Self-checking means that on a periodic basis (every cycle, upon machine start up, etc.) a safety monitoring circuit or device performs diagnostics to ensure individual components have not failed. Existing faults (or failures) detected will result in the machine being shutdown, preventing further operation.
Some applications may require that not only are standard faults detected, but also diminished performance faults that occur should be detected. When an application depends on reaction times (such as a stopping function on a guarded machine), the diminished performance becomes an issue, due to the engineering of the safeguard involved stopping time calculations to determine the safe guarding distance.
It is not uncommon to find “sticky” or “sluggish” valves. A monitoring system that just “checks” to see if both valves shifted may not truly be monitoring the system’s safety. Self-monitoring valves have the necessary logic built-in to check for both types of faults.
Redundancy refers to placing two components that perform the same operation into a system in such a manner that failure of either component will not inhibit the performance of the critical function of the second component. Diverse redundancy refers to a system that uses two dissimilar components in performing redundant functions to end up with the same result.