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

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

Passive sensors have already been around since the early days 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 a toothed tone wheel that rotates with the average person wheel. wind anemometer Each tooth on the tone wheel acts being an actuator for the wheel speed sensor. As the tone wheel rotates, one’s teeth go in and from the proximity of the sensor. The result 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 can vary from vehicle to vehicle due to: winding type, air gap, magnetic strength of sensor, metal properties of the tone wheel and wheel speed. Unfortunately these types of passive systems have been susceptible to false cycling, which is the word used to describe an ABS cycle even though the road conditions usually do not dictate the need because of this cycle. This condition is quite prone to happen at slower speeds. This will happen due to wheel damage or rust build-up on the tone wheel. An incorrect sensor air gap may cause this condition, and also debris on the wheel speed sensor tip.

Active wheel sensors have been in use since 1999. This style of speed sensor helps to increase performance, durability and low speed accuracy. Active sensors do not appear to have exactly the same false cycle issue of passive sensors do. Most vehicles with active sensors still work with a toothed tone wheel which acts as the trigger mechanism for the sensor. Some vehicles work with a magnetic encoder instead of a normal tone wheel. The encoders have north/south pole magnets imbedded into the ring. The ring is then pressed on the axle shaft as being a tone wheel. In either case the result is 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. Based on the tone ring or magnetic encoder position, this 7 mA signal is fired up or off. The output of the sensor delivered 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.

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