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| Mjolinor:
I am just having some difficulty visualising this problem. It is related to an electronic ignition I am making. The situation is: I have a rotating disc with two magnets opposite each other. I have a pick up plate with two coils opposite each other. Manufacturing error means that these magnets and coils are not likely to be exactly 180 degrees apart. With the coils in series will I get a single sine wave that is exactly right irrespective of the actual angle between the magnets and between the pickups provided that angle is small? It is just baking my noodle a bit. Sometimes I conclude that there may be a slight DC offset, sometimes I think it's a sine wave and sometimes I think I may get two peaks. My brain is proper mush trying to get this straight. My gut says it is a single perfect sine wave. |
| rs20:
*ASSUMING* each magnet/coil interaction would produce a pure sine individually, the result of adding the two (or four, or any number actually) sine waves of any phase will just be another simple sine wave, with no DC offset. Now, given your description of the physical system, I would expect a pulse as each magnet went past a coil, not a nice sine wave, so that assumption above would be false. |
| Mjolinor:
I don't think the sine bit is important. I will just detect a level or a level change or a peak but let's assume pulses then. If the angle is small are the pulses 180 degrees apart. By small I mean that the magnet is still passing one coil when the second one passes. If you assume they are 178 and 182 (that is much more than it will be) then on one pass both coils will be level at the top of the wave together, almost half a rotation later (178 degrees) one will be at the max and 4 degrees behind the other when it passes but the peak will still be 2 degrees before that. So that is exactly 180 degrees between pulse max and in fact it should be 180 degrees between any two points on the generated waveform. I think, perhaps, maybe. |
| Ian.M:
Nope. As soon as one pickup gives a non-sinusoidal waveform you risk broadening or double-peaking of the pulse and if they don't get exactly the same field strength the pulse may be assymetric. Also if the two magnets are not exactly 180 degrees apart and exactly the same strength and radial distance, you'll get pulse to pulse jitter. As its a royal PITA to adjust the rotating magnets, a more sane design would be a single magnet + non-magnetic counterweight, as the pickup radial positions can easily be adjusted with mounting screws through two slotted mounting holes on the pickup and an eccentric screw in another hole, or a notch in the pickup and nearby hole in the baseplate to lever against with a 'tweaker' (made from a screwdriver with the end ground into a round pin at one edge of the blade), to adjust its position. Use a soft O-ring belt drive from a synchronous motor and a heavy flywheel to minimise wow and flutter and adjustment should be as simple as scoping the pickups in turn with a DSO, triggered from whichever you decide is the master, while you tweak the others for equally spaced pulses. |
| Mjolinor:
The more sane way to do it would be to stick one magnet and one coil on the crank instead of the cam. I am still not convinced. I think that whatever variation there is in pulse strength / position there will be a point on the wave edge that is always 180 degrees. When both coils are together then the rising edge will be steeper than when the coils are staggered. I think that maximum will always be the point that is 180. That's just about the hardest to detect typically. |
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