Issue with old DC servo

[HenkaJ]

Hello everyone,

I was wondering if any experts on this forum could help me with an issue I'm having with my early 2000's Körner micromachining center. This particular machine was manufactured in Germany around the year 2001 by MASCHINENFABRIK KÖRNER GMBH that has since stopped operations, so support from that side is pretty hard to come by.

The issue I'm having has to do with the Z-axis servo motor and its tendency to oscillate or hum at a very high frequency at certain points of its travel. The humming only happens when the Z-axis is idle and in the right spot along with the servo's rotation. I've tested the axis with an indicator attached to the axis, and you can creep up on these areas with the jog function, moving at 0.01mm increments. When the servo is at this spot you can see the indicator move about 5 microns in both directions very rapidly. It stays there oscillating between the positions and can only be stopped by jogging the axis.

Here is some information about the servo motor:

Make: Dunkermotoren
Model: GR80x80 brushed DC servo 224 watt
Encoder: 1000 ppr encoder with two channels (4000 total pulses)
Brake: Yup

The servo drives are digital, manufactured by the same company that made the machine. I've attached a picture of the Z-axis PID settings that can be adjusted inside the software.

PID settings

 

[GENERALDISARRAY]

being a micro-machine, does it have scales also?

 

[Vancbiker]

IME with digital servo drives from Fanuc, Mitsu, etc., a servo system that used to work well, but starts to hunt, tuning is not the issue. Playing with tuning may get it to stop, but the trouble has usually been that mechanically, something has changed. The resulting loss of damping as the mechanical assembly wears causes the system to lose stability. If the mechanical condition is still acceptable then tuning may be your answer. First thing to look at is to ensure that there are no mechanical defects at the root of the trouble.

 

[DDoug]

Op says 'Brushed DC"....did you look at the brushes ?
Clean out the armature with compressed air ?

 

[BugRobotics]

Are problem areas repeating at the same distance apart, more than once, along the Z axis travel?

• Would be good to clean the brushes if you haven't done so already.
• Check the wear in the Z axis by moving to the problem areas and moving the table around checking with an indicator and comparing to other areas that aren't exhibiting the issue.
• Check lubrication (manually add lubrication around the problem areas).
• Check any feedback (scales and/or encoders) with an oscilloscope in the problem areas.
• Inspect all linear rail mounting screws and ball carriages (assuming this is on rails and not ways)
• Inspect belts and pulleys as well as servo mounts

I have a much older (1991) DC servo system and had to replace a lot of capacitors on a number of the servo control PCBs. Many of the caps were very leaky, all were out of spec and a few were dead shorts. I wouldn't rule that out but if this is only happening in a few areas I'd be checking mechanical stuff as Vancbiker mentioned.

 

[HenkaJ]

Thank you everyone for your suggestions.

The problem was an improper torque setting on the bolts that hold the servo. I was going to pull off the Z-axis servo to see if I could replicate the oscillation when the servo spins freely, and upon loosening the first bolt, I noticed it was set to very low torque. I checked the proper torque setting and tightened the bolts, and the oscillation was gone.

I'll still have to run some milling tests to see if there's an improvement on surface finish, but now Z-axis sits completely still when idle so I'm hoping there's an improvement.

I'll post an update when I've done some milling on the machine soon.