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My 2nd Onstep - harmonic driven :)


simingx@...
 
Edited

After my first Onstep on my EM10 failed due to the mechanicals of the EM10 being not up to snuff, I have embarked on a new project :)

Main output gearheads are SHF 17-80-2SO, 80:1 harmonic drives.
The steppers are the StepperOnline NEMA 17 motors with 30:1 "High Precision" gearing. Total ratio is then 2400:1 which gives ~0.17arcsec of resolution (according to the spreadsheet).
Mechanical parts were designed (painfully) in TinkerCAD and manufactured locally. Not cheap though...
Tangent arm is a Taobao/Aliexpress "Hercules" unit as I was too lazy to design one. :p

There is absolutely no backlash in the drivetrain, and I cannot stall the motors with my hand strength, even with a Vref of just 0.8A (The motors are rated for 1.6A). Note that this is only possible because the harmonic drives have an extremely high efficiency, in the region of 90% or so.

Links to components:
Harmonic drives: https://www.ebay.com/itm/Harmonic-Drive-Gear-Reducer-HD-17-80-Ratio-80-1-Cross-roller-bearing-input-8mm/254100423402?ssPageName=STRK%3AMEBIDX%3AIT&_trksid=p2057872.m2749.l2649
The seller occasionally has some of these pop up, so even if it says "Sold Out" keep an eye out for them on his page, you might get lucky :)

Motors: https://www.omc-stepperonline.com/precision-planetary-gearbox/
Many ratios to choose from, but NEMA 17 is the smallest size that has the "precision" planetary box. Do note that the gearboxes have a keyed 8mm output shaft, so remember to buy matching pulleys if you are going to use them in another application.

Tangent arm: https://www.aliexpress.com/item/S8177-2-Stronghold-Tangent-Assembly-set/32822479423.html?ws_ab_test=searchweb0_0,searchweb201602_1_10065_10068_319_10059_10884_317_10887_10696_321_322_10084_453_10083_454_10103_10618_10307_537_536,searchweb201603_52,ppcSwitch_0&algo_expid=17c42620-c5ea-4d24-90ab-4fd6a7320079-2&algo_pvid=17c42620-c5ea-4d24-90ab-4fd6a7320079&transAbTest=ae803_3
[Update: I ended up changing to a Astrotrac wedge as the Aliexpress one wasn't sturdy enough. Discontinued but looks like 365astronomy still has them: https://www.365astronomy.com/AstroTrac-TW3100-Telescope-Mounting-Wedge.html
Edit: 19 July 2019: First light report: https://groups.io/g/onstep/topic/first_light_of_my_onstep_ed/32323884?p=,,,20,0,0,0::recentpostdate%2Fsticky,,,20,2,0,32323884
Edit: 25 July 2019: First images here: https://groups.io/g/onstep/message/11726


Testing under the stars will have to wait a while more, but I do really hope it works as the mount does not need counterweights...


Khalid Baheyeldin
 

Thanks for letting the group know. This is a great build. Amazing that backlash is zero.
I added it to the show case page on the Wiki.

Please provide links to the parts (motor, tangent arm, harmonic drive) so it is easier for others to find them. It would be best to edit the original post, so the info is in one place, rather than a thread.

Good to know there are motors out there that are smaller, have 30:1 reduction, and can run at a relatively cool 0.8A, yet have zero backlash. They should work with EQ5 mounts with a 1:1 pulley, and provide a fine 0.19" resolution (with 1/8 microstep, or 0.09" with 1/16).


tnut55
 

Cool mount.

Mark


On Sat, May 4, 2019 at 12:10 PM, Khalid Baheyeldin
<kbahey@...> wrote:
Thanks for letting the group know. This is a great build. Amazing that backlash is zero.
I added it to the show case page on the Wiki.

Please provide links to the parts (motor, tangent arm, harmonic drive) so it is easier for others to find them. It would be best to edit the original post, so the info is in one place, rather than a thread.

Good to know there are motors out there that are smaller, have 30:1 reduction, and can run at a relatively cool 0.8A, yet have zero backlash. They should work with EQ5 mounts with a 1:1 pulley, and provide a fine 0.19" resolution (with 1/8 microstep, or 0.09" with 1/16).


simingx@...
 

Hi Khalid,
I've edited the post to add links to the parts.
I left out though, that the "precision" gearboxes have an 8mm shaft (with key) so you have to find pulleys with the corresponding hole.
I'm not sure also that the motors will have enough torque to run a EQ5-style mount at 0.8A, someone will have to try it.

