The is a project for an Endurance rig.

I need to switch a solenoid ON for 0.5 sec ( variable between 0.5 to 5 sec based on requirement ) and then switch it OFF for 0.5 sec. This constitutes one cycle and typically will need to run for about 5 Million cycles non stop.

The control system is mostly PC based and not with Real Time OS due to cost considerations. It runs on WIN7 OS and LabVIEW app.

In use for many years and all well except for a rare and odd failure where either due to a program lock up or failed Solid State Relay module ,the solenoid remains in ON condition for more than 5 seconds leading to serious implications like pump seizure, motor tripping etc. The component under endurance is lost.

I am hence looking for a fail safe mechanism to sense if the solenoid is ON for more than 5 sec and if YES, trip the supply and raise an alarm. Options I have thought :

1. Use a regular industrial On-Delay timer and route the solenoid 24V dc power via the relay contacts and also power the timer itself from the incoming supply to the solenoid. But in this for every cycle the timer will power OFF and start again. Simple. But not very elegant ??

2. Have an Auxillary 24V dc power to power a MCU / Timer driving a relay. Route the solenoid supply through this Relay. Use the solenoid supply as trigger to reset a timing cycle and if the solenoid power remains ON for more than the programmed time, the MCU / Timer times out cutting the solenoid power and also raising an alarm. Only the Auxillary power cycling will reset the alarm.

Any other ideas to try ??