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My astronomy project:
Stepper motor focus driver


Content:

  1. Collecting information
  2. Tools for developement
  3. To be continued

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

1, Collecting information:

Now when I'm almost finished with my mechanical design of a stepper motor focuser I need to build a hardware driver and control logic for that motor and it must also be able to use INDI drivers. I have to draw some plan for it and I start to collect some information that I need to be able to build this.

They who built a controller board from the ground and up looks to use the Arduino processor and there is something called OnStep to use. I have to investigate further. Now I have at least started with this. And the construction must be a stand alone solution, it must not be dependent of the Raspberry to do all the control of the stepper motor. The Raspberry is already on the limit to handle all the needed tasks.


Stepper motor focuser driver

I have a NEMA 17 stepper motor in my boxes that I plan to use. Maybe it's an overkill and a NEMA 14 stepper motor is better suited for this. But I start with this stepper motor, the camera system is heavy and I plan for a weight of 5 kg. I want it to operate at 13 Volts direct, not as I have for the earlier focuser that need 8 Volts.

The data I have found for this stepper motor (17HD48002H-22B):

  • Step angle 1.8 degree
  • Step accuracy 5%
  • Holding torque, 0.59NM (59Ncm)
  • Current 1.7 Ampere

With my earlier USB-Focus unit I have learned a lot about motor focusers. When having the driver setup to half step mode the torque is much higher, you can use a smaller stepper motor. The extra torque comes from that all the motor's coils have a current run through. With this setup it takes a lot more power, you can never let it go down to zero because then it loose its position if it's between two full steps. Without current it takes a half step back or forth. The rest positions are only in full step. Even better had been to have it setup for half step but only stop at full step, but I'm not sure there are any driver that let this be done. I want to save power and then I have it to work in full steps with its lower torque, one big reason to have a big NEMA 17 stepper motor.

More to read about stepper motors:


Arduino Focuser Pro2:

After I have read about different technique to control the stepper motor for focus purpose I start with the Focuser Pro2 concept. My friend have already built one of these with an Arduino NANO as hardware. It can be used both in Windows ASCOM and Linux INDI.

From this page there is a PDF documentation that have all the needed information. You will learn a lot from this documentation so I recommend to read it even if you not build your own focuser.

There are a lot of options to choose. One of them is the temperature compensation. I will not use that function because I use auto focus function which I found work much better.

I will use a NEMA 17 stepper motor in full step mode, the one I have take a little bit too high current for this. Then there is no need to hold the coils with current to hold it in position. Less heat and no power consumption in rest. It also gives the highest torque. The stepper motor will be a bipolar type.

  • NEMA 17 stepper motor, bipolar
  • Full step mode
  • Buttons for IN and OUT focus is handy and I will include them
  • Home position switch, maybe
  • Reset switch
  • No display, everything is setup and controlled from INDI driver in Astroberry
  • Communication over USB only
  • It must handle from 12 to 15 volt input because I have it battery powered

That's from my own experience, but you find information about it in the PDF documentation above.

From the beginning I planned to buy one of the PCB, but later I found that I want to make some smaller changes and not use all the options. Then it was better to use some standard board. I follow the schematic on page 7 of this PDF.

List of devices that I need to purchase:

  • NANO CH340G (micro controller)
  • DRV8825_HW203 (stepper motor driver)
  • IN / OUT push buttons
  • Reset switch

And lot of other small devices. But there is something important missing what I can see. The NANO CH340G has a serial port, but how to connect the USB ? And they don't recommend the FTDI version. I have now read the first 70 pages of 209, maybe the answer comes later.

If you too find it interesting I have collected some tutorial videos here:

Now it's time to order the different parts I need for this project.


After I wrote this I got a lot of ideas from the INDI forum. Especially the Ardufocus looks interesting, see the list in the beginning. It can handle two focus motors and it looks easy to address the processor's output to different pins.

Whatever solutions I choose there will be an Arduino, here is more to read about it:

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