Controlled motors

(Apologies about my accidental half post)

Hi all!

I'm a physics researcher in Australia, and I'm looking at building a demonstration called a driven pendulum. Basically, it involves freely attaching a pendulum stick to the outside of a circular disk (say, 10cm radius, though the option of bigger is better) which is then attached to a motor. As the motor turns, it rotates the disk which drives the pendulum which creates really interesting motion. The pendulum stick could be anywhere from 30cm to a metre, depending on what is workable in the construction - the metre size would probably be a closer to my preferred choice.

What I'm uncertain about is what sort of motor to use. I need it to rotate with a constant, but adjustable, speed ranging from 0-3 rotations per second (higher is fine, but not really necessary). I'm just not sure how I would control a motor's speed like that or what type of motor to use. It has to be something that can operate with a reasonable load, consisting of the pendulum arm and the bob itself, which could be swinging reasonably fast (say max 1 rev/s at 1m arm length, worst case scenario) It would be necessary for the rotating disk not to be affected by this swings, at least ont significantly.

Does anyone have any suggestions on what I should be looking at for this kind of job? Any help would be much appreciated! Thanks for your time.

Cheers, Joel

--Ever seen a pendulum clock called a synchronome? Interesting variation on your theme; might suit the bill. Pendulum is driven by a falling weight; after weight falls it's automatically raised back up by a whack from a solenoid. Dead simple and makes for a really accurate clock. Used in RR stations in bygone days; maybe check at alt.horology for more info?

Hi,

Thanks for that info - sounds interesting, and I'll google it to have a look. But actually, I'm specifically interested in the chaotic motion of the pendulum. It's for a physics demonstration where we're looking at chaos and how by changing the driving frequency you can get different types of motion. This type of driven pendulum is well understood, so it would be interesting to actually build, but I can't find much information on the web about it being done before.

Cheers, Joel

--Check over at the Exploratorium store; IIRC they sell a model of one such and have an exhibit of same.

budget?

Hi,

The Physics Department is paying, but I would probably be looking at somewhere around the A$100 mark (which is about US$60-70). Obviously, the less the better :), but that would be a comfortable amount. Is something doable for that, do you think?

Cheers, Joel

I think a trip to the surplus store may be in order.

look for bodine motors. Something like that will be teaching your kid's kid's in the year 2080. You will need a suitable controller, but hopefully whomever sells you the motor, will have a controller.

Something like this is adaptable to other projects as well.

How 'bout a exercise treadmill? Has enough torque for a human being, has variable speed control; you should be able to pick one up cheaply at a garage sale, etc.

John

This is basically an application for a 'torquer' - you don't want gears - the surplus market may have something, but otherwise you could look at Pancake Motors (sometimes called 'Flat' or 'Flat Style'), which have many of the right properties. See

Dave