Is there a way to check whether a sensor has failed using the PLC?

[aqueousdan]

I have a section of rollers which have a sensor at the end. When product is moved onto the sensor then the rollers stop.

If this sensor happened to fail the product would fall and potentially cause damage.

Can anyone think of a way to monitor this sensor to check whether it is working or not.

The sensor is a simple 3 wire PNP sensor.

I could have another sensor afterwards but if the 24V was missing from both then the fault would still happen.

 

[janner_10]

You could make it a through beam and n/c so you always have a signal and the product breaks the signal.

Have 2 next to each other and if the 2 don't match (within whatever time span suits the process) then a problem is detected.

 

[Ronnie Sullivan]

You have to use positive feedback. If no signal the conveyor can't run.
The product would stop the signal and so would losing the sensor or 24V

 

[gbradley]

When the sensor is closed, start a timer with preset longer than your cycle.
If the timer times out, then the sensor is stuck on.

 

[widelto]

I have seen sensors with internal diagnostics, mainly with fieldbus like devicenet and ASI.
It´s probably that they also exist in other fieldbuses.

 

[JesperMP]

A simple "self-diagnostics" is to have a sensor with both N.O. and N.C. outputs.
Then monitor that only ON+OFF and OFF+ON are allowable.

 

[Pete.S.]

I have a section of rollers which have a sensor at the end. When product is moved onto the sensor then the rollers stop.

If this sensor happened to fail the product would fall and potentially cause damage.

Can anyone think of a way to monitor this sensor to check whether it is working or not.

The sensor is a simple 3 wire PNP sensor.

I could have another sensor afterwards but if the 24V was missing from both then the fault would still happen.

Is it a photo sensor? Actually it doesn't matter.

When damage can occur it is advisable to add an additional sensor of a different type, for instance a mechanical limit switch. They are very reliable, much more so than any electronic switch.

Add a limit switch with a roller or whatever works with the products. Place so it will get activated if the product goes too far.

Wire it electrically so it's normally closed and the rollers will stop electrically when the switch is activated. Wire the other contact pair to an input to the PLC.

This way it's electrically impossible and much safer than just wire it as an input to the PLC. If the input is activated you can use that as an alarm condition - because under normal operation the product will never go that far. So that indicates that something is wrong with the photo sensor (or whatever it was). You also use that signal to actually stop the rollers from PLC or put the machine in stop mode or whatever you can do to prevent it from restarting immediately if the product is removed from the limit switch.

If the rollers can go both forward and backwards wire the switch to only stop the rollers in the dangerous direction.

 

[mellis]

It's been said many different ways already. But here is my attempt to clarify.
Simply choose the switch and how it's wired so that it is failsafe. That is, the conveyor should stop if product is detected, if the sensor loses power, or the sensor is knocked out of alignment.

One easy way to achieve this with a photo eye is a through beam type with a transmitter/receiver pair wired so the PLC input is ON only when the light makes it from the transmitter to the receiver. Now when the sensor fails, you have to repair it to get the conveyor to run. There are still failure modes that this will not catch. A redundant sensor can drastically reduce the probability of running product off the end of the conveyor for these cases. I'd mount them so that both sensors are triggered at the same time. That way you can verify operation of both sensors.

I will take issue at the suggestion of adding a mechanical switch that only gets actuated if the primary switch fails. There are two problems, first the idea that a mechanical switch is more reliable than an electronic switch. While it is true that a high quality mechanical switch will outperform a shoddy electronic switch, in general the opposite is true. Electronic switches are rated for a much higher cycle life. The second problem is relying on a switch that does not get actuated unless there is a failure. This switch is not verified during normal operation. It could be completely destroyed and you wouldn't know it until the primary sensor fails and the product runs off the end of the conveyor.

 

[aqueousdan]

Many thanks for your replies guys.

Some of the answers don't apply to my situation which I should have been clearer about. We have stacks of boards moving down a conveyor which are at different lengths so a timer for example would be harder to implement.

I have it so that if there is no signal received then the conveyor wont run, however as someone else has said here, there are situations where that wouldn't be safe.

I have decided to use a safety relay and a small light curtain and take the PLC out of the equation. So far it is working well.

Thanks again!

 

[Pete.S.]

Many thanks for your replies guys.

Some of the answers don't apply to my situation which I should have been clearer about. We have stacks of boards moving down a conveyor which are at different lengths so a timer for example would be harder to implement.

I have it so that if there is no signal received then the conveyor wont run, however as someone else has said here, there are situations where that wouldn't be safe.

I have decided to use a safety relay and a small light curtain and take the PLC out of the equation. So far it is working well.

Thanks again!

If the safety of personal is involved in some way that is the right decision.
Can't trust a standard PLC with safety.