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27

Automotive Steering Systems

Tomy Sebastian, Mohammad S. Islam, and Sayeed Mir

Delphi Corporation, Saginaw, Michigan

27.1 INTRODUCTION

The steering system in an automobile converts the drivers rotational input at the steering

wheel or hand wheel into a change in the steering angle of the vehicles road wheels to

control the direction of motion. Effort is required to turn the steering wheel due to the

presence of friction between the tires and the road surface. In earlier steering systems, the

driver provided the required torque to steer the vehicle. For heavier vehicles, the driver is

unable to provide sufficient torque to steer the vehicle. In such cases an additional mech-

anism is needed to assist the driver. Hydraulically assisted power steering was introduced

around the 1950s. Developments in power and control electronics and in electric machines

led to the development of electrically assisted steering. This chapter will discuss various

types of steering mechanisms.

27.2 STEERING SYSTEM

Based on the operating principle, steering systems can be classified