abs

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.