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ABSTRACT
As one of the key technologies of electrified vehicles,
regenerative braking offers the capability of fuel saving by
converting the kinetic energy of the moving vehicle into
electric energy during deceleration. To coordinate the
regenerative brake and friction brake, improving regeneration
efficiency and guaranteeing brake performance and brake
safety, development of special brake systems for electrified
vehicles is needed.
This paper presents a new type of electrically-controlled
regenerative braking system (EABS) that has been developed
for electrified passenger vehicles, which has the potential to
be brought into production in China. By utilizing as much as
possible mature components, integrating cooperative
regeneration with ABS/TCS functions, EABS can achieve
high regeneration efficiency and brake safety while providing
system reliability, low development cost and development
risk. This article describes the layout of the newly developed
regenerative braking system. The operation modes and
control methods of the system are introduced. Road test data
from a commercialized electric vehicle prove the good
performance of this system. The energy consumption of
vehicle reduced by EABS developed is over 25% under ECE
driving cycle.
INTRODUCTION
As social needs for the global environment conservation and
energy-saving growing rapidly, various types of environment
friendly vehicles such as hybrid and plug-in hybrid vehicles,
fuel-cell vehicles and electric vehicles have become the
research focus. As one of the key technologies of electrified
vehicles, regenerative braking can improve fuel economy by
converting the kinetic energy of the moving vehicle into
electric energy during deceleration.
To extend driving range, improving energy efficiency
effectively, most of the electrified vehicles now are equipped
with regenerative braking systems. However, as the
capability of regenerative braking is limited by the states of
the motor and the battery, it is not enough to meet all the
braking demand under various operating conditions.
Therefore an additional friction braking mechanism is
needed, i.e. the regenerative brake cooperates with the
friction brake to meet the total braking demand of electrified
vehicle.
The regenerative braking systems can be generally classified
into two types, namely the parallel type and serial type. In
parallel systems, the brake pressure is coupled to the brake
pedal, and the regenerative braking is added directly to the
friction brake operated by brake pedal, which results in a low
regeneration efficiency and poor brake comfort. With serial
type a blending between friction brake and regenerative brake
can be obtained, leading to the vehicle deceleration
corresponding to the driver's braking demand. This type
offers a high regeneration efficiency and good braking feel,
but requests several modifications in the hydraulic brake line
to mechanically decouple the connection between the brake
pedal and the brake pressure.
Development of the Electrically-Controlled
Regenerative Braking System for Electrified
Passenger Vehicle
2013-01-1463
Published
04/08/2013
Junzhi Zhang, Chen Lv, Xiaowei Yue and Mingzhe Qiu
Dept. of Automotive Eng, Tsinghua Univ
Jinfang Gou and Chengkun He
Chinese Academy of Sciences
Copyright © 2013 SAE International
doi:10.4271/2013-01-1463
Since the serial type regenerative braking systems are much
more advantageous over the parallel ones respecting to the
regeneration efficiency and the braking feel, automotive
makers and component suppliers worldwide have proposed
serial-type solutions respectively for electrified vehicles.
Toyota, TRW and Honda have developed EHB (Electro
Hydraulic Brake) systems [2,3,4], which have been brought
into series production and implemented successfully in
commercialized HEVs, such as Toyota Prius, Ford Escape,
and Honda Insight. EHB systems, decoupling the brake pedal
with brake pressure by using stroke simulators and adjusting
the brake pressure provided by an additionally set pressure-
supply unit via valves control, can achieve the by-wire brake
function. The regenerative braking system adopted by
Continental Teves, using a pedal feel simulator and its cut-off
device, and modulating the brake pressure by controlling a
active booster, can realize the pedal-decoupled concept [5].
The Electrically-Driven intelligent Brake (EDiB) system
developed by Hitachi has been employed successfully in the
Leaf electric vehicle. In the EDiB system, which based on a
newly developed master cylinder without using the
conventional vacuum booster, the brake pressure is adjusted
by an electrically driven motor through a ball screw, and the
pedal feel is also adjustable by using a pressure-generating
mechanism and a pedal-force compensator [6].
