The paper describes an application of three-level, hierarchical control to a simple assembly line for micro-bearings. This approach ensures that the whole control system is flexible, reliable, easily maintained, and inexpensively installed. Each work station controller, located at the lowest level, performs an individual assembly operation. The station controller is a dedicated small programmable logic controller (PLC) based on Petri nets. A number of separate assembly operations are formed into an assembly line which is controlled by an assembly line controller which is in the middle level. A low-cost local area network (LAN) is employed to interconnect the station controllers and line controllers. The lower-level communication is implemented at much reduced cost. To guarantee correct operations of the station controllers and the associated communication to their line controller, an emulation and simulation algorithm in the stations has been designed in order to test and verify the correctness of on-line control algorithms and their communication prior to direct transfer to running on the practical assembly line.
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