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Engineering, Mechanical
With the rapid development of electric vehicles (EVs) in the direction of advanced driving assistance systems and autonomous vehicles, the demands for X-by-wire systems are huge. For example, the brake-by-wire (BBW) system must have a fast enough step response, the adjustable ability of higher accuracy and fault tolerance to ensure the safety of vehicles. The BBW system of “safety, comfort, and low carbon” has become a troublesome issue in the development of the industry. In EVs, the moving parts that can achieve automatic control are more than 200. As an important assurance system for the safe driving of vehicles, the BBW system has the advantages of accurately and independently controlling the pressure of each wheel cylinder and a fast response.
- BBW system
- design flow
- control methods
- control strategies
1. Structures of Brake-By-Wire (BBW) System
The Goodyear company proposed the idea of a brake-by-wire (BBW) system for the first time in 1979, and the Lorrell company successfully completed experimental tests of the electric brake system on an A-10 attack aircraft in 1982, thus opening the first page of the development of BBW System. Various forms of BBW systems were designed during this time. According to the different types of sources and regulations for braking force, it is mainly divided into an electro-hydraulic brake (EHB) system based on a servo motor and an EHB system based on high-pressure accumulator and electro-mechanical brake (EMB) system. The BBW system uses wires to replace parts or all of the brake pipelines, and the controller manipulates the electronic control elements to control the braking force. Essentially, it is an energy transformation system, where the power unit amplifies the output force through hydraulic or mechanical mechanisms [10]. At present, research efforts aim to shorten the response time of the brake and improve the accuracy of the braking control [11].
1.1. Electro-Hydraulic Brake System Based on Servo Motor
In the 1980s, based on the mature automotive ESC system, many researchers designed some schemes of an EHB system based on a servo motor. This system drives the master cylinder piston by increasing the motor’s rated speed and displacing the motor plunger pump or controlling the motor and deceleration mechanism. It realizes the normal operation of the brake system without changing the overall structural arrangement of the brake system [10]. To increase market share, original equipment manufacturers (OEMs) prefer to adopt this kind of solution. Continental’s MK C1 system is a typical motor structure and deceleration mechanism, which makes the master cylinder produce hydraulic pressure, and then the motor rotates and turns the torque into the thrust of linear motion under the controller. This method has a simple principle and is easy to control [18]. Hitachi’s e-ACT system is an electric intelligent brake system. When the driver steps on the brake pedal, the brake master cylinder builds up hydraulic pressure through the pedal push rod and the piston pump and achieves brake [19]. Song designed a hydraulic brake system based on the electronic stability program (ESP), and the arrangement of the system is an X-type circuit. To control the pressure of the four wheels, two symmetrically arranged piston pumps are driven by a motor [20]. Yu applied for a new EHB system structure, which consists of a DC motor, lead screw and the mechanism of gear and pinion. This system produces hydraulic pressure by pushing the plunger of the master cylinder [21]. Wu studied a hydraulic brake system of a dual hydraulic cylinder that can achieve backup when the four wheels lose efficacy and brake through a motor drive the brake master cylinder. The system has small changes and is convenient to arrange [22].
1.2. Electro-Hydraulic Brake System Based on High-Pressure Accumulator
From the 1990s to the early 2000s, motor technology was still immature. To solve the requirements of high pressure and fast flow for brake systems, the EHB system, which uses a high-pressure accumulator as an energy store and energy supply device, was a solution; the system builds up high pressure by the motor plunger pump and saves in it the high-pressure accumulator in advance. The high pressure, which can provide the brake fluid at a faster speed and reduce the response time of the brake, is released when the brake starts [11]. Bosch, Advics and other companies have started to supply relevant products for Mercedes-Benz, General, Ford, Toyota and other OEMs. The electronically controlled braking system of Advics is the earliest EHB system with a high-pressure accumulator. This system cancels the vacuum booster, and the pedal is directly connected to the master cylinder [23]. TRW designed a slip control boost (SCB) brake system based on a high-pressure accumulator; it mainly consists of a new type of brake master cylinder and an electro-hydraulic control unit [14]. The same solution based on high-pressure accumulators and solenoid valves is as follows. Bosch studied a sensotronic brake control (SBC) system, which is a semi-separated structure. The accumulator saves the high-pressure brake fluid, which is provided by the pump; meanwhile, the system obtains the brake effect by releasing the brake fluid to the wheel cylinders [24]. Jin designed an EHB system, and it consists of the simulator of pedal feel, electronic control unit and hydraulic control unit. Jin’s system can achieve three different working conditions by controlling the different states of solenoid valves [25].
