Summary:
Since the birth of the automobile, the vehicle braking system has always played a decisive role in realizing the smooth handling of the automobile and ensuring the safety of the automobile. The braking system itself has also continued to evolve with changes in industrial technology and the development of the automotive industry.
According to the difference of braking scenarios, the braking system can be divided into the following two categories:
- service brake
- Parking brake
The development route of driving brake and parking brake, the picture comes from Soochow Securities
This article will summarize the development history of these two types of braking systems, provide readers with a bird's-eye view perspective, and lay the foundation for subsequent articles to expand the working principles of mainstream products of braking systems and their applications in intelligent driving.
1. Development of service braking system
The service braking system generally consists of a braking transmission and a brake. The brake transmission device includes various components and pipelines that transmit braking energy to the brakes, such as brake pedals, brake master cylinders, wheel cylinders and connecting pipelines. A brake is a component that produces a force that resists the movement or tendency of a vehicle, generally through friction between a stationary element and the working surface of a rotating element.
A typical service braking system, image courtesy of Al-Jazirah
This series of articles focuses on passenger cars, and the development of passenger car service braking systems can be summarized into three periods:
-
hydraulic braking period
-
Electromechanical + hydraulic fusion braking period
-
bbw period
1.1. Hydraulic braking period
The most primitive braking system is to apply force directly to the brakes by manipulating a group of simple mechanical devices by the driver. The mass of the initial vehicle is relatively small and the speed is relatively low. Mechanical braking can meet the needs of vehicle braking.
As vehicles become heavier and heavier, mechanical brakes can no longer meet the basic braking requirements; coupled with the development of science and technology and the development of the automobile industry, new breakthroughs have been made in the design of vehicle braking systems. Another major innovation after braking.
The Duesenberg Eight car was the first to use hydraulic brakes for cars, and Chrysler's four-wheel hydraulic brakes came out in 1924. General Motors and Ford adopted hydraulic braking technology in 1934 and 1939 respectively. With several generations of product upgrades, hydraulic power-assisted braking systems have gradually become mainstream.
The hydraulic braking system of a general passenger car consists of the following parts:
- brake pedal
- vacuum booster
- brake fluid
- brake hose
- brake master cylinder
- brake wheel cylinder
- Wheel brakes (disc or drum)
The disc brake shown below is an example. When the driver steps on the brake pedal, due to the leverage, the pedal force is transmitted to the vacuum booster through the first stage of amplification; the vacuum booster transmits the braking force to the master cylinder through the second stage of amplification; The brake fluid of the master cylinder is pushed into the wheel cylinder and under the action of pressure, a greater braking force is generated, and the wheel end calipers are pushed to tighten the brake disc to hinder the rotation of the brake disc, thereby achieving braking.
Working principle diagram of hydraulic brake system
Due to the balance between reliability and cost, many entry-level family cars today still use hydraulic brake systems with vacuum booster pumps as brake assist.
1.2. Electromechanical + hydraulic fusion braking period
With the development of the automobile industry, the maximum speed of automobiles is increasing year by year, and the degree of injury caused by traffic accidents is also increasing. Therefore, while improving the braking performance of the braking system, the industry is also exploring ways to improve safety.
In fact, since the 1930s, the automobile industry began to conduct research on anti-lock brake systems (Anti-lock Brake System, ABS), but limited by the technical level at that time, there has been no mass-produced solution. Until the late 1970s, the development of digital electronic computer technology and the advancement of hydraulic control technology had a revolutionary impact on the ABS system. The German Bosch company launched ABS based on hydraulic control, and the control effect was quite satisfactory. Bosch also officially mass-produced it in 1978. Since then, Bosch, ITT Automotive, Kelesy-Hayes, Wabco and many other companies have continuously strengthened the research on ABS. Various new types of ABS have emerged in an endless stream, with continuous optimization of performance and gradually lower prices, so that more and more cars use ABS as the standard. configure.
