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Motor options for a barn door tracker

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#1 sternenhimmel

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Posted 17 June 2013 - 11:17 AM

Hey everyone,

I recently finished constructing a simple barn door tracker out of some hard wood plywood, a threaded rod bent to a radius, and some gears and hinges I had laying around.

I would eventually like the device to be driven by a motor. I'd like to first see if I can make use of what I have lying around before buying something specific, but I can't make up my mind on what drive would be most suitable.

Currently I have a 2" diameter plastic gear driving the threaded shaft, and will be mounting a motor with a suitably smaller gear to mesh with this larger gear.

Here are the motors I have laying around:
2 x large 400 steps/rev stepper motors (though they vibrate on stepping, I'd want to dampen this some how) ~ 6mm shafts
1 x small stepper motor, unknown resolution, short shaft, spur gear on shaft
2-3 DC motors of various sizes, several have gears already
1 x high-torque micro servo (Futaba 3114: http://www.futaba-rc...os/analog.html)

I also want to be able to build a speed controller that is adjustable with a potentiometer such that I can obtain the exact sidereel rotation. I also would like to do this without programming a microcontroller, if possible. As I understand it, the steppers would require the most complex circuit, followed by the servo, and finally the DC motor. I know PWM would be needed to drive both the servo and the DC motor (for adjustable speed), but I am unsure of which would preform better. The required shaft rotation speed is about 0.9 RPM, so the motor speed should be 5-10 RPM.

According to this video, their cheapo servo motor has trouble at low speeds via their method: http://www.youtube.c...h?v=VJfBJJ2Y1kA

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#2 sternenhimmel

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Posted 17 June 2013 - 11:43 AM

Sorry, this could probably be moved to the DIY Forum, as that would be a more appropriate audience.

#3 orlyandico

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Posted 17 June 2013 - 12:03 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...

#4 sternenhimmel

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Posted 17 June 2013 - 12:40 PM

Thanks orlyandico,

I'm not sure if it's clear or not in the photo, but I have the shaft curved to the appropriate radius, although the accuracy of this curve may be in question and could use error correction if I end up doing long exposures with telephoto lenses.

I've been running steppers off of a raspberry pi, but I don't own an arduino at the moment (not that I'm against the idea of owning one). I do own a handful of PIC MCUs, I just don't have a lot of MCU programming experience.

#5 orlyandico

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Posted 17 June 2013 - 01:00 PM

Arduino is a lot simpler than RasPi. And since the clone China boards are like $15 it's really a no-brainer, they are actually cheaper than analog control circuitry.

You don't even need to use the $20 Pololu stepper boards, because those are fancy micro-stepping controllers. There are cheaper options out there.

#6 sternenhimmel

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Posted 17 June 2013 - 02:08 PM

Pulled the trigger and got an Arduino Leonardo after a little research. Should be here in a few days. I have some stepper driver boards that can sink a good amount of current. They've worked well in my testing with the RaspPi.

If I use the steppers, I'll need a small gear with a large (6mm) inner diameter, and it would need a set screw, as the shaft is round and not notched. Any tips on where to get spur gears on the cheap? (all my gears so far have come from scrapping old printers)

#7 orlyandico

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Posted 18 June 2013 - 12:28 AM

You can get some toothed belts off eBay.

Also check out the AccelStepper library, great work.

#8 Geo.

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Posted 18 June 2013 - 06:42 PM

Electronic Goldmine has a good gear selection. www.goldmine-elec-products.com/

#9 sternenhimmel

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Posted 20 June 2013 - 03:18 AM

Excellent website Geo. Thank you!

I've stumbled upon that stepper library but haven't downloaded it yet. I also heard of something called AFmotor which will do microstepping?

My arduino did arrive today though, and I couldn't help playing with despite the many other things I need to get done. In any case, it will serve a stepper driver quite well and I believe I can get first light this weekend. Without microstepping, there are small vibrations from the steps, but I think these are either insignificant or could be dampened with some rubber washers on the motor mount.

The stepper I'm using right now is a small one I got out of an ink jet printer. The big ones I have were too big and collided with the threaded shaft. This one has such a short drive shaft that it has to be mounted above the gear as it cannot reach up through the wood. I temporarily mounted it and applied tension with a rubber band just to test my code and ensure it had enough torque to move the arm with a full load.

