Fabio Cremona and Marten Lohstroh and David Broman and Stavros Tripakis and Edward A. Lee and Michael Masin

EECS Department, University of California, Berkeley

Technical Report No. UCB/EECS-2017-6

April 6, 2017

http://www2.eecs.berkeley.edu/Pubs/TechRpts/2017/EECS-2017-6.pdf

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 (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.

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BibTeX citation:

@techreport{Cremona:EECS-2017-6,
    Author= {Cremona, Fabio and Lohstroh, Marten and Broman, David and Tripakis, Stavros and Lee, Edward A. and Masin, Michael},
    Title= {Hybrid Co-simulation: It's About Time},
    Year= {2017},
    Month= {Apr},
    Url= {http://www2.eecs.berkeley.edu/Pubs/TechRpts/2017/EECS-2017-6.html},
    Number= {UCB/EECS-2017-6},
    Abstract= {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 (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.},
}

EndNote citation:

%0 Report
%A Cremona, Fabio 
%A Lohstroh, Marten 
%A Broman, David 
%A Tripakis, Stavros 
%A Lee, Edward A. 
%A Masin, Michael 
%T Hybrid Co-simulation: It's About Time
%I EECS Department, University of California, Berkeley
%D 2017
%8 April 6
%@ UCB/EECS-2017-6
%U http://www2.eecs.berkeley.edu/Pubs/TechRpts/2017/EECS-2017-6.html
%F Cremona:EECS-2017-6