Need Help!- Tachometer on top of resolver?

[Dustin L]

I have a hopefully simple question. I had a servo act up on my Amada punch press it is a fanuc dc 6l. It is not holding its retract (home) position at start up. We are constantly adjusting the parameter by quite a bit like every day. I know one of the brushes in what I am pretty sure is the tach was sticking and not holding tight to the commutator. I called Fanuc and asked if new brushes were available for my resolver and they had no idea what I was talking about. They said there is no brushes for the resolver, so after some google image searching I hope I am right at saying the end piece with the plastic ring and four brushes is the tach and the next part on the shaft is the resolver. The resolver is labeled I just assumed (i know i know) the tach was part of it. Anyways my real question is shouldn't the brushes being bad also cause problems during normal operation and not just the homing sequence.
Thanks for any input

[Al_The_Man]

A tach does not normally have any effect on position, especially just homing, on this machine you can have either a resolver or encoder, a resolver has the appearance of a small sealed motor approx. 1" in dia. with 6 leads attached.
I would suspect that being on a Punch where a great deal of vibration is present that the resolver coupler may be loose or the resolver changing position due to the vibration, or the motor coupler could cause it also.
I also assume we are talking about one axis only.
Al.

[Dustin L]

First off thanks for the reply Al, the part I am calling a resolver is closer to 2 inches in diameter and is directly on the motor shaft under the cap. I call it a resolver because it is labeled exactly that Resolver A290-0601-T803 3X I believe the newer number is A290-0651-V503 and priced around 1,800. If this were faulty at all wouldn't it be messing up during operation? Could the origin limit switch be the culprit? I also checked the possiblity of the resolver moving with a scribed line and it does not. I will however be going through and doing the same with all the couplers.
Thanks again

[Dustin L]

Also Al how come when I click on new posts I don't see mine in the updated list?


Nevermind I just used the Todays post and now I remember.

[Al_The_Man]

Originally Posted by Dustin L If this were faulty at all wouldn't it be messing up during operation? Could the origin limit switch be the culprit?

The home limit switch could be a problem, but does it increment a home position every time or does it move back and forth at different homing times?
I would more inclined to suspect the motor shaft moving.
Al.

[Dustin L]

The only time it really needs to origin is on startup after that it no longer needs to this servo runs the auto index which rotates the punch in the turret. When it screws up I adjust the parameter to correctly home it then it will work for quite some time but when it messes up again it stays consistent. Once it homes to an incorrect spot it will keep homing to that in correct spot until I adjust the parameter. Make sense?

[Al_The_Man]

One way is to make a scribe mark on the motor shaft itself, it may be easier on the rear end?
Then check to see if the mark has moved when it goes to an improper home posn.
If it goes to the same spot then it is something after the motor which has changed posn.
If it does not coincide and the error is the same every time, jumping between the same + & - value, then the home switch may be right on the edge of the home point, although this does not usually occur with resolvers, as much as encoders?
Al.