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Variable Reluctance sensor waveform explanation
injb:
Hi...I could use some help understanding this waveform (attached). It's from a variable reluctance sensor that picks up a screw in the flywheel of an engine, so that the computer knows the precise angle of the crankshaft.
I can imagine the metal part getting closer, and the voltage going up, and then peaking when it gets closest to the sensor, then dropping off again. But that's clearly not what happens. After it peaks and changes direction, it goes negative, and peaks and changes direction again.
I've read that the zero crossing point is where the metal actually passes closest to the sensor. That kind of makes sense because that makes it symmetrical. But that makes it hard to understand what happens just before and after that centre point. Why does the voltage change direction at those 2 points just before and after the centre?
kim.dd:
The sensor is build using a coil with a magnet at its back. So a change in flux is going to cause a changing voltage in the pickup coil.
This page has an animation of the field:
http://www.movingmagnet.com/en/variable-reluctance-sensors/
When the metal is at the center you can see the flux reach zero, so the voltage in the coil should also be zero?
Sparker:
If I understand correctly this is basicly a (few turns of a) coil sensing magnetic field. Then it should be true that the voltage induced in the coil is proportional to the derivative of the magnetic flux:
https://en.wikipedia.org/wiki/Faraday%27s_law_of_induction
It looks like that's what you see here. First the sensor feels the field becoming stronger and you see positive output, then as it passes the center of your magnet the field becomes constant at some point and the sensor output becomes zero. After that the field is climbing down and you get negative output. I am not totally sure about the specific construction of your sensor/engine and if the flux becomes zero in the middle or what exactly happens... but I guess it should explain the negative output a bit.
Also it seems to me, if you integrate your sensor signal over a full period, you will get zero. If we get something different from zero, it will mean that the magnetic flux through the sensor is rising on the average with every engine rotation, which makes no sense here. :-// That's why you have a second little positive pulse, its area combined with the area of the first pulse compensate the area of the negative pulse.
injb:
--- Quote from: kim.dd on November 14, 2018, 06:10:35 am ---The sensor is build using a coil with a magnet at its back. So a change in flux is going to cause a changing voltage in the pickup coil.
This page has an animation of the field:
http://www.movingmagnet.com/en/variable-reluctance-sensors/
When the metal is at the center you can see the flux reach zero, so the voltage in the coil should also be zero?
--- End quote ---
Thanks. I don't really understand the animation, but I'll use it as a starting point anyway.
--- Quote from: Sparker on November 14, 2018, 06:37:25 am ---
...
It looks like that's what you see here. First the sensor feels the field becoming stronger and you see positive output, then as it passes the center of your magnet the field becomes constant at some point and the sensor output becomes zero.
...
--- End quote ---
Interesting, thanks. You've touched on what confuses me here. The voltage reaches a maximum at some point before the object reaches the centre of the magnet. Why? And if the object doesn't stop moving, why would the field become constant?
Regarding the construction of this particular sensor, I think it's pretty basic. I saw a video on youtube where a guy attaches a simple coil inductor to a scope, and waves it past the little magnet on the bottom of a work light, and he gets the same waveform I've shown here. So I don't think there's anything about this sensor in particular that's causing it.
One interesting thing is that as you pointed out, it's not totally symmetrical. The factory manual for this car shows it as asymmetrical too, but with the positive peak being higher.
T3sl4co1l:
If you think about the integral of the waveform (the flux), you will get what you were expecting, I think. :)
Tim
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