequency with which the simulators of each part communicate can have a negative impact in the accuracy of the global simulation results. In fact, the computed behavior can be qualitatively different, compared to the actual behavior of the original system, laying waste to potentially many hours of computation. It is therefore important to develop methods that answer whether a given communication frequency guarantees trustworthy co-simulation results. In this paper, we take a small step in that direction. We develop a technique to approximate the lowest frequency for which a particular set of simulation tools can exchange values in a co-simulation and obtain results that can be trusted.

2016

• [PDF] Mustafiz, Sadaf, Cláudio Gomes, Bruno Barroca, and Hans Vangheluwe. “Modular Design of Hybrid Languages by Explicit Modeling of Semantic Adaptation.” In Proceedings of the Symposium on Theory of Modeling & Simulation: DEVS Integrative M&S Symposium, 29:1--29:8. DEVS ’16. Pasadena, California: IEEE, 2016. https://doi.org/10.23919/TMS.2016.7918835.
  Abstract

  The engineering of a complex cyber-physical system (CPS) involves the creation and simulation of hybrid models often encompassing multiple levels of abstraction and combining different formalisms, often not expressible in any single existing formalisms. Modular language engineering is thus essential for effective and efficient development of new formalisms, appropriate for the task. In our work, each modeling language is represented as a language specification fragment: a modular representation of the syntax and semantics. We propose a white-box technique for explicitly modeling the definition and composition of the fragments. In this paper, we focus on the composition of the operational semantics (i.e., the semantic adaptation) of hybrid languages. This enables automatic synthesis of a simulator for the hybrid language. Our approach is demonstrated by creating two well-known hybrid languages as a composition of Timed Finite State Automata (TFSA) and Causal Block Diagrams (CBD) – hybrid TFSA and hybrid CBD.

• [PDF] Mustafiz, Sadaf, Bruno Barroca, Cláudio Gomes, and Hans Vangheluwe. “Towards Modular Language Design Using Language Fragments: The Hybrid Systems Case Study.” In 13th International Conference on Information Technology - New Generations (ITNG), edited by Shahram Latifi, 785-97. Las Vegas, NV USA: Springer, Cham, 2016. https://doi.org/10.1007/978-3-319-32467-8_68.
  Abstract

  Cyber-physical systems can be best represented using hybrid models that contain specifications of both continuous and discrete event abstractions. The syntax and semantics of such hybrid languages should ideally be defined by reusing the syntax and semantics of each components’ formalisms. In language composition, semantic adaptation is needed to ensure correct realization of the concepts that are part of the intricacies of the hybrid language. In this paper, we present a technique for the composition of heterogeneous languages by explicitly modelling the semantic adaptation between them. Each modelling language is represented as a language specification fragment (LSF): a modular representation of the syntax and semantics. The basis of our technique is to reuse the operational semantics as defined in existing simulators. Our approach is demonstrated by means of a hybrid language composed of timed finite state machines (TFSA) and causal block diagrams (CBD).

• [PDF] Lawrence, David P. Y., Cláudio Gomes, Joachim Denil, Hans Vangheluwe, and Didier Buchs. “Coupling Petri Nets with Deterministic Formalisms Using Co-Simulation.” In Symposium on Theory of Modeling & Simulation: DEVS Integrative M&S Symposium, 6:1--6:8. Pasadena, CA, USA, 2016.
  Abstract

  Modelling cyber-physical systems is often seen as a highly multi-disciplinary activity. Therefore, efficient methodologies are required to be able to represent environment, plant and control models using the most appropriate formalisms. On the one hand, the Petri net formalism is appropriate to model the environment of a complex cyber-physical system, especially since it inherently supports non-determinism and concurrency. On the other hand, plant models, based on the laws of physics, are often represented using differential equations. In order to produce a relevant simulation of the overall system, the controller, the plant and the environment models are composed. Co-simulation and its industrial standard, the Functional Mock-up Interface, is one such generic technique that allows for the coupling of different executable models. However, coupling a non-deterministic model with other types of models would be equivalent to build the state space of the given composed system, which in some cases is too complex. In this paper we discuss the co-simulation of a Petri net environment model with a system described by causal block diagrams. To reduce the size of the state space explored, we use a constraint language based on automata and trace matching. The approach is applied to the simulation of a simplified train system.

• [PDF] Gomes, Cláudio, Joachim Denil, and Hans Vangheluwe. “Causal-Block Diagrams.” University of Antwerp, 2016. http://msdl.cs.mcgill.ca/people/claudio/pub/Gomes2016a.pdf.
  Abstract

  The description of a complex system in terms of constituent components and their interaction is one of the most natural and intuitive ways of decomposition. Causal Block Diagram (CBD) models combine subsystem blocks in a network of relationships between input signals and output signals. Popular modeling and simulation tools such as Matlab/Simulink® implement different variants from the family of Causal Block Diagram formalisms. This chapter gives an overview of modeling and simulation of systems with software and physical components using Causal Block Diagrams. It describes the syntax and - both declarative and operational - semantics of CBDs incrementally. Starting from simple algebraic models (no notion of time), we introduce, first a discrete notion of time (leading to discrete-time CBDs) and subsequently, a continuous notion of time (leading to continuous-time CBDs). Each new variant builds on the previous ones. Because of the heavy dependency of CBDs on numerical techniques, we give an intuitive introduction to this important field, pointing out main solutions as well as pitfalls. After reading this chapter, the reader will be able to judge when to use the CBD formalism and how to use it, as well as the main issues often encountered with the description of physical systems and the implementation of CBD simulators.

