Design of Flexible Manufacturing Systems
A flexible manufacturing system (FMS) is a production system consisting of a set of identical and/or complementary numerically controlled machines which are connected through an automated transportation system.
Each process in an FMS is controlled by a dedicated computer (FMS cell computer). This is often imbedded in a large hierarchical network of computers. An FMS is capable of processing workpieces of a certain workpiece-spectrum in an arbitrary sequence with negligible setup delays between operations. This is possible since in an FMS a set of preadjusted tools is available through a centralized tool magazine with short access time and/or through local tool magazines at the machines with direct access. Furthermore, workpieces are clamped on pallets at separate setup tables thus allowing them to be quickly positioned at the machines. Due to the automation of tool exchange operations, the usually time-consuming interruptions for tool exchanges (including tool preparation, if necessary) are performed while the machine is operating. While a workpiece is being machined by a given tool, the next tool is already picked up by a robotic arm from the tool magazine. After completion of the process it is exchanged with the previous tool. This procedure generally requires only a few seconds. In the case of a workpiece exchange, this time is necessary anyway while the new NC program required to control the next operation to be performed at the machine is loaded.
It is customary to process workpieces of several different product types and production orders simultaneously in an FMS. Usually the workpieces are mounted on pallets with the help of fixtures. Every workpiece is characterized by its specific process advancement, which continuously has to be updated and stored individually by the control system of the FMS. Intermediate storing of unfinished workpieces between operations can be accomplished at central storage areas (central buffer) or, if possible, at local storage areas at the machines. A set of (ca. 20-200) local tools is assigned to each of the CNC (Computer numerical control) machines (machining centers) in an FMS for direct access. They can be stored in cassettes, drums, discs or chain magazines. For processing parts which require tools not available at the local tool storage, external supply is necessary. In general, this is achieved by exchanging tools which are no longer required. This tool exchange can involve a single tool, or it can involve complete cassettes or discs armed with those tools which are next required at the machine. The exchange procedure can be performed either automatically, through a tool supply system, or manually.