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A new focuser motor on my 1-1/4" JMI RCF

Howard Dutton
 
Edited

This focuser had a Pololu 1000:1 micro gear motor (dc motor,)  it now has a 75:1 reduced micro stepper motor.  From gearbox to focuser shaft there's an additional 3:1 reduction so overall 225:1 now.  I still intend to add an RJ11 jack inside the housing and 3D print a spacer plate but this works acceptably well.  I used the prototype "next version" MaxPCB to test this, it has SSS TMC2130's for the focusers/rotator (one common SPI bus to all stepper drivers) and had never been tested either (OnStep support or h/w) but fortunately everything worked as expected.  For the stepper motor I used 3x the rated current to make enough torque so I can't sit there and move the focuser all day but need to stay under a 33% duty cycle, but that's easy to do and the prospect of overheating doesn't really worry me (based on how warm the motor became during testing.)  The micro-usb plug is there to give a sense of scale along with the old DC gear motor which is similar in size to the stepper except the stepper is about 1/2 the length.



Thinking about my 2" JMI EV focuser (which has the same 1000:1 DC gear motor for now) the 75:1 reduction on this stepper is probably not enough but these micro gearboxes are the same 10mm x 12mm size and mounting screws/holes are the same.  The 1000:1 gearbox would mount on the tiny stepper motor but the pinion gear on the stepper shaft was too large so unfortunately that wouldn't work (could pull/swap the gears but those things are "microscopic".)  Probably the 1000:1 reduction would be too much anyway and something along the lines of 200:1 or 300:1 would be more optimal which I intend to try.  I have the gear boxes coming, in two ratios, fingers crossed the motor pinion gears are the same on at least one of them.  There is also a 125:1 micro motor on eBay I might try, slightly bigger stepper motor is probably more powerful too so that might very well work as-is.

I could still try the 75:1, I'm only guessing it's not enough until I test with the focuser/common loads it supports, and perhaps I could look at a different pinion gear on the gearbox output shaft to up the 3:1 ratio to 4:1 or perhaps even 5:1 which would make success more likely.

Howard Dutton
 
Edited

On Mon, Nov 4, 2019 at 04:04 AM, Howard Dutton wrote:
Thinking about my 2" JMI EV focuser (which has the same 1000:1 DC gear motor for now) the 75:1 reduction on this stepper is probably not enough but these micro gearboxes are the same 10mm x 12mm size and mounting screws/holes are the same.  The 1000:1 gearbox would mount on the tiny stepper motor but the pinion gear on the stepper shaft was too large so unfortunately that wouldn't work (could pull/swap the gears but those things are "microscopic".)  Probably the 1000:1 reduction would be too much anyway and something along the lines of 200:1 or 300:1 would be more optimal which I intend to try.  I have the gear boxes coming, in two ratios, fingers crossed the motor pinion gears are the same on at least one of them.  There is also a 125:1 micro motor on eBay I might try, slightly bigger stepper motor is probably more powerful too so that might very well work as-is.
Tested tonight, and my worries were unfounded...  put the 75:1 micro gear motor stepper in my JMI EV2N and my Photometrics Quantix 6303e on top, which is really way too much camera for this focuser, and the motor has no problem lifting it up and down.  Putting the motor in was a five minute job.  Amazing, a stepper motor size of my fingernail can do that but it does.  Focusing speed is fine probably can slow it down a bit in-fact.  Full-stepping the motor gives about 1.5 microns/step which is a nice place to be no need to microstep and can power off at standstill (remember I'm using 3x the rated current when moving.)  Guess the lesson here is that perhaps a NEMA17 stepper motor would be a bit oversized for the typical focuser, LOL.  Would have done this years ago if I'd known it would be so easy and cheap:

Ant No
 

That looks like such an improbable load. Well done.

