Computer Controlled Telescope - Stepper Motors
I had a few problems in choosing stepper motors, mostly related to not knowing
which ones would be powerful enough to turn my 'scope. I already had a small
stepper on my RA drive that happily turned the telescope for tracking. The
difference with the new system was that the telescope was required to slew from
one object to another so the motor had to turn at a much faster rate without
stalling, in practice that meant a bigger motor with greater torque otherwise
slew rates would be unacceptably slow.
The Metric Factor
The information on Mel's Website refers to motor torque expressed in
"ounce-inches" but in the UK and Europe we use
"Newton-metres" or usually mNm - milli-Newton metres. After dredging
up some physics I calculated the factor to be 1oz-in
= 7.1 mNm. This was confirmed by a visit to the very useful
Website of Warner Electric, who
make steppers.
The Motors
The motors I eventually chose were both 500mNm (70 oz-in) holding torque, frame
size 23, rated at 5.1V, 1A, 8-wires and 1.8 degree step angle. I bought one of
them from RS Components part number 440-442
for 
£42.51 (inc Tax). The other, almost identical motor, came from a very old,
very bulky dot-matrix printer and had the name "Slo-Syn" on the back
which, after searching the Web turned up Warner
Electric. Warner were very helpful, after an e-mail to them they sent me a
data sheet showing the wiring diagram for the motor which saved a lot
of time trying to figure out the connections.
Other Surplus Motors
I spent some time digging around the junk where I work and in my junk box for
motors. A few possibilities turned up, some were 7 degree step angle and some
were too small. One large motor, again out of a printer, made it to the
"commissioning stage" of my drive. After a lot of wasted time I ended
up buying the motor described above, the reason was this. The motor would not
microstep evenly no matter what I put in for the PWM values, it always had one
huge step. The motor drew large amounts of current despite adjusting MSpause
etc. In the end I'm sure the motor was faulty, my conclusion - don't waste a lot
of time with surplus steppers if they behave uncooperatively - get a different
motor/buy a new one!
Motor Frame Sizes
Stepper motors mostly come in standard NEMA frame sizes. This is very useful for
connecting the motor to a gearbox if you are going to use one. I chose to buy
gearboxes which meant that my motors fitted up to them very easily by use of a
small kit available with the gearbox. Bear this in mind if you get hold of a
surplus stepper - some of the ones I found did not have standard frame sizes.
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