3D printed star tracker

[pipnina]

I think one thing to consider is that barn door trackers are very limited and, imo, not very good and actually kind of bad value. If this star tracker was even twice as expensive at 100 euros or so, chances are its performance would outstrip the typical barn door by so much, it would still be a worthwhile price. Especially as the cheapest manufactured tracker otherwise costs €300+.

My gen1 star adventurer makes a little clock ticking noise, the stepper in there is probably only using a resolution of something like 2 steps per arcsecond or so, this project is targeting about 4 ticks/second as a homebrew project!

Your eccentric gear looks very promising to me, I think it could be optimised by getting the number of teeth in the inside gear to be as close to the outside gear without locking them together, then making the eccentric movement roughly the same as one tooth pitch. If this is right it becomes a bit like a cheaty strain wave gear in my mind?

[Thalestris24]

4 minutes ago, vlaiv said:

Well - I don't really count my time or amount of money I spent developing it.

Since I plan to release specs to general public - it's more "the budget" needed to complete the project or BOM if you like.

I'm really developing this so that anyone who has 3d printer or access to one - can build themselves working star tracker that they can use for DSLR+lens or smart phone astrophotography.

Sort of 21st century barn door tracker kind of thing, but instead of going to "dad's workshop" to whip up some sort of contraption out of wood and scrap metal - one would do few clicks online to get needed goods and then its 3d printing and soldering time

 

Very altruistic of you! Maybe you could offer a 'deluxe' version too? I've not done any calculations but wondering whether it could be possible to simplify it with a direct drive rather than gearing? If it's just a camera and lens you wouldn't need the torque that you'd presumably get from gearing? Just thinking out loud...

Louise

[vlaiv]

11 minutes ago, pipnina said:

My gen1 star adventurer makes a little clock ticking noise, the stepper in there is probably only using a resolution of something like 2 steps per arcsecond or so, this project is targeting about 4 ticks/second as a homebrew project!

Probably something like that.

AzGti has resolution of 0.625"/step - so that is very close to 2 steps per arc second.

I think I'll be taking a step back and going with 1"/step instead - so a little bit coarser than commercial star trackers, after all - this is, as you say homebrew / DIY stuff. That way, I can simplify things considerably and simply go for 3 stages of 80 / 15 GT2 belt / pulley combinations. 80 tooth gt2 fits in 50mm diameter - so the whole thing should not be large - and it will certainly be slim enough.

15 minutes ago, pipnina said:

Your eccentric gear looks very promising to me, I think it could be optimised by getting the number of teeth in the inside gear to be as close to the outside gear without locking them together, then making the eccentric movement roughly the same as one tooth pitch. If this is right it becomes a bit like a cheaty strain wave gear in my mind?

It turns out that it is quite hard to do so efficiently. Tooth height is about 2.25 x module, so for say module 1mm - we need more than 2 tooth difference - just to achieve clearance on the opposite side. Then there is issue of interference. It turns out that if we have internal and external gear that have small difference in tooth count - we will have interference unless we use large pressure angle and introduce all sorts of backlash and additional clearances.

For above demonstration I used outer gear with 80 teeth and inner with 75 teeth and I needed to use 28 degrees pressure angle and to shorten the teeth somewhat to avoid interference.

That makes whole thing something like 1:16 reduction, if I'm calculating things correctly as 80 / (80-75) - each wobble of "runner" gear (one revolution of input shaft) advances it by 5 teeth on a circle of 80 teeth.

Cycloidal drive of the same size can be made to much higher reduction ratio and it operates on the same principle - except it does not use teeth but rather "lobes" and semi circles.

so same thing - except for the "shape" of the "teeth"

 

[vlaiv]

20 minutes ago, Thalestris24 said:

Very altruistic of you! Maybe you could offer a 'deluxe' version too? I've not done any calculations but wondering whether it could be possible to simplify it with a direct drive rather than gearing? If it's just a camera and lens you wouldn't need the torque that you'd presumably get from gearing? Just thinking out loud...

Louise

What would you consider to be deluxe version?

I'm certainly going to explore behavior of that split ring compound planetary system - and see how it behaves from periodic error and smoothness point of view. I was thinking of maybe doing "poor man's EQ5" kind of thing as one of my next projects - something that will have ASCOM control and be completely motorized with ability to be guided and of course decent payload.

As far as calculations for direct drive go - it might almost be doable with 256 micro steps and 0.9 degree motor - but do keep in mind that torque at those settings is seriously small and one would need to "rev up" things before switching to such fine control. Anyway - here is quick calculation

360 x 60 x 60  = 360 x 3600 = 1296000 arc seconds per circle

400 steps x 256 micro steps - 102400 steps per circle

Ratio of the two is 12.65625" / step

Now, that might be better than using x500 rule and static tripod, but I'd add at least one belted stage like 5:1 to improve things.