Ant👣


On 5 Nov 2019 01:24, "Howard Dutton" <hjd1964@...> wrote:
On Mon, Nov 4, 2019 at 04:04 AM, Howard Dutton wrote:
Thinking about my 2" JMI EV focuser (which has the same 1000:1 DC gear motor for now) the 75:1 reduction on this stepper is probably not enough but these micro gearboxes are the same 10mm x 12mm size and mounting screws/holes are the same.  The 1000:1 gearbox would mount on the tiny stepper motor but the pinion gear on the stepper shaft was too large so unfortunately that wouldn't work (could pull/swap the gears but those things are "microscopic".)  Probably the 1000:1 reduction would be too much anyway and something along the lines of 200:1 or 300:1 would be more optimal which I intend to try.  I have the gear boxes coming, in two ratios, fingers crossed the motor pinion gears are the same on at least one of them.  There is also a 125:1 micro motor on eBay I might try, slightly bigger stepper motor is probably more powerful too so that might very well work as-is.
Tested tonight, and my worries were unfounded...  put the 75:1 micro gear motor stepper in my JMI EV2N and my Photometrics Quantix 6303e on top, which is really way too much camera for this focuser, and the motor has no problem lifting it up and down.  Putting the motor in was a five minute job.  Amazing, a stepper motor size of my fingernail can do that but it does.  Focusing speed is fine probably can slow it down a bit in-fact.  Full-stepping the motor gives about 1.5 microns/step which is a nice place to be no need to microstep and can power off at standstill (remember I'm using 3x the rated current when moving.)  Guess the lesson here is that perhaps a NEMA17 stepper motor would be a bit oversized the typical focuser, LOL.  Would have done this years ago if I'd known it would be so easy and cheap:



Howard Dutton
 

Thank you, It'll be nice to have precise repeatable focusing on my 10" OTA that rides on the G11 in my observatory.  The 1-1/4" RCF focuser I plan to adapt to my 50mm guide-scope on my portable rig.

Anyway, this morning I 3D printed a spacer for the 1-1/4" RCF focuser motor aluminum mounting housing since it needs to be up off the focuser back about 3/16".  I was using aluminum and cardboard shims for testing but this is much nicer looking.  Lucky the RJ22 jack width is basically the same as the micro gear motor width and so the jack will be mounted inside the motor housing (both focusers.)  Then I just need to make up RJ22 4P4C cables of the appropriate length and plug everything right in.  Simple.  I have the RJ crimpers so this is cheap and easy to do.

Howard Dutton
 

I just finished off these two focusers.  They now both have RJ22 jacks inside the housing and so a detachable cable can plug them in.

I discovered two things during my final setup/testing:

1. The actual current I was sending to the tiny stepper motor was less than I thought, I didn't have the drivers set properly at Vref = 2.5V.  This is a good thing, the motors are not being overpowered and don't need to be run out of spec.  I now use the full rated current, peak = 276mA when moving and drop back to 80mA at standstill.

2. I spent about 3 hours figuring out that a socket header for the CS signal (spi bus) to focuser2 wasn't making contact.  It was a bad socket header LOL, was loosing my mind trying to figure that one out...  Tested good for continuity even since I was pressing on the pin with the meter lead.  Let the pressure go and it would loose contact.

Corey Mooney
 

Hi Howard,

You've inspired me to make some progress toward motorizing my focuser ( 8" EAA rig on an OnStep mount).
Thanks for the tip on the little geared stepper motors. I ended up ordering a couple of these 10mm 1:275 steppers
Your results are promising. 

Could be fun to try and make a mini 3D printed alt-az mount using these itty-bitty motors to drive little baby worms or GT2 pullies. 

-Corey


On Fri, Nov 8, 2019 at 8:47 AM Howard Dutton <hjd1964@...> wrote:
I just finished off these two focusers.  They now both have RJ22 jacks inside the housing and so a detachable cable can plug them in.

I discovered two things during my final setup/testing:

1. The actual current I was sending to the tiny stepper motor was less than I thought, I didn't have the drivers set properly at Vref = 2.5V.  This is a good thing, the motors are not being overpowered and don't need to be run out of spec.  I now use the full rated current, peak = 276mA when moving and drop back to 80mA at standstill.

2. I spent about 3 hours figuring out that a socket header for the CS signal (spi bus) to focuser2 wasn't making contact.  It was a bad socket header LOL, was loosing my mind trying to figure that one out...  Tested good for continuity even since I was pressing on the pin with the meter lead.  Let the pressure go and it would loose contact.