Re: ci700

Khalid Baheyeldin

On Mon, Dec 16, 2019 at 12:35 AM, --Oz-- wrote:
I am looking to buy the stm32 Kit C to use on my ci700 that has Gemini servo motors with 250ppr encoders and the arduino pro mini (with with A4950 motor drivers. This setup would be 180t * 25 * 250 * 4 = 4,500,000 ticks for turn (half of the G11/Gemini setup), I am guessing that is ok.

Few questions:
1. for the arduino pro mini, should I order the 3V or 5V version to use with the bluepill stm32?
Retaining the servos is possible, but before you embark on this, consider the challenges:

1. Rarity: I only know of one person (German or Austrian?) who have made it work a few years back. There was possibly another person asking questions (Asia), but not sure if he actually implemented it or not. On the other hand, we have hundreds of people using stepper motor setups.

2. Complexity: you require two additional microcontrollers, one for each servo motor, and additional firmware on them.

3. Tuning: you need to do the PID thing described in dcservo's documentation and Youtube videos.

Based on the above, I think that you are better off with just stepper motors, like the vast majority of OnStep users. Ignore the encoders.

If I decide to use stepper motors, looks like one guy used nema 17 (17HS4401) and 16t/40t pulleys with belts, where do i find these parts (links):
2. Motors? 
Read the entire Construction page. It has lots of helpful links.
Use the spreadsheet on the construction page to run the numbers.

OMC Stepper, as simingx said. Also eBay and AliExpress.

Motors that are 0.9 degree are better, and they should have adequate holding torque (36 Ncm or more), and low inductance.

This 0.9 degree, 0.9A motor is popular with EQ5 mounts.
With sufficient reduction (3:1), it should be able to drive CI700 (G11 like mount)

An alternative approach is to get a motor with an integrated gearbox with more reduction (e.g. 10:1 to 15:1).
That way, you can use 1:1 pulleys with a lesser number of teeth (e.g. 25T to 25T), so they are easier to fit on the worm shaft side.
With the added benefit that the reduction increases resolution and torque at the same time.
Again, run the numbers in the spreadsheet.

3. belts/pulleys?
eBay or AliExpress. Also Amazon.
The Construction page has belt size calculators, ...etc.

4. What stepper motor driver is most recommended (LV8729)?
Depends on the motor current.
If it is low (1A or less) then LV8729 will work well, and the TMC2130.
If it is above 1A, then you need the S109 or the TMC5160.

Again available on AliExpress, eBay and Amazon.

5. If you dont have BHC or the SHC, how do you control the mount (align-stars/goto/etc)?
The STM32 kit C has a Wemos, which is an ESP8266 and allows full control from an Android App. Very convenient.

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