However, the configurations of those regenerative braking
systems introduced above are complicated with several newly
developed components added, and many modifications of
hydraulic brake line are also required, which results in an
increase of system price, mounting difficult and development
risk.
The present authors have been dedicated to the research and
development of regenerative braking for a long period and
made some progress [7,8,9,10,11]. 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 EABS, which is based
on the proven ABS technique, has been developed and will
be soon brought into series production in China. Adding two
pedal stroke simulators and high-speed solenoid valves on the
layout of conventional 4-channel hydraulic ABS modulator,
this new solution can realize a high efficiency cooperative
control of regenerative and hydraulic brakes with good pedal
feel.
The EABS composition, control method, braking
performance and regeneration effect from test results of an
electric vehicle equipped with the EABS developed are
discussed in this paper.
EABS SYSTEM CONFIGURATION
System Outline
The overall structure of the regenerative braking system is
shown in figure 1. The main component of this newly
developed regenerative braking system is the EABS control
unit, which is mounted easily in the brake line between
master cylinder and wheel cylinders, replacing the position of
ABS control unit in the ordinary hydraulic braking system.
The brake pedal force is still assisted by the vacuum booster
to build up the brake pressure, without a pressure-supply unit
set additionally, unlike the EHB. An electric control unit in
the EABS communicates with VCU (Vehicle Control Unit)
via CAN bus and adjusts the brake pressure rapidly and
accurately as demand. The modulated wheel cylinder
pressure cooperates with the regenerative braking torque of
electric motor, realizing the cooperative regeneration.
Figure 1. Overall structure of the regenerative braking
system
EABS Control Unit
Figure 2. EABS control unit
As the key component of the newly developed regenerative
braking system, the EABS control unit is comprised of two
parts: the electric braking control unit (BCU) and the
hydraulic pressure modulator, as figure 2 shows.
Downloaded from SAE International by Tsinghua University, Wednesday, June 24, 2015
The EABS hydraulic pressure modulator is set as X-split
type, as shown in the dotted box in figure 3. Taking the front-
wheel-drive vehicle as example, since the brake blending can
only be carried out in the front axle, the two rear wheel brake
lines, namely LR (Left Rear) and RR (Right Rear), are
connected directly to the master cylinder, i.e. the rear wheel
pressure keep the same value with master cylinder all the
time.
Figure 3. Layout of EABS hydraulic pressure modulator
Without using a high-pressure accumulator, which is the
hydraulic pressure supply unit in EHB, the brake pressure in
EABS is still generated in master cylinder via brake pedal
depressing.
The EABS hydraulic pressure modulator is comprised of
high-speed valves, hydraulic pump, hydraulic pressure
sensors and pedal stroke simulators. Among the fourteen
valves in total, four NO (Normally Open) valves and four NC
(Normally Closed) valves in the downstream of the brake line
are used for ABS/TCS control, which is the same layout with
that of the conventional ABS/TCS. Above the eight valves in
downstream, another six valves, namely two NO valves in the
main routes (MV1,MV2), two NC valves in the bypass routes
(BV1,BV2) and two NC valves for regeneration control
(RV1,RV2), are added for the cooperative regen brake
control in the front wheel brake lines. The two main routes
valves (MV1,MV2) and the two bypass valves (BV1,BV2)
are on-off controlled, while the eight ABS/TCS valves and
the two valves for regeneration control are pulse width
modulated (PWM).
Two pedal stroke simulators with structure of piston-spring,
are set in the LF and RF brake lines respectively, providing
the feedback force to guarantee driver's good pedal feel
during brake blending. One hydraulic pressure sensor for
driver's braking request detecting is mounted at the inlet port
of the hydraulic modulator, while another two sensors
monitoring the front wheel cylinder pressures are set at the
outlet port of LF and RF brake lines of the hydraulic
modulator. The electric control unit BCU is integrated with
the hydraulic pressure modulator. This enables the brake line
pressure to be controlled as demanded.
The development of EABS utilizes the existing available
resources, adding few proven components to the layout of
conventional ABS, reducing the development cost and
development risk significantly mounting ease equal to that of
conventional brake systems.
