Re: servo hunting

Elliot Burke wrote:

> From: "Elliot Burke" <elliot@...>
>
> My experience with servos is limited, so I'll ask the group here:
> If a servo is controlled digitally, is it inevitable or even usual that it
> will hunt between two adjacent values? The situation envisioned is if the
> encoder is at a point where it near a transition between two values, will
> noise or other causes make it switch between them?
> If this is so, then why wouldn't hysterisis be used to prevent this?

There are a number of ways to combat this, but, in fact, it often is
just no problem. There is no specific code in EMC to prevent this,
but at least in my system, which has fairly fine resolution, there is almost
no hunting. Every once in a whilke the Y axis will hum for a moment,
but that is about it. The Z axis does in fact hunt all the time, but at only
about 1 Hz. So, it is rocking back and forth .000025" between two
encoder transitions. This is actually caused by the weight of the quill.
As the quill sags under its own weight, the servo needs to increase
upwards force until it brings the encoder's position back to zero error,
then it relaxes, and the cycle starts again. Certainly no problem, and
I guarantee it is not a problem in machining, having the quill wobbling
about 25 micro-inches.

> A colleague is using microsteppers and having great difficulties getting
> quiet positioning. The residual vibration level of desired is in the sub
> microinch level. Positioning accuracy or precision doesn't have to be that
> low, he just doesn't want it moving around once he puts it somewhere.

Well, he probably shouldn't be using steppers at all. Most microsteppers
that have closed-loop current control have a large amount of fluctuation
in instantaneous current, which will cause vibration in the most sensitive
systems. A servo system could be specifically tuned to minimize hunting
and similar vibrations. Or, it could be brought to position and then turned
off, assuming the leadscrew or other drive could be trusted to not creep.

Jon