Anthony,
Without any feedback you will always have speed errors due to loading
factors, even if your clock source is very precision. For any system
of this type to work in a precision fashion requires negative
feedback. There are motors available that have a built-in tachometer.
These can be used to give quite decent regulation. I often get these
type motors off of *old* discarded floppy drives.

Sometimes a small motor used as a generator can be attached to the
main motor. This is not as good as the built-in tach/generator, but is
often workable.

Slotted disks and opto devices can be used to sense rotational speed,
and if a quadrature output is used, you can also get positional
information. This is all handled digitally.

Back EMF of the motor can also be used to determine actual speed,
although some care must be exercised in making this measurement,
as the PWM method will allow speed measurements to be taken only
after a settling time. But it is do-able.

Hope this helps.
Fr. Tom McGahee

 At 08:02 PM 26/08/98 EDT, you wrote:
> >I am new to PICing and I was wondering if anyone on the list would be so
kind
> >as to share some wisdom.  I am building a project with a 17c44 to drive a
> >pulse width modulated motor controller.  I am currently using an RC ocilator
> >as clock input but I am sure that a crystal type clock would been more
> >accurate, wouldn't it?
> >I am not planning to use encoders, but I do need very precise motor control.
> >Using PWM motor controllers, wouldn't using the most accurate clock increase
> >the precision of motor control?
> >
> >Anthony
> >
> >(And yes! I AM a nebie, and proud of it.  You have to start somewhere.)
> >
> >
>