A New Petri-Net-Based Synthesis Technique for Supervisory Control of Discrete Event Systems

A new Petri-net-based top-down synthesis technique for supervisory control of Discrete Event Systems (DES) is proposed to solve the forbidden state problem. The supervisors obtained are compiled supervisors, whose control policy is represented as a net structure, as opposed to mapping supervisors, whose control policy is computed as a feedback function of the marking of the system. The compiled supervisors obtained by using the technique proposed in this paper are both nonblocking and maximally permissive. The supervisors to be synthesised consist of a controlled Automation Petri Net model of the system. Automation Petri Nets (APN) include the following extensions to the ordinary Petri net framework: sensor readings as firing conditions at transitions and actions assigned to places. Ladder logic diagram (LLD) code is used to implement the supervisors on programmable logic controllers (PLC). It is important to note that the supervisors obtained are correct by construction; therefore there is no need for verification. The supervisory control synthesis technique proposed in this paper is applicable to both high-level discrete event control, where the role of the supervisor is to coordinate control of in the discrete manufacturing sense machines, workcells, etc., and low-level discrete event control, where the role of the supervisor is to arrange low-level interaction between control devices, such as motors and actuators. In this paper, the applicability of the proposed technique to low-level discrete event control is demonstrated by considering an experimental discrete manufacturing system.