PLC Commanded Intelligent Logistics Model Based on Discrete Event Systems

The main issue of this paper is to design a controller for Discrete Event Systems (DES), based on the Supervisory Control Theory (SCT), applied in transport or logistic areas. This theory is used mostly for command and control of many cybernetic, manufacturing, communication, military or transportation as well as many others. The controller designed for the DES applications is currently named as a supervisor. The SCT based applications involve many dedicated languages and automation concepts taken from the classic control domain. By using SCT, any logistic chain can be modeled by using automation sequences. The automation itself is a true event sequence generator. The desired and controlled behavior of that logistic system is clearly defined by its specifications. By using SCT elements, any supervisor can be particularly synthesized, avoiding the transportation to reach some undesired states. After defining all operations, the resulted supervisor can be implemented in practice to perform real control tasks. Programmable Logic Controllers (PLCs) are widely applied for control tasks in today industry, and, by consequent, the supervisor for an industrial logistic chain could be implemented by means of PLCs. The language which is mostly used for programming PLCs is based on the Ladder Logic Diagram (LLD) Any supervisor described in an automation sequence form could be easily converted to an LLD code. As an example, in this paper, a small manufacturing system that consists of two machines and a buffer is considered. All pieces must be vehiculated between the machines on a simple logistic sequence, depending on their state. The implementation of the supervisor is also provided.