Introduction to One-box and Two-box Braking Systems

Introduction to One-box and Two-box Braking Systems

   Recently, another Tesla high-speed collision incident has caused a stir. Is the braking of electric vehicles safe enough? It has rekindled public attention and discussion. Today, I will explain the braking system of electric vehicles from two aspects: the difference between the braking systems of electric vehicles and traditional vehicles and the technical application of electric vehicle braking systems, so as to provide readers with technical reference for rationally looking at issues related to the braking system.

01 Introduction to the passenger car braking systems

   Whether it is a traditional fuel vehicle or a new energy vehicle, the basic braking system consists of the following components:

The transmission path of braking force is three stages: pedal mechanical force → brake fluid pressure → caliper mechanical force:

1) The force from the driver's foot is first amplified by the brake pedal lever ratio, and then is amplified by the secondary amplification of the booster. Then it is passed to the master cylinder input the push rod.

2)The master cylinder input push rod pushes the piston to convert mechanical force into brake fluid hydraulic pressure. The brake fluid hydraulic pressure is then transmitted to the brake caliper through the pipeline and push the caliper piston.

3) The piston of the brake caliper pushes the friction plates to conform the rotating brake disc to produce friction, which acts on the wheels as the braking torque.

   There are no differences in principles and applications between electric vehicles and fuel vehicles when it comes to brake pedals and brakes. The main differences between different types of vehicles are concentrated in the "booster + master cylinder + ESP" module. The reason why "booster + master cylinder + ESP" are put together here is because the integration levels of these three modules are different in different technical solutions.

02 The structure of the braking system of the fuel vehicle

The structure of the braking system of a traditional fuel vehicle is shown in the figure below.

"Booster + master cylinder" is an assembly, and ESP is a separate module.The "booster" here is actually a vacuum booster. The principle is that the inside of the booster is divided into two cavities by a diaphragm: the atmospheric cavity and the vacuum cavity. When not braking, both the large chamber and the vacuum chamber are connected to the vacuum source to form a vacuum negative pressure. After the brake pedal is stepped on, the vacuum chamber continues to maintain the vacuum. The large atmosphere chamber is connected to the outside world and begins to intake air. Then the pressure difference between the two chambers acts on the diaphragm to form the vacuum-assisted force, which ultimately acts on the input push rod of the master cylinder. The amount of the vacuum-assisted force is in a fixed proportion to the input force of the pedal.The vacuum source comes from the engine. There are two ways of providing vacuum from the engine: one is the vacuum formed during the air intake process of the engine intake manifold, and the other is the vacuum pump driven by the engine crankshaft.The specific structure of the master cylinder with vacuum booster assembly is shown in the figure below.

For the above-mentioned vacuum assist system, the typical failure modes are as follows:

1) Brake pedal: Brake pedal fracture is a very rare and low-level failure mode. Regulations also define this part as a part that is not prone to failure. The main pedal-related failure is the failure of the brake light switch (BLS). BLS failure has no impact on basic hydraulic braking, but it will affect electronic braking functions such as ABS/TCS/VDC, EMS, and logical judgments related to the brake light switch. Of course, the lighting of the brake tail light will also be affected;

2)Vacuum booster: The most serious result of vacuum booster failure is no vacuum boost, such as booster leakage, vacuum tube leakage, etc. The driver's intuitive feeling is that the brakes are hard. Because of the lack of vacuum assist, the driver needs to exert several times more force than usual to achieve the vehicle's deceleration under normal circumstances.

3)Master cylinder: The failure of the master cylinder is concentrated in two forms: leakage and stuck. The former will cause the pedal stroke to become longer and softer, but the vehicle cannot establish normal deceleration; the latter will directly cause the brake pedal to be unable to be depressed.

4)ESP module: Failures in the brake light switch, powertrain, wheel speed sensor, power supply, CAN network and so on ,which will affect ESP related functions (ABS/TCS/VDC/HHC/AVH/HDC, etc.),.But due to the ABS/TCS/ The VDC function will only intervene under extreme vehicle conditions, so the failure of the ESP function will not affect basic braking. That is to say, light/moderate braking on a good road surface has little effect, but ABS fails during heavy braking and the wheels are prone to locking. The most dangerous road conditions in this case are ice, snow or gravel roads with low adhesion coefficient. The front and rear wheels can easily slip and lose control when braking or driving.

5)Brakes: There are many brake failures, especially those related to braking NVH, but the failures that really seriously affect driving safety are mainly the leakage of brake fluid in the calipers and the deterioration of the friction pads. Caliper brake fluid leakage is similar to the aforementioned master cylinder leakage. The performance degradation of the friction pad is mostly caused by thermal degradation. After the degradation, the braking efficiency decreases and the vehicle deceleration is far lower than the driver's expectation. The driver feels that the car cannot be braked.

6)Others: pipeline failure (leakage), wheel speed sensor failure, EPB failure, etc.

03 Electric vehicle braking system structure

Since the vacuum booster requires the engine to provide vacuum, new energy vehicles cannot use this system that relies on the engine to obtain vacuum when driving purely electric.