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#10 sternenhimmel

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Posted 16 July 2013 - 01:26 AM

I'm finding that having my barndoor tracker on a ballhead is incredibly inconvenient when it comes to trying to align the mount to the celestial axis. It's impossible to easily make small adjustments, and there's significant drift when I release that requires compensation. Thus, I'm thinking of getting decent pan-tilt head, but honestly, the whole point of this project was to go the more cost-effective route instead of buying an equatorial mount. Thus, I don't want to spend a ton of money on a pan-tilt head and, if I do, I want to be using it for other things too.

Do you guys have suggestions on alternatives? I would really like to be able to make small adjustments, though I know nothing like that ever comes cheap. I also considered making my own pan-tilt (alt-az) assembly out of either aluminum or hardwood, then using gravity and a machine screw for fine adjustments.

I've also updated the electronics on this project significantly. I've decided to go with a Nema-17 form factor, bipolar stepper. It wasn't free, but it has a shaft long enough to be bottom mounted and a face small enough not to interfere with the threaded rod. I also got a 64/16 (4) gear ratio now, with nicely fitting gears from http://www.sdp-si.com/. Much smoother.

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(The bottom piece of wood will be replaced when I get everything configured and prototyped correctly -- mostly for aesthetic reasons)

I also swapped out the two small hinges for one beefier hinge. However, the hing still has more play in it than I'd like (I would use a rubber band to tension against the play axis). Where on earth can I find a hinge with zero or very little play?

New gears:
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#11 orlyandico

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Posted 16 July 2013 - 03:07 AM

they say that piano hinges have the least play...

as for your alt-az adjustment... a variation of this ought to work..

http://www.covington...ro/woodenwedge/

#12 highertheflyer

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Posted 16 July 2013 - 11:59 AM

Aligning with the hinge pin would be an important part of the setup.
Yet I've not tried this, I offer a thought...
Replace the hinge pin with tubing, then build a means to align and hold a laser pointer at its lower end, to thus direct its beam inside the axis, towards the celestial north pole.
Fine adjusters would be necessary for aligning the laser's beam within the confines of the tubing, but that's a good thing!
Now you have a starting point to align a hinge pinning to the celestial axis, and to drive the camera along its way.
Just my two cents,
Jim

#13 sternenhimmel

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Posted 16 July 2013 - 12:53 PM

I can't tell if that was a joke about piano hinges, or true fact...

A wedge is an excellent idea. Sturdy, and already at the approximate angle needed. I might modify the design to make elevation adjustments a little easier than adjusting the legs.

As far as aligning the hinge and the laser goes:
http://www.cloudynig...5959601/page...

#14 sternenhimmel

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Posted 19 July 2013 - 09:02 PM

So I ended up building what is essentially a second barndoor, except I used a straight bolt and a coupler as my means of vertical adjustment. This serves as my "wedge" for alignment to the celestial pole. It's essentially a very crude wooden equatorial mount now.

I have a new question for anyone that is electronics savy. In my first night out, I managed to get about 20-30 minutes out of the circuit before the 9V battery dropped below 7V and the voltage regulator output began to waver. Clearly 9Vs are not a good option, but I also feel like I should be limiting the current to the stepper motor, otherwise it will just keep drawing more current, right?

Should I use PWM to reduce the current being fed to the stepper motor? I could connect a potentiometer so that I can adjust the duty cycle if I'm not getting enough torque.

#15 Phil Sherman

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Posted 21 July 2013 - 09:24 AM

I'd get a heftier battery. Try a 12V 4AH or 7HA AGM or gel cell type battery and use a DC-DC switching regulator to drop the voltage down to 7V. These regulators can be purchased for a few dollars on eBay and are around 90% efficient vs the 60% efficiency of a 12V to 7V linear regulator.

If you want to stay with 9V batteries, use rechargables and set up your "battery" to use at least three in parallel so you can swap them out as they run out of juice.

Phil

#16 Christopher Erickson

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Posted 21 July 2013 - 06:13 PM

Here is a very similar barn-door tracker I built back in 1985.

I drove it with a synchro motor and a small, variable-frequency 12VDC to 120VAC inverter.

I squeezed the hinge tubes with vise-grips to eliminate play.

The distance between the bent screw and the hinge center was dictated by the two gears I had and the speed of the synchro motor at 60hz. However it ended up that I didn't get the distance perfect and the variable-frequency DC-AC power supply had to be set to about 58hz for it to track the sky correctly.

Note the home-made stop-switch that would kill the motor just before it came fully-closed.

