Co-design Approaches to Dependable Networked Control Systems

Chapter 2

Computing-aware Control 1

2.1. Overview

To implement a controller, the basic idea consists of running the whole set of control equations in a unique periodic real-time task whose clock gives the controller sampling rate. In fact, all parts of the control algorithm do not have an equal weight and urgency w.r.t. the control performance. To minimize the latency, a control law can be basically implemented as two real-time blocks; the urgent one sends the control signal directly computed from the sampled measures, while updating the state estimation or parameters can be delayed or even more computed less frequently [ÅST 97].

In fact, a complex system involves sub-systems with different dynamics which must be further coordinated [TÖR 98]. Assigning different periods and priorities to different blocks according to their relative weight allows for a better control of critical latencies and for a more efficient use of the computing resource [SIM 98]. However, in such cases finding adequate periods for each block is out of the scope of current control theory and must be done through case studies, simulation and experiments.

Latencies have several sources: the first one comes from the computation duration itself, and worst-case-execution times (WCET) are difficult to get. In multi-tasking systems, they come from pre-emption due to concurrent tasks with higher priority, from precedence constraints and from synchronization. Another source of delays is the communication medium and ...