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First some basic theory: If a conductive material is passed through a magnetic field, there is a braking effect caused by the generation of current in the conductor. With a powerful magnet and Aluminium, this effect is quite noticeable. For example, a Neodymium magnet rolls very slowly down an aluminium
sheet.
For a clock, a small conductive wheel could be geared up and forced to turn through a strong magnetic field. The speed of the wheel will be slowed down, depending on the magnetic field, and the resistance of the wheel.
No tick, tock, but a very smooth turning of the seconds hand.
Cardioid
http://www.math.hmc...urves/cardioid.html
... to explain the "cardioid-shaped strip" annotation. [reensure, Jun 05 2006]
Torque curve of typical eddy current brake.
http://www.mech.ed....er/niall1/index.htm
See graph in section 3. [Ling, Jun 06 2006]
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Annotation:
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Let me see if I get it: You wind up a mainspring, and in place of the escapement there is just a smooth wheel, braked by a magnet, which (by feedback) regulates the rotation to one per second, or minute, or whatever. |
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Too complex, much better to use that energy (and electronics) to drive a small motor. |
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neelandan, I don't see the need for electronics. The magnet is a permanent magnet. Everything remains 'constant', and so the speed is 'constant'. Trim can be made by gap, or material dimensions. |
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Be careful, Ling - Rolex is watching you. |
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...very, very expensively... |
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Size the cardioid-shaped strip and wheel right, and this would work while still providing a second hand "slip". |
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[BrauBeaton], I can guarantee that it will be correct twice a day (sorry, it's an old joke). |
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[reensure], I'm intrigued about why you need a cardioid strip. Please tell more. |
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The point of a pendulum (spring or gravity type) is that the frequency is nearly constant as the driving force varies. Unless you had some additional means of ensuring constant torque, wouldn't your clock slow down as the spring winds down? |
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Might be reasonably accurate if powered by a falling weight on a string, though. |
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An eddy current brake has a strange torque curve. Below a certain critical speed, the retarding effect increases with speed.
That is, at v.low speeds, the braking force is hardly anything.
At slightly higher speeds, up to a point, the braking force increases.
This means that there is a sort of speed regulation automatically built in; but I do not claim to surpass a pendulum's regularity. |
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See the link if you are interested: the torque curve is similar to that of a squirrel cage induction machine, if you compare it with 'slip' in mind. |
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Interesting link. I agree that the torque curve is strange. I'm not sure that it helps much. In the region below the critical speed, torque is directly proportional to speed, or, which is the same thing, speed is directly proportional to torque. I assume this is because the metal is essentially an ohmic conductor. This is similar to dynamic mechanical friction which (theoretically) increases in proportion to speed. The difference is that there is no equivalent here to static friction. |
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Above the critical speed, it would be highly unstable, as torque decreases with speed. |
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I like your idea, but I still think you need to supply a constant torque, meaning the inductive brake will not regulate the speed on its own. |
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//cardioid strip// I think [reensure] is getting your example illustrating inductive braking theory (a rolling magnet) mixed up with your Idea (a conductive wheel rotating in a magnetic field, but remaining in a fixed position). |
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Yes, I know what you mean. It's far from perfect. But from my knowledge of induction machines, the shape of the torque curve depends on the (equivalent)resistance of the rotor. This is usually done on traction or hoisting applications, by using a slip ring on the rotor and switching in different resistances.
A higher resistance means a flatter curve, less peak, and more speed drop when a load is applied. It is usually applied at low speed/ high torque instances. Once the machine is moving, the resistances are gradually shorted out, and the torque curve comes back to the one that is in the link. |
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Now, when a high resistance motor is slightly loaded, the speed will drop noticeably.
But when a low resistance motor is slightly loaded, the speed will drop only a little, because the torque curve is so steep ( a small change in slip gives a rapid increase in motor torque, which nearly compensates for the increased load torque). It's not perfect, of course. |
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So I'll come to the point...finally, the resistance of the wheel is very important for the escapement. It think it needs to be as low as possible, to give as near a constant speed as possible, with varying 'load' torques. I doubt whether much work has been done on this particular aspect of eddy current brakes (except they are well known for giving constant torque at varying speeds, when over the critical point). |
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By the way, one low resistance material that I know of is a superconductor :) |
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But apart from that, it would be a good excuse for using larger quantities of precious metals (silver) in expensive watches. |
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