My astronomy project:
Stepper motor focus driver

Content:

1. Collecting information
2. Hardware tools for development
3. Arduino Nano and DRV 8825
4. Prototype setup
5. Setup Arduino system
6. Schematic / setup
7. Start assembling circuit board
8. Setup INDI for MyFocuserPro2
9. Enclosure
10. 5 kg load test
11. High impedance stepper motor
12. To be continued

Note:
I take no responsibility or liability for what are written here, you use the information on your own risk!

11, High impedance stepper motor:

The stepper motor I have used until now have a low impedance and doesn't fit very well for a 12 Volt system. The DC resistance of it is 4 ohm. Now I have bought another one with 9 ohm DC resistance. With too low impedance the hardware driver can't use its full range and that's what cause some of the problems.

NEMA 17 stepper motors:

The new one to the right. It has less torque and I'm not sure if it will be strong enough for a heavier camera. I kept the NEMA 17 standard to make it easy to install. The weight of the big motor is 360 gram and the new about half of that.

The new stepper motor doesn't take up much space. But it's much weaker compare to the older one. I must increase the current to it.

I first increased the current to 700 mAmp which is the limit according to the data sheet of the stepper motor. But it got hot after a while. I lowered the current to 500 mAmp, much better and the motor had still torque enough to move the focuser. Still it got a bit hot, it doesn't matter much because it isn't in use for longer periods. Only in use when doing auto focusing, about 3 minutes per hour. The potentiometer where the current is adjusted is a bit sensitive to electric disturbance, use a screw driver of plastic material. I read the current from the built-in ampere meter I have at my power splitter. When in rest the system take 700 mAmp, when the focus motor running it increase to 1200 mAmp.

To get the motor even cooler and save the battery I disabled the coil Power when in rest. I changed the setup to no hold current (INDI driver setup).

Because of that it can come out of sync by half a step (in 1/2 micro step mode) each time if it stop at a half step. It doesn't matter because I let the system do auto focus. I must sync the home position sometimes with this solution, if I add a home switch later it will be solved automatic. With a focus temperature compensation it's more important that it has a hold current. Alternative is to setup it in full step mode, but that has some drawbacks, less torque and the stepper motor get very noisy. If I in the future find a bigger stepper motor optimized for 12 Volt I maybe change the motor once again.

Now we have winter and dark nights, one of my first photos with the push pull focuser, Whirlpool Galaxy, M51. Taken from home at Bortle class 9 environment. Even at this early stage I can see that the focuser works well, even in automatic mode.