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Design and performance analysis of a novel regenerative braking system for electrified passenger vehicles.

ABSTRACT

A novel type of regenerative braking system for electric vehicles is proposed in this paper. Four pressure-difference-limit valves, two relief valves and two brake pedal simulators, are added to the layout of a conventional four-channel hydraulic modulator. The cooperation of relief valves and hydraulic pumps provides a stabilized high-pressure source. Pressure-difference-limit valves ensure that the pressure in each wheel cylinder can be modulated separately at a high precision. Besides, the functions of anti-lock braking system and electronic stability program are integrated in this regenerative braking system. The models of regenerative braking controller and vehicle dynamics are built in MATLAB/Simulink. Hydraulic brake model is built in AMESim through a parameterized and modularized method. Meanwhile, the control strategy of hydraulic brake modulation and brake force distribution are designed. Simulations are conducted via co-simulation interface between MATLAB and AMESim under scenarios of typical braking and ECE driving cycle. Simulation results show that regenerative and hydraulic braking forces are coordinated well during typical braking process, verifying the feasibility and effectiveness of the models built and strategies proposed. Under an ECE driving cycle, the proposed RBS can recover more than 75% of the total recoverable braking energy, which lengthen the vehicle's driving range by more than 24%.

CITATION: Yuan, Y., Zhang, J., Lv, C., and Li, Y., "Design and Performance Analysis of a Novel Regenerative Braking System for Electrified Passenger Vehicles," SAE Int. J. Mater. Manf. 9(3):2016, doi:10.4271/2016-01-0438.

INTRODUCTION

Electrified vehicles such as hybrid vehicles, fuel-cell vehicles and electric vehicles have gained great research focus due to their environment friendly property. Regenerative braking system has become one of the key components of electrified vehicles since this technology improves driving range by recovering electric energy during deceleration [1, 2].

During regenerative braking, driving motor is operated as a generator which converts kinetic energy to electricity and exerts a brake torque on the vehicle simultaneously. However, the capability of regenerative braking is restricted by factors such as the states of the motor and the battery, therefore a spare friction braking mechanism is equipped as a supplement. Hence regenerative brake cooperates with friction brake to satisfy the total braking demand.

Nowadays, research and development of regenerative braking systems are mainly conducted on system configuration and control strategies.

Toyota, TRW and Honda have developed RBS based on EHB (Electro Hydraulic Brake) systems. EHB is a by-wire brake system, which imports stroke simulators and an additional set of pressure supply unit, can decouple the main cylinders with wheel cylinders via valves control. The EHB based RBS has been implemented successfully in commercialized HEVs, such as Toyota Prius, Ford Escape, and Honda Insight [3, 4, 5].

Gao [6] proposed two regenerative braking strategies for a passenger car equipped with a front axle motor. However, neither of the strategies can achieve good balance between regeneration efficiency and braking efficiency. Chu [7] designed a strategy based on the ideal front-rear braking force distribution curve; both braking stability and fuel economy of the vehicle were considered. Zhang [8] developed a regenerative braking system based on the Electronic Stability Program (ESP) platform. Control strategies were proposed, based on road test results, a fuel economy enhancement of more than 25% was achieved under the Economic Commission for Europe (ECE) driving cycle.

The present authors have been dedicated to the research and development of regenerative braking for a long period and made some progress [9, 10, 11, 12]. In order to further improve the regeneration performance while reducing the system cost and development risk, a new type of electrically controlled regenerative braking system named EESP, which is based on the proven ESP technique, has been developed.

To realize the cooperative control of regenerative braking and hydraulic braking, 4 pressure-difference-limit valves, 2 relief valves, and 2 brake pedal simulators, are added to the layout of a conventional 4-channel hydraulic modulator.

The system composition, control method, braking performance and regeneration effect from co-simulation results of an electric vehicle equipped with the RBS developed are discussed.

EESP SYSTEM CONFIGURATION

System Outline

As illustrated in Figure 1, a complete RBS is generally composed of motor brake subsystem, hydraulic brake subsystem, brake control unit (BCU), vehicle control unit (VCU) and CAN network, but individual diversities merely exist in hydraulic brake configuration.

The overall structure of the EESP hydraulic brake subsystem is shown in Figure 2.

To realize the cooperative control of regenerative braking and hydraulic braking, four pressure-difference-limit valves, two relief valves, and two brake pedal simulators, are added to the layout of a conventional four-channel hydraulic modulator.

