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I'm working on designing a motor drive for a telescope mount that is giving me some trouble. I'm trying to decide between using a dc motor with feedback and a stepper motor. Speeds need to be really slow (a constant 0.05-0.1rpm, exact speed TBD), so gearboxes or gearmotors will be necessary. The main drawback of steppers is power consumption as this will be battery powered, so dc motors seem prefereable for that reason.

The issue of course with dc motors is their speed isn't particularily constant, so some sort of feedback is necessary. An encoder is the obvious choice, though finding something that physically connects up has been tough. I did come accross someone who took an existing motor drive and added on speed control by counting flyback pulses https://www.cloudynights.com/topic/702124-motor-speed-controller-for-eq-1-eq-2-ra-axis-motor-drive/. It looks like they had some success, and I see some improvements to be made, but I'm curious if there's any inherent limitations to this method. I can't see any issues with it offhand (though I'm no electrical engineer), but I also can't find any other instances of this method of speed control being used.

Are there any limitations or issues with such a design that I'm overlooking?

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    \$\begingroup\$ The linked story presents a respectable example of successful reverse and forward engineering done by a hobbyist who knows what he does. The limitation of the method is that one really must know what he does in many areas of electronics. and nobody guarantees your motors produce such easily detectable switching pulses. An optical pulse generator needs some room to be fit to the motor axis and some skillful mechanical work. I guess the hobbyist wanted to avoid just the mechanical work, In theory one could pick also the magnetic field fluctuations or even the vibrations of the motor, but \$\endgroup\$
    – archaido
    Commented Oct 9 at 8:30
  • \$\begingroup\$ (continued) he fortunately detected the existence of the pulses before trying to something else. The pulses apper so clearly just with that motor and that motor driver circuit. To do the same you must find how to imitate the behaviour of that circuit & motor combination and, of course, how to keep phones and other radio transmitters and noise sources out. \$\endgroup\$
    – archaido
    Commented Oct 9 at 8:31
  • \$\begingroup\$ Thanks for the input. I have the exact same unit, so was thinking about trying to recreate what was shown. Noise sources are definitely a concern though, so I'll probably keep investigating mechanically implemented solutions, which is where I'm more comfortable anyways. \$\endgroup\$
    – Brenticus
    Commented Oct 9 at 19:46
  • \$\begingroup\$ If you have an oscilloscope you can easily check how strong pulses you get when the motor commutator switches, does a transmitting phone cause problems and could the problem be eliminated by lowpass filtering without losing the timing pulses. Without an oscilloscope it's practically impossible to know what pulses one gets from an improvised source. \$\endgroup\$
    – archaido
    Commented Oct 9 at 20:01

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