Hybrid Co-simulation : It ' s About Time Fabio Cremona

Model-based design methodologies are commonly used in industry for the development of complex cyber-physical systems (CPS). There are many different languages, tools, and formalisms for model-based design, each with its strengths and weaknesses. Instead of accepting some weaknesses of a particular tool, an alternative is to embrace heterogeneity, and to develop tool integration platforms and protocols to leverage the strengths from different environments. A fairly recent attempt in this direction is the Functional Mock-up Interface This work is partially based on previous work published by the authors [7, 8, 15], but the contributions presented in this article stand on their own. This work was supported in part by the TerraSwarm Research Center, one of six centers administered by the STARnet phase of the Focus Center Research Program (FCRP) a Semiconductor Research Corporation program sponsored by MARCO and DARPA, by the National Science Foundation (NSF) awards #1446619 (Mathematical Theory of CPS), #1329759 (COSMOI) and #1139138 (ExCAPE), and by iCyPhy (the Industrial Cyber-Physical Systems Research Center) and the following companies: Denso, National Instruments, and Toyota. This work was also supported by the Swedish Research Council #623-2013-8591 and by the Academy of Finland. Fabio Cremona University of California, Berkeley, USA E-mail: f.cremona@eecs.berkeley.edu Marten Lohstroh University of California, Berkeley, USA E-mail: marten@eecs.berkeley.edu David Broman KTH Royal Institute of Technology, Sweden E-mail: dbro@kth.se Edward A. Lee University of California, Berkeley, USA E-mail: eal@eecs.berkeley.edu Michael Masin IBM Research – Haifa, Israel E-mail: michaelm@il.ibm.com Stavros Tripakis University of California, Berkeley, USA & Aalto University, Finland E-mail: stavros@eecs.berkeley.edu 2 Fabio Cremona et al. (FMI) standard that includes support for co-simulation. Although this standard has reached acceptance in industry, it provides only limited support for simulating systems that mix continuous and discrete behavior, which are typical of CPS. This paper identifies the representation of time as a key problem, because the FMI representation does not support well the discrete events that typically occur at the cyber-physical boundary. We analyze alternatives for representing time in hybrid co-simulation and conclude that a superdense model of time using integers only solves many of these problems. We show how an execution engine can pick an adequate time resolution, and how disparities between time representations internal to co-simulated components and the resulting effects of time quantization can be managed. We propose a concrete extension to the FMI standard for supporting hybrid co-simulation that includes integer time, automatic choice of time resolution, and the use of absent signals. We explain how these extensions can be implemented modularly within the frameworks of existing simulation environments.