Two-box solution for automobile brake-by-wire-EEWORLD

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According to different brake actuators, the wire control brake system can be divided into two categories: hydraulic and mechanical. Among them, the electronic hydraulic brake system (EHB) is based on the traditional hydraulic brake system, replaces some mechanical components with electronic devices, uses brake fluid to transmit force, and has a hydraulic backup brake system. It is the current mainstream technology. According to the degree of integration, EHB is divided into two solutions: Two-box and One-box.

Twobox/one box system, domestic suppliers all have corresponding products, including Bosch, Continental, ZF, Nissin, Hitachi (including CBI), Mobis, ADVICS, etc., and Wanxiang, Asia Pacific, Bethel, GLUBO, Nason, Tongyu, etc. The technical concepts are similar, and the main differences are in mass production scale and product maturity.

As the new energy vehicle market expands, the "eBooster + ESC" combination has become the most mainstream two-box solution on the market. In addition to realizing basic brake assist and stability control functions, the solution also coordinates brake energy recovery to ensure that the driver's pedal feel is consistent when switching between electric braking and hydraulic braking. In addition, with the popularization of advanced driver assistance systems and automatic parking systems, "eBooster + ESC" also plays a role in brake redundancy.

This article will introduce the implementation of the basic braking function of the two-box solution.

*Driver brake control

* External ECU brake control

* Brake light control

Two-box solution system architecture of eBooster and ESC

01 Driver Brake Control

In order to realize the driver's brake assist function, eBooster first needs to correctly detect the driver's braking intention. The pedal travel sensor (Pedal Travel Sensor) installed at the eBooster push rod monitors the depth of the driver's pedal depression, and then feeds back to the eBooster's DBR-F (Driver Brake Request-Brake Force) module to judge the driver's intention.

ESC+eBooster driver brake control function diagram

After confirming the driver's braking intention, eBooster does not provide assistance directly, but instead feeds back the braking intention to ESC through network communication. ESC is responsible for allocating hydraulic braking force and drive motor force.

As a vehicle stability control system, the main goal of ESC is to ensure that the distribution of braking force does not cause the risk of vehicle instability. For example, if the driver suddenly steps on the accelerator and causes the wheels to lock, the ESC system will stop requesting braking force from the drive motor while activating the ABS, and focus on maintaining stability by adjusting the wheel cylinder hydraulic pressure.

In addition, the ESC braking force distribution can also realize the braking energy recovery function. With the popularization of new energy vehicles, braking energy recovery has become an important function. Through this function, in addition to using hydraulic pressure for friction braking during braking, the car also realizes partial braking force through the cooperation of high-voltage batteries and drive motors. Negative torque generates reverse current to charge the high-voltage battery, and finally converts part of the vehicle's kinetic energy into chemical energy and stores it in the high-voltage battery. In this way, energy recovery can be achieved to achieve the purpose of energy saving and emission reduction.

ESC hydraulic circuit diagram

In order to achieve braking force distribution, the ESC hardware must first be able to remove the "direct connection" between the master cylinder brake fluid and the wheel cylinder brake fluid, which is achieved by the ESC large-capacity accumulator and the control of the wheel-end solenoid valve. With the support of the large-capacity accumulator, when the driver steps on the brake pedal, the eBooster controls the master cylinder hydraulic pressure to enter the wheel cylinder to generate braking force. At the same time, the driving electric brake force slowly increases with the increase in the depth of the brake pedal. During this process, the brake fluid from the master cylinder will not flow directly into the wheel cylinder, but a part of it will be temporarily stored in the accumulator. The brake fluid in the accumulator will not generate braking force, thereby achieving dynamic coordinated control of the electric brake force and the hydraulic brake force during braking.

