Linear inductive position sensor - Page 1
Forum section: electronics / beginners

author: Howey
author_rank: Newbie
timestamp: June 17, 2025, 10:52:07 pm
content: Hi,I am a mechanic, trying to wrap my head around a sensor on a truck gearbox. It is described in the manual as an inductive position sensor. It is fixed in place, and looks like any other 2 wire position sensor, except that there is a hole in its centre that allows a hollow metal pin, that is fixed to a gear selector fork, to move into it.The operation explanation in the manual just says that the position of the pin changes the inductance of the coil, and the ECU reads that and interprets a position.Delving beyond this has led me down so many rabbit holes that I don't know which way is up. What I have been able to work out is that the ECU delivers 5v DC to the sensor at 100Hz on/off.I had one person tell me with authority, that the ECU is reading the back EMF spike that happens when the power supply is switched off, and figures a position by the intensity of the spike. I can see how that might work, but I can't find anything to confirm it. Another piece of information says that the ECU provides the switched 5v, and measures the amount of time it takes for 5v to appear on the return line, which in turn is a result of the inductance of the coil at that time. I can also see how this might work, but can't find anything to confirm.I've attached a picture of the sensor. From what I can tell, it has a single coil, so not differentiating between decoupled voltages, but finding specifics has not been easy. Any insight appreciated.Ideally I would like to be able to explain the operation, and demonstrate its operation on an oscilloscope.

author: Circlotron
author_rank: Super Contributor
timestamp: June 17, 2025, 11:50:03 pm
content: There might be a resistor in the return line to the ECU and the voltage drop, hence the current is read from the resistor. Variable inductance = variable 100Hz current.

author: Whales
author_rank: Super Contributor
timestamp: June 18, 2025, 12:05:06 am
content: QuoteYes it could be either of those techniques. They are both different ways of measuring the inductance of the coil. The coil's inductance depends on the material inside it (air or steel).An inductor does not like changes in current. If you try and change the amount of current flowing through it then it will onlyslowlycomply. The more inductance the more it will try and slow changes in current down.The two techniques you mention might be related to two of the techniques I mention below, but there is some info missing so I won't claim 100%.(1) Cycle voltage-short-voltage-short at a fixed frequency, measure current with a series resistor.For this to work you need a relatively small resistor is series with the coil to measure its current ("resistive current shunt").Let's say the ECU puts 5V on the coil for 10 microseconds, then 0V (short) on the coil for 10 microseconds, then repeats. During the 5V period the amount of current travelling through the coil will slowly rise. During the 0V phase it will slowly drop. If you graph this it will look a bit like atriangle waveform.The "height" of the triangle wave on your graph depends on how quickly the current can change. This depends on the inductance. This depends on the coil's core material.My choice of 10usec is arbitrary, in reality you'd change the speed of driving depending on the size of coil. Tweak till the graph looks relatable and friendly.(2) Cycle voltage-open-voltage-open at a fixed frequency, measure flyback voltage with a parallel resistor.For this to work you need a relatively large resistor in parallel with the coil ("resistive load").Apply 5V to the coil for 10usec (arbitrary guess), then disconnect your 5V completely (this isdifferentto "applying 0V"). The coil will want to keep current flowing, but the only place it can now go is through the resistor still attached to the coil. The coil has to create a voltage to force the desired amount of current through the resistor (ohm's law V=IR). This voltage can actually be much bigger than the original 5V (depending on many different parameters), but it can also be the same or smaller.The voltage generated depends on the amount of current flowing. The amount of current flowing depends on the inductance of the coil. The inductance of the coil depends on the coil's core material.(3) Use the inductor as part of an oscillator, measure the oscillation freq.If you put an inductor and a capacitor in parallel then they will want to resonate at a particular frequency that depends on the amount of inductance and the amount of capacitance. They won't do it on their own, to make them constantly oscillate you need to add energy (I won't go into detail here, big topic). If you do this you make a device that outputs an oscillating voltage (on-off-on-off) forever.The oscillation frequency depends on the resonant frequency of the inductor+capacitor circuit. The resonant freq depends on the inductance of the coil. The inductance of the coil depends on the coil's core material.

author: Howey
author_rank: Newbie
timestamp: June 18, 2025, 03:36:39 am
content: Thank you. I managed to weasel my way in to get more information, and now it makes sense.There is a 100 ohm pull down resistor in the ECU. The time to go from 0V to 1/3 of V supply is the measured portion. Vmin and Vmax are used as fault triggers if below or above thresholds.

author: Poroit
author_rank: Frequent Contributor
timestamp: June 18, 2025, 04:27:04 am
content: G'day Howey,This link may be of interest.https://www.youtube.com/watch?app=desktop&v=TTLNAaYplH4