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Author Topic: Servo drift due to EM noise. Optical encoders not preventing it.  (Read 938 times)

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Servo drift due to EM noise. Optical encoders not preventing it.
« on: December 06, 2017, 07:49:54 PM »
I am a bit confused. On my mill I have servos that have optical encoders. The EM noise from my spindle has always been a problem, and I get servo drift on my Z axis. I thought that optical encoders should be talking to Mach3 to prevent drift. However when I observe the drift, via a micrometer mounted vertically in my vice, I can see the Z axis dropping, but the readout on Mach3 does not show this change. Am I wrong to assume the software should be sensing this through the optical encoders and preventing the drift? Am I missing a setting somewhere?
Re: Servo drift due to EM noise. Optical encoders not preventing it.
« Reply #1 on: December 06, 2017, 08:21:24 PM »
Hi,
Mach is an open loop controller, it issues pulses to your servo drive and that uses the optical encoder to close the loop.
Mach does not read the encoder directly neither does it 'adjust its pulses' to accommodate it.
It sound to me like noise is getting onto the Step and/or Dir lines to your servo drive.

Craig
My wife left with my best friend...
     and I miss him!
Re: Servo drift due to EM noise. Optical encoders not preventing it.
« Reply #2 on: December 06, 2017, 08:27:40 PM »
I see. I was under the impression it was the opposite. So, if I can't get rid of the EM noise, what would be the next solution? I tried swapping the servo cables and even wrapped them in foil to ground. It worked for a while, but it seems the longer im working the worse it gets. Fresh in the morning, I get no interference, but after about an hour it all goes pear shaped.
Re: Servo drift due to EM noise. Optical encoders not preventing it.
« Reply #3 on: December 06, 2017, 08:46:11 PM »
Hi,
best solution is to stop the spindle from generating the interference in the first place. What sort of spindle is it?
Where does its electrical supply come from? Is its supply separate from the supplies for the servos/PC/drives etc?

Craig
My wife left with my best friend...
     and I miss him!
Re: Servo drift due to EM noise. Optical encoders not preventing it.
« Reply #4 on: December 06, 2017, 09:01:42 PM »
It's a Seig X3 small mill. So China wiring at it's finest. Ive moved the servo controller box to the other side of the room and that seemed to work, but for some reason when it is cold out, the effects are stronger, and I can actually watch the drift happen. In the summer, the drift is so minor, the job does not get effected. I'm only doing engraving.
Re: Servo drift due to EM noise. Optical encoders not preventing it.
« Reply #5 on: December 06, 2017, 09:30:09 PM »
Hi,
two mechanisms for noise, conducted and radiated.

Conducted noise is often overlooked. It is where the 'hash' impressed on an electrical supply by a device, in your case a spindle,
which then gets conducted to other devices on the same power supply, a servo drive for instance. As an experiment
run your spindle from an extension lead plugged in as far removed (electrically) from the supply for your PC and servos as you
can.

The other mechanism is radiated where the hash generated by a device is radiated to other devices and circuits by
electromagnetic radiation, ie radio waves. There are numerous techniques for reducing the sensitivity of a circuit to
radiated interference, a search of this forum alone will result in hundreds of ideas. Still the best bet is to try to reduce
the hash generated by the spindle/VFD. Line reactors are not popular with hobbyists, you have to pay for them, shock
horror but they are effective.

Craig
My wife left with my best friend...
     and I miss him!