3.1 Electronic vacuum pump solution

The logic of the electronic vacuum pump solution is: since there is no engine to provide a vacuum source, then parts that can be evacuated independently are provided. The principle is very simple, that is, the motor drives the blade to rotate and vacuum. There are also plunger types, but they are not widely used. Therefore, the electronic vacuum pump solution directly provides vacuum for the engine at the hardware level. Electronic vacuum pumps are divided into independent pumps (the only source of vacuum and higher hardware requirements) and auxiliary pumps.

   The obvious advantage of this solution is that the amount of modification is small, and it is very suitable for sharing the braking systems of fuel vehicles and new energy vehicles on the same platform. The disadvantages of this solution are also obvious:

1) Arrangement problems caused by noise and vibration of electronic vacuum pumps;

2) The mainstream electronic vacuum pump market is almost monopolized, prices are high, and the quality of other manufacturers' products is unstable;

3) The conventional ESP has a low active pressure-building capability and cannot provide strong support for energy recovery and intelligent driving;

4) The failure or unreasonable strategy of the electronic vacuum pump leads to the failure or reduction of vacuum assist. Overall, the electronic vacuum pump solution is actually a low-cost solution. Judging from the trend of technological development, it is a transitional solution.

3.2 Electronic booster solution (two-box)

   With the promotion of new energy vehicles and the development of intelligent driving technology, the interaction between the braking system and the outside world is becoming more and more important. The cruising range of new energy vehicles puts forward higher requirements for energy recovery. The coasting recovery in energy recovery is related to the stability of the vehicle's low attachment. Braking recovery requires a braking system to dominate hydraulic braking and motor recovery braking. The development of intelligent driving has also put forward higher requirements for the pressure-building capability and response of the braking system. At the same time, the redundant design of autonomous driving also requires that the braking system must have a backup function. Therefore, Bosch has launched a solution of electronic booster that does not rely on vacuum, which is commonly called iBooster electronic booster. The structure of the electronic booster is very different from that of vacuum booster, but in essence it is still designed to simulate an empty booster. The difference from a vacuum booster is that the boost is provided by a built-in motor. The following figure can fully illustrate the power-assisting method of the electronic booster: the motor rotates to drive the gear to rotate. After reducing the speed and increasing the torque, the rotational motion is finally converted into linear motion through the worm gear, and finally, together with the force transmitted from the pedal, it drives the master cylinder input push rod. Build hydraulic pressure. The master cylinder part is the same as the traditional vacuum booster, and the valve seat that determines the boost ratio of the booster is basically the same structure and principle as the traditional vacuum booster. Since the booster and ESP are two independent modules in this solution, the industry calls it the two-box solution.

   Regarding the judgment of iBooster assist: The ECU will internally store one or more sets of pedal feel curves calibrated during the vehicle development process (such as pedal stroke vs. deceleration, pedal stroke vs. brake assist, etc.). When the driver depresses the brake pedal, the iBooster's internal stroke sensor infers the driver's braking intention based on the displacement of the brake pedal, further calculates the target assist amount, and then comprehensively considers the energy recovery amount/ABS working status, etc. Get the ultimate boost of iBooster motor execution. Thanks to iBooster's powerful power assist capability, electronically controlled semi-decoupled control method and the natural dual backup of Two-Box (iBooster and ESP), this braking system solution has great advantages in energy recovery and intelligent driving. This is also the reason why iBooster can be promoted quickly on the market. Up to now,a large number of models such as all Tesla series, almost all Volkswagen new energy vehicles, all Honda Accord series (including fuel vehicles), all Geely Lynk & Co new energy vehicles, Mercedes-Benz S-Class, Weilai, Xpeng have used the iBooster solution.

Of course, this type of system also has certain shortcomings:

1) The brake pedal feel will be worse than that of the traditional vacuum booster system. Theoretically, the coordination principle of the boost ratio between the electronic booster and the traditional vacuum booster is the same (both have rubber feedback disc structures), but in fact the boost of the electronic booster The size is a series of calculation and execution processes. During the execution process, the sensor's signal collection, controller calculation, and motor execution will produce certain errors and delays. In addition, the coordination between energy recovery and hydraulic braking will also Further increasing the difficulty of control, this "simulation" process is not as "smooth" as the purely physical dynamic balance of forces on traditional vacuum boosters.

2) The more complex things are, the greater the probability of failure. IBooster is strongly related to external ESP, intelligent driving, and power systems. Related system failures and CAN network failures may affect the iBooster's power-assisted function.

3.3 one-box solution

   one-box is mainly defined for two-box. When Bosch developed the two-box solution of iBooster+ESP, the mainland company was also developing another more integrated solution in response to the needs of the OEM: integrating ESP and electronic booster, becoming a module ,which is commonly known as one-box.