• [PDF] Gomes, Cláudio. “Foundations for Continuous Time Hierarchical Co-Simulation.” In ACM Student Research Competition (MoDELS). Saint Malo, France: ACM New York, NY, USA, 2016.
  Abstract

  Complex systems have to decomposed into sub-systems which are developed by specialized teams. Modeling and simulation techniques help each team achieve a locally optimal solution but they fail to help all teams achieve a globally optimal solution. This is due to each team having its own models made in its own tools, and external suppliers having intellectual property. The result is that it is difficult to simulate the coupled system. Co-simulation is proposed as a way to enable such simulation. Simulators communicate and collaborate as black boxes. The technique has been used in a number of domains improving the overall cost of engineered systems. In these use cases, a minimum common denominator is assumed for the capabilities of simulators. Leveraging the optional capabilities of simulators improves the performance/accuracy of a co-simulation, but the number of decisions the orchestration mechanism has to make grows exponentially. Different, conflicting concerns have to be addressed in an optimal way. We propose a way to deal with each concern independently and possibly reuse existing orchestration algorithms that perform better with respect to the conflicting concerns. Our approach leverages Model Driven Development techniques to process a co-simulation scenario into a canonical/trivial version, where fewer decisions remain to be made. Along the process, conflicting concerns will be addressed as an optimization problem, and an appropriate cost function identified. If successful, the result of this research allows co-simulation scenarios that may offer real-time guarantees, bounds in the maximum error made, or packet size in the communication between simulators.

2015

• [PDF] Gomes, Cláudio. “Foundations for Co-Simulation -- IWT Proposal.” Antwerp: University of Antwerp, 2015.
• [PDF] Gomes, Cláudio, Bruno Barroca, and Vasco Amaral. “TrNet: A Language and for Efficient and Analysable Model Transformations.” Technical Report, 2015.
• [PDF] Lúcio, Levi, Bentley James Oakes, Cláudio Gomes, Gehan Selim, Juergen Dingel, James R. Cordy, and Hans Vangheluwe. “SyVOLT: Full Model Transformation Verification Using Contracts.” In 8th International Conference on Model Driven Engineering Languages and Systems - Demo, 6019-6635. Ottawa, Canada: Springer International Publishing, 2015.
  Abstract

  We introduce SyVOLT, a plugin for the Eclipse development environment for the verification of structural pre/post-condition contracts on model transformations. The plugin allows the user to build transformations in our transformation language DSLTrans using a visual editor. The pre-/post-condition contracts to be proved on the transformation can also be built in a similar interface. Our contract proving process is exhaustive, meaning that if a contract is said to hold, then the contract will hold for all input models of a transformation. If the contract does not hold, then the counter-examples (i.e., input models) where the contract fails will be presented. Demo: https://www.youtube.com/watch?v=8PrR5RhPptY

2014

• [PDF] Gomes, Cláudio, Bruno Barroca, and Vasco Amaral. “Classification of Model Transformation Tools: Pattern Matching Techniques.” In Model-Driven Engineering Languages and Systems, edited by Juergen Dingel, Wolfram Schulte, Isidro Ramos, Silvia Abrahão, and Emilio Insfran, Vol. 8767. Lecture Notes in Computer Science. Springer International Publishing, 2014. https://doi.org/10.1007/978-3-319-11653-2_38.
  Abstract

  While comparing different model transformation languages (MTLs), it is common to refer to their syntactic and semantic features and overlook their supporting tools’ performance. Performance is one of the aspects that can hamper the application of MDD to industrial scenarios. An highly declarative MTL might simply not scale well when using large models due to its supporting implementation. In this paper, we focus on the several pattern matching techniques (including optimization techniques) employed in the most popular transformation tools, and discuss their effectiveness w.r.t. the expressive power of the languages used. Because pattern matching is the most costly operation in a transformation execution, we present a classification of the existing model transformation tools according to the pattern matching optimization techniques they implement. Our classification complements existing ones that are more focused at syntactic and semantic features of the languages supported by those tools.

• [PDF] Gomes, Cláudio. “A Framework for Efficient Model Transformations.” Master Thesis, New University of Lisbon, 2014.
  Abstract

  The reported productivity gains while using models and model transformations to develop entire systems, after almost a decade of experience applying model-driven approaches for system development, are already undeniable benefits of this approach. However, the slowness of higher-level, rule based model transformation languages hinders the applicability of this approach to industrial scales. Lower-level, and efficient, languages can be used but productivity and easy maintenance seize to exist. The abstraction penalty problem is not new, it also exists for high-level, object oriented languages but everyone is using them now. Why is not everyone using rule based model transformation languages then? In this thesis, we propose a framework, comprised of a language and its respective environment, designed to tackle the most performance critical operation of high-level model transformation languages: the pattern matching. This framework shows that it is possible to mitigate the performance penalty while still using high-level model transformation languages.