Not Sensing versus No Sensor

I'm feeling my way along here so bear with me. Would it be possible to test it sometime before it becomes inaccessible? Turck, has a small battery powered tester which could operate the sensor independent of the PLC.
 
I've had a problem like this in the past and what i did was create a startup screen on the HMI that forces the operators each morning to flag the photoeyes before the line can be started.

I created a HMI screen that had the locations of the photoeyes around 20 and before the machine could be started they all had to be flagged. Once they were flagged, the system could be started. It worked well and it also forced operators and maintenance to get familiar with the location of all the photoeyes.

This is an effective approach that I've seen before (and, in pharma).

You basically create a simple test sequence/protocol that exercises all pertinent sensors/actuators. Incorporate this protocol in a new or existing Standard Operating Procedure. Execute the protocol after maintenance is performed or at the start of each shift, if necessary.

The intent of the protocol is to demonstrate that the machine is ready to perform its intended function. This should flag the improper maintenance scenario that you cited, as well as others that you can not yet imagine.
 
Not sure this could be an option as you have said a new sensor is not an option. What I have done for removable apparatus that must be connected for opperation but also removable for maintenance is to use a 4 or 5 pin twist lock plug and receptacle or amphenol connector or any type of voltage current rated mutlipin connector. Then just jumped from the power pin to another pin to use for feedback. Now if the connectors are not needed the eye is not in place and you can adjust your logic accordingly. I hope this be an option as it worked very well for me in the past.
 

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