20-10-2012, 04:00 PM
MODELLING & DEVELOPMENT OF ANTILOCK BRAKING SYSTEM
MODELLING & DEVELOPMENT.pdf (Size: 2.15 MB / Downloads: 119)
ABSTARCT
Antilock braking systems are used in modern cars to prevent the wheels from locking after brakes are applied. The dynamics of the controller needed for antilock braking system depends on various factors. The vehicle model often is in nonlinear form. Controller needs to provide a controlled torque necessary to maintain optimum value of the wheel slip ratio. The slip ratio is represented in terms of vehicle speed and wheel rotation.
In present work first of all system dynamic equations are explained and a slip ratio is expressed in terms of system variables namely vehicle linear velocity and angular velocity of the wheel. By applying a bias braking force system, response is obtained using Simulink models. Using the linear control strategies like P - type, PD - type, PI - type, PID - type the effectiveness of maintaining desired slip ratio is tested. It is always observed that a steady state error of 10% occurring in all the control system models.
INTRODUCTION
Anti-lock brake systems (ABS) prevent brakes from locking during braking. Under normal braking conditions the driver controls the brakes. However, during severe braking or on slippery roadways, when the driver causes the wheels to approach lockup, the antilock system takes over. ABS modulates the brake line pressure independent of the pedal force, to bring the wheel speed back to the slip level range that is necessary for optimal braking performance. An antilock system consists of wheel speed sensors, a hydraulic modulator, and an electronic control unit. The ABS has a feedback control system that modulates the brake pressure in response to wheel deceleration and wheel angular velocity to prevent the controlled wheel from locking. The system shuts down when the vehicle speed is below a pre-set threshold.
Wheel-Speed Sensors
Electro-magnetic or Hall-effect pulse pickups with toothed wheels mounted directly on the rotating components of the drivetrain or wheel hubs. As the wheel turns the toothed wheel (pulse ring) generates an AC voltage at the wheel-speed sensor. The voltage frequency is directly proportional to the wheel's rotational speed.
Electronic Control Unit (ECU)
The electronic control unit receives, amplifies and filters the sensor signals for calculating the wheel rotational speed and acceleration. This unit also uses the speeds of two diagonally opposed wheels to calculate an estimate for the speed of the vehicle. The slip at each wheel is derived by comparing this reference speed with the speeds of the individual wheels. The "wheel acceleration" and "wheel slip" signals serve to alert the ECU to any locking tendency.
The microcomputers respond to such an alert by sending a signal to trigger the pressure control valve solenoids of the pressure modulator to modulate the brake pressure in the individual wheel-brake cylinders. The ECU also incorporates a number of features for error recognition for the entire ABS system (wheel-speed sensors, the ECU itself, pressure-control valves, wiring harness). The ECU reacts to a recognized defect or error by switching off the malfunctioning part of the system or shutting down the entire ABS.
SCOPE & OBJECTIVE OF PRESENT WORK
During the design of ABS, nonlinear vehicle dynamics and unknown environment characters as well as parameters, change due to mechanical wear have to be considered. PID controller are very easy to understand and easy to implement. However PID loop require continuous monitoring and adjustments. In this line there is a scope to understand improved PID controllers with mathematical models.
The present work, it is planned to understand and obtain the dynamic solution of quarter car vehicle model to obtain the time varying vehicle velocity and wheel. After identification of system dynamics a slip factor defined at each instance of time will be modified to desired value by means of a control scheme. Various feedback control schemes can be used for this purpose.
VEHICLE DYNAMICS
Basically, a complete vehicle model that includes all relevant characteristics of the vehicle is too complicated for use in the control system design. Therefore, for simplification a model capturing the essential features of the vehicle system has to be employed for the controller design. The design considered here belongs to a quarter vehicle model as shown in Fig 2.1. This model has been already used to design the controller for ABS.