Wheel RPM sensor

Wheel RPM sensor

The vehicle speed measurement system consists of a wheel or toothed crownwheel (phonic wheel) fitted on the hub or wheel axle, and a variable reluctance sensor. This assembly generates an electrical signal whose frequency is proportional to the speed of rotation.

1. Brass bush
2. Permanent magnet
3. Plastic casing of the sensor
4. Winding or coil
5. Polar core
6. Toothed crownwheel or phonic wheel
7. Coaxial twin cable

1. Sensor
2. Phonic wheel
3. Hub
4. Vertical link
5. Steering arm

The signals are obtained from magnetic flow lines which close through the teeth of a metal toothed wheel facing the sensor, which rotates with the wheel. The passage from full to empty, due to the presence or absence of the tooth, causes a change in the magnetic flow sufficient to generate an induced alternating voltage, derived from the count of teeth located on a ring (or phonic wheel). Measuring the period gives the speed data, while comparing successive periods gives the wheel acceleration/deceleration data. The specified gap for obtaining correct signals, between the end of the sensor and the phonic wheel, must be 0.9 ± 0.4 mm. This gap is not adjustable so, if the gap is outside the tolerance limits, check the condition of the sensor and the phonic wheel.