DIY controller for my CG-4 dual axis motors

[SalehRam]

Hello,

I've got a dual axis motor drive for my CG-4 mount. It is not working now and I want to make a DIY one, as buying a new kit is not an option for me now, plus I get to add an ST4 port for auto guiding... However I'm running into some issues getting the information right, and I could really use some help getting things right so I can do the controller...

After some research, I know the following info about the motors that came with it:

1. They are 42PM48 and looking around I found the same model with different name which I think is the generic one (42BYJ-48)
2. They have a step angle of 7.5" (since 360/48=7.5) and they are attached to gearbox
3. According to what I understand the gearbox reduction ratio is taken from the number of the teeth on the RA axis worm gear, which is in my case (Celestron CG-4 mount) 130 teeth, so the gearbox is 130:1
4. With the reduction ratio applied, the final angle I get is 0.0576 (since 7.5/130=0.0576)
5. I know that an object appears to move 15" per hour in the sky (since 360/24=15). That lead to an object appear to move at 0.25" per minute or 0.00416" per second...

 

The problems I currently have are:

• The movement of an object per second is more than 10 times slower than what the motors will give me per step, so either my estimations for the gearbox reduction ratio are wrong, or there is something in the original controller's micro-controller that is making this work.
• Based on just visual observation for the behavior of the motors, I hear/feel multiple pulses per second (something around 6) and I take each pulse is a movement for the stepper. Based on this I assume that the gear reduction ratio is much more than 130 and I am getting the reduction ratio wrong...
• The motors I have are having 4 wires so it is a bipolar model, I cannot make half step (or if it is possible I am not aware about since I'm not expert in stepper motors :S )
• Doing steps every certain amount of time to track the objects I'm afraid it will ruin the long exposure images if it is much coarse or slow, so I'm not sure how the original controller do the movements.
• I was looking for a way to know more about the config of the actual control box so I can match it with the DIY model that I'm going to build, but when I opened the box I saw a lot of diodes, resistors and transistors, in addition to the main micro-controller which is STC (https://www.ebay.com/b/STC-Microcontrollers-Programmers/159680/bn_17672964) - I don't know how to work with micro-controllers directly other than the ATMega chip through the Arduino Uno interface. But for my project I'm planning to use an Arduino Nano.

 

What I am currently looking to solve is:

• First to know more accurate details about the gearbox reduction ratio (or how can I get this information myself with practice or any other means)
• Then what is the best way to move the motor to achieve accurate tracking results in terms of matching the motor steps and speed with the actual object's apparent sky movement...

 

Once I get those 2 things I believe I'll be in a better situation to build the circuit and do tests. I have some stepper motor controllers but I'm looking to either do this with 2x H-bridges or 2x L298N dual H-Bridge drivers not sure which one will be more practical...

[Dr_Ju_ju]

Can you post a picture of the motors please?, as the Info I can find on them, suggests that they are high current drive types with will require beefier driver modules as opposed to the standard A4988\DRV8255 types...

[drjolo]

I do not know if you already did, but you may take a look into AstroEQ project https://www.astroeq.co.uk/tutorials.php . This is based on Arduino Mega and two bipolar motors with microstepping controllers. There is also an option to add ST4 port. Mount is controlled from PC using EQASCOM driver. The gear ratio you can use may be in wide range - all can be configured in the software. Some time ago I have added AstroEQ to my CG4 mount 

 

[benzomobile]

That's correct. I did the same with PicGotoPlus control system and bipolar or unipolar  steppers@200 s/t, without gearbox. 

In slew mode I reach 400X  very stably and with good torque.

24 (or 48 too) s/t steppers  with  1:130 ratio are ok  for  low speeds, but they doesn't reach   high ones.

 

Beppe