TimothyMoulder
Member
In my recent "whaddafuh" post, I got alot of good suggestions, and some criticism on shotgunning the repair. No arguement, I don't like shotgunning a repair any more than the next tech. And I'm running out of chickens...
Anywho, I thought I'd open a discussion on servo troubleshooting techniques - this is one area I wish I was stronger in. My difficulty in troubleshooting servos comes from not knowing which end of this serpent is the head, and which is the tail.
Let's say I have a following error fault on a previously working system. I find looking at the encoder on an o-scope to be somewhat less than helpful, because
1. It could be the mechanical load. Changes in the load mechanism can render the original tuning incapable of operating the drive successfully.
2. It could be the motor/encoder. I can take off the motor, disable it and turn it by hand to verify the pulses are there - but are they correct? To get an accurate count, I have to move a fixed distance and compare expected vs real counts. And I can't do that because I'm failing for following error at every attempt.
3. It could be the drive. It may be getting the encoder pulses, but not seeing them. Far, far worse, it may be getting the pulses, but not seeing ALL of them. Or maybe I'm not putting out all the pulses in the first place(see #2).
So to troubleshoot at all, I gotta start somewhere. I go mechanical, and have the maintenance guys certify that the mechanism is okay - 4 out of 10 cases, the thing needs greased, another 2 in 10 it has something loose or misadjusted.
I used to go after the motor; it's the easiest change, and you get a fresh motor and new encoder in the deal. However, it's almost never the motor, only a few times in several years. So these days, I usually go after the drive instead. About 2 in 10, the drive corrects it - these things are pretty fragile.
After that, last resort, I start with the tuning and limits. I can usually get it up that way if all else fails. On those occasions I've not been able to, something else has been at fault - the motor or drive on the shelf was bad.
So that's my approach. Yeah, it's shotgunning, but that's the logic behind it. I'd like to hear how others tackle these things, particularly Peter and Ron.
How about it guys, which end do you grab first?
TM
Anywho, I thought I'd open a discussion on servo troubleshooting techniques - this is one area I wish I was stronger in. My difficulty in troubleshooting servos comes from not knowing which end of this serpent is the head, and which is the tail.
Let's say I have a following error fault on a previously working system. I find looking at the encoder on an o-scope to be somewhat less than helpful, because
1. It could be the mechanical load. Changes in the load mechanism can render the original tuning incapable of operating the drive successfully.
2. It could be the motor/encoder. I can take off the motor, disable it and turn it by hand to verify the pulses are there - but are they correct? To get an accurate count, I have to move a fixed distance and compare expected vs real counts. And I can't do that because I'm failing for following error at every attempt.
3. It could be the drive. It may be getting the encoder pulses, but not seeing them. Far, far worse, it may be getting the pulses, but not seeing ALL of them. Or maybe I'm not putting out all the pulses in the first place(see #2).
So to troubleshoot at all, I gotta start somewhere. I go mechanical, and have the maintenance guys certify that the mechanism is okay - 4 out of 10 cases, the thing needs greased, another 2 in 10 it has something loose or misadjusted.
I used to go after the motor; it's the easiest change, and you get a fresh motor and new encoder in the deal. However, it's almost never the motor, only a few times in several years. So these days, I usually go after the drive instead. About 2 in 10, the drive corrects it - these things are pretty fragile.
After that, last resort, I start with the tuning and limits. I can usually get it up that way if all else fails. On those occasions I've not been able to, something else has been at fault - the motor or drive on the shelf was bad.
So that's my approach. Yeah, it's shotgunning, but that's the logic behind it. I'd like to hear how others tackle these things, particularly Peter and Ron.
How about it guys, which end do you grab first?
TM