Barn door star tracker

[Steve_C]

Originally Posted by N1

Would an egg timer have enough torque to drive this thing?

I did see a design using an egg timer! It used a length of fishing line to drive the arm from the timer, using the weight of the camera to provide tension on the fishing line! I doubt it would be accurate enough and you would have no way of making minor adjustments.

See http://www.mpas.asn.au/Astro/Tips/di.../dingaling.htm

[Steve_C]

Originally Posted by Russman

Hi Steve, Thanks for pointing that out! I will have to get that as well!

The link I gave has a microcontroller chip with on board memory, as well as a motor driver chip capable of delivering up to 4A. If it can produce the correct pulse frequency, I think it would work well.

[Steve_C]

Originally Posted by Russman

I ventured out into the yard to give my single arm barndoor and new DIY mount a go.

Where did you get the gears for your mount, and what motor did you use?

Plans for my single arm barn door are coming together, slowly. 32 TPI rod seems to be hard to get at a reasonable price, however I can get 1/4" BSW rod (20TPI) at Bunnings, along with appropriate nuts. Will this work? I realise the radius will need to be bigger as will the arm. I could use 3/16 rod at 24 TPI, but Bunnings don't seem to have 3/16 nuts available without screws, perhaps the 3/16 rod would be too thin. Thoughts?

[OzEclipse (Joe Cali)]

Originally Posted by Steve_C

Where did you get the gears for your mount, and what motor did you use?

Plans for my single arm barn door are coming together, slowly. 32 TPI rod seems to be hard to get at a reasonable price, however I can get 1/4" BSW rod (20TPI) at Bunnings, along with appropriate nuts. Will this work? I realise the radius will need to be bigger as will the arm. I could use 3/16 rod at 24 TPI, but Bunnings don't seem to have 3/16 nuts available without screws, perhaps the 3/16 rod would be too thin. Thoughts?

Hi Steve,

If you re-read my post earlier in this thread where I replied to your Q about motors, I stated that DC motors are difficult to gear down. It's because of their high native output speed.

Let's say you build a tracker with a 20TPI 1/4" whitworth rod and a 10 inch arm radius, every turn of the nut advances the arm by

206265 ÷ 20 TPI ÷10 inch = 1031 arc sec per turn of the nut. But the sky only moves at 15.04 arc sec per sec. So you need to turn the nut at 1 turn every 68.6 seconds.

If you were to order this 6rpm motor.

http://www.ebay.com/itm/12V-6RPM-Oup...item2a52ae43c2

You will need approximately 6:1 gear reduction in between the motor gearhead output and the gear that turns the shaft.

You could make the required reduction a bit less by making or buying a pulse width modulated DC motor speed controller but if you reduce the motor speed too much, you'll eventually lose torque.


I realize that steppers are a slightly more difficult to control electronically but they are far far easier to deal with on the mechanical side. If you use one of the Arduino steppers with 64:1 gear box that are available on ebay, you can drive the nut with a simple pulley or 1:1 gear. No reduction required.

The stepper controller mentioned earlier in this thread will run the arduino motors. If the speed adjustment on the circuit is too course, send in a pulse from a 555 timer board. Replace the timing capacitor with a polyester capacitor, they are much more temperature stable. When you have the right resistance, remove the course potentiometer and replace it with a metal film resistor and a small range trimpot to allow fine tuning of the motor speed.

Cheers

Joe

[OzEclipse (Joe Cali)]

An afterthought, I think those PWM DC speed controllers will give decent torque at 1/6 speed reduction. I don't know how stable or accurate the speed control is when using these controllers. Using 1/6th of the rated voltage will kill the torque although the 6rpm motors have such a high reduction already, the loss of torque may not be an issue.

Joe

[Russman (Russ)]

Originally Posted by Steve_C

Where did you get the gears for your mount, and what motor did you use?

Plans for my single arm barn door are coming together, slowly. 32 TPI rod seems to be hard to get at a reasonable price, however I can get 1/4" BSW rod (20TPI) at Bunnings, along with appropriate nuts. Will this work? I realise the radius will need to be bigger as will the arm. I could use 3/16 rod at 24 TPI, but Bunnings don't seem to have 3/16 nuts available without screws, perhaps the 3/16 rod would be too thin. Thoughts?

I got my motor from ebay, gears from jaycar and other stuff from work. I work in a nut and bolt shop. PS threaded rod is very cheap, I'd stick to M6 as its easier conversions etc. I'll try and put together a detailed description of how I made mine over the next couple of days. PS. i'd stay away from anything lesser diameter than 6mm, I think it might be too flimsy.

