Supervisory Control Using Variable Lookahead Policies

This paper deals with the efficient on-line calculation of supervisory controls for discrete event systems (DES\´s) in the framework of limited lookahead control policies (or LLPs) that we introduced in previous papers. In the LLP scheme, the control action after a given trace of events has been executed is calculated on-line on the basis of an N-step ahead projection of the behavior of the DES. In this paper, we adopt a forward calculation procedure over the N-level tree of interest for the determination of the control action at each step. This forward procedure improves upon previous work by avoiding the explicit consideration of all the nodes of the N-level tree. The forward search ends whenever a control decision can be made unambiguously or whenever the boundary of the N-level tree is reached, whichever comes first. This motivate the name "Variable Policy" (or VLP) for this efficient implementation of the LLP supervisory control scheme. This paper presents a general VLP algorithm and studies the properties of several special cases of it. The paper also presents computational results regarding the application of the VLP algorithms to a time-varying DES.