Vehicle Speed Sensor

Basic Description

Several automotive systems rely on information about the vehicle speed in order to function properly. Transmissions rely on vehicle speed information for optimizing shift strategy. Cruise control systems must monitor vehicle speed in order to make appropriate throttle adjustments. ABS systems, Electronic power-assisted steering systems and engine control systems all utilize vehicle speed information in one way or another. Vehicle speed sensors have traditionally been mounted on the transmission or rear differential, where they monitored the rotation speed of the drive shaft. These sensors can provide incorrect information about the vehicle speed if one or more tires do not have good traction or if the vehicle is skidding. Recently, it has become more common to calculate the vehicle speed based on information from the wheel speed sensors associated with the ABS system.

Vehicle speed sensors are usually either inductive or optical sensors. The most common inductive sensors consist of a rod magnet on top of a magnetic pin that is surrounded by a fixed coil. This sensor is mounted a fixed distance from a ferromagnetic rotor with teeth. As the rotor turns and a tooth comes into the proximity of the rod, the magnetic flux in the coil changes. This change in flux results in a voltage pulse across the coil. The vehicle's engine control module counts these voltage pulses and computes the vehicle's speed. Optical sensors also generate pulses at a frequency corresponding to the rotor rotation; except instead of measuring magnetic flux, the optical sensor measures reflected light. The rotor either has light and dark marks and the optical sensor detects the reflected light using photosensors.

Manufacturers

Beru, Bosch, Continental, Delphi, Honeywell, MSC

For More Information

[1] Vehicle Speed Sensor Description & Related Information , Engine-Light-Help.com

[2] Position/Speed Sensors, Toyota Motors Sales document at Autoshop101.com

[3] How Speedometers Work, William Harris, HowStuffWorks.com, July 10, 2007.

[4] VSS or Vehicle Speed Sensor, YouTube.com, Jan. 2008.