Do servo systems sing?

[ToyMaker]

I've been thinking about stepper- and servo-driven machines. Stepper systems sing when in motion at the frequency of the step rate.
In a stepper system when an axis is not moving there is power applied to the motor. Any force applied to the head is met by the holding torque of the motor.
In a servo system when an axis is not moving there is NO power applied to the motor. So any force greater than stiction will move the head at least one encoder pulse distance. The servo amp will then kick in to move the head back to the commanded position and then remove power. Repeat.
If nothing else changes the head will oscillate between the two positions. Is this oscillation audible? Do servo systems sing?

robotic regards,

Tom
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I asked God for all things that I might enjoy life.
God said, No. - I gave you life so that you might enjoy all things.
- - unk.

[balsaman]

Yes, they sing. On my system it's barely audible.

Eric

[HuFlungDung]

Mine sing, too. Not much when stopped, but you get a real tune when they run

Its not loud though. Some of the really old PWM drives worked at a low frequency, like 10khz or something, and these were really annoying, because they had a steady high pitched whistle to them.

I am pretty sure that there is a small holding current applied to a stationary servo motor, too, because the position needs to be constantly checked. Depending on the encoder resolution, this can be a significant movement, or not. One system I have uses 1000 line encoders, and it makes kind of scratchy sort of sound as it hunts to check position. But an 8000 line resoluton system makes no noise when stopped.

[ESjaavik]

It also depends on the setings of your servo drive. The PID controller needs tuning. If you make it too "touchy" it will sing, and when decelerating hard to a stop, you can get a "boingg" sound. This could be a sign that you have the settings wrong. That will also influence the accuracy of your work.

There should be a "deadband" when position is reached. It should not hunt for position, neither should there be any holding current when stationary unless needed to keep it. One example is the Z-axis where a current is needed to counteract the weight of your spindle tool and the axis mechanics. If it hunts, that will create unnecessary heat. It may also find resonance in the structure of your machine and then it can be real bad. It's quite a bit of headscratching to get it tuned, but when done it's very satisfying to see it run full speed ahead then stop almost instantly and sit there. Then if you try to push it, it will not budge. Not only satisfying, but outright fun! I can sit there watching it over and over again. :-)

And as Hu points out, you may hear the chopper when it's moving under load. About as you can with a stepper at standstill. Because as you correctly point out a stepper will be fed current even when stopped.

[Boros]

Good quality/ high end servos systems make no noise at all if tuned correctly.

The only sound that can be heard is that of the bearings. i.e. Motor ball barings, ballscrew and/or belt, linear bearings.

Thanx