Decentralized control is a promising method with many advantages while an effective design method for it is desired. Design in conventional methods is complicated because the correspondence between a system role and its controllers is unclear. To solve this problem, we have proposed the idea of "functionality" for a bilateral control system. The idea is to divide the system role into minimum components. The design method based on this idea is clear since controllers correspond to respective functions. Furthermore, the design method is simple since each function is a minimum component. This study expands the idea of the functionality to design of decentralized control systems. Subsystems are constructed based on functions. Functions for normal operation and for exception handling are treated in a unified manner. The system can automatically sustain important functions by a priority order of functions. This method realizes simple and explicit controller design for a decentralized control system with exception handling. Simulation and experimental results show the flexibility, fault tolerance and expandability of the decentralized system.
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