I'm doing work to an electrical engineer graduation subject about the transducer. I'm studying the magnetic transducer just to understand how it works and I had a little doubt.

Let's look at the image first to be more clear:

enter image description here

As you can see the image shows a Hall effect sensor that measures the crankshaft angular speed.

I understood that when there's a significant magnetic field or flux sensed by the Hall sensor its output, in the case of a digital sensor, will be high, otherwise, it will be low voltage.

I reflected on the Hall voltage formula: $$ V_H= R_H \cdot(\frac{B.I}{t}) $$

VH is the Hall voltage in volts, RH is the Hall effect coefficient, I is the current flowing through the sensor in amps, t is the thickness of the sensor in mm, and B is the magnetic flux density in Teslas.

In my point of view, only the magnetic flux is varying somehow in accordance with the crankshaft spinning, but I don't know how to explain it.

Could someone help me with this question?

  • 1
    \$\begingroup\$ The teeth on the wheel are ferromagnetic, causing a detectable change in the magnetic field when they move closer or farther from the detector. \$\endgroup\$
    – Eugene Sh.
    Commented Mar 8, 2021 at 18:45
  • \$\begingroup\$ It might help to draw a diagram of the two states with minimum distance of the tooth and maximum distance of the tooth and then add the flux lines. You'll see that the flux density changes quite a bit. The actual hall sensor is much smaller than the cross section of the sensor face. \$\endgroup\$ Commented Mar 8, 2021 at 19:11
  • 1
    \$\begingroup\$ en.wikipedia.org/wiki/Variable_reluctance_sensor \$\endgroup\$ Commented Mar 8, 2021 at 20:12
  • \$\begingroup\$ Variable reluctance as Bruce says. \$\endgroup\$
    – user16324
    Commented Mar 8, 2021 at 20:17

2 Answers 2


The permanent magnet on the sensor creates a magnetic field. The teeth will create a change in the magnetic field that is detected by the hall sensor as Eugene explained.

You are going too far in your thinking, the system only detects pulses as the teeth are passing in front. It's a simple comparator that compares the field when there is a tooth or not, and the output signal is a square wave.

The logic system (MCU) will simply check the frequency of that square wave to calculate the speed of the motor, you need to know the number of teeth.

It doesn't use the analog magnetic flux.


Either the teeth are magnetic or there are strong magnets fitted into the flywheel usually at TDC (Top Dead Centre) and some amount of degrees before (or after) 10, 20 or 30 maybe.

The ecu can then use the duration between the two pulses to calculate the speed of the engine (rev/min) and the rate of change which is used to predict fuel delivery.


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