Variable reluctance sensor

A variable reluctance sensor (VRS) is 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 gear teeth of the rotating wheel (or other target features) pass by the face of the magnet, the amount of magnetic flux passing through the magnet and consequently the coil varies. When the gear tooth is close to the sensor, the flux is at a maximum. When the tooth is further away, the flux drops off. The moving target results in a time-varying flux that induces a proportional voltage in the coil. Subsequent electronics are then used to massage this signal to get a digital waveform that can be more readily counted and timed. Although VR sensors are based on very mature technology, they still offer several significant advantages. The first is cost--wire and magnets are relatively inexpensive. Unfortunately, the low cost of the transducer is frequently offset by the cost of the additional signal-processing circuitry needed to recover a useful signal.

And because the magnitude of the signal developed by the VR sensor is proportional to target speed, it is difficult to design circuitry to accommodate very-low-speed signals. A given VR-sensing system has a definite limit as to how slow the target can move and still develop a usable signal.

One area in which VR sensors excel, however, is in high-temperature applications. Because operating temperature is limited by the characteristics of the materials used in the device, with appropriate construction VR sensors can be made to operate at temperatures in excess of 300°C. An example of such an extreme application is sensing the turbine speed of a jet engine.

References

• Infolytica - Variable Reluctance Sensor
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