EMBEDDED SYSTEMS

ANTI-LOCK BRAKING SYSTEM

ABSTRACT:

Embedded system is the recent emerging

technology in the field of electronics. An

important application of embedded system

is Anti-lock brake system (ABS) in

automobiles.

Anti-lock Braking systems were developed

to reduce skidding and maintain steering

control when brakes are used in an

emergency situation. Anti-lock Brakes are

essentially an enhanced or improved version

of ordinary brakes. Simply put, the antilock

brake system is designed to prevent the

brakes from locking up and skidding when

braking hard or when braking on wet or

slick surfaces.

ABS brakes work by automatically

actuating the brakes on and off during

emergency stops. They are effective in

helping drivers avoid accidents. In certain

situations, ABS brakes can help to shorten

stopping distances. This adds a significant

margin of safety for everyday driving by

preventing dangerous skids and allowing the

driver to maintain steering control while

trying to stop. This paper reveals an

overview of embedded systems and its

application in automobiles.

INTRODUCTION

An embedded system performs one or a few

pre-defined tasks, usually with very specific

requirements. Since the system is dedicated

to specific tasks, design engineers can

optimize it, reducing the size and cost of the

product. Embedded systems are often mass-

produced, benefiting from economies of

scale.

In terms of complexity embedded systems

can range from very simple with a single

microcontroller chip, to very complex with

multiple units, peripherals and networks

mounted inside a large chassis or enclosure.

CHARACTERISTICS OF EMBEDDED

SYSTEM

Some have real-time performance

constraints that must be met, for

reason such as safety and usability.

Some may have low or no

performance requirements, allowing

the system hardware to be simplified

to reduce costs.

An embedded system is not always a

separate block - very often it is

physically built-in to the device it is

controlling.

It often runs with limited computer

hardware resources.

CPU PLATFORM

Embedded processors can be broken into

two distinct categories: microprocessors

(µP) and microcontrollers (µC).

Microcontrollers have built-in peripherals

on the chip, reducing size of the system.

DEBUGGING

Embedded Debugging may be performed at

different levels, depending on the facilities

available, ranging from assembly- or

source-level debugging with an in-circuit

emulator or in-circuit debugger.

RELIABILITY

Embedded systems often reside in machines

that are expected to run continuously for

years without errors and in some cases

recover by themselves if an error occurs.

Therefore the software is usually developed

and tested more carefully than that for

personal computers, and unreliable

mechanical moving parts such as disk

drives, switches or buttons are avoided.

APPLICATIONS

Automatic teller machines (atms).

Avionics, such as inertial guidance

systems, flight control

hardware/software.

Cellular telephones and telephone

switches.

Engine controllers and antilock

brake controllers for automobiles.

Handheld calculators, computers,

medical equipment.

Household appliances.

Computer peripherals such as routers

and printers.

ANTI-LOCK BRAKING SYSTEM

Antilock braking systems (ABSs) are

electronic systems that monitor and control

wheel slip during vehicle braking. ABSs can

improve vehicle control during braking, and

reduce stopping distances on slippery road

surfaces by limiting wheel slip and

minimizing lockup.

Antilock Brakes system is designed to

prevent the brakes from locking up and

skidding when braking hard or when

braking on wet or slick surfaces. Antilock

brakes do not necessarily reduce the

stopping distance, and in fact may actually

increase stopping slightly on dry pavement.

But on wet or slick pavement, antilock

brakes may reduce the stopping distance up

to 25% or more, which could be the

difference between a safe stop and an

accident.

A tire that is just on the verge of slipping

(10 to 20% slippage) produces more friction

with respect to the road than one, which is

locked, and skidding (100% slippage). Once

traction is lost, friction is reduced, the tire

skids and the vehicle takes longer to stop.

COMPONENTS

The anti-lock brake controller is also known

as the CAB (Controller Anti-lock Brake).

A typical ABS is composed of

A central electronic unit.

Four speed sensors (one for each

wheel).

Two or more hydraulic valves on the

brake circuit.

ABS malfunction indicator lamp.

WHEEL SPEED SENSOR

The wheel speed sensors (WSS) consist of a

magnetic pickup and a toothed sensor ring.

As the wheel turns, teeth on the sensor ring

move through the pickup magnetic field.

This reverses the polarity of the magnetic

field and induces an alternating current

(AC) voltage in the pickup windings. The

number of voltage pulses per second that are

induced in the pickup changes in direct

proportion to wheel speed. So as speed

increases, the frequency and amplitude of

the wheel speed sensor goes up.

MODULATOR VALVES

ABS modulator valves regulate the air

pressure to the brakes during ABS action.

When not receiving commands from the

ECU, the modulator valve allows air to flow

freely and has no effect on the brake

pressure.

The ECU commands the modulator value

to either:

Change the air pressure to the brake

chamber, or

Hold the existing pressure.

The modulator valve usually has three ports:

Supply port: It receives air

from a quick release or relay

valve.

