Basic knowledge, working principle and basic composition of ABS

one,ABSThe basics of
In order to understand the necessity of adopting ABS, the following basic knowledge about braking is introduced.
(one)carMain evaluation indexes of braking performance
There are many requirements for the braking performance of automobiles, so there are many evaluation indicators, and the following three aspects are usually mentioned. They are:
1.Braking efficiency
Braking efficiency mainly refers to braking distance and braking deceleration, and usually refers to braking distance in practice.Braking distance is when the driver starts to step onBrake pedalThe distance traveled to the complete stop of the car. The shorter the braking distance, the more beneficial it is to avoid traffic accidents. It is the most basic evaluation index of braking performance.
2.Directional stability of the car during braking
The directional stability of a car during braking generally refers to the ability of the car to maintain a straight line and drive in a predetermined curve during braking. If the car slips, flicks, or turns around when braking, it will not be possible to maintain the original driving direction, which will make the car lose its directional stability; if the car brakes in a curve, the car will no longer follow the original curve. When driving on roads, rushing into other lanes or off the road, or even driving straight, it is impossible to avoid obstacles and steering the steering wheel does not work, which means that the car loses its steering control ability (steering maneuverability). During the braking process of a car, loss of directional stability and loss of steering control ability are both important causes of traffic accidents.
3 Consistency of braking efficiency
The constancy of braking performance mainly refers to resistance to thermal degradation. Thermal decay resistance refers to when the car brakes under heavy working conditions (as follows: long-term continuous braking on long slopes),BrakeAfter the temperature rises, the degree of maintenance of its braking performance. It is an important issue that must be carefully considered in the design of brakes and material selection.
Among the above three indexes, after the first two indexes adopt ABS device, its performance will be significantly improved and the improvement can play a good role in avoiding traffic accidents. Therefore, ABS is a very important active safety device in automobiles.
(2) Advantages of ABS
From the above analysis, it can be seen that in the braking process of the car, the damage is greater when the wheels are locked, and when the slip rate is about 20%, the longitudinal adhesion coefficient between the wheels and the road surface is the largest, and the maximum ground braking force can be obtained. The braking distance is shortened on the ground; at the same time, when the slip rate is about 20%, the lateral adhesion coefficient between the wheels and the road surface is also larger, so that the car can better maintain the directional stability and steering control ability when braking.In order to ensure driving safety, obtain the best braking performance, prevent the wheels from dragging when braking, and control the wheel slip rate within a narrow range near the ideal slip rate, people have vigorously developed, developed and promoted anti-lock braking.Braking SystemNamely ABS.
ABS is based on the original traditional braking system, adding a set of control system to prevent wheel brake lock. This device is in the process of braking, when the wheel tends to lock, that is, when the wheel slip rate enters the unstable zone , It will quickly reduce the brake system pressure and restore the wheel slip rate to a stable zone close to the ideal slip rate. The brake system pressure can be adjusted automatically and at a high frequency (the frequency is up to more than ten times per second). Keep the wheel slip rate in a narrow range near the ideal slip rate, so as to make full use of the longitudinal peak adhesion coefficient between the wheel and the road surface and the high lateral adhesion coefficient, to prevent wheel lock and obtain the best braking performance.
It should be pointed out that when the traditional braking system is used for braking, although the driver also knows that intermittently stepping on and releasing the brake pedal to prevent the wheels from locking, the experienced driver cannot accurately judge and Control, especially during emergency braking, it is impossible to control the wheel slip rate within the ideal range, which will often cause the wheels to lock up, especially when the car is braking on icy, rainy and slippery roads. It is easy to produce sideslip, tail flick and loss of steering control ability. At this time, the driver often has a nervous mood and lacks a sense of safety.
In summary, the advantages of ABS are:
(1) Maintain directional stability during braking;
(2) Maintain steering control ability when braking;
(3) Shorten the braking distance
(4) ReducetireWear
(5) Reduce the driver's nervousness.
