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

There are two types of wheel speed sensors currently used: magnetic inductive, more commonly referred to as passive sensors and magneto resistive, or active wheel speed sensors. Both of sensors execute a similar job, but they function in very different ways. Both function effectively within their own ways, but the newer active wheel sensor is normally regarded as 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 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. wind speed sensor Because the tone wheel rotates, the teeth go in and out of the proximity of the sensor. The effect is an alternating electric current (AC) voltage that’s 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 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 susceptible to false cycling, which is the word used to describe an ABS cycle even though the road conditions do not dictate the need for this cycle. This condition is quite prone 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 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 use a toothed tone wheel which acts because the trigger mechanism for the sensor. Some vehicles use a magnetic encoder in place of a normal tone wheel. The encoders have north/south pole magnets imbedded in to the ring. The ring is then pressed on the axle shaft as being a tone wheel. In either case the result is really a digital square wave signal.

On this system, the ABM sends battery voltage to the speed sensor to power it up. The sensor, subsequently, supplies the ABM a constant 7 milliamp (mA) signal on a signal return circuit. With regards to 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 really 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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