Servo tuning for an axis that behaves different in one direction versus the othe

Hi Guys,

I've re-controlled a Homag CNC router to use Linux CNC with MESA controls. It's was a complicated project, but for the last year it's been working well enough for me to be running commercial parts on it.

All of the motion hardware (mechanical / servos / drives) is what the Germans put in the machine 20 years ago. It uses Bosch ECODRIVE servo drives and has ball screws on x-axis and z-axis and a rack and pinion on y-axis.

The x-axis and z-axis work fine. The y-axis is "jumpy" when moving in the y+ direction. And smooth in the y- direction. I can clearly the position errors ramp up in halscope and then it appears to catch up and get back into position. With my hand on the gantry I can feel it jump / jump / jump going in the y+ and you can clearly see it in the tool marks. If I rapid at anywhere near full speed in y+ it will get wildly out of control and start slamming back and fourth and attempting to rip the whole 8,000 lb machine apart. But it rapids super smoothly in y-.

Mechanically the axis seems fine and there isn't anything like a dust collection hose that drags differently when going in a different direction. I've manually excessively lubricated the rack and pinion and it perhaps helps a little bit, but certainly doesn't solve the problem.

I recall probably spending about 10 hours attempting to tune the PID parameters and was never able to get it moving smoothly in y+. But I'm at the point now where I need to get it cutting smoother. So I am looking for any suggestions of things to try?

Thank you!

Do the motors have tachometers? A dirty tachometer commutator might explain the behaviour

I don't think so. It appears to just be a three phase brushless motor with integral encoder. Here is how the schematic shows the y-axis motor:
 

Looking up the part number, it is described as:
MHD041B-144-NP0-UN by Rexroth, Indramat, Bosch is a Synchronous Motor in the MHD Series. The MHD041B-144-NP0-UN features a Motor Frame Size of 82mm Flange Size, 50mm Centering Diameter and a Motor Frame Length of 2.7 Nm. This Motor has a Motor Encoder of Digital servo feedback and comes with a Output Shaft of Shaft with keyway according to DIN 6885-1 (with shaft sealing ring).

Thanks

 

I am not sure what I was thinking this morning, it is x-axis (the 120" long one) that is rack and pinion. y-axis and z-axis are ball screw and are working fine. (most of my work these days has been on MSY lathes and now a tube laser ... it seems every machine in my shop has different axis names ... and then I screw up the simplest axis named machined I own)

I am going to take tommylight's suggestion and give the rack and pinion a good cleaning. And look at if there is anything that could be lose / binding elsewhere in that axis.

Thanks