I tried running the motors in StealthChop mode for Gotos, but they stalled... so back to spreadCycle for now.
The planetary gearboxes are also quite noisy at high speeds...


simingx@...
 

I'm sorry I had to edit it again.. the gearheads should be SHF 17-80-2SO. It's 1:45AM here...


Khalid Baheyeldin
 

On Sat, May 4, 2019 at 01:41 PM, <simingx@...> wrote:
Hi Khalid,
I've edited the post to add links to the parts.

Thanks. One confusing bit is that you say NEMA 14, then in the edit you say only NEMA 17 have the precision gears.
If the 14 is an error, then please edit again to fix that part.

I left out though, that the "precision" gearboxes have an 8mm shaft (with key) so you have to find pulleys with the corresponding hole.
That is in the motor spec, so someone buying it has the info before ordering the pulley. So OK to leave that info out.

I'm not sure also that the motors will have enough torque to run a EQ5-style mount at 0.8A, someone will have to try it.
Assuming everything else is equal: your overall reduction is 30 x 80. For an EQ5 it is 30 X 144. So anything that can run your mount should be able to run an EQ5 as well, since there is more reduction for the EQ5.

I tried running the motors in StealthChop mode for Gotos, but they stalled... so back to spreadCycle for now.
I will leave this for those who use TMC2130 to comment on whether this is expected or unusual.


simingx@...
 

Hi Khalid,
14 was an error, the correct size should be NEMA 17. 
Worm drives have a very low efficiency (~10-15% as the reduction works on a "sliding" action). This is the reason why mounts based on them need counterweights, so that the motor "sees" only a little bit of counter torque from the rest of the drive train.

Harmonic drives (and other "meshing/pushing" gears such as planetary gearboxes) can transmit much more of the torque to the output, therefore they do not need counterweights.

However they can still benefit from counterweights, if you balance the setup properly you can support as much load as the cross roller bearing in the harmonic drive will support :)
Pretty amazing for something that size!

The benefits of the harmonic drives in our application (besides the zero backlash) are:
They have a nice large output flange to attach stuff onto
They have a low profile so they don't extend the motor too much
They have a nice output roller bearing that can support substantial weight, thus making it easier to design the mechanical parts


Butchf
 

Very cool.  you did a great job.


simingx@...
 

A quick test tonight: swinging my Borg 107FL. Absolutely no effort although I did need to use 24V for the motors. I think they can do with a slightly higher Vref..
There is some play in the tangent assembly though, but that's not surprising given that it's only supported on one end. I should be able to hack design something out before I bring it out under the stars...


Butchf
 

Great Job


simingx@...
 

A quick guiding graph from last night, as you can see the alignment is horrible but the mount is really responsive!
Hope it works out during actual conditions...


Jay Murphy
 

Nice! Yeah with some better alignment you'll be on the money. I can't wait to start field testing my CGE build.


On Mon, May 27, 2019, 9:14 PM <simingx@...> wrote:
A quick guiding graph from last night, as you can see the alignment is horrible but the mount is really responsive!
Hope it works out during actual conditions...


Zubuyer Kaolin
 

I have a question; how do you polar align on this setup?


simingx@...
 

I do it via drift alignment, pretty much the only option at my latitude. The "Refine PA" function works pretty well too :)


James McPherson
 

Just wanted to say that this is awesome. I'm going to follow in your footsteps and give this a try.


simingx@...
 

On Sat, Oct 12, 2019 at 08:54 AM, James McPherson wrote:
Just wanted to say that this is awesome. I'm going to follow in your footsteps and give this a try.
Go for it, it's well worth the effort for not carrying counterweights around :)


Howard Dutton
 

Just dragging this nice little harmonic drive mount back up so Jerry can see it.


simingx@...
 

I haven’t had as much time under the stars as I’d like, but it worked very well for the recent solar eclipse :) 


Jerry
 

Ahh. Nice and simple. Now just stick a couple small brushless servo motors on there. ;)

And way lighter than my mount. Lol. 

-Jerry

On Jan 6, 2020, at 6:39 AM, simingx@... wrote:

I haven’t had as much time under the stars as I’d like, but it worked very well for the recent solar eclipse :) 
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Howard Dutton
 

RA is a little jumpy in the guiding graph, fast gear noise I guess.  How's the backlash behavior... heavily loaded so responsive in many orientations for RA but what about Dec?