A Petri-Net Based FA (Factory Automation) Controller for Flexible and Maintainable Control Specifications

摘要

In this paper, a new type controller based on a Petry-Net like control language named C-net is proposed. By using C-net, control specifications can be easily described as a directed graph according to a time chart representing work station's actions. Complicated control specifications of work stations can be easily comprehended and changed.C-net can be directly executed by a microprocessor based controller named SCR (Station Controller). A C-net interpreter executes a defined C-net by interpreting it cyclically (this cycle is called an execution cycle). In real time controll systems, a response time of the controller is an essential factor. An efficient execution schema of C-net interpretation called a limitted scanning method is also presented. In order to evaluate the efficiency of this schema, a performance evaluating model is built. Through the analysis of this model, the expression for calculating a response time of SCR is formulated, and this theoretical response time shows good coincidence with measured response time. By using the presented method SCR can excute the C-net model within 15 msec/execution cycle (on HMCS 68000), which describes control specifications of 10 machine tools-work station, where one of actions avaragely finishes every half a second. (This execution time doesn't depend on a scale of a C-net model.)SCR has been successfully applied to several applications. A parts assembly station which consists of one industrial robot and six machine tools is dercribed as a example. In this application, initial software development man-hour was reduced about 50% compared with relay radder diagram method. Besides, maintainances as trouble shooting and changing of control specifications have been achieved easily.