My Planned Onstep Conversion using Teknic ClearPath Motors on a G11 mount


Peter Boreland
 

I've not seen anyone use the Teknic ClearPath motors in this forum, so I thought I'd start this thread to chronicle this adventure.

The motor I've settled on is model  CPM-SDSK-2310S-EQN. It has a NEMA23 frame, 1/4" shaft, is driven by TTL signals step and direction, has a programmable output that can provide an index for PEC, has a built in 6400 tick optical encoder, and has its own power input (settled on 48V). 

The main reason I chose to adopt this motor is the ability to obtain 0.56" step accuracy with a heavy mount load. As I understand things, microstepping has limitations in terms of achieving this degree of resolution under load, and there is the possibility of step to step variation and lost steps. The downside is the 6400 tick optical encoder. Ideally, I would like to do better than this for the Dec axis, therefore I'm considering using  closed-loop stepper motor controller like the MKS SERVO42B (NEMA17)  or the SERVO57B (NEMA23) motor where step-to-step accuracy is not as important. 

Any thoughts would be appreciated!

Peter


Khalid Baheyeldin
 

First impression here is that this is overkill.
Other G11 users have used regular NEMA17 with great success.

Besides, if you increase the load, the motors are not the only things that matter.
The gears and bearings are also important.


Drew 🔭📷🚴‍♂️
 

On Wed, Dec 8, 2021 at 12:04 PM, Khalid Baheyeldin wrote:
Besides, if you increase the load, the motors are not the only things that matter.
The gears and bearings are also important.
I absolutely agree. Most of these mounts came with much weaker motors than we typically use for OnStep. When these original motors hit a mechanical issue they would behave as a clutch and simply stall.

Using higher torque motors to "power thru" a mechanical issue will only lead to disaster. I know, i have been there, done that and have the tee-shirt (or at least the ruined parts).


Peter Boreland
 

On Wed, Dec 8, 2021 at 09:04 AM, Khalid Baheyeldin wrote:
Besides, if you increase the load, the motors are not the only things that matter.
The gears and bearings are also important.
Khalid,

That's well understood.  In theory the G11 can carry 70lbs including weights for imaging. I will be in this range with my scope and associated equipment. I know several people who carry more weight than this and get good guiding results. I'm expecting the NEMA17 motor to handle the Dec OK. The Clearpath motor is for the RA and is the smallest size available. I already have it fitted and it sticks out no further then the original Losmandy motor. It would not work in a tucked design however. I'm at the limit of what I think can be achieved with a gearbox based design (higher frequency noise components) at with a consistent a total rms tracking error of around 0.5" +/- 0.15" depending on seeing etc; hence, the move over to a direct drive implementation. Also, Gemini  2 has proven unreliable for me, and would like an indexed PEC implementation.

Peter


Peter Boreland
 

On Wed, Dec 8, 2021 at 09:36 AM, Drew 🔭📷🚴‍♂️ wrote:
I absolutely agree. Most of these mounts came with much weaker motors than we typically use for OnStep. When these original motors hit a mechanical issue they would behave as a clutch and simply stall.

Using higher torque motors to "power thru" a mechanical issue will only lead to disaster. I know, i have been there, done that and have the tee-shirt (or at least the ruined parts).
This is of course absolutely correct on both points. One thing about the Clearpath design is the ability to shut the motor down in software based on current draw. It can be programmed and tuned via USB connection. So experimenting will be needed, but that's the engineering fun of it I guess. 


Khalid Baheyeldin
 

On Wed, Dec 8, 2021 at 12:47 PM, Peter Boreland wrote:
I will be in this range with my scope and associated equipment. I know several people who carry more weight than this and get good guiding results. I'm expecting the NEMA17 motor to handle the Dec OK. The Clearpath motor is for the RA and is the smallest size available. I already have it fitted and it sticks out no further then the original Losmandy motor. It would not work in a tucked design however.
A well balanced scope can be driven by a small motor, provided the slewing speeds are not too high.
The example we always quote is that the 200 inch mirror Hale telescope at Palomar (530 tons) used to be driven with a 1/12 horse power motor in RA for decades. It now uses a 1 HP motor, but still the point stands. 

