M542 driver - motor vibration at low speeds

**birchy** · 19-11-2013

OK...so I've tried the motor on a different drive - no joy. Then tried a different motor on same drive and still have the problem. I only have 3 motors, all bought at the same time. Will try the 3rd motor tonight but am not holding much hope...

I've played around with various settings (acceleration, max velocity, etc) but had no joy there, either. I then tried changing the gear ratio in software to 6:1 and had no problems as the motor runs faster even at low feed rates. The question is - should the motor be smooth throughout the range or will there be inherent "bad" spots due to the Chinese drives and motors? I'm convinced this is a stepping issue that causes resonance but only at low speeds. If I increase the step rate to 3200, the drive sounds much "cleaner" but I get a "realtime" error in LinuxCNC. What step rate is "normal" for this hardware?

**Jonathan** · 19-11-2013

Slides 22 onwards in this document give a good explanation of how resonance affects machine frames. If you read that then what is happening here should be obvious. The other lecture notes in that series are well worth reading too.

Originally Posted by birchy

The question is - should the motor be smooth throughout the range or will there be inherent "bad" spots due to the Chinese drives and motors?

You can't blame this on the Chinese (or in John S's case linux), since the Chinese also sell drivers with resonance damping functions (e.g. AM882). That doesn't mean you need to change drivers though. I'd try testing cutting something at the speeds where it sounds bad and see if it affects the finish noticeably. Also, thoroughly test the motor tuning, e.g. by running random G-code. If it doesn't stall, goes fast enough and gets an acceptable finish then you can safely ignore the 'problem'.

At what step frequency do you start getting the real time error? Unless you're very unlucky with the computer it should be fine around 25kHz - quite likely more.