58 5. ACTIVE CONTROL FOR VEHICLE ROLLOVER AVOIDANCE
the wheel brake cylinder to the high-pressure accumulator, to the master cylinder, and to the
reservoir are interrupted and the pressure of wheel brake cylinder is kept constant. (3) Pressure
reduction. e outlet valve and cut valve act in opposition state to each other. e cut valve and
the inlet valve closed, and the outlet valve is open. So, the wheel brake cylinder is connected
to the reservoir. e brake fluid flows out of the wheel brake cylinder into the reservoir via the
return line and the brake pressure drops.
It is necessary to include a failsafe operation for EHB system which contained many
electronic elements, like sensors and solenoid valves. In this operation, all the valves hold initial
state. e inlet valve and the outlet valve are closed and the cut valve holds open. So, the wheel
brake cylinder is connected to the master cylinder, like the conventional hydraulic brake system.
e dynamic equations of the mode of pressure build-up and pressure reduction can be
expressed as
dp
w
dt
D
C
b
A
b
K
b
V
b
p
0
V
0
V
0
C qt
p
w
n
(5.4)
dp
w
dt
D
C
b
A
b
K
b
V
b
p
n
w
; (5.5)
where C
b
is the flow coefficient of solenoid valve; A
b
is the area of orifice throttle; K
b
is the
bulk modulus of the brake oil; V
b
denotes volume of the wheel brake cylinder; n is the index of
solenoid valve; p
w
is the pressure of wheel brake cylinder; q is average flow rate; is adiabatic
index; and p
0
and V
0
denotes initial pressure and volume of the gas chamber of accumulator.
To valid the model of EHB system, a test platform is developed, as shown in Figure 5.9.
e parameters of the model have been determined by means of the regressive analytics. e
experimental results of the model parameters for each wheel are shown in Table 5.2.
Table 5.2: e experimental results of EHB model parameters
Parameter Wheel 1 Wheel 2 Wheel 3 Wheel 4
n
0.83 0.83
0.85 0.85
C
b
A
b
/(K
b
V
b
)
28.2 28.2 58.5 58.5
q/m1/s
4.68 4.68 4.55 4.55
5.5 INTEGRATED CHASSIS SYSTEM
e four rollover avoidance control mechanisms mentioned above can effectively improve the
stability of vehicle rollover. However, any single use of these mechanisms will have a certain
adverse impact on the normal operation of the vehicle. For example, active steering systems
could reduce or reverse the unstable roll maneuver by controlling the steering angle but change
5.5. INTEGRATED CHASSIS SYSTEM 59
Figure 5.9: e test platform of Electro Hydraulic Brake system.
the intention of driver operation; and different braking could reduce vehicle speed and use more
energy. erefore, to optimize the performance of rollover prevention control, combinations of
these different techniques will be a better option. Yim et al. designed a controller that used
an active anti-roll bar and an electronic stability program (ESP) for rollover prevention [57],
and to enhance robustness of the controller, differential braking and an active suspension system
were adopted as actuators that generated yaw and roll moments, respectively [8]. Doumiati et al.
aimed at stabilizing the vehicle while achieving a desired yaw rate and proposed a suitable gain
scheduled Linear Parameter Varying control strategy via coordination of active front steering
and rear braking [58]. Moreover, Yoon et al. described a unified chassis control scheme which
aimed to prevent vehicle rollover, and to improve vehicle maneuverability and its lateral stability
by integrating electronic stability control (ESC) and active front steering (AFS). And the control
scheme was evaluated on a virtual test track [59].
Liu describes an integrated chassis control framework for a novel three-axle electric bus
with active rear steering (ARS) axle and four motors at the middle and rear wheels. e in-
tegrated chassis control framework for the targeted electric bus is shown in Figure 5.10. e
integrated framework consists of four parts: (1) an active speed limiting controller is designed
for anti-body slip control and rollover prevention; (2) an ARS controller is designed for coordi-
nating the tyre wear between the driving wheels; (3) an inter-axle torque distribution controller
is designed for optimal torque distribution between the axles, considering anti-wheel slip and
battery power limitations; and (4) a data acquisition and estimation module for collecting the
measured and estimated vehicle states [60].
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