1. Introduction
The traditional automotive hydraulic braking system consists of a vacuum booster with a master cylinder, an Anti-lock Braking System (ABS)/Electronic Stability Control System (ESC), four wheel-cylinders and hydraulic pipelines. The vacuum booster amplifies the driver’s force on the brake pedal, reducing the driver’s fatigue. ABS can regulate the hydraulic pressures of the four wheel-cylinders independently through the control of solenoid valves and motors, thereby achieving anti-lock control of the wheels. ESC can actively build braking forces to specific wheels, thereby applying yaw moment to the vehicle to achieve vehicle stability control. Applications of these products have greatly improved driving safety and maneuverability.
In recent years, with the continuous development of the economy, the transportation system is also constantly changing. In order to solve the conflict between the rapid increase in vehicle ownership and limited land resources, Intelligent Transportation System (ITS) technology was born. ITS is an important area of sustainable urban development for it can bring people more intelligent and convenient travel methods, reduce accident rates, reduce energy consumption, and thus promote urban efficiency, safety, and environmental protection. The early ITS focused on electronic and informatization, and the main applications of this stage of ITS are electronic toll collection systems, traffic signal control, and electronic police systems. As the ITS continued to develop, it is now focusing on networking and collaboration, and the main applications are intelligent networked vehicles. In the future, ITS may focus on autonomous driving and vehicle–infrastructure collaboration. With the help of big data, artificial intelligence, and 6G technology, the applications will evolve into autonomous intelligent transportation systems for the Internet of Everything, intelligent vehicle–infrastructure collaboration systems, etc. [
1,
2,
3,
4]. In this context, ITS requires autonomous driving of vehicles, such as the ability to park automatically in parking lots and emergency braking on public roads [
5]. This requires vehicles to have high-performance active braking capabilities and redundant brake backup capabilities.
New energy vehicles are one of the important means to achieve sustainable development. New energy vehicles use pure electric drive, which can replace traditional internal combustion vehicles, thus getting rid of the dependence on fossil fuels. At the same time, new energy vehicles do not produce greenhouse gases such as carbon dioxide during driving, which can effectively reduce carbon emissions and alleviate global warming trends. In addition, new energy vehicles do not produce toxic and harmful gases and particulate pollutants during driving and can effectively alleviate urban emission pollution. The development of automobile electrification has put forward new requirements for automotive braking technologies. For example, the popularity of electric vehicles has led to the lack of vacuum sources. New brake systems need to get rid of the dependence on a vacuum source. At the same time, electric vehicles require brake systems to achieve a higher energy recovery rate to increase driving range.
However, traditional braking systems can no longer meet these requirements. In order to comply with these new requirements, automotive hydraulic braking systems are undergoing drastic changes. The wire-controlled Electro-Hydraulic Braking System (EHB) has emerged at this moment. It uses a high-performance motor and reduction mechanisms instead of a vacuum source for service brake and can achieve high-performance active braking functions. As a key component of ITS technology and new energy vehicles, EHB technology can achieve high-performance active braking and high-efficiency energy recovery, which can effectively support the development of ITS and new energy vehicles and is the key to sustainable development. EHB can be divided into Two-box and One-box solutions.
The Two-box EHB solution only replaces the vacuum booster with an electric booster, leaving the original ESC with no or minor changes. The Two-box EHB itself can achieve mechanical decoupling of the brake pedal and wheel-cylinder braking forces through mechanical clearance or through the modified ESC, thereby achieving a 100% energy recovery rate [
6] within the deceleration range of about 0.3 g.
The original intention of developing the Two-box EHB is basically to realize wire control of the brake booster, and it is a substitute for a vacuum booster. Therefore, the Two-box EHB solution has a simple structure and low overall development difficulty; car manufacturers only need to replace the original vacuum booster with a Two-box EHB, making little changes to the original vehicle. Based on the above advantages, Two-box EHB was the first to be applied massively and occupied most of the brake-by-wire market share. However, Two-box EHB still has many obvious disadvantages, such as the difficulty in achieving brake decoupling alone and being impossible to use in high-level autonomous driving scenarios due to its lack of vehicle stability control functionality. These problems forced researchers to continue research and then invented the One-box EHB solution.
The One-box EHB solution integrates the electric booster with an ABS/ESC and can independently perform all braking functions, including service brake, pedal feel simulation, failure backup, active brake, wheel-cylinder hydraulic control, ABS, ESC, and other vehicle safety and comfort control functions. At the same time, One-box EHB completely isolates the brake pedal and wheel-cylinders through the solenoid valves and thus has a complete decoupling capability. In theory, it can achieve a 100% energy recovery rate in any deceleration range. In addition, by equipping One-box EHB with additional redundant braking units, high-level autonomous driving can be achieved.