Among the eighteen valves in total, eight normally closed valves (e.g. FL-InV) in the downstream of the brake line are used for ABS/ESP control, which is the similar layout with that of conventional ABS/ESP, nonetheless four inlet valves are replaced by four pressure-differencelimit valves which are more precise in pressure modulation.

Above the eight valves in downstream, four normally open valves named isolation valve (e.g. FL-ISV) are mounted to isolate master cylinder with wheel cylinders. Thus pedal feel is decoupled with brake force. Two relief valves (RV-A, RV-B) with the crack pressure of 15 MPa are equipped in the bypass of the layout. The pumps cooperate with two relief valves serving as a stabilized high pressure source.

Two switch valves (SV-A, SV-B) and two stroke simulators are mounted at the outlet of the master cylinder providing conventional brake pedal feel. The four isolation valves (e.g. FL-ISV) and the two switch valves (SV-A, SV-B) are on-off controlled, while the eight ABS/ESP valves are pulse width modulated (PWM).

The research object in this paper is a front driven electric vehicle, but as described in figure 2, the four wheel cylinders and their inlet as well as outlet valves have the equivalent configuration, therefore this RBS can be equipped on electrified vehicles of all driven types.

Operation Modes of EESP

Benefiting from the relatively individual structure of four wheel cylinders, EESP can realize various brake functions of hydraulic brake, cooperative regenerative brake and ABS/ESP function.

Hydraulic Brake Mode

Under some conditions regenerative brake cannot be implemented such as the battery is full charged, hence normal hydraulic brake is needed. Under these conditions, all of the components of EESP hydraulic pressure modulator are not energized. The brake lines connections are the same with the conventional hydraulic brake system. As shown in Figure 3, the hydraulic fluid from master cylinder directly enters the wheel cylinders without modulation, building up the wheel pressures as demand.

Cooperative Regenerative Brake Mode

Under cooperative regenerative brake mode, the total braking demand is meted by the addition of regenerative braking force and friction braking force. The four isolation valves and two switch valves are energized, the hydraulic brake fluid flows into corresponding stroke simulators respectively as illustrated in Figure 4. The connections between master cylinder and the downstream part are cut off, which enables the mechanism decoupling of brake pedal and front wheel cylinders, provides the driver a good pedal feel. Meanwhile, the pump motor, working at a proper rotation speed, coordinates with the two relief valves providing a stabilized high pressure, which is similar as the function of high pressure accumulator in EHB system. The pressure in the pipeline between the pump and inlet valves is just identical with the crack pressure of the two relief valves i.e. 15 MPa.

At the start stage of brake, the regenerative braking torque generated by driving motor is applied on the front axle, recovering the kinetic energy of the vehicle. However the generation capability of driving motor is limited by factors such as the change rate of demand torque and vehicle speed, when the regenerative brake torque cannot meet the brake request of front wheels, the hydraulic brake force needs to be exerted.

ABS/ESP Control

When the wheel speed is detected as having a tendency of locking during deceleration procedure, the ABS control function is activated. At this scenario, the upstream six valves i.e. switch valves and isolation valves are not working. The ABS controller integrated in BCU undertakes the responsibility of modulation of the eight valves in downstream to realize the pressure increase, decrease and hold control of each wheel cylinder individually.

When an emergent steering happens to avoid collision or at other critical moments, the vehicle may lose stability suddenly following with a large value yaw velocity, which would activate the ESP control mode. The BCU controls the six valves in the upstream of the hydraulic pressure modulator to be energized, cutting off master cylinder and downstream part. The pump motor works to build up a rapid high pressure. With the same logic of the conventional ESP control, the pressure-difference-limit inlet valve is energized to lead accurate brake fluid into corresponding wheel cylinder fulfilling single-wheel brake to correct the trajectory of the vehicle.

SYSTEM MODELING AND CONTROL ALGORITHM DESIGN

A control system of regenerative braking needs to be established for the simulation and analysis of the regenerative braking procedure. In this study, the model of vehicle dynamics, the tires, the electric motor, the battery and the braking control unit are set up in MATLAB/Simulink. The model of the whole hydraulic brake system is set up in AMESim simultaneously. The detailed methods for modeling the vehicle dynamics and the tire have been described in [9]. The modeling of the motor and the battery is based on the foundation of [10]. The hydraulic brake system is modeled in AMESim and the procedure is expounded as follows.

The modeling procedure of the hydraulic braking system in AMESim is composed of two parts, i.e. pressure-difference-limit valve modeling and pipeline construction.

