Animation Fundamentals: Magnets
Magnets are often used in animation, but not as often as you might think. This is because magnets are surprisingly difficult to work with. Time after time an enthusiastic modeler will get "clever" and attempt to solve an animation problem with magnets, only to be disappointed by the behavior: the animated object—usually a small vehicle—moves along in jerks. It will lurch forward, pause, then lurch again. Darn! What's wrong with it?
Here's the problem: as the hidden magnet moves, the vehicle remains stationary until the magnetic pull overcomes the friction inherent in the vehicle's wheels, axles, etc., plus any surface irregularities. When that friction is overcome, the vehicle starts moving, accelerates, and quickly catches up with the moving magnet, sometimes even overshooting it a bit. Once caught up, friction brings it to a halt, and the cycle is repeated, over and over again.
Is there anything that can be done to stop this? Yes and no. I say "yes and no" because whatever you've already built will probably need to be rebuilt employing one of the following two approaches. However, success depends on whatever it is you're attempting to accomplish, and there are a great many things that are quite simply impossible to do with magnets. Sorry to disappoint, but trust me, BTDT, got a lot of T-shirts.
Balanced Friction
The first trick is to substantially increase the friction in the system, as counterintuitive as that may seem. Note, however, there comes a point of having too much friction; a delicate balance must be struck, and this can only be achieved by trial and error. Too much friction, or not enough, will result in the lurching behavior, which in this case will often be more like jittering. The level of friction required is such that, if your vehicle is wheeled, the wheels will more than likely need be stationary; depending on the vehicle, this may look awkward. Also, the surface upon which the vehicle moves needs to be quite smooth; an irregular surface will not work. Thus, the technique is limited to certain applications.
I employed this technique to make a kiddie train ride. The train has no wheels; instead, the loco and cars have styrene "sleds" with embedded magnets (below left) that slide along brass wire rails (below right).
A chain-powered mechanism moves a magnet underneath the track. Success relied on achieving just the right amount of friction: I tried a number of different sled sizes and materials until I found the perfect combination. The balance is so delicate that I must occasionally polish the rails, as oxidation increases the drag, and the train begins to jitter. Also note that the train isn't pulled by the locomotive; each car has its own magnetic sled and drive magnet underneath.
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Brute Force
The other approach I've found that overcomes the lurching effect is to use brute force: create a magnetic field so powerful that it forces the vehicle to stay with the drive magnet. This requires the strongest magnets you can find, positioned as closely together as possible. It also relies on the surface upon which the vehicle rolls to be as smooth as possible; irregularities will cause lurching or jittering. One final caveat: it only works at slow speeds; increase the speed, and it will begin lurching. (As it happens, at slow speed the vehicle is still jittering, but the amount of jitter is too small to see.) Thus, the technique is limited to certain applications.
I successfully employed the brute force technique for my road compactor: after modifying the kit so that the rollers rotated smoothly, I fitted it with a pair of super-magnets (above left) mounted on a spacer to result in the smallest air gap possible under them. Note that this was a challenge, because the magnets had to be oriented the same way; this meant they naturally repelled one another, and getting them installed involved an hour of frustration and a lot of super glue. The moving magnet (above right) is quite a bit larger, and is mounted in a sled connected to a chain drive. Between them is a sheet of 0.020" thick black styrene.
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Other Uses
All of this may make it seem as though magnets are almost useless. Under a lot of circumstances involving free movement across fair distances, they pretty much are. However, magnets have a great many other applications: they're excellent for positioning and affixing objects to one another non-permanently. They're good at moving tiny objects back and forth tiny distances—consider the rocking chair—and they're able to move ice skaters around a rink fairly convincingly. They give self-powered vehicles the ability to negotiate roads, as they control the steering. And, for many decades they've been indispensable for automatic coupling. But all of these are highly specialized applications, as opposed to more ordinary animations that most often spring to mind.
As I said, a lot of this is probably counterintuitive, and I imagine some modelers simply won't believe me; they'll need to try for themselves. And they'll fail, just as I have many times. Or, they may get lucky and succeed, in which case I'd bet what they did fits into one of the two methods I've detailed above. Having said all of this, if you find another way that works, please let me know, and I'll be more than happy to publish it here, with all due credit.