[pixelsaurus (Mike)]

Originally Posted by N1

Would an egg timer have enough torque to drive this thing?

You mean something like this?

http://www.instructables.com/id/Stan...o-photography/

[Steve_C]

Originally Posted by Russman

threaded rod is very cheap, I'd stick to M6 as its easier conversions etc. I'll try and put together a detailed description of how I made mine over the next couple of days. PS. i'd stay away from anything lesser diameter than 6mm, I think it might be too flimsy.

One nut and bolt shop near me wants $41 per meter for M5. I can get 1/4 20tpi (steel or brass) from Bunnings for $7.50. Do you know of a cheap source for M5 or M6 rod?

[Steve_C]

Hi Steve,

If you re-read my post earlier in this thread where I replied to your Q about motors, I stated that DC motors are difficult to gear down. It's because of their high native output speed.

Surely if I got a 4rpm o/p motor off ebay and used 4:1 gearing that would work as per Gary's article?

Let's say you build a tracker with a 20TPI 1/4" whitworth rod and a 10 inch arm radius, every turn of the nut advances the arm by

206265 ÷ 20 TPI ÷10 inch = 1031 arc sec per turn of the nut. But the sky only moves at 15.04 arc sec per sec. So you need to turn the nut at 1 turn every 68.6 seconds.

I need to understand the maths for this, long time since I studied math! Gary's article mentions a formula R= rpm/(.004375*tpi) to get the rod radius and rod position from the hinge, but I don't understand where the .004375 constant comes from.

If you were to order this 6rpm motor.

http://www.ebay.com/itm/12V-6RPM-Oup...item2a52ae43c2

You will need approximately 6:1 gear reduction in between the motor gearhead output and the gear that turns the shaft.

I understand that, and it is fairly easy to do. I plan to go with a stepper, either a28BYJ-48 or a Nema 17 format, using the driver / controller board I mentioned previously. Just need to get the arm radius and arm postion calculations correct. I'll look at the spreadsheet mentioned elsewhere. I haven't decided on motor voltage yet, I have some spare 11.1V li-pol batteries from some Rc gear I can use, I'll probably regulate them down to around 6V.

[Steve_C]

Let's say you build a tracker with a 20TPI 1/4" whitworth rod and a 10 inch arm radius, every turn of the nut advances the arm by
206265 ÷ 20 TPI ÷10 inch = 1031 arc sec per turn of the nut. But the sky only moves at 15.04 arc sec per sec. So you need to turn the nut at 1 turn every 68.6 seconds.

Where does the figure of 206265 come from?

[N1 (Mirko)]

Originally Posted by pixelsaurus

You mean something like this?

http://www.instructables.com/id/Stan...o-photography/

Ingenious in a way but does not support a DSLR according to the comments further down.

[Russman (Russ)]

Originally Posted by Steve_C

One nut and bolt shop near me wants $41 per meter for M5. I can get 1/4 20tpi (steel or brass) from Bunnings for $7.50. Do you know of a cheap source for M5 or M6 rod?

That's a joke! A 1 meter length of M6 Zinc all thread shouldn't cost more than $10 I'd probably sell Zinc for $5 & brass for $10 over the counter at work.

I used the following items on my barn door;
http://www.ebay.com.au/itm/12V-DC-2R...item3cb880f8f0

http://www.jaycar.com.au/PRODUCTS/El...e-Set/p/YG2632
Used the 2:1 gears

https://encrypted-tbn1.gstatic.com/i...tkSNXKFzQ6hswg

The M6 flange nut was pressed into the larger gear and I drilled a hole in the bottom board of the barn door to match the OD of the flange nut which locates the gear into position and keeps it engaged with the smaller drive gear.

I also used a control knob (think its called a potentiometer) to fine tune the speed as I found that the 2rpm motor from china wasn't very accurate, might have something to do with my voltage supply??. I drew a white dot on one of the gear teeth and a dot on the board, turn it on, set a timer and adjust to 1rpm. I can get it pretty accurate to within +- within 1 second.

I'll take some photos and post them up soon.

If you want to PM me and we can discuss posting to you. Ive got a fair collection of threaded bits and pieces in my workshop!

[OzEclipse (Joe Cali)]

Originally Posted by Steve_C

Where does the figure of 206265 come from?

Hi Steve

It's the number of arc seconds in a radian.

60 x 60 x 180 ÷ pi

Using radian measure for small angle trig allows really simple calcs that are just as precise as using trig functions because the tan and sin functions of small angles are the same as their radian angle value.

