REVERSE SCHEDULING AND ITS USE FOR INTELLIGENT FMS JOB ASSIGNMENTS

Abstract

In flexible manufacturing systems, one of the factors the level of success depends on is the effectiveness of the methods used for solving scheduling problems. Heuristic approaches are commonly used to generate practical solutions for specific scheduling problems. These approaches are different and heavily affected by the constraints that the environment imposes. Among the tools used are computer simulations, expert systems and optimisation methods to get more efficient schedules. But the question is: how good these schedules are? It is very difficult to estimate the goodness of the schedule. It is hard to get criteria to measure the goodness and, if they exist, it is not easy to use them. In this paper the idea of reverse scheduling is introduced as a method for solving the FMS scheduling problem and to estimate its goodness. The idea of reverse scheduling was proposed by professor J. Somlo and it is in some respect similar to the idea of reverse engineering. At the use of the reverse scheduling method the ideal schedule is first generated. Such generation is done in a way that the resources of the system are fully utilised. Then, the input data sets are generated which can give the ideal schedule. These data sets are given by fictitious process plan results. After that, using the generated process plans, a traditional 'forward' scheduling is performed. The obtained schedule is compared with the ideal using of a single performance index or combination of indices. On this basis different scheduling algorithms utilising different priority rules are compared. The goodness of the priority rules is estimated, and expert systems strategies are developed. To use the result of reverse scheduling, statistical properties of the process plans of FMS tasks should be estimated. The knowledge base, which has been developed using the reverse scheduling will be able to recommend the suitable scheduling strategies to be used in similar situations.