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BullsEye Focuser - DIY Arduino + Trinamic TMC2209 based focuser with temperature measurement


kbrown
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Decided to make another DIY electronic focuser for my new WO FLT 91. It will be Arduino Nano based again but this time I'll be using a Trinamic TMC2209 based stepper driver (Same as in my Prusa Mk3). The focuser on the scope is a rack and pinion style and I'll be driving it with a flexible coupler directly on the focuser shaft. I'm hoping to utilise the Stall Guard feature of the TMC2209 to auto-home the focuser and also as sensorless limit switch. I'll be adding a Class A PT100 based RTD as well to measure the focuser temperature. I found one with M4 thread that I can bolt directly on the focuser body.

I've pretty much done with the mechanical design. Still need to finalise the electronics and solder it together + write the firmware and INDI driver.

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Posted (edited)

Ouch. Ran into a bit of a road block with this. Turns out my chosen NEMA 11 12Ncm motor is not quite strong enough to drive the focuser attached directly on the (coarse side) of the focuser shaft. It very  nearly works with a bit of help by hand but not on its own. Not even with some over current and slow acceleration etc... Bummer...

So back to drawing board. A few options I can think of include:

  1. Use a bigger and stronger stepper (NEMA 17 for example) the same way on the coarse side of the focuser shaft.
  2. Use the existing NEMA 11 stepper on the 10:1 side of the focuser and deal with backlash in the arduino code.
  3. Use the existing NEMA 11 stepper with pulleys and a belt to gain better gear ratio while minimising backlash on the coarse side of the focuser.
  4. Use a NEMA 11 stepper with a gearbox on it to gain more torque on the coarse side of the focuser and deal with backlash in the arduino code.

Any other practical ideas you guys can think of?

Edited by kbrown
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3 hours ago, skybadger said:

Double the voltage and use current control. 

 

 

Had another look at the specs of the motor I have and it's rated 0.67A @ 6.2V so I'm already nearly double the voltage driving it with 12V. Increased the current going into it as much as I dared and it did start moving the focuser but it was occasionally skipping steps and got rather hot too.

Think I'll try a NEMA 17 stepper which I already have to see if it's better. Just need to print a new L-Bracket for it.

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Posted (edited)
2 hours ago, skybadger said:

Are you running microstepped or at the base step size ?

This is the first time I'm using a TMC2209 driver and the TMCStepper Arduino library so I'm not too familiar with it yet. I'm trying to configure it via the UART interface which gives me loads of options but in regards microstepping I haven't been able to really go below 8 microsteps as it seems to start misbehaving even without any load. So far I've had the best results with 16 or 32 microsteps.

 

I know in theory I should get more torque with native steps but I'm not sure if that's the case with the TMC2209.

Edited by kbrown
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Gave a NEMA 17 stepper a go and it's much better. Think I will go with it and re-design the enclosure around it. Here's a quick test:

 

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Did a bit more pondering and experimenting with this. The rated torque of the stepper motor in the above video is supposed to be 26Ncm which feels just about enough with the 16th microsteps I was running it with (AFAIR). But it didn't take much resistance by hand for it to start skipping steps which was a bit of a bummer. Just as an experiment I also tried with another NEMA17 motor that was rated about 19.8Ncm. This wasn't enough to move the focuser without helping it by hand at all so I guess I'm just above the threshold with the 26Ncm motor.

I'm currently waiting for a delivery for yet another NEMA17 motor that is supposed to be rated 45Ncm. I'm hoping that will be enough to drive the focuser at 32 microsteps. That's the sort of resolution I'm aiming for minimum without any sort of transmission. One microstep would be around 33 microns of travel while my worst case scenario critical focus zone is just above 50 microns.

Hope it works out okay. If not then I'll have to look into a geared stepper or a pulley and belt system. Feels a bit like I'm balancing on the edge of failure at the moment...

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The new stepper arrived today and does seem to do the job well even at 32 microsteps. I had this test running for several minutes and the dial gauge always stopped at the same spots. Feels like this will work nicely now! :)

 

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  • 3 weeks later...

Been trying to source a Baader Diamond Steeltrack focuser for my Newt for over 9 months and now I finally got one as it was back in stock for a brief moment at FLO. Turns out my first NEMA 11 version of my DIY autofocuser is perfect for this. Just had to design and print a new bracket and get a HTD 3M pulley and belt. Work is keeping me too busy from writing the arduino code and indi driver though. Only should have to do it once though as the electronics on these two are pretty much the same.

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  • 3 weeks later...
Posted (edited)

Finally I've had some time to mess about the arduino code for this focuser. Took a bit of trial and error plus some black magic voodoo to get the auto homing dialled in but it is working fairly well now. What it does is upon powerup it will rack the focuser out as far as it can. Once it reaches the mechanical limit the TMC2209 stall guard will indicate an error. From there it will rack the focuser back in to the other extreme while measuring the length of the travel. Once it reaches the inward mechanical limit it'll back out a tiny bit and call that home or the zero position. Here's a little demo of it:

 

Edited by kbrown
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I love your project. I’m hoping to start tinkering around with raspberry pico in next few days. Can’t wait to see how your Focuser develops. You’ve made a great progress so far, keep it up!

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