Compared with Two-box EHB, One-box EHB has a compacter structure, higher integration, and smaller size, which is more conducive to vehicle layout; it has a higher degree of decoupling, which is more conducive to increasing the driving range of electric vehicles; it has a higher degree of scalability, which is more conducive to achieving high-level autonomous driving. Based on the above advantages, as a new generation of brake-by-wire systems, One-box EHB is currently a hot research topic for researchers and engineering developers.
However, there are many difficulties in the process of developing One-box EHB. These difficulties restrict the rapid implementation and mass application of One-box EHB. They are described below.
First of all, One-box EHB integrates the electric booster and wheel-cylinder pressure control unit and, at the same time, needs to meet braking regulations, fail-safe redundancy requirements, and various functional requirements. How to achieve highly integrated system architecture design under numerous constraints is a major challenge.
Secondly, One-box EHB is composed of mechanical, electronic, and hydraulic systems. These three systems are coupled to and interfere with each other. The entire braking system has many components, and there exist unknown disturbances. There are difficulties in how to achieve robust master cylinder and wheel-cylinder hydraulic pressure control under deep coupling and high disturbance conditions.
Finally, coordinated electro-hydraulic composite braking control requires good braking stability, high energy recovery rate, good braking smoothness, and braking feeling but is also restricted by actuator capacity limitations, braking regulations, etc. How to control the four-wheel braking forces and the drive motors to achieve multi-objective-optimized composite braking control is worthy of in-depth study by researchers.
Therefore, this article conducts an in-depth study on the current development status of the above research hotspots so as to understand the current research level and provide directions and suggestions for future research on One-box EHB.
Section 2 of this article studies the current status of the system architecture design of One-box EHB and intelligent driving applications.
Section 3 studies the current status of master cylinder pressure control, friction model and its compensation, master cylinder pressure estimation, and sensor-less control.
Section 4 studies the current status of solenoid valve modeling, solenoid valve, and wheel-cylinder pressure control.
Section 5 studies the current status of electro-hydraulic composite braking systems. The key concluding remarks are given in
Section 6.
Figure 1 gives a structured overview of this review work.
Table 1 is the summary of the important concepts in the paper.
3. Master Cylinder Pressure Control
As a replacement for the traditional vacuum booster, One-box EHB is required to undertake conventional service braking functions. It also needs to take the responsibilities of electro-hydraulic coordinated composite braking control, active emergency braking, and vehicle comfort braking functions. Therefore, the robust and precise control of the master cylinder pressure is very important, and its control quality will directly affect driving safety and comfort. However, the One-box EHB system is relatively complex. The mechanical, electronic, and hydraulic parts are coupled with each other. The whole system has many links and long chains. There are many nonlinear factors, time-varying factors, and external disturbances. The robust and precise control of hydraulic pressure is a big challenge. Therefore, researchers have conducted a lot of research on this topic.
3.1. Pressure Control
The braking system has pressure–volume (PV) characteristics; that is, the braking system has a certain hydraulic stiffness, and the pressure of the system obeys a specific relationship with the volume of brake fluid entering the braking system from the brake master cylinder. Therefore, by controlling the stroke of the master cylinder, the pressure of the brake system can be controlled. I.-J. Yang et al. [
31] designed a master cylinder stroke control algorithm, and experiments proved that the algorithm can achieve fast and comfortable braking.
However, for master cylinder pressure control, master cylinder stroke control is indirect and belongs to open-loop pressure control. Since the PV characteristics of the braking system will be affected by the external environment and disturbances, simple master cylinder stroke control cannot meet the pressure control accuracy requirements of One-box EHB, and a feedback control algorithm for the master cylinder pressure still needs to be studied. Fabio Todeschini et al. [
32] developed a hybrid position-pressure switching controller. At the beginning of the control, the position controller is deployed; when the pressure approaches the target, the controller switches to the pressure control.
The EHB system has nonlinear characteristics, and it is difficult to establish an accurate mathematical model of the system. Zhizhong Wang [
33] et al. developed a gain-scheduling PI controller to control the actuator. Lei Chen et al. [
34] used the error and error derivatives as input variables to design a fuzzy incremental PID controller for hydraulic pressure control.
A cascade controller connects multiple controllers in series. Among them, the primary control object is the control object of the entire system, and the secondary control object is related to the primary control object. By introducing the secondary controller, the disturbance on the secondary control object can be overcome; thereby, the secondary controller can cooperate with and complement the primary controller to improve the control effect. The EHB system has a long control chain. By reasonably designing multi-level series controllers, the disturbances in each controller can be effectively suppressed, thereby improving the hydraulic pressure control effect.
Jian Zhao et al. [
35] designed a four-step cascade brake pressure controller based on pressure-position-speed-current. Among them, the pressure control adopts open-loop control according to the PV characteristics of the braking system; the position and speed closed-loop controller adopts PI controller; the d-axis current in the current controller adopts PI control; and the q-axis current adopts MPC control.