   The One-box integrates brake assist and ESP functions. The same thing as the two-box is that the brake assist is provided by the motor. The main difference is that the force transmitted by the two-box to the master cylinder input push rod is the sum of the driver's input force and the motor assist, and the proportional relationship between the two is the result of a mechanical balance, while the braking force provided by the one-box all comes from the motor, without superimposing the braking force provided by the driver. The force provided by the driver through the brake pedal is eventually converted into hydraulic pressure and leaked into the one-box's built-in pedal feel simulator. The pedal feel simulator is actually a piston spring mechanism used to simulate the brake pedal feel and provide the driver with force and stroke feedback.

The one-box assistance process can be simply described as:

1) The displacement generated by the pedal is obtained by the sensor and then input to the ECU;

2) The ECU calculates the driver's braking demand and then drives the motor to establish hydraulic pressure;

3) Hydraulic pressure enters the four wheel cylinders through the ABS inlet valve and ultimately generates braking force.

Therefore, under normal circumstances, the pedal force and the braking force ultimately provided by the one-box are mechanically decoupled.

   The most obvious benefit of this integration is the small number of parts and the low volumetric weight. The completely decoupled design makes it possible to theoretically adjust the deceleration relationship corresponding to any desired pedal force or stroke through software, that is, the pedal feel is largely determined by software. The disadvantage is that the force feedback on the pedal is isolated from the wheel, and the driver cannot sense the status of the wheel through the pedal. For example, when ABS is working, the driver cannot sense through the vibration of the pedal. Referring to the experience of the pedal feel problem of the two-box, the pedal feel of the completely decoupled one-box is worthy of attention. In addition, for L3 and above intelligent driving, one-box needs to plug in an ESP module as a redundant backup. This is where one-box is useless in advanced intelligent driving. As for failure, after the electronic booster fails, the two-box can also actively build pressure for braking by ESP, but the one-box does not have a backup system in the brake booster part (unless a low-performance ESP is plugged in).

04 One-Box system features

   The One-Box wire-controlled hydraulic braking system integrates traditional braking functions such as TCS (traction control system), ESC, ABS, and EPB. In addition, third-party control software can be integrated, such as tire pressure monitoring, EBD (Electronic Brake Force Distribution), AEB (Automatic Brake Assist System), AVH (Automatic Parking System) and other functions to achieve the development of integrated control of wire-controlled chassis domains.The main functions are:

1) Base Brake Control (BBC)

It automatically identifies the driver's braking demand by detecting the input of the brake pedal stroke sensor, establishes the corresponding hydraulic braking force according to the pedal displacement, and controls the brake hydraulic pressure to achieve brake-by-wire.

2) Anti-lock Braking System (ABS)

During the emergency braking process, the four-wheel braking pressure is controlled, and the wheel cylinder hydraulic pressure is controlled according to the wheel speed to prevent wheel locking, improve braking strength, and ensure vehicle driving stability.

3)Traction Control System(TCS)

During strong driving, such as starting or accelerating, the engine torque is adjusted to apply braking pressure to the slipping wheels to prevent excessive slipping of the driving wheels.

4)Electronic Stability Control (ESC)

When the vehicle turns, control the over-steer or under-steer of the vehicle.

5)Brake Energy Recovery System(CRBS)

During the braking process, the motor torque battery status and brake pedal status are detected in real time, and coordinated braking energy recovery is achieved by adjusting the braking pressure and motor recovery torque to improve the vehicle's cruising range.

6)Support AEB braking request

Receives ADAS module commands to implement functions such as Prefill and Warning Brake Deceleration; rapidly boosts pressure to improve AEB automatic emergency braking and shorten the distance during AEB emergency braking. The 300+ms saved through quick response can significantly reduce AEB false triggering probability;

7)Support ACC vertical control request

According to the commands of the ACC module, control the powertrain or braking system to achieve acceleration and deceleration;

8)Support APA/RPA vertical control request

According to the commands of the APA/RPA module, the powertrain or braking system is controlled to achieve acceleration and deceleration. By responding to the vehicle trajectory instructions, the vehicle is accurately controlled in the longitudinal direction of braking and driving, and the driver can automatically park in the car.

9)CST(Comfort-Stop) Comfortable parking

10)BSW

By detecting the information from the rain sensor, a certain pressure is established on the wheel cylinder and the water film on the brake disc is wiped to improve the braking performance in rainy days;

11)D-EPB

Dual-control EPB solves the parking redundancy problem of electric vehicles;

12) Redundant backup brake EPB-A

The rear wheel/front wheel EPB actuator acts as a backup service brake.

13)All-terrain and creep

Various off-road surfaces to improve passability and safety

14)HFC

Provides additional wheel cylinder pressure to the driver when the driver fully depresses the brake pedal and the vehicle does not reach maximum deceleration.

05 Comparison of one-box and two-box

   For the one-box or two-box system, Chinese domestic suppliers such as Wanxiang, Asia Pacific, Bethel, Grubo, Nason, and Tongyu all have corresponding products. The main foreign suppliers of one-box or two-box system include Bosch, Continental, ZF Friedrichhshafen, Nissin, Hitachi (including CBI), Mobis, Advics, etc. The product technology concepts of these suppliers are similar, and the main differences lie in mass production scale and product maturity.