Context, Foundations and Impact of Cyber-Physical Systems

Definitions of CPS

martint Thursday April 12, 2018

In an early posting, I addressed the question of what constitutes a Cyber-Physical System and How Do I Know When I See One? In this posting, I would like to follow up and add some more reflection.

First of all, the introduction of the concept of CPS has had quite some impact. The original US initiative was recognized by the US President’s Council of Advisors on Science and Technology, eventually resulting in the launch of a multidisciplinary research program on CPS by the National Science Foundation (a program that is still active), [Ref: Cyphers D2.1]. Since its introduction, the use of CPS has spread and it has also been adopted within several industrial domains, notably in the form of Industry 4.0 initiative in Germany, as a manufacturing domain interpretation of CPS. A large number of definitions of CPS have been introduced over the years. Let us here focus on three representative ones.

  1. “The integration of physical systems and processes with networked computing has led to the emergence of a new generation of engineered systems: Cyber-Physical Systems (CPS). Such systems use computations and communication deeply embedded in and interacting with physical processes to add new capabilities to physical systems. These CPS range from minuscule (pace makers) to large-scale (the national power-grid).” - This US definition was part of the initial CPS initiative [Ref: Cyphers D2.1]
  2. “A Cyber-Physical System (CPS) is a system with embedded software (as part of devices, buildings, means of transport, transport routes, production systems, medical processes, logistic processes, coordination processes and management processes), which:
    • directly records physical data using sensors and affect physical processes using actuators;
    • evaluates and saves recorded data, and actively or reactively interacts both with the physical and digital world;
    • is connected with other CPS and in global networks via digital communication facilities (wireless and/or wired, local and/or global);
    • uses globally available data and services;
    • has a series of dedicated, multi-modal human-machine interfaces.”,

    This more detailed definition was provided by acatech, the German National Academy of Science and Engineering in its work on an Integrated Research Agenda for CPS [Ref: Acatech]
  3. Cyber-Physical Systems or "smart" systems are co-engineered interacting networks of physical and computational components. This represents a more recent definition as part of the NIST CPS framework initiative [Ref: NIST 2017 ]

The different emphasis between the definitions (1 and 3 on one hand, and 2 on the other), provides two common but different perspectives to CPS; one emphasizing co-design and multidisciplinarity (definitions 1 and 3), and the other, the IT/cyber side capabilities of CPS (definition 2). Further definitions of CPS often highlight characteristics such as a large scale nature and capabilities related to adaptivity and intelligence.

Considering the etymological origin of the word, it is interesting and important to note the dual interpretation of the word “cyber”. The mainstream interpretation of the term “cyber” refers to the use of computers or computer networks, see e.g. [Ref: M-W, 2018]. However, the term originates from Norbert Wiener who coined cybernetics from the Greek “kybernetike”, meaning "governance", referring to feedback systems, [Ref: Wiener]. Today, both interpretations are relevant for CPS.

A key aspect of CPS is the potential for integrating information technologies, operational technologies in terms of embedded systems and control systems, and physical systems, to form new or improved functionalities.

It is important to realize that such integration concerns more than just matching apparent interfaces and combining cyber and physical parts. The composition has the purpose to achieve overall functionality and end-to-end system properties such as safety, availability and extensibility. Composition is thus multi-dimensional. A key challenge in developing CPS is that our current engineering methods and tools only provide limited support for such multi-dimensional integration -
see the Foundations workshop which addressed these and other challenges.

The diversity of definitions, and since more and more systems are becoming “CPS”, led the CyPhERS project to propose a characterization of types of CPS (the definitions tend to be rather general). The characterization recognizes that there are different types of CPS (e.g. from centralized to decentralized, with or without humans, at lower or higher levels of automation, etc.) and the fact that people using the term often have a bias, or emphasis when using it. This emphasis may, for example, refer to a viewpoint considering cloud and edge computing as core aspects of a CPS, see e.g. [Ref: Cyphers D2.1, Cyphers D2.2, Törngren]. Since CPS per definition represents heterogeneous systems, it is natural that they will have to be described by multiple views. A characterization of CPS can, for example, draw upon the CPS architectural framework by NIST, emphasising development of safety-critical CPS with common viewpoints such as functions and interfaces, as well CPS specific aspects such as timing and composition [Ref: NIST 2017 ].

As a perspective, it is interesting to relate to ongoing methodological work on Systems Engineering (SE) by INCOSE. In this work it was noted that there was little consensus on how to define what a “system” is among leading SE experts! It was however possible to identify a core set of traits that provide a common notion of what characterizes a “system”. These traits include the following: (i) there are relationships between the parts (of a system); (ii) there are interactions between the parts; (iii) there is more than one part; and (iv) there are "emergent properties" – that is, properties of the whole system not possessed by the individual parts acting separately [Ref: Sillitto].

One might then draw the conclusion that a CPS, in addition to these basic criteria, also needs to contain cyber- (computers) and physical parts, that are connected through feedback. Most CPS will in addition be communicating with other CPS, and have many types of interactions among the cyber (computer)- and physical parts, as well as with the environment in which the CPS acts, [Ref: Törngren, CPS, 2018]1. The CPS will also typically encompass multiple layers, having to deal with mul-tiple time horizons and abstraction levels.

From an ongoing survey of academic research literature on CPS (carried out thin the Platforms4CPS project) we conclude that the use of the term within the academic research community is dominat-ed by computer science – thus of the cyber side in terms of IT. We also however note that CPS as a concept, but under other names, is prevalent in many other engineering disciplines, for example under the umbrella of smart industry, mechatronics and even 5G efforts.

Considering research that is labelled as CPS, there appears to be less work that embraces the physical side and actual co-design of cyber-and physical systems (along the lines of definitions 1 and 3). Further areas where research are needed were identified by the Platforms4CPS workshop on the Foundations of CPS, [Ref: Platforms4CPS D4.3], including Humans as part of CPS, Dealing with CPS complexity, Au-tonomy and AI as part of CPS, and Composability for CPS.

Are there factors by which a CPS can be distinguished from other types of systems, e.g. those la-beled as IoT? In general the terms may well be used to refer to the same kind of system. Often the terms reflect specific perspectives (e.g. IoT emphasizing communication). IoT is by many academics seen as a subset of CPS as an intellectual discipline, since a CPS may, or may not, include internet connections, [Ref: Lee and Seshia]. An IoT system on the other hand may not necessarily in-volve feedback, but may still encompass multiple considerations of the physical world.

1 Martin Törngren and Ulf Sellgren. Complexity Challenges in Development of Cyber-Physical Systems (accepted for publication). To appear in Principles of modeling, Springer Festschrift" LNCS post-proceedings – Essays dedicated to Edward Lee. 2018