On the formalisation of integrating watchdogs into discrete event controller structures

This paper reports a low-cost online fault detection approach for supervisory controllers in the framework of Supervisory Control Theory (SCT). For the cases when sensors dedicated to fault detection increase significantly the cost of controllers, or failure events are even impossible to detect by a direct way, methods based on the well-known watchdog structures are proposed. To successfully integrate watchdogs in the SCT framework, their discrete-event model is defined, and fault-detection techniques proposed in this paper are based on the extension of controller models previously designed using conventional supervisory synthesis methods. Fault-detection strategies are presented for centralized and distributed supervisory control environments, in the latter case providing solutions for avoiding problems according to fault propagation. Proposed techniques give full authority to the system designer in defining failure handling procedures and are proved not to influence the operation of the processes when no fault occurs. Since the extension of the controller models is defined by a formal and systematic manner, suitable algorithms based on the presented techniques can be constructed to allow automatic integration of fault-detection capabilities into existing controller structures.