Anti Lock Braking System (ABS) of cars
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6/10/2010
PROJECT REPORT
ANTI-LOCK BRAKING SYSTEM
SUBMITTED BY:
SUBMITTED TO:
NS BILAL WASEEM
SIR AKHTAR KHURSHID
NS ESAAM JAMIL NS SOHAIL TANVEER
SUBJECT:
NS BILAL TAHIR
CONTROL SYSTEMS
NS HAFIZ M. NOUMAN [DE-30, ME (A)] 1|P a g e
DATED: JUNE 10, 2010
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1. ACKNOWLEDGEMENT:
First
of all thanks to Allah Almighty who has given human beings brilliant
minds and made us the crown of the creation. Secondly, we would like to honor our respected sir Akhtar Khurshid for his advice and encouragement in making this project. We would also like to thank him for always being available for our guidance and help inside and outside the class. Finally,
we would like to thanks our parents for their love. We would also
like to acknowledge with appreciation of their support in our studies.
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TABLE OF CONTENTS:
TOPICS
PAGE NO.
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INTRODUCTION:
An Anti-Lock braking system (ABS) is a safety system on motor vehicles which prevents the wheels from locking while braking. The Anti-lock Braking System is designed to maintain vehicle control, directional stability and optimum deceleration under severe braking conditions on most road surfaces It does so by monitoring the rotational speed of each wheel and controlling the brake line pressure to each wheel during braking. This prevents the wheels from locking up. A rotating road wheel allows the driver to maintain steering control under heavy braking.
4. HISTORY: 1929 Anti-lock braking systems were first developed for aircraft, by Gabriel Voisin. 1950 Dunlop's Maxaret introduced a system and still in use on some aircraft
models. 1960 A fully mechanical system used in the Ferguson P99 racing car, the Jensen FF
and the Ford Zodiac, but saw no further use; the system proved expensive and, in automobile use, somewhat unreliable. 1964 Limited form of ABS in Austin 1800, utilizing a valve which could adjust front
to rear brake force distribution when a wheel locked. 1971 Chrysler and Bendix Corporation introduced imperial called "Sure Brake". 1971 ABS Systems based Mercedes design were later introduced on other cars. 1975 Ford also introduced ABS on the Lincoln Continental Mark III and the Ford
LTD station wagon, called "Sure Trak 1978 Bosch and Mercedes-Benz introduced the first completely electronic 4-
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wheel multi-channel ABS system in trucks and the Mercedes-Benz S-Class. 1988 BMW became the world's first motorcycle manufacturer to introduce an
electronic/hydraulic ABS system, this on their BMW K100. 1992 Honda launched its first ABS system, this on the ST1100 Pan European. 1997 Suzuki launched its GS F1200SA (Bandit) with ABS.
Today ABS has become standard equipment even for small cars.
5. INSPIRATIONS BEHIND ABS: The basic design of a braking system has been around and in use in other applications for many years. The brakes in a car use the simple principle of hydraulics. This principle reduces the amount of work required by the user. Figure below illustrates the basic design of a modern braking system.
Figure:
Basic braking system design
The problem with the traditional braking system is that the force exerted by the brakes on the wheel cannot exceed the force of friction between the wheel and the road. If the braking force exceeds the force of friction from the road the vehicle will begin to slide. This problem brought about the invention of the anti5|P a g e
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locking breaking system (ABS). The ABS detects drastic changes in the speed of the wheels. When a sharp deceleration is detected the ABS will reduce the hydraulic pressure supplied to the braking system until the wheel begins to accelerate again. When the acceleration is detected the pressure is again increased until an unusual amount of deceleration is detected. The process is repeated until the user removes their foot from the brake pedal or the vehicle comes to a complete stop.
6. BASIC COMPONENTS OF ABS AND THEIR FUNCTIONS: The ABS consists of speed sensors, valves, a pump, and a controller. The location of these devices within a vehicle is shown in figure below:
Figure:
Anti-Lock Braking System Design
a) SPEED SENSORS: The speed sensor is used to determine the acceleration or deceleration of the wheel. A picture of this sensor is shown in figure right:
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These sensors use a magnet and a coil of wire to generate a signal. The rotation of the wheel or differential induces a magnetic field around the sensor. The fluctuations of this magnetic field generate a voltage into the sensor. A schematic of this system is shown in figure below. The ABS controller interprets this signal.
Figure:
Speed Sensor Schematic
Since the voltage inducted on the sensor is a result of the rotating wheel, this sensor can become inaccurate at slow speeds. The slower rotation of the wheel can cause inaccurate fluctuations in the magnetic field and thus cause inaccurate readings to the controller.
b) VALVES: The valves within an ABS serve three distinct functions. The first function of the valves is to open and allow the hydraulic fluid from the brake pedal or the pump to reach the braking system. The second function of the valves is to maintain the current pressure provided to the braking system. This is accomplished by closing the valve to resist further pressure from the brake pedal. The third function of these valves is to reduce the amount of hydraulic pressure at the braking system. This is accomplished by opening the valves to allow the hydraulic fluid to be released from the braking system. A picture of a standard ABS valve and pumping system is show in figure below. The majority of problems with the valve system occur due to clogged valves. When a valve is clogged it is unable to open, close, or change position.
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An inoperable valve will prevent the system from modulating the valves and controlling pressure supplied to the brakes.
