Using a motor to rotate a sphere from the inside 3

The concept of something for the motor to react against comes to mind.

The base probably has a slowly rotating magnet that will align poles with the magnet in the globe (causing globe to continually rotate.)Have you seen the hot plates with magnetic stirrers? Assume the mechanism is similar, but at a much slower speed.

Install an on/off switch so you can stop it where you want.

Incidentally the levitating system is non trivial

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

I have a small levitating globe supplied as a desk toy.

The levitating system comprises a powerful permanent magnet on an arm arching over the globe, surrounded by an electromagnet, plus a powerful permanent magnet in the globe's north pole. (Plus a pair of permanent magnets, south pole and base, to keep the globe from rolling off the desk when the levitation fails.)

I don't know if feedback comes from the electromagnet itself or some transducer, but there's a fairly fancy circuit, or a microcontroller, hidden in there somewhere. The levitated air gap is about 8 mm, but you have to manually position the globe within about a half mm of that in order for the electronics to 'capture' the globe, else it jumps up and sticks at the north pole, or falls and sticks at the south pole.

Once the globe is captured, it is free to rotate about the one axis. You can carefully spin it, or even more carefully spin it during the capture process, and it will keep spinning for quite a while.

I'm guessing you could rotate it with an external magnetic field, but that would be fancy and might interfere with the levitation. A gentle puff or stream of air from a hidden nozzle would be sufficient, and simpler.

As for positioning, or rotating about other than vertical axes, I'd expect the required hacking effort to go up rapidly.


Mike Halloran
Pembroke Pines, FL, USA

The control problem with a dangling sphere is much less complex than one levitating over the support system.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

Well, you could look at how satellites do it.

Internal gyroscopes as I recall.

If you can levitate it with magnets or whatever, your motor drives a blower that exits through a tangential hole. You could suck the air in through the bottom.

JHG

Heck, just put a motor in side with the "stator" attached to the globe.

Turn motor one way the stator will react in the opposite direction.

Two motors gets you two-axis control.

However anything done inside the globe means it needs power and control signals.

If there is already a levitation control system it would seem less expensive and less complex to incorporate motion control into the levitation control.

The key is conservation of angular momentum.

So, all you have to have is a well balanced motor housing with a double ending shaft and tie both shaft ends to the equator, instead of a normal motor spinning the shaft you have the shaft fixed to the sphere and the motor is spinning. The single axis control should be very simply

Isphere*W+Im*wm=0

which states the conservation of angular momentum

All you need is to hold the motor speed constant

2 axis control can be accomplished with an orthogonal copy of the above so you can get any orientation of spin you want.

That ignores air drag

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

Probably negligible for his speeds; if not a simple servo control can correct for it.

I just had this vision of the motor accelerating away to infinity and beyond

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

thanks guys for the tips.

I was thinking kind of along your lines zekeman on the conservation of angular momentum. i'm looking for the solution with the least amount of components and power usage. Since only 12oz can go in the globe. Of course at the same time I don't want to sacrifice the precision of the mechanism. zekeman's idea should work fine if i could find double shafted small motors. But i'm not sure if that would give me as much control as a servo motor allows. Wouldn't this work the same way with servo motors? Except in the opposing axis.

something like the rotating person with bicycle wheel experiment popped up in my mind:

now if i can use this analogy correctly. The globe is the near frictionless spin disk the man stands on. And so if i can have something inside the globe that works just like the bicycle wheel, the globe should spin? So i would have some motor driving a small wheel that could be flipped depending on the direction i want the globe to spin.

Would this be overkill and not as efficient as the two motor solution?

Without bothering to look at the youtube I will assume it to be a demonstration of the physics of a gyroscope.

To implement that you would need a motor to spin the gryo and a linear actuator for each axis of motion.

My suggestion relies on the same principle as Zekeman's. Although I don't understand why he thinks a double shaft motor is necessary.

The principle is simply action-reaction. I doesn't matter if the shaft or the "stator" of the motor is fixed to the globe.

yep mintjulep your post was pretty much what I was talking about. sorry i didn't get it completely in the first read. lol i wasn't too sure what a stator was till now.

seems like the gyroscope idea(with the motor, gyro and linear actuator) though might be more difficult to implement. would you guys agree?

sorry for the double post but...

I think the reason zekeman suggested the double shaft is to allow greater control. With simply two motors with the stators attached, control might be more difficult, especially with a conventional motor. Once you apply the force to an axis, how would you stop the globe from rotating when you know it's rotated to a point that you want? One way is to have the motor turn the other way, but having to adjust like that seems quite difficult. Especially since there is no friction involved, with a little force the globe can spin almost forever. Its definitely mechanically impossible to stop the globe from moving at one point. Please point out if I'm wrong here.

the same problem occurs for the bicycle gyroscope idea. In the video, when the guy flips the bicycle wheel vertical to the ground, the man stops spinning. But not immediately, he de-accelerated due to friction. Unfortunately, the globe is nearly frictionless with some small air drag. Thus, there is no way to stop the spinning unless i reverse the spin of the gyro. And that brings me back to the problem above with the stator.

this is a problem i need to mull over :/

Ah, a new requirement.

Now we have to be able to accurately control and stop rotation. That wasn't clear (at least to me) before.

Pairs of counter-rotating motors for each axis. Control by differentiating the speeds.

Getting ever more complicated.

Go play with one of those mini RC helicopters.

Crazy idea -

How about a toy helicopter with three tail rotor blades all in different axis. Get a gyro autopilot to maximize control.

Fake it.

Project the image onto the surface of the sphere from the outside. Then it just becomes an image manipulation problem, with the sphere able to rotate in any direction irrespective of the image.

Craig Pretty
Tru-Design Plastics