Since the birth of automobiles, vehicle braking systems have played a vital role in vehicle safety. In recent years, with the advancement of vehicle technology and the increase in vehicle driving speed, this importance has become more and more obvious. Many automotive engineers have devoted a lot of effort into research on improving vehicle braking performance. Current research on automobile braking mainly focuses on braking control, including the theory and methods of braking control, as well as the adoption of new technologies.
1. History of brake control system
The most primitive brake control is just the driver operating a set of simple mechanical devices to apply force to the brakes. At this time, the mass of the vehicle It is relatively small and has a relatively low speed. Although mechanical brakes have met the needs of vehicle braking, as the weight of the car increases, the power assist device has become very necessary for mechanical brakes. At this time, the vacuum assist device begins to appear. The Cadillac V16 car produced in 1932 with a mass of 2860kg used four-wheel drum brakes with a diameter of 419.1mm and a vacuum booster controlled by the brake pedal. Lincoln also launched the V12 sedan in 1932, which used drum brakes that controlled vacuum boosters through four soft cables.
With the development of science and technology and the development of the automobile industry, especially the development of military vehicles and military technology, vehicle braking has made new breakthroughs. Hydraulic braking is another step after mechanical braking. Major innovation. The Duesenberg Eight car was the first to use hydraulic brakes in cars. Chrysler's four-wheel hydraulic brakes were introduced in 1924. General Motors and Ford adopted hydraulic braking technology in 1934 and 1939 respectively. It wasn't until the 1950s that hydraulically assisted brakes became a reality.
In the late 1980s, with the development of electronic technology, the most significant achievement in the world's automotive technology field was the practicality and promotion of anti-lock braking systems (ABS). ABS integrates microelectronics technology, precision processing technology, and hydraulic control technology. It is a high-tech product with electromechanical integration. Its installation greatly improves the car's active safety and handling. Anti-lock devices generally include three parts: sensors, controllers (electronic computers) and pressure regulators. The sensor receives motion parameters, such as wheel angular velocity, angular acceleration, vehicle speed, etc. and transmits them to the control device. The control device performs calculations and compares them with the specified values ??before issuing instructions to the pressure regulator.
In 1936, Bosch applied for a patent for an electro-hydraulic controlled ABS device, which promoted the application of anti-lock braking systems in automobiles. In 1969, Ford used vacuum-assisted ABS brakes; in 1971, Chrysler cars used four-wheel electronically controlled ABS devices. These early ABS devices had limited performance, suboptimal reliability, and high cost.
In 1979, Merbenz launched a reliable, fully digital electronic system-controlled ABS braking device with an independent hydraulic booster. In 1985, the United States developed an "integrated" ABS anti-lock device with a digital display microprocessor, composite master cylinder, hydraulic brake booster, solenoid valve and actuator. With the emergence of large-scale integrated circuit and very large-scale integrated circuit technology, as well as the rapid development of electronic information processing technology, ABS has become a mature product with reliable performance and declining costs, and has broad application prospects. In 1992, the world's annual output of ABS exceeded 10 million units, and the world's automotive ABS installation rate exceeded 20%. Some countries and regions (such as Europe, Japan, the United States, etc.) have formulated regulations to make ABS standard equipment in automobiles.
2. Current status of brake control system
When considering the basic braking function, hydraulic control is still the most reliable and economical method. Even after the anti-lock braking (ABS) function is added, the traditional "hydraulic braking system" still has an advantage. But the addition of traction control, vehicle stability control and some of the new technologies being considered for "smart cars" make basic brakes seem trivial in terms of complexity and economy.
Traditional brake control systems only do one thing, which is to distribute fluid pressure evenly.
When the brake pedal is depressed, the master cylinder sends an equal amount of fluid to the lines leading to each brake and balances the front and rear through a proportional valve. ABS or another brake intervention system adjusts the fluid pressure according to the needs of each brake.
At present, the vehicle anti-lock braking control system (ABS) has developed into a mature product and has been widely used in various vehicles. However, these products are basically based on wheel acceleration and deceleration thresholds. And designed with reference to the slip rate method. Although the method is simple and practical, it is difficult to debug. Different vehicles require different matching technologies and are verified on many different roads; theoretically, the wheel slip rate during the entire control process is not maintained at the optimal slip rate. , did not achieve the best braking effect.
In addition, due to the many limitations of programming logic threshold ABS, the vehicle dynamics control system (VDC) has been developed based on ABS in recent years. The best ABS combined with dynamic control is the ABS with slip rate control target. It is in the form of continuous control to maintain the best and stable slip rate during the braking process. In theory, it is an ideal ABS control system.
The difficulty of slip rate control is to determine the optimal slip rate under various road conditions. Another difficulty is the measurement of vehicle speed. It should be a low-cost and reliable technology and can eventually be developed into products used. For ABS that targets slip rate, control accuracy is not a very prominent issue, and it is difficult to achieve high-precision control; because the road surface and vehicle motion state change greatly, and various interferences have a greater impact, so The important issue is the stability of the control, that is, the system robustness, which should be maintained without losing control under various conditions. The anti-lock system requires high reliability, otherwise it will lead to personal injury, death, and vehicle damage.