Figure:
ABS Valves and Pumping System
c) PUMPS: The pump in the ABS is used to restore the pressure to the hydraulic brakes after the valves have released it. A signal from the controller will release the valve at the detection of wheel slip. After a valve release the pressure supplied from the user, the pump is used to restore a desired amount of pressure to the braking system. The controller will modulate the pumps status in order to provide the desire amount of pressure and reduce slipping. A picture of the pumping system is shown in figure above. Similar to the valves, the major limitation or mode of failure is due to blockage within the pump. A blockage within the pump will prevent the pump from supplying the correct pressure to the pumping system. d)
CONTROLLER:
The entire system is observed and manipulated by the ABS controller. A detailed control system used in ABS is shown in figure below.
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Figure:
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ABS Control System
7. MATHEMATICAL MODELLING AND TRANSFER FUNCTIONS: The parts described above produce the control loop shown in figure below:
Figure:
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Control Loop Described in the Background Section
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For
the purpose of the analysis the pump and valves will be combined in the system to form the actuator and the controller will be modified to a P, PD, or a PID controller. The control loop shown in figure below will be used for the remainder of the analysis.
Figure:
Control Loop Evaluated for the ABS
The basic equation of rotational motion will be used to evaluate the plant. This formula is shown below in Equation 1.
Where J is the moment of inertia of tire about the axis of rotation is the angular acceleration of the tire is the angular velocity of the tire b is the rotational damping of the tire (bearings). For
the purpose of this analysis the values below will be used: Tire Weight = 75 Kg Tire Radius = 33 cm
From
these values the moment of inertia is calculated to be J = 4.1 Kg*m^2. Since the wheel has bearing and the bearings are well lubricated the assumption is that the rotational damping factor is b = 1. Equation 2, Equation 3, and Equation 4 show how the equation above is converted into a transfer function with the appropriate J, b, and (0) values. 10 | P a g e
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Laplace (Equation1) = J [s(s) (0)] + b(s) = T (s)
Where (0) is the initial angular velocity
The valves and pumping system are combined in this analysis to represent the actuator. The actuator can be modeled as a servo system. The servo system has the transfer function shown in Equation 5.
The same values used to define Equation 4 are valid for Equation 5. Equation 6 is the servo motor transfer function with the appropriate values for the rotational damping and the moment of inertia.
The sensor used for the ABS application converts the velocity of the rotating wheel into a voltage. This application can be modeled as a tachometer. Equation 7 is the transfer function of a tachometer.
The new control loop obtained from the transfer functions above is shown in figure below. 11 | P a g e
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ABS Control Loop Including System Transfer Functions
The controller in the revised control loop is then tested under P, PD, and PID control. In the analysis the gain of the sensor (Ktach) is assumed to be 25. This assumption is based on the fact that the voltage produced by the sensor is small. The controller will require a higher voltage signal in order to receive the information from the sensor. Therefore the voltage from the sensor must be amplified and thus the gain is high. The gain of the actuator (Kact) is assumed to be 0.25 in the controller analysis. This is due to the high pressures of the hydraulic system. The valve and pumping system will not have to do much work in order to achieve the desired pressures at the braking system. The results of the P control analysis are shown in figures below. In first figure the proportional gain (Kp) is 1000 and the transfer function shown in Equation 8 is used. In latter figure the proportional gain (Kp) is 1 and the transfer function shown in Equation 9 is used. As seen in the graphs, the braking effects are liner with a higher proportional gain. When Kp is reduced the braking pattern is non-linear. The non-linear results are due to slipping. Figure
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in next page:
Anti-lock Braking System
Figure:
ABS Proportional Control w/ Kp = 1000
Figure:
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ABS Proportional Control w/ Kp = 1
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The results of the PD control analysis are shown in figure below and the results of the PID control analysis are shown in last figure. These graphs show that linear braking can be achieved through the use of PD and PID control.
Figure:
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ABS Proportional and Derivative Control
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ABS Proportional, Derivative, and Integral Integral Control
8. CONCLUSION/DISCUSSION: The control analysis shows that a proportional (P), proportional / derivative (PD), and the proportional / Integral / Derivative (PID) is able to control the ABS. Since all of the control systems will achieve the desired results, the best design must be determined by another method. Since the proportional control is the least complex, the ABS controller should use this method. The ABS is responsible for preventing car accidents and personal injury. A simplified system should reduce the complexity, cost, as well as increase the easy of serviceability.
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9. REFERENCES: Online resources for ABS Braking Systems Georgia Institute of Technology George W. Woodruff School of Technology Article Introduction and History www.scribd.com Motorola for the article Anti-Lock Braking Systems http://www.motorola.com/webapp/sps/site/application.jsp?nodeId=04M0ym4Ky 5P Controls Tutorial for Matlab http://www.engin.umich.edu/group/ctm/index.html How Stuff Works for the article How Anti-Lock Brakes Work http://www.howstuffworks.com/ Magneto-Resistive Wheel Speed Sensors, New Active Wheel Speed Sensors Changing ABS Diagnostic Procedures, David W. Gilbert. http://www.asashop.org/autoinc/aug2001/mech.cfm Heavy Truck Pneumatic Braking System Modeling, Analysis and Anti-lock Braking Robust Controller Design, Prof. Umit Ozgumer. http://car.eng.ohio-state.edu/consortium/proposals/hdv/h02pneu.pdf
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