No Output from Rotary Encoder

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I have just purchased a rotary encoder but I have had no luck getting any output.

Wiring…
Red is 24vdc.
Black 0v
Green A
White B

It provides elevator position feedback to a PLC (Wecon). The PLC has read previous encoders (looked identical to the one purchased, and they worked fine).

I have tried with and without resistors on the data output. I don’t have access to an oscilloscope currently.

Any suggestions on how to verify the encoder is working would be appreciated.

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Hi Rob,

Welcome to the forum!!

What did your circuit diagram look like? Do you know if the previous encoders had inbuilt pull-up resistors?

Liam.

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Hi Rob.
Apparently the output of the encoder is NPN open collector on O/P A and B so to test to you will need about 10k pull up resistors. You will also need to power the encoder. Pull ups connected to 24V should be OK but when connected to logic they should be connected to 5V or 3.3V depending on the flavour of the logic. If connected to Arduino it has built in pull ups so just activate them.

Anyway, back to testing. With everything connected (don’t forget to connect all grounds together if using separate supplies) connect a multimeter between ground and O/P A or B. Turn the encoder VERY SLOWLY (or you will miss the switching) and you should get a reading fluctuating between PSU value and Gnd. Repeat for other output. Don’t forget this device gives 400 PPR so you will have to turn VERY slowly. By the way, if you can find one an analog meter is better for this sort of thing as you don’t have to wait for the meter to do its thing.
Cheers Bob

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Hi Rob

A result re suggestions would be nice too.
Cheers Bob

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Hi Robert,

Sorry for the late reply. The encoder is installed on a funicular located at another location.

I was able to test this morning and I can get an output with the resistors in circuit. However, the PLC is still refusing to read the input, with or without the resistors.

So I need to dig deeper. Thanks for the heads up.

Rob

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Hi Rob. I was just wondering how you got on. I don’t suppose the sense is reversed is it. That is the PLC may be expecting a high when the encoder switches are on and according to the data sheet for that linked unit the output is NPN open collector so the pull ups are required and the output is Low when the switches are on. An oscilloscope would be nice here for diagnosis. Don’t suppose you have one on hand ??
Cheers Bob

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Hi Robert,

I’m only checking with a digital MM so it’s a bit challenging. I can read 24v pulses with just the pull up resistors only. When I connect to the plc I read lower voltage pulses (around 8-9) without the pull ups, and very little voltage (hardly registers) with the pull ups.

I will try reversing the inputs with and without oil ups next time I’m on-site (over the weekend). I also have another brand of encoder to try.

Would love access to an oscilloscope- that will be the next step.

Thanks for your help.

Rob

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Hi Rob
That doesn’t sound right. Are you sure all the wiring is OK and more importantly correct. I have used this sort of thing with PLCs on coil winding machines to count turns at 10 counts per rev. Not rotary encoders as such but steel discs with holed drilled in them and 2 magnetic sensors. Using this in an encoder configuration enabled direction to be sensed and counting in reverse (in the case of a miswind where turns have to be removed). These sensors were also open collector NPN and worked fine but I feel much slower that your 400 PPR unit.

That is not the problem is it ?? Are these encoders the same number of pulses per revolution (PPR) as the ones they are replacing. If not they may be too fast for the PLC. 400 PPR can be quite a high frequency with a shaft of any speed. It was a long time ago but I seemed to remember the encoder function of the PLC (Mitsubishi) was quite modest regarding the highest frequency it could handle.
Cheers Bob
PS: A digital multimeter will be practically useless with the encoder turning at anything much more than standstill as the meter is too slow to respond. Any type of meter will struggle. An oscilloscope is the ONLY instrument you will get any meaningful result from.

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Hi Bob,

The plc input is rated to 100khz. I have an oscilloscope on order so hopefully it will arrive soon.

Rob

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Hi Rob

That is fairly quick. I think the units we used were the Mitsubishi FX series, They had a little display screen and have been proven to be pretty robust. I think from memory the frequency limit for encoder use (2 inputs) was in the order of 2 or 20kHz . I think this was dependant on the number of inputs used for counting. Best with only 1. Our requirement was relatively slow as we were counting the actual coil former revolutions not a motor. Anyway they have been winding coils now for more than 15yrs. Had a couple of different models on automatic TIG welders also. Another device we used extensively for lighting control and automated testing set-ups was the Mitsubishi Alpha 2 series controllers. They also have proven reliable.
Cheers Bob

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Hi Bob,

I have been testing with a new 360pr encoder I just received and it is identical to the original.
AB Phases:

  • Directly connected to the PLC inputs = 0vdc
  • Connected to a 10k pull up resistor only = 24vdc
  • Connect to the resistor and PLC = 7.3vdc
    I can actually get the pulse to sit at its peak (if I am very careful with positioning) so the voltage reading is quite stable now.

The PLC is expecting 24vdc. I have tested the PLC inputs with 24vdc and it senses a static inpout (i.e. not configured as a counter).

Does any of that make sense? I cant see many more options. I will be reaching out the the PLC supplier to see if they can assist.

Thanks in advance
Rob

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Hi Rob.
Sounds like the PLC input has an impedance of about 5k which would become the lower leg of a voltage divider. That is ridiculously low for a PLC input. Are you sure you haven’t got a phantom resistor to ground somewhere??? Check everything around that PLC input circuitry. Check the PLC input DC resistance to ground with everything off.

The other thing to check is the 10k pull up resistors. Make sure they are 10k. If they happened to be 10MΩ the 10MΩ of your meter would be the lower leg of a divider and the maximum you would read is 12V even without the PLC connected.
Cheers Bob
EDIT: Check the pull ups anyway but I think they will be OK. You have already said you read 24V without the PLC connected so they would not be 10MΩ.

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Hi Bob,
Just a quick update… no luck with any of the suggested changes from the PLC manufacturer. The PLC has both hardware and software counters, so i setup to try all possibilities but no luck. I am sourcing another PLC to trial as I suspect its fried.

Note: the inout impedance on the software counters reads 5.38k, and the hardware counters read 38k. I suspect your onto somethng here. Will let you know how the new hardware works out.

regards

Rob

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Hi Rob.
Are you sure the output of this encoder is the same configuration as the original. Why are you fitting a new one ???. Suspecting the PLC is a bit drastic especially as you have already established that it behaves normally when you apply 24V to the inputs.

If you measure 5k it has to be driven from a low impedance source. Maybe the encoder you are replacing sourced 24V without pull ups in which case it would be low Z. The encoder you originally specified in your original post is NPN “open collector” which requires pull ups. The source impedance will effectively be the value of the pull ups (10k). You have since tried another encoder whose outputs are unknown to us outsiders. To work they have to be the same as the original or have to be made look the same.

There is a way to convert the source Z before you go rushing off changing the PLC. Get a couple or 1 dual rail to rail OP amps which are happy with 24V and connect as unity gain voltage followers. This will give you a high input Z and about 75Ω output source Z and the 5k won’t bother it.
Cheers Bob
Add on: PLCs are normally built for industrial use and are pretty rugged. If designed for 24V input I don’t see what you could have done to “fry” (isn’t that a great word, extensively used these days) it.

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Thanks Bob.

I have 2 different encoders - one is identical to the opriginal. Nothing works. Very odd.

I iwll see if I can source the voltager follower.

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You have got me confused. This doesn’t tell anyone exactly what sort of output they are and which you have been trying.
The “voltage follower” is not a device, it is the method of connecting an OP amp for high Z input and low Z output and unity (1) gain.
Cheers Bob

Maybe it is the PLC but you originally stated that it was OK with 24V applied.

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Yes binary inputs seem ok.

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Hi Rob
Just had a look at some of your PLC models. You don’t say what model you have so I just picked one from the middle of the group and had a look at the handbook.

It appears that the 5k is about correct as the input is an opto isolator with a series resistor of 4k7 plus an unknown value in parallel with the opto LEDs. With his type of input the encoder (or any other input) has to supply (or be the source) 24V at sufficient current to drive the opto LEDs. Or in other words the raw 24V which you have said works.

In this case the open collector outputs of your

will not work. The outputs have to be the source of 24V.

If you want to use this encoder you will need the OP amps connected as a unity gain voltage follower.Need to be “rail to rail” and be happy with 24V. Pretty simple. Connect 24V to supply pins. Connect output to inverting input. Connect output to PLC input. Connect encoder output (NPN OPEN COLLECTOR TYPE) to non inverting input. Connect 10k pull ups from 24V to encoder output/non inverting input. That should give you a 24V source with enough grunt to easily drive PLC input opto isolators.
Cheers Bob

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Thanks Bob

Is there another encoder you are aware of that would work out of the box?

Rob

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Hi Rob
I can’t and won’t start recommending anything without being able to make sure that everything else is OK. This I can’t do remotely

For a start, why are you replacing the encoder ?? Have you made sure this is the culprit.
Why are you experimenting and not just replacing with one with the same output.
The situation where you tried a 400PPR unit then a 360PPR unit makes me suspect that you are experimenting somewhat. What is the PPR of the original unit ??? If it is going to work in an existing situation the PPR has to be the same.

In the absence of sufficient information my replies have mostly been guesswork. There are giant holes in the info provided like make and model of all the bits you are playing with and some sort of circuit showing what is going on would be extremely useful.
Cheers Bob

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