Electronically Controlled Air Suspension System

Electronically Controlled Air Suspension System

1. INTRODUCTION Today we are living in the world of science and technology. Today's technology is becoming past and tomorrows is struggling for it's existence. Automobile engineering is also fastest growing field. The automobile suspension provides ground for extensive research and development. Conventional suspension systems such as leaf spring, coil spring suspension lack prime requirement of variable spring rate for various load conditions thus giving less vehicle controllability and passenger comfort. Consumer's satisfaction is a keyword in today's business. So scientists and engineers have developed an air suspension system which provides more comfort and better vehicle controllability. I have started this seminar with critical review of conventional suspension systems, their drawbacks and alternative provided by air suspension. I have described general arrangement of air suspension system and its components and their functions. The second part deals with air-suspension system and is electronic control. Since active suspension is an advanced part comprising of model prototype studies and extensive force analysis on chassis and complex mathematics, I have avoided that Instead I have focused on sensors used in electronic suspension and design parameters, to be controlled electronically. At the end there is a distinct comparison between the conventional construction, I hope that air suspension will be a common feature of future Indian vehicles.

J.T.Mahajan Polytechnic College, Faizpur


The basic function of an automobile suspension is to isolate passenger from shocks and vibration. Road surface decides forces acting on vehicle chassis and resulting vibrations. Various suspension systems with different arrangements are used to perform the function.

1.1

THE SUSPENSION SYSTEMS IN AUTOMOBILES CAN BE DIVIDED TWO MAIN PARTS

a)

Rear Suspension system and

b)

Front suspension system.

a)

Rear Suspension System

Functions i)

To support rear weight of vehicle.

ii)

To isolate vehicle pay load from shocks and vibrations.

b)

Front suspension system In addition to functions described above front suspension system has

to perform two more functions. i)

To provide steering control and wheel alignment.

ii)

To maintain steering control during severe braking. Before going into air suspension system let us see conventional

suspension systems.



1.2

CONVENTIONAL SUSPENSION SYSTEM It consists of coil spring, torsion bars, leaf springs.

1)

Leaf springs

:

Widely employed in trucks and cars in India.

2)

Torsion bars

:

e.g. pugeuot (America)

3)

Machperson strut

:

Widely used in front suspension system.



2.

NEED OF AIR SUSPENSION

2.1

MAIN DRAWBACKS OF CONVENTIONAL SYSTEMS

1)

Rear Wheel Bounce Experienced on laden and unloaded trucks, trailers, causing driver

fatigue, noise, excessive tyre wear and punishing structural stresses. 2)

Uncomfortable Pitching On lightly laden buses and trucks causing passenger discomfort or

damage to fragile freight and chassis components. 3)

Change in Headlight Angle Occurring on partly laden trucks, causing awkward misalignment

between trucks and loading platform. 4)

Load Imbalance Occurring on partly loaded trucks, causing awkward misalignment

between truck's loading platform.

2.2

HISTORY OF AIR SUSPENSION SYSTEM The idea of suspending a vehicle on air goes back over 100 years in

the mid of 19th century. Many experiments were carried out but the absence of a durable rubber material for air springs severely limited development. It was only in 1980's when air -springs were successfully introduced in automobiles. Experiments are still going on to optimise smooth ride and comfort.



3. AIR SUSPENSION SYSTEM AND COMPONENTS

Micro computer

Air spring

Fig. 1: Air suspension system & its components

3.1 AIR SPRINGS Generally air spring is incorporated at each wheel. a) Types 1)

Single convolution

2)

Double convolution

3)

Triple convolution

4)

Rolling diaphragm type

5)

Un-restricted diaphragm type 6) Rolling lobe spring.

Working Air flow in spring is controlled by levelling valve so as to change spring rate of spring to suit load conditions. To prevent any damage to spring, a guard is provided.



