Building an alt/az mount (aka gimbal) for satellite tracking.

Discuss using and implementing the NearZero motor controller and the RoboWheel hubmotor.
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Joined: Thu Dec 31, 2020 1:09 pm

Building an alt/az mount (aka gimbal) for satellite tracking.

Post by bobrich »

Hey all,

Sorry for the wall of text, would like to get your take on an approach here.

I'm just starting to dip my toes into motion control and robotics and have a project that I'd like to start with. I'd like to build an alt/az mount for tracking satellites with directional antennas or lightweight optics. I was going to go a standard servo route but was super inspired by the work of this guy building an incredible mount for astrophotography

Now just one of the encoders Bruce uses in that rig are about 4x my total budget, so my goals are much more pedestrian. But after a modicum of tinkering I've collected this set of criteria:

- Would like the system to be direct drive for all the reasons that direct drive is awesome.

- Payload would be less than 5lbs and could be balanced.

- 30 degrees per second max rotation speed would be plenty

- Would like to shoot for < 10 arcminutes of resolution in pointing accuracy. For RF I only need +/- 2 degrees so this is a stretch goal.

- I don't know the right way to parameterize it or think about it, but for tracking I would like the velocity control to have very small tolerances (e.g. little to no wow and flutter). I just took delivery of a great little controller from MJBots to get a hands-on impression of BLDC motors and how they work. This seems like it might be the trickiest part.

So, questions:

Any initial feedback on the idea itself would be appreciated. I am improving my understanding of orbital mechanics, TLE parsing, etc with libraries like skyfield and hope that they converge in me having a good understanding of where to point once I have a good understanding of how to point.

What kind of motor should I start with? The MJBots dev kit has a 600W 330kv motor which has plenty of torque for the payloads I have in mind, but probably too much cogging. Gimbal-wound BLDC motors seem like they would be better from a cogging standpoint but it's hard to find ones with anything approaching 1Nm of torque. Hub motors obviously would have sufficient torque, but I have zero experience with them and don't know if they would be suitable here.

Based on my limited understanding, the NearZero controller looks just about perfect for what I'm trying to do (except for that whole getting my grubby hands on one part :) ). If that's wrong, though, would appreciate any pointers there as well.


Edit: Just bought a 10 pack of used 5208-200 gimbal motors off of ebay for $100. Now I just need a hub motor to tinker with and a controller to drive it.

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Joined: Tue Aug 25, 2020 4:21 pm

Re: Building an alt/az mount (aka gimbal) for satellite tracking.

Post by slomobile »

I've been contemplating a similar project. Helix antenna mounted with telescope/network camera.

I've decided against an alt/az mount in favor of a Stewart platform because it has 1 additional motor to share the load, all drive/encoder trains are built identically, they are all preloaded by gravity eliminating backlash without tight clearances. The platform provides an intuitive keep out area without ungainly balance weights hanging in walking paths. Alt/Az has the advantage of keeping the image upright, but at the cost of gimbal lock and impossible acceleration requirements at singularity.

Smooth motion is elusive. Large diameter,ow kv, high winding resistance large pole count gimbal motors are appropriate. that might help you decide. I have not been able to eliminate wow and flutter. Let us know if you are able to mitigate it. I suspect it will require field oriented control.

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