From Control Loops to Real-Time Programs
Paul Caspi, Oded Maler · Birkhäuser Boston eBooks · 2005
This article discusses what we consider one of the central aspects of embedded systems: the realization of control systems by software. Although computers are today the most popular medium for implementing controllers, we feel that the state of understanding of this topic is not satisfactory, mostly due to the fact that it is situated in the frontier between two different cultures and world views (control and informatics) which are not easy to reconcile. The purpose of this article is to clarify these issues and present them in a uniform and, hopefully, coherent manner. The article is organized as follows. We start with a short high-level discussion of the two phenomena involved, control and computation. In Section 2 we explain the basic issues related to the realization of controllers by software using a simple proportional-integral-derivative (PID) controller as an example. In Section 3 we move to more complex multi-periodic control loops and describe various approaches for scheduling them on a sequential computer. Section 4 is devoted to discrete-event (and hybrid) systems and their software implementation. Finally, in Section 5 we briefly discuss distributed control and fault tolerance.