The issue of supervisory control of discrete event systems (DES) based on Petri nets (PN) has been investigated by using various methods. However, there exists a problem that the supervisor, introduced to enforce the given specification, causes a deadlock in the closed-loop system, especially when the deadlock occurs in the closed-loop system in which the original plant model is live. To address this problem, we propose a hybrid approach to design a deadlock-free PN controller for DES with given specification. The approach mainly consists of two stages. Firstly, a supervisor is designed to enforce the given specification using the reported place invariants method. Then, we regard the obtained closed-loop system with deadlock introduced by the supervisor as a new plant net and adopt the theory of regions to design a deadlock-free PN controller. The controller designed by our approach is guaranteed to be maximally permissive when the original plant PN is fully controllable and observable. Even for partially controllable and observable plant PN, it is possible to obtain a suboptimal solution.
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