Re: Constant Velocity unwinder

--- In CAD_CAM_EDM_DRO@yahoogroups.com, "Fernando" <fer_mayrl@...> wrote:

>
> Thanks Mariss,
> Not that proficient in electronics, but I will look into it.
> I will let you know.
> Regards and thanks a lot
> Fernando.

Mariss used the word node which makes it sound worse than it is :)

"Form a summing node of the tach feedback (- voltage)
with a velocity command (+ voltage)"

just means stick the voltage from the tach and the voltage from a pot
you will use to set the speed into a op-amp summing amplifier.
Because the voltages are different signs the output of the op-amp will
only be zero when they are both exactly the same otherwise it will be
outputing a voltage to tell the VFD to do something. The P part is
just the gain of that amplifier and the I and D are obtained by adding
some capacitors etc, I gets rid of the steady state error and D
responds to the dynamics (I doubt you need D).

I have done something quite similar to this recently except it was PI
not PID and I am trying to maintain tension in EDM wire rather than
speed but the principle is the same. My circuit is breadboarded up
and working, I can try and find the schematics if I produced any. It
was driving a DC motor directly (well via a transister) but you could
drive the 0-10v control on the VFD with no problems. You could also
find a frequency to voltage converter circuit on the web to convert
and encoder to a voltage if you fancied a non contact feedback method.

Unlike a typical servo system you need no reversal of direction and as
you seem to be countering the effects of spool size I suspect that the
system does not need to have particularly good dynamic response etc so
it should be fairly non critical.

Graham