Pressure-Difference-Limit Valve

Pressure-difference-limit means that when a high speed on-off valve is controlled under current PWM mode, the pressure difference across the valve is linearly related to the control current. This control method can simplify the valve control procedure and reduce noise significantly.

The configuration of a normally closed inlet valve mentioned is shown in Figure 5. Regarding the initial position of valve core as the origin, the OX coordinate system was established, as Figure 6 shows.

After being energized, the normally closed valve opens, and before the core leaves the valve seat, the electromagnetic force [F.sub.e], the restoring spring force [F.sub.s], the hydraulic force [F.sub.h] and the supportive force [F.sub.N] of the valve seat are exerted on the valve core. The axial balance equation of the valve core in the valve-closed state can be expressed as

[F.sub.e] - [F.sub.s] + [F.sub.h] + [F.sub.N]sin[alpha] = 0 (1)

where [alpha] is half of the cone angle of the valve seat as showed in Figure 6.

However in order to achieve the pressure-difference-limit control method, the valve core should work at a critical closed state, which means the valve is about to open, the supportive force disappears ([F.sub.N] = 0) but the valve core displacement [x.sub.v] is still zero. Hence the equation (1) can be rewritten as

[F.sub.e] - [F.sub.s] + [F.sub.h] = 0 (2)

The electromagnetic force acting on the valve core is determined by the coil current, the number of coil turns N, the air gap length l and the magnetic reluctance [R.sub.g] of the air gap [13]. The electromagnetic force can be expressed by the relation

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (3)

Since the valve core works at the critical closed position, the influence of [x.sub.v] on the air gap length is ignored, hence the air gap length is proportional to the coil current. Then equation (3) can be transformed to

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (4)

here [K.sub.i] is the current-force coefficient.

When the valve core moves, the restoring spring force is given by the relation below.

[F.sub.s] = [k.sub.s]([x.sub.0] + [x.sub.v]) (5)

Where [x.sub.0] is the pre-tension displacement of the spring. At the critical closed state ([x.sub.v] = 0), the spring force can be expressed as

[F.sub.s] = [k.sub.s][x.sub.0] (6)

At the critical closed position, the hydraulic force acting on the valve core is determined by the pressure difference across the valve [DELTA]p and the axial shadow surface area As of the steel ball, which can be expressed as

[F.sub.h] = [DELTA]p * [A.sub.s] (7)

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (8)

where [R.sub.B is the radius of the steel ball.

By combining equations (2) to (7), the balance equation (1) can be represented as

Hence the relationship between the coil current and the pressure difference can be expressed as

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (10)

The critical linear relationship between control current and pressure difference at the valve core working position is proved by equation (10). The model of pressure-difference-limit valve is built in AMESim based on equation (10) as shown in Figure 7.

The parameters of the valve built is listed in Table 1.

Pipeline Construction

The pipeline construction is based on Figure 2. AMESim provides a more physical and graphical modeling method than MATLAB/Simulink. The pumps, pump motor, clippers, reservoirs and relief valves are selected from AMESim prototypical library, and are set with proper parameters. The overall hydraulic brake system is shown in Figure 8. The MATLAB/Simulink interfaces are also modeled to fulfill the co-simulation between MATLAB/Simulink and AMESim. Since brake pedal feel is not researched in this paper, the master cylinder and stroke simulators are omitted.

Model Validation

Diminutive simulations on newly built valves are conducted to verify the validation of the model. The valve is exposed under a constant pressure source of 15MPa. The value of control current is calculated based on the subtraction of target pressure and constant pressure source. Two different pressure increase procedure is simulated. The results are presented in Figure 9. The simulation result indicates that the pressure-difference-limit valve can track the target pressure well with proper overshoot. This valve model can be used for researching RBS equipped with this module.

Regeneration Control Strategy

Braking force distribution strategy and hydraulic pressure modulation algorithm compose regeneration control strategy.

Braking Force Distribution Strategy

As shown in Figure 10, a constant brake force imitates the engine brake of conventional vehicles, and the front brake force is divided into 2 parts. At the beginning of the brake, the brake force demand of front axle can be satisfied by motor generation torque, i.e. regenerative braking. When the brake demand of front axle hits the motor limit, the hydraulic brake force is exerted. The whole brake force of rear axle is achieved only by hydraulic brake force.

The brake force distribution (BFD) is identical with the BFD of conventional vehicles. The brake force of rear axle is proportional to the sum of regenerative brake force and the hydraulic brake force of front axle.

