How Does an ABS Speed Sensor Work?

by S.F. Heron

Antilock Braking Systems

Anti-lock braking systems (ABS) are common on newer model cars. ABS controls the braking ability of the car under select driving conditions that result in a loss of traction. This system features a controller, valves, and speed sensor that work in conjunction to determine the safe braking ability of your car. ABS speed sensors monitor the rotation of each tire, making sure that each wheel is rotating properly. Any slippage or difference between the wheels triggers the ABS system.

Parts of the ABS

ABS begins with sensors deciding if the wheels are losing traction. Once this decision is made, the controller (like a computer) engages a valve system in the vehicle that controls available brake fluids in the lines. Since brakes work based on pressure generated by fluid compressed in the brake lines, a pump engages in the braking system to rapidly apply the brakes. This application occurs faster than any human can apply the brakes. Many drivers feel a pulsing feeling in the brake pedal or hear a grinding noise when the ABS engages on their vehicle.

Evaluating Speed

ABS speed sensors are located at the wheel hubs of the vehicle. These sensors constantly monitor the rotation of each wheel to evaluate whether the ABS system needs to override manual braking of the vehicle in slippery conditions. The speed sensor calculates revolutions as well as evaluates continuity between all wheels. Any detection of differences in rotation can result in ABS engaging to control braking.

Generating the Speed Sensor Signal

Speed sensors are comprised of a magnet wrapped in a coil and a toothed sensor ring mounted around the CV joint hub. An electrical field given off by the contact between the magnet and the toothed ring creates a signal. The composition of the signal includes measurements of the number of pulses per second created by the electrical field existing between the magnet coil and sensor ring. This signal converts to a digital signal and is transmitted to the ABS controller.

Sending Messages to the ABS Controller

Speed sensors convert the pulses created by the contact between the magnet and sensor ring through a coil attached to the sensor. This voltage is passed along to the controller. The controller counts the number of pulses to determine wheel speed and evaluates whether the ABS system should intercede to control braking.

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