|Type:||ABS sensor inductive|
|Signal type:||Amplitude and frequency varying|
|Signal level:||0.4 V to 8 V|
A anti lock braking system (ABS) sensor is used for determining wheel rotation speed to prevent lock up when braking. The inductive ABS sensor consists of a permanent magnet with a coil around it. The magnetic field strength changes when a magnetism sensitive object passes trough the magnetic field of the magnet. This changing of the magnetic field induces a voltage in the coil. The polarity of the induced voltage depends on the direction of the moving object, moving away or towards the sensor. This sensor doesn't need a power supply.
Figure 1: Schematic view of ABS sensor
In most cases the object used to influence the magnetic field is a disk or ring with evenly distributed teeth, mounted on the driveshaft. When the driveshaft is rotating, the teeth are passing the sensor and the pattern in which they are placed is visible in the ABS sensor signal. Every period of the signal is a tooth passing the sensor. The frequency and amplitude of the signal depend on the rotation speed of the driveshaft and the amount of teeth on the disk or ring.
The ABS sensor signal is monitored by a special ECU for the ABS system, the CAB (Controller Anti-lock Brake). The ECU uses the ABS sensor signal in two ways. When the wheel rotates fast enough for the ABS sensor signal to become larger than a certain threshold, the ECU determines the wheel speed using the rising and falling edges of the signal. When a wheel clearly has a lower speed than the other wheels or decelerates faster than the vehicle could, the ECU lowers the brake pressure for that particular wheel. This will result in a wheel speed increase and prevents skidding. When necessary, the brake pressure is continuously adjusted multiple times per second until the car nearly stops and the signal voltage falls below the threshold.
Correct functioning of the ABS sensor can be checked by measuring the following signal voltages, see figure 2:
|1||Signal at the positive side of the sensor||2 V1|
|Signal at the negative side of the sensor|
Figure 2: Measuring diagram
The lab scope is connected to the ABS sensor via a Measure lead TP-C1812B and Back Probe TP-BP85. The lab scope is set to normal scope mode with the trigger-timeout at infinite. When a one-shot measurement is started with these settings, the measurement is performed when the wheel is turned (by hand).
Figure 4 shows a waveform of a ABS sensor which is measured with the wheel turned by hand. This signal can be downloaded and used to correctly set up the lab scope or as reference signal.
Figure 4: Lab scope measurement of ABS sensor
Channel 1 (red) shows the signal from the ABS sensor. The signal starts with a low amplitude and frequency because the wheel rotates very slowly. Halfway during the measurement the wheel rotates at a speed that is sustained during the rest of the measurement.
The amplitude of the signal changes with the wheel rotation speed: with higher rotation speed the amplitude increases. The shown example waveform has an amplitude of 0.5 V at maximum. This is quite low because the wheel is turned by hand at low speed. The voltage will be higher when driving, but this is sufficient to test the ABS sensor. The exact amplitude of the signal is not important because the ECU uses the raising and falling edges to determine the wheel speed.
Signal values may differ on different types of anti lock braking systems and ABS sensors. Consult ATIS for information on specific anti lock braking systems and ABS sensors.
The following signal deviations can indicate a problem:
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The information in this application note is carefully checked and is considered to be reliable, however TiePie engineering assumes no responsibility for any inaccuracies.