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    Analogue-digital systems with modes of physical


    Edwin Beggs1 and John V. Tucker2

    1 Department of Mathematics, College of Science,Swansea University, Singleton Park, Swansea, SA2 8PP,

    Wales, United

    2 Department of Computer Science, College of Science,Swansea University, Singleton Park, Swansea, SA2 8PP,

    Wales, United

    Abstract. Complex environments, processes and systems may exhibitseveral distinct modes of physical behaviour or operation. Thus, for ex-ample, in their design, a set of mathematical models may be needed, eachmodel having its own domain of application and representing a particularmode of behaviour or operation of physical reality. The models may beof disparate kinds discrete or continuous in data, time and space. Fur-thermore, some physical modes may not have a reliable model. Physicalmeasurements determine modes of operation. We explore the question:What is a mode of behaviour? How do we specify algorithms and software

    that monitor or govern a complex physical situation with many modes?

    How do we specify a portfolio of modes, and the computational problem

    of transitioning from using one mode to another mode as physical modes

    change? We propose a general definition of an analogue-digital systemwith modes. We show how any diverse set of modes with or withoutmodels can be bound together, and how the transitions between modescan be determined, by constructing a topological data type based upona simplicial complex. We illustrate the ideas of physical modes and ourtheory by reflecting on simple examples, including driverless racing cars.

    No plan survives contact with the enemy.

    After Helmuth von Moltke the Elder (1800-1891)

    There are known knowns; there are things we know that we know. Thereare known unknowns; that is to say, there are things that we now knowwe dont know. But there are also unknown unknowns there are thingswe do not know we dont know.

    Donald Rumsfeld (2002)

  • 2 Beggs and Tucker

    1 Introduction

    A typical analogue-digital system is a system in which a continuous physicalenvironment, process or component is monitored or governed by a discrete al-gorithmic process. This simple description covers an astonishingly large rangeof systems, for example: classical control systems for machines, industrial plant,and buildings; networked products and vehicles; human monitoring and surveil-lance systems; and scientific experiments, mediated by software, that measurephysical quantities. Many applications involve hybrid systems and cyber-physicalsystems3 These are types of analogue-digital systems whose deployment is vast inscope and whose formal theories and design methods have matured; in particular,hybrid systems have a solid theoretical basis [25], which we will discuss shortly(in subsection 1.3). We will take a fresh look at such systems and raise and offeranswers to some foundational questions for a general theory of analogue-digitalsystems, reflecting on the interface between the physical and the algorithmic,and defining formally a new class of complex analogue-digital systems havingdistinct modes of physical behaviour.

    1.1 Observations on real-world analogue-digital systems

    To begin, consider from first principles, a real-world analogue-digital system thatconsists of physical equipment that is controlled by software on a processor. Itis analogue-digital because the system involves continuous and discrete data.Typically, the data characterising the equipment is represented by real numbers,and the data processed by the program is represented by bits. The equipmentis made from physical components that exchange data with the controlling soft-ware. An important design criterion is the

    Principle 0: Robustness. The design of the software and its underlying al-gorithms must be able to cope with any data that may be output from, or inputto, the physical components.

    Typically, the design of the software is based upon(a) mathematical models generally called dynamical systems of the be-

    haviour of the physical equipment;(b) exception handlers if none of the mathematical models apply to a mode

    of behaviour;(c) logical and algebraic models of the behaviour of the programs.

    Of course, in the case of (c), the behaviour of the programs is derived from thesemantics of the specification and programming languages employed; we will notconsider this factor. Consider (a): how the software depends on models of thebehaviour of the equipment by, or derived from, dynamical systems.

    3 A cyber-physical system generalises the concept of an embedded system to a net-work of interacting devices with physical input and output [23]. For example, cyber-physical systems and the internet of things are central to speculations on the futureof manufacturing in [15].

  • Analogue-digital systems with modes of physical behaviour 3

    To create the software for an analogue-digital system, the physical must be re-placed by an abstract specification that documents certain operations, tests andproperties and that constitutes a data type interface between physical quantitiesand the algorithms and software. The interface must enable portability and veri-fication so that the software can be certified as reliable as possible according tosome best practice engineering standard. For the designer of an analogue-digitalsystem, specification and validation presents certain problems outside softwareengineering. The reliability of an analogue-digital system depends upon the ab-stract assumptions about the physical system, the sensors and actuators. Theseassumptions are based on dynamical systems that model physical reality.

    A dynamical system is a mathematical model of an entity, process, or envi-ronment whose behaviour changes in time. The model represents behaviour bymeans of states that change over time. The model is likely to have parametersthat are external inputs, which represent either

    (i) external influences that are not predictable physical factors to which thesystem must respond or, conversely,

    (ii) known parameters or instructions designed to control the system.

    The state of a dynamical system can be based on continuous data, discrete dataor a combination of both.

    Complex environments, processes and systems may exhibit quite distinctmodes of physical behaviour or operation. Thus, for the design of complex phys-ical environments, processes and systems, a set of mathematical models maybe needed, each model having its own physical domain of application, and rep-resenting a particular mode of behaviour or operation of physical reality. Themodels may be of disparate kinds. Certainly, no single mathematical model isadequate. Furthermore, the models may not cover adequately all the possiblemodes of behaviour, i.e., not all physical modes may have a reliable model.

    1.2 Modelling analogue-digital systems with modes

    This paper is about complex analogue-digital systems with many modes. Weaddress the following questions: In a complex analogue-digital system,

    What is a mode of behaviour?

    How can modes of physical behaviour, with a portfolio of different models, bespecified for the use of algorithms and software?

    How can modes of physical behaviour, without sound models, be specified forthe use of algorithms and software?

    How are all the modes bound or linked together to make a robust system? Asthe system operates, one model becomes less relevant and another more relevantmodel must be chosen to replace it. What conditions govern the transition fromone mode to another?

    We introduce initial concept of an analogue-digital system of the form

    physical system + interface and protocol for data exchange + algorithm.

  • 4 Beggs and Tucker

    Specifically, we propose the algorithms and software treat the physical systemas an oracle, whose queries are mediated by a protocol governing an interface.4

    A good deal is known about the basic properties of such a physical oracle modelin cases where the physical systems are very simple and can be faithfully cap-tured by a single dynamical system and, therefore, have one and only one mode[6,7,8,9,10,12]. Here we generalise this concept of an analogue-digital system byintroducing modes that may have models or need to be handled as exceptionalsituations. See the schematic in Figure 1 with four modes.










    physical system program

    Figure 1: Illustrating four modes

    We propose a theoretical framework for the development of such analogue-digital systems with modes of physical behaviour and their evaluates their fitness.Essentially, we define the concept of a mode to be a data type representinga physical behaviour and equipped with criteria that evaluate its accuracy orrelevance.

    To the interface and data exchange protocol are added a set ofmode transitionfunctions that specify what needs to be done to make a transition from onemode to another. We create a simple geometric description of the system, onewhich describes both the relationship between the various modes of operationof the system and when transitions between the modes are necessary. Such ageometry needs a topological space and so we construct a simplicial complex a space made of points, lines, triangles, tetrahedra etc., glued together usingsimple rules that we call a nerve. The points represent the modes, and thelines or triangles joining them are used to show how well each mode describesthe physical system. As th