Arduino Help

[bluemaxroe]

Hi everyone.

 

I am in the early stages of creating a sky tracker.    I am asking for help from anyone who has good knowledge of stepper motor control with Arduino....specifically Arduino Nano.

I am a product designer, not a programmer - although i have dabbled.

the dilemma i have at the moment is as follows.

 

I have a 5 wire Unipolar Stepper motor.   which i would like to control with a Arduino Nano...for me the Uno is too big.    I have not yet picked out a Stepper driver chip/board.

I would love to make the driver board myself.   but if i did i would need circuit board design help too.    

the stepper motor in question is this one -  http://www.moatech.com/chi/prod_stepping_13.html  - the SP-35rc-600s

 

I will need to control it by millisecond per step, rather than by potentiometer.  

 

let me know if you are interested to help.

 

Andy

[kens]

There's no big difference between the Nano and the Uno unless you want to use a shield. If you can I'd get something with a crystal clock for more accuracy. I built a controller for my mount using a couple of ULN2003  drivers. But you need to be handy with a soldering iron and wiring diagrams. The rest is all code. Unipolar motors are especially easy to code for. Details here http://www.astronomyforum.net/atm-diy-telescope-making-forum/195812-auto-guiding-old-super-polaris-mount.html

[wimvb]

For the stepper motor driver, I think this is the easiest solution:

https://www.sparkfun.com/products/12779

The product page links to a tutorial.

Other tutorials are easy to find on Google.

[ringz]

Hi Andy,

Can you say which design of sky tracker are you thinking of making?

The design could help you decide how hard you will need to drive the stepper for example.

 

Martyn.

[bluemaxroe]

Hi Martyn

Imagine a Vixen Polarie but it has the camera offset from the rotational centre instead of the polarscope.

The Stepper motor will be geared down so that 1 step is approximately 2 arc seconds of rotational movement by the head.   this gearing down will also give the torque required for it to hold DSLR and 200-300mm lens.     I am planning a counterweight to help with balance.

With my present design the 3.75 degree per step motor will b needed to step once every 10 "ish" milliseconds...the actual figure goes into micro seconds but you get the idea.

 

 

[Dave_D]

http://www.engblaze.com/microcontroller-tutorial-avr-and-arduino-timer-interrupts/#intro this is an interesting read. using standard arduino methods of pulsing a stepper are crude to say the least. coding the arduino AVR chip to control timers to trigger the stepper should (if i understand the theory correctly) make a far more accurate tracker. 

[bluemaxroe]

Thanks Dave D.   It's always good to approach differently.     Speaking of which I was thinking I should use a more common 5v stepper motor.    It would simplify things a touch.  This is the stepper I was thinking.... 28BYJ-48 ..anyone know how much strength it has?

 

thanks

[ringz]

The  28BYJ-48 has quite good torque depending on how you drive it. One thing to mention though is that the attached gearbox has a strange reduction ratio ( 63.68395:1 ) so you would need to take this into account in your calculations on step rate. You do realise how small these motors are though, about 27mm in diameter.

 

Martyn.

[bluemaxroe]

does size matter Martyn?   but seriously, i am looking for a compact design and this one seemed to fit that bill.  however, if NEMA 14 or 17 is a better/tougher/torquier choice than revisions would be better made now than down the line when i have designed and modelled the 3d printed casings needed.

[bluemaxroe]

PS  ringz.   where did you get the gear ratio?   the datasheet says  1/64....or are they just being lazy with the fingers on the numberpad?

[ringz]

The gear ratio appears on lots of websites that I've visited but this one http://42bots.com/tutorials/28byj-48-stepper-motor-with-uln2003-driver-and-arduino-uno/ was the one I used to refresh my memory.

[bluemaxroe]

Quick Visual of where i am at the moment..  loads of working out to do yet.  but enjoying it.

[Gina]

I got caught out by the 64:1 gearbox in the spec when I tried it for driving a clock.  Went over to a NEMA14.  But they're great for applications where the exact ratio doesn't matter such as focusers.

[bluemaxroe]

bit more work tonight.  re modeled the boxing and added the ULN2003 board and the Arduino Nano.   changed the case to clear so that you can see the innards.

Using the 28BYJ-48 shouldn't matter as long as i use the correct gear ratio in my calculations for the "delay" in my sketch right?