1.3. Electro-Mechanical Brake System
In the early 2000s, to further reduce the response time of active braking, shorten the distance of emergency brakes and simplify the brake system, the suppliers of brakes and research institutes began to research some new brake systems, such as EMB. The output torque, which is provided by a motor, is directly transmitted to the friction components of the brake through a mechanism of slowing down and increasing torque, such as a gear deceleration mechanism and a ball screw mechanism, and the friction braking torque is generated [9]. The EMB of Bosch is a form of a two-stage planetary gear deceleration mechanism and ball screw mechanism, in which the motor can be a transverse flux motor or a permanent magnet synchronous motor and transmit torque by gear. This EMB has some advantages, such as its compact structure and good reliability [26]. The Continental’s EMB cleverly integrates the ball screw mechanism and planetary gear deceleration mechanism through the middle-ribbed support tube, the internally ribbed tube and the internally ribbed rotor. Meanwhile, Continental designed the rotor locking device [27]. Siemens studied a booster structure of lever driving by using the lever principle, adjusted the brake gap using a mechanical gap in time mechanism, and measured the position of the push rod using a position sensor, so the pressure control is more accurate [28]. Tsinghua University built a complete text of a bench system of an EMB system based on the direct-drive configuration and designed a torque motor controller [29]. Tongji University designed a model machine of the EMB system based on direct-drive configuration [30]. Li designed an EMB system that also uses a combination of planetary gears and ball screws [31]. Gong designed a new EMB system, which provides braking force for the brake system through a linear motor directly drives the amplification mechanism [32].
Various BBW systems have been proposed, as the traditional brake systems limit the design of most BBW systems. To meet the demands of the market and the reliability, it is very meaningful to improve the performance of the BBW system, simplify the structure of the BBW system and explore a new structure of the BBW system. With the increasingly urgent requirements of high efficiency, energy-saving and a high response performance, “direct-drive” and “near zero drive” provide a new technology route for BBW systems.
2. Design Flow of the BBW System
With the performance requirements of the BBW system improving continuously, the development and application of new types of BBW systems require guidance from an advanced and efficient design flow. From demands analysis to optimization designs, the designers need to adopt a variety of disciplinary knowledge from the overall consideration to make the BBW’s comprehensive index reliability, security, practicability, maintainability and other aspects meet the requirements and achieve better [33]. Among them, the design of key components and the design of a single discipline are two main factors in the design process.
2.1. The Design of Key Components
2.1.1. The Pressure Supply Unit
The design of the key components mainly revolves around the pressure supply unit and the pressure regulation unit that realize the accurate control of the brake wheel cylinder pressure. The pressure supply unit is the source of the braking force, while the source of the power unit is often a motor. According to the difference in the force amplification mechanism and the motion transformation mechanism, it is mainly divided into mechanical and hydraulic motion mechanisms. The main forms of machinery include a rotating motor combined with the mechanism that slows down and increases torque and the motion transformation mechanism, linear motor, a mechanical amplification mechanism of force, etc. [32]. KI designed an EMB system using electrical actuators, helical gears, etc. The system converts the motor’s rotary motion into linear motion through the helical gear, and the linear motion generates a clamping force between the brake piece and the brake disc [36]. Gong designed a new type of linear actuator based on a linear motor, which provides a driving device for the brake system using the linear motor’s characteristics of linear driving and the lever principle [32]. Han designed an electro-wedge brake system that consists of a motor, worm gears, a caliper and a wedge and generates braking force by controlling electronic actuators and self-excited wedge mechanisms [37]. The type of hydraulic mainly includes motor pump, motor-assisted master cylinders, etc. [38]. Zong studied a general scheme of an EHB system. The source of the EHB system consists of a motor pump and a high-pressure accumulator, and the hydraulic electric pump provides high-pressure brake fluid for the accumulator and continuously accumulates its energy [39]. Shangguan designed an integrated electronic–hydraulic brake system, in which the driving motor drives the ball screw by the deceleration pair gear, and it pushes the piston of the servo master cylinder to build up pressure [38]. Gong designed a direct-drive EHB unit based on an electromagnetic linear actuator. Gong’s unit directly drives an unequal-diameter hydraulic cylinder to provide hydraulic pressure for wheel cylinders by the linear motor [40].