Second, the working principle of ABS
whenIgnition SwitchWhen turned on (ON), ABS protectionRelayThere will be current flowing in the electromagnetic coil, and the system enters the self-checking state. After a short self-inspection, if a fault is found in the system that affects its normal operation, it will maintain its working state during the self-inspection, that is, turn off the ABS system.At this time, each in the pressure regulatorThe electromagnetic valveAll solenoid coils are not energized, and all solenoid valves are maintained in a state of increased braking pressure, and the car resumes normal braking state work. After self-inspection, no faults that affect the normal operation of the system are found, and the ABS enters the waiting state.
When the car is running, the speed of each wheelsensorContinuously input the wheel speed signal of each wheel to the ABS computer.When the vehicle speed exceeds 8km/h, if the driver depresses the brake pedal to brake, the brake lightswitchclosure,AccumulatorGive a voltage signal to the ABS computer. After the ABS computer receives the battery voltage signal, it determines that the car enters the braking state.It will be based on wheel speedsensorThe input information analyzes and judges the motion state of the four wheels.
In the braking process, when the braking of each wheel does not appear to be locked, ABS does not work. At this time, the braking process is exactly the same as the conventional braking process. In the braking process, when the ABS computer determines that there is a wheel brake tending to lock, it will start anti-lock control on the corresponding control channel, and control the wheel slip rate within the optimal range until the car speed is very high. Low or stop.
During the braking process, if the car is turning sharply at high speed, when the lateral acceleration of the car reaches a certain value, the pair of contacts in the lateral acceleration switch will be disconnected, and the ABS computer will no longer have the battery voltage signal. It is determined that the lateral acceleration of the vehicle has exceeded the set limit value, and its anti-lock braking control process will be corrected to make the ABS work more effectively.
The ABS protection relay is used to prevent damage to the ABS computer when the battery polarity is reversed or the battery voltage is too high.
Three, the basic composition of ABS
Although modern ABS adopts different control methods, methods and structural forms, in addition to the original traditional conventional braking device, generally ABS is composed of three parts: sensors, electronic controllers and actuators.The sensors are mainly wheelsRotating speedSensors and actuators mainly refer to brake pressure regulators.
1. Wheel speed sensor
The wheel speed sensor is the most important sensor in ABS. The wheel speed sensor is often referred to as the wheel speed sensor for short. Its function is to detect the movement state of the wheel and obtain the wheel speed (speed) signal.
2.Electronic controller
ABS electronic controller (Electronic Control Unit), commonly used ECU said, referred to as ABS computer. Its main function is to receive input signals such as wheel speed sensors, calculate wheel speed, reference vehicle speed, wheel deceleration power, slip rate, etc., and make judgments, output control instructions, and control brake pressure regulators to work. In addition, the ABS computer also has functions such as monitoring. If there is a failure, the ABS will stop working and the ABS warning light will be turned on.
3.Brake pressure regulator
The brake pressure regulator is the main actuator in ABS. Its function is to receive instructions from the ABS computer, drive the solenoid valve in the regulator (or motor rotation, etc.) to adjust the pressure of the brake system to increase, maintain or decrease it, and realize the control function of the brake system pressure.
Since ABS is formed by adding a set of control devices on the basis of the original traditional braking system, ABS is also based on the traditional conventional braking process. In the braking process, when the wheels have not yet tended to drag, the braking process is exactly the same as the conventional braking process; only when the wheels tend to lock, ABS will perform the braking pressure on the wheels that tend to lock. adjust.
Generally, ABS will only adjust the brake pressure of the wheels that tend to lock during the braking process when the speed of the car reaches a certain level (such as 5 km/h or 8 km/h). When the car speed drops to a certain level, because the car speed is very low, the wheel brake lock has little adverse effect on the car's braking performance. In order to make the car stop as soon as possible, ABS will automatically terminate the anti-lock brake pressure adjustment , Its wheels may still be locked by the brakes.
During braking, if the conventional braking system fails, ABS will lose control. If only the ABS fails and the conventional braking system is normal, the braking process of the car will still proceed as usual, but the anti-lock braking control effect will be lost. Modern ABS can generally monitor the working conditions of the system and has fail-safe and self-diagnostic functions. Once a fault that affects the normal operation of ABS is found, it will automatically turn off ABS, resume normal braking, and turn on the ABS warning light. A warning signal is issued to the driver to remind the driver to make repairs in time.