I'm at the limit of what I think can be achieved with a gearbox based design (higher frequency noise components) at with a consistent a total rms tracking error of around 0.5" +/- 0.15" depending on seeing etc; hence, the move over to a direct drive implementation. Also, Gemini  2 has proven unreliable for me, and would like an indexed PEC implementation.
If you are going to autoguide, then gearboxes don't matter at all.
I have an 18:1 gearbox on a tiny NEMA11 motor, and they drive my Vixen SXD with a C8, large 240mm guide scope, Canon DSLR, and guide camera. It guides pretty well, limited by the sky really.

Not saying you should use gearboxes. Rather, they can be overcome by autoguiding.
Use direct drive motors by all means, but NEMA23 are not needed, and the encoders on the Clearpath add no value.

Bottom line is that you can run the math and go into what theoretical limits all you want. But in the end what matters is actual practical information from those who did it.


Peter Boreland
 

On Wed, Dec 8, 2021 at 10:01 AM, Khalid Baheyeldin wrote:
If you are going to autoguide, then gearboxes don't matter at all.
I have an 18:1 gearbox on a tiny NEMA11 motor, and they drive my Vixen SXD with a C8, large 240mm guide scope, Canon DSLR, and guide camera. It guides pretty well, limited by the sky really.

Not saying you should use gearboxes. Rather, they can be overcome by autoguiding.
Use direct drive motors by all means, but NEMA23 are not needed, and the encoders on the Clearpath add no valu
I agree a well balance setup requires very little effort to turn the worm, but on the issue of guiding it is the higher frequency stuff attributable to gears that places a limit on what can be achieve I feel. While these higher frequencies are individually small, they can and do add constructively. In my experience guiding can not really remove them. I've attached an image of my Ra time response from two weeks ago here in the Northeast when seeing was poor. Every significant spike smaller in time than 76s is attributable to the gear/motor train. This is what I'm hoping to remove. No PEC was running.


John Scherer
 

Peter, what optics and what focal length are you intending to use on this mount?


Robert Benward
 

Peter,
Your choice of motors are interesting.  I have not heard anyone mention this, but I believe this is a DC servo motor, that, with the encoders, achieve a stepper like interface.  Am I right in my understanding?  I believe the encoders provide the only feedback the system has. 

It sounds like you already own it and it is installed on your mount.  Keep us posted, I would like to hear how you make out with it.   

Best of luck,
Bob


Rockmover
 

Hi Peter and great to hear about another OnStep G11 user! 

IMHO the G11 is a really nice little mount, and OnStep is a fantastic upgrade.  I've been using my G11 pretty much solid the past year remotely and overall am really happy with it.  

I agree with the above from Khalid and Dave, I have a Nema 23 on my AP1200 mount, and will be using it on the RA axis of an even larger mount I'm now working on, but for a little G11 a NEMA 17 is perfectly suited in my experience (I have had a few different scopes and setups on it).  The limits of the mount will be the issue as the loading gets higher, and not your installed motor torque.  

With that said, I personally would go with this one - PKP246MD15A2 as it has both good resolution and torque for the 17 series.  Smaller Nema 17's are ok, but personally I wouldn't consider using them (especially if your going to drive it directly with no belt reduction).  But again, that is simply my opinion for various reasons. 


As for limits, I am now a big fan of adding the mechanical limit switch's to the mounts.  Software can (and does) often have bugs, and the hardware solution that Howard has implemented is very nice.  Limit switches are some extra install time, but only cost a few dollars, and well worth the effort I believe.

I am actually in the process of finishing a 3D print for my Ed Byers mount. It will use both the homing switch and limit switches with a cheap non contact optical sensor from amazon (<$5).  Once that's done I'm definitely adding them to my G11 mount (hopefully in the next coupe weeks).   In any case, I'd be happy to send you my 3D files for it as I progress. 