The rest of the calc is just working out the fraction of a radian.

Joe

[OzEclipse (Joe Cali)]

Originally Posted by Steve_C

I need to understand the maths for this, long time since I studied math! Gary's article mentions a formula R= rpm/(.004375*tpi) to get the rod radius and rod position from the hinge, but I don't understand where the .004375 constant comes from.

I had often wondered that myself. I have worked it out. Turns out, he's using the same radian measure trig shortcut that I am.

The angle the board moves through is RPM ÷ { TPI x R(inches) }

In one minute the board has to move
60s x 15.04 arc seconds per second = 902.4 arc sec
902.4 ÷ 206265 (arc sec per radian) = 0.004375 radians.

0.004375 = RPM ÷ { TPI x R(inches) }

rearranging

R(inches) = RPM ÷ { TPI x 0.004375 }

If you want to go down the dc motor route, I'd be tempted to use a 6rpm motor and a 4:1 gear. Then use a PWM dc motor speed controller to achieve the 2/3 speed reduction.

Originally Posted by Steve_C

Surely if I got a 4rpm o/p motor off ebay and used 4:1 gearing that would work as per Gary's article?

It should work if everything is made VERY precisely. It will work. If not, you really need some motor speed adjustment to trim out the construction errors.

Originally Posted by Steve_C

I understand that, and it is fairly easy to do. I plan to go with a stepper, either a28BYJ-48 or a Nema 17 format, using the driver / controller board I mentioned previously. Just need to get the arm radius and arm postion calculations correct. I'll look at the spreadsheet mentioned elsewhere. I haven't decided on motor voltage yet, I have some spare 11.1V li-pol batteries from some Rc gear I can use, I'll probably regulate them down to around 6V.

The spreadsheet referred to is mine. I ran my spreadsheet for Russ based on some of the construction parameters he wanted to use. Then I created a second output spreadsheet to display the results. I don't mind giving the spreadsheet out but it's not user friendly. I think I'd spend more time explaining it or fielding complaints if people run it the wrong way and build their drive on wrong parameters than I would if I just run it and give constructors the output and error chart. Happy to do the same for you.


Hope this helps

Cheers

Joe

[OzEclipse (Joe Cali)]

Originally Posted by Steve_C

One nut and bolt shop near me wants $41 per meter for M5. I can get 1/4 20tpi (steel or brass) from Bunnings for $7.50. Do you know of a cheap source for M5 or M6 rod?

Suggest you just change the radius to suit the 20TPI 1/4" - 11.43 inches

[Steve_C]

Originally Posted by OzEclipse

Suggest you just change the radius to suit the 20TPI 1/4"

That's what I plan to do, just makes it a bit longer. The 1/4 rod at Bunnings seemed to bend fairly easily too.

[Steve_C]

Joe,

I plan on using a stepper motor now, just working out the voltage/current requirements. I'm going to get the motor and electronics first, to get that working before I do the mechanics.

If I went with a DC motor, the voltage would be made adjustable to fine tune speed.

For some reason the forum is not letting me quote posts that have been quoted by me before.

[Howard (Howard)]

Can't argue with the previously posted photo's using a DC motor. Great results.
However, for those who do still wish to pursue stepper motors ... I have never tried this, but the following webpage is a guy who built a DIY HiFi turntable. About half way down the page he drives a stepper motor by two 90 degree offset sine waves created on pc with free software. The motor is hooked up to amp/audio card output and when you 'play' the sine wave(s) apparently it drives the stepper motor at whatever the frequency you put into the sine waves. If you have a pc in the field it might work, but all the faffing around might be just as tough to do as find/buy those pcb boards off eBay?
For what its worth the link is an interesting read though ...
http://www.altmann.haan.de/turntable/

[OzEclipse (Joe Cali)]

Howard, It's a good idea. The only downside I see is that you are stuck with needing a laptop which reduces portability and increases setup time.

However, once tuned, the frequencies could be stored to a sound file on an MP3 player, smartphone etc You'd only need to install a battery, power supply and two channel power amplifier card in the base of the barndoor to drive the stepper. joe

[Steve_C]

Originally Posted by Russman

If you want to PM me and we can discuss posting to you. Ive got a fair collection of threaded bits and pieces in my workshop!

Thanks, I have plenty of bits and pieces as well, just no threaded rod.

I remembered another place in Sydney where I have bought stuff from before, I'll see if they have the rod at a reasonable price.

Steve