How does a magnetic sensor measure the angular speed of a crankshaft? How does its spinning affect the magnetic flux?

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:

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?

• The teeth on the wheel are ferromagnetic, causing a detectable change in the magnetic field when they move closer or farther from the detector. Commented Mar 8, 2021 at 18:45
• 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. Commented Mar 8, 2021 at 19:11
• en.wikipedia.org/wiki/Variable_reluctance_sensor Commented Mar 8, 2021 at 20:12
• Variable reluctance as Bruce says.
– user16324
Commented Mar 8, 2021 at 20:17