A wheel speed sensor is a small device near each wheel that measures how fast that wheel is rotating. This information is essential because a vehicle can lose steering control if one wheel locks during hard braking. The anti-lock braking system, or ABS, uses wheel speed data many times per second to decide whether to reduce brake pressure at a specific wheel.
Without accurate wheel speed signals, ABS, traction control, and stability control cannot respond correctly.
Key Facts
- Wheel speed can be found from pulse rate: wheel speed in rev/s = pulses per second / teeth on tone ring.
- Vehicle speed from one wheel can be estimated by v = 2πr f, where r is tire radius and f is wheel rotation frequency in rev/s.
- A passive variable reluctance sensor creates an AC voltage as metal teeth pass the sensor tip.
- An active Hall effect sensor uses power and produces a digital on-off signal as magnetic poles pass by.
- ABS compares wheel speeds to detect rapid deceleration that suggests a wheel is about to lock.
- Slip ratio can be estimated by slip = (vehicle speed - wheel surface speed) / vehicle speed.
Vocabulary
- Wheel speed sensor
- A sensor mounted near a wheel hub that measures how quickly the wheel is rotating and sends that information to control modules.
- Tone ring
- A toothed or magnetic ring that rotates with the wheel so the sensor can detect repeated changes.
- Hall effect
- A physical effect where a voltage changes when a magnetic field passes through a powered semiconductor sensor.
- ABS
- The anti-lock braking system that adjusts brake pressure to help prevent wheel lockup during hard braking.
- Signal pulse
- A repeated electrical change produced each time a tooth or magnetic pole passes the sensor.
Common Mistakes to Avoid
- Counting tire rotations instead of sensor pulses, which is wrong because the sensor may produce many pulses for each wheel revolution depending on the number of teeth or magnetic poles.
- Assuming all wheel speed sensors make the same type of signal, which is wrong because passive sensors usually output AC voltage while active sensors often output a digital square wave.
- Ignoring air gap between the sensor and ring, which is wrong because too large or uneven a gap can weaken or distort the signal.
- Using one wheel speed as the true vehicle speed during braking, which is wrong because a slipping or locking wheel rotates slower than the vehicle is actually moving.
Practice Questions
- 1 A tone ring has 48 teeth and the sensor reads 960 pulses per second. What is the wheel speed in revolutions per second and revolutions per minute?
- 2 A tire has a radius of 0.32 m and rotates at 12 rev/s. Estimate the vehicle speed in m/s using v = 2πr f.
- 3 During hard braking, the left front wheel speed suddenly drops much faster than the other three wheel speeds. Explain how the ABS control module should respond and why.