Magnetic Inductance

Magnetic inductance sensors consist of a coil of wire around an iron core plus a permanent magnet. The magnet can be either stationary or movable. If the magnet is the moving member, as it passes the coil the magnetic lines of force cut through the coil and a voltage is produced. Since the north and south poles of the magnet alternate as they pass the coil, the voltage polarity also alternates. As the speed of the magnet rotating past the coil is increased a larger voltage is produced and the frequency of the voltage polarity changes is increased. This same type of sensor can also work if the magnet is stationary and attached to the core of the coil. When a toothed reluctor, or rotor (made from a magnetic material) is rotated past the coil and magnet, the magnetic lines of force move and cut through the coil. The lines of force cutting through the coil will produce the same type of voltage output as when the magnet was moving.




This type of sensor is commonly used as a wheel speed sensor on ABS equipped vehicles. This sensor is also used in the distributor to determine RPM and crankshaft position. Since the voltage output of this sensor is varying continually and is low at low speeds, the computer must be able to sense the small voltage. If electrical interference is allowed to combine with the signal voltage, the computer could be fooled. To prevent stray electrical interference, the signal wire usually has a ground shield formed around it like the knock sensor.


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