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FORUMS Photography Talk by Genre Astronomy & Celestial Talk 
Thread started 09 Sep 2013 (Monday) 04:57
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Tracking mount (Barn door)

 
alliben
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Sep 11, 2013 06:57 |  #16

967stuart wrote in post #16287164 (external link)
Looking into the parts that guy sells, he seems to be using a stepper motor which needs a circuit board to 'pulse' to give the correct 1rpm...

Yes, he uses a stepper motor with a circuit board. If I understand correctly, the rpm of a DC motor might fluctuate as the batteries get weaker - or it gets colder, whereas with a stepper, the rpm is constant until the batteries just plain quit.

He uses a quartz timer for the stepper which is very accurate.

For comet Hyakutake, I built a manual only barn door tracker. I then built a DC motorized tracker for Hale Bopp. I had to monitor and adjust the rpm carefully. Now with the stepper motor, the rpm's seem to be more consistent.

BTW, what parts have you found on ebay?




  
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967stuart
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Sep 11, 2013 07:33 |  #17

alliben wrote in post #16287394 (external link)
Yes, he uses a stepper motor with a circuit board. If I understand correctly, the rpm of a DC motor might fluctuate as the batteries get weaker - or it gets colder, whereas with a stepper, the rpm is constant until the batteries just plain quit.

He uses a quartz timer for the stepper which is very accurate.

For comet Hyakutake, I built a manual only barn door tracker. I then built a DC motorized tracker for Hale Bopp. I had to monitor and adjust the rpm carefully. Now with the stepper motor, the rpm's seem to be more consistent.

BTW, what parts have you found on ebay?

Well the parts I thought might be usable have turned out to be not upto the job.
I have emailed a hobbycraft shop (who sell motors and such), I'm hoping they have the parts I'm looking for.

Someone suggested a stepper motor with a gearing system, they seemed confident that it would give me 1rpm, will post back as soon as they get back to me with an answer.




  
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pwm2
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Sep 11, 2013 07:47 |  #18

It's hard to control the speed of a DC motor well enough unless you have a sensor on it and have a microprocessor perform adjustments. Without control, you need to limit your self on long exposure times you can make before the drift becomes too high - you might manage 2-5% speed error. Remember that the speed error is affected by temperature so it is hard to sit at home and trim it "perfectly".

If using a microprocessor then stepper motors becomes the obvious choice since they are designed with the purpose of caring about exactly how far they have rotated.

There are synchronous motors with step-down gear that are very nice - but wants AC which isn't readily available with good frequency precision outdoors.


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pwm2
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Sep 11, 2013 07:50 |  #19

967stuart wrote in post #16287456 (external link)
Well the parts I thought might be usable have turned out to be not upto the job.
I have emailed a hobbycraft shop (who sell motors and such), I'm hoping they have the parts I'm looking for.

Someone suggested a stepper motor with a gearing system, they seemed confident that it would give me 1rpm, will post back as soon as they get back to me with an answer.

The stepper motor only gives you 1rpm if you have electronics that generates drive pulses to it with the specific frequency that specific motor needs with that specific gear connected. So if the gear is 10:1 and the stepper motor needs 48 pules/turn then you would need to supply 480 pulses/minute.


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alliben
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Sep 11, 2013 08:14 |  #20

pwm2 wrote in post #16287500 (external link)
The stepper motor only gives you 1rpm if you have electronics that generates drive pulses to it with the specific frequency that specific motor needs with that specific gear connected. So if the gear is 10:1 and the stepper motor needs 48 pules/turn then you would need to supply 480 pulses/minute.

The gentleman I referenced has this all worked out. He builds the circuits to run the geared stepper motor at precisely 1 rpm.




  
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967stuart
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Sep 11, 2013 08:14 |  #21

pwm2 wrote in post #16287500 (external link)
The stepper motor only gives you 1rpm if you have electronics that generates drive pulses to it with the specific frequency that specific motor needs with that specific gear connected. So if the gear is 10:1 and the stepper motor needs 48 pules/turn then you would need to supply 480 pulses/minute.

How would I go about getting these electronics to control the pulses? (is it something that has to be made or is there a bit of kit for sale somewhere?)




  
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rfdesigner
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Sep 11, 2013 13:04 |  #22

967stuart wrote in post #16287456 (external link)
Well the parts I thought might be usable have turned out to be not upto the job.
I have emailed a hobbycraft shop (who sell motors and such), I'm hoping they have the parts I'm looking for.

Someone suggested a stepper motor with a gearing system, they seemed confident that it would give me 1rpm, will post back as soon as they get back to me with an answer.

My homebrew drive uses microstepped 1.8degree motors direct driving the worms.. very very smooth and accurate.

The electronics is surprisingly simple. The code is a little more complex. Personally, for a barn door, I'd go for a DC motor with a regulated supply off a 12V battery.. then you can plug it into a car charger (the standard means of powering scopes)

The next most easy alternative would be a synchronous motor requiring a 50Hz/60Hz supply, then build a simple crystal derived 50/60Hz source.

Derek


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alliben
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Sep 12, 2013 16:32 |  #23

I've purchased from Dennis and am pleased with the transaction.

He has curved brass rods, stepper motors, quartz timed driver boards set up specifically for barn door trackers, and a wealth of useful information.

https://sites.google.c​om/site/distar97/ (external link)




  
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the ­ jimmy
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Sep 12, 2013 20:07 |  #24

alliben wrote in post #16291724 (external link)
I've purchased from Dennis and am pleased with the transaction.

He has curved brass rods, stepper motors, quartz timed driver boards set up specifically for barn door trackers, and a wealth of useful information.

https://sites.google.c​om/site/distar97/ (external link)

Great find, have never seen this site even though I have googled barn door many times.




  
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rfdesigner
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Sep 13, 2013 02:27 |  #25

alliben wrote in post #16291724 (external link)
I've purchased from Dennis and am pleased with the transaction.

He has curved brass rods, stepper motors, quartz timed driver boards set up specifically for barn door trackers, and a wealth of useful information.

https://sites.google.c​om/site/distar97/ (external link)

Excellent. Do keep us informed on how you get on.. this could end up being a regular recommendation.

Derek


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967stuart
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Sep 17, 2013 03:22 |  #26

Guy's, I'm having some real issues finding a 10-32 Threaded rod as it's metric here in the UK.

I can easily get a M6 rod to use, does anyone know if I would need to adjust the distance of the rod to the hinge ?

I guess I would have to change the speed of the Motor also.

Here's what I was going to buy -
http://www.screwfix.co​m …6-x-300mm-pack-of-5/29072 (external link)




  
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pwm2
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Sep 17, 2013 03:32 |  #27

Well, that M6 rod will result in a movement of 1mm / turn.

So if the original software expects a different ratio then you need a firmware modification or you need a mechanical change that give that scales the movement corresponding to that ratio change.

Changing the rods distance from the hinge will affect the number of turns the rod needs to rotate for a specific angle change, so scaling the size of your barn door should do the trick.


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967stuart
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Sep 17, 2013 04:37 |  #28

pwm2 wrote in post #16303003 (external link)
Well, that M6 rod will result in a movement of 1mm / turn.

So if the original software expects a different ratio then you need a firmware modification or you need a mechanical change that give that scales the movement corresponding to that ratio change.

Changing the rods distance from the hinge will affect the number of turns the rod needs to rotate for a specific angle change, so scaling the size of your barn door should do the trick.

Thanks.

I'm going to be using an Aurduino
http://www.ebay.co.uk …_trksid=p3984.m​1497.l2649 (external link)

A driver board with a small stepper motor.
http://www.ebay.co.uk …_trksid=p3984.m​1497.l2649 (external link)

Do you think just changing the rotations would do the trick (rather than adjusting the rod position?)




  
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the ­ jimmy
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Sep 17, 2013 06:01 |  #29

967stuart wrote in post #16303054 (external link)
Do you think just changing the rotations would do the trick (rather than adjusting the rod position?)

Here is the math required to figure this out.

R=RPM/(0.004375*tpi) where 'R' is distance between the centre of the hinge and the hole for the drive screw and 'tpi' is threads per inch on the lead screw. The RPM is measured at the drive screw, not the motor.

HERE is a thread that I got this from, you may find other useful information




  
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pwm2
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Sep 17, 2013 07:46 |  #30

967stuart wrote in post #16303054 (external link)
Thanks.

I'm going to be using an Aurduino
http://www.ebay.co.uk …_trksid=p3984.m​1497.l2649 (external link)

A driver board with a small stepper motor.
http://www.ebay.co.uk …_trksid=p3984.m​1497.l2649 (external link)

Do you think just changing the rotations would do the trick (rather than adjusting the rod position?)

Yes, it doesn't matter if you change the gearbox, or the motor speed or the distance between rod and hinge - any solution that affects the angle change/minute will do.


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Tracking mount (Barn door)
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