I. Overview of Fms Problems

The design and use of flexible manufacturing systems (FMSs) involve some intricate operations research problems. FMS design problems include, for example, determining the appropriate number of machine tools of each type, the capacity of the material handling system, and the size of buffers. FMS planning problems include the determination of which parts should be simultaneously machined, the optimal partition of machine tools into groups, allocations of pallets and fixtures to part types, and the assignment of operations and associated cutting tools among the limited-capacity tool magazines of the machine tools. FMS scheduling problems include determining the optimal input sequence of parts and an optimal sequence at each machine tool given the current part mix. FMS control problems are those concerned with, for example, monitoring the system to be sure that requirements and due dates are being met and that unreliability problems are taken care of. This paper defines and describes these FMS problems in detail for OR/MS researchers to work on. K e y w o r d s and phrases FMS design problems, FMS planning problems, FMS scheduling problems, FMS control problems. I n t r o d u c t i o n Flexible manufacturing systems (FMSs), being somewhat similar and yet different from conventional manufacturing systems, provide new and different problems for the OR/MS community to solve. In this paper, short descriptions are provided of the various FMS management problems that need to be addressed at different stages of an FMS's life cycle, from conception through to implementation and operation. Some of these problems have been examined to some extent from varying points of view and using different OR models and various solution techniques. © J.C. Baltzer A.G., Scientific Publishing Company 4 K.E. Stecke, FMS design, planning, scheduling, and control problems This paper describes the problems and decisions that have to be addressed during the design, planning, scheduling and, finally, the actual control of an FMS. A companion paper [6] provides some information on models that can and have been used to evaluate some of these decisions. An initial issue that has to be addressed is whether or not flexible manufacturing is really applicable to the proposed application. There are technological considerations that affect this decision. Also, often, some sort of cost justification is attempted, but this problem is difficult and the numbers can be manipulated to justify any decision. What is also important are the strategic abilities that flexible manufacturing provides. It is very difficult to quantify the abilities to respond to the demand for the FMS products, or to quickly introduce new part numbers. Nevertheless, cost justification is required and some of the papers in this volume do address this issue in an appropriate manner. When the management decision has been made that flexible manufacturing is the way the company will go for production in a particular department, then all of the following problems and issues have to be addressed. 1. FMS design p rob l ems In developing an FMS design, there is a partial ordering to some of the decisions that have to be made. Some decisions must precede others in time. We partition these into initial specification decisions and subsequent implementation decisions. These decisions are now defined and described further if additional explanation is required. 1.1. INITIAL SPECIFICATION DECISIONS (1) First, the manufacturing requirements need tQ be specified. Determine the range or families of components or part types to be produced. From all of the part types which the factory produces, identify a subset to be manufactured and/or assembled on the FMS. This is only an initial specification of what it is desired to manufacture on the system. This will change in time. Some of the benefits of flexible manufacturing includes the ability to respond to changing markets, and to quickly and efficiently incorporate design or process changes, or to use new materials. New part numbers can be added in the future. From this broad idea of what the FMS will manufacture, the capacity and functional requirements can be specified, in terms of machine time and cutting tools required. In particular: K.E. Stecke, FMS design, planning, scheduling, and control problems 5 (2) Determine how these part types shall be manufactured. This consideration eventually specifies the numbers and types of machine tools and robots that are required. First, a process plan has to be determined for each part type. Each process plan defines not only which type (or sometimes, types) of machine tool shall perform each operation, but also determines the appropriate cutting tools and cutting conditions (i.e. the speed and feed rate of each cutting tool as well as the depth of the cut). These are required to determine the processing time of each operation in the process plan. With this information, the capacity required (both tool magazine capacity and processing time capacity), in terms of the number of machine tools of each type, can be determined. A process plan also provides a partial precedence ordering among theoperations. (3) Specify what types of different flexibilities are required or desired and the amounts of each. Browne et al. [1 ] defined and described eight types of flexibilities that all FMSs theoretically can have. Buzacott [2] begins to quantify some of these flexibilities. All FMSs have varying amounts of each. In addition, no FMS utilizes all of its potential flexibility. This is because, in general, 'more flexibility' will be both more expensive and more difficult to utilize or to take advantage of. For example, flexible routing is more difficult to manage than fixed routing. Also, real-time control of the flow of parts through the system (where a decision on an act is made whenever it is near the time for some decision or action to occur, and is based on the particular state of the FMS near the time of that action) is much more difficult to implement than a fixed, static schedule of work flow. (4) Determine the type of FMS that shall be developed. In Browne et al. [1 ], the different types of flexibilities are used to classify different FMS types (according to how 'flexible' an FMS is). Of course, these FMS types range from somewhat inflexible (perhaps a flexible transfer line having a fixed process flow) to very flexible FMSs, having a widely varying process flow, even for parts of identical part type. This decision will help specify the amount of automation that will be included in the system and the type of control strategies for the different system components. 6 K.E. Stecke, FMS design, planning, scheduling, and control problems (5) Specify the type, then capacity, of the material handling system. The parts can be automatically transported throughout the FMS via roller conveyors, two-way tow-line tracks, or wire-guided carts, for example. In the latter two cases, the number of carts has to be determined. Stecke and Browne [5] further sub-classify various types of FMSs according to their methods of handling materials. (6) First the type, and then the size, of the buffers has to be specified. The buffer provides a queueing place for in-process inventory. There can be a central buffer area or a small, local buffer at each machine tool or both or neither. Some systems provide no area for a buffer and in-process inventory remains on the material handling system. Whether storage is centralized or local, the buffer size has to be determined. The trade-offs involve, in part, the space and the cost of the buffers (too much space) versus having enough buffers to help keep the machine tools utilized. (7) The hierarchy among the computers controlling the different aspects of production has to be specified. This control structure also specifies which computers communicate with which (their interconnections) and at what levels of the control. Data transmission devices are specified. There may be a computer controlling the maclfining operations of the machine tools, which determines and then downloads the appropriate part program that describes each cutting operation to the machine tool. There may be another computer controlling the material handling system. There may be still another supervisory computer controlling these computers. The structure may be CNC and/or DNC, for example. (8) The vendors have to be chosen.