Delivery port: It sends air to the

brake chambers.

Exhaust port: It vents air from

the brake chamber(s).

Typically, when an ECU detects impending

wheel lockup, it activates the solenoids to

close the supply port and open the exhaust

port. When enough air is vented to prevent

wheel lockup, the exhaust valve will close.

ELECTRONIC CONTROL UNIT

The ABS electronic control module is a

microprocessor that functions like the

engine control computer. It uses input from

its sensors to regulate hydraulic pressure

during braking to prevent wheel lockup.

The key inputs for the ABS control module

come from the wheel speed sensors and a

brake pedal switch. The switch signals the

control module when the brakes are being

applied, which causes it to go from a

"standby" mode to an active mode.

When ABS braking is needed, the control

module kicks into action and orders the

hydraulic unit to modulate brake pressure as

needed. On systems that have a pump, it

also energizes the pump and relay. Like any

other electronic control module, the ABS

module is vulnerable to damage caused by

electrical overloads, impacts and extreme

temperatures.

MALFUNCTION INDICATOR LAMP

(MIL)

Illuminated in key position 2

Goes out with engine running

Illuminated when a fault is present

Will indicate low available voltage

(< 10.5 V)

Flickering off and on while engine is

running indicates a voltage supply

problem

WORKING

The electronic unit constantly monitors the

rotation speed of each wheel. When it

senses that any number of wheels are

rotating considerably slower than the others

it moves the valves to decrease the pressure

on the braking circuit, effectively reducing

the braking force on that wheel. Wheel(s)

then turn faster and when they turn too fast,

the force is reapplied. This process is

repeated continuously, and this causes the

characteristic pulsing feel through the brake

pedal.

The sensors can become contaminated with

metallic dust and fail to detect wheel slip;

this is not always picked up by the internal

ABS controller diagnostic.

ABS CONTROL MODE

PRESSURE HOLD

Vehicle speed signal indicates wheel is

about to lock.

Pressure still applied, but can’t increase

because ABS control module has activated

value y8.

PRESSURE RELEASE

To reduce pressure at locking wheels (the

wheel that has stopped spinning):

ABS control module:

Activates return pump

Activates Valve y8 so no more

pressure can be applied

Activates valve y9 so pressure at

brake can be reduced

These valves can be pulsed very rapidly.

You can feel this stage of operation through

the brake pedal.

TRACTION CONTROL

The ABS equipment may also be used to

implement traction control on acceleration

of the vehicle. If, when accelerating, the tire

loses traction with the ground, the ABS

controller can detect the situation and take

suitable action so that traction is regained.

Manufacturers often offer this as a

separately priced option even though the

infrastructure is largely shared with ABS.

More sophisticated versions of this can also

control throttle levels and brakes

simultaneously.

DRIVING WITH ABS

When vehicle first exceeds 5mph after a

start-up, a self-test is performed. With foot

on brake pedal, a pulsation will be felt.

Driver applies brakes:

– If wheels do not approach lock-up, then

ABS is not activated and brakes function as

in a non-ABS vehicle.

– As vehicle is braked, ABS may be able to

prevent wheel lock-up by simply holding

the brake fluid pressure from going any

higher. The driver would not sense this

stage of control.

– If driver feels brake pedal pulsate, then

ABS is reducing brake fluid pressure in

order to insure that wheels continue rotate.

This action may be accompanied by pump

noise.

Remember! ABS gives the driver the ability

to steer the vehicle in an emergency braking

situation.

ABS DOs AND DO NOT

DOs

DO keep your foot on the brake

pedal

DO allow enough distance to stop.

DO practice driving with ABS.

DO consult your vehicle owner’s

manual.

DO NOTS

DO NOT drive an ABS-equipped

vehicle more aggressively.

DO NOT pump the brakes.

DO NOT forget to steer.

DO NOT be alarmed by clicking

noises.

CONCLUSION

Maximum braking effect is achieved with

the wheels on the limit of friction, whereas

ABS works by releasing the brakes as the

wheels break traction, so a skilled driver

should be able to exceed the braking

performance of an ABS system in a straight

line on a consistent surface. When activated,

the ABS causes the brake pedal to pulse

noticeably. When drivers do encounter an

emergency that causes them to brake hard

and thus encounter this pulsing for the first

time, many are believed to reduce pedal

pressure and thus lengthen braking

distances, contributing to a higher level of

accidents than the superior emergency

stopping capabilities of ABS would

otherwise promise. Nevertheless, ABS can

significantly improve safety and control for

drivers in on-road situations if they know

not to release the brakes when they feel the

pulsing of ABS.

PRESENTED BY,

M.NAVEEN KUMAR (11ECL197)

K.MOHAN PREETH (11ECR083)

SECOND YEAR ECE DEPARTMENT,

KONGU ENGINEERING COLLEGE,

PERUNDURAI, ERODE.

MAIL ID:mnaveenkongu@gmail.com

CONTACT NO:7200459528