Zhi-qiang XU

(Guangdong University of Science and Technology， Dongguan 523083， China)

Abstract: The Electric-control Hydraulic Brake (EHB) system of automobile is a brake control mode which combines the technology of wire control within hydraulic control unit. The dynamic performance of EHB directly affects the braking effect of the whole automobile. In order to improve the control efficiency of EHB system, in this paper, the working characteristics of EHB system are analyzed firstly, and then the simulation model of EHB system is established by using AMESim software. Based on the proposed model, the control strategy of EHB system is designed with PWM and logic threshold method. Finally, the pressure following and ABS braking effect are tested. The test results show that the designed control method can achieve the effective braking of the vehicle, which has a good effect on the response characteristics of the target pressure and greatly improves the braking performance of the vehicle.

Key words: Electric-control Hydraulic Brake, ABS, Pressure following, PWM control

With the development of automobile electronic technology, the brake system is developing to linear control, which makes the electro-hydraulic brake technology mature and perfect. EHB system combines wire control technology with hydraulic brake unit, cancels vacuum booster, and gets rid of the limitation of conventional brake system in hardware structure. The EHB system has fast response speed and great rising gradient of braking pressure, which can complete a variety of functions such as ABS, TCS and ESP, and has good pressure control and braking performance[1-2]. For pure electric vehicle or hybrid vehicle, EHB system is also conductive to the realization of braking energy recovery function, and it is easier to achieve automatic driving, which has a better application prospect.

The EHB system obtains the brake pedal signal through the electronic control unit, so as to identify the driver’s brake intention, and determine whether the brake type belongs to light brake, general brake or emergency brake. Then the hydraulic control unit is used to achieve the establishment and release of the brake pressure, and finally the hydraulic mechanism completes the brake operation [3-5]. The performance of EHB control system directly influences the braking effect of the whole vehicle. In the process of braking, the EHB system needs to switch between the three states of pressurization, pressure protection and pressure reduction, and the pressure in the wheel cylinder often presents problems such as delayed hydraulic impact, which will influence the effectiveness of braking. Therefore, it is the focus of researchers to find an effective braking control algorithm. In this paper, a control strategy based on PWM and logic threshold method is proposed by analyzing the dynamic characteristics of EHB system, and the pressure following characteristics of the system and the effectiveness of ABS mode are analyzed through simulation and test, and the braking effect of EHB system is evaluated.

1 Structure and working principle ofEHB system

EHB system uses electronic components to replace some mechanical components in the original brake system, which are powered by hydraulic system and linear control by electronic system [6]. The structure of EHB system mainly includes brake pedal, displacement sensor, motor, oil pump, accumulator, pressure control valve, brake and other components, as shown in Fig.1.

1.Brake pedal; 2.Displacement sensor; 3.Hydraulic pump; 4.High pressure accumulator; 5.Switch valve; 6.Pressure relief valve; 7.Booster valve; 8.Balance valve

In the above figure, brake pedal and displacement sensor constitute the brake signal acquisition part of EHB system, hydraulic pump and accumulator constitute the pressure source part of EHB system, each pressure control valve constitutes the hydraulic control unit of EHB system, booster valve and pressure reducing valve are used for the establishment and release of brake pressure, and balance valve is used to balance the brake pressure of left and right wheel cylinders of vehicle. When the EHB system fails, the oil inlet valve and the oil outlet valve will be closed, and the switching valve is open. By depressing the brake pedal, the driver makes the brake fluid flow into the wheel cylinder through the switching valve, and realizes the emergency braking by using the traditional hydraulic braking method.

When EHB works normally, the sensor collects the brake command from the driver, converts the collected displacement or speed data into electrical signal, and then transmits it to the brake control unit. The controller judges the driver’s brake intention according to the brake signal, sends the corresponding command to the hydraulic control unit, uses PWM control method to change the opening state of the booster valve and the pressure reducing valve, and controls the access wheel The brake fluid flow of the cylinder completes the establishment and release of the brake pressure and realizes the precise control of the pressure in the braking process. The hydraulic control unit can also receive the dynamic data from ABS, ASR and ESP through CAN bus technology. After analysis and processing, the control signal is sent to the corresponding control unit to optimize the vehicle control.