Here are a couple G11 videos you may find useful. 
https://www.youtube.com/watch?v=DKkbr5xx2r4
https://www.youtube.com/watch?v=yEvuvyM_T84
https://www.youtube.com/watch?v=r9pGQRS9bSE&t=328s

And finally, on the encoder side, that all depends on if your doing visual or photography.  If its visual, then not losing steps is indeed a nice feature since you can unlock the scope and move it around by hand. But the encoders then need to be mounted on the shafts, and not the motors. If your doing photography, then you are almost certainly always going to do a plate solve when slewing the scope to the exact spot you want. Thus encoders are all but worthless.  It doesn't matter if you lose some steps along the way as you slew to an object.  Plate solving simply corrects that on its first iteration.  Then once your imaging, your guide star is the feedback and thus encoders again don't help in any meaning way.

Adding homing switches is a much better option than adding encoders IMHO anyway.  I have CUI ATM22 - 14 bit 16384 plus encoders - on my AP1200 mount, but don't even use them anymore.  If things get messed up you can simply go home, and then plate solve if needed.  

Just my 2 cents, and good luck with your build!


Robert Benward
 

On Wed, Dec 8, 2021 at 04:02 PM, Rockmover wrote:
And finally, on the encoder side,
I think we are missing a big point here, what Peter selected is a servo motor that uses encoders to close the loop.  The encoders are necessary for the servo motor system to work, and are self contained and not an option.  Once the loop is closed, it imitates a stepper by responding to step and direction inputs.  It will not work without the encoders.  This is why it cost $300.

https://teknic.com/model-info/CPM-SDSK-2310S-EQN/

Bob


Khalid Baheyeldin
 

On Wed, Dec 8, 2021 at 06:02 PM, Robert Benward wrote:
I think we are missing a big point here, what Peter selected is a servo motor that uses encoders to close the loop.  The encoders are necessary for the servo motor system to work, and are self contained and not an option.  Once the loop is closed, it imitates a stepper by responding to step and direction inputs.  It will not work without the encoders.  This is why it cost $300.
And that reveals another issue with such a design ...

With an encoder of only 6400 ticks, positioning will be way too coarse.
If 6400 is assume per rotation, that is 3.375 arc MINUTES resolution).
Not suitable for astrophotography at all, if you rely on the 'servo' for positioning.

Again, the advise from all of us is to use what is been proven to work for many others, rather than what is better on paper, but not proven for THIS particular application.


Robert Benward
 

Khalid, Peter,
That's around a 400 step motor with a 16 microstep.  6400/400=16. 

I don't agree with your resolution. You divided the whole worm wheel by 6400.  The losmandy worm is 1 degree per rev, and about 3600 arcseconds per rev.  So I think that is a little more than 1/2arcsecond resolution.

But, I do agree, take advantage of the work that has been proven, unless of course you want to have fun.   I just converted my G11.  I am using a NEMA 17 400 step motor with a precision 10:1 planetary (<15arcmin backlash).  The planetary gives me plenty of torque, although slew rate will be probably be limited to 1.5deg/sec max.

Bob


Khalid Baheyeldin
 

On Wed, Dec 8, 2021 at 07:56 PM, Robert Benward wrote:
That's around a 400 step motor with a 16 microstep.  6400/400=16. 

I don't agree with your resolution. You divided the whole worm wheel by 6400.  The losmandy worm is 1 degree per rev, and about 3600 arcseconds per rev.  So I think that is a little more than 1/2arcsecond resolution.
It depends on what 6400 ticks means. Is it per degree, or per rotation (CPR).
If it is per rotation, then it is pretty low resolution.
If it is per degree, then yes, it is around 0.5", which is great for low to moderate focal length/pixel scale.
Will it be enough for more? Who knows ...


Robert Benward
 

Agreed.  In the case of the Losmandy, 6400 counts connected directly will lead to the 0.5".  On your other point, once you go this way you loose the latitude to change any resolution.  If you go with a stepper, you still can mess with microstepping.  This servo motor, although it takes dir & steps, it will not take microstepping.

