What’s the Difference Between Passive and Active Wheel Speed Sensors

There are two forms of wheel speed sensors currently used: magnetic inductive, additionally referred to as passive sensors and magneto resistive, or active wheel speed sensors. Both of sensors do a similar job, however they function in completely different ways. Both function effectively within their own ways, but the newer active wheel sensor is generally thought to be the more reliable.

Passive sensors have been around since the start of the anti-lock brake system (ABS). These sensors function on the generator principle. The speed sensors work with the toothed tone wheels to monitor and offer the anti-lock brake module (ABM) with wheel speed information. The actuator is really a toothed tone wheel that rotates with the average person wheel. Each tooth on the tone wheel acts being an actuator for the wheel speed sensor. Because the tone wheel rotates, the teeth go in and out from the proximity of the sensor. cup anemometer The effect is an alternating current (AC) voltage that is generated in the sensor coil by magnetic lines of force fluctuating as the tone passes by the magnetic sensor.

The output of the wheel speed sensor may differ from vehicle to vehicle because of: winding type, air gap, magnetic strength of sensor, metal properties of the tone wheel and wheel speed. Unfortunately these kind of passive systems have been prone to false cycling, which is the word used to spell it out an ABS cycle even though the road conditions usually do not dictate the need because of this cycle. This condition is quite susceptible to happen at slower speeds. This tends to happen because of wheel damage or rust build-up on the tone wheel. An incorrect sensor air gap will cause this condition, as well as debris on the wheel speed sensor tip.

Active wheel sensors have been around in use since 1999. This form of speed sensor helps to increase performance, durability and low speed accuracy. Active sensors do not appear to have the same false cycle issue of passive sensors do. Most vehicles with active sensors still work with a toothed tone wheel which acts because the trigger mechanism for the sensor. Some vehicles use a magnetic encoder instead of a traditional tone wheel. The encoders have north/south pole magnets imbedded in to the ring. The ring is then pressed on the axle shaft just like a tone wheel. In any case the result is really a digital square wave signal.

With this system, the ABM sends battery voltage to the speed sensor to power it up. The sensor, in turn, supplies the ABM a continuing 7 milliamp (mA) signal on a signal return circuit. With respect to the tone ring or magnetic encoder position, this 7 mA signal is turned on or off. The output of the sensor sent to the ABM is a Direct Current (DC) voltage signal with changing voltage and current levels. The ABM monitors the changing digital signal from each wheel speed sensor and is interpreted as wheel speed.

Leave a Reply

Your email address will not be published. Required fields are marked *