View Single Post
  #1  
Old 30-09-2020, 10:25 AM
Greggles (Gregory)
Registered User

Greggles is offline
 
Join Date: Aug 2017
Location: Sydney, Australia
Posts: 50
Stepper motor upgrade - EQ Platform

Hi every one,

Just want to share my project to upgrade the motor on my 10"dobsonian DIY eq Platform from a EQ1 DC motor to a stepper motor run by arduino.

Firstly I would like to say a big thank you to Rod and Bojan who helped me out, I couldn't of done it without your support and encouragement.

I was always so scared to move to a stepper motor as I felt overwhelmed and confused trying to figure out all the bits and bobs to get one to work for an EQ platform. The way I learnt how to do it (besides all the help I got) was to take a step back and learn Arduino and then learn about steppers and then put it all together to work with an EQ platform. The code I wrote started as a switch turning on a led and I kept developing it to eventually work for my platform.

Functionality:

When turned on the motor will give a 2 second warning beep then the motor will proceed at a faster speed in reverse until the push button is triggered when the platform reaches the starting position. The platform will then move forward at a faster speed for 700 steps (eliminate the push button being activated again due to slow tracking speed) then begin to track at normal until it reaches the end where another button is pushed putting it in reverse and repeating the process.

I know some calculate actual amounts of rotation to signify when a reset needs to be done eliminating switches but I chose to go with switches as I like the ability to reset early or begin tracking at any point by reaching down and hitting the button.


Parts:

So the motor im using is a nema17 stepper motor with a 27:1 gear box Big thank you to Rod who shipped this out to me. What a legend. It is run by a DRV8825 stepper motor driver forced into fast decay mode in order to help with missed steps, an Arduino Nano, 12v power supply, buck converter, acouple of push buttons and a buzzer.

Putting it all together:

I first developed everything on a breadboard to ensure it was working and work on the code. Once I got it all I working I needed to move it to a more permanent board. Being my first electronics project I ended up going with a solder able breadboard as to reduce the possibility of me making any errors compared to strip/proto board.

I then soldered everything onto the board and mounted it to the platform. I connected the two push buttons onto the edges of the platform so they were triggered when the platform was at the start and end positions. And connected the motor as a direct drive with a coupling designed for CNC routers.

Motor powered by a 12v power supply. And board powered by the same power supply reduced to 5v through the buck converter.

If any one is interested in the Arduino sketch/code i'm happy to post this too.

Ready to go I connected it all together hooked it up andÖ. Oops. Something wasn't running right. The motor seemed to be slowing down and speeding up. Increasing the amps to the motor seemed to help but but I was running and A4498 at this stage rated for only 1A so I swapped to a DRV8825 at 1.5A. Unfortunately the problem persisted, seemingly worse. The motor was running at 1/16 microsteps and the motor was quite loud and when the speed issues where happening you could hear when it go louder and then subsequently quieter. Changing to 1/32 microsteps helped significantly, but it does seem I have some slight speeding up and slowing down at times. Not sure if this is PE or skipping steps or what. I have ordered a S109 driver just in case it helps with skipping steps as DRV8825 doesnt have the best reputation. If its PE there's a high precision 30:1 stepper motor on sale at the moment thatís looking inticing Not sure if this would help?

Alternately, as per Bojans advice, I could look into overhauling the whole drive system with belt drives etc.

Getting the perfect speed wasn't as easy as I though. The speed calculated as per my dimension was pretty close but not perfect. Using an eliminated reticent eyepiece with a 2x barlow I began making tweaks to the time between steps to get it as perfect as I could, this took for ever as often I would have to wait 10+ minutes to see any deviation. The other issue I was having was being unsure if some deviation was due to the PE, missing steps or platform slipping or not.

Iíve got the speed quite perfect when half way through tracking but from the start position the platform seemed to be tracking a little fast. Very frustrating. But not much I can do about it besides possibly get new more accurate sectors or redeveloping the platform.

Over all the change to a stepper motor is great. The precision is leaps and bounds over the DC motor I have.

- Once calibrated I donít have to spend 30 minutes every single session fiddling with the potentiometer of the DC motor trying to get it right.
- will confirm tonight but with a 2X barlow and 12.5mm eyepiece star will stay in field of view for 45 mins+.
- kept approximately centre with deviations back and forth for approx 15+ minutes.
- dslr with 1.5x barlow exposure time moved from 4 seconds to approx 8 seconds for relatively consistent photos. 10 seconds which was my goal seems to be just too long and picks up the errors in the drive more often.

Pretty ordinary weather here in Sydney but once I get some deep space shots with the new motor I will post them here as well as on my Instagram @astrogreggles

Hope this was helpful/interesting. Please let me know if you have any questions and comments.

Cheers Greg
Attached Thumbnails
Click for full-size image (eq1.jpg)
182.8 KB51 views
Click for full-size image (eq2.jpg)
193.7 KB42 views
Click for full-size image (eq3.jpg)
203.2 KB36 views
Reply With Quote