Bob


John Petterson
 

Peter,

As others have already said, that motor looks like overkill to me too although I have not yet converted my G11.  I have added OnStep to both a GM8 and an AZ8, and I detect very little difference in the effort required to move these 3 mounts manually so I don't expect much of an issue with the same motors I used on the other two.  I ended up adding encoders from Astro Device to both of these mounts and probably will also add them to the G11 when I get that one done.  The encoders allow me to nudge the mount to anything I want to look at without losing the alignment, and also help to recover from any lost steps in the movement.

John


Peter Boreland
 

On Wed, Dec 8, 2021 at 12:26 PM, John Scherer wrote:
Peter, what optics and what focal length are you intending to use on this mount?
John,

My current setup is the Esprit 100mm, so 550mm focal length. I'll be mounting my 12" truss Newt for galaxy season.  That has a focal length of I believe of 1200mm/F4. It weights in at 50lbs, but there there is all the other stuff. The focuser is very heavy, at least 7 or 8 lbs, I use OAG so no guide scope. 

Peter


Khalid Baheyeldin
 

On Thu, Dec 9, 2021 at 12:00 PM, Peter Boreland wrote:
My current setup is the Esprit 100mm, so 550mm focal length. I'll be mounting my 12" truss Newt for galaxy season.  That has a focal length of I believe of 1200mm/F4. It weights in at 50lbs, but there there is all the other stuff. The focuser is very heavy, at least 7 or 8 lbs, I use OAG so no guide scope.
Your ClearPath motors have a minimum resolution of ~ 0.5", and you can't go lower since you only have a Step/Dir interface (no way to control microstepping, and no intermediate gear reduction).

Maybe that is fine with 550mm focal length.
But I would be surprised if you end up with subpar quality at 1200mm.

As I said earlier, I do image at 1310mm (C8 + reducer + 2 inch adapter), with an 18:1 gearbox.

This is the Lagoon Nebula, single exposure of 300 seconds, autoguided. No stacking, no darks, no processing apart from stretching.


Peter Boreland
 

On Wed, Dec 8, 2021 at 01:02 PM, Rockmover wrote:

Rockmover,

The PKP246MD15A2  looks like a quality motor. I'll be sure to order one for my Dec.

Great set of videos. I see you went belt drive. Might I ask what pulley and belt parts you used?

I might go this way right off the bat for the Dec axis. Really I want at least 0.14" step resolution. My testing has shown a significant performance increase in guiding. Going from 0.56" to 0.14" saw at least a 0.35" improvement in Dec guiding performance for my G11 (this makes sense as PHD2 has less opportunity to over correct in Dec). I achieved this by changing the optical encoder in the Losmandy motor from 256 to 1024. In regard to Ra, I saw no significant performance improvement reducing the step size below 0.56", hence I'm OK with what I can achieve with the Clearpath motor. 

As for limits, I am now a big fan of adding the mechanical limit switch's to the mounts.
I think this is a must. I have had several near disasters running Gemini 2, with it doing all manner and strange things oftens pointing the scope at the ground and running through limits. I just have to figure out a good way to mount the limit switches. How did you do it?

"And finally, on the encoder side,"

To be clear the encoder is mounted in the motor. It's the highest resolution one I could find built into a motor. Apart from not losing steps, the reason  I like this design is the motor will hold its position precisely. I have no experience with microstepping as such, but from what I've read the motor has very little holding power (torgue) and thus there is a practical limit of the number of micro steps one can really use (16/32).  The good thing is I'll have the opportunity to test this setup against a stepper motor. That said I see real advantage to your tucked motor design for Ra. The Clearpath motor is too large to achieve this. It might be a case of builder it both way and running a comparison test, if I can draw myself away on a clear night from imaging. That said your geared approach would increase the motor torque, where as I'm direct connected to the worm.

I do have the three post knob. Are the old knobs available? Would you print off an extra tightener? But then again I could unscrew the posts.

Peter


Howard Dutton
 
Edited

On Thu, Dec 9, 2021 at 09:36 AM, Peter Boreland wrote:
The PKP246MD15A2  looks like a quality motor. I'll be sure to order one for my Dec.
And its been tested with TMC2130 drivers: