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Motor options for a barn door tracker

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#26 sternenhimmel

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Posted 24 July 2013 - 10:57 PM

Good advice. I implemented a 10ms on/ 65ms off full-stepping. This brings the current consumption for the motor down to < 100 mA. I'm also using a 2000 mAh 7.4V LP-E5 rechargeable battery for now and am driving the arduino off a 9V since it only pulls 30-50mA.

I've already got a 4 to 1 gearing on the stepper, and the stepper is 200 steps/rotation. With full-stepping that gives me 1.8 degrees/step, and with gearing, that's 0.45 degrees/step. My rod is 1/4"-20, so something like 0.006 mm/step which is something like 0.00122 degrees/min (for a 28.8cm arm). I imagine that is more than accurate enough for my purposes.

The bigger question now is whether the motor vibrations will be an issue, and if I can even polar align accurately enough to make stepper accuracy matter.

Clear skies the next few nights :)

#27 orlyandico

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Posted 25 July 2013 - 04:32 AM

Chris, the stepper motor in that link has a 1/64 gear head and is unipolar with 48 ppr. I have like 10 of them. The only downside is that since they aren't hybrid steppers, they can only do about 200pps maximum - you're not going to be slewing anywhere with these motors.. But for tracking they are fine.

So $3.20 with shipping for both the gear head stepper and the ULN2003 board can't be beat.

Also.. I am not an embedded developer. But I have several different embedded boards to play with. The strength of Arduino is the huge community and the very fine libraries.

Check out Mike McCauley's AccelStepper. Its a wonderful piece of code. While it is true that the Arduino forums are littered with newbies writing spaghetti code, there's also a lot of wisdom there. It does get annoying when I go ask a question and get responses meant for newbies. Maybe part of the disdain (?) of traditional developers for Arduino is because it makes embedded programming so approachable that every rasta hippie is doing it? :o

But overall I think the Arduino experience is a very positive one.

And I would not go back to the days of HC11s and uploading HEX files over a serial line. Arduino still does that... But all the complexity is hidden.

#28 Christopher Erickson

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Posted 25 July 2013 - 04:38 AM

Chris, the stepper motor in that link has a 1/64 gear head and is unipolar with 48 ppr. I have like 10 of them.

So $3.20 with shipping for both the gear head stepper and the ULN2003 board can't be beat.


I guess I missed that in the specs so that isn't too bad.

Personally I would still prefer a worm gear reduction instead of spur gears. Worms can't be overrun by force on the output shaft.

#29 orlyandico

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Posted 25 July 2013 - 04:45 AM

I don't think there are any worm drive gear head steppers... All I've seen are spur and planetary gearheads.

And the cheap steppers are all tin can. You want hybrid (for decent slewing speed) and you're talking big bucks.

The Vexta Pk243's with 10:1 gearheads I'm using on my AP600 GoTo conversion are $250 a pop new. Good thing there's eBay...

#30 Christopher Erickson

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Posted 25 July 2013 - 12:31 PM

From a $5 junk inkjet printer.

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#31 sternenhimmel

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Posted 25 July 2013 - 03:30 PM

I've gotten some decent steppers from ink jet printers. The reason I didn't use any of them personally was because they had small shafts and were only 50 steps/revolution. I was willing to put out the extra few bucks to just buy something new that fit my design requirements. If I had the right tools, I could have made do with what I had.

However, I totally support and practice the philosophy of harvesting components from old hardware. I especially recommend de-soldering goodies from circuit boards.

#32 Groo

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Posted 15 October 2013 - 03:04 PM

one huge advantage of using a stepper motor and micro-controller, is that you can get away without a bent shaft.

with a straight shaft the speed has to be varied as you go along the shaft (this is what the Astrotrac does). So in your shoes, I'd use an Arduino, an A4988 Pololu stepper motor controller, the AccelStepper library, and one of those steppers (you could micro-step them to smooth them out, but frankly at 400 steps/rev it will not be an issue).

You can also do guiding and periodic error correction with the Arduino...


Are you able to tell me what rate I would need to increase the rotation speed with the Arduino if using a straight rod?

#33 Christopher Erickson

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Posted 15 October 2013 - 03:21 PM

one huge advantage of using a stepper motor and micro-controller, is that you can get away without a bent shaft.

with a straight shaft the speed has to be varied as you go along the shaft (this is what the Astrotrac does). So in your shoes, I'd use an Arduino, an A4988 Pololu stepper motor controller, the AccelStepper library, and one of those steppers (you could micro-step them to smooth them out, but frankly at 400 steps/rev it will not be an issue).

You can also do guiding and periodic error correction with the Arduino...


Are you able to tell me what rate I would need to increase the rotation speed with the Arduino if using a straight rod?


It would be a mathematical function that would vary the speed based on the current angle of the screw to the axis. The speed would be different when the wedge was mostly-closed versus mostly-open. The radius of the wedge, thread pitch of the drive screw and ratio of the motor drive and gearing would all be inputs into that mathematical function.

A curved screw allows the drive speed to be constant regardless of the position of the wedge and makes everything else simpler to deal with.

#34 Phil Sherman

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Posted 16 October 2013 - 08:17 AM

An alternative to a curved screw is a curved pillar block on the fixed side of the barn door. The radius of curvature needs to be the same as the distance from the hinge to the (closed door) contact point of the screw. Now, as the hinge opens, the screw will push against the pillar block whose contact surface is always the same distance from the hinge.

The downside of this approach is that the length of time that you can track is controlled by the size of the pillar block. A hole cut in the moving portion of the barn door for the pillar block will allow you to start with the door completely closed.

Phil

#35 Groo

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Posted 16 October 2013 - 10:40 AM

if I make a form/template to curve the threaded bolt, the curve of the template needs to match the center of the bolt correct not the inside or outside?

#36 Phil Sherman

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Posted 17 October 2013 - 09:38 AM

The center of the bolt needs to be the correct distance from the hinge. The bending form needs to be sized to match the inside edge of the bolt, 1/8" smaller than the hinge to bolt dimension if you're using a 1/4" bolt.

Unfortunately, that form won't work. A steel bolt is an elastic material (when being bent) and if you bend it against a form that size, you'll end up with a bolt with a larger radius of curvature. You'll need to use a smaller form to bend it or significantly heat the rod while it's held against the form to relieve stress and have it keep the shape you want.

Phil






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