3.2

LEVELING VALVE

a)

Levelling valves are used to maintain constant platform height by controlling the passage of air to and from the air-springs. Three valves are used, one control mounted above the front axle controlling front springs and one mounted above each end of v-frame controlling the respective springs.



b)

Operation of valve

Levelling valve is a three way valve 1)

Compressed air supply to valve

2)

Compressed air supply to spring

3)

Exhaust

3.3

ISOLATOR VALVE It's function is to prevent loss of roll stiffness whilst cornering. This is

accomplished by restricting the transfer of air from one spring to other except when air is being admitted to exhaust from springs due to activation of he levelling valve.

3.4

AIR FILTER It is in-corporate between auxiliary reservoir and levelling valve. Sintered

bronze filters are fitted in the valve ports to protect the internal mechanism from damage by foreign matter which may be carried back from the interior of the spring.

3.5

DRIER Any moisture in the air in air-spring will corrode the interior of the spring.

Hence drier is fitted in suspension air ckt. It contains silica gel which absorbs moisture from air. Exhaust air from air spring is used as a re-generative agent to remove moisture from silica gel and make it fresh.



3.6

PRESSURE REGULATOR It is connected in the pipe line from the auxiliary reservoir, safeguard the

braking system by allowing air into suspension system only when sufficient pressure is available in the braking system.

3.7

COMPRESSOR It is connected to storage reservoir. It is operated from engine. There is a

single air for air-brake and suspension system. In some cars e.g. Nissan motors D.C. motor is used to drive compressor.



4. TWO BASIC TYPES OF AIR SUSPENSION SYSTEMS 1)

The brake installation and air suspension have the same container pressure. The suspension is coupled secondary to the brake installation.

2)

Air suspension operates at higher pressure than the brake installation. The compressed air from brakes is taken directly after the frost protection pump and before the four way safety valve. The overflow valve from the braking has a higher opening pressure than safety valve in order to safeguard priority of the brake system.

4.1. BASIC PRINCIPLES OF AIR SPRING ARRANGEMENT 1)

All air suspension on guided wheels It is clear that any irregularities in road surfaces causing vibrations are

directly related with spring-rate. In this case air spring rate determines natural frequency of the vehicle directly. 2)

All air suspension with swinging axles The forces on the wheel and on the air spring work on different lever arms. For this reason forces and deflections of different magnitude act on the

wheel and on the air spring. In order to determine the natural frequency of vibration; reduced spring are must be determined at the wheel Centre.



5. ELECTRONIC CONTROL OF AN AIR SUSPENSION SYSTEM 5.1. NECESSITY Spring rate and damping coefficient need accurate settings so as to give better ride performance. Road conditions are prime factors which affect socks and vibrations. By use of sensors and feed-back loop control, pre-programmed micro-processor can perform accurate control of air suspension ckt. Hence to achieve better vehicle controllability and ride comfort electronic control is employed.

5.2

CONTROL SYSTEM In incorporates electronic sensors and micro-processor unit.

5.3

COMPONENTS

OF

ELECTRONICALLY

CONTROLLED

SUSPENSION SYSTEM a)

AIR SUSPENSION UNIT Shock absorbers and air spring are designed with integrated construction. A small motor built into the piston rod operates a control valve which

varies damping coefficient over-three different settings (described later). The front suspension unit has taper rolling guide in order to give the spring rate a suitable no linear characteristics.



b) SUB-TANK It is equipped at each wheel. When low spring rate is desired a portion of the air in the air spring is divided into the sub-tank. The structure is as follows 1)

The supply valve controls inflow and outflow of air for regulating

the

vehicle height. 2)

The cut-off valve controls the flow of air between the suspension unit and sub-tank to select the proper spring rate. It also incorporates a valve that functions to maintain a minimum pressure level in the chamber. It also prevents a vacuum from developing in the chamber when the

suspension unit is fully extended.

c)

COMPRESSOR Similar as explained earlier. In addition a thermostat is provided which

stops the operation of the compressor if the humidity inside the compressor goes above certain level.



d)

PRESSURE SWITCH It maintains pressure inside piping below 10 kgf/sq.cm.

e)

VEHICLE HEIGHT SENSOR 3 in no.2 at front suspension unit. 1 at rear unit. It is equipped with potentiometer that provides reading in increment of 1 mm allowing fine tuning.

