The Thrumold Series VRS (Variable Reluctance Speed) sensors are designed to detect engine timing and transmission speedby determining the speed and position of rotating shafts in heavy-duty, off-road, and construction vehicles, providing information to help improve engine combustion process efficiency. These passive VRS sensors are simple, rugged devices that do not require an external voltage source for operation. A permanent magnet in the sensor establishes a fixed magnetic field. The approach and passing of a ferrous metal target near the sensor’s pole piece (sensing area) changes the flux of the magnetic field, dynamically changing its strength. This change in magnetic field strength induces a current into a coil winding which is attached to the output terminals. The output signal of a VRS sensor is an ac voltage that varies in amplitude and frequency as the speed of the monitored device changes, and is usually expressed in peak to peak voltage (Vp-p). One complete cycle occurs as each tooth passes the sensor’s pole piece. If a standard gear were used as a target, this output signal would resemble a sine wave if viewed on an oscilloscope.