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

Shane M Dwyer 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.
>
>
>

There are two reasons. 1. linear scales of low resoultion are quite
costly.
And, a rotary encoder of low resolution will have much higher positioning
resolution than the affordable linear encoder.

Work it out: Standard linear scales used on DROs have resolution of
approximately .0002" (actual res .005 mm) A 36" scale like that goes for
$300+. Now, a 250 cycle/rev encoder that sells for $18 - 40 provides
1000 counts/revolution in quadrature, and with a 5 TPI screw gives
5000 counts/inch, or .0002" resolution. With a 10 TPI screw, you get
.0001" res. Yes, this is RESOLUTION, not ACCURACY, I know
that! With a decent rolled ballscrew, you can correct for small
deviations in screw pitch. So, this is almost a 10:1 cost savings.
Very significant for many home shop builders.

(One hidden advantage of the shaft encoder is it is using a steel part
(the screw) as the actual measuring instrument, while the linear scales
are typically glass. So, the screw is automatically compensating for
the ambient temperature of the shop. Steel parts made in summer or winter
will come out the same size, when moved into the 68 degree metrology
lab. this won't work with a glass scale. it also won't work real well
with aluminum or plastic parts.)

Jon