Also, i did wonder, but when each delay is 10ms or so do i need to factor the time the motor actually physically takes to do the step as well?   am i being silly? or not, and if not how do i find out what the extra time is to add to the design of my code.

 

[bluemaxroe]

With this configuration, without any adjustment for actual movement time the motor will need to move 1 whole step every 6900 microseconds, give or take.     This is not a whole number by any stretch, which means that it generates a 3200 microsecond positive error every 10 minutes.   i think this would be easy enough to perhaps program in a compensation delay of 320 microseconds every minute. 

Will be very interesting to see what length exposures i can get with this.  I am guessing that polar alignment is the biggest error maker, but i am thinking / hoping for 20mins exposures at 150mm/200mm.

 

thoughts anyone?

[RuralBill]

On 20 July 2016 at 19:57, bluemaxroe said:

Thanks Dave D.   It's always good to approach differently.     Speaking of which I was thinking I should use a more common 5v stepper motor.    It would simplify things a touch.  This is the stepper I was thinking.... 28BYJ-48 ..anyone know how much strength it has?

 

thanks

They are not that powerful, they also come in both 12v and 5v versions, so bear that in mind, I tried the 12v one on a focuser, and it would not drive the coarse focus knob with barely any tension...so not sure it would be good enough, also I took one apart and the gears are all plastic and poorly built with movement between them, so with a bit of load  they can easily slip.

i spent a lot of effort to build my auto focuser with one of these motors based on good things I had read about these cheap steppers, and wish I had not bothered, in my opinion they are rubbish, unless you are driving very very minimal weight.

sorry to be the bearer of bad news, but just based on my poor experience....find another motor, at the end of the day these motors are very cheap, around £2 from China, so remember the saying, "you get what you pay for", it is so true here.

Bill

[RuralBill]

On 20 July 2016 at 21:57, bluemaxroe said:

PS  ringz.   where did you get the gear ratio?   the datasheet says  1/64....or are they just being lazy with the fingers on the numberpad?

They are 48 step step motors geared down to give around 512  or 2048 steps per Rev depending on which version you have

[RuralBill]

On 20 July 2016 at 23:41, ringz said:

The gear ratio appears on lots of websites that I've visited but this one http://42bots.com/tutorials/28byj-48-stepper-motor-with-uln2003-driver-and-arduino-uno/ was the one I used to refresh my memory.

Hmmmmm, I don't agree with that site, they are 48 step motors for sure, hence the number 48 in the model number of the motor, the thing that is not completely agreed on is the gearbox ratio, mine was giving 2048 steps per Rev at the output, I also had one that only gave around 512, so there is obviously different versions around, in fact there are three version that all look identical, and made by different manufactures, they are not all the same.

i have taken two part now and the insides and gears are different on them both, but both still plastic and not that well made, but one is better than the other, if you do a search for these motors there is a website showing the three different versions and there differences inside with images, but can't seem to find it at this point I time, I will keep looking for you.

[bluemaxroe]

that would make it 1:10.6666 ratio though? right?

[RuralBill]

1 minute ago, bluemaxroe said:

that would make it 1:10.6666 ratio though? right?

Yes, I am trying to find the website I mention above, it has lots of info.

also those motors can only be full or half stepped, using the driver board you mention.

[RuralBill]

Here you go, this is one of the ones I had, 512 steps, 32 step motor with a 1/16 gearbox

https://www.adafruit.com/product/858

and there are a few other different variations too, and when you buy them from China you will get different ones again.....

[RuralBill]

And another model from the same company

https://blog.adafruit.com/2012/05/30/new-product-small-reduction-stepper-motor-5vdc-48-step-164-gearing/

so all in all just bear in mind that you need to make sure you get the correct one for your needs, but personally I would find a better one......

[RuralBill]

Here is the website with the comparisons, found it at last

https://grahamwideman.wikispaces.com/Motors-+28BYJ-48+Stepper+motor+notes

Bill

[bluemaxroe]

errghhh, this i did not know.   i think i will re-design the casing again to enable differernt types of motor to be used....attached from the outside.  this will also enable me to change if the torque is not enough, or if i want to use something that will cope with even bigger payload.

 

[RuralBill]

1 hour ago, bluemaxroe said:

errghhh, this i did not know.   i think i will re-design the casing again to enable differernt types of motor to be used....attached from the outside.  this will also enable me to change if the torque is not enough, or if i want to use something that will cope with even bigger payload.

 

Quote

I think a different motor would be a good idea...

Bill