Variable Reluctance sensor waveform explanation - Page 1
Forum section: electronics / beginners

author: injb
author_rank: Regular Contributor
timestamp: November 13, 2018, 11:45:51 pm
content: 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 directionagain.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?

author: kim.dd
author_rank: Newbie
timestamp: November 14, 2018, 06:10:35 am
content: 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?

author: Sparker
author_rank: Regular Contributor
timestamp: November 14, 2018, 06:37:25 am
content: 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_inductionIt 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.

author: injb
author_rank: Regular Contributor
timestamp: November 14, 2018, 01:10:50 pm
content: Quote from: kim.dd on November 14, 2018, 06:10:35 amThanks. 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 amInteresting, 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.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 14, 2018, 01:40:26 pm
content: If you think about the integral of the waveform (the flux), you will get what you were expecting, I think.Tim

author: injb
author_rank: Regular Contributor
timestamp: November 14, 2018, 01:52:02 pm
content: Quote from: T3sl4co1l on November 14, 2018, 01:40:26 pmThanks, but I don't really understand that aspect of it properly. I thought it should be possible to understand it in simpler terms, but maybe it's not.The way I was looking at it, the object moving towards the sensor is what makes the voltage rise initially. Is that not right? If it is, then is it not possible to explain what makes it fall again in similar terms?

author: injb
author_rank: Regular Contributor
timestamp: November 14, 2018, 03:12:54 pm
content: From reading and thinking a bit more, I think it must have to do with the angle between the object's movement and the magnetic field lines, and the distance. So at long distances, the distance is the main factor, and as the object gets closer, the signal gets stronger. But at some point the object's direction of movement starts to line up with (or against, I'm not sure) the magnetic field, and so the signal starts to get weaker despite the object getting closer to the sensor. The peaks of the waveform represent that point. Is that the gist of it?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 14, 2018, 04:16:10 pm
content: It's not a simple situation to understand, however you cut it -- the field around the object is very inhomogeneous, the induced voltage is the derivative of the change in field (Faraday's law), good old fashioned field geometry, and etc.I would guess the double-peaked response comes from a flat shaped object, where the first corner passes (field is more intense around corners of permeable objects), then as the corner is (nearly) underneath the centerline of the sensor, voltage crosses through zero (flux change is zero), then it reverses as the field is less along the breadth of the object, then it peaks again in the opposite manner as it leaves.Suffice it to say, there's a blip of some sort, and detecting it with a high-pass filter and comparator should be adequate. More advanced signal analysis could be done (e.g., curve fitting to find the geometric center of the blip, at any RPM and ambient noise level), but that would be fairly heroic effort for little gain.Tim

author: max_torque
author_rank: Super Contributor
timestamp: November 14, 2018, 06:23:23 pm
content: That sudden voltage swing, caused by and increasing inductance suddenly becoming a decreasing one is the reason VR sensors are used for position determination. It's fairly trivial to have some basic electronics look for a zero crossing, and the sudden swing means that mechanical position is fairly rapidly reflected in an electrical one, ie the system can determine the tooth centre very accurately.The issue with VR sensors is that the peak of their output waveform IS dependant upon the speed with which the target passes the sensor (the sudden voltage swing occurs at exactly the same point, but it swings from a higher voltage as the speed increases. To compensate, devices are designed to allow a repeatable and stable zero crossing detection while the peak voltage can vary enormously. If you google "VR SENSOR AMPLIFIER IC" you will see such devices and how they are used