2.1.2. The Pressure Regulation Unit
The pressure regulation unit is a key component in controlling the braking force, and it plays an important role in the realization of active safety, such as ABS, ESP, etc. The EHB system usually uses linear valves, on-off valves and other valve-controlled pressure regulation methods; motor booster master cylinders, motor servo pumps and other pump-controlled pressure regulation methods; and the methods of coordinated pumps and valves [41]. Li controlled the hydraulic pressure of the wheel cylinder with a direct-drive valve that consisted of an electromagnetic linear actuator based on the Halbach permanent magnet array and valve core [42]. Chu studied a current-response characteristic of a high-speed switching electromagnetic valve and a realization method of the accurate control of the brake pressure [43]. According to the command of the brake, Xiong controlled the forward and reverse rotation of a rotating motor to drive a deceleration mechanism, push the main cylinder piston and increase or reduce the pressure [44]. Yao separately controlled a motor pump and a high-speed switching solenoid valve and achieved a rapid response and accurate control of the wheel cylinder’s hydraulic pressure [45]. The EMB system usually controls the forward and reverse movements of a rotary or linear motor to control the clamping force of the brake. Continental applied the patent of an EMB structure, which realizes the brake when the rotating motor is forward and releases the brake in the reverse direction [27]. Gong designed a new brake system based on a linear motor. When the linear motor moves forward, the brake piece is pushed to complete the clamping brake; when the linear motor moves in the reverse direction, the braking force is released [32].
2.2. The Design of Single Discipline
In terms of the designs and analysis methods of brake systems, the research about single-discipline issues is constantly advancing from designing basic structure parameters to analyzing the dynamic performance. In addition, the rise in energy consumption and temperature has gradually attracted the attention of researchers. Iqbal designed a new bilayer multi-pole electromagnetic brake, which was derived from an analytical model of the brake and compared with simulation results by finite element modeling [35]. Wang studied an adaptive dual-loop brake pressure control method to verify the dynamic performance of the system [46]. Li used an adaptive weighted particle swarm optimization algorithm to optimize the nonlinear flow controllability of a solenoid valve, and after optimization, the flow controllability of the solenoid valve increased by 119.7% [47]. He analyzed the effect of system parameters and structure parameters on the energy consumption characteristics of an EHB system [48]. Deng studied the influences of parameters in the friction torque model and analyzed the proportion of the friction torque at each component and the influence of the rotation rate and clamping force on these friction torques [49]. Zhao studied a model based on the overall rise in brake temperature based on the temperature rise and temperature fall models [50]. The brake system is a highly integrated mechanical–electrical–hydraulic system, especially when considering the cross-coupling of the internal electromagnetic field, temperature field, structure field, flow field and other multi-physical field effects. To improve the efficiency of energy transmission, brake systems put forward higher requirements for the effectiveness of the system design flow. Li analyzed the interaction mechanisms and the characteristics of the electromagnetic, fluid and temperature of a double-sided axial permanent-magnet eddy current brake [51].
Considering the dynamic performances of the system, improving the system efficiency and simplifying the system arrangements, the effective design flows of a BBW system of integration for mechanical–electrical systems or mechanical–electrical– hydraulic systems are of great significance to the development of the BBW system.
3. Lower Control Technology of BBW System
The execution control technology of the BBW system is the key to the rapid response and control accuracy of the system. An accurate model, observation of difficult model factors and the effectiveness of control methods are the research hotspots [52]. The development trend of execution control technology is reflected in: identifying the system parameters to improve the accuracy of the model, observing states to obtain the disturbances of a difficult model and combining composite control algorithms with complementary advantages of different algorithms to achieve effective compensation control.
