Page 385 - Automotive Engineering Powertrain Chassis System and Vehicle Body
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CHAP TER 1 2. 1 Braking systems
Many experimental studies, including Newcomb the control unit, its associated software and the array of
(1981), Newcomb and Spurr (1974), Mortimer (1976) sensors that combine to define the overall effectiveness of
and Spurr (1972), have been undertaken that have led to the system. The controller must operate in closed-loop
improved understanding of driver behaviour during fashion, be able to take into account the in-use variation of
braking. These have focused on the study of limb dy- the system parameters and fail safe.
namics, pedal effort, braking kinematics and response to
external stimuli such as obstacles and road signs. This has 12.1.6.3 Anti-lock braking systems
given rise to the development of mathematical models
that embody a representation of the driver into a model Under normal braking conditions, the driver of a vehicle
of the vehicle dynamics. Any such model, typified by makes use of the linear portion of the brake slip vs brake
McLean et al. (1976), contains elements that describe force characteristic (Figure 12.1-7). The brake force
the dynamics of the vehicle, the braking system, the coefficient, m, builds from zero in the free rolling state to
neuro-muscular system and force characteristics of the a maximum, m p , at around 20% slip and within this region
driver and finally the motion detection system/sensory the wheel is both stable and controllable. When braking
characteristics of the driver together with feedback loops under extreme conditions the driver may demand
as appropriate to the model in question. The adaptive a brake torque that is greater than that which is capable
nature of the driver that is captured in such models re- of being reacted by the wheel. This results in a torque
quires enhancement but simulation of vehicle braking imbalance that causes the wheel slip to increase and the
performance with the driver can yield deceleration wheel rapidly decelerates to the full lock condition and in
characteristics that match closely those from this state, the brake force coefficient is approximately
experiment. 0.7 m p . If the front wheels have locked, then steering
control is lost and if rear wheel lock takes place then the
12.1.6.2 Brake by wire vehicle becomes unstable. Simultaneously, the ability of
the vehicle to generate side force markedly reduces
The driver behind brake-by-wire systems has arisen from (Figure 12.1-7), and this explains why limiting wheel
the ongoing development of modern braking systems slip, thereby avoiding wheel lock, is more critical for
such as anti-lock and traction control systems (TCSs) steering and directional stability of the car than for
along with the need to effect their seamless integration stopping distance alone.
within the overall chassis control strategy. There are two The purpose of ABS is to control the rate at which
strategies currently receiving attention. individual wheels accelerate and decelerate through the
The first utilizes a conventional hydraulically actu- regulation of the line pressure applied to each founda-
ated braking system, that includes the brake fluid, tion brake. The control signals, generated by the con-
brake lines and conventional actuators, together with troller and applied to the brake pressure modulating
a significant number of electro-hydraulic components unit, are derived from the analysis of the outputs taken
(Jonner et al., 1996). from wheel speed sensors. Thus, when active, the ABS
The second relies upon a full electro-mechanical system makes optimum use of the available friction between
(Bill, 1991; Maron et al., 1997; Schenk et al., 1995) in the tyres and the road surface.
which the brake force is generated directly by electro-
mechanical foundation brake actuators. The electro- 12.1.6.4 Traction control systems
mechanical system potentially requires little maintenance
due to the removal of the hydraulic fluid as the means of Traction control systems aim (TCSs) to control and
energy transmission and this conveniently combines with maintain vehicle stability during acceleration manoeu-
a reduction in the amount of hardware demanded by the vres, by, for example, preventing wheel spin when ac-
brake system which in turn leads to an overall weight re- celerating on a low friction surface or on a steep up-grade.
duction. Such systems may also contribute towards the This is achieved by the optimization of individual wheel
enhancement of passenger safety as the location of the torques through the control of some combination of fuel
pedal assembly within the vehicle can be optimized so that mixture, ignition and driven wheel brake torque. TCSs
the likelihood of lower leg injury is minimized during are able to utilize components used in ABS and in-
impact events. As with all advanced control systems, it is tegration of the two systems is becoming commonplace.
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