Modern cars are equipped with a large number of anti-lock braking systems. ABS not only has the braking function of ordinary braking system, but also can prevent the wheel from locking, so that the car can still turn under braking conditions, ensure the stability of the braking direction of the car and prevent side slip and deviation. ABS is the most advanced braking device with the best braking effect in automobiles at present.
When the ordinary braking system brakes on wet roads or in emergency braking, the wheels are easily locked safely because the braking force exceeds the friction between the tires and the ground.
First, the application of ABS
The world's first anti-lock braking system, ABS(anti-ilock Brake System), came out on 1950, and was first applied to aircraft in aviation field. 1968 began to be used in automobiles. In 1970s, the application of ABS in automobiles was promoted because the front wheels or front and rear wheels of new cars produced in seven European and American countries began to adopt disc brakes. After 1980, computer-controlled ABS gradually expanded rapidly in cars in Europe, America, Asia and Japan. So far, some high-end luxury cars, such as Mercedes-Benz, BMW, Yadea, Porsche and Opel in West Germany, Rolls-Royce, Jetta, Lu Hua, Bentley, Ferrari in Italy, Porsche in France, TX3, 300Z and Red Comet in Ford in the United States, Emgrand, Baron and Dodge in new york. By 1993, 46% of ABS has been installed in cars in the United States, and now nearly 75% of cars produced around the world use ABS.
At present, many companies in the world have produced ABS, such as Bendiks, Bendiks, Bosch, Morgan Davis, Hayes Chelsea, Su Mai Tom, Honda, Japanese Unlimited, etc., and some of them are integral and non-integral. It is expected that with the rapid development of automobiles, more manufacturers will produce them.
Second, the function of ABS.
Braking performance is one of the main performances of automobile, which is related to driving safety. The most basic indexes for evaluating automobile braking performance are braking acceleration, braking distance, braking time and directional stability during braking.
The stability of the direction when braking means that the car can still drive in the specified direction when braking. It is very dangerous if the wheels are completely locked because of the emergency braking of the car (especially when driving at high speed). If the front wheel is locked, the car will lose its steering ability; If the rear wheel is locked, it will turn its tail or turn around (running sideways), which is very harmful to driving safety, especially in the case of wet road.
The braking force of a car depends on the friction of the brake, but the braking force that can slow down the braking of the car is also restricted by the ground adhesion coefficient. When the braking force generated by the brake increases to a certain value, the automobile tires will slip on the ground. Its slip rate
δ= (V t -V a )/V t × 100 %
Where: δ-slip rate;
Vt- theoretical speed of the car;
The actual speed of the car.
According to the test, when the wheel slip ratio δ = 15%-20%, the adhesion coefficient reaches the maximum. Therefore, in order to obtain the best braking effect, the wheel slip rate must be controlled in the range of 15%-20%.
The function of ABS is to reduce the braking force when the wheel is about to lock, and increase the braking force when the wheel is not locked, so as to make the braking effect the best.
Three, two control modes of ABS
1. two-parameter control
ABS controlled by two parameters consists of speed sensor (speed radar), wheel speed sensor, control device (computer) and actuator.
Its working principle is that the vehicle speed and wheel speed signals are respectively input into the computer by the vehicle speed sensor and the wheel speed sensor, and the actual slip rate is calculated by the computer and compared with the ideal slip rate 15%-20%, and then the braking force of the brake is increased or decreased by the solenoid valve.
This kind of resistance sensor usually uses Doppler velocimetry radar. When the car is running, the Doppler radar antenna constantly emits electromagnetic waves to the ground at a certain frequency, and at the same time receives the reflected electromagnetic waves. By measuring the difference between automotive radar transmission and reception, the speed of the car can be accurately calculated. The wheel speed sensor is installed in the transmission housing and driven by the transmission output shaft. It is a pulse motor, and its frequency is directly proportional to the wheel speed.
The actuator consists of solenoid valve and relay. The solenoid valve adjusts the braking force to maintain the ideal slip rate.
This kind of ABS can ensure the ideal control of slip rate and has good anti-lock braking performance, but due to the addition of a speed measuring radar, the structure is more complicated and the cost is higher. take for example
Patent examined by Shen Shusheng, an automobile magazine (patent number 9222 1809.9).
2. Single parameter control
The purpose is to control the angular deceleration of the wheel, control the braking force of the wheel and realize anti-lock braking. Its structure is mainly composed of wheel speed sensor, controller (computer) and solenoid valve.
The wheel speed sensor consists of a sensor and a ring gear steel ring.
1, cable 2, permanent magnet 3, shell 4, induction coil 5, pole shaft 6 and gear ring.
2. In order to accurately measure the wheel speed, there should be a gap of 1 mm between the sensor head and the wheel gear ring. In order to avoid the influence of water, mud and dust on the sensor, the sensor should be filled with butter before installation.
Solenoid valve is used for pressure adjustment of wheel brake. For a four-channel braking system, one wheel has a solenoid valve; Three-channel braking system, one for each front wheel and one for two rear wheels. The solenoid valve has three hydraulic holes, which are respectively connected with the master brake cylinder and the wheel brake cylinder, and can realize the pressure adjustment functions of boosting, maintaining and reducing pressure. The working principle is as follows.
1) When the booster solenoid valve does not work, the master cylinder interface is directly connected with each brake cylinder interface. Due to the force of the main spring, the oil inlet valve opens and the braking pressure increases.
2) Pressure maintenance When the pressure in the wheel brake cylinder increases to a certain value, the oil inlet valve is cut off and closed. The bracket is kept in the middle state, and the three holes are sealed with each other to maintain the braking pressure.
3) When the step-down solenoid valve works, the bracket overcomes the elasticity of two springs, opens the unloading meat, and reduces the pressure of the brake cylinder. Once the pressure is reduced, the solenoid valve is switched to the pressure holding state or the pressurization preparation state.
The main task of the control device ECU is to calculate, analyze, amplify and judge the signals sent back by the sensors of each wheel, and then the output stage outputs command signals to the solenoid valve to perform the task of brake pressure adjustment. The electronic control device consists of an input stage A, a controller B, an output stage C and a voltage stabilizing protection device D. ..
The electronic controller drives the solenoid valve at a frequency of 4- 10 1tz, which the driver cannot do.
This single-parameter control mode of ABS is widely used at present because of its simple structure and low cost.
Most Chrysler luxury cars in the United States are equipped with ABS with this single parameter control mode. It is equipped with wheel speed sensors on all four wheels of the car.
The distribution valve is a three-channel distribution valve located under the brake oil pump assembly.
The wheel shaft is equipped with a gear ring with 45 teeth or 100 teeth, and the sensing head of the wheel speed sensor is installed at the top of the gear ring. When the wheel rotates, the sensor constantly generates a voltage signal, which is input into the computer, and compared with the ideal speed in RoM, the speed increase or deceleration of the wheel is calculated, and the command of boosting or depressurizing is sent to the solenoid valve to control the braking force of the cylinder.
Fourthly, the problems existing in the use of ABS.
(1) After replacing the brakes or components of the hydraulic brake system, the air in the brake pipeline should be exhausted to avoid affecting the normal operation of the brake system.
(2) Vehicles equipped with ABS should change the brake fluid once a year. Otherwise, the brake fluid has strong hygroscopicity, which will not only reduce the boiling point and cause corrosion, but also reduce the braking efficiency.
(3) Unplug the power supply before checking the ABS anti-lock braking system.