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  #21  
Old 14-12-2018, 09:27 AM
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DavidTrap (David)
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Originally Posted by pmrid View Post
G'day David.
Long time between shouts!
I'm sneaking up on this project very slowly and carefully. I thought I was being adventurous when I bought a Raspberry Pi. But Arduino is something else again. I've ordered the core elements most people seem to recommend - an Arduino Leonardo, a L298P, LCD1602 and a Temp Sensor DS18B20. For the massive investment of US$32.95. Buying locally would have been more than double. But while I wait for those bits to arrive, I am also expecting the imminent arrival of a couple of motors and control boards.

What eludes me for the moment is the mechanical issue of how to get the motor to drive my focuser - I ought to have bought a geared motor I suppose but that's definitely being wise after the event. Do you use belts or gears to drive your focuser? And how did you determine the appropriate gearing ratios?

Peter
Hi Peter,

Apologies for a slow reply,

Definitely high finance on these things

I am using the motor that came with the micro touch controller. After much trial and error I worked out the wiring connections. This setup uses a gear mechanism to connect to the focusing shaft. The shaft it connects to is the small knob of a 10:1 reduction system anyway, so there is a lot of reduction built into the system.

From what I read in his documentation, the NEMA17 motor is direct drive. You'll need some sort of reduction using belts and pulleys (probably be easier for adjustment than gears) to give enough accuracy. There is another stepper motor that has a reduction gearbox built in - you might be able to connect that directly to a focusing shaft. The Robofocus motors have a reduction gearbox and connect directly to the focuser shaft.

He has lots of calculations about critical focus zones in his documentation. I skipped over that as I was using an exisiting motor and gear mechanism - I just needed the controller to drive the actual motor.

I do plan to make up another controller and motor system for another telescope and a camera lens, so I might be able to offer some more advice after that - although that will be in the new year. I'm planning on 3d printing some brackets to hold the motor, belts and pulleys.

Hope you don't get too drenched in the upcoming weather event.

DT
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  #22  
Old 14-12-2018, 09:30 AM
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DavidTrap (David)
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Thanks for the added vote for Mr. Brown's project.

I've now ordered a Nema 17 Stepper Motor 17HS13-0404S-PG27 with a 27:1 planetary gearbox. So with the f8 x 200 mm scope this is meant for, that should give me 40-odd steps within the CFZ.

Now we sit back and wait a bit while all these bits and pieces wing their way here from all over the globe.

Obviously I have a bit of reading to do in the meantime.

Peter
Looks like Chris has got you on the right path. I'm assuming his direct motor connection is to the 10:1 reduction shaft of that focuser.

The documentation is fantastic for an open source project!

DT
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  #23  
Old 14-12-2018, 12:17 PM
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One of the many interesting bits of knowledge Gary has been kind enough to pass on to me is that the 12VDC connection to the Arduino Leonardo board causes the NEMA17 motor to run quite hot and to get around this he has scaled the input voltage back to 6VDC at which point that problem goes away.

I've found a waterproof buck converter modules that will provide me with 2-3 Amps of 6 volts - and another at 9 volts and I can take power from my 12 volts supply source at the scope. That's real handy. I'll start at 9vdc and see how that works and then retreat to 6 if heat is still an issue.

We live and learn.

Peter
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  #24  
Old 14-12-2018, 01:41 PM
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There is an option to turn off the coil power in the ASCOM driver
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Old 14-12-2018, 05:21 PM
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Originally Posted by DavidTrap View Post
Looks like Chris has got you on the right path. I'm assuming his direct motor connection is to the 10:1 reduction shaft of that focuser.

The documentation is fantastic for an open source project!

DT
No it couples to the main focuser. 16 microsteps within 200 steps per revolution gives plenty of microsteps in the critical focus zone on my scopes. The documentation with myFocuserPro has excellent detail and explanations of it all.

Also I used a TMC2130 stepper driver which is very smooth and quiet. Much better than drv8825s etc - but they all work. And as David mentioned the ascom driver let's you turn off power to the stepper so you can manually focus - not sure why you'd want to do that though as the controller loses its place.

