- High Cost Service Applications
- Heavy Loads
- Corrosive or Washdown
- Temperature Extremes
- Wet or Dirty Environments
- Electrically Noisy Environments
Variable Reluctance Speed Sensor Technology
Variable-reluctance sensors are preferred for industrial and automotive environments, because they sustain mechanical vibration and higher temperature operation. In most applications, they sense a steel target that is part of a rotating assembly. Because the unprocessed signal amplitude is proportional to target speed, a sensor whose signal-processing circuitry is designed for high speed ceases to function at some lower rate of rotation.
Variable Reluctance sensors are often referred to a ‘passive’ magnetic sensor because it does not need to be powered. Variable Reluctance sensors are ‘self generating’
The variable reluctance wheel speed sensor is basically a permanent magnet with wire wrapped around it. It is usually a simple circuit of only two wires where in most cases polarity is not important. The physics behind the operation include magnetic induction.
A toothed ring on the wheel passes by the speed sensor and disrupts this magnetic field. The disruption in the field causes the wheel speed sensor to produce a sinusoidal (AC) voltage signal. The frequency and amplitude of the AC voltage signal are proportional to the speed of the wheel. The amplitude of the wheel speed signal is also directly related to the distance between the wheel speed sensor coil and the toothed ring. The distance is referred to as the air gap. This gap is critical to ensure AC output at lower speeds.
It is important to remember that air gap can change on a vehicle with loose wheel bearings or worn parts. Improper air gap can cause what is called sensor dropout. This can occur when the sensor will not produce output at lower speeds. This situation was found on some vehicles with an electronic speedometer. The customer would complain that while slowing to a stop, the speedometer would "drop off" or return to zero, maybe at around 8 to 10 mph. Too large of an air gap was the problem.
- Low cost, robust proven speed and position sensing technology.
- Self-generating electrical signal requires no external power supply.
- Fewer wiring connections contribute to excellent reliability.
- Meets a wide range of output, resistance, and inductance requirements so the sensor can be tailored to meet specific customer requirements.