However, due to the high nonlinear characteristics in the EHB system and hydraulic circuit, the traditional PID controller is not fully capable of pressure control, and the use of PID controllers in multiple cascade closed-loop controls requires a lot of calibration work, which results in difficulties in engineering practice. To solve these problems, Weihong Yang et al. [
36] designed a pressure-position-current cascade controller. Among them, the pressure controller adopts the fuzzy PI algorithm based on feedforward. Experiments show that compared with traditional PID control, this algorithm can effectively improve the control effect of hydraulic pressure.
On this basis, Jian Zhao et al. [
37] further proposed a pressure-position-current cascade control algorithm. In order to solve the harmful effects of the dynamic friction in the mechanism, the large inertia of the system, and the nonlinear characteristics of the hydraulic system on the control, the authors introduced a three-step nonlinear control method in the position closed-loop control and decomposed the nonlinear control problem into static control, feedforward control based on reference dynamics and state-dependent feedback control. Hardware-in-the-loop (HiL) experiments prove that this control algorithm can effectively improve control accuracy and response speed.
The EHB system and the ABS system are interrelated and together form a complete automotive braking system. When the ABS is working, during the pressure decrease process of ABS control, the brake fluid will be pumped from the wheel-cylinder back to the EHB’s master cylinder; during the pressure increase process of ABS control, the brake fluid will be poured from the EHB’s master cylinder into the wheel-cylinder. The modulation process of wheel-cylinder pressure will cause the master cylinder pressure to fluctuate violently, causing noise and affecting the durability of the system. Therefore, it is important to consider master cylinder pressure control during ABS activation. Jingtian Wang et al. [
38] proposed that during the ABS control process, the EHB needs to provide a pressure greater than the four-wheel wheel-cylinder pressure in real-time, but it needs to be smaller than the hydraulic pressure required by the driver. The real-time pressure of the four-wheel-cylinder is obtained by installing four wheel-cylinder pressure sensors, and based on the rules, the maximum wheel-cylinder pressure envelope value and the maximum wheel-cylinder pressure pre-charge value are designed as the target input of the EHB master cylinder pressure. The authors then proposed a three-closed-loop pressure optimization control strategy of master cylinder pressure-piston displacement-motor current. HIL experiments show that under different road adhesion conditions, this strategy can effectively reduce pressure fluctuations when ABS is activated without affecting the control performance of ABS itself.
3.2. Friction Model and Its Compensation
The mechanical transmission mechanism of EHB comes in many forms, but all of them have the nonlinear characteristic of friction, which causes crawling, dead zone, oscillation, and other problems in the master cylinder hydraulic pressure control process. However, system friction is a nonlinear time-varying disturbance, which is affected by external factors such as the piston speed, piston movement direction, transmission load, and temperature, causing difficulties in pressure control. How to accurately model system friction and eliminate or compensate for the effects of friction through control methods is a major challenge in EHB hydraulic pressure control.
3.2.1. Experiments and Identification
Bing Wang et al. [
39] proposed an EHB friction model that combines static friction and viscous friction. When testing static friction, control the motor to slowly increase the current. The motor torque, when the motor’s speed changes from 0 to a non-zero value, is the static friction of the system. When testing viscous friction, control the motor to move at a constant speed at different speeds. The measured motor torque at different speeds is the viscous friction of the system.
Ricardo de Castro et al. [
40] studied the characteristics of the internal friction of the EHB through a slow ramp experiment of the motor current and found that due to the existence of static friction, the hydraulic force has a crawling phenomenon. In addition, the size of the friction force is related to the load, and the friction force is much larger than the load, which shows that friction force disturbance plays an important role in the EHB control system.
On this basis, Yuan Ji et al. [
41] added the motor current slow sine experiment and obtained the kinetic friction and static friction curves. They found that the kinetic friction and static friction are asymmetrical with respect to the motor speed direction, and have a linear relationship with the load. At the same time, there is a nonlinear relationship between dynamic friction and piston speed. On this basis, the authors propose an asymmetric continuous friction model, which combines the continuity advantages of the Lure model and introduces linear load-dependent nonlinear characteristics.
Wei Han et al. [
42] conducted an in-depth study on the relationship between friction force, piston position, and motor speed under the conditions of pressure increase and pressure decrease. By controlling the piston to move at a constant speed at different speeds, the friction relationship under different conditions is obtained through experiments, and it is concluded that the relationship between friction and rotational speed obeys Coulomb friction, viscous friction, and Stribeck effect, and has load-dependent properties. Also, there is friction asymmetry during the pressure increase and decrease processes. In order to reduce the difficulty of identifying friction model parameters, the authors chose to compensate based on Coulomb friction and viscous friction models.
Considering the balance between model accuracy and refinement, Zhentao Chen et al. [
43] used the Karnopp friction model to describe the system friction characteristics of EHB. Since experiments show that break-away force and slip friction are positively related to hydraulic pressure, hydraulic force is introduced into the Karnopp friction model. The parameters of the model were identified through experiments with slowly increasing current.