ESC braking force distribution diagram

However, the "separation" of the master cylinder brake fluid and the wheel cylinder brake fluid caused by the dynamic distribution of ESC braking force will cause the driver's brake pedal feel to change. The PFC (Pedal Force Compensation) module of eBooster can ensure a consistent pedal feel. Its core principle is that during the driver's braking process, eBooster controls the size of the boost (as shown in the figure below) to always ensure that the reaction force of the pedal fed back to the driver's foot is constant at the same pedal depth, so that the driver cannot feel whether it is motor braking or brake fluid braking at this time, thereby achieving a consistent pedal feel and giving the driver the most comfortable experience.

The principle of eBooster maintaining consistent pedal feel when achieving brake energy recovery

In the figure above, the spring force Fsprings is constant. In order to achieve constant pedal force Fpedal at a certain pedal depth, the PFC module needs to know the size of the braking force Fhydraulic generated by the current hydraulic energy, so as to adjust the appropriate boost Fboost. At this time, due to the influence of the accumulator fluid control and wheel-end solenoid valve control during the braking force distribution process, the master cylinder pressure value collected by the master cylinder pressure sensor in the ESC system cannot correspond to the actual hydraulic braking force, so the ESC needs to send a "virtual" master cylinder pressure value to the eBooster to determine the size of Fhydraulic. The virtual master cylinder pressure value is obtained by looking up the pre-calibrated pv curve in the DBR-T (Driver Brake Request-Brake Torque) module of the ESC, and is fed back to the PFC module of the eBooster through communication to determine the size of the boost motor output required to achieve the target pedal feel.

02 Brake light control

The brake light control strategy is related to the degradation status of the eBooster system.

When the eBooster is in full function, the brake lights are controlled by the eBooster system according to the driver's pedal status. At this time, ESC will only request to light up the brake lights when the stability function or the auxiliary function that does not rely on the driver's braking activates active pressure building.

Brake light control with eBooster fully functional

When the eBooster power assist function fails, eBooster requests the activation of the HBC function. At this time, the driver's braking request is realized by the ESC actively building pressure. At this time, the brake lights under all working conditions are fully controlled by ESC, including the driver's braking conditions and the conditions where the stability function and auxiliary function actively build pressure.

Brake light control when eBooster function is downgraded

03 External ECU brake control

Since the dynamic response speed of eBooster pressure building is faster than that of ESC active pressure building and the NVH performance is better, eBooster is the main actuator when the external ECU (such as ADAS ECU) requests the braking system to brake. This can also reduce the load of active pressure building in the entire life cycle of the ESC system.

eBooster+ESC realizes EBR (External Brake Request)

The EBR-C (External Brake Request-Controller) module in the ESC is responsible for receiving brake requests from the external ECU, converting the brake requests into target master cylinder pressure values and sending them to the EBR-E (External Brake Request-Execution) module in the eBooster through the communication network. The eBooster then calculates the target assist value to implement the brake request.

During this process, eBooster will also provide real-time feedback of the actual output pressure value to ESC. For example, when the eBooster reaches the Runout point and its power-assisting ability drops significantly (see the figure below), ESC will actively build up pressure for braking compensation.

eBooster runout point

04 Conclusion

Thanks to iBooster's powerful boosting ability, 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, which is why iBooster can be quickly promoted in the market. So far, a large number of models such as Tesla's entire series, almost all Volkswagen's new energy vehicles, Honda's entire series of Accord (including fuel vehicles), Geely Lynk & Co's entire series of new energy vehicles, Mercedes-Benz S-Class, Weilai, and Xiaopeng have used the iBooster solution.

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

1) The brake pedal feel will be worse than that of the traditional vacuum booster system. In theory, the coordination principle of the boost ratio of the electronic booster and the traditional vacuum booster is the same (both have a rubber feedback plate structure), but in fact the boost size of the electronic booster has gone through a series of calculations and executions. During the execution process, the sensor's signal collection, the controller's calculations, and the motor's execution will all produce certain errors and delays. In addition, the coordination between energy recovery and hydraulic braking will further increase the difficulty of control. After all, such a "simulation" process is not as "smooth" as the dynamic balance of purely physical forces on the traditional vacuum booster.