High accuracy pan tilt turret

Hi, I want to build a high accuracy pan tilt turret The thing is I want it to be very accurate and usable at longer ranges 0.1 miliradian (~ 0.4 MOA for those not familiar with metric units) accuracy is expected - obviously the less the better. Speed isn't really an issue - if it can do 0.5 RPM max it's ok

Mechanicly it will look something like this :

I got two options: A)the 28BYJ-48 motor with an inbuilt gearbox (5.625 deg/48 = ~0.087deg per step) - not ideal but could be further geared down with a belt gear and it has got plenty of torque

B)typical 200steps per rev step motor but microstepped to 1/64 or 1/128

What would be better in your opinion ?? Is it even possible to microstep the 28BYJ-48 motor? How accurate are microstepped motors ?

How accurate are microstepped motors

Definitely not 1/128th of a step accurate. I'd be very suprised to find anything that can realistically guarantee locked positioning greater than 1/8th of a step. Microstepping is about reducing harmonics and smoothing the motion, not about increasing accuracy.

If this were my project I'd probably still use a stepper because they're easy to control but I'd use a gearbox or pulleys etc. to multiply the rotational accuracy, not by directly attaching the stepper shaft to the device.

Good luck!

[edit - fixed obvious misspellings]

I built something like this with two 200 step per revolution steppers and a 30:1 worm gear reduction.

I used 8x micro stepping also, this gave me about 48,000 points per revolution.

Speed is also actually fairly fast, here is a video controlling the assembly with an xbox controller:

https://vimeo.com/51856361

Here is my design, the cad files are in the comments:

http://stevenbrace.co.uk/2012/06/diy-arduino-motorised-time-lapse-head/

I can answer questions if you have any.

C.

dude you’re awesome - that’s exactly what I need
it even has a parts list

Never considered a worm gear… in fact I didn’t want to use gears at all because they are hard to find, but I guess I don’t have other choice

Is there any noticable backlash??
Have you tried using it for long range photography??

Have you tried using it for long range photography??

Does this have to be Real Smooth for Video, or are these still shots??

kerimil: dude you're awesome - that's exactly what I need it even has a parts list

Never considered a worm gear... in fact I didn't want to use gears at all because they are hard to find, but I guess I don't have other choice

Is there any noticable backlash?? Have you tried using it for long range photography??

If you can get the gears to mesh tight I have found no noticeable backlash. With a long zoom you might observe stepping in the image, I recommend maybe using a 60:1 ratio on the worm gear to get extra precision.

Also another tip would be to explore the other worm gears and worm wheels available on spd si, the ones I used are quite expensive and there are cheaper alternatives, especially if you go for imperial sizes. If you want to be really precise you could also replace the steppers with geared steppers. But you might want to do some other things also like using thicker shafts to reduce twist along the shaft and also reduce risk of recoil.

C.

Does this have to be Real Smooth for Video, or are these still shots??

The smoother the better. I think you’re trying to suggest a geared dc motor? they might be more suited for smooth motion - I know

I’ve spent an hour trying to find worm gears on the internet and the cheapest I are 15$ each - so 30$ total. that’s what I meant when I said they are hard to source :slight_smile:
I am starting to see an emerging pattern - the mechanical side of any project turns out to be the most expensive :slight_smile:

I can get the 28BYJ-48 motor for about 4$… Not sure if that’s becausethey are:
A)low quality and not very precise
B)made in china
ohh well anyway 8$ for too motors is dirt cheap so I might as well give them a try

I found cpwlibrary, which supports up to 1/64 step mode, and I built a simple driver... with a 200 step motor it supposedly can give 12800

It IS very smooth - though I can't tell if it's really 12800 discrete and precise steps as I can't really test it properly ATM as I lack space and it's almost midnight

Guess I just found use for that laser module I bought a while ago

servocity has a new worm gear drive assembly.

http://www.servocity.com/html/vertical_shaft_worm_drive_gear.html

I am done some test and microstepping does work but there are problems with accuracy - individual steps aren't equal... they are repeatable - meaning that I always end up in the same place (so if I move 2500 steps CW and then 2500 CCW I end up exactly in the same spot) but steps aren't exactly equal

The difference is minimal and I could mitigate that effect by using external gears but I am leaning towards 28-byj motors... does anybody know if they can be microstepped ?

There is no advantage to microstepping that motor. The steps would be uneven and in groups. Instead of using 8 microsteps why not gear another 8x? This would be the best solution if maximum speed is not an issue.

Ok thx I think I understand now - the motor is already very accurate so getting accuracy better than its 0.088 deg per step isn't likely - I guess I have to use external belt gear for extra precision