Fault checking in PLC

[plcteur 0]

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?

[b_carlton 61]

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.

[plcteur 0]

Thank you