No abstract

Model-based design methodologies are commonly used in industry for the development of complex cyber-physical systems (CPSs). 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 Communicated by Prof. J. Sztipanovits, M. Broy, and H. Daembkes. This work is partially based on previous work published by the authors [7,8,15] 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.

No abstract

The Internet of Things leverages Internet technology in cyber-physical systems (CPSs), but the protocols and principles of the Internet were designed for interacting with information systems, not cyberphysical systems. For one, timeliness is not a factor in any widespread Internet technology, with qualityof-service features having been routinely omitted for decades. In addition, for things, safety, freedom from physical harm, is even more important than information security, the focus on the Internet. Nevertheless, properties of the Internet are valuable in CPSs, including a global namespace, reliable (eventual) delivery of messages, end-to-end security through asymmetric encryption, certificate-based authentication, and the ability to aggregate data from a multiplicity of sources in the cloud. This paper discusses and surveys architectural approaches, communication protocols, and programming models that promise to bridge the gap, enabling the use of the Internet technologies even in safety-critical, cyber-physical applications such as factory automation and transportation. Specifically, we argue that smart gateways hosted on edge computers complement cloud-based services; they can provide tighter control over timing and security that is robust against network outages, play an active role in managing interactions between things, and isolate safetycritical services from best-effort services. We explain how time sensitive network technology can be leveraged to reliably orchestrate a multiplicity of things, and how augmenting our programming models with a well-defined notion of time can make systems more deterministic and more testable.

This paper presents a master algorithm for co-simulation of hybrid systems using the Functional Mock-up Interface (FMI) standard. Our algorithm introduces step revision to achieve an accurate and precise handling of mixtures of continuous-time and discrete-event signals, particularly in the situation where components are unable to accurately extrapolate their input. Step revision provides an efficient means to respect the error bounds of numerical approximation algorithms that operate inside co-simulated FMUs. We first explain the most fundamental issues associated with hybrid co-simulation and analyze them in the framework of FMI. We demonstrate the necessity for step revision to address some of these issues and formally describe a master algorithm that supports it. Finally, we present experimental results obtained through our reference implementation that is part of our publicly available open-source toolchain called FIDE