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Magnetic Levitation |
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Visual Levitation |
Visual Levitation ImplementationAs an electronics hobbyist I love to see how things are built. Having designed my own levitator, I am always interested in improvements and may yet build my own new and improved model. (I know my toy is pretty lame!) This page has notes and comments about how the Visual Levitation device was built. Lifting CoilThe lifting coil measures between 19 and 20 ohms of resistance. It appears to be a single coil of 28 AWG magnet wire (not double strand or litz wire). So assuming no reactance/inductance we know the current draw in the coil is no more than 632ma max probably closer to 600ma and we can probably assume some loss in the H-bridge. I kind of figured his coil would draw more current at max, but I guess since it was optimized to run with the magnets doing all the work not the coils, it makes sense. Coil DimensionsThe coil assembly is approximately 63 mm in diameter and 87 mm tall. It provides 24 mm of flying height with the current adjustment. More detail is shown when you click on these photos.
Iron CoreThere is a plain 3/4" cylinder of iron core. As far as I can tell, it is ordinary iron stock. Although the good Dr. seems to have thought of everything, I don't see any attempt to reduce eddy currents. Perhaps it could be improved with this in mind. Typically it means cutting a slot down the side of the core, or using laminations such as say ten 16-penny nails with the ends cut off. Or maybe he did think of it, and eddy currents are negligible in this application (I doubt it). Flux ConcentratorThe patent describes a mu-metal flux concentrator on top of the coil, in a top hat arrangement. However, the commercial levitator does not include it. I would expect the flux concentrator makes no significant difference. There would still be a few inches of free space from this pole to the object, which would dominate the total magnetic reluctance in the flux path. ![]() Hall Effect SensorsThe patent suggests using two Hall-effect sensors under the lifting coil for increased sensitivity. The x-ray images apparently show two sensors, but it is difficult to make out. Certainly all the sensors are at the bottom of the lifting coil. There is obviously no sensor at the top of the coil assembly. I've seen no evidence of magnetic shielding around the sensors. The x-ray image does not show anything obvious. Since there is none at the top, I will assume it is not at the bottom, either. I wonder if it has problems with sensor saturation. The best device I've found looks like it saturates at about .16 Tesla. This is achievable with very strong magnets. However, the levitator works so well that either it has a sensor with a high dynamic range, or it mainly operates at lower levels. Mondo MagnetThe magnet and calibration rod are shown below.
Still checking to see if the unit uses a Neodymium Magnet, or one of the older Samarium types. Due to the fact Neodymium is more powerful than Samarium, you could possibly use a smaller magnet with the same lifting capability. The NdFeB is the Neodymium super magnets. Power Fail FeatureIn case of power failure, this levitator will park the object under the lifting coil. This is inherent to the operating point; the object is normally just above the magnetic balance point and the coil is normally pushing it downward. When power is lost, the result is that it jumps upward. A more sophisticated version of this levitator (not the kit) is sold for corporate displays and museums, and it will automatically re-launch the object when power is restored. Printed Circuit BoardThe control electronics are neatly manufactured with both feed-through and surface-mount components. Here are photographs of both sides.
Power SupplyThe power pack on my VisLev kit is rated at 12vdc and 1A. Since it never gets warm, I assume the kit is drawing much less than that.
PWM FrequencyThe coil is driven by a PWM frequency of 29us or ~34.5kHz. A friend found a potentialy useful H-bridge (Allegro 3959 or 3952), it has an adjustable PWM frequency, unlike the LMD18201. Frequency is the problem, too much inductance at high frequencies and it will draw too much current from the H-bridge, and at low frequency the coil gets hot. Need to find the happy medium. Ring Magnet SuspensionSo I read through patent application 20040052029, and one interesting thing is the "ring magnet" suspension. Of course this device doesn't use one (it is a follow-on patent) but a broad diffuse suspension seems counter to what I figured out. I still believe a good levitator needs a point source lifting from above, such as from a tapered iron core. This provides the lateral pull to return the object back to the middle of the lifting axis. |
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Last Update 2008-06-08
©1998-2023 Barry Hansen |