whaddafuh?

A dummy's explanation

I'm pretty limited with encoder troubleshooting. We have them all over the place here, but I've never seen a problem with one. Can someone elaborate exactly how you would use the oscilloscope to troubleshoot this? Particularly, what leads out of the encoder would you check, and shouldn't the encoder be turned to watch the pulse? I guess my main question is, which leads do you look at, and what do you expect to see when the encoder is not moving, further, if you are looking at a steady signal, why will a good VOM not give you the same information? Thanks.

Russ
 
Depending on how much functionality you have from the device, you can check:

A & B - Pulses present, phasing (when going one way, A pulse starts before B Pulse, going the other, B before A)

Z (index) pulse - occurrs once per revolution. May be used - may not be used. Depends on set up.

voltages at correct levels, square waves, clean waveforms (low noise).

All of this is only relevant if you can get the thing to move.

I recently learned about a command available on some controllers, in Trio it's the DAC command. It allows you to output a voltage to the drive's command signal, irrespective of the feedback. This is a spiffy way of testing the above, since it is immune to errors relating to the encoder which stop normal positioning cold.

TM
 
TimothyMoulder said:
Depending on how much functionality you have from the device, you can check:

A & B - Pulses present, phasing (when going one way, A pulse starts before B Pulse, going the other, B before A)

Z (index) pulse - occurrs once per revolution. May be used - may not be used. Depends on set up.

voltages at correct levels, square waves, clean waveforms (low noise).

All of this is only relevant if you can get the thing to move.
Click to expand...
I often disconnect things and test the devices separately. In a recent tech support call our controller was blamed because a valve didn't work. The mechanical guys would not even trust the open loop or DAC command you suggest below. Fortunately the electricians had a mulimeter that was also a current source. They could connect this directly to the valve without the motion controller and test the valve. The multimeter could also check the output of the motion controller when different open loop or DAC commands are issued.

TimothyMoulder said:
I recently learned about a command available on some controllers, in Trio it's the DAC command. It allows you to output a voltage to the drive's command signal, irrespective of the feedback. This is a spiffy way of testing the above, since it is immune to errors relating to the encoder which stop normal positioning cold.
TM
Click to expand...
Yes, in fact we have two forms of DAC or open loop commands. One that cares about errors and another that doesn't. The one that doesn't is used for diagnostic work. The version that cares about error is used in operational mode. Believe it or not there is still a lot of open loop control out there.

Another thing we have is graphs that can plot what is happening like a AB trend but ours is faster with higher resolution. We also have an event or error log that can record events or error as the millisecond level.

TM, You have too many threads going.
 
At the last mill I worked in the first thing I did was get a scope. Others couldn't understand my insistance on having a good O-scope. :(

I teach my students how to use them in a troubleshooting class.

I remember the holes for safety wires too. Uncle does do a few things right.
 
russrmartin said:
... if you are looking at a steady signal, why will a good VOM not give you the same information? Thanks.
Russ
Click to expand...
The main issue here is intermittent connections. Sensors are running at 24DC or 120AC, with a pretty broad threshold. A quick dropout of a few milliseconds might go unnoticed.
Encoder signals are lower voltage and the slightest intermittent will show up as a missed pulse, so poor connections in connectors will cause problems here first.

What's the best way to diagnose an intermittent, and prove you fixed it? First, you need to 'see' it. Wiggling a cable with a bad connection will produce 'glitches'. An osciliscope is the best tool to see a glitch. A meter with a Bargraph option will also show it.
A digital voltmeter uses an A/D converter. The processing takes time. It's hard to see a glitch with a digital. More expensive voltmeters have a Bargraph to overcome this limitation. There's no need for accuracy with a bargraph, so a fast A/D circuit is used for this part of the display.

One poster said his problem occurred when the cable was put back into the cable tray. Extra signals were coupled into the cable. A scope would clearly show the extra signal riding on the main one.
Todays scopes have a storage feature. You can save a trace to floppy or USB. Read and store your signals when they're good, so you can compare when a problem is suspected.

There's a large variety of Encoders, and I'm not familiar with what's typical. I can't tell you which pins to connect to.
This part isn't that critical anyway. Trying to look at phase or timing isn't necessary. The controller will tell you hard faults and phase errors.
The point of this discussion is to use a scope to make sure the quality of the signals aren't marginal.
 
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