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Drive Design

Keep in mind that these are general recommendationsThe stepper motor performance (possible mid-band resonance issues,) stepper driver performance, drive voltages, acceleration settings, mass, quality of mount construction (drive efficiency,) and any tendency of to be out of balance (and probably other things not coming to mind at the moment) all make it difficult to know how a design will perform before final testing.

  • Use the stock steppers motors:  In some cases you can use an older mount's stock stepper motors (not designed for goto) to make a slow goto drive.  These steppers are usually tin-cans with a 120:1, 150:1, or even 300:1 reduction built-in (which in combination with a typical 144:1 worm/wheel gives a very high overall reduction.)  To reach goto speeds requires operating the stepper motor at speeds far above optimal.  Fast enough and the motor stalls (stops moving but vibrates making a "singing" noise.)  To reach higher speeds overcoming inductance by operating at higher voltages (relative to the stepper's design voltage) helps.  Unfortunately these older drives often have uni-polar stepper motors which which were designed for 8 or 12V which limits performance (high inductance.)  Usually these efforts result in about 0.25 to 0.75 deg/s speeds.  I call 0.5 deg/s the lower limit for goto (very slow but useful.)
  • Use a Hybrid stepper: Depending on the NEMA frame size (and available torque across various speeds) and overall reduction they are driving into, goto speeds for a typical amateur telescope can range from 2 deg/s up to dangerously fast.  Into a 360:1 reduction a NEMA17 400 step motor is known to work well with no additional reduction (for G11 mounts, 60 lbs payload.)  About 360:1 is where a 400 step motor's arc-sec/micro-step reaches a nice level (0.28".)  This is based on a 32x micro-stepping mode do not count on 128x or 256x micro-stepping to improve tracking much there is very little torque & accuracy between those tiny steps.  Also, 360:1 is about where torque from a NEMA17 reaches respectable proportions into a low efficiency worm-wheel [See Figure 2.]  More optimal from a goto speed (and efficiency) standpoint would be a 200 step motor into a 720:1 or 1440:1 reduction [See Figure 1.]  Note that not all hybrid stepper motors are designed to operate at high speeds with the typical 12VDC or 24VDC power supply levels we run at so pick a low design voltage stepper motor (so it's running at several times the design voltage) if the reduction ratio is higher.  Just be sure pick a stepper motor that stays within the current limits (with consideration to heat dissipation) of the stepper driver you plan to use with it.
  • Drive design and Torque: Note that 1 lb-ft is a force of 1 pound applied 1 foot from an axis of rotation.  So if applying 4 lbs of force 2 feet from an axis of rotation moves your telescope around with confidence (even when balance isn't optimized) that's about 8 lb-ft.  Perhaps double that once more to be on the safe side, about 16 lb-ft for this example.  The charts below give numbers for two fairly common types of NEMA17 frame stepper motors.  Smaller lower torque and larger higher torque steppers exist.

    Figure1 :

    Figure 2:

  • The Oriental Motor website: This is an excellent resource for comparing stepper motor coil resistances/design voltages and performance (torque vs. speed charts.)  The Hurst website and others have similar information on Tin-can stepper motors.
  • Power Transfer: Depending on your mount's design, you may use transfer gears, or belts and pulleys, or even directly drive a worm/wheel, or friction rollers, or band drives.  Timing belts and pulleys have the advantage of almost no backlash and are low cost so they get used quite a bit.