2 Mathematical model of EHB system

(1) Model of accumulator

If the spring force inside the accumulator is ignored, the relationship between the internal pressure of the accumulator and the average flow rate of the hydraulic oil is as follows [7-8]:

(1)

Where,γis adiabatic exponent;p0is initial pressure of accumulator;V0is initial volume of accumulator;pAis working pressure of accumulator;qis average flow rate of hydraulic oil.

(2) Dynamic balance equation of solenoid valve

core

(2)

Where,xis spool displacement;mis spool quality;A0is cross sectional area of valve port;F0is spring preload;Kis spring stiffness;Cvis damping ratio.

(3) Brake cylinder model

The relationship between the fluid flow into the wheel cylinder and the brake fluid pressure is as follows:

(3)

Where,Eis bulk modulus of elasticity of hydraulic oil;Vis volume sum of wheel cylinder and pipeline;pwis brake wheel cylinder internal pressure.

(4) Braking force model of tire

(4)

Where,Jis moment of inertia of wheel;ωis angular speed of wheel;Fzis braking force of the ground;Ris wheel radius;Tbis braking torque;Tis resistance moment of wheel.

3 Simulation analyses of EHB system

3.1 Simulation model of EHB system

In order to study the performance of EHB system, the performance of EHB system is studied by means of joint simulation. AMESim software is used to establish the hydraulic system model of EHB system, as shown in Fig.2.

Fig.2 AMESim model of EHB system

3.2 Pressure following control algorithm of EHB system

The rationality of pressure following algorithm is the basis of ensuring the braking effect of EHB system. In this paper, the motor and solenoid valve are controlled by PWM method, and the brake pressure is adjusted in real time to achieve the rapidity and accuracy of brake response [9-10]. The control algorithm block diagram is shown in Fig.3.

In this figure:Vpis pedal speed;Sis the brake pedal travel;Ptagis the target pressure of the wheel cylinder;Pactis the actual pressure value of wheel cylinder;PWMrefis the reference value of PWM;PWMcmdis the command value of PWM.

The controller collects the brake pressure of the wheel cylinder, gets the error between the actual value and the target value according to the reference pressure of the pedal signal, then judges the working state of the EHB system according to the error, and calculates the duty cycle PWM of each wheel cylinder solenoid control valve. When the difference value is large, PWM will increase, the working time of the solenoid valve gets increased, when the difference value is small, PWM will decrease, and the working time of the solenoid valve gets shorten, so as to reduce the pressure oscillation process. The specific flow of the algorithm is shown in Fig.4.

Fig.3 Block diagram of brake pressure following control algorithm

Fig.4 Workflow of pressure following algorithm

3.3 Simulation of pressure following characteristics

In order to verify the effectiveness of the braking force tracking algorithm, step signal and brake signal are used as the target pressure of the wheel cylinder, and PID controller is added to optimize the control model. Specific parameters of simulation are as follows: the motor speed is 1 000 r/min, the signal duty cycle of solenoid control valve is 0.1-0.9, the frequency of solenoid valve is 25 Hz, the control parameters of PID areKp=4，Ki=1，Kd=0.1, respectively, and the simulation is carried out with the step signal of 14 MPa as the target pressure, and the results are shown in Fig.5. The simulation results show that the overshoot of the pressure response curve is small and the response speed is fast.

Fig.5 Pressure curve under step signal input

The brake signal is used as the target pressure value for simulation, and the results are shown in Fig.6. It can be seen that the control algorithm has a strong tracking ability, can well achieve the following of the wheel cylinder pressure to the target signal, and has a good effect on the response characteristics of the target pressure.

