Good point, John, thanks for expanding on the issue...

- Chuck

> -----Original Message-----
> From: John Payson [SMTP:supercat@MCS.NET]
> Sent: Tuesday, January 27, 1998 3:14 PM
> To:   PICLIST@MITVMA.MIT.EDU
> Subject:      Re: Counting encoder pulses
>
> > The reason I ask, is that depending on the above, entirely different
> > processing is needed.  In the mouse biz, the encoders are not
> detented,
> > and as a result, the mouse can be "parked" at the transition point
> of
> > the shaft encoders (the photodetectors (darlingtons) drive the uC
> pins
> > directly).  So, jittery or "creeping" cursor motion became a problem
> in
> > early designs.  The way around this was to throw away a valid
> transition
> > on direction reversal - this is essentially firmware hysteresis .
> But,
> > in your case, I suspect that you never want to lose a count on
> direction
> > change in order to isolate noise...
>
> While it would seem nice not to have the encoder be "off" by a count
> on dir-
> ectional changes the effect is non-cumulative and in fact may in some
> cases
> be a GOOD thing depending upon how the encoder is located relative to
> the
> object in question and its driving force.  If your encoder is on a
> motor which
> is moving the telescope, the backlash of the encoder's hysteresis will
> work
> to counteract mechanical backlash in the system.  This is the only way
> of
> mounting which can be made reliable around direction changes, and even
> here
> you may still have to take measurements of mechanical backlash (which
> will
> probably be greater than one encoder pulse) and compensate.
> Alternatively,
> you could always approach target positions from the same direction,
> analagous
> to the way many musical instruments are tuned (e.g. on a violin, you
> always
> tune by raising a string to the correct pitch; if the pitch is too
> high, you
> loosen the string below the correct pitch and then tighten it to raise
> the
> pitch).  In this case, the one-count offset on direction changes will
> be a
> non-issue.