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