Fig.6 Pressure curve under brake signal input

4 Research on ABS braking mode of EHBsystem

4.1 Control algorithm in ABS mode

The function of ABS system is to make full use of the adhesion between the wheel and the road surface when the car is braking, so that the wheel is in the best braking state, while shortening the braking distance, ensuring the stability of the car when braking, and preventing the car from sideslip and deviation. At present, the ABS function of automobile mainly adopts the logic threshold control method, which controls the tire slip rate and the wheel angular deceleration at the same time. By judging the running state of the wheel, the corresponding measures of pressurization, pressure maintaining and decompression are taken to keep the tire slip rate in the optimal range, so as to improve the driving stability of the vehicle. In order to study the ABS function of EHB system, this paper adopts the four phase logic threshold method for control, and its regulation process is shown in Fig.7.

Fig.7 Pressure regulation process of ABS system

In the above figure, 1- 4 represents the four phases of the control process. A1 and A2 are the thresholds for wheel deceleration. S1 and S2 are the thresholds for wheel slip. In the process of braking, when the wheel deceleration is less than the threshold value, the wheel cylinder will enter the pressure maintaining state. At this time, due to the hydraulic hysteresis, the wheel slip rate will still rise in a certain period of time until the slip rate is greater than the threshold value, which is the phase 1. When the wheel slip rate is greater than S1, it enters into the decompression stage. With the decrease of the wheel cylinder pressure, the acceleration of the wheel will increase until the acceleration reaches the upper door limit. This stage is the 2-phase. When the acceleration of the wheel is greater than A2, it enters the pressure maintaining stage again, and the slip rate gets decreased accordingly, which is the 3-phase. When the slip rate reaches S2, ABS enters the 4-phase control logic, and the whole control cycle ends when the slip rate reaches S1 again. In this control cycle, A1 determines the time when ABS starts to work, A2 determines the time when the wheel cylinder pressure is maintained, S1 plays a role in the beginning of the control cycle, and S2 ensures that the slip rate is controlled within a reasonable range.

4.2 Simulation analysis of ABS mode

In order to verify the effectiveness of ABS function in EHB system, the simulation analysis of ABS mode is carried out according to the above control logic. In the simulation process, the vehicle speed is 100 km/h, and the adhesion coefficient of the driving road is 0.6. At this time, the changes of the vehicle speed, wheel speed and slip ratio of left-front wheel are shown in Fig.8 and Fig.9.

Fig.8 Slip rate change of front-left wheel

Fig.9 Changes of vehicle speed and wheel speed

From the simulation results, it can be seen that the time from the start of braking to the stop of the vehicle is about 5 s. In the process of braking, there is an intermittent braking process, but there is no wheel lock. The wheel and the vehicle basically stop moving at the same time, which better realizes the ABS function. When the tire slip ratio changes from 0.1～0.2, the adhesion of the road surface will be fully utilized and the braking stability of the vehicle is greatly improved.

5 EHB system test

According to the structure of the electro-hydraulic brake system, a test platform is built. Using the control strategy adopted, take FR wheel as an example, carry out the pressurization and decompression test in the basic braking process, and compare with the simulation results, as shown in Fig.10 and Fig.11.

Fig.10 Pressurization test results

Fig.11 Decompression test results

It can be seen that the pressurization and decompression process can reach the set value in about 0.1 s, and maintain the pressure according to the reference pressure signal. There is almost no overshoot in the whole braking process, which can well describe the working process of pressure establishment and pressure release. The test and simulation results are basically consistent, which shows that the pressure response of EHB system is faster and more accurate, and the proposed control method meets the system braking requirements.

6 Conclusions

(1)The dynamic performance of the electro-hydraulic braking system directly influences the braking effect and braking stability of the whole vehicle. This paper firstly analyzes the dynamic characteristics of EHB system, establishes AMESim simulation model, puts forward the control strategy based on PWM and logic threshold method, and analyzes the pressure following characteristics of the system and the effectiveness of ABS braking mode to achieve the evaluation of EHB system braking effect.

(2)The research results show that the model and control method can achieve the effective braking of the vehicle and have a good follow-up effect for the target pressure. At the same time, EHB system can realize ABS function well. In the process of emergency braking, the braking potential of the vehicle has been fully exploited, which greatly improves the braking stability of the vehicle.