汽车专业英语(英文）教学课件汇总完整版电子教案.ppt

Lesson 24 Antilock Braking System The purpose of an electric antilock，or antiskid braking system(ABS)is to prevent wheel lockup under heavy braking conditions on virtually any type of road surface. It has been used on aircraft for years，and some domestic cars were offered with an early form of antilock braking in the late 1960s and 1970s. Modern antilock systems may be the greatest advance in vehicle safety since the seatbelt. The most efficient takes place when the wheels are still revolving. If the driver slams on the brakes and the wheels are locked，the followings occur: ①driving stability is lost，and the vehicle skids， ②the vehicle cannot be steered， ③the braking distance increases， ④the risk of an accident increases. Although the hardware used in an ABS varies with each vehicle manufacture，the system mainly consists of an electronic control unit，a hydraulic actuator，and wheel speed sensors at each wheel. 1 ABS wheel speed sensors The anti-lock braking system needs some way of knowing when a wheel is about to lock up. The wheel speed sensors，which are located at each wheel provide this information .They transmit wheel speed information to the computer. A small alternating current signal voltage is generated in the sensor through magnetic induction，by making a toothed exciter ring pass a stationary magnetic sensor. The frequency of the signal varies directly with the wheel’s rotation rate. This signal is used by the computer to determine the speed of the wheel. 2 ABS computer The ABS computer uses the signals from the wheel speed sensor to control the operation of the hydraulic actuator. If a wheel-locking tendency is detected，the computer will command appropriate wheel cylinder valve positions to modulate brake fluid pressure in some or all of the hydraulic circuits to prevent wheel lockup and provide optimum braking. 3 ABS warning light If the control unit detects a malfunction in the system，it will illuminate an ABS warning light