Variable Reluctance Linear Position Sensors and Switches Information
Variable reluctance (VR) linear position sensors and switches are noncontact devices which measure the position and speed of moving metal components.
The sensor head of these devices essentially consists of a coil of wire wrapped around a magnet. As gear teeth (or other target features) pass by the face of the magnet, they cause a time-varying magnetic flux that induces a proportional voltage in the coil. Electronics massage this signal to get a digital waveform that can be counted and timed. This image illustrates the sensor and toothed target.
Image Credit: Moving Magnet Technologies S.A.
VR sensors are a type of inductive sensor, meaning they can detect the presence of metal objects without touching them. These sensors are also deemed passible sensors because they require no power supply.
- LVDTs (linear-variable-differential transformers) are VR sensors with high outputs and reasonable temperature performance.
- LVRTs (linear-variable-reluctance transducers) are less expensive than LVDTs but have lower outputs and may require support electronics.
VR sensors have a number of important specifications used to describe performance.
- Operating distance, air gap, or mechanical gap describes the distance between the end of the sensor and the metal object being detected. For most VR sensors, this distance can extend up to 3 mm.
- Accuracy describes the percent deviation from the actual/real value of the measurement.
- Operating temperature specifies the range of temperatures the sensor is designed to operate effectively.
Features for VR sensors may be important to consider based on the application.
- Output describes the means by which the sensor signal is translated to a usable form. Analog outputs use current, voltage, or electric charge signals, while digital sensors use binary or other computer code.
- Sensors can be connected via cable (often includes "bare" stripped leads) or via an integral connector for a specific system.
- Sensors can be shielded (protected against EMI and RFI), weld field immune, short circuit protected, and/or intrinsically safe (cannot cause ignition of atmospheric mixtures).
Because the magnitude of the signal developed by the VR sensor is proportional to target speed, Hall Effect sensors are many times the better choice. Although VR sensors have low cost transducers, additional circuitry may be required to accommodate the very-low-speed signals. However, these sensors excel in high-temperature applications. VR sensors can be made to operate at temperatures in excess of 300°C. They are used in automotive engines, aircraft turbines, oil pumping equipment, ships at sea, and industrial applications utilizing conveyor belts.