Adaptive Design of Petri Net Controllers for Error Recovery in Automated Manufacturing Systems

The concept of Petri net controllers is extended to include automatic error recovery and adaptive design. The Petri net controller provides control of an automated manufacturing system and the execution of error recovery. In our Petri net controller, a place that represents an operation or a state of a machine is attached to two functions and a constant so that it can represent a system working with both normal states and abnormal states. In addition it is possible to detect an error with the controller using watchdog timers. Four basic Petri net augmentation methods for error recovery are investigated: input conditioning, alternate path, feedback error recovery, and forward error recovery. We demonstrate that when these methods are used to augment the Petri net controller, some important properties of the controller are guaranteed to be preserved. This is shown by developing and proving four theorems. These properties include boundedness or safeness, liveness, reversibility, and the essentially decision-free property. An example of augmentations for error recovery for a piston insertion cell with two robots is given.