Star Tracker using geared dc motor with encoder

Projects General Guidance
1

Hi guys,

I'm looking to build a star tracker for astrophotography using a Geared DC Motor with Encoder for speed feedback.

I have built one previously using a nema17 stepper but these are quite power hungry, heavy and large in size.

So the design will be basically a geared dc motor driving a worm gear. See this video for a look at the kind of design I'm going for. Vixen Polarie - an inside view - YouTube

My question is around working out how to control the motor in a way that I can get the required speed reduction after gearing and still maintain enough torque to drive the tracker.

I am currently looking to use a 6v Motor with an output after gearing of around 12rpm.

So "Sidereal Rate" is the speed of rotation I need to match which is 15 degrees / hour.

Which equates to an output of 0.0007 rpm. Which is a reduction of 17,280:1 still required.

My worm wheel ratio is looking to be 120:1 ratio with a 4:1 ratio of gearing between output of geared dc motor and worm gear shaft giving a total reduction ratio of 480:1.

This means I would need to drive that Motor at 0.025 RPM in order to get my final required speed on the output. Which I am doubtful is even possible without even more gear reduction.

Now assuming that the above driving RPM of 0.025 is not an issue, I also need to know how I would control the speed of the motor using the encoder.

I am currently thinking I would use the encoder ticks, to calculate current speed and then use that speed to control a pwm'd voltage that drives the motor so that the output shaft moves at exactly 15 deg/hr.

So I guess my questions are these:

  1. How low an RPM can I expect to drive that motor at given its maximum output speed is 12rpm.
  2. Is my idea on how to use the encoder ticks to drive the motor feasible or the best solution?
  3. Do you have any other ideas as to how I could achieve the required reduction in rpm? using different motors?

I hope I've included enough information that someone may be able to provide some advice on the project. If I haven't please let me know what additional info you may need and I'll do my best to provide it.

Thanks in advance.

2

Since you can rely on the 15 degrees per hour being pretty much constant, you really should pick a motor and a gearbox that will produce the 15 degrees per hour at the optimal RPM of the motor. My guess is that you need two gear boxes. One for the camera end, which can handle high torque. This would be a metal gear box. The other gearbox would be at the motor end and would take care of the rest of the needed reduction. This box deals with lower torque and higher speed and could be plastic gears. You might be able to build this gearbox using gear sets available somewhere:

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The encoder should work quite well. It should be all about having a precision clock and counting steps and putting up a PID. Having a 4 hour exposure time means counting the exact number of steps for 4 hours, right? I mean, if you adjust the motor speed every 10 s, according to a ratio n steps per 10 seconds, you end up in a big error unless the n is a precise integer, not just a high precision float. n probably is not an integer, due to the length of the sidereal day.