3 wire Inductive Prox troubleshooting information

[arpus4KM]

So spent a morning at a customer and ran into an issue that had me a little baffled, and at this point my assumption (without pulling out an extra bit of test gear like an oscilloscope) is that the 24vdc powering their prox is combining with other issues like induced voltage or insulation leak to cause part of their problem.


anyway. the sensor is a balluff BES 326 series pnp 3 wire. there are two sensors (not identical, don't know what the other is either, too deep) and they share 24v and 0v wirenutted together.

then the offending sensor will stay illuminated and show anywhere from 1-2.4vdc on the signal out wire (black/N.O.) and fluctate with other equipment action. The actual sensor itself and wiring being held outside the cabinet is not running next to any other source of power from the wire nuts to the sensor held in hand. but yet the sensor stays lit even without an object in front of it..... Triggering the prox by putting it in front of metal gives us a 23v output and the light stays brightly lit of course.



The questions remains, how would a 3-wire prox sensor light up and show output voltage without anything connected to its output wire, just power to the sensor and thats it. they also attempted to try a couple different sensors, all balluff, that react the same as well.






The final fix for them was to just put an interposing relay in between and eat the little bit of voltage in the line through the coil so it won't keep specific drive inputs on. But this doesn't help explain the weird electrical event going on.

 

[NetNathan]

Is this on an input to the PLC? On certain plc modules the inputs and outputs will "bleed" voltage..... Easiest way of explaining.
May need a relay for the prox.

 

[arpus4KM]

Is this on an input to the PLC? On certain plc modules the inputs and outputs will "bleed" voltage..... Easiest way of explaining.
May need a relay for the prox.

yes, so they have it running to an input and then jumped from the input down to the drive input as well.

The odd thing is that this has two sensors, for two drives, next to each other. They land on different cards and do the same thing. Input and jumpered to drive input, but only one has an issue.

I was basically attributing it to a possible cable issue, but we didn't have the ability to stop the line and swap over prox output wire controls and verify that the problem followed it.

if it was an input card issue, I would think that it would happen on both drives, not just one. The input card was replaced with new by them before i got there too.




anyway, I would think, regardless of plc input or drive input... if all that is connected is the 24Vdc and 0vdc and the output of the sensor is left disconnected, there should be no led lighting with nothing in front of it. that is the thing throwing me off. never seen that unless it's literally a pulsing 24vdc that is screwing with the internal electronics because it isn't a clean 24vdc.

 

[parky]

Not that this is the problem in your case but most sensors will switch i.e. light on without the output connected. generally the output will switch without a load with the exception of a two wire proximity as this requires a load to ground.
1.2v seems too low to switch a PLC input on generally it would require at least 10-12v for switching. I have seen something similar but this was power supply problems, a system was a build back i.e. sensor to PLC & a delay timer in the PLC, some engineers had replaced the sensor twice & the input card on the PLC, the problem was the timer in the PLC never timed out (never seemed to even start) so it was like the sensor was detecting, the input to the PLC was on but not starting the timer, they called me in, checked the 24v supply & it was 22.4V so should have been ok, however, I noticed the timer was starting (in monitor mode a few ms but then resetting, put a scope on the prox output it was almost half a sinewave, turned out to be the capacitors in the PSU had started to fail, so I suspect either noise (does not explain such a low voltage switching the input), defective proximity or possibly a failing PSU.

 

[arpus4KM]

Not that this is the problem in your case but most sensors will switch i.e. light on without the output connected. generally the output will switch without a load with the exception of a two wire proximity as this requires a load to ground.
1.2v seems too low to switch a PLC input on generally it would require at least 10-12v for switching. I have seen something similar but this was power supply problems, a system was a build back i.e. sensor to PLC & a delay timer in the PLC, some engineers had replaced the sensor twice & the input card on the PLC, the problem was the timer in the PLC never timed out (never seemed to even start) so it was like the sensor was detecting, the input to the PLC was on but not starting the timer, they called me in, checked the 24v supply & it was 22.4V so should have been ok, however, I noticed the timer was starting (in monitor mode a few ms but then resetting, put a scope on the prox output it was almost half a sinewave, turned out to be the capacitors in the PSU had started to fail, so I suspect either noise (does not explain such a low voltage switching the input), defective proximity or possibly a failing PSU.

its not an issue switching on, with or without load connected. I can measure a good 23v output signal when triggering the sensor.

and the PLC is not the issue on this, the plc inputs works fine, it sees a low enough drop in voltage to allow a change of state.

the issue comes from an SEW drive taking an input for a home signal. the 2.4vdc is still high enough to prevent it from seeing a change of state, it's basically latching itself into a high state and not allowing a homing process to complete unless it goes to 0v.



balluff support has only pointed towards a very dirty power supply that could cause all kinds of weird issues..... I may have to point them towards that, just wish I knew exactly what the issue was and why it was happening with the sensor.