Polar scope was a finder from an old Sears 40mm refractor from the 60's. The brass tube around the polar scope came from a plumber's junk box.

The camera ball head is being held on with an adjuster-screw from a pair of old 50's-era roller-skates.

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#17 sternenhimmel

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Posted 24 July 2013 - 01:15 PM

Thanks for the tips and the inspiration!

A heftier battery would indeed be a good start. A switching regulator would also be a smart decision. I was using the 7805 mostly because I had it laying around and I needed 5V for the arduino. However, the stepper motor draws a decent amount of current and the regulator would get incredibly hot.

I'll add a switching type regulator to my next digikey order and explore some different battery options. I'd like to get the current through the stepper down to around 200 mA as well.

Chris, that's a nice looking barndoor tracker right there. Small, simple, clean. I'll try using vice grips on the hinge to see if I can get rid of some of the play.

I'm really hoping to get some quality wide-field (up to 105mm) imaging done with this tracker.

#18 orlyandico

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Posted 24 July 2013 - 01:20 PM

Oh I found some 12-36V to 5V DC-DC converters on ebay for $1.75, shipping included von Rot China (as these things tend to be)

http://www.ebay.com/...&_trksid=p39...

#19 Christopher Erickson

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Posted 24 July 2013 - 01:51 PM

Thanks for the tips and the inspiration!

A heftier battery would indeed be a good start. A switching regulator would also be a smart decision. I was using the 7805 mostly because I had it laying around and I needed 5V for the arduino. However, the stepper motor draws a decent amount of current and the regulator would get incredibly hot.

I'll add a switching type regulator to my next digikey order and explore some different battery options. I'd like to get the current through the stepper down to around 200 mA as well.

Chris, that's a nice looking barndoor tracker right there. Small, simple, clean. I'll try using vice grips on the hinge to see if I can get rid of some of the play.

I'm really hoping to get some quality wide-field (up to 105mm) imaging done with this tracker.


A small ZX-24r microcontroller from Zbasic.net combined with a Parallax BOE & Terminal AppMod and a $2 stepper-driver chip combined and a nine volt battery will do everything you need to drive a small gearhead stepper motor and give you the ability to adjust the tracking rate to whatever you need. The BASIC coding would be straightforward and the electronic circuit could be assembled without any soldering. The whole works would fit in a tiny plastic box mounted to the tracker and be completely self-contained with a tiny display and four buttons for configuring the tracking speed and such. You could even add external buttons or switches for tracking corrections. This would be smaller and easier for the beginner than an Arduino solution.

#20 orlyandico

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Posted 24 July 2013 - 02:12 PM

I've looked at the ZX24r and I don't really see how it is simpler for the beginner than an Arduino..

sure it's Basic but the weird mangled C++ dialect used by Arduino is pretty simple, too.. and between a China Arduino Nano fur under $10 and a "special package" containing a ULN2003 chip and stepper motor with 1/64 gear reduction.. you're all set.

#21 Christopher Erickson

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Posted 24 July 2013 - 02:40 PM

I've looked at the ZX24r and I don't really see how it is simpler for the beginner than an Arduino..

sure it's Basic but the weird mangled C++ dialect used by Arduino is pretty simple, too.. and between a China Arduino Nano fur under $10 and a "special package" containing a ULN2003 chip and stepper motor with 1/64 gear reduction.. you're all set.


That's essentially true but most people have more experience with BASIC than with C or with Arduino, which was designed for "artists, musicians and other non-technical people."

With Arduino, I see a world where about one percent are talented programmers and everyone else struggles at just above the script-kiddie level, trying to modify and use code created by others.

If I were to tackle this project I would probably choose an ATmega328P chip and use WinAVR C to program it.

In fact I am tempted to whip out a circuit design and the support code and post it here, if there were any interest. I designed something similar for a college heliostat table derotator controller and the professors preferred that I do it in BASIC for them instead of Arduino. I could probably take that code and with a few simple code modifications, turn it into a barn door controller.

My advise is to use whatever you are comfortable with!

#22 sternenhimmel

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Posted 24 July 2013 - 03:50 PM

I think the Arduino is a good choice if people are interested in programming microcontrollers, but are intimidated by the learning curve associated with both the electronics and programming aspects. As they learn and write more complicated programs, they can delve further and branch out.

The arduino also serves as a development platform so that people can test ideas on the fly and see what would work and what wouldn't work. I don't have to move the chip from my programmer to the circuit each time I make a change.

