Suggestions on a custom dual axis drive?

[el_draco]

Getting toward the business end of a project and need to address the requirements for a dual axis drive system, but not a lightweight.

I need to control a English Fork that weighs about 150kg, (90mm polar axis) and the Dec axis' are 50mm. The OTA weighs in at 130kg+ and 2500mm long. Permanent observatory.... as you'd guess.

I am figuring the starting point is a couple of Byers type gear and worms but that's as far as I have got at the moment.

Anyone out there have experience working on hefty mounts who can share a pearl of wisdom, or 20..., I'm all ears

Rom

[Wavytone]

You could use 12V stepping motors and use toothed belts to achieve a reduction of say 20:1 to drive the worm. The toothed belts will keep it quiet and smooth in operation.

you'll have to make or buy stepping motor driver circuits, not hard if you are handy with a soldering iron and understand electronics.

[el_draco]

I've been exploring the possibility of friction drives. Seems to be some pretty compelling reasons to consider this, including scope for local manufacture.

[Wavytone]

Agreed. With the exception of a Byers drive, virtually anything based on commercial gears will have massive backlash. Possible alternatives:

- a final reduction that has no backlash, ie a friction or tape drive;
- harmonic drive; or
- an arrangement where the drive is permanently loaded in one direction so reversing doesn't experience backlash (hard to arrange and not compact).

The trouble with friction drives is coming up with an arrangement that allows slewing and yet does track at the sidereal rate without slipping. A clutch is one way, but not the only way. To drive the shaft, use a miniature differential gearbox with two motors driving into it (the output being the sum of the rotation of the two input shafts). One input shaft would be the RA rate driven by a stepper, you could drive the other shaft either manually with a wheel, or with another motor with variable speed. You can find differential gearboxes, a Chinese one would be OK.

Regarding the friction drives, I wouldn't get too obsessed with the possibility of slip or inaccuracy of the reduction ratio because in tracking stars, atmospheric refraction means the apparent rate in hour angle is slight variable, depending on elevation.

[el_draco]

Yeah, I am aware of this and figure tracking would deal with the elevation error. I like the simplicity of friction drives and the idea of avoiding $2k++ per drive axis is very appealing as well. Had not thought about using a differential gearbox for different slewing rates. That's gotta be a good line for further research.

Rom