# Rotating Platform That Starts and Stops Incrementally

I'm not sure this is the right place to pose this question, as it probably encompasses more than one field - EE, ME, programming, etc. But I'll ask the question anyway and hope that someone might have at least a general starting place.

Basically I would like to build a circular platform. On the platform will be four different objects. I would like the platform to rotate exactly 90 degrees when triggered via a computer (or by hand for a lower tech option) and then stop automatically.

Ideally it would be nice for the entire system to be automatic, but this isn't entirely necessary.

I'm assuming I will need some kind of electrical motor and belt for the rotation... beyond that I'm not sure how to go about implementing the automatic stop at 90 degrees. If someone were to start it by hand, I suppose they could also stop it, but it is importnt that it stop at exactly 90 degrees...

Any suggestions or ideas would be appreciated!

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What you ask can't be done. Forget about it. Your spec of exactly 90 degrees is impossible, which should have been obvious. Perhaps you want the site for theoretical mathematics. We do engineering here and deal with real world problems. –  Olin Lathrop Apr 12 '12 at 12:14
I think he will be OK with circa 90 degrees with 1% tolerance, maybe even more. –  noah1989 Apr 12 '12 at 12:18
@noah: Not according to his spec he won't, and you have no way to otherwise judge at what level he is OK. Making up specs is even worse than requiring impossible ones. –  Olin Lathrop Apr 12 '12 at 12:20
Ignore Olin, he's still grumpy after seeing that Zener Diode schematic. How close do you need to get to 90 degrees? Is 89-91 ok? What about 85-95 ? –  Rocketmagnet Apr 12 '12 at 12:44
I wouldn't ignore Olin - he brings up a valid point. A stepper motor would be my first suggestion, but depending on the accuracy he needs there's a whole world of other solutions out there that he might have to look into. He even said - it's important that it stop exactly at 90 degrees. It's important, so how exact is it? –  AngryEE Apr 12 '12 at 15:21
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You'll be pleased to hear that this is not a hard task - as long as there are not difficult conditions that you have not revealed. The more detail you can provide the more cerain we can be that the answer really suits.

(1) It is "easily" accomplished using a "stepper motor" which has a preset number of steps per revolution.

or

(2) Using a single sensor (Hall cell or reed switch) and magnets on the table the table can be stopped at each magnet. Depending on rotation speed and how exact (exactly 90 degrees" is this may need slightly more electronics.

Other solutions possible.

How fast must it turn?
What table mass?
What diameter?
What environment?

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