Geared or Non-Geared stepper motors for focuser ?

[Astro-Geek]

I thought I'd start yet another new thread to concentrate on this specific aspect of remote focusers.  (sorry ! 👀)

Further to my previous threads on my build of Robert's myFocuserPro2 , I'm now putting it through its paces on my scopes.

I made my attachment brackets modular with a standardised quick-release fixing method so that I can attach either of my two Nema 17 motors to any scope.

The two Nemas are a simple non-geared Nema 17 with a 2 amp rating, and a 27:1 geared Nema with a 0.4 amp rating.

Either of them runs happily via Robert's myFocuserPro2 controller.

Robert's general  recommendation is for the 27:1 geared model, and many users expressed a preference for the the geared model due to it's greater holding power with the coils energised.

I have to admit though, I much prefer the simpler non-geared Nema 17.  When I run it in half-stepping mode it hums along nicely, has plenty of torque to turn my SCT crayford dual speed focuser, even with the tension knob set to quite firm to avoid slippage of the mechanism.

The motor is attached directly via a flexible drive, so there is no gear train slack.

The range of movement is still very fine, with plus/minus 100 steps getting you quite close, then plus/minus 10 steps being as fine as I need to go, so the plus/minus 1 step is almost imperceptibly fine.

When I try the same SCT crayford focuser with the 27:1 geared motor, the reduction in speed becomes more of a hindrance than an advantage, and the gearbox adds significant weight and bulk.  Also, when the geared one is connected to the focuser shaft, it's impossible to turn the manual knob, even with the coil current off, whereas the non-geared Nema can still be turned manually, if the coil power is off.

When I searched the subject of focusing motors, opinions varied widely, with one person describing the 27:1 as "Mega - Uber - Overkill" ! and others feeling that even the Nema 17 itself was overkill, and that smaller steppers could be used.

All opinions on this gratefully received....  🤓

[Dr_Ju_ju]

One big advantage of the geared motors is the holding torque of the motor with NO power applied, thereby cutting down the possibility of things getting hot, especially motors with a constant current.....

I've used Nema 14 & 17's, and both work well, the larger the loads tend more towards using the 17's.

I've just taken delivery of a Nema 11 PG27 for use on a DSLR rig, its only 28x28mm but quite long @ 86mm.

 

[markse68]

I’ve just started a new project to build a motorised manual focuser for my dob and given it’s an f7.3 I reckon I should be able to get away with direct driving the coarse focus knob of a Moonlite with a Nema17 400 step- I found a nice Sanyo one on eBay cheap, and it should have enough torque and resolution- hopefully 😉

The idea of constantly fighting gear train backlash  when changing direction didn’t appeal at all - but then I’m only going to be using it visually so hopefully holding torque won’t be an issue with the motor disabled for battery saving

Edited September 27, 2019 by markse68

[JamesF]

Good that so many other people have tried this project.  Should mean there's plenty of advice to be had when I start mine

James

[Astro-Geek]

1 hour ago, Dr_Ju_ju said:

One big advantage of the geared motors is the holding torque of the motor with NO power applied, thereby cutting down the possibility of things getting hot, especially motors with a constant current.....

I've used Nema 14 & 17's, and both work well, the larger the loads tend more towards using the 17's.

I've just taken delivery of a Nema 11 PG27 for use on a DSLR rig, its only 28x28mm but quite long @ 86mm.

 

I can appreciate the strong holding torque of the geared motor, even without the focuser powered up.

That's the problem for me though, it's quite immovable manually with the motor clamped to the shaft.

So far I've experimented with coil holding current on and off, (via the remote option in Robert's operating software).

My motor doesn't seem to get hot when the coil is left on, but even when I turn it off, the normal friction resistance of my Crayford holds my ZWO camera with no problems.  Maybe with a heavy DSLR it may be a problem though, I must admit.

[stash_old]

Most "hot" steppers are caused by having Microstepping which requires current to "hold" the position otherwise the Stepper will "snap" to a detent position. Unless you have set he current wrong - quote from REPRAP site "Rule of thumb, if you can put your hand on the motor but have to remove it, it's hot but not too hot. If you can't touch it without being burned... probably a bit too hot, back off on the current. If you can keep your hand on it... you're fine."

In the end if it works with your kit its right decision - geared or not. 🙂

[Astro-Geek]

31 minutes ago, stash_old said:

Most "hot" steppers are caused by having Microstepping which requires current to "hold" the position otherwise the Stepper will "snap" to a detent position. Unless you have set he current wrong - quote from REPRAP site "Rule of thumb, if you can put your hand on the motor but have to remove it, it's hot but not too hot. If you can't touch it without being burned... probably a bit too hot, back off on the current. If you can keep your hand on it... you're fine."

