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Stepper Resonance
I have a 4-axis Clipper system. 2 axes are closed-loop stepper, and 2 axes are servo. I am having what appears to be a resonance issue with one of the stepper axes. It is a ballscrew axis, and most of the time it works great. It is tuned well, but occasionally, it will start a move and make a horrible sound, then fault on following error. The sound is like it is resonating badly, but the stepper is not stalling. The unusual part is that the move parameters never change and the load never changes. Any ideas as to what might be causing this?
Make sure your ball screw is properly lubricated. Make sure your motors are tuned (see this post for some guidelines) to produce a maximum following error that is within your specifications. Excessive Kp (Ixx30) or Kd (Ixx31) can sometimes cause buzzing in the motor. Furthermore, if you are hitting a natural frequency of your mechanical system, you can try to filter it out with a notch or low-pass filter. You can design these within the PMAC Tuning Pro2 software, which comes with PeWin32Pro2. Under "PID Interactive Tuning" you can select "Low Pass Filter Calculator" or "Notch Filter Calculator". You can enter the frequency which you want to filter out and then select "Calculate" and "Implement." One way to determine what frequency you need to filter out is by using the FFT feature of the tuning plot windows (this is the far right button on the top toolbar in any tuning plot window). You can try to program a move that causes your "horrible noise" and see if there is a peak in the FFT. You can look on the horizontal axis of the FFT result plot to determine the frequency [Hz] at which the peak occurred. This is the frequency you can enter into the filter calculator programs.

Would you be able to gather the position of your steppers when this resonance happens (since you mentioned closed loop stepper) and upload it?

Sina Sattari
Hardware Engineering Manager
Delta Tau Data Systems, Inc.
The problem I have with the suggestions is that the problem appears to be random. I cannot duplicate it. The machine is in production 24/7, and this problem happens 3-4 times a day. I could gather on each move, then upload only if the move gets aborted. The ballscrew has been lubricated, but that did not help. I am using IMS steppers and drives, if that helps.
We have had some success reducing this phenomena on 'sloppy' mechanics by using an exponential filter in the ECT. This works very well with low resolution steppers ( say 1/10 stepping). Vibrations introduced by the motor which happen interstep, but at a rate which the servo loop will try and 'pull in' can cause this effect, and a filter in the ECT seems to fix this.
Thanks bBrinj for letting us know.
Sina Sattari
Hardware Engineering Manager
Delta Tau Data Systems, Inc.

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