Large diameter Axial Flux DD motor for telescope


#1

I am currently working on a design for an Alt-Az telescope mount that utilises large (12"ish) axial flux motor (home made) with 18 coils (24 magnets) and a high precision encoder with sub arc-second solution. Obviously the primary function is to move the scope at a slow rate to compensate for the Earth’s rotation which is 1 rev per day (of course). The scope itself with all the cameras etc mounted weighs around 20kg. so a fair amount of torque is needed…especially to accelerate it and move it to a new target. I think somewhere in the order of 10N/m would be sufficient and I believe a motor that size could easily provide that at 24V and maybe a handful of amps.

My question … Is the ODRIVE suitable for this? I am a Python programmer by trade and therefore this is a very attractive solution for that reason. Would the current firmware already do this slow, precise movement - or would I need to work on that too? Lastly would the encoder input work with something like the Renishaw ring encoders?


#2

I’m not an astronomy expert, but why wouldn’t you design the mount such that the scope is balanced?(adding counterweight if necessary) Given a balanced load and the very low accelerations you’re looking at, I’d think you’d be able to get by with very low torque levels and consequently low power levels.

Given the positioning precision level you’re looking for, I’d suspect that you’ll have more issues with motor cogging and mechanical. I wonder about adding a viscous damper to smooth some of that out…

Yes, I think ODrive is worth exploring for this job.


#3

Maybe I didn’t explain things quite right. The scope will be balanced. It is purely the weight that needs to be accelerated to move to new points around the sky. As for damping… is that not possible in the drive set-up. As I plan on making my own axial flux motor I have the opportunity to use as many poles and in any position I wish. As I say my initial thoughts are to have 18 coils and 24 magnets. What are the main factors in reducing cogging in a motor and can I reduce it with the arrangement of the rotor & stator or is it all down to tuning?


#4

The most important part will be to have an encoder that is at least 10x higher res than the position error you hope to track. If you have that, cogging won’t be an issue, as the controller can compensate very rapidly compared to how quickly the cogging torque is changing over angle.


#5

And we have a prototype anticogging algorithm, which may help also. But like madcowswe said, the encoder resolution is going to be more important.