f)

STEERING ANGLE SENSOR For accurate detection of straight line condition of the vehicle . Neutral position sensor is added to steering angle sensor. This prevents any Un-necessary vehicle pitching by temporarily suspending the vehicle height control when vehicle is turning.



g) Warning Lamp When the control module senses a system defect, the module turns on the air suspension warning lamp in the roof console to inform the driver that a problem exists. If the air suspension system is working normally, the warning lamp will be on for one second when the ignition switch is turned from the Off to the Run position. After this time, the warning lamp should remain off. This lamp does not operate with the ignition switch in the start position. The warning lamp is used during the self-diagnostic procedure and the spring fill sequence.



6. CONTROL UNIT It consists of 1)

Micro-computer

2)

Power supply

3)

Analogue to digital converter

4)

Input signal circuits

5)

Actuator for damping coefficient selection and drive circuit for each valve.

6.1

MICRO-COMPUTER FEATURES

1)

8-bit processor

2)

ROM - 13 KB

3)

RAM - 252 Byte The program is designed with good compatibility between control and

timing requirement. The electronic ckts. make extensive use of FET's and transistors with bilt in surge absorber. This results in lower ckt costs and improved packing density. Fail safe function and self diagnostic functions are also added. Fail safe system suspends control over the symbolic system of the affected device, wile maintaining he maximum control possible over other systems so as to assure safe operation.



7. COMPARISON OF AIR SUSPENSION SYSTEM & CONVENTIONAL SUSPENSION SYSTEM

Sr. No. 1. 2.

Parameter

Air Suspension

Conventional

Load Carrying Capacity Ability to Absorb Shocks &

Higher Better

Suspension Lower Good

3. 4.

Vibrations Car Height Control Squat Dive &

Better Better

No Poor

5. 6. 7. 8. 9. 10.

Characteristics Cost Maintenance Passenger Comfort Electronic Control Life Noise Level

Costly More Excellent Possible More Less

Cheaper Less Not Satisfactory Not possible Less Higher

Roll



8. CONCLUSION

From the critical comparison of suspension systems, it becomes clear that air suspension system provides a better alternative for conventional system ensuring better car ability and better passenger comfort.

The requirement of this suspension system is better road conditions.

With improvement in road conditions, I hope that air suspended vehicles will be a common feature of future Indian vehicle.



REFERENCE1)

Course & Anglin, Automotive Mechanics, by: McGraw Hill Publication (1991)

2)

Ellinger, H.E., Automobile Chasis and Design

3)

Dunlop, Pneuride, Technical Information Manual, (1990)

4)

Journal of Air-Suspension and Shock Absorbers, Wheels and Tyres (1990)

5)

R. Bosch VOT-KKB 1011, Leveling valves for air commercial vehicles. (1994)


suspension on


SYNOPSIS

In this paper. I have tried to give details about advanced suspension system which is nothing but air suspension system and its advantages over conventional system.

It includes details about air suspension systems, its types, components of system and electronic control of air suspension system which used to control the different specifications of suspension system and lastly comparison between air suspension system and conventional suspension system.



CONTENTS

SYNOPSIS 1.

2.

3.

4.

INTRODUCTION 1.1

Types of Suspension System

1.2

Conventional Suspension System

NEED FOR AIR SUSPENSION 2.1

Main Drawback of Conventional System

2.2

History of A.S.S.

A.S.S. AND COMPONENTS 3.1

Air Spring

3.2

Levelling Valve

3.3

Isolator Valve

3.4

Air Filter and Drier

3.5

Pressure Regulator and Compressor

TWO BASIC TYPES OF A.S.S 4.1

5.

1

4

5

9

Basic Principles of Air Spring

ELECTRONIC CONTROL OF A.S.S. 5.1

Necessity

5.2

Control System

5.3

Components of Electronically Controlled S.S.

10

6.

CONTROL UNIT

14

7.

COMPARISON OF A.S.S & CONVENTIONAL S.S.

15

8.

CONCLUSION

16

REFERENCE

17


Electronically Controlled Air Suspension System

Recommend Documents