Jo wrote:

The nice thing about the current system is that it is an independant module
that I can remove easily for manual focusing and I am reluctant to start
adding limit switches to the Meade focuser.

Depends how you fit the switches - they can just as easily attach to
your module and touch the focusser tube wherever they can find. Its only
for a reference. A digital position servo would be pretty easy.


Not sure about required resolution at the focuser tube..but I do know that
one turn of the DC motor makes no visible difference to the focus, it takes
quite a few turns before any optical changes occur..and that is after taking
precautions that I'm not looking at backlash in the gear train.. Here is my
plan:

I may be able to count revs by monitoring the DC feed to the
motor and watching for commutator breaks.

If you don't HAVE to use second order effects like that, why bother -
you only need a single flag pulse per rev of the motor and a (ideally) a
direction signal.

OR a rubber wheel on the focusser tube, then a digital pot chip with a
push button up/dn counter....linear output volts with position.


2) Given that I can get a supply of tachometer pulses back from the motor in
step 1) I need to count them. A digital counter is one solution but I have
thought of something simpler. I integrate the pulses and compare the
integrated signal with the output from my multiturn pot that is tied to a
stabilised reference voltage. The comparitor output is then used to
determine the pulse width and polarity of the drive to the motor.

In the above positional resolution will depend on:
a) Motor turns per unit movement of the focuser tube.
b) "Goodness" of the integrator circuit and comparitor.
c) Resolution of the multiturn potentiometer.
d) Stability of the reference voltage.

The "simpler" method is the road to madness - the integrator would droop
too much in practice. The pot won't be a problem, a zener would be
stable enough, but board leakage would knacker the integrator for long
period integration.

If you are an amateur and WANT to mess around, go ahead. If one of my
staff offered this as a method though.....

Steve