RE: [CAD_CAM_EDM_DRO] RE: dither, hunting. was re: G320s, New and Improved!

Alan,
just as a piece of information... hydraulic servo systems sometimes use
very fine (high frequency) dithering to keep the seals moving all the time,
where by not letting them settle and encountering stiction (stick slip
effect)... if the servo has to break the seal stiction to move only a small
amount, it will generally over shoot and have to continually make
corrections to try and get to where it wants to be... whereas if it is
dithering and the seals are floating, it is easy to move that small distance
required to accurately position [whatever]...

Bryan

-----Original Message-----
From: Alan Marconett KM6VV [mailto:KM6VV@...]
Sent: Monday, 10 September, 2001 2:26 PM
To: CAD_CAM_EDM_DRO@yahoogroups.com
Subject: [CAD_CAM_EDM_DRO] RE: dither, hunting. was re: G320s, New and
Improved!


Hi Jon, list,

I seem to recall a servo system we had in the submarine (I can't
remember which), and it had dither intentionally introduced in it, to
increase accuracy (I think). That was a long time ago! Is what you are
describing possibly called "HUNTING", an not dither, which I thought was
intentional? (I said it was a l-o-n-g time ago!)

Alan KM6VV
(back from vacation #2, now to get some rest!)


Jon Elson wrote:

>
> cadcamcenter@... wrote:
>
> >
> > > First off there is virtually no dither noise. I do have one axis
> > that you
> > > can hear the motor shift back and forth very slightly and at a very
> > slow
> > > rate (maybe one or two cycles per second) when it is stopped in
> > certain
> > > positions. There is absolutely no humming noise that I used to get
> > on the
> > > old G320's.
> >
> > The presence of dither means that the servo is always seeking
> > the "correct" position means that servo is always overcorrecting,
> > then undercorrecting. Re: the old servo vs stepper "controversy".
>
> It can be argued that any servo which dithers with no other input is not
> properly tuned. I can say that my servos do NOT dither, except under
> very rare circumstances when a mechanical resonance has been excited.
> That, in itself, is indication that I could tune it better, but it is very
> much
> "Good Enough".

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