3.2.2. Compensation and Control
After considering Coulomb friction, viscous friction, and Stribeck effect, combined with the influence of hydraulic load, Ricardo de Castro et al. [
40] designed a friction map model and approximated it through a linear-in-the-parameter model. Experimental results show that the friction model has certain effectiveness and performance, can reduce fitting errors, and can allow a certain degree of parameter uncertainty.
Since the Coulomb friction and viscous friction models ignore static friction, Wei Han et al. [
42] introduced high-frequency sinusoidal flutter signal compensation to eliminate the influence of static friction. At the same time, due to the introduction of model-based friction compensation, the amplitude of the flutter compensation can be very small, thus ensuring the comfort of braking. Finally, the authors fuse the sliding mode controller with the conditional integrator.
The system has disturbances such as model mismatch, time-varying parameters, and unexpected disturbances. Zhentao Chen et al. [
43] designed a disturbance observer and proposed an explicit model predictive controller. Experiments show that this control method improves hydraulic pressure control accuracy and disturbance suppression capabilities, and the real-time performance of the algorithm is verified on the microchip.
Table 4 shows the summary of master cylinder pressure control schemes.
3.3. Pressure Estimation and Sensorless Pressure Control
The pressure sensor is a key component of the EHB system, providing master cylinder pressure signal feedback for EHB to carry out precise pressure control. However, the unit price of the pressure sensor is relatively high. If the master cylinder hydraulic pressure can be accurately estimated through the pressure estimation algorithm, the pressure sensor can be omitted, reducing the cost of the EHB and improving product competitiveness. In addition, if the pressure sensor fails, the EHB must rely on the pressure estimation algorithm for redundant braking. Therefore, it is very necessary to study the master cylinder hydraulic pressure estimation and master cylinder pressure control without a hydraulic pressure sensor providing a feedback signal.
The position of the master cylinder piston can be calculated by the angle of motor rotation together with the transmission ratio of the system. The torque output by the motor can be calculated by the motor current. These two signals are the main basis for estimating the master cylinder pressure. Wei Han et al. [
53] established a control-oriented system model for EHB and regarded the dead zone of static friction, Coulomb friction, and PV characteristics as disturbances. Based on this model, an interconnected pressure estimator is designed, which includes an estimator and a nonlinear parameterized disturbance observer.
However, there are many uncertain signals when estimating the master cylinder hydraulic pressure. The PV characteristics of the system are affected by the direction and speed of the piston movement. The friction of mechanical structures is affected by temperature, pressure, and speed. These uncertain disturbances bring greater challenges to the accurate estimation of master cylinder pressure. The master cylinder pressure cannot be accurately estimated through a single PV characteristic curve or system dynamics model. Wei Han et al. [
54] proposed a master cylinder pressure estimator based on actuator characteristics and vehicle dynamics. The dynamics of wheel rotation are obtained through the wheel speed sensor, and the pressure is estimated based on the vehicle and wheel dynamics models. Compared with the estimator based only on actuator characteristics, the estimation accuracy of the joint estimator is improved, and the performance and robustness of the control method are verified through experiments.
The friction coefficient varies greatly during the braking process and will thus affect the pressure estimation by vehicle and wheel dynamics. Biaofei Shi et al. [
55] obtained a modified brake line friction coefficient model through a large number of real vehicle braking tests under different initial brake disc temperatures, different vehicle speeds, and different braking pressures. The master cylinder pressure is estimated based on vehicle longitudinal dynamics and wheel dynamics, and the estimation method can adapt to road gradient changes with the help of inertial sensors.
4. Wheel-Cylinder Pressure Control
The brake-by-wire system must have the ability to independently modulate the four-wheel braking forces to achieve ABS, TCS, AYC, and other functions. Using solenoid valves to control brake hydraulic pressures is a reliable and low-cost solution and is widely used in hydraulic-based brake-by-wire systems. Each wheel brake is equipped with a pair of an inlet valve and an outlet valve, which can increase, hold, and decrease the wheel-cylinder pressure. The control quality of the inlet valve and the outlet valve determines the accuracy of wheel-cylinder pressure control and is the most important actuator for wheel-cylinder pressure control. Therefore, many researchers have conducted a lot of research on wheel-cylinder pressure control based on solenoid valves.
4.1. Modeling of Solenoid Valves
The solenoid valve system is a dynamic system coupled with multiple physical fields such as electromagnetic field, flow field, temperature field, and structural field. However, solenoid valves have many small parts and are fully enclosed. Directly observing various physical quantities in solenoid valves through experiments is very complex and costly. Therefore, the researchers chose to accurately model the solenoid valve system to deepen their understanding of the controlled object.
Figure 7 is the typical architecture diagram of a solenoid valve.
