| Under the great pressure of environmental protection,energy saving,safety and traffic congestion,electric and autonomous vehicles have become the main focus,megatrend and key competitive areas for modern automotive industry.This trend has also driven the fast development of vehicle braking systems towards more integration with brake-by-wire features,in order to satisfy the emerging demands from electric and/or autonomous vehicles.For electric vehicles,the braking system needs to decouple between human driver operation and auto braking function,such that the braking forces can be independently controlled to achieve maximum braking energy recovery without sacrafise the braking pedal feel by human drivers.For autonomous vehicles,braking system needs to achieve fast and accurate active braking control functions to meet the requirement for ADAS systems,such as ACC,AEB,.etc,;In addition,as one of the core safety critical systems,braking system also need to have higher fault-tolerant capabilities for safe and reliable performance.The structure and working principle of the traditional braking system can hardly meet the functional requirements such as the electrification and intelligentization of the above-mentioned vehicles.It is difficult to realize the decoupling of the human braking and the braking of each wheel,as well as the fast and accurate four-wheel active braking when maintaining long-term pressure.The fault-tolerant ability of the system is also difficult to meet the growing demand for autonomous cars.In addition,throughout the history of the braking system,from ABS to the introduction of ESC,new system is often introduced by functional superposition from old system.However,with the increase of functional requirements,problems such as functions redundancy,processor resources waste and control objectives conflict arise.In this paper,based on the National High Technology Research and Development Program of China(863 Program)-"Development of Chassis and Dynamics Control System for Electric Vehicles"(No: 2012AA10904)and National Natural Science Foundation of China-"Research on Integrated Modeling and Integrated Control of Intelligent Electric Vehicles"(No: U1564211),etc.,a novel electro-hydraulic brake system that can meet the needs of modern vehicle development is proposed,the pressure control strategy and the control algorithm which is based on the control structure of the novel electro-hydraulic brake system are elaborately studied.The research focuses mainly on: the design of the new electrohydraulic brake system;the control method based on the control structure of the novel electro-hydraulic brake system;and the pressure control method of the novel electrohydraulic brake system.Based on the theoretical research,the proposed control method is verified by setting up corresponding simulation platform and hardware in-loop experimental platform.In this paper,we mainly focus on the following aspects:(1)Design and braking principle analysis of New Electro-Hydraulic Braking System.Based on the functional requirements of electric and autonomous vehicle,a new electro-hydraulic braking system based on the dual-motor / dual-single-chamber-cylinders configuration was proposed.Compared with all the current configuration,our system has advantages of flexible controlling,fast pressure response,high control precision and good fault tolerant capability.Different working mode like parallel and multiplexed time-division pressure control can be realized for a better braking performance.After applying for a good number of patents,we do research on the mechanism of the yaw moment perturbation caused by the braking process,the performance indexes of the system are also analyzed and calculated.At the same time,to facilitate the design of the key parameters of the braking system,a meticulous model is established,and the system parameters are determined through optimization.(2)Research on system-level control framework and braking force distribution method.In this paper,we adopt a novel top-down control structure on the braking system and focuses the emphasis on control command analysis,control target calculation and brake force distribution.At the control command analysis layer,drivers’ longitudinal and lateral intent recognition by considering vehicle dynamic response is taken into account.At the control target calculation layer,the lateral dynamic and rollover stability of the vehicle are considered,and the influence of yaw moment on vehicle lateral and rollover is also analyzed.Based on the feedforward and feedback approach,the target longitudinal force of the vehicle is determined;based on discrete sliding mode control method,additional yaw moment is calculated;At the braking force control distribution layer,based on the constraint optimization,the control target is distributed optimally to the four wheels,while good target follow-up characteristics and road adhesion coefficient utilization are realized.(3)Research on pressure controlling of novel electro-hydraulic brake system.Considering that our system has dual-motor / dual-single-chamber-cylinders and the braking cylinder is connected with 4 wheel cylinders through 4 normally closed solenoid valves,our system is significantly flexible in adjusting the wheel cylinder pressure.Under the parallel brake pressure control mode,pressure demand and brake cylinder working conditions are considered to determine whether the brake cylinder is a high pressure or low pressure source and the corresponding target pressure,at the same time with the control of linear solenoid valve,precise control of wheel cylinder pressure can be achieved;when under multiplexed time-division pressure control mode(dual-channel,four-channel),we realized the precisely pressure control by considering the target pressure demand of each wheel cylinder,and coordinating the control of the brake cylinder and the solenoid valves.In multiplexed time-division mode,pressure of every wheel cylinder is adjusted in sequence,so that the control difficulty is decreased greatly.By considering the nonlinear factors(such as system friction,etc.)of the mechanical transmission,we use a feedforward plus compensation control method,in combination with pressure feedback,to achieve precise control of the brake cylinder pressure,which laid a good foundation for parallel and multiplexed time-division pressure control.(4)Control Algorithm Verification Based on simulation and HIL Platform.For validating the system structure and the control method,we built a simulation platform based on MATLAB/Simulink and Car Sim.To further improve the confidence of our validation process,we built a HIL experimental platform based on d SPACE/Micro Auto Box II and Simulator platform,and validated the high-level control algorithm and low-level pressure control algorithm.Simulation and experimental results show that under the three pressure control modes,the system can achieve a fast pressure response and high control accuracy.Meanwhile,the proposed control method can achieve good control performance to balance vehicle’s braking performance,braking energy recovery,lateral stability and rollover stability in real-time.In summary,the main innovations achieved in this paper are as follows:(1)A novel electro-hydraulic braking system based on dual-motor / dual-single-chamber-cylinders braking system configuration is proposed,parameters of the system which are the basis of meeting the designed performance index are obtained through system matching.The braking system has various working modes including parallel and multiplexed time-division modes,also including the characteristics of fast pressure response,high pressure control precision,diversified control modes(flexible control)and fault tolerance when comparing with the current mainstream braking system.(2)The control architecture and braking force distribution method for the novel system is proposed,it adopts a hierarchical framework,which includes control command parsing layer,control target decision layer,braking force distribution layer,braking force executive layer,state observing layer and system fault diagnosis layer and so on,leading to the great function scalability.In this paper,the target longitudinal force of the vehicle is determined based on the feedforward and feedback methods.The discrete sliding mode controller is used to determine the additional yaw moment,and optimal distribution of control target between wheels is achieved based on the constraint optimization method,ensuring good target following characteristics and good road surface adhesion coefficient utilization.(3)A new pressure control method is proposed for the novel electro-hydraulic system,which includes parallel brake pressure control,dual-channel and four-channel time-division pressure control.By coordinating the control of the brake cylinder and the solenoid valves,the fast response and high accuracy can be obtained.When under time-division pressure control mode,the system response performance can be further enhanced,which is of great value for system fault tolerant.When under parallel control mode,the pressure of all cylinders can be adjusted continuously,the subtle disturbance of yaw moment can be avoided.Under different requirements,different pressure control mode can be adopted,which guarantees the control flexibility and fault tolerance capability of the novel braking system. |
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