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Presentation Outline
What is an Anti-Lock Braking System (ABS)?
Motivation for ABS Development
History of ABS
Overview
Principles for ABS Operation
ABS Components Overview
ABS Components
How does ABS work?
System Diagram
ABS Configurations
Design Challenges
Limitations
1. What is ABS?
An anti-lock brake system is a feedback control system that modulates brake pressure in response to measured wheel deceleration, preventing the controlled wheels from becoming fully locked.
2. Motivation for ABS
Under hard braking, an ideal braking system should:
provide the shortest stopping distances on all surfaces
maintain vehicle stability and steerability
Anti-lock braking systems were developed to
best meet these needs.
3. History of ABS
1936: German company Bosch is awarded a patent for an Apparatus for preventing lock-braking of wheels
in a motor vehicle.
1936-: Bosch and Mercedes-Benz partner - R&D into ABS.
1972: WABCO partners with Mercedes-Benz developing first ABS for trucks.
1978: First production-line installation of ABS into Mercedes and BMW vehicles.
1981: 100,000 Bosch ABS installed.
1985: First ABS installed on US vehicles.
1986: 1M Bosch ABS installed.
1987: Traction control - in conjunction with ABS - used on passenger vehicles.
1989: ABS hydraulic unit combined with standard hydraulic
brake unit
1992: 10M Bosch ABS installed.
1995: Electronic Stability - in conjunction with ABS and
TCS - for passenger cars.
1999: 50M Bosch ABS installed.
2000: 6 of 10 new cars on the road are ABS equipped.
2003: 100M Bosch ABS installed.
Anti-Lock Braking Systems (ABS) are designed to maintain driver control and stability of the car during emergency braking. Locked wheels will slow a car down but will not provide steering ability. ABS allows maximum braking to be applied while retaining the ability to
'steer out of trouble . The operation of ABS can slightly reduce stopping distance in some cases like on wet road surfaces, but it can increase the stopping distance in others, as may be the case in deep snow or gravel.
An ABS system monitors four wheel speed sensors to evaluate wheel slippage. Slip can be determined by calculating the ratio of wheel speed to vehicle speed, which is continuously calculated from the four individual wheel speeds. During a braking event, the function of the control system is to maintain maximum possible wheel grip on the road - without the wheel locking - by adjusting the hydraulic fluid pressure to each brake by way of electronically controlled solenoid valves.
5. Principles for ABS Operation
Design Goals:
attain minimum stopping distance
maintain stability and steerability
Design Solution:
Develop a system that rapidly modulates the
braking force under hard braking conditions to:
maintain the ideal tire slip percentage to
maximize braking force ( 15%), recalling that Fbrake = BN
permit the vehicle to be steered with
stability maintained by preventing skidding
Typical ABS Components:
Wheel Speed Sensors (up to 4)
Electronic Control Unit (ECU)
Brake Master Cylinder, Hydraulic Modulator Unit with Pump and Valves
Vehicles Physical Brakes
ABS Components
Wheel Speed Sensor (WSS)
Hydraulic Modulator Unit
a) Wheel Speed Sensor (WSS)
Teeth on the sensor ring
rotate past the magnetic
sensor, causing a reversal
of the magnetic field
polarity, resulting in a signal
with frequency related to
the angular velocity of the
axle.
Electronic Control Unit
The signal from the WSS is
proportional to angular velocity. By
differentiating this signal, acceleration
of each wheel is known. If a wheel is
decelerating too quickly the brake
pressure is modulated.
A fifth input to the ECU is from a
brake pedal switch. This signal can
shift program execution from a
standby to an active state
b) Hydraulic Modulator Unit
The hydraulic modulator unit
contains the ABS pump as
well as solenoid valves for
each brake line.
The fifth line - far right - is
from the brake master
cylinder, which is connected
to the brake pedal.
Valves and Brakes
The valves modulate the brake
pressure up to 20 times per
second, effectively realizing
the ideal tire slip percentage.
ABS pumps the brakes much faster than any driver could.
7. How does ABS work? Basically, there are sensors at each of the four wheels (or in the case of the
less sophisticated three-channel system, one on each of the fronts and only one
for the pair of rears). These sensors watch the rotation of the wheels. When any
one of the wheels stops rotating due to too much brake application, the sensors
tell the car's computer, which then releases some of the brake line pressure that
you've applied - allowing the wheel to turn again. Then, just as fast as it released
the pressure, the computer allows the pressure to be applied again - which stops
the rotation of the wheel again. Then it releases it again. And so on. With most
ABS, this releasing and re-application - or pulsing - of the brake pressure
happens 20 or more times per second.
Practically speaking, this keeps the wheel just at the limit - the threshold - before
locking up and skidding. ABS prevents you from ever locking up the brakes and
skidding - no matter how hard you apply the brakes. Obviously, this is going to
mean much more steering control.
With ABS, all you have to do in an emergency is quickly squeeze the brake
pedal as hard as you can and hold it there. And when I say hard, I mean HARD.
Let the system do the finesse work for you. This may not be as easy as it sounds.
After years of being told (and practising) not to press too hard on the brake pedal,
this may not feel very natural.
8. System Diagram
9. ABS Configurations
Depending on the ABS application, there are several typical layouts.
Because of the safety-related nature of the ABS application, special emphasis is placed on functions
designed to detect system faults and ensure that a fail-
safe state occurs during faults. These functions may be
implemented with techniques such as dual processors
and specialized self test and watchdog modules.
ABS systems typically include eight electric solenoid
valves and other electrical loads, such as relays and
pump motors, that require considerable drive currents.
Electronic drive circuitry is often required to sense any
failure of these components during operation.
Three points should be obvious, but don't appear to
when looking at the type of crashes some drivers
have with ABS-equipped vehicles. Contrary to
popular belief, ABS :-
does not allow you to drive faster; does not allow you to brake later; and does not allow you to corner faster.