Design of brake-by-wire brake system for an unmanned minibus based on ESC

The ESC-based brake-by-wire system introduced in this paper is one of the most economical and mature methods to realize the intelligent control of automobile brakes.

With the continuous development and maturity of automotive electronic technology, the brake-by-wire system came into being. Due to its high integration and easy layout, the brake-by-wire system can not only meet the ESC function, but also realize ACC, AEB and other functions, so as to meet the needs of intelligent driving function development. Automotive ESC controls the size and direction of tire longitudinal force and lateral force to ensure that the car runs stably under braking, driving, sharp steering and even other extreme conditions, and enhances the safety of the car. Automotive ESC provides a certain basis for automotive brake-by-wire by realizing precise control of the wheel cylinder pressure of each wheel.

1、Selection and structure of brake-by-wire system

1.1 Selection of brake-by-wire system
The unmanned minibus needs to realize L4 unmanned driving functions in a specific operating area, such as autonomous planning of routes and selection of stops, etc., requiring its brake-by-wire response delay to be ≤0.5 s. Since the vehicle's ESC is based on the traditional hydraulic braking system, it has the advantages of low cost, short delay, complete failure redundancy, real-time independent control of four-wheel braking, etc., and can be used to accurately execute the braking command issued by the automatic driving controller to realize Active control of vehicle deceleration or brake pressure. Therefore, the driverless minibus uses an ESC-based brake-by-wire system.

1.2 Brake-by-wire system architecture
The car's ESC-based brake-by-wire system architecture is shown in Figure 1

including:

• oil storage cup 1,
• electronic hydraulic control unit (HCU) 2
•  pressure sensor 3
• pressure acquisition board 4 
• combination sensor 5 
• brake calipers 6
• Brake disc 7
• brake hard pipe 8
• brake hose 9
• oil-resistant hose 10
• wiring harness 11
• CAN signal 12, etc.

Wherein the electronic hydraulic control unit (HCU) 2 includes a motor, a controller, and a solenoid valve. Its main functions are as follows:
1) Respond to the target deceleration request sent by the top-level controller of the vehicle (that is, the vehicle VCU): the deceleration range is 0-6.0 m/s2, the deceleration response time is ≤0.5 s, and the deceleration pressure build-up time is ≤0.6 s. The response time refers to the time from when the VCU of the whole vehicle sends a braking request to when the vehicle speed begins to decrease precipitously; the pressure build-up time refers to the time from when the VCU of the whole vehicle sends a braking request to when the vehicle reaches the target deceleration.
2) On normal cement or asphalt roads, the brake-by-wire accuracy is required to be max(0.2 m/s2, 10%), that is, take the maximum value between 0.2 m/s2 and (10%×target deceleration)
 

1.3 Control algorithm architecture of brake-by-wire system
1.3.1 Brake pressure model

The basis of the control algorithm of the ESC-based brake-by-wire system is the brake pressure model.

1) Brake pressure model design. The brake pressure model is designed as follows: firstly build the hardware model of the motor and various controllers in the HCU based on the characteristics of the HCU, and then compare the different target decelerations calculated according to the vehicle parameters of the unmanned minibus with the required The relationship curve of brake pressure is imported into the above-mentioned brake pressure hardware model, and finally the brake pressure required for different target decelerations can be achieved through the matching design of the opening of the motor and controller in the model.

2) Brake pressure model control. When the HCU receives the braking signal, the designed braking pressure model performs feed-forward control and performs feedback control according to the wheel cylinder pressure signal. The HCU selects the appropriate control command to generate the target pressure to brake the vehicle so that the vehicle reaches Target deceleration, while ensuring the consistency, stability and smoothness of braking deceleration.

1.3.2 Control Algorithm Architecture

The control algorithm based on the ESC brake-by-wire system is mainly divided into active braking control (relevant state quantity calculation and entry and exit condition judgment) module, upper controller (target deceleration controller) module and lower controller (Active brake pressure controller) module, its architecture is shown in Figure 2.

Among them, the control logic of the upper target deceleration controller and the lower active brake pressure controller is shown in Figure 3.

The role of the upper-level target deceleration controller is to convert the target deceleration into a target pressure; the role of the lower-level active brake pressure controller is to solve the appropriate motor and solenoid valve commands to achieve the target pressure requested by the upper-level controller.

The control logic of the upper-level target deceleration controller: according to the vehicle longitudinal dynamics model, calculate the reference target pressure required to achieve the target deceleration as the feedforward link in the control process; according to the deviation between the target deceleration and the actual deceleration, The target braking pressure is corrected to obtain the corrected braking pressure, which is used as the feedback link in the control process; finally, the comprehensive target pressure is obtained according to the reference braking pressure and the corrected braking pressure.

The control logic of the lower active brake pressure controller: first, calculate the basic opening of each solenoid valve and the basic opening of the motor according to the forward pressure model; then, calculate the corrected opening of each solenoid valve according to the pressure deviation feedback and the corrected opening of the motor; finally, the combined opening of the solenoid valve and the motor is obtained by superimposing the basic opening and the corrected opening.

2、Selection and structure of brake-by-wire system

The components of the above-mentioned brake-by-wire system are assembled into the whole vehicle, and the above-mentioned theoretical design is verified to complete the final design of the brake-by-wire system of the whole vehicle.

For the aforementioned unmanned minibus, the dynamic verification of the brake-by-wire system is carried out on a flat, high-adhesion pavement, and the ambient temperature is about 30 °C.

This verification test item is the deceleration step change. The deceleration step change test reflects the typical pressurization-holding-decompression process and simulates the typical braking and deceleration conditions of the vehicle. When braking, the initial speed is about 15 km/h, and the target deceleration is 1.0-6.0 m/s2. For each target deceleration, record the deceleration response time, deceleration pressure build-up time, and brake-by-wire accuracy. The technical requirements and test results of the verification test are shown in Table 1.

The comparison of the main functions in 1.2 and the technical requirements and test results in Table 1 shows that the system can follow the target deceleration in time and accurately under different target decelerations, and the two time indicators also meet the technical requirements and achieve the expected goal .

3、Conclusion

This paper expounds the design and development process of the brake-by-wire system of a 4 m unmanned small passenger car, mainly introduces the architecture, main functions, technical indicators and control algorithm architecture of the brake-by-wire system based on ESC, and conducts verification tests.

The results show that the ESC-based brake-by-wire system fully meets the requirements of the braking response time ≤ 0.5 s and the pressure building time requirements under each deceleration gradient.