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

There are two types of wheel speed sensors currently in use: magnetic inductive, more commonly referred to as passive sensors and magneto resistive, or active wheel speed sensors. wind sensor 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 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 use 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. As the tone wheel rotates, one’s teeth go in and out of your proximity of the sensor. The result is an alternating electric 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 due to: 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 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 will happen due to 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 work with a toothed tone wheel which acts because the trigger mechanism for the sensor. Some vehicles use a magnetic encoder in place 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 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 continuing 7 milliamp (mA) signal on a signal return circuit. According 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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