Hi my client has an existing fruit washer whereby a series of polaroid photo reflective sensors monitor different stages of the conveyor . Where they should be reading the plastic crates they are sometimes reading the water spraying overhead and giving a false signal to the plc . Sensors used are Polarised Retrorelfective QS18VPSLP Banner PNP . 3 5 metre sensing range . Light and Dark Operation with a reflector used for the beam to bounce back onto the sensor .Ive read up a bit on laser type sensors with background suppression . I wonder if this would help eliminate the issue with false readings of the water spraying and mist . If tuned or reached to be more accurate over the plastic crate that's passing .
Any feedback would be greatly appreciated

 

Judging from the photo of the sensor (I presume it's the/one of the sensor(s) in use on the machine) it appears to be not very new, and the screw heads look pretty worn. Either the sensor come loose a lot or it's been moved (possible vertically?) to try to improve the operation.

Which brings us to the big question: Is this a new problem (meaning the system worked well for a while and has started misbehaving), or, has this problem existed since the commissioning of the machine (which was how long ago?)?

If it's a new problem, then what has changed? Has the water spray changed (increased, or decreased)? Has the spray pattern changes? Have the spray nozzle orifices become plugged/choked over time and need cleaning/replacement? Is this an intermittent/failing PLC input? Have all of the sensor wiring terminations (crimp terminals, if any, as well as terminal boards been checked to see if they are all properly terminated and terminal board screws are properly tightened? Are you 100% certain the sensor is in good, working order/condition?

If it's been a continual problem since commissioning it would seem to be an improperly chosen sensor for this particular application, or improper location/mounting, or possibly even poor wiring terminations/connections along the loop for this sensor. Has anyone contacted a vendor or supplier of similar switches for assistance with determining if there's a better solution? (Often times manufacturers' representatives can be very helpful (if he/she is experienced with the application or similar applications. Good manufacturers' representatives will contact the companies they represent for application assistance, also. Or, sometimes directly contacting the sensor manufacturer can also be very helpful.

We don't know what's been done to try to resolve this problem, and what the results were. We don't know how long the problem has persisted, or when it began. There's just a lot we don't know. But I often find in these kinds of situations the easy checks/verifications get overlooked (terminations; terminal board screws; wiring condition; etc.) and a lot of time gets spent doing a semi-serious "deep dive" into all manner of unlikely, though possible, causes without really analyzing the situation to determine what all the possible causes might be and ranking them in order of ease of checking/verification and likelihood of being the cause of the problem.

I once went to a quarry that had a lot of false trips of the equipment in the past couple of months because of a large induction motor's embedded RTDs sending erroneous signals to the motor protection relay. When I arrived I found one of the cables connecting one of the RTDs to be new, and it used a different wire color scheme than the others. The local mechanic (not an electrician) had run the new cable (not even a shielded control signal cable) and just connected the wires in the order he thought was correct--and that's what he kept telling his supervisors ("It's connected correctly--I checked it! It's that damn motor that's bad!") who were endlessly whinging about the electric motor being defective, or the protective relay. A few quick checks with a multimeter and the wiring was corrected in about 1 hour 40 minutes and the machine was started, and as far as I know, is still running today. (And the plant manager didn't want to sign my time-sheet because he was still claiming it was a warranty issue (on an eight year-old electric motor) and because it took so little time to fix that it must have been something else that I was "hiding" from the mechanic and plant personnel. He eventually signed my time sheet--very grudgingly.)

The point of that story is that something had changed--a portion of the interconnecting cable (which should have been in hard conduit but wasn't) had gotten damaged a couple of months ago, and an untrained person ran a new cable and didn't know how to verify the connections from the device to the protective relay, and kept blaming the motor or the protective relay as the problem (both were fine; a quick PI verified the motor was okay, and the protective relay diagnostics said the input channel was working correctly as verified with a potentiometer and a short piece of wire). But, no one stopped to think what happened just prior to the problem's start--which was the key to the whole solution.

NOTICE: I have no experience with this type of sensor; just a lot of application and troubleshooting experience on many other types of sensors and machines. Hopefully the questions will prompt a step back and a better understanding of when the problem began and what the possible solutions might be.

Best of luck, and please let us know what you discovered and how you resolved the problem!