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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