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Low-speed direct-drive 3-phase motor and controller

uzernaam

Dec 12, 2021
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Summary: I want to build some large high-torque very-low-speed axial flux 3-phase motors (including controllers) to direct the altitude and azimuth of a telescope to track stars accurately for long-exposure astrophotography.

Apologies if I’ve posted in the wrong section.


Greetings! -

I'm a new user here hoping to get myself mixed in with some smart people who can point me in the right direction. I've taken on an ambitious project and am very excited to get started.

I've been an amateur astronomer for a while, and have built a few telescopes from scratch (even ground my own mirrors!) and I want to take it to the next level.

So to begin, I saw this video and was hooked.

I was marveling at how smooth and accurate the direct-drive motor system works and want something similar.

This telescope carries a $200,000 price tag, but I know that with perseverance I can get away with building it myself and still get good results. I don’t expect this project to be completed overnight; I’m sure I will be tinkering and improving things over months and months.

Because I’m just a single person without a lot of resources, I have to keep things to a budget as much as possible.

So I want to make a 2-motor alt-az telescope mount. The motors on that telescope are large axial flux motors using permanent magnets and coils. Someone at that company designed the motor controllers so that they can provide very slow but very steady motion so that the telescope can track the stars extremely accurately. The specification they list is less than 0.07 arcsecond deviation (1 / 52,000th of a degree) from perfect while tracking the entire night. This kind of tracking accuracy is an astrophotographer’s dream.

My idea is rather than engineer some complex custom motor controller from scratch, why not use the computational abilities of a raspberry pi to generate sine waves on the audio output, feed those into some rather simple audio amplifiers, and feed the amp outputs into the coils that control the motor?

Sounds easy? Of course, we'd need some way to tell the motor to speed up, slow down, or otherwise slew to a certain location in the sky. Absolute location accuracy would require encoders, but maybe I can get away with something simpler?

I know there are commercial telescope equatorial mounts which do a good job on their own - but using a guide star, small tracking scope and a digital camera sensor attached to the guide scope there is software that feeds back corrections to these commercial telescope mounts to keep them even more accurately on target than they do without feedback. I'm thinking a similar arrangement would allow me to point my scope to a target, start tracking it via software, and not have to worry about the expense and complication of using high-resolution encoders on the motor axes (for now).

Anyway, that's the idea. At this moment I feel confident enough to begin several areas of this project, but programming a raspberry pi to control a motor is a bit over my head, although I do have some amateur electronics, computer, and programming experience.

I'm really wondering what all the people here who are smarter than I am think of this? Any advice?
coils.jpg
 

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Harald Kapp

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Nov 17, 2011
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Have you tried searching for "diy telescope tracker"? Lots of amateur projects to be found on the internet.
 

uzernaam

Dec 12, 2021
3
Joined
Dec 12, 2021
Messages
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The projects I've seen so far all employ gears.
Gears are problematic, such as backlash and periodic errors.

I'm looking for a silky-smooth direct drive system that fits my budget and specs even if I have to build most of it myself.
 
Last edited:

uzernaam

Dec 12, 2021
3
Joined
Dec 12, 2021
Messages
3
Here's a little more of what I'm looking to create:
(Skip to 11:20 where they talk about the direct drive system)

 
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