New motors for EQ3-II mount (or is it CG4?)

[bojan]

After getting hold of this little mount for free (thanks again MattT :-) ), I decided to replace RA motor with smaller one, to make the mount easier to pack and transport (I may want to use it for TSE 2028.. hopefully).

I decided to employ the second clutch I have (leftover from OnStep-motorized EXOSII, the first one was used on my other EQ3 ).

This time I machined the timing pulley (48T), so the belt transmission height is as low as possible, 36 mm).
Bracket for stepper is printed (same idea as for my C11 focuser, only slightly modified to fit this application).
The electronics will be Arduino nano, I used the modified sketch (arduinian for f/w code) published here. The stepper driver I am using is based on ULN2803 (8x) to support 2 motors.. I am now working on the code to add DEC motor (which is not really needed, but sometimes it is useful).
I intend to place electronics inside the mount, there should be enough space.. if not then in separate box (hand pad, with buttons).

[Nikolas (Nik)]

Great project, let us know how it all ends up

[bojan]

Motor mounted in place.
The belt tension can be adjusted by rotating the motor in holder, which can then be tightened by side screw.
I have to shorten the clutch screw a bit, to prevent the clash with small timing pulley (on the motor shaft).
Now, waiting for ULN2803 to arrive from AliExpress, and first phase of the project is done.

The next stage will be adding the DEC motor (both on mount and in the code).


NB:
I could have used NEMA17 thin stepper motor (400 s/r), height would be same/similar, but it would be more complicated to adjust the belt tension.. and resolution would be 5x worse so I decided not to go this way.

[bojan]

Arduino stepper driver, Schematic.

[bojan]

Electronics, even smaller than Nano (doesn't have USB adapter). It should fit inside mount (DEC).
I will have couple of holes drilled on DEC cavity (around polar finder) for connectors (power, hand-pad, motor(s-)).

[bojan]

Code, modified to support two motors and joystick, tested - works OK (it came from here).
Now I have to think how to mount and couple DEC motor to worm.
This is not must-have, only nice to have.

[Leo.G (Leo)]

How are you finding the motors driving capacity Bojan?
We have several here and found they aren't always matched internal gearing (even bought in sets from the same place at the same time) and they have an internal clutch.
My son tried one driving a project (big project) constructed out of K'Nex plastic rods, it was only on the steering mechanism and the motor was close to useless. We're now going to purchase a pair of NEMA 14 motors for the project. Luckily they will fit.
I'd be rather curious to hear how well it drives the head.

[bojan]

The motors I have here are pretty good for the purpose.
I am using them here, also for another small mount, and focusers (links are here and here).
They are definitely not good for high(er) loads.. they are designed for moving light things only (somewhere I read originally they were used for directing air flow in cars and aircons).
But with suitable transmission.. could be used for many applications. Especially because they are cheap ans small.

[Leo.G (Leo)]

They certainly are small and cheap. I ended up buying a force gauge to work out exactly how much torque was required to turn the steering mechanism and worked out some NEMA 14 motors will work well.
Were we to strip the smaller unit like you're using and stiffen up the clutch they would likely work but there's enough room (and there's not much of that anywhere in my sons build) for the stronger units.

[bojan]

Leo,

I am not aware those motors have clutch.. I opened couple of them in the past (to re-arrange the coils, I wanted to turn it into bipolar to use Pololu drivers) and there was nothing like clutch inside.
However, if you turn the output shaft too hard, the rotor could start to rotate through gearbox.. or if you are unlucky, the last gear may become loose from shaft (it is hot-pressed onto knurled shaft) .. and then the motor is basically destroyed.

[Leo.G (Leo)]

We have read about a friction clutch in some of them when the stepper was slipping in the application but can't find anything now. Maybe it was just too much load for the little motor.
Though I did use one of the little 28byj-48 stepper motors for the remote focus on my WO DDG focuser but haven't used the scope since fitting it and haven't done the driver for it yet, one day.