4.1. DESCRIPTION OF THE NEWLY PROPOSED BBW SYSTEM 45
4.1 DESCRIPTION OF THE NEWLY PROPOSED BBW
SYSTEM
A novel BBW system, which can be implemented on all types of vehicles, including traditional
internal combustion engine (ICE) vehicles and different forms of electrified and intelligent ones,
is schematically depicted in Fig. 4.1. Unlike the standard BBW system—for example, Toyota’s
electronically controlled brake (ECB), [96]—the high pressure accumulator and linear valves are
not needed in the proposed system; all of the valves in the proposed system are normal on/off
valves. Only two relief valves (RV1, RV2) are added, unlike the standard electronic stability
program (ESP) systems [97], the differences between proposed BBW system and conventional
BBW system have been highlighted in Fig. 4.1. e pressures of the master cylinder and each
wheel cylinder are measured by the corresponding pressure sensors.
To achieve good tracking performance of the target pressure, the wheel cylinder pressure
should be modulated precisely. e state machine for wheel cylinder pressure modulation is
illustrated in Fig. 4.2. e wheel cylinder pressure is threshold controlled and has three modes,
including pressure increase, pressure hold and pressure decrease. ese modes can shift from one
to another by judging the logic thresholds, which are the various differences between the actual
and target wheel cylinder pressures. e operation principle of the developed BBW system is
explained in details below.
During the normal brake mode, the four normally open isolation valves are activated and
closed. en, the master cylinder pressure, which represents driver’s deceleration demand, is
physically isolated from the brake pressures of the downstream wheel cylinders. e braking
fluid in the master cylinder is directly passed into the pedal stroke simulator, ensuring good
brake pedaling feeling. As all of the wheel cylinders have the same braking circuit layout, we
take the example of the front right wheel to explain the pressure modulation process.
During the pressure increase process, two hydraulic pumps are driven by the hydraulic
pump-motor, which is controlled by PWM signals. Brake fluid is pumped directly from the
tank instead of the master cylinder, leaving the brake pedaling feel unaffected. e opening of
the normally closed relief valve RV1 is controlled to keep the upper stream pressure of the inlet
valve IV1 constant or at the upper limit, depending on the relief valve (RV1) control method.
By adjusting the opening of the inlet valve IV1, the wheel cylinder pressure can be modulated.
e precise pressure increase is achieved by the coordinated control of the hydraulic pump, relief
valve, and inlet valve. e detailed control algorithms for relief and inlet valves will be investi-
gated in Sections 4.3 and 4.4.
During the pressure decrease process, the hydraulic pumps stop and the relief valve RV1 is
fully open. At the same time, the IV1 is fully closed, and the outlet valve OV1 is controlled with
PWM signals, which modulates the decreasing ratio of the brake pressure in wheel cylinder. e
hydraulic fluid flows from the wheel cylinder to the tank directly.
During the pressure hold process, both inlet valve IV1 and outlet valve OV1 are fully
closed. e pump operates at a low speed, and the relief valve RV1 is opened to a certain level.
46 4. CONTROLLER DESIGN OF CYBER-PHYSICAL VEHICLE SYSTEMS
Tank
Motor
Pump
Backup Circuit Normal Circuit
M
Braking Pedal
MC
MCP
WCP
WCP
FR FL RL RR
ISV1 NO ISV2 NO ISV3 NO
RV2 NC
RV1 NC
IV2 NO
IV1 NO
OV1 NC
OV2 NC OV3 NC
OV4 NC
IV4 NO
IV3 NO
ISV4 NO
PSS
Figure 4.1: Diagram of the structure of the proposed BBW system. MC: master cylinder; MCP:
master cylinder pressure sensor; PSS: braking pedal stroke simulator; ISV : isolation valve; RV :
relief valve; IV : inlet valve; OV : outlet valve; WCP: wheel cylinder pressure sensor; FR: front
right wheel; FL: front left wheel; RL: rear left wheel; RR: rear right wheel; NO: normally open
valve; NC: normally closed valve.
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