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

Hi Bryan,

That sounds familiar! Although I didn't have any hydraulic systems. I
think it may have be to make a sensor more accurate... I don't remember
enough about it. In fact, I think that system was replaced/updated
early on.

Alan KM6VV


Gail & Bryan Harries wrote:

>
> 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".