Xiang Gao et al. [
56] established a nonlinear dynamic model of the solenoid valve based on theoretical analysis and finite element simulation. The model consists of three sub-models, namely the mechanical sub-model, the electromagnetic sub-model, and the fluid dynamics sub-model. These sub-models are coupled through force and valve spool displacement. The mechanical model is a single-degree-of-freedom mass-spring-damper system that follows Newton’s second law. Electromagnetic models are divided into electric field models and magnetic field models. The electric field model is a resistance-inductance system and follows Kirchhoff’s law; the magnetic field model follows Kirchhoff’s second law of the magnetic circuit and is represented by two-dimensional finite element simulation results. For the fluid dynamics sub-model, the valve spool is mainly affected by hydrostatic force, hydraulic force, and viscous force. The fluid momentum of the control body of the valve is analyzed, and the adaptation parameters are analyzed through finite element analysis to obtain the hydraulic force on the valve spool. Through experimental verification, this model has high accuracy and can be used to help study wheel-cylinder pressure control strategies.
Figure 7.
Structure of the solenoid valve [
57].
Figure 7.
Structure of the solenoid valve [
57].
Aihong Meng et al. [
57] established a flow field simulation model through CFD tools and conducted joint simulation with the brake model built with AMESim, so they obtained the dynamic pressure and opening data of the solenoid valve under real working conditions. The simulation model can be used as a reference for control strategy optimization.
4.2. High-Frequency On-Off Control
The main control actuator for wheel-cylinder pressure control is the inlet valve. The inlet valve is a two-position, two-way solenoid valve. When the coil is powered off, the valve spool is in the fully open position by the restoring force of the spring, allowing brake fluid to flow into the wheel-cylinder; when the coil is powered on, the electromagnetic force overcomes the spring force, and drive the spool into the closed position, and brake fluid cannot flow into the wheel-cylinder. Through the high-frequency pulse control of the coil, the valve spool is quickly switched between the fully open and fully closed positions, and the time period of each opening is controlled so that a very small amount of brake fluid enters the wheel-cylinder during the opening process, so that the wheel-cylinder braking pressure can be finely adjusted.
Aldo Sorniotti et al. [
58] tested the step response of the inlet valve on the HiL brake test bench and completed the ABS anti-lock simulation experiment verification based on the high-frequency switching control method. Kevin O’Dea [
59] simulated ABS based on high-frequency switch control. The experimental results showed that the estimation accuracy of wheel-cylinder pressure estimation using solenoid valves did not affect the ABS braking performance. The high-frequency switch control has high robustness to the upper layer vehicle braking stability algorithm. Ding Nenggen et al. [
60] measured the inlet valve characteristics under different pressure increase–hold cycles and found that rapid switching of pressure states can be achieved by using different increasing and holding times, and the pressure increase rate can be flexibly controlled. This enables fine regulation of wheel-cylinder pressure increase.
Zhicheng Chen et al. [
61] studied the impact of PWM frequency on control under high-frequency switching control and believed that the working range of lower frequencies is wider, but the control effect within the control cycle is poor; the working range of higher frequencies is narrower, but the control effect within the control loop is better. At the same time, the characteristics of the solenoid valve under different duty cycles were further studied, and it was found that there was a delay during the pressure increase process. In order to make the wheel-cylinder pressure increase delay as small as possible, a larger pressure increase rate is required. However, a large pressure increase rate can easily cause overshoot and oscillation. Therefore, the authors proposed a variable gain PID controller. When the hydraulic pressure control error is large, the controller gain is relatively large; when the hydraulic pressure control error is small, the controller gain is relatively small. Experimental results show that the controller can achieve rapid pressure increase when the hydraulic pressure tracking error is large and can achieve precise control when the hydraulic pressure tracking error is small.
The high-frequency switching control frequency of solenoid valves is generally around 10–100 Hz, which is easy to implement, simple to control, and has high robustness. However, under this control strategy, the solenoid valve is in a high-frequency switching state, and its valve spool switches back and forth between the fully open and fully closed positions, rapidly hitting the mechanical limits on both sides, which will cause noise. At the same time, the hydraulic pressure increases step by step under high-frequency switching, and the control is relatively rough, making it impossible to finely adjust the wheel braking force and slip rate. At the same time, during the process of closing the valve, hydraulic pressure oscillation will occur due to the water hammer effect, which will have a negative impact on hydraulic pressure control, noise, and pedal feel.
