Need Help!- Diagnosing servo problem

[Jared]

We have an older (1990) Mazak Laser that has had some geometry issues for the past couple years and might be getting worse. The problem only occurs when the the laser path changes directions. It makes the corners and even round holes of the parts look pretty bad. The best way I can describe the geometry is a wavy line that starts after it turns a corner and then fades out within a distance of about .250" to .500". At first we thought it was a ballscrew backlash but upon running a test we eliminated the ballscrew as the problem. Recently, I took the cover off the X axis servo and noticed the ballscrew "bouncing" back and forth when it changed directions. This lead us to believe that perhaps the servo motor was worn out. We also had a problem about two years ago that caused us to loose all the parameters in the control and we had to reinput them. Some people around here think that perhaps some of the parameters are not correct or need some adjustment. The link below is to a video clip that I took of the ballscrew's "bouncing" movement when changing directions.

I would greatly appreciate any help I can get on resolving this issue.


http://www.amfminc.com/VID00003.AVI

[H.O]

Hi,
It sure looks like your PID gains are way to low. That motor looks fairly large so it should feel very "stiff", almost like it's locked in place if you try to move it by hand.

If it feels "springy", like it looks in the video you need to tune the PID parameters on the servo drives.

[Jared]

H.O,

Thanks for the reply. I'm really glad to hear that it might be as easy as adjusting a few parameters. I guess my problem now would just be knowing exactly which parameters to adjust. I've attached a pdf of my parameter list if you wouldn't mind taking a quick look.
Again, thanks so much.

[H.O]

Hi,
Well, parameters 1036 & 1037 and maby 1040 & 1041 would be the only ones in that list that looks right to me. Take a look at those, try changing them a little and see if it makes any difference.

Aren't there any trim-pots etc on the servo-amps? Or is it all just one integrated unit? What I'd look for is trim-pots or parameters called P, I and D or Gain and Damping or something like that.

/Henrik.