Edit:
The calcs are as follows
CriticalFocusZone = 2.2 x FR^2. For my scope FR=5, so CFZ = 55um

If I direct drive the focuser (no gears) on my scope
- 1 turn of focuser moves it 15mm
With a 200 step/rev stepper using 16 microtstepping
- that gives 3200 microsteps per revolution which moves the focuser 15mm
- or 12 microstep within the CFZ of 55um. This is more than enough

So with your stepper with 27:1 gearing you don't even need to use microstepping. I prefer to use microstepping because then I don't get the backlash problems with the 27:1 gearbox.

Last edited by ChrisV; 14-12-2018 at 07:47 PM.
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  #26  
Old 15-12-2018, 06:04 AM
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1/4 microstepping is my next target. For the best number of steps in the CFZ I am targeting, I need to get my Nema17s to run 800 s/rev. But how is it actually achieved? Is it in the choice of controller or the Arduino sketch programming?

Peter
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  #27  
Old 15-12-2018, 07:11 AM
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Originally Posted by pmrid View Post
1/4 microstepping is my next target. For the best number of steps in the CFZ I am targeting, I need to get my Nema17s to run 800 s/rev. But how is it actually achieved? Is it in the choice of controller or the Arduino sketch programming?

Peter
You will need driver capable of microstepping.But, it is also possible to do microstepping with software only, by driving coils in PWM mode.
Not sure if anybody did this, considering the Pololu drivers (and their's derivatives) are so cheap and so available.

Last edited by bojan; 15-12-2018 at 07:22 AM.
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  #28  
Old 16-12-2018, 09:05 AM
RobC (Rob)
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Peter,

Here is my 2 cents worth.

I have built three of these devices to date and have written the Arduino and ACOM compliant code to drive them. Two of the motors were based on the Nema 17 27:1 Stepper and the third on the Nema 11 27:1 Stepper. I have built two controller units. One based on the LN298 driver and the other on the micro stepping DRV8825.

I am not a fan of micro stepping because of the torque and heating problems. Turning the power off may cause the motor to slip to its natural position and the focuser could move from the CFZ. Although I used the DRV 8825 controller I do not micro step it. I full and half step the LN298 and DRV 8825. At full step mode the 27:1 steppers have roughly 5400 steps and 10800 steps in half step mode.

I plagiarized and modified the Arduino source code from the Texas thread. The ASCOM compliant focus driver code I wrote from scratch.

I use my focuser controller with both SGP and APT.

I have attached a photo of the Nema 17 connected to the course focus side of a 2.5" moonlite on the back of my North Group carbon fibre ED 127.

If I can be of any assistance to you please contact me.

Cheers

Rob
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  #29  
Old 31-12-2018, 01:18 AM
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Small steps but hopefully in the right direction.
I'm looking forward to catching some time with RobC in the New Year but in the meantime, I've pressed ahead with the PnP Focuser project (GitHub) and have managed to get a response out of it.
Thanks go to Gary for his advice here.
The problem had been that I could not install the "Upload" file to the Arduino Leonardo. I pressed the reset button on the Leonardo and got a fuller response but noted in the fine print that it had been unable to locate what proved to be a key file - avrdude.conf. It seems that there was an executable in part of the Arduio IDE setup called - you guessed it - avrdude.exe. I ran that and it created the required conf file for me. I then transported that to the right directory, did another reset on the Leonardo and ran Upload again - and BINGO! It worked.
I've now managed to get the Leonardo, 298P driver shield and LCD talking nicely and the temp sensor wired in and working as well. I haven't connected any motors yet.
That's my next heart-stopping step!

Peter

EDIT: Well I couldn't wait so I wired in the motor (a plain NEMA17 27:1 planetary gear job) sometime around 3 a.m. and Lo and Behold!! The bleedin' thing worked. Thanks Santa!

Last edited by pmrid; 31-12-2018 at 10:23 AM.
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