4.3. Pressure Difference Control
When the valve spool of the solenoid valve is in the closed position, the spool is subject to electromagnetic force, hydraulic force, spring restoring force, and valve seat support force. The hydraulic force comes from the pressure difference between the master cylinder and the wheel-cylinder. If the electromagnetic force applied to the valve core happens to be balanced with the hydraulic force and spring restoring force, although the valve core and the valve seat are in contact, there is no interaction force between them, and the valve spool is in a critical state. In this critical state, keep the electromagnetic force unchanged, and continue to increase the hydraulic pressure of the master cylinder, the hydraulic pressure on the valve core will become larger, the force balance of the spool will be broken, and it will leave the valve seat and move in the opening direction. At this time, the brake fluid enters the wheel-cylinder from the master cylinder. As the wheel-cylinder pressure gradually increases, the hydraulic pressure on the spool gradually becomes smaller until it returns to the original equilibrium state and the spool returns to the critical state again. Since the electromagnetic force does not change at this time, the hydraulic pressure difference does not change either; that is, the pressure difference at both ends of the spool does not change. In other words, the coil current and electromagnetic force are in one-to-one correspondence with the pressure difference across the spool. Under the premise that the master cylinder pressure can be obtained through the sensor, the wheel-cylinder pressure can be controlled in an open loop by controlling the current, which also provides a new method for wheel-cylinder pressure control.
Junzhi Zhang et al. [
62] conducted theoretical analysis and simulation verification on this critical state pressure difference control method. The results showed that there is a linear relationship between the coil current and the critical pressure difference, but it takes a certain time to reach the critical pressure difference. In addition, the authors also found through simulation experiments that although the mass of the valve spool has no effect on the critical pressure difference, it will significantly extend the time for the valve core to reach the critical state. At the same time, Junzhi Zhang et al. [
63] verified through HiL experiments that compared with traditional PWM control, this critical pressure difference control method has higher accuracy in controlling brake hydraulic pressure.
Figure 8 shows the details of the valve spool and seat and the linear relationship between the coil current and the critical pressure difference.
In fact, open-loop control of wheel-cylinder pressure using only critical pressure difference characteristics cannot meet the requirements of dynamic adjustment of wheel-cylinder pressure by upper-level algorithms such as ABS. Passively waiting for the brake hydraulic pressure to reach the critical state takes a long time, and other control methods need to be combined to meet the upper-level control’s demand for rapid response to hydraulic pressure. Houhua Jing et al. [
64] added PID feedback control based on the critical pressure difference characteristics and achieved better target hydraulic follow response speed and control accuracy.
Since the solenoid valve has manufacturing errors and the working conditions, such as ambient temperature change in real-time, and it may be affected by external disturbance, using critical pressure difference characteristics for open-loop control will also lead to low control accuracy. The use of closed-loop feedback control can better compensate for uncertain external changes, and the hydraulic pressure control has high robustness. In order to achieve stronger control robustness and control accuracy and to compensate for valve inconsistencies and external disturbances, Chen Lv et al. [
65] introduced a hydraulic pressure control algorithm based on sliding mode control on the basis of retaining the original open-loop critical pressure difference characteristic control. Experiments show that this control algorithm that combines critical pressure difference characteristics and sliding mode control has higher hydraulic control accuracy and control robustness than pure critical pressure difference characteristic control or simple sliding mode control.
4.4. Valve Spool Displacement Control
The wheel-cylinder has PV characteristics; that is, there is a certain relationship between the pressure of the wheel-cylinder and the volume of brake fluid entering the wheel-cylinder. If the volume of brake fluid entering the wheel-cylinder can be controlled, the pressure in the wheel-cylinder can be dynamically controlled. The flow rate through the inlet valve is not only related to the pressure difference between the two ends of the spool but also related to the displacement of the spool. According to the law of orifice throttling flow rate equation, the greater the valve core displacement, the greater the flow rate. If the valve core displacement can be well controlled, the flow rate of the inlet valve can be effectively controlled, thereby controlling the pressure of the wheel-cylinder. Therefore, the wheel-cylinder pressure control problem can be converted into a spool displacement control problem.
WANG Weiwei et al. [
66] established a control simulation model of the inlet valve. Through simulation, they studied how the valve spool can be suspended at a certain position with different duty ratios under 2~4 kHz frequency PWM control, thus achieving the goal of achieving the third spool state other than fully open or fully closed in the traditional high-frequency switch control. This method broadens the functional application range of the inlet valves and provides a reference for further improving the accuracy of wheel-cylinder pressure control.
Xun Zhao et al. [
67] designed a nonlinear sliding mode observer for the spool displacement based on the voltage and current of the coil and obtained the brake fluid flow through the hydraulic model of the solenoid valve and the braking system. Taking into account the nonlinear characteristics of electromagnetic force, hydraulic force and solenoid valve flow under different valve spool openings, a sliding mode controller is designed to adjust the valve spool position to accurately linearly control the wheel-cylinder pressure. Tests show that this method can effectively linearly control the wheel-cylinder pressure, enable the low-cost on–off valve to function as a proportional valve and reduce noise and pedal vibration.
Zhao Xiangyang et al. [
68] established a mathematical model of the solenoid valve, established the state equation of the solenoid valve, used the root mean square volume Kalman filter algorithm to estimate the valve spool position of the solenoid valve, and calculated brake fluid flow and wheel-cylinder pressure based on this estimated valve spool position. Then, a spool position controller is designed based on the sliding mode variable structure control algorithm. HiL experiments show that the wheel-cylinder pressure estimation is accurate, and the control algorithm can accurately track the control target.
