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Pan/Tilt with stepper motors

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HarveyH42

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While out last minute christmas shopping, I saw a cheap toy that gave me some ideas. Haven't really thought out all the details or direction I'll finally take, but I'll need a Pan & Tilt device.

Want to use steppers, because I have a bunch of them, have driver chips (haven't pulled them, or looked up the part numbers, but they go with 4 steppers I pulled out of a slot machine). Anyway, the load should be in the 10-12 ounce ranges, shouldn't be a strain; video camera, laser pointer, and a $15 dollar 'toy'.

I'll be using an AVR microcontroller, maybe two, to controller the steppers and the 'device'. Would like to do it wireless, but think that might add to the complexity and expense. I've got a big spool of 4 conductor telephone wire that might work well enough.

What I'm really looking for is how to setup the pan/tilt using steppers. I've seen a few using servos, might be able to use something from there, but would like to find something more stepper specific.
 
Servo's are easier, because they give absolute positioning - if you're using steppers you need some way to know where they are. This may be as simple as just looking at a monitor and moving them till they point where you want - otherwise you need to arrange some type of positional feedback, particularly during power up. You could even crudely use limit switches, and move the camera until it hits the switches - then you have an accurate starting point.

Even cruder is the method the old Commodore 1541 disk drives used - they simply stepped the motor hard into the end stop, making sure they did it more than enough to ensure it reached the end (this was why they made horrible banging noises when you inserted a disk!).
 
I should need to know absolute position electronically, just need to limit travel. I'll pointing it remotely from what I see through the video camera. Won't even matter where it starts from.
 
Travel limiting is easy with a couple switch sensors, just find out where the stops are going to be and put push button switches there so it knows when it's at it's limit. Since you're using steppers this will also give you absolute position. Just write the program so that it goes all the way to the left, and bottom till it hits the stops then move it from bottom to top counting the steps till it hits the next stop and you have the number of stops from bottom to top, do the same thing left and right and then just divide by two and run the stepper that number of steps from the 'home' position. Quick an dirty automatic centering.

Just a note, if you want any kind of precise ability to aim the camera/device you're going to want to use some kind of gearhead on the stepper. Otherwise your aiming ability is going to be limited the number of degrees per step (unless you can micro step the motor)
 
Here's a really low-end pan-tilt head. If you search around for "trackerpod", you can probably find some more links.

**broken link removed**

Both motors are in the base and drive the head using a pulley/tendon configuration. This allows it to slip if forced, but it also results in some pretty lousy accuracy/slop. Differing pulley diameters give it some mechanical advantage. "centering" is done through simian-visual feedback (i.e. keep clicking on the UI program until it looks centered, and don't ram it into the limits after that).
 
My skills with mechanical stuff and fabrication are kind of minimal, the results aren't pretty, but functional (divine grace,, meaning te most critical areas surprisingly turn out well...).

Basically, I don't anticipate a high level of precision, well beyond my abilities, so going to design around it. This is more of a cheap (hopefully) toy, but what I learn in the process will be useful for some future works.

Not sure how much holding strength I can expect out of the steppers, but I'd like to just adjust the position remotely, and have the camera+other stuff sit there until I need to change the position. I don't need it to scan, as my property is pretty well covered with fixed cameras.
 
Steppers are good for holding strength. If you're going to use stepper directly just keep the load light, as they can't be assured of stepping under significant starting loads.
 
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