State recovery for input/output asynchronous machines using output feedback

In this paper, a control mechanism is proposed for state fault-tolerance of asynchronous sequential machines. A disturbance input may penetrate into the asynchronous machine and provoke unauthorized state transitions. The considered asynchronous machine is of input/output type, where access to the state of the machine is not available. The objective is to design output feedback controllers that automatically counteract unauthorized state transitions. Necessary and sufficient conditions for the existence of appropriate controllers are presented in a theoretical framework. As a case study, the closed-loop system of an asynchronous machine with the proposed control scheme is implemented in VHDL code.