Z axis servo "giving up" on position...help?

Any help is appreciated!

I have a Lagun 3 axis CNC mill that I've retrofitted with a Mesa 5i25 and 2x 7i29 motor controllers. Also a 7i37 and SPINX1 but I don't think those are related to this problem.

About a year ago I did my PID tuning and haven't had any real problems with it. Right now my values on Z is P 7000 I 12 D 24 deadband .0001 no feed forward. Max output 95 output scale 100 encoder scale 20000. Today while drilling a few holes it would seem to give up on getting to the commanded position, to the point where a 3/8" deep hole was only drilled about 1/8" before the drill retracted and moved on to the next operation.

Now what seems weird about this axis compared to others is that with no obstruction it will go to any commanded position. However, if I pull the belt guard and manually move the pulleys it will resist my movement, but if I push it past about 5* rotation the motor will stay wherever its pushed to, it will not attempt to move back to the position it was initially at. On the AXIS interface the movement is registered - sorta. It does register movement but only a few thousandths which is why I suspect that its not trying to move back to the original position, it certainly isn't registering a movement of 1/8" or more. On my X/Y axis if I rotate against the servo it will spring back, almost violently to the position it started at and will register all movement past the position it was at until released and it goes back to that position.

I know that my encoder is functioning because it does accurately move to commanded positions, and shaft movement does register on the 7i29's encoder LED's.

Help!

Sounds like an encoder noise issue

Are you using a differential output encoder?

if so, is the 7I29 jumpered for differential inputs?

Is the encoder filter bit turned on? (it should be)

Is the Z axis encoder 5V OK at the encoder?

If you are running master or some pre-release version of 2.6 you can
easily change the encoder digital filter time constant

andypugh wrote:

greenbuggy wrote:

However, if I pull the belt guard and manually move the pulleys it will resist my movement, but if I push it past about 5* rotation the motor will stay wherever its pushed to, it will not attempt to move back to the position it was initially at.

Is there any chance that this is actually a pulley slipping on the shaft?

It was. I feel like a big idiot, next time I'll use loctite. I checked for slippage on the ballscrew shaft but not the servo shaft, and after tearing the caps off the servo to double check all connections and wires I realized my error. Must be some of the chattery work I've been doing lately - I've never had a compression style lock pulley ever slip on a shaft before. I'm tempted to get one that has a keyway in the inside of the pulley swage to make *extra* sure it never happens again.