Re: [CAD_CAM_EDM_DRO] Re: Why not lineal feedback, instead of rotary encoders?

Torsten, good description....Shane, imagine that a servo will "hunt" between
the adjacent "spaces" in its encoder, because it doesn't know where it is
unless it's "banging its head" against both sides.That's just how it works.
That's the singing sound you will hear when a controlled servo is idling.
With the encoder on the motor, that hunting traverses only a tiny fraction
of a degree, which is proportional to the precision of (quantity of counts
per revolution) the encoder. Now imagine the encoder instead mounted to the
screw. Any and all slop between the motor and the table will have to be
traversed many times a minute to allow the "banging" previously described.
It's been described by those who've tried it as pretty ugly, and sometimes
totally self destructing.

That is a natural questions though, and one many of us asked a short while
ago when getting up to speed. There are also more technologically advanced
servo drives with dual control loops however that can use both encoders on
the motor and the table, however I haven't heard much about them except that
they still don't correct completely a problem that is naturallyand more
inexpensively eliminated by simply removing all the backlash in the first
place.

Does that sound correct guys?

R. William Kichman, P.E.
Kichman Engineering Associates
103 Old Furnace Road
Cornwall, PA 17016-0643
tel/fax 717/270-0714

----- Original Message -----
From: "Torsten" <torsten@...>
To: <CAD_CAM_EDM_DRO@yahoogroups.com>
Sent: Tuesday, August 12, 2003 7:28 PM
Subject: [CAD_CAM_EDM_DRO] Re: Why not lineal feedback, instead of rotary
encoders?


> --- In CAD_CAM_EDM_DRO@yahoogroups.com, "Shane M Dwyer"
> <dwyersm@p...> wrote:
> > I have been looking around the groups, www, commercial vendor
> sites,
> > etc and have noticed that positional location feedback is always
> > determined by an encoder mounted to the screw or motor. I can't
> > figure out why the actual position of the object (table, workpiece,
> > tool, or whatever)is not determined by a linear transducer, eg a
> > scale not unlike one used with a dro.
> > Wouldn't this be more precise. We are intending mostly to measure
> > linear - planar, and not a rotating object's final position
> afterall.
> >
> > And "final position" doesn't have to mean stationary, it means
> final
> > before the next reading, position update, polling, and so on.
> >
> > It is the object's location that we are currently interested in,
> not
> > the rotation of or position of the screw or screw motor spindle.
> >
> > Measuring the screw's location (ie. number of turns or part thereof)
> > is not quite the same as the actual location of the work - due to
> > sloppiness in anything, the screw, bearings, screwnuts, collars,
> > retainers, gibs, gears, belt drives, transmissions, friction, etc .
> >
> > A positional feedback system derived from measurement upon the
> actual
> > article we are forcibly locating would then be somewhat immune to
> the
> > abovementioned sloppiness, wouldnt it?
> >
> > So why isnt it done?
>
> You have pretty much answered your own question.
> Backlash in components are a bad thing for feedback.
> Servos constantly hunt for position this can be
> easily observed on a badly ajusted servo it will oscilate
> back and forth causing a wild shaking of the motor.
> Once the feedback indicates a motor is out of position it
> is given a current in the opposite direction often overshooting
> its target wich is causing the ocilations.
> Having the encoder mounted on the shaft of the motor eliminates
> and backlash and allows for better controll of its position.
> Hope this helps
>
>
>
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