Vixen GP-DX motorization for Mak 180 #drv8825 #tmc5160 #esp32 #ds3231


Antoine Cesbron
 

Hi All,

6 year ago I had to stop astronomy, but now, I can re-start !
I started with a ETX-90 and finished with a Dob-Newton 300mm, but that was far to big, heavy...

For my 40 and 41 birthday, I just offer me a Mak 180, and buy a Vixen GP-DX from 2005.
All will be received in a couple of week.
I'll update the electronics components of the mount.

Short presentation :
After 15 years as acoustic engineer, I'm going back to school for a year to learn software development.
I own a CNC 4060, a 3D printer 300*300*400mm, a small laser cutter... well, I'm a maker.
Solidworks is my favorite video game!

My goal:
Use my smartphone via Bluetooth for localisation, settings and Goto.
Use a Nunchuck for manual mouvements and focusser. 

To start, this is my BOM (I already have some), may you please check and tell me if I miss something or if I can do better:
1x ESP32
1x DS3231
2x TMC5160
2x Nema 17/48, 0.9°, 1.68A
2x 12 teeth pulley
2x 80 teeth pulley
2x 6mm belt
1x DRV8825
1x 28BYJ-48 (focusser)
1x Nunchuck


Questions:
- ESP32 VS Teensy 4.0:
Please confirm that the "only difference" will be on maximum slew rate speed?
The Configuration editor (attached) show me that I will not be able to reach 2°/s slew rate using ESP32, but a bit less. I'm right?
Will it still be comfortable at that speed?

- Equatorial mount as Azimutal use:
I live in an appartement, and I like with friends, to quickly put the scope on the balcony and scan the sky, but on my balcony, I can't access to the north pole...
Setting the EQ mount at 90°, permit in theory, to use it as an azimutal mount.
Is it possible on OnStep to set two configurations to use the goto and tracking on a EQ mount used as EQ or Azim?

More questions will come !
Thanks in advance for your help.
Antoine


Antoine Cesbron
 

Well, it as been fast !
I juste found a Teensy 4.1.
So, forget questions about ESP32.
Updated BOM :

*1xTeensy 4.1
*1x D1 Mini Pro
1x DS3231
2x TMC5160
2x Nema 17/48, 0.9°, 1.68A
2x 12 teeth pulley
2x 80 teeth pulley
2x 6mm belt
1x DRV8825
1x 28BYJ-48 (focusser)
1x Nunchuck

I'm right ?


George Cushing
 

Generally, we use a 3:1 GR1 for this type of mount and you may find 80T are difficult to fit. I use 48T:16T for 3:1. 60T:20T will work too.  Using 64 µsteps and 60T:12T results in an estimated 0.30 tracking resolution.

        Gear Reduction   Tracking
  STEPS/arc-deg Stepper-Steps MICROSTEPS GR1-Pulleys GR2-Mount Trkg Res
RA 34129.92 400 32 6.666 144 0.22
Dec 40960 400 64 4 144 0.30

Once you get to 0.5 arcsecs/sec chasing better resolution really bears little fruit.

You can use the TMC2130 with the 1.68A motors. These are usually set for 0.8-0.9A Vref well under their 1.2A RMS rating. Saves you about $9 a driver. 

I'd suggest that you use one of the cheap TMC22XX drivers for the focuser. They can be controlled by UART signals and are better technology than the TI DVR8825. The next version of OnStep will be taking advantage of UART driver control to bring the cost of drivers down by about 80%.

Hope this helps.


Antoine Cesbron
 

Hi George, 

Thanks you.
I'll go back to 60T, and just ordered a TMC2209.
The TMC5160 are already ordered.

May you point me to a standard wiring schematic ?
Thanks.


Antoine Cesbron
 

Thinking of it, using as you, the 48T:16T belt gear is far enough.
Maybe you know the right belt length for Vixen GPDX mod?
I'll receive it in two weeks, I can't measure now.
Thanks. 


Dave Schwartz
 

On my original generic EQ-5 conversion I used 138mm belts. The brackets (from Thingiverse) didn't provide any extra spacing between the motor and the worm block but I used a large washer about 2mm thick to get the right spacing.

On a Meade LXD55 conversion I am just completing, the brackets I designed (uploaded into the Files area) provide a 3mm spacing from the worm block but I had to use 140mm belts, which makes sense due to the 1mm extra distance.

These mounts are both Vixen GP clones.

