Fiat 1996 Brava Service Manual page 569

Bravo-Brava
9 8 range
Braking system
Active sensors
33.
WHEEL RPM (ACTIVE) SENSORS
The ABS Bosch 5.3 system adopted on the 98 range, uses new design sensors known as "active" for de­
tecting the speed of the wheels.
The operation of "active sensors" is based on the variation of the internal electrical resistance according
to the intensity and the direction of the lines of force of an external magnetic field, producing a square
wave signal whose frequency varies according to the rotation speed of the wheel, but whose range is
constant.
The external magnetic field is produced by a so-called "multi-polar ring", consisting of an elastomer
equipped with a certain quantity of magnetic particles w h i c h , by means of a special magnetization tech­
nique, are positioned in such a way as to form different magnets w i t h alternating North - South polari­
ties around the circumference.
The multi-polar ring can be fitted on the wheel bearing seal or on the (rear wheel) hub.
The sensor is supplied by the ABS electronic control unit and produces a square wave signal whose
range is almost constant and whose frequency is proportional to the rotation speed of the wheel.
The use of this new type of sensor offers the following advantages compared with traditional magnetic
induction type sensors:
- less sensitivity to the distance between the sensor and the magnetic ring;
- improved immunity to electro-magnetic fields;
- the capacity of the sensor to measure the speed of the wheel d o w n to zero (rather than 2.5 kph for
passive sensors);
- a reduction in the weight and size on the vehicle.
1. Active sensor body
2. Multi-polar ring
3. Gap
4. Example of an output signal from the wheel rpm sensor
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