Hydraulic Pressure Modulation Algorithm

Figure 11 illustrates the control block diagram of the EESP hydraulic pressure modulation. The command value of regenerative brake torque ([T.sub.regen_cmd]) calculated by BCU according to the total brake demand ([T.sub.total]), with regard to the vehicle and battery information, is sent to the MCU (motor control unit) and implemented by the drive motor.

Meanwhile, according to the actual motor torque, the target wheel cylinder pressure ([p.sub.w_tgt]) is calculated. A PID controller takes control of the difference (e) between the target wheel pressure ([p.sub.w_tgt]) and the actual value ([p.sub.w_act]) detected by the wheel pressure sensor, calculating the actuation commands of the valves (Valve_cmd) in the EESP modulator, realizing the closed-loop control of the wheel cylinder pressures. Thus, the regenerative brake and hydraulic brake are applied together to meet the total deceleration requirement of the vehicle.

SIMULATIONS

To evaluate the control performance of the adopted regeneration brake control algorithm, simulations are carried out via MATLAB/Simulink-AMESim co-simulation interface using the models described in section 3.

Simulation Scenarios Set-Up

Two scenarios are designed to fulfill the co-simulation.

For the first scenario, a typical braking condition is adopted. The initial braking speed of the vehicle is set to 40 km/h. The master cylinder pressure, which indicates the driver's brake demand, is taken as a ramp input, which stabilizes at 3 MPa. The road is assumed to have no slope and to have a dry surface with a high adhesion coefficient of 0.8.

The second scenario of simulation is the ECE driving cycle, which reflects the fuel economy improvement of an electric vehicle equipped with EESP, which is of practical significance. During test, a continuous ECE driving cycle, taken as vehicle operating target, is carried out.

Simulation Results

Typical Braking

The simulation results of typical braking are presented in Fig 12. The maximum regenerative braking torque is 120.41 Nm as shown in Figure 12 (a), which is slightly lower than the electric motor torque limit (145Nm), thus indicating high regeneration efficiency. The total energy regenerated is 57.86 KJ, which is 75.03% of the total recoverable energy as shown in Figure 12 (c). The equivalent front-left wheel cylinder matches the pressure in master cylinder precisely indicating the validation of regenerative brake strategy as illustrated in Figure 12 (b).

ECE Driving Cycle

To evaluate the improvement in fuel economy of the electric vehicle enhanced by EESP, the contribution rate [delta] is adopted as an evaluation parameter, which can be expressed as

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (11)

where [E.sub.reg] is the regenerated energy at the DC bus of the whole driving cycle, [E.sub.drive] is the consumed energy at the DC bus of the whole driving cycle, [[eta].sub.charge] is the charge efficiency of the battery, taken as 0.95 and [[eta].sub.discharge] is the discharge efficiency of the battery, taken as 0.95.

During simulation, the speed of vehicle is controlled by driver model, tracing the standard ECE speed trajectory. The result of speed tracking is shown in Figure 13. The data, which contains information of energy consumption and regeneration, is extracted from simulation result and listed in Table 2. According to equation (11), with cooperative regenerative braking fulfilled by EESP, the average value of the fuel economy contribution rates is 24.22% i.e. the driving range extended by EESP is about 24% under ECE driving cycle.

SUMMARY/CONCLUSIONS

A novel type of regenerative braking system, which integrates regenerative braking with ESP function, is proposed in this paper. The main cylinder decouples with brake force and a stabilized high pressure source exits in this system. Pressure-difference-limit control method is imported into this configuration to simplify the control logic and reduce valve noise. The models of different components of this regenerative braking system are built in MATLAB/Simulink and AMESim separately. Meanwhile, the control strategy of hydraulic brake modulation and brake force distribution are designed. Simulations are conducted via co-simulation interface between MATLAB and AMESim under scenarios of typical braking and ECE driving cycle. Simulation results show that regenerative and hydraulic braking forces are coordinated well during typical braking process, verifying the feasibility and effectiveness of the models built and strategies proposed. And under an ECE driving cycle, the proposed RBS can recover more than 75% of the total recoverable braking energy, which lengthen the vehicle's driving range by about 25%.

Further studies will be carried out in some areas such as the following: methods of eliminating jerks during the exit of regenerative braking and optimizing parameters of hydraulic components such as the reservoir capacity and pump etc.

REFERENCES

[1.] Tur, Okan, Ozgur Ustun, and Nejat Tuncay R.. "An introduction to regenerative braking of electric vehicles as anti-lock braking system." In Intelligent Vehicles Symposium, 2007 IEEE, pp. 944-948. IEEE, 2007.

