19 ANTI-LOCK BRAKES ABS Wheel Speed Sensor Electromagnetic pulse pick up with toothed wheels mounted directly to the rotating components of the drive train or wheel hubs. These devices provide a digital signal whose frequency is proportional to wheel rotational velocity. The ECU derives the wheel speeds from the signal sent by the wheel speed sensor. Inductive wheel speed sensor (DF6) has a stator pole with coil windings and installed directly above the Toner Wheel. The Toner Wheel is generally fitted on to the wheel hub. The stator pole is linked to a permanent magnet, thus inducing a magnetic field into the toner wheel . The al ternat ing teeth and gap that accompanies the wheel rotat ion induces corresponding fluctuation in the magnetic field through the stator pole. This variation in magnetic field induces an alternating current at the end of the winding. The frequency of the current is proportional to the speed of the wheel. The voltage is zero when the vehicle is stationary. The minimum detectable rotation rate is decided by Fig. 11 factors like tooth geometry, gap, voltage rise rate and the ECU sensitivity to incoming signals. The sensor has to be installed on a stable mount to prevent oscillation pattern from the brakes resulting in distorting sensor signals. The wheel speed sensor also has a coating of grease to prevent it from dirt and water spray. Fig. 10 Front wheel speed sensor
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