Variable reluctance rotational speed sensor (VRS) with analog output
Phoenix America

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.