Design of real-time periodic control systems through synchronization and fixed priorities

Control systems are often designed using a set of co-operating periodic modules running under control of a real-time operating system. A correct behaviour of the closed-loop controller requires that the system meets timing constraints like periods and latencies, which can be expressed as deadlines. The control system timing requirements are captured through a partition in control paths by which priorities are assigned according to their relative urgency. Latencies are managed through precedence constraints and more or less tight synchronization between modules. The implementation uses the fixed-priority based pre-emption service of an off-the-shelf real-time operating system. Such a system can be modelled with timed event graphs, and its temporal behaviour can be analysed using the underlying (max, plus) algebra. Examples coming from a uni-processor robot controller are provided