Strong co-observability conditions for decentralized supervisory control of discrete event systems

We study nonblocking decentralized supervisory control of discrete event systems. A notion of normality which is a weaker version of strong decomposability has been defined in our previous work (2001). However, a nonblocking decentralized supervisor which achieves the supremal L_m (G)-closed, controllable, and normal sublanguage may be still unnecessarily restrictive. The main purpose of the paper is to synthesize a more permissive nonblocking decentralized supervisor. We introduce stronger versions of C&P co-observability and D&A co-observability, called strong C&P co-observability and strong D&A co-observability, respectively. Strong C&P co-observability and strong D&A co-observability are weaker than normality. Moreover, strong D&A co-observability is preserved under union. So a nonblocking decentralized supervisor exists under the AND rule for the supremal L_m(G)-closed, controllable, and strongly D&A co-observable sublanguage. We present a procedure for computing this supremal sublanguage