In the end if it works with your kit its right decision - geared or not. 🙂

Wow......

I hadn't realised I was erring on the safe side so much !

Before I slightly reduced the current from my drv8825 board (in accordance with Robert's instructions), the motor was just slightly warm, after about 5 minutes holding......

[Dr_Ju_ju]

…..  but if you use a non geared motor, you will invariably start to use micro-stepping to ensure you get fine enough positional control, especially if step size is too large and if there's any backlash in the system, which there probably will be....

[stash_old]

 

IMO you could Just get a 0.9 degree Stepper ,1.2amp and use 1:2 Microstepping as a compromise. 🙂  Plenty of fine control  ,especially if direct connection via shaft , and its not going to get that hot.

Maybe on SGL some person has written up he/her project with maths and data to support all the different methods in a real situation - but then life is too short. 🙂

Bottom line you can always change it - as no doubt most of us do LOL.

 

[Astro-Geek]

Further testing developments................... 🤓

The direct non-geared Nema 17 gave very fine movements when coupled direct to my SCT Crayford Focuser, even at full stepping.

I was focusing my Skymax 180 Mak and didn't even need to go down to single steps, plus/minus 10 gave the smallest discernible adjustments.

However.....  There is another problem instead.

When I tried it on my 150 achro refractor, the Crayford focuser on that is much firmer (though still smooth and adjusted correctly).

Consequently, the non-geared Nema 17 wasn't man enough to step, so it did need gearing.

I then fitted the 27:1 to it, and it steps comfortably in full steps at full speed.

It still has step size set to 50 microns, which is the maximum with the firmware version on it (290 - the latest).

If I could up it a bit, it would then be a good all-rounder for all of my scopes.  (maybe I could look through Robert's code, and find the range limiter ? )

[Astro-Geek]

More thoughts.....

The difference between the power of the Nema geared and non-geared motors is quite quantifiable.

My non-geared motor has a torque of  45 NCM, or just under half a Newton Metre,

My 27:1 geared motor has a torque of 3 NM, three Newton Metres, six times as much, despite drawing a quarter of the current, (0.4A, instead of 2A).

The 5:1 geared motor (that I was tempted to get) has a torque of 97 NCM, just under one Newton Metre, so less than a third of the 27:1 (quite logically I suppose !)

Second thoughts on trying to edit Robert's firmware code too, the stepsize limitation to 1-50 is within his windows application, not the Arduino firmware.

[RadekK]

There are many pros and cons, but I would say that lack of backlash in non-geared setup makes difference.

[Gina]

You can set compensation for backlash in some drivers.

[Astro-Geek]

 

1 hour ago, Gina said:

You can set compensation for backlash in some drivers.

Yes, Robert's very comprehensive software/firmware does have provision for backlash compensation, even different settings for each direction.

 

1 hour ago, RadekK said:

There are many pros and cons, but I would say that lack of backlash in non-geared setup makes difference.

True, a significant advantage for absolutely precise positioning.

----------------------------

Further testing developments....................

My cunning plan to use geared and non-geared motors interchangeably with the same single controller box has the disadvantage of only having one hardware setting, (the drv8825 driver board's pot) for the motor current.  My two Nemas have widely different current ratings, (0.4 and 2 amps), so it's best that I stick to just one motor, to be able to set it and leave it.

Also - re-reading Robert's PDF, the 0.4 amp geared motor puts far less stress on the drv8825 driver board, whereas the non-geared Nema's 2 amps is right on the limit.

I therefore think I'll just use the 27:1 Nema on all of my scopes, (as per the original recommendation in the PDF !  "Mega Uber Overkill" regardless !! 😀

Edited September 29, 2019 by Astro-Geek

[Dr_Ju_ju]

You can get geared & non-geared motors, that use the same current, especially at the lower rating.

I use motors from https://www.omc-stepperonline.com, you can just select type\size\torque etc. and I usually get them from either UK or Germany, 

[markse68]

Have you tried running the 2A motor on 0.4A Astro Geek? It’s probably too low but I think the 2A rating is max cont current- you can run them lower with reduced torque , but cooler and quieter if it’s enough

edit- apologies I should have read the rest of your posts more carefully 🤦‍♂️

“Consequently, the non-geared Nema 17 wasn't man enough to step, so it did need gearing.”

 

Edited October 1, 2019 by markse68

[Astro-Geek]

2 hours ago, Dr_Ju_ju said:

You can get geared & non-geared motors, that use the same current, especially at the lower rating.

I use motors from https://www.omc-stepperonline.com, you can just select type\size\torque etc. and I usually get them from either UK or Germany, 

Thanks for the link Julian.

It actually looks like the same firm I've been buying them from via Ebay.

