Testing encoders? When/Why do they fail?

[fergusda]

I seem to be having a string of bad luck with encoders on surplus servo motors, and want to ask for some advice to make sure I am not doing something wrong. I am a novice, and have no business doing what I am doing, but we all have to start somewherer, right?

When I read the voltage at A or B channel on my working servo motor, the voltage varies from 0-5V as I rotate the shaft (as I would expect). I have 4 encoders that read a steady 2.5V on A, A-,B,or B- through the shaft rotation, and don't seem work properly. These are all made by DRC, and function smoothly, with all electrical connections good.

So, my questions:
1: Does a steady voltage through encoder rotation indicate a faulty encoder?
2: Do encoders fail often in this manner? Why?
3: Are encoders particularly sensitive to static electricity, poor wiring, greasy fingers, being sat on, used like a hacky-sack, etc?
4: Can they be repaired, or should they be replaced?
5: Does anyone else have really bad luck with surplus motors/encoders off of E-Bay?

Thanks all, for your insight!
-Daniel Ferguson

[Al_The_Man]

Originally Posted by fergusda I have 4 encoders that read a steady 2.5V on A, A-,B,or B- through the shaft rotation, and don't seem work properly. These are all made by DRC, and function smoothly, with all electrical connections good.

Most likely the encoders are open collector, this would mean they require a pull up resistor to measure anything meaning-full.
I have purchased both encoders and encoder/motor combinations off ebay, I have not had all that luck with used encoders, probably for obvious reasons, many go surplus because they are giving problems, and due to the fact they often have to be sent out to be repaired, they are often just replaced, the old ones end up on ebay.
The other type of encoder you may run into is the type that has the opto-isolator output instead of squaring up electronics, these are harder to test.
I only buy new off ebay now, motors are a different story, I have not yet had one with a bad encoder.
When an encoder fails, if electronic, probably not worth getting it repaired, if due to glass contamination, then you can some times get back in business by using alcohol and non-lint tissue.
If I buy CNC electronic equipment off ebay, I take it as a good sign if the connecting wiring was cut at the terminal strip with the ends still on.
It usually means that the part was removed from a working machine, rather that taken out for replacement.
Al.

[Mariss Freimanis]

1) Yes
2) Yes when it's miswired
3) No
4) Usually not repairable if the LED/phototransistors are damaged
5) I've heard of it happening before

My guess is a blown encoder is the reason the motor is taken out of service and placed on ebay.

Mariss

[daedalus]

i use heds encoders a lot, and the problems we have tend to fall into one of two categories:

1) failed led/phototransistor, usually constant output
2) worn encoder disc due to axial play in motor crashing the disc into the encoder casing. Usually can be seen by inspecting the disc under a magnifying glass, and starts out as an intermittent failiure.

if your encoder isnt the correct type for your environment (ip rated for use on a mill), then you may end up with grease / dirt getting into the encoder. I have fixed intermittent encoders with spray air / alcahol swabs under these conditions, but ideally you should use the right enclosure for the job.

[Al_The_Man]

Originally Posted by Mariss Freimanis 1) Yes Mariss

What about the constant or obscure voltage obtained when reading an open collector with no pull up?
Al.

[Mariss Freimanis]

The symptom listed was 2.5V on the output where 0/5V should be.

1) A failed open-collector should result in 0V out because silicon nearly always fails shorted.

2) A phototransistor outputs a quasi-sinewave analog output signal. This signal is normally "squared-up" with a comparator using positive feedback to provide hysterisis. The comparator output is constrained to only two states, "on" or "off". Its output is then buffered by a line driver, making the output 0/5V regardless.

3) A 2.5V output where 0/5V should be tells me the driver is a totem-pole type (active pull-up/pull-down) and it has packed it in. Most likely both output transistors are "on", giving a voltage stuck between 0/5V.

Mariss

[Al_The_Man]

The point I was making is that with a working encoder that has an open collector output and if testing it with no pull up resistor and using a very high impedance voltmeter, as many modern ones are, you are likely to get various resultsin voltage readings, as I myself have experienced.
By connecting the pull up the correct 0/5 or 0/12 results, as the case may be.
The other type of encoder I was refering to, made by Heidenhain and others has no squaring up electronics, the output diagram shows opto output and is confirmed by readings.
Heidenhain make both types and can only be determined by part number.
Al.

[fergusda]

Thanks. everyone for your input. The problem seems more than likely due to failed electronics, and likely why they were surplused.

I spoke with U.S. Encoders which has an office a few minuted from my home, and will speak with their rep in person about replacing the guts after the holiday. He offered to test my encoders, but I expect it's best just to replace them.

Either way, I will quit pulling out my hair trying to make a bad part work and get on with my project.

-Thanks
-Daniel Ferguson