OPERATION MODES OF EABS
EABS can realize various brake functions of hydraulic brake,
cooperative regenerative brake and ABS/TCS, and also offers
the fail-safe mode.
Hydraulic Brake
Under the normal hydraulic state, all of the components of
EABS hydraulic pressure modulator are not energized. The
brake lines connections are the same with the conventional
hydraulic brake system. The hydraulic fluid from master
cylinder, passing through the EABS modulator, directly
enters the wheel cylinders to build up the wheel pressures
without adjusted.
Cooperative Regenerative Brake
The total braking demand of vehicle is provided by
regenerative braking force and friction braking force under
cooperative regeneration mode. With the two main route
valves and the two bypass valves energized, the connections
between master cylinder and the two front wheel cylinders
are cut off, and the fluid in the two front wheel brake lines are
led into the corresponding pedal stroke simulators
respectively, which enables the mechanism decoupling of
brake pedal and front wheel cylinders, as figure 4 (a) shows.
Meanwhile, the regenerative braking torque is applied on the
front axle, recovering the kinetic energy of the vehicle.
As the regenerative capability of electric motor has
limitation, when the motor's regenerative brake torque cannot
meet the brake request of front wheels, the hydraulic brake
force is need to be exerted. As shown in figure 4 (b), the main
route valves and the bypass valves keep being energized,
while the two regeneration control valves (RV1,RV2) are
PWM controlled, leading the fluid stored in the pedal stroke
simulators and master cylinder entering the front wheel
cylinders to supplement the rest part of the braking demand.
At this situation, compared with EHB systems, whose brake
pedal is always decoupled with brake pressure, the brake
pedal and the front wheel cylinders in EABS are non-
decoupled again via bypass brake lines.
ABS / TCS Control
When BCU detecting the wheel speed a locked tendency
during deceleration procedure, the ABS control mode would
be activated. At that moment, the six valves in the upstream
are not energized. According to the motion state of each
wheel, BCU controls the four inlet valves and four outlet
valves in downstream to realize the pressure increase,
decrease and hold control of each wheel cylinder
individually.
When driver depresses the accelerator pedal fiercely on a
low-adhesion road, the drive wheels may slip, which would
make the TCS function activated. Taking the front-wheel-
drive vehicle as example, once the TCS control enabled, the
regenerative braking torque of electric motor will be
removed. The BCU controls the six valves in the upstream of
the hydraulic pressure modulator to be energized, cutting off
the main route and connecting the bypass route. With the
same logic of the conventional TCS control, the fluid of the
master cylinder is pumped into the front wheel cylinders by
pump motor via bypass route. And by controlling the on-off
state of the inlet and outlet valves of the slip wheel, the slip
ratio could be controlled decreasing to the target level.
Fail Safe Mode
To guarantee the brake safety of vehicle in the maximum
extent, EABS offers two levels of the fail-safe mode based on
system control and structure.
On the system control level, once a malfunction of the
electric driving system occurring during regenerative braking,
the BCU would remove the regenerative brake torque of the
electric motor and cut off power supply of all the components
in the EABS modulator instantly, recovering the hydraulic
brake.
On the structure level, as the two rear brake lines are not
decoupled to the brake pedal, the rear wheel hydraulic brake
always exists when the driver operates the brake pedal. This
means that the rear braking will remain consistent with the
driver's pedal input regard less of the amount of regenerative
braking being carried out on the front axle. Therefore, when
driver depresses the brake pedal, even if the front wheel
brake fails, the rear wheel brake can still provide a
deceleration of approximately 0.2g, covering the normal
deceleration demand.
COOPERATIVE REGENERATIVE
BRAKING CONTROL STRATEGY
AND ALGORITHM
Braking Force Distribution Strategy
As shown in figure 5, during deceleration process, brake
forces of the vehicle are divided into two parts: the brake
force imitating the engine brake of the conventional vehicle
and the brake force generated by operating the brake pedal.
