Variable Reluctance

Variable reluctance sensors are used to measure position and speed of moving metal components. This sensor consists of a permanent magnet, a ferromagnetic pole piece, a pickup coil, and a rotating toothed wheel.

As the wheel rotates, the reluctance of the flux path through the coil changes, and the flux linkage through the coil changes, which results in a change in voltage that is measured by an external circuit. The path of the flux generated by the permanent magnet varies as the toothed ring rotates in the field of the VR sensor.

The flux linkage varies periodically as the teeth pass the sensor. The flux linkage variation converts to a voltage signal. For example, the rotational speed of the ring of 600 RPM with 36 teeth around the ring could translates to a period of 2.8 ms for the sensor's output signal.

The major disadvantage of variable reluctance sensors is the decreasing signal strength as wheel rotation slows and approaches lockup.

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