Table 5 shows the summary of wheel-cylinder pressure control schemes.
4.5. Wheel-Cylinder Pressure Estimation
There are usually two ways to estimate wheel-cylinder pressure: one is estimating actuator characteristics through solenoid valves and pressure models, and the other is estimating braking force based on tire models and wheel dynamics. The estimation method based on actuator characteristics is interfered with by the consistency of the actuator and unknown external disturbances, and there is error accumulation. Estimation methods based on wheel dynamics are affected by vehicle and brake system model accuracy and actual signal oscillations. A single method cannot accurately estimate wheel-cylinder pressure. Qixiang Zhang et al. [
69] designed a wheel-cylinder pressure estimation algorithm based on the orifice throttling flow rate equation and wheel-cylinder PV characteristics. Guirong Jiang et al. [
70] used the extended Kalman filter to combine the two estimation methods and used wheel speed variables as observed variables to correct the hydraulic model. Experimental results show that this method can accurately estimate the wheel-cylinder pressure during the wheel-cylinder pressure regulation process.
Haichao Liu et al. [
71] established a solenoid valve hydraulic model and a wheel dynamics model and combined the two models to design a wheel-cylinder hydraulic pressure estimation algorithm based on the extended Kalman filter. Experiments show that this method shows high estimation accuracy in both the overall trend and the process of hydraulic pressure changes.
Lingtao Wei et al. [
72] established a linear valve model and a switch valve model, respectively, based on the characteristics of solenoid valves. The two models were respectively subjected to Kalman filtering with the tire longitudinal force model based on slip rate and adhesion coefficient, and the two were fused using the IMM model to obtain the estimated wheel-cylinder pressure. Real vehicle experimental results show that the linear valve model is more accurate in most cases, while the switch valve model is more accurate under high hydraulic conditions, and the IMM fusion estimation algorithm can achieve higher precision wheel-cylinder hydraulic pressure estimation than the Kalman filter alone.
4.6. Four Wheel Pressures Coordinated Control
The method of independently controlling the four-wheel-cylinder pressures of the One-box EHB system is different from that of traditional ESC. Under the traditional ESC architecture, the back pressure for wheel-cylinder pressure modification is provided by the motor driving the plunger pump. When increasing the pressure, the brake fluid pumped by the plunger pump flows into the wheel-cylinder through the inlet valve; when decreasing the pressure, the brake fluid flows out of the wheel-cylinder and flows into the accumulator through the outlet valve and is pumped back to the original circuit by the plunger pump to participate in the circulation of brake fluid. During this process, the two circuits are independent of each other, and the back pressures of the inlet valves of the two wheel-cylinders in each circuit are the same and need to be modified independently. For the traditional ESC architecture, because the pressure of the plunger pump fluctuates and the control accuracy is poor, it is only necessary to maintain a high back pressure through the plunger pump, and the fine modification of the wheel-cylinder pressures is completed by the fine control of the solenoid valves.
As for the One-box EHB system, since the pressure increase in the pressure-building unit is generated by a high-performance motor-driven transmission mechanism that directly pushes the hydraulic cylinder piston, its hydraulic pressure control accuracy is high and does not produce pressure fluctuations and noise when the building pressure. Moreover, the One-box EHB system is equipped with a hydraulic pressure sensor, and thus the wheel-cylinder pressure estimation can be more accurate, so using the One-box EHB system for wheel-cylinder pressure control can theoretically achieve a better control quality.
However, there are also challenges in independently controlling wheel-cylinder pressures in a One-box EHB system. The inlet valves of the four wheel-cylinders are connected to each other, and their back pressures are coupled through the pressure of the pressure-building unit. The coordination and scheduling of the four-wheel pressures need to be considered. The original PV characteristics change during the wheel-cylinder pressure modification process, and traditional piston position control is no longer applicable. Real-time flow changes during the wheel-cylinder pressure modification process will cause greater disturbance to the pressure control of the pressure-building unit. Therefore, it is necessary to study the coordinated control of each actuator under four-wheel-cylinder pressure control.
Jian Zhang et al. [
73] designed single-wheel pressure modification and multi-wheel pressure modification methods. For single-wheel pressure modification, the inlet valve of this wheel is opened, and the pressure is controlled by the pressure-building unit, while the corresponding inlet valves of other wheel-cylinders are closed. For multi-wheel pressure modification, the inlet valve of the wheel with the greatest pressure demand is opened, and the pressure is controlled by the pressure-building unit. The inlet valves of the other wheels are controlled according to their respective pressure demands.