Figure 5. Distribution between regenerative brake and
hydraulic brake
The brake force imitating the engine brake is provided by the
regenerative brake torque of electric motor. And its amount is
determined by the vehicle speed and accelerator pedal
operation. The application of this part of brake force is to
provide driver of the electrified vehicle a good driving feel
similar with the conventional one and also to further exploit
Figure 4. Regenerative braking mode of EABS
the regeneration potential. The brake force generated by
operating brake pedal is supplied by regenerative and friction
brakes during cooperative regeneration, equaling to the brake
amount required by the driver.
Front and Rear Braking Force Allocation
Usually in an ordinary vehicle, the front-rear brake force
distribution (BFD) is a fixed value, which is determined by
parameters of the installed brake devices. However, for an
electrified vehicle, as the regenerative brake is introduced, the
front-rear brake force distribution needs to be reconsidered to
guarantee the brake safety and regeneration efficiency.
Since no additional power-supply unit used in EHB is set in
the EABS and pedal stroke simulators are only set in front
wheel brake lines and rear brakes are not decoupled with
brake pedal, the adjustment of the front-rear brake force
distribution would lead to the fluctuation of the brake pedal.
In order to ensure the good pedal feel, a fixed front-rear brake
force distribution is adopted in EABS. The brake blending of
regenerative and hydraulic brakes is carried out only on front
axle, and hydraulic brake is always applied on rear axle
which can guarantee the brake stability and avoid issues of
steering-loss. Finally the fixed front-rear brake force
distribution is implemented, as figure 6 shows.
Figure 6. Front and Rear braking force allocation
Regenerative and Hydraulic Brakes
Distribution
To improve the regeneration efficiency as much as possible,
the maximum regeneration principle is applied for brake
blending on front wheels. As shown in figure 5, only
regenerative brake is exerted on front axle at first, and the
front wheel hydraulic brake is supplemented once the
regenerative brake cannot meet the braking demand of front
axle while rear wheel hydraulic brake is always applied
during deceleration process. The total brake force of the
regenerative and hydraulic brakes keeps consistent with the
driver's braking intention.
Cooperative Regenerative Braking
Algorithm
Figure 7 illustrates the control block diagram of the EABS.
Driver operating the brake pedal, the driver's total brake
demand (Ttotal) is detected via pressure sensor at the master
cylinder. Then the command value of regenerative brake
torque (Tregen_cmd) calculated by BCU with regard to the
vehicle's information based on the regenerative braking
control strategy 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
(pw_tgt) is calculated. A PID controller takes control of the
difference (e) between the target wheel pressure (pw_tgt) and
the actual value (pw_act) detected by the wheel pressure
sensor, calculating the actuation commands of the valves
(Valve_cmd) in the EABS 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.
Figure 7. Cooperative regenerative braking algorithm
VEHICLE TEST
Normal Deceleration Process Test
Performance of cooperative control of regenerative and
hydraulic brakes is tested on an electric vehicle under normal
braking process, as figure 8 shows. With the driver's foot off
the accelerator at 0s, the master cylinder pressure remains
about 0Mpa until 0.8s, indicating that no brake operation is
taken. Meanwhile, a slight regenerative brake torque is
applied to imitate the engine brake of a conventional ICE
vehicle during coasting. From 0.8s-1.0s, the master cylinder
keeps at a relatively low level, corresponding to a small
Downloaded from SAE International by Tsinghua University, Wednesday, June 24, 2015
braking demand of driver. During this period, with no
hydraulic brake applied, the regenerative brake employed
alone can fully meet the brake request of the front wheels.
Later, the braking demand increases with the growth of the
master cylinder pressure. As the capability of regeneration is
limited by the relatively high motor speed, the hydraulic
brake is exerted in the front wheel cylinders to supplement
the rest part of the braking demand, leading to the rise of
deceleration consistent with the driver's brake intention. As
the motor's regeneration capability grows up with vehicle
slowing down, regenerative brake torque increases gradually
with the wheel hydraulic pressure decreasing
correspondingly, and the deceleration remains stable. When
vehicle speed decreases to a relatively low level after 4.5s,
the capability of regeneration drops rapidly, and the hydraulic
braking pressure builds up quickly, taking over all the
braking demand.