 

[johnd_125]

I've seen terminal block style em relays on PLC outputs light dimly even when the PLC output was not on. The relay contacts did not change state with the dim light. It was due to the relay NOT having a built in suppressor. Getting the same relay, but with the suppression built in, made the light stay off when it was supposed to.

Does the light on the switch behave correctly with a relay wired to its signal output? If so there is a possibility your switch needs to see a minimum load and you don't have that with just the PLC input. You might try wiring a small resistor in parallel with the PLC input. PLC inputs draw very low current and may not be enough to pull the output voltage down to near zero.

 

[arpus4KM]

I've seen terminal block style em relays on PLC outputs light dimly even when the PLC output was not on. The relay contacts did not change state with the dim light. It was due to the relay NOT having a built in suppressor. Getting the same relay, but with the suppression built in, made the light stay off when it was supposed to.

Does the light on the switch behave correctly with a relay wired to its signal output? If so there is a possibility your switch needs to see a minimum load and you don't have that with just the PLC input. You might try wiring a small resistor in parallel with the PLC input. PLC inputs draw very low current and may not be enough to pull the output voltage down to near zero.

you mean like a Triac output?

I've asked the questions about loading it down with a resistor, or possibly using surge suppression, and the response I got was that it was entirely unnecessary. This same sensor can be used on a TB3-CP80 tester and it functions normally, no issues whether the output wire is connected or not.

 

[Lare]

you mean like a Triac output?

I've asked the questions about loading it down with a resistor, or possibly using surge suppression, and the response I got was that it was entirely unnecessary. This same sensor can be used on a TB3-CP80 tester and it functions normally, no issues whether the output wire is connected or not.

PLC inputs take only uA amounts of current and sometimes leak current can be issue. Try with resistor as suggested or if change is slow enought then add relay between PLC and prox.

 

[OkiePC]

It sort of sounds like the SEW drive DI is either damaged or made to be extremely sensitive so that it can detect an extremely short pulse (for example an encoder z signal.)

You said the other system behaves normally...is it also wired to the same type of input on the same model VFD?

 

[arpus4KM]

It sort of sounds like the SEW drive DI is either damaged or made to be extremely sensitive so that it can detect an extremely short pulse (for example an encoder z signal.)

You said the other system behaves normally...is it also wired to the same type of input on the same model VFD?

correct.



the unfortunate thing is I can see the other sensor (in the middle of a conveyor line), I can see it is a different sensor, but I can't see what it is for sure.


both drives are SEW. If I get called back, I'm hoping they do it when no production is running so I can actually take things apart instead of measuring and guessing. Go figure, they call when production runs full bore and line stoppages just cost too much money for something they can bypass twice a shift.

PLC inputs take only uA amounts of current and sometimes leak current can be issue. Try with resistor as suggested or if change is slow enought then add relay between PLC and prox.

yep, that was the typical fix i've always done when theres any induced voltage causing inputs to flicker and rewiring isn't an option. Same thing they implemented before I got there, so good on em' for getting that rolling.


and yes, I had thought of a resistor, but the fix was already in place. and the thing that I'm hoping to figure out is why it was working before, and it's working on an identical system. but this one suddenly stopped working like normal.


Hence why I'm leaning towards a power supply issue causing this specific brand/type of sensor to go whacky.

 

[parky]

I agree with the PSU problem, unfortunately, I accidently posted before I finished, what fooled the engineers on the conveyor I posted about was that the other sensors were fed off the same PSU, the answer was quite simple in this case, the sensor that was switching (100Hz) was only about 6 ft from the panel the others were over 20-30 ft away, the PSU according to the scope had a ripple causing it to drop from the peaks of 24v down to around 10v, this caused the switching, because the other sensors had longer leads, the capacitance of these was enough to smooth out some of the ripples, I measured other PLC inputs from the other sensors & the ripple was a lot lower i.e. seemed to be over 15 v an interesting scenario was that one particular sensor when fitted, reduced the switching on that build back but still caused a problem so along with a failing PSU, length of cables & slight variations in spec (although same sensor type) created a situation that fooled the engineers, my point is some faults might on the surface not what you would expect.