Better to get them straight from the source though, and their website is very well laid out with the various choices, with the option of selecting the UK warehouse too.

[Astro-Geek]

46 minutes ago, markse68 said:

Have you tried running the 2A motor on 0.4A Astro Geek? It’s probably too low but I think the 2A rating is max cont current- you can run them lower with reduced torque , but cooler and quieter if it’s enough

edit- apologies I should have read the rest of your posts more carefully 🤦‍♂️

“Consequently, the non-geared Nema 17 wasn't man enough to step, so it did need gearing.”

 

No need for apologies, my posts on this thread have shown how I've blundered around with this.

I find it tricky to judge the set current, via that tiny preset on the DRV8825 board, turning it till the motor "sounds happy"....

I've just acquired another geared Nema for testing/comparison,  a secondhand 5:1 Nema 17 with a rated max current of 1.68 amps.

It worked ok, but was a lot heavier than the PG27 with max 0.4 amps, so I'm still admitting that the 27:1 recommended in Robert's PDF is certainly the best all-rounder for me.  🤓

 

[markse68]

You just have to measure the voltage on the middle pin of the pot on the driver board then you can fairly precisely set the current- should be in the data sheet. I’m using the Trinamic chip and it is spec’d in theirs anyway

[Astro-Geek]

16 hours ago, markse68 said:

You just have to measure the voltage on the middle pin of the pot on the driver board then you can fairly precisely set the current- should be in the data sheet. I’m using the Trinamic chip and it is spec’d in theirs anyway

Thanks Mark, I'll give that a try.

[brown_rb]

On 28/09/2019 at 08:46, Astro-Geek said:

It still has step size set to 50 microns, which is the maximum with the firmware version on it (290 - the latest).

If I could up it a bit, it would then be a good all-rounder for all of my scopes.  (maybe I could look through Robert's code, and find the range limiter ? )

Version 291 in testing will increase this limit to 100.0

https://www.dropbox.com/s/98taxxjlhocs158/Firmware%20myFP2%20291.zip?dl=0

Cheers

Robert

[Astro-Geek]

3 hours ago, brown_rb said:

Version 291 in testing will increase this limit to 100.0

https://www.dropbox.com/s/98taxxjlhocs158/Firmware myFP2 291.zip?dl=0

Cheers

Robert

I'm much obliged to you Robert. 👍🍺

That will make the 27:1 gearbox a really good all-rounder choice for all of my focusers, the "stiffer" ones as well as the lighter ones.

Having now made many tests with the non-geared 2 amp, a 5:1 geared 1.6 amp, and the recommended 27:1 0.4 amp, the PG27 seems to be a really good combination of torque and minimum amps drain, (as your PDF had said).

There's only one very minor thing that still intrigues me, when I test the manual hardware in/out buttons, the step position counter on the LCD display suddenly reverts to 0 after a dozen steps or so,  (with the LED lights flashing correctly).

The position counter performs perfectly when controlled by the computer, with all the stepping increments, in and out, and the numeric step position is always mirrored correctly in the LCD window on the box.

 

[brown_rb]

On 04/10/2019 at 20:07, Astro-Geek said:

There's only one very minor thing that still intrigues me, when I test the manual hardware in/out buttons, the step position counter on the LCD display suddenly reverts to 0 after a dozen steps or so,  (with the LED lights flashing correctly).

The position counter performs perfectly when controlled by the computer, with all the stepping increments, in and out, and the numeric step position is always mirrored correctly in the LCD window on the box.

 

If this is happening please email me directly (address is in firmware file) so we can get to the bottom of this

Regards

Robert

[Astro-Geek]

Thanks Robert.

regards,

Paul

[Phobos 226]

On 04/10/2019 at 08:07, Astro-Geek said:

That will make the 27:1 gearbox a really good all-rounder choice for all of my focusers, the "stiffer" ones as well as the lighter ones.

Having just set up one of the 0.4A 27:1 Nema 17's from stepperonline I've found it works perfectly with a DSLR and I could turn the current setting on my LV8729 right down low so everything is virtually silent while still managing more weight than I have available to add to the focuser right now. I'm happy this is future proof for whatever weight I want to add.

With a direct coupling to the coarse shaft on the focuser (don't use the fine focus) on my Skywatcher Crayford I can full-step for 2.2 microns of accuracy on my f5 130PDS, which is plenty enough inside the Critical Focus Zone.

As for the backlash, yes it's there but not nearly as bad as I had anticipated and it can be compensated for in software/firmware. Having absolute positional accuracy (provided no missed steps) with full-stepping, the reduction in power requirements, less heat from fully energised coils and the huge amounts of torque in reserve is an acceptable compromise for me.

So there's another +1 for the 27:1 from me, if it's of any help.