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Re: Changing the servo motor encoders from 256 to 512 or 1024. Any reason not to do this?

 

Page 31 of the Gemini-1 users manual has a table. It says the G11 and? GM8 have 6400 steps per worm revolution (25 gearbox x 256 motor encoder revolution).? There are 360 degrees per revolution and the G11 ring gear has 360 teeth, so that's one degree per worm revolution.? So each degree having 3600 arcsec per degree is divided by the steps per worm revolution that is 6400.? ? So that's 3600/6400 arcsec per step = 0.56 arcsec per step.??

As for reasons not to go with a higher motor encoder: the Gemini-1 clock is 1.5 MHz (per the G-1 manual).? I don't know the clock speed of a G-2.? You might find the encoder detector chip unable to keep up with the codes at a high slew rate....but that's only a factor of 2...so it might be just fine.? It's worth a test.? Getting the motor encoder out is a major soldering challenge.? Replacing the fragile encoder disk is easier but really...these motors are hard to get apart and repairs of them are rare.? (Brendan Smith and David Partridge are expert in this subject.? I tried and had no success.)??

To double the step accuracy, you can also replace the 25:1 gearbox with a 50:1 gearbox.? [I'm only familiar with the McLennan versions.? The McLennan company didn't recommend using their 50:1 gearbox with a HiTorque motor because it's shaft is ~3.2 mm and the small 50:1 gearbox pinion gear is 3.0 mm...they thought enlarging the pinion ID would get too close to the teeth.? Alternatively, Stuart Hutchins recommended using the motor rotation and a file to reduce the shaft OD to 3.0 mm to fit the pinion ID.? Stuart thinks this is the best way to keep everything I have not tried that idea...the question is what is best to modify. ].?

Best,
Michael

On Sat, Jun 12, 2021, 6:11 AM pcboreland via <pcboreland=[email protected]> wrote:
Here is my math:?(360*3600)/(256*25*360) = 0.5625 arc sec per step.

This seems rather coarse. We have 2.3 million steps per rotation of the Ra shaft. From talking with Sideral Technologies high end mounts have a greater number of steps. If it is possible to change the encoder used in the motor this could be increased to 4.6 or even 9.2 million steps. So 0.28 or 0.14 arc sec per step. This I think should have a significant impact on improving guiding performance, especially in the Dec axis, as right now the minimum correction is 0.56 arc sec. My imaging system is 0.76 arc sec per pixel. I would welcome any thoughts. Perhaps Losmandy can comment on wether the controller will work with a higher resolution encoder. They cost around $65 per axis to do the upgrade, so well worth running a test I feel.
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