Figure 8. Test results of normal braking process
Normal deceleration vehicle test results show that:
1. EABS, with simple configuration, distinguished from the
mainstream brake-by-wire system EHB, can achieve the
cooperative control of regenerative and hydraulic brakes with
a good brake pedal feel, demonstrating the feasibility of the
system layout.
2. Hydraulic braking force adjusted rapidly and accurately by
EABS modulator, cooperates with the regenerative braking
torque, providing a total brake force corresponding to driver's
brake demand, leading to the good brake performance
obtained, which validates the control strategy and algorithm
developed.
3. The regeneration capability offered by electric motor is
utilized completely, converting the kinetic energy of vehicle
into electric energy effectively, achieving the high efficiency
of regenerative braking, improving the vehicle energy
efficiency remarkably.
ECE Driving Cycle Test
Indicating the operating conditions in urban areas of a
vehicle, the ECE driving cycle is adopted to carry out the
road test for studying the fuel economy improvement of an
electric vehicle equipped with EABS, which is of practical
significance. During test, four continuous ECE driving
cycles, taken as operating target, are carried out. Figure 9
shows the situations of the road test.
Figure 9. Road test under the ECE driving cycle
To evaluate the improvement in fuel economy of the electric
vehicle enhanced by EABS, the contribution rate δ is adopted
as an evaluation parameter [11], which can be expressed as
(1)
where Ereg is the regenerated energy at the DC bus of the
whole driving cycle, Edrive is the consumed energy at the DC
bus of the whole driving cycle, ηcharge is the charge
efficiency of the battery, taken to be 0.95 and ηdischarge is the
discharge efficiency of the battery, taken to be 0.95.
Table 1. Test results of the ECE driving cycle
The road test results under ECE driving cycle are shown in
Table 1. According to equation (1), with cooperative
regenerative braking by EABS, during the four ECE driving
cycles, the maximum value of the fuel economy contribution
rates is 25.70%, the minimum value is 24.57%, and the
average value is 25.08%, i.e. the ability of vehicle's range
extending offered by EABS is above 25% under ECE driving
cycle.
SUMMARY/CONCLUSIONS
Based on the conventional hydraulic ABS techniques, by
utilizing as much as possible known and proven components,
a new type of electrically-controlled regenerative braking
system EABS, distinguished from the layout of the
mainstream EHB system, has been developed and will be
soon brought into production in China. Integrating
cooperative regenerative braking with ABS/TCS functions,
this brake system achieves a high regeneration efficiency and
brake safety while providing system reliability, low
development cost and vehicle mounting ease equal to that of
conventional brake systems.
Cooperative regenerative braking control strategy and
algorithm have been developed. The hydraulic braking force,
which is closed-loop controlled, cooperates with the
regenerative braking torque, realizing the brake blending
consistent with driver's brake demand, which results in the
good brake performance and high regeneration efficiency
achieved.
Road tests under normal braking process and the ECE driving
cycle were carried out in an electric vehicle equipped with the
developed EABS. The normal deceleration test results show
the good cooperative regenerative braking control effect
offered by EABS, validating the control strategy and
algorithm developed. And the test results of the ECE driving
cycle also demonstrates that the fuel economy of the electric
vehicle improved by EABS is above 25%.
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CONTACT INFORMATION
Prof. Zhang Junzhi
State Key Laboratory of Automotive Safety and Energy
Tsinghua University
Beijing
China
jzhzhang@mail.tsinghua.edu.cn
Dr. Lv Chen
State Key Laboratory of Automotive Safety and Energy
Tsinghua University
Beijing
China
henrylvchen@163.com
ACKNOWLEDGMENTS
This article is supported by the Natural Science Foundation
of China [project no. 51075225] and National High Tech
Project “863” [project no. 2011AA11A243].
The Engineering Meetings Board has approved this paper for publication. It has
successfully completed SAE's peer review process under the supervision of the session
organizer. This process requires a minimum of three (3) reviews by industry experts.
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