Oscilloscope measurement and analysis of automobile ABS wheel speed sensor signal

The anti-lock brake system (ABS) of a car is to automatically control the braking force when the car brakes, so that the wheels are not locked and are in a state of rolling and sliding (slip rate is about 20%), so as to ensure that the adhesion between the wheels and the ground is at the maximum.

The function of the wheel speed sensor in ABS is to measure the rotation speed of the car wheels. The wheel speed sensor detects the frequency signal (rotation speed signal) of each wheel rotation, and then transmits this signal to the ABS computer. When the vehicle speed reaches the set value, emergency braking is applied and the ABS system starts to work. When the ABS computer controls the wheels to brake and release, the wheel speed sensor transmits the detected distance signal of the tire from braking to rotation to the ABS computer, so that the ABS controls the brakes to achieve the optimal braking distance. Common wheel speed sensors mainly include magnetic wheel speed sensors and Hall wheel speed sensors.

The magnetoelectric wheel speed sensor is designed based on the principle of electromagnetic induction. It has a simple structure, low cost, is not afraid of mud and dirt, and does not require power supply. It is widely used in ABS anti-lock braking systems. However, it also has some disadvantages, such as low frequency response. When the vehicle speed is too high, the frequency response of the sensor cannot keep up, and it is easy to generate false signals. In addition, the anti-electromagnetic interference ability is poor. The waveform measured by the oscilloscope is in the shape of a sine wave, and the higher the wheel speed, the greater the output signal voltage amplitude.

The Hall-type wheel speed sensor is made using the Hall effect principle. Its output signal voltage amplitude is not affected by the rotation speed, has a high frequency response, and is highly resistant to electromagnetic interference, but it must be powered. We use this sensor as an example for measurement:

First, connect a BNC to banana head cable to one channel of the oscilloscope. The red end is connected to the piercing needle as the positive pole, and connected to the signal line of the sensor. The black end can be connected to the alligator clip or the piercing needle as the negative pole and grounded.

Open the oscilloscope channel menu, set the channel attenuation ratio to 1X, adjust the vertical scale to 1V/div, and the time base to about 10ms. You can then adjust it according to the actual situation.

Since the Hall-type wheel speed sensor is powered by a power supply, the waveform observed at this time should have a DC current of about 11-12V.

Lift the vehicle with a jack, turn the wheel, and use an automotive oscilloscope to measure the sensor signal output waveform.

The Hall effect sensor consists of an almost completely closed magnetic circuit of a permanent magnet or magnetic pole, and a soft magnetic impeller rotates through the gap between the magnet and the magnetic pole. When the window on the impeller allows the magnetic field to pass through and pass to the Hall effect sensor unimpeded, the magnetic field is interrupted (because the blades are the medium that conducts the magnetic field to the sensor). The impeller allows the magnetic field to pass through and blocks the magnetic field when the window opens and closes, causing the Hall effect sensor to turn on and off like a switch. This is why some car manufacturers call Hall effect sensors and some other similar electronic devices Hall switches. This device is actually a switching device. Therefore, the signal waveform of the Hall effect sensor is actually one pulse after another, that is, a square wave.

The faster the wheel speed, the faster the frequency of the signal waveform, but the AC voltage value remains unchanged, changing from 0 V to 1 V. If the wheel speed is slowed down, you can see that the frequency of the signal waveform also decreases.

If the digital ABS sensor has only 0 volt output, you should first check whether it has power supply. Then confirm whether the signal frequency of the sensor keeps up with the wheel speed, otherwise it means failure.