Therefore, developing a robust ABS control system becomes key. Nowadays, a variety of robust control systems are applied to the control logic of ABS. In addition to the traditional logic threshold method for comparison purposes, gain scheduling PID control, variable structure control and fuzzy control are commonly used robust control systems, which are currently used continuous control systems targeting slip rate. The fuzzy control method is a control based on empirical rules and has nothing to do with the system model. It has good robustness and flexibility of control rules. However, it is difficult to adjust the control parameters. In terms of theory, it is basically a trial and error method. . However, for most control based on target values, the control rules have certain rules.
In addition, other control methods are also used, such as methods based on state gates and linear feedback theory, fuzzy neural network control systems, etc. Various control methods are not applied individually to cars, but are usually implemented in combination with several control methods. For example, fuzzy control and PID can be combined to achieve good control effects by taking into account the robustness of fuzzy control and the high precision of PID control.
Wheel drive slippage and brake locking are very similar problems. When the car starts or accelerates, the drive wheels rotate at high speed due to excessive driving force, exceeding the friction limit and causing slipping. At this time, the wheels also do not have enough lateral force to maintain the stability of the vehicle, and the tangential force of the wheels is also reduced, affecting acceleration performance. It can be seen from this that preventing wheel slippage and locking is to control the slip rate of the car, so the drive anti-skid system (ASR) was developed based on ABS.
ASR is a logical and functional extension of ABS. After adding the ASR function to ABS, the main change is to add a drive anti-skid logic system to the electronic control unit to monitor the speed of the drive wheels. ASR mostly borrows the hardware of ABS, and the two are integrated and developed into an ABS/ASR system.
At present, ABS/ASR has been commonly used in new European trucks, and the European general regulation EEC/71/320 has made it mandatory for certain cargoes with a total mass greater than 3.5t. Used on vehicles, heavy-duty vehicles are installed first. However, ABS/ASR only solves the problem of utilizing the adhesion coefficient during emergency braking and can obtain shorter braking distance and braking direction stability, but it cannot solve all the defects in the braking system. Therefore, the ABS/ASR function can control the braking intensity at the same time.
ABS will only control braking in extreme situations (the wheels are completely locked). During partial braking, the electronic brake can control the pressure of a single brake cylinder, so the reaction time is shortened and ensures that the brakes are stopped at any time. Get the correct brake pressure in a flash. In recent years, the rapid development of electronic technology and computer control technology has brought opportunities for the development of EBS. Germany has taken the lead in developing ABS/ASR systems and putting them on the market since the 1980s, and has been at the forefront of the world in the research and development of EBS.
In 1993, the German Bosch Company jointly installed EBS on Scania tractors and trailers for the first time. However, EBS is a brand-new system that has great potential and will definitely bring revolutionary changes to current and future braking systems.
3. Development of brake control systems
Today, ABS/ASR has become standard equipment in automobiles in developed countries such as Europe, America and Japan.
The development of vehicle braking control systems is mainly the development of control technology. On the one hand, it is to expand the control range and increase control functions; on the other hand, it is to adopt the optimal control theory to implement servo control and high-precision control.
In the first aspect, in addition to ASR, the expansion of ABS functions also extends suspension and steering control, making ABS not only an anti-lock braking system, but a more comprehensive vehicle control system. Brake developers have also proposed the idea of ??integrating ABS/TCS and VDC with intelligent transportation systems in the future. With the development of electronically controlled transmission, suspension systems and steering devices, a network of connections between electronic control systems will be created, resulting in some new functions. For example, the use of electronically controlled clutches can greatly improve the efficiency of starting a car from a standstill; During braking, the vehicle can be prevented from pitching by inputting a drive command to the electronic suspension system.
In the second aspect, some intelligent control technologies such as neural network control technology are relatively new control technologies, and some people have already applied them to automobile braking control systems. ABS/ASR cannot solve all problems in automobile braking. Therefore, the further development of ABS/ASR into electronically controlled braking systems (EBS) will be an important direction in the development of control systems. However, it is not a simple problem to apply EBS in practice. In addition to technology, the cost of the system and related regulations are key to its application.
After more than a hundred years of development, the form of the automobile braking system has been basically fixed. With the development of electronics, especially large-scale and ultra-large-scale integrated circuits, the form of automobile braking systems will also change. For example, the Casey Hayes (K-H) Company installed an electro-hydraulic (EH) braking system on an experimental vehicle, which completely changed the operating mechanism of the brakes. By using four proportional valves and power electronic control devices, K-H's EBM can take into account basic braking, ABS, traction control, cruise control braking intervention, etc. without adding any additional devices. The potential advantage of the EBM system is that it can distribute the basic braking force more effectively than standard brakes, thereby shortening the braking distance by 5%. The development of a completely oil-free, complete circuit brake BBW (Brake-By-Wire) makes the traditional hydraulic brake device a thing of the past.
