Relay ladder logic diagnosis

A wide variety of industrial systems are controlled and supervised by programmable logic controllers (PLCs). Although the PLC is particularly well adapted to problems requiring sequential control or the verification of interlocks prior to further actions taking place, they are very poor at supporting diagnostics when problems occur. On the other hand, the PLC does contain valuable information to assist with diagnosis though, it is just very difficult for an engineer or operator to understand it directly. This paper describes the automatic development of a troubleshooting system which can be build using the precise and deterministic knowledge embedded within the PLC and extending it with heuristic knowledge.