The superiority of ABS is manifested in the following aspects:
- It can effectively use the adhesion between the tire and the road surface and shorten the braking distance, especially on icy and snowy roads, it can be shortened by 10%-15%;
- During the braking process, the wheels can still roll, which maintains the maneuverability of the front wheels, prevents the sideslip of the rear wheels, and maintains the stability of the driving direction;
- Due to the anti-locking of the brake, the wheel will not lock and slip, which reduces the wear of the tire, can increase the service life of the tire and reduce the pollution in the air
However, ABS also has limitations. The first thing that bears the brunt is that the premise of ABS activation is that the driver's braking will cause the wheels to lock, which means that the stability control of ABS under driving conditions (such as acceleration and skidding on ice) is helpless. In order to solve the stability problem of this driving condition, in 1986, Bosch launched the traction control system TCS, which integrated the brake anti-lock braking system and the traction control system and applied it to the Mercedes S-class sedan, which marked the The coming of ABS/TCS integration era.
Since ABS only works when braking, and TCS only works when driving, the integration of ABS/TCS can only solve the problem of vehicle longitudinal stability, but cannot solve the problems caused by extreme working conditions such as driving and braking steering, high-speed steering, etc. lateral stability problems. Bosch developed the first-generation stability control system VDC based on ABS/TCS in 1992 to solve the problem of vehicle lateral stability, and named the product integrating ABS/TCS and VDC functions as ESP ( Electronic Stability Program).
The birth of Bosch ESP is an epoch-making product in the history of automobile development. In 1995, the ESP system was mass-produced and applied to the Mercedes-Benz S-Class for the first time. Later, other manufacturers also launched similar products, but they could no longer use the abbreviation of ESP, so they were named in various ways, but they were all collectively called Electronic Stability Control System ESC (Electric Stability Controller). Studies have shown that the VDC function can reduce 80% of traffic accidents caused by sideslip and reduce the number of serious car accidents by 50%. Therefore, the ESC that inherits the VDC function is called "the ESC that can save lives" by many world-renowned automobile manufacturers and research institutions.
Key Components of VDC Control System
1.3. Brake-by-wire period
The wire control technology originates from the aircraft control system, which converts the pilot's control commands into electrical signals, and transmits them directly to the autonomous steering gear through cables. The biggest advantage of wire control technology is precise and rapid response, but the disadvantage is that a large number of electronic and electrical components replace traditional mechanical components, reliability is challenged, and relatively high costs are required to reduce the failure rate of wire control systems. It is also based on this point that although the automotive industry explored the application of wire control technology as early as the end of the 1920s, there was no mass production.
The development of new energy vehicles, especially pure electric vehicles, has brought a turn for the popularization of wire control technology in vehicles. Since the engine is replaced by an electric motor, the use of vacuum boosters that rely on the engine to generate the vacuum source is limited. On the other hand, due to the existence of the driving battery, the traditional braking system converts kinetic energy into heat energy through friction and consumes it, which is not very energy-saving. The braking system needs to explore the conversion of kinetic energy into chemical energy and store it in the battery in order to recover energy. way of recycling.
Under such a trend, as a global leader in chassis-by-wire technology, Bosch, Germany has launched a new generation of brake booster products: the intelligent booster iBooster after in-depth research and development.
iBooster does not depend on the vacuum source and replaces the traditional vacuum pump and vacuum hose. It is smaller in size and lighter in weight of the entire braking system. It does not need to consume energy to establish a vacuum source. It only consumes power when braking, and at the same time it can recover energy, thereby To achieve the purpose of energy saving and carbon reduction, it is applicable to all powertrains, including hybrid and electric vehicles, and is more in line with future development trends. Therefore, it is favored by mid-to-high-end models and its market share is getting higher and higher.
On the other hand, the advantages of precise and rapid response of wire control technology have been continued. iBooster uses sensors to perceive the strength and speed of the driver stepping on the brake pedal, and sends the signal to the electronic control unit after processing. Work, so as to achieve electronically controlled braking, faster response and precise pressure control.
Bosch second generation iBooster
After Bosch, new smart booster manufacturers have emerged in the domestic and foreign markets. The mainstream ones are Continental, ZF and China Nason. The names of each manufacturer vary, but they are collectively called eBooster.
At present, many mainstream mid-to-high-end new energy vehicles are equipped with the "eBooster+ESC" combination, also known as the "Two-box" solution, which realizes the basic braking function and the stability function respectively. In order to further reduce costs, an integrated basic braking The "One-box" solution that combines functions and stability functions has begun to be favored by OEMs.