Wei Han et al. [
74] designed a brake-by-wire system with a minimum hydraulic circuit, which consists of an electro-mechanical brake actuator and a hydraulic control unit. The hydraulic control unit includes four normally open solenoid valves, which are connected to four brake wheel-cylinders. When wheel-cylinder pressure modification is required, the brake actuator reaches a specific hydraulic pressure and adjusts the wheel-cylinder pressures in real-time through valve action. The authors set the average waiting time of wheel-cylinder pressure as the evaluation index and propose a wheel-cylinder pressure control method based on queuing theory. Experimental results show that this method can achieve better wheel-cylinder pressure control.
Haizhen Liu et al. [
75] designed a new brake-by-wire system architecture. This architecture has two pressure-building units, each of which is connected to four normally closed solenoid valves, which are respectively connected to the four wheel-cylinders. One of the pressure-building units is responsible for pressure increasing, and the other is responsible for pressure decreasing. For this system architecture, the authors proposes three control states, namely, pressure increasing, pressure decreasing, and coexistence of pressure increasing and decreasing. If all four wheels need to increase the pressure, they are in a pressure-increasing state; if all four wheels need to decrease the pressure, they are in a pressure-decreasing state; if the four wheels need to increase and decrease the pressure separately, they are in a coexisting state. This time-division wheel-cylinder pressure control through two pressure-building units can realize the synchronous or semi-synchronous pressure requirements of the four wheel-cylinders while ensuring high control accuracy and short response time.
Yuan Ji et al. [
76] designed a DRAC master cylinder pressure control method to solve the problem of parameter uncertainty and disturbance to maintain a stable back pressure. At the same time, the authors designed an ANNC valve control method that can learn system dynamics and open-loop control characteristics online. After experimental verification, compared with pump-based pressure control, this control method has higher control accuracy.
6. Conclusions
As the current development trend of hydraulic braking systems, One-box EHB has many advantages, such as low cost, high integration, and the ability to achieve complete decoupling. At the same time, the new structure also brings new challenges, and researchers have conducted in-depth research on topics such as design, control, and application. The following conclusions summarize these studies and propose future research prospects.
6.1. System Architecture Design
In terms of system architecture design, the research on a single One-box EHB system has been relatively complete, which can meet the new requirements for automotive braking systems under the development of ITS and automobile electrification. It has complete functions, and each solution has appropriate identification and processing mechanisms for typical single-point failure modes of braking systems and can ensure the failure backup of the automotive braking system.
However, due to the higher redundancy requirements of high-level autonomous driving on active braking functions, there is still few literature and research on related system architectures, which restricts the development of intelligent vehicles from level L2 to L3 and even higher levels. System architectures suitable for high-level autonomous vehicles should be further studied.
6.2. Master Cylinder Pressure Control
Regarding the master cylinder pressure control, since the mechanical structures of the pressure-building units of One-box EHB and Two-box EHB are relatively similar, the master cylinder pressure control of One-box EHB can basically follow or refer to the control method under the Two-box EHB architecture. Its development is relatively mature, and researchers have conducted a large number of studies on issues such as internal friction, robust control, and pressure estimation.
However, during the process of regulating wheel-cylinder pressures, the master cylinder pressure control provides the back pressure. At this time, the master cylinder pressure control quality will directly affect the wheel-cylinder pressure control quality. During the process of wheel-cylinder pressure regulation, the parameters of the braking system will change significantly, which will have a serious impact on the piston position control, speed control, etc., within the master cylinder pressure control. Regarding the master cylinder pressure control during the dynamic regulation process of wheel-cylinder pressures, the current research is not in-depth enough, and further research is needed.
6.3. Wheel-Cylinder Pressure Control
For wheel-cylinder pressure control, the key lies in the understanding and control of the solenoid valves. Its basic principle is similar to that of the traditional ESC solenoid valves. Researchers have carried out a lot of research on this issue, including high-frequency switching control, critical differential pressure control, flow control, valve spool position control, etc.
However, the hydraulic layout in the One-box EHB system architecture is different from the traditional ESC. In the process of dynamic control of the four wheel-cylinder pressures, the coordinated control of the four wheel-cylinder pressures is also different from the single-valve control in the ESC. There are few relevant studies on this topic. At the same time, One-box EHB has derived new types of solenoid valves. These solenoid valves have different functions and different performance requirements. The research on the design methodologies of these new solenoid valves is currently confidential to each company, and there is little public literature.
6.4. Electro-Hydraulic Composite Braking Control
Electro-hydraulic composite braking has attracted the attention of researchers as an effective means to extend the driving range of electric vehicles. With the emergence of new brake-by-wire systems with decoupling capabilities, coordinated composite braking technology has improved the vehicle’s regenerative braking capabilities, and researchers have also conducted in-depth research on this.
However, some of these studies were divorced from actual brake actuators and only designed composite brake control strategies from a theoretical level without actual experimental verifications. Since a coordinated composite braking strategy deeply depends on the characteristics and capability boundaries of the brake actuators and their decoupling components, designing an appropriate composite braking strategy based on the system characteristics of One-box EHB and meet the needs of the electric vehicles is an important topic that needs further research.