[Thalestris24]

1 minute ago, vlaiv said:

What would you consider to be deluxe version?

I'm certainly going to explore behavior of that split ring compound planetary system - and see how it behaves from periodic error and smoothness point of view. I was thinking of maybe doing "poor man's EQ5" kind of thing as one of my next projects - something that will have ASCOM control and be completely motorized with ability to be guided and of course decent payload.

As far as calculations for direct drive go - it might almost be doable with 256 micro steps and 0.9 degree motor - but do keep in mind that torque at those settings is seriously small and one would need to "rev up" things before switching to such fine control. Anyway - here is quick calculation

360 x 60 x 60  = 360 x 3600 = 1296000 arc seconds per circle

400 steps x 256 micro steps - 102400 steps per circle

Ratio of the two is 12.65625" / step

Now, that might be better than using x500 rule and static tripod, but I'd add at least one belted stage like 5:1 to improve things.

A belt drive would be interesting! My heq5pro was excellent with its belt drive mod! Of course, with proper eq mounts you're into dual axis motors anyway, really - a whole different ball game!

Louise

[pipnina]

Re: Belt drive

If you used a reduction from motor shaft to an intermediary shaft with one belt, then put the smaller size pulley on the intermediary shaft above the larger pulley, you can then have a second thicker belt attach between that small pulley and a very large pulley for the RA axis shaft.

I found pretty cheap pulleys that are very thick (2cm thick 45cm length). Thicker pulleys means less stretch effect as well, and if tensioned properly, no appreciable backlash.

For a "poor man's EQ5", maybe doubling up the pulleys would be necessary to reach the 10KG imaging limit without stretching... Printed pulleys probably would let us get a bigger reduction than the pulleys I've seen available commercially too.

For a larger mount especially however... A clutch or brake needs to be considered. Worm mounts naturally cannot move without the motor driving them as the worm gear can drive a gear but cannot be driven by it. This protects our equipment from the consequences of poor balancing and random power or software failures!

For the small tracker belts could still work though, maybe easier as the free-fall problem isn't as much of a concern for small 1-3kg payloads.

[vlaiv]

19 minutes ago, pipnina said:

If you used a reduction from motor shaft to an intermediary shaft with one belt, then put the smaller size pulley on the intermediary shaft above the larger pulley, you can then have a second thicker belt attach between that small pulley and a very large pulley for the RA axis shaft.

Good idea - I can actually use larger pulley on RA shaft for additional boost in resolution and use smaller ones in between - two of them in total.

People can then opt to purchase smaller pulley sets or print them (but they will need to print large RA pulley anyway).

I still need to make room for electronics - or that might go into separate box - since all we need at tracker is just power to stepper motor and that is just a single cord with 4 wires. I think that AZ-EQ5 has something similar going to DEC motor:

23 minutes ago, pipnina said:

For a larger mount especially however... A clutch or brake needs to be considered. Worm mounts naturally cannot move without the motor driving them as the worm gear can drive a gear but cannot be driven by it. This protects our equipment from the consequences of poor balancing and random power or software failures!

Yes, just making mechanical side of things without motor assemblies will be a bit of a challenge.

It will require some additional DIY skills. I think that most will be done with aluminum tubing which will need to be cut to size and some small holes drilled in.

Then there is matter of counterweight shaft and counterweights.

I know how to make that - in theory, but never done in practice. Say brass or even steel rod is taken and then on one side is drilled with drill, tapped and piece of threaded rod is screwed / glued in. Similarly on other side, short nut is screwed in similar hole to act as stop.

Dumbbell weights can be used for counter weights - but those also need drilling and tapping and possibly 3d printing some sort of centering insert as most have 30mm bore diameter and we'll be using something like 20mm shaft.

That sort of project is really on next level, and I do hope to do it one day - but it will present whole set of new challenges.

[RobinH]

I have used weight lifting type weights then filled the large centre hole with polymorph. when its cooled you can file it flat then drill a hole to suit. I used these for my binocular parallel mount weights. They can often be bought from charity shops or car boot sales.

Robin

[vlaiv]

Belts arrived, and I made first test of printed pulleys and how they work with belts:

I'm happy with results so far.

Idea is to have 3 stages. Two stages will be 5:1 reduction (80 teeth to 16 teeth) and last stage will be 10:1 (160:16). That will give total of 250:1 or 0.81"/step.

3 belts are required - 2x200mm and 1x360mm - those are supposed to be standard sizes.

Whole thing sounds like it's playing space invaders - very distinct "8bit" squeal is heard from the motor, but I guess that is from 18 steps per second or step about each 50ms. One can almost hear pitch changing like sine wave as motor is doing micro stepping.

Major hurdle now is to design easy to assemble and rigid box with ability to adjust tension on belts. I won't be using idles idlers unless I absolutely must - as those require additional bearings.