3.1. The Method of Modeling
From the perspective of system modeling, the existence of nonlinear factors, such as dead-zone and friction, in the BBW system and the interference factors, such as modeling and no modeling, make it impossible to obtain accurate parameters of a model. Researchers obtain the parameters of a model with a black box or state observation, which are indirect identification methods. Tao studied a method of dead-zone identification for a proportional directional valve based on the pressure change in the cylinder inflatable chamber [53]. Wu created a LuGre friction model to represent the friction characteristics of the system in the electric-power-assisted braking system and used genetic algorithms to conduct the parameter identification of the LuGre model [22]. Li searched for an optimal solution using the genetic algorithm, considered the hydraulic unit and brake wheel cylinder as a gray box and used the pressure and flow test data to identify the unknown parameters in the ESP system [54]. Jin identified some key parameters of the model using the method of linear regression parameter identification and validated the model of the EHB system with experimental data of a test bench [55]. Zhang constructed an extended state observer to estimate the no modeling dynamics of the test bench system, which improves the modeling accuracy [56]. Ma designed a linear extended state observer to estimate uncertain dynamics [57].
3.2. The Method of Control
An effective strategy for compensation control is a development trend of solving nonlinear problems. Todeschini fully considered the characteristics of the saturation and dead-zone for the hydraulic components and ensured the accuracy of system pressure control by designing a compensation controller [58]. Li studied a brake pressure compensation control method based on the sliding mode control (SMC) algorithm in the electronic booster hydraulic brake system to ensure the vehicle brake safety effectively [59]. Xiong used a chatter compensation method to control the hydraulic pressure of the integrated EHB system, and the method can mitigate the oscillation and low-speed creeping problems caused by friction under different working conditions [60].
From the perspective of control algorithm, the algorithm of traditional control, such as PID control, decoupling control and other methods, and the algorithm of modern control, such as adaptive control, robust control and other methods, are constantly being applied in the field of BBW systems. Zong used a PID control method based on the feedback of brake pressure to help the brake system achieve good hydraulic control effects [39]. Todeschini studied a hybrid position-pressure switching controller that aimed at coping with the highly nonlinear and time-varying nature of the EHB system, which effectively improved the robustness of the brake system to hydraulic nonlinear interference and reduced the influence of valve dead-zone [61]. Xiong designed adaptive sliding mode hydraulic pressure control based on desired state and integral anti-windup compensation, and the controller improved the robustness of the wheel cylinder pressure control [62]. Yang used an SMC method to improve the robustness of an electric booster system, and the brake system has stronger anti-interference [63]. Tanelli studied a nonlinear output feedback control law for active braking control systems, which can effectively improve the stability of a vehicle under braking conditions [64]. Yang adopted a kind of time-sharing control strategy to realize the purpose of independent and accurate hydraulic pressure regulation of each wheel brake cylinder in various brake conditions of a vehicle [65]. Chen designed the hydraulic pressure controller based on fuzzy PI control to improve the robustness of the PI controller [66]. Wang used a closed-loop control method based on the feedback of brake pressure and designed a hydraulic PI controller with gain-scheduling. The distributed EHB system achieved good hydraulic control effects in the full working range [67]. Yu studied an optimized self-adaptive PID controller based on the Taguchi method to adapt the changes of target pressure and brake characteristics and ensured that the pressure could respond rapidly in the early process and track accurately in the later process [68].
Intelligent control methods, such as reinforcement learning and neural networks, are also integrated into the control of the BBW system, especially the application of complex algorithms, which greatly improves the performance of the BBW system. Zhao designed a hydraulic pressure SMC method based on a radial basis function (RBF) neural network. The adaptive law of the RBF neural network adjusted the parameters of the sliding mode controller of the system and achieved accurate control of the hydraulic pressure of the system [69]. Cao studied a controller that combines a neural network and SMC, and the RBF neural network is used to adaptively adjust the switching gain of the sliding mode controller, which effectively reduces the instability of the system and improved the robustness of the system [70]. Yang developed an integrated time-series model based on multivariate deep recurrent neural networks with long short-term memory units for the dynamic estimation of the brake pressure of EVs, which can achieve a more reliable multistep prediction with higher accuracy [71]. Kim designed an application of the brain’s limbic system based on control, and through a genetic algorithm, Kim optimized the control parameters, which improves the control speed, reference tracking and robustness to the disturbance of the system [72].
Deeply researching the coupling effects of various nonlinear factors in the BBW system, designing observation methods for multiple nonlinear factors and creating more accurate control, which considers compensating for various nonlinear factors at the lower execution, are of great significance for improving the response speed and accurate control of the brake system.
This entry is adapted from the peer-reviewed paper 10.3390/act11030080
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