Hi,

I am trying to find a way to program a PLC, for detecting any faulty sensors or actuators, using ladder logic. I have been able to come up with a possible strategy to monitor the sensor and actuator health off-line, but I can't figure out a way to do so on-line. The main problem is

How do I make my controller differentiate between a sensor/actuator in idle or OFF state from a faulty one?

Some strategies:

Time - the input should turn on/off occasionally withing a certain amount of time. No activity - stuck on or off - use a timer which will time out if a change doesn't happen on the input.

Sequence - the sensor doesn't respond in a sequence when sensors preceding or following it in time have actuated.

Critical applications - for very critical applications use redundant inputs. Shut down if they don't match within certain tolerences.

Thank you