Edited March 16, 2023 by vlaiv
typo

[Thalestris24]

35 minutes ago, vlaiv said:

Belts arrived, and I made first test of printed pulleys and how they work with belts:

I'm happy with results so far.

Idea is to have 3 stages. Two stages will be 5:1 reduction (80 teeth to 16 teeth) and last stage will be 10:1 (160:16). That will give total of 250:1 or 0.81"/step.

3 belts are required - 2x200mm and 1x360mm - those are supposed to be standard sizes.

Whole thing sounds like it's playing space invaders - very distinct "8bit" squeal is heard from the motor, but I guess that is from 18 steps per second or step about each 50ms. One can almost hear pitch changing like sine wave as motor is doing micro stepping.

Major hurdle now is to design easy to assemble and rigid box with ability to adjust tension on belts. I won't be using idles unless I absolutely must - as those require additional bearings.

The noise might be linked to the driver hardware - similar to some 3D printers. Still, looking good!

Louise

[Thalestris24]

Have attached pdfs for heq5 and neq6 Rowan belt kits - in case of interest. Both have the worm drives, of course - illustrated better in the neq6 pdf.

Louise

HEQ5-Belt-Kit.pdf

NEQ6-Belt-Kit.pdf

[pipnina]

1 hour ago, vlaiv said:

Belts arrived, and I made first test of printed pulleys and how they work with belts:

 

I'm happy with results so far.

Idea is to have 3 stages. Two stages will be 5:1 reduction (80 teeth to 16 teeth) and last stage will be 10:1 (160:16). That will give total of 250:1 or 0.81"/step.

3 belts are required - 2x200mm and 1x360mm - those are supposed to be standard sizes.

Whole thing sounds like it's playing space invaders - very distinct "8bit" squeal is heard from the motor, but I guess that is from 18 steps per second or step about each 50ms. One can almost hear pitch changing like sine wave as motor is doing micro stepping.

Major hurdle now is to design easy to assemble and rigid box with ability to adjust tension on belts. I won't be using idles idlers unless I absolutely must - as those require additional bearings.

Perhaps the 3d printed belt tensioner for 3d printers could be adapted a bit to fit the motor, allowing people to just twist a little thumb screw and tension/relax the belt?

Or at least, the principle of it anyway. https://www.thingiverse.com/thing:4718212

[Thalestris24]

12 minutes ago, pipnina said:

Perhaps the 3d printed belt tensioner for 3d printers could be adapted a bit to fit the motor, allowing people to just twist a little thumb screw and tension/relax the belt?

Or at least, the principle of it anyway. https://www.thingiverse.com/thing:4718212

I'm thinking a belt tensioner not needed. When I did the heq5 belt mod I recall needing a couple of goes to get the belt tension right. After that I never needed to adjust it again. For a star tracker with no significant load, even less need?

Louise

[pipnina]

1 minute ago, Thalestris24 said:

I'm thinking a belt tensioner not needed. When I did the heq5 belt mod I recall needing a couple of goes to get the belt tension right. After that I never needed to adjust it again. For a star tracker with no significant load, even less need?

Louise

That's fair. I just know that when belts are assessed on printers it seems to show tighter is better, reduces vibration and ghosting etc. There isn't a huge load there either really, but if high tension proves important it might be a relatively easy fix.

Vlaiv already has a long slot for the motor to be moved around so in theory hand tightening could be done easily enough. Like you say it took me a few goes to get the rowan mod right but I managed it by hand...

I dunno

[vlaiv]

19 minutes ago, pipnina said:

Perhaps the 3d printed belt tensioner for 3d printers could be adapted a bit to fit the motor, allowing people to just twist a little thumb screw and tension/relax the belt?

Or at least, the principle of it anyway. https://www.thingiverse.com/thing:4718212

That is very similar to idler - or in fact - it is an idler with screw to pull it back.

Two more bearings and in principle, like Louise said - no need for constant tension adjustment, nor for very big pull on belt. Here, belts are rather small compared to those on 3D printer.

Standard way is to have grooves instead of fastening holes for motor and other things and you manually pull them back until there is sufficient tension on belt so that it engages teeth on pulley properly and that is about it.

Here is solution that I employed in above example:

Motor is fastened thru those slots - but only barely until everything is assembled - then motor is pulled back for tension to form on belt and then bolted down properly.

In fact - I might even put a bit more tension than needed in this proof of concept - top and bottom plate do have some flex in them and it seems that whole assembly is ever so slightly bent because over tensioning the belt.

 

[vlaiv]

Problem with this approach is that I'll have 3D printed pulleys to tension next - and those are not very stiff, or rather - they should be supported on both sides of their rotational axis before attempting to put tension on the belt - otherwise axis will tilt.

I'll probably solve it by building "cages" that will hold pulleys and then those will move in slots in the base.