4. Full circuit braking (BBW)
BBW is the development direction of future braking control systems. Fully electric braking is different from traditional braking systems because it transmits electricity instead of hydraulic oil or compressed air. It can omit many pipelines and sensors and shorten braking reaction time. The structure of all-electric braking mainly includes the following parts:
a) Electric brake. Its structure is basically similar to that of hydraulic brakes, with disc and drum types, and the actuator is an electric motor;
b) Electric brake control unit (ECU). Receives the signal from the brake pedal to control the brake braking; receives the parking brake signal to control the parking brake; receives the wheel sensor signal to identify whether the wheel is locked or slipping, etc., and controls the wheel braking force to achieve anti-lock braking and The drive is anti-slip.
Since the control systems of various control systems such as satellite positioning, navigation systems, automatic transmission systems, continuously variable steering systems, suspension systems, etc. are highly integrated with the braking control system, the ECU must also take into account the control of these systems;
c) Wheel speed sensor. Obtain the wheel speed accurately, reliably and timely;
d) Wiring harness. Transmit energy and electrical control signals to the system;
e) Power supply. Provide energy for the entire electric braking system. Compatible with other systems. It can be a variety of power sources, including renewable energy.
It can be seen from the structure that this full-circuit braking system has advantages that other traditional braking control systems cannot match:
a) The entire braking system has a simple structure and eliminates the need for The brake oil tank, brake master cylinder and booster device in the traditional braking system are eliminated. Hydraulic valves, complex pipeline systems and other components reduce the quality of the entire vehicle;
b) Short braking response time improves braking performance;
c) No brake fluid , easy to maintain;
d) The manufacturing, assembly and testing of the system assembly are simple and fast, and the braking sub-assembly has a modular structure;
e) The system is connected by wires, and the system has good durability. ;
f) Easy to improve, various electrical control functions can be added with slight improvements.
The all-electric brake control system is a brand-new system, which has brought great changes to the brake control system and provided conditions for future intelligent vehicle control. However, there are still many problems that need to be solved in order to be fully promoted:
The first is the issue of driving energy. Using a full-circuit brake control system requires more energy. A disc brake requires approximately 1kW of driving energy. The current vehicle 12V power system cannot provide such a large amount of energy. Therefore, in the future, the vehicle power system will use high-voltage power to increase the energy supply to meet the braking energy requirements. At the same time, the safety issues caused by high voltage need to be solved.
The second step is to handle the failure of the control system. One difficulty faced by the all-electric braking control system is the handling of braking failure. Because there is no independent active backup braking system, a backup system is needed to ensure braking safety. Regardless of ECU component failure, sensor failure, brake itself, or wiring harness failure, the basic braking performance can be guaranteed. A key technology to realize all-electric braking control is the information exchange protocol when the system fails, such as TTP/C.
Once the system fails, information will be sent immediately to ensure that information transmission complies with regulations. The most suitable method is multi-channel time division (TDMA), which can ensure that there will be no unpredictable information lag. The TTP/C protocol is developed based on TDMA. The third is anti-interference processing. There will be various interference signals during the operation of the vehicle. How to eliminate the impact of these interference signals? There are currently a variety of anti-interference control systems, which are basically divided into two types: symmetrical and asymmetrical anti-interference control systems.
The symmetrical anti-interference control system uses two identical CPUs and the same calculation program to process the braking signal. The asymmetric anti-interference control system uses two different CPUs and different calculation programs to process the braking signal. Both methods have advantages and disadvantages. In addition, how to modularize the software and hardware of the electric brake control system to adapt to the needs of different types of vehicles; how to modularize the chassis is an important problem. Only by comprehensively considering braking, steering, suspension, navigation and other systems, modularizing the algorithm, and establishing a data bus system can we obtain the best control system at the lowest cost.
The electric brake control system was first used in hybrid braking system vehicles, using two braking systems: hydraulic braking and electric braking. This hybrid braking system is a transitional solution to fully electric braking systems. Since two sets of braking systems exist, the structure is complex and the cost is high.
With the advancement of technology, the above-mentioned problems will gradually be solved, and the all-electric braking control system will truly replace the traditional hydraulic-based braking control system. Figure 3 is the configuration scheme of this all-electric braking control system.
5. Conclusion
In summary, modern automobile brake control technology is developing in the direction of electronic brake control.
Due to its huge advantages, all-electric brake control will replace the traditional hydraulic brake control system. At the same time, with the development of other automotive electronic technologies, especially very large-scale integrated circuits, the cost and size of electronic components continue to decrease.
The automotive electronic brake control system will be integrated with other automotive electronic systems such as automotive electronic suspension systems, automotive active directional swing stabilization systems, electronic navigation systems, driverless systems, etc. to become a comprehensive vehicle Electronic control system, there will be no isolated braking control system in future cars, various control units are concentrated in one ECU, and will gradually replace conventional control systems to achieve intelligent vehicle control.
However, the development of automobile brake control technology is restricted by the development of the entire automobile industry. Attracted by a huge existing and potential automobile market, various advanced electronic technologies, biotechnology, information technology and various intelligent technologies are constantly being applied to automobile braking control systems. At the same time, various relevant international and domestic regulations need to be improved, so that cars equipped with new braking technology will be truly applied in mass production of cars.