Another advantage of the "One-box" solution compared to the eBooster is that the pedal is decoupled, the driver's pedal force does not act on the master cylinder, the pedal feel is realized by the simulator, and the braking force is realized by the servo motor, so the pedal feel can be adjusted freely greater degree. The mainstream products of "One-box" in the current market are Bosch's latest generation IPB, Continental MK C1, Bethel WCBS and so on.
Under today's wave of electrification and intelligence of automobiles, with the increasing popularity of assisted driving systems (such as ACC, AEB, etc.) and the gradual implementation of automatic driving systems, new demands for more and more intelligent scenarios have been derived. . Driven by this wave, automobiles have also put forward higher requirements for the braking system. How to ensure the safety of the automatic driving system is the primary issue.
Functional safety requires that when the E/E system fails, the vehicle needs to be able to enter a safe state in time to avoid unreasonable risks of personal injury. This requires that the automatic driving system needs a redundant design in order to liberate the driver while ensuring In the event of a single failure the system is still able to take over until it reaches a safe state. Brake system redundancy is essential to fulfill this requirement.
In summary, it can be seen that the brake-by-wire system will have a broader stage to play under the wave of vehicle electrification and intelligence. At the same time, the brake-by-wire system itself needs to continue to evolve to meet the requirements of the new era.
2. Development of parking brake
The development trajectory of the parking brake is similar to that of the service brake, that is, the transformation from the traditional mechanical system to the electromechanical system.
Traditional mechanical hand brake is made up of brake lever, stay wire, braking mechanism and return spring. The brake lever uses the lever principle, so that the driver can pull it to a fixed position with a small pulling force, and then lock and park the vehicle through the locking teeth.
Schematic diagram of mechanical handbrake, picture from West China Securities Research Institute
At present, the electric parking system (EPB, Electric Parking Brake) is gradually replacing the handbrake. According to the forecast, the market penetration rate is expected to reach 90% in 2025. EPB consists of two parts:
- The parking actuator (caliper and motor that controls the caliper) that generates the parking force
- The electronic control unit (ECU) that controls the parking actuator, consisting of software and hardware.
Electronic handbrakes are gradually replacing mechanical handbrakes
At present, the market share of EPB is mainly occupied by foreign brands such as Bosch, Continental, and ZF. However, because EPB is a static parking brake system, the requirements are lower than that of the service brake system, and the process of localization replacement is expected to be accelerated. Domestic self-owned brands such as Bethel, relying on the advantages of localization, high cost performance and quick response, have realized partial localization substitution, and are gradually developing into the field of high-end passenger vehicles.
The electronic parking system also has a new stage in the wave of vehicle electrification and intelligence. For example, Remote Parking Control (RPA, Remote Parking Control) and Automated Valet Parking (AVP, Automated Valet Parking) currently have relatively mature implementation solutions, but many OEMs believe that the "Two-box" solution can be used to achieve brake redundancy requirements The cost is relatively high. In this context, the autonomous parking brake redundancy scheme of the integrated electronic parking system EPB, which is "modified" based on the standard EPB system, is being favored by more and more OEMs. (The design of this brake redundancy scheme will be described in detail in subsequent articles.)
Further, under the evolution of automatic driving, the service brake system and the parking brake system also show a trend of "combining two into one".
The current brake-by-wire system is mainly a hydraulic brake-by-wire (Electro-Hydraulic Brake, EHB) that is not purely controlled by wire. The calculation unit realizes wire control, but the execution unit still retains the traditional hydraulic working method.
If the four hydraulic wheel cylinders are replaced by four motors and calipers, the braking command is transmitted to the motor at the wheel end, and the motor controls the calipers to directly apply the braking force at the wheel end, then complete wire control will be achieved. This solution is called mechanical wire Control brake (Electro-Mechanical Brake, EMB). At present, this solution is limited by the cost and has not yet achieved mass production, but with the gradual evolution of automatic driving to L5, EMB, as the ultimate solution for brake-by-wire, will give full play to its advantages over EHB. A distinction is made between the service brake system and the parking brake system.
Author | Pan Jiang
Produced| Yan Zhi