Testing and Cause of Failure for Encoders and Hall Effect sensors

[ChrisH]

I have a question about the use of optical encoders and hall effect sensors in our robots. When they fail, how do you test to determine that they have failed and what was the failure mode?

For example, if we are using an encoder to control the position of our arm, and the arm stops behaving properly (not moving on command, not stopping on command, or oscillating when before it was holding still). How would we determine whether the failure was in software or in hardware? If it in hardware (like a fried encoder) how would we determine whether the source of failure was a mechanical overload or some electrical problem?

Please keep the replies simple. I'm just a mechanical, but when something breaks I'm the one who has to correct the problem and prevent recurrence. I hate to use stuff where I don't even know how to check whether or not it is working in the first place.

Do we need to add an oscilloscope to our pit equipment? If so how do I hook it up to get the answer I need? I know what to do for limit switches and pots, but encoders seem to take a little more than my trusty VOM.