Development of a rule-based finite capacity scheduling system

Abstract

This paper discusses the use of the finite capacity planning model as a basis for a job-shop scheduling system. The suitability of this approach for constructing short term schedules with very short lead times is examined. This paper introduces a rule-based scheduling system whose operation, unlike previous monolithic schedulers, is based around distinct though interlinked processes. Scheduling herein is defined as the selection of a set of orders for manufacture, and the allocation of processing time to each on an acyclic network of processing resources. The objectives are, firstly, compliance with all customer deadlines and, secondly, the efficient utilisation of available machine time and prevailing machine setups.Key words : Finite capacity scheduling, rule-based systems. INTRODUCTIONProduction scheduling can be seen as forming a link between the areas of Computer Aided Design (CAD) andComputer Aided Manufacture (CAM), whose objectives are the implementation of all managerial directives relating to the manufacturing domain. Taking a more algorithmic approach, scheduling may be thought of as the problem of constructing a master production schedule (MPS) from a set of pending orders and a set of resource allocations. Early scheduling systems used priority dispatch rules 1 , forming a deterministic method whereby orders are not scheduled until certain constraints have been fulfilled. Later heuristic systems (e.g. XCON & ISIS 2) were intended more for medium or long term planning, rather than short term scheduling.Much work has also gone into the examination of Flexible Manufacturing Systems (FMS) -domains with multiple machines connected to an automated transportation system. This, however, only covers a fraction of manufacturing domains. This paper describes the development of a scheduling system for use in a manufacturing domain which is viewed as a directed graph, with nodes representing processing machines and edges between nodes representing the movement of material between these machines. Each order has a predetermined "route" which is a list of nodes to be visited by that order, the object of the scheduling system being to produce a schedule at very short notice (within 5 minutes) to suit the current factory status. The most important feature of this system is that orders which fall into a category called "urgent orders" must be manufactured promptly.Other aims are the efficient use of machine setups and maximum utilisation of the available resources. SCHEDULINGMany different systems have been developed to solve a variety of scheduling problems. The solution described in this paper differs from existing systems in many ways, and most of these differences are due to the need for reactive 3 scheduling.The much referenced XCON, which proved the applicability of expert system techniques to solving real world problems, would not prove a good model on which to base this project. XCON configured the