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 "> So why isnt it done?

Dear Sir:

You are on the right track if precision is your goal. In fact,
better systems DO use `direct' feedback with the result that the
accuracy improves. However problems do occur; the compliance and
backlash following the motor can cause the motor to hunt. This is
solved by using velocity feedback from the motor in a second loop
whose function is stability of the system.

If precision is NOT as important, using the motor encoder easily
solves the stability problem.

Regards,
Jack C.