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[3.] Nakamura, E., Soga, M., Sakai, A., Otomo, A. et al., "Development of Electronically Controlled Brake System for Hybrid Vehicle," SAE Technical Paper 2002-01-0300, 2002, doi:10.4271/2002-01-0300.

[4.] Blaise, G., and Ann, A., "Slip Control Boost Control System", US Patent Application, 20090077963A1, 2009.

[5.] Kimura, Y. and Murakami, M., "Analysis of Piston Friction - Effects of Cylinder Bore Temperature Distribution and Oil Temperature," SAE Int. J. Fuels Lubr. 5(1):1-6, 2012, doi:10.4271/2011-01-1746.

[6.] Gao, Y. and Ehsani, M., "Electronic Braking System of EV And HEV---Integration of Regenerative Braking, Automatic Braking Force Control and ABS," SAE Technical Paper 2001-01-2478, 2001, doi:10.4271/2001-01-2478.

[7.] Gao Y, Chu L, Ehsani M. "Design and control principles of hybrid braking system for EV, HEV, and FCV". Vehicle Power Propulsion Conf 2007:384-91..

[8.] Zhang J., Lv J., Gou J., and Kong D., "Cooperative control of regenerative braking and hydraulic braking of an electrified passenger car," Proceedings of the Institution of Mechanical Engineers, Part D: Journal of Automobile Engineering 226.10 (2012): 1289-1302..

[9.] Zhang J., Chen X., Zhang P.. "Integrated control of braking energy regeneration and pneumatic anti-lock braking," Proc IMechE Part D: J Automob Eng 2010;224:587-610.

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[12.] Zhang, Junzhi, Ye, Yuan, Chen Lv, et al. "Modeling and analysis of regenerative braking system for electric vehicle based on AMESim." Mechatronics and Automation (ICMA), 2015 IEEE International Conference on. IEEE, 2015.

[13.] Lin R. "Research and digital simulation of high speed on/off valve". Master's Thesis, Wuhan University of Technology, Wuhan, Hubei, People's Republic of China, 2005.

CONTACT INFORMATION

Dr. Yuan, Ye

State Key Laboratory of Automotive Safety and Energy, Tsinghua

University, Beijing, China

yuanye_yuanye@126.com

Prof. Zhang, Junzhi

State Key Laboratory of Automotive Safety and Energy, Tsinghua

University, Beijing, China

Collaborative Innovation Center of Electric Vehicles in Beijing

jzhzhang@mail.tsinghua.edu.cn

Dr. Lv, Chen

State Key Laboratory of Automotive Safety and Energy, Tsinghua

University, Beijing, China

lv-c10@mails.tsinghua.edu.cn

Dr. Li, Yutong

State Key Laboratory of Automotive Safety and Energy, Tsinghua

University, Beijing, China

einstein_li@sina.com

ACKNOWLEDGMENTS

This paper is supported by the Natural Science Foundation of China [project no. 51475253] and National Key Technology Research and Development Program of the Ministry of Science and Technology of China [project no. 2013BAG08B01].

DEFINITIONS/ABBREVIATIONS

ABS - Anti-lock brake syem

ESP - Electronic Stability Program

RBS - Regenerative brake system

EHB - Electro Hydraulic Brake

[F.sub.e] - Electromagnetic force exerted on valve core

[F.sub.s] - Restoring spring force exerted on valve core

[F.sub.h] - Hydraulic force exerted on valve core

[F.sub.N] - Supportive force of valve seat

[alpha] - Semi-angle of the valve cone seat

[[DELTA].sub.p] - Pressure difference across the valve

[A.sub.s] - Axial shadow surface area of the steel ball

Ye Yuan, Junzhi Zhang, Chen Lv, and Yutong Li

State Key Lab of ASE, Tsinghua Univ.

Table 1. Parameter list of the valve

Parameters                    Value

Seat Diameter (Hole)           0.65 mm
Seat Semi-Angle               57[degrees]
Ball Diameter                  1.588 mm
Maximum Displacement Of Core   0.3 mm
Spring Stiffness               0.34 N/min

Table 2. Simulation results of energy consumption and regeneration

Index  Consumed Energy  Regenerated Energy  Contribution
       (KWh)            (KWh)               Rate(%)

Value  0.1524           0.0409              24.22
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Author:Yuan, Ye; Zhang, Junzhi; Lv, Chen; Li, Yutong
Publication:SAE International Journal of Materials and Manufacturing
Article Type:Report
Date:Aug 1, 2016
Words:3920
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