Based on this experience, I would say that the length of the belt is 134mm plus twice the thickness of any shim or bracket wall between the motor and worm block.

Fortunately, when I found that 138mm was too tight for my new brackets, I had 140mm belts on hand because I had to do the purchase multiple times to get it right when doing my EQ-5. With shipping from China being much less reliable now than then, I would recommend you by multiple sets of belts bracketing the value you think you need... the cost of the belts you don't need is your insurance against having to wait for multiple orders.

On 2022-09-20 1:12 p.m., Antoine Cesbron wrote:
Thinking of it, using as you, the 48T:16T belt gear is far enough.
Maybe you know the right belt length for Vixen GPDX mod?
I'll receive it in two weeks, I can't measure now.
Thanks.


hitosi sato
 

Hi Antoine

I'm sure you already know this, but just in case, here's a tip about the 28BYJ-48 modification.
This motor is a 5-wire uni-polar motor, so you need to make some modifications to run it with a bipolar driver such as the DRV8825.
There is a lot of information on YouTube etc., so I won't go into details.
You need to open the blue cover and cut one wiring pattern on the small PCB.
The red wire is not connected to the motor driver, and the remaining 4 wires are connected to the driver.
This motor has an internal reduction gear, and it has 2048 steps per revolution for full step operation, but there is some backlash.
If the performance is not enough, it would be better to replace it with a 200-step motor for better results.
Hence it is better to have a motor driver with good micro stepping performance.
The DRV8825 is not recommended because it does not perform finer than 1/8 micro step.

My focuser has worked well enough for my limited use.
I plan to install it on my Crayford Focuser to test its performance.

Hitoshi


George Cushing
 

I found these NEMA 14 and 17 400 step motors for under $5. The 17s were stage lighting take offs.

image.png



Antoine Cesbron
 

Thanks for your replies. 
I have time and will re-think about my focuser solution. 

I'll look for a 400 steps, micro sized motor.
I can spend 20 bucks in a better controler and motor (I'll buy another TMC5160 to have the 3 sames drivers in the setup).


hitosi sato
 

Hi.
To calculate the focuser setting, CFZ is calculated from the distance the tube moves in one rotation of the focus knob and the F value of the optics.
You need to have about 10 steps within the CFZ distance to get a precise focuser setting.
I used the "Myfocuserpro" document as a reference.
If Crayford Focuser has a fine knob, 28BYJ-48 might work.
In any case, you need to find the required number of steps from CFZ.

Hitoshi


Antoine Cesbron
 

Hi, 

Parts are ordered, I buy the controller (TMC5160), but wait for the focuser stepper, I'll manage it in a second time.

I have question about "Soft limits".
As a drawing is easier than long words story, I quickly draw "my balcony" (the EQ mount come from GradCAD) to show you what I plan to do and to show the limits I'm talking about.

(This is 3D PDF, you have to accept to open it.)
On "OnStep_EQ_AZ_Limits_1.pdf" you can see the risk that the ocular touch the balcony if I want to continue turn left.
To continue to turn left, I have to "full rotate" to be safe "OnStep_EQ_AZ_Limits_2.pdf".

Is it possible to set "Soft limits" in a case like mine ?
Two axis positions at a time need to be used to check  limits.
Ex :
If altitude is lower than "X deg", Azimuth can rotate 360 deg (useless but permitted)
When altitude reach "X1 deg", Azimuth rotation is maxi "Y1 deg".
When altitude reach "X2 deg", Azimuth rotation is maxi "Y2 deg".
When altitude reach "X3 deg", Azimuth rotation is maxi "Y3 deg"...

It can permit to avoid ocular hitting the balcony.
The other interrogation is, did OnStep can manage those informations on a Goto request ?

Excited to receive the Teensy to start playing with software !
And excited to take skills in software development, because finally, all my requests are reachable with the right code.
Thanks for your help.


Howard Dutton
 

On Thu, Sep 22, 2022 at 02:01 AM, Antoine Cesbron wrote:
Is it possible to set "Soft limits" in a case like mine ?
Two axis positions at a time need to be used to check  limits.
Ex :
If altitude is lower than "X deg", Azimuth can rotate 360 deg (useless but permitted)
When altitude reach "X1 deg", Azimuth rotation is maxi "Y1 deg".
When altitude reach "X2 deg", Azimuth rotation is maxi "Y2 deg".
When altitude reach "X3 deg", Azimuth rotation is maxi "Y3 deg"...
OnStep doesn't support that.