In my final design, I'll probably end up with a PIC or ATmega driven circuit and move my Arduino to other projects under development.

On a stepper related note: Currently I'm half-stepping my motor, but would be interested in microstepping for smoother function. Can microstepping be easily implemented in software without any additional ICs?

#23 Christopher Erickson

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Posted 24 July 2013 - 04:34 PM

I think the Arduino is a good choice if people are interested in programming microcontrollers, but are intimidated by the learning curve associated with both the electronics and programming aspects. As they learn and write more complicated programs, they can delve further and branch out.

The arduino also serves as a development platform so that people can test ideas on the fly and see what would work and what wouldn't work. I don't have to move the chip from my programmer to the circuit each time I make a change.

In my final design, I'll probably end up with a PIC or ATmega driven circuit and move my Arduino to other projects under development.

On a stepper related note: Currently I'm half-stepping my motor, but would be interested in microstepping for smoother function. Can microstepping be easily implemented in software without any additional ICs?


If you are planning low-power, low heat and operating on minimal battery power, you really want to avoid half-stepping and microstepping. Shutting off the stepper holding current between steps will save you a significant amount of battery capacity versus keeping the coils energized to hold half or microsteps.

Personally I would suggest going with a gear reduction head on the stepper motor instead. Same end-result related to smooth driving without all of the wasted power and heat.

There are a lot of cheap ink jet printers out there that have bipolar steppers in them with worm drive gears and plastic worm wheels. Looking for junked printers in your area (or on eBay) could reward you with a good bipolar stepper worm drive assembly for almost nothing. You could post a wanted-ad on Craig's List too.

And yes, there are some good microstep driver chips out there but I have always avoided them in preference for gear reduction instead.

Gear reduction will give you a lot more torque and weight-handling capacity too.

#24 orlyandico

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Posted 24 July 2013 - 09:38 PM

There really is no need to go looking through junky printers when you can get this for $3.20

http://www.ebay.com/...r-board-uln2003

(and that's including shipping)

Actually the nice thing about an Arduino is if you buy an Uno (with the DIP28 packaged Atmega328, not the SMD package) you can pull out the Atmega328, wire it up to a super-minimal circuit (2 small caps, 1 bypass cap, and a crystal) and it will still work.

http://www.instructa...-on-breadboard/

"Arduino" is nothing more than a tiny bootloader for the AVR, and a nice IDE that hides the ugliness of AVRdude and the cross-compiler.

Here's another wrinkle.. the good folks at Digilent have come up with a new board, the DP32. It is not physically compatible with Arduino, but it is software-compatible and uses the MPIDE (Arduino-compatible) IDE from Digilent.

The nice thing? the DP32 uses a PICMX that is also a DIP28 and can also be put in a minimal circuit like the Atmega328. The difference? 50 MHz, 512K of flash, and 32K of RAM - enough to implement a full-blown telescope controller with GoTo. If you want to go there..

#25 Christopher Erickson

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Posted 24 July 2013 - 10:51 PM

There really is no need to go looking through junky printers when you can get this for $3.20

http://www.ebay.com/...r-board-uln2003

(and that's including shipping)

Actually the nice thing about an Arduino is if you buy an Uno (with the DIP28 packaged Atmega328, not the SMD package) you can pull out the Atmega328, wire it up to a super-minimal circuit (2 small caps, 1 bypass cap, and a crystal) and it will still work.

http://www.instructa...-on-breadboard/

"Arduino" is nothing more than a tiny bootloader for the AVR, and a nice IDE that hides the ugliness of AVRdude and the cross-compiler.

Here's another wrinkle.. the good folks at Digilent have come up with a new board, the DP32. It is not physically compatible with Arduino, but it is software-compatible and uses the MPIDE (Arduino-compatible) IDE from Digilent.

The nice thing? the DP32 uses a PICMX that is also a DIP28 and can also be put in a minimal circuit like the Atmega328. The difference? 50 MHz, 512K of flash, and 32K of RAM - enough to implement a full-blown telescope controller with GoTo. If you want to go there..


That $3.20 item isn't a gearhead stepper or a worm drive stepper and that was the whole-reason to look at some discarded inkjet printers.

Arduino is fine if that is what someone wants to use. However over the years I have seen a lot of ardent campaigning for various microcontrollers and program languages by people that have never actually rolled any code or built anything functional. Present company excepted, of course!

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