Variable reluctance sensor

A variable reluctance sensor (commonly called a VR sensor) is a transducer that, when combined with very basic electronic circuitry, detects the change in presence or proximity of ferrous objects.

With more complex circuitry and the addition of software and specific mechanical hardware, a VR sensor can also provide measurements of linear velocity, angular velocity, frequency, etc.

A VR sensor used as a simple proximity sensor can determine the position of a mechanical link in a piece of industrial equipment.

A Crankshaft position sensor (in an automobile engine) is used to provide the angular position of the crankshaft to the Engine control unit. The Engine control unit can then calculate engine speed (angular velocity).

Speed sensors used in automobile transmissions, are used to measure the rotational speed (angular velocity) of shafts within the transmission. The Engine control unit or Transmission control unit (depending on the particular automobile) uses these sensors to determine when to shift from one gear to the next.

A pickup used in an electric guitar (or other musical instrument) detect vibrations of the metallic "strings". See Pickup (music technology) for details of this application.

This sensor consists of a permanent magnet, a ferromagnetic pole piece, and a coil of wire.

VR sensor interface circuits VR sensors need waveform shaping for their output to be digitally readable. The normal output of a VR sensor is an analog signal, shaped much like a sine wave. The frequency and amplitude of the analog signal is proportional to the target's velocity. This waveform needs to be squared up, and flattened off by a comparator like electronic chip to be digitally readable. While discrete VR sensor interface circuits can be implemented, the semiconductor industry also offers integrated solutions. Examples are the MAX9924 to MAX9927 VR sensor interface IC from Maxim Integrated products, LM1815 VR sensor amplifier from National Semiconductor and NCV1124 from ON semiconductor. An integrated VR sensor interface circuit like the MAX9924 features a differential input stage to provide enhanced noise immunity, Precision Amplifier and Comparator with user enabled Internal Adaptive Peak Threshold or user programmed external threshold to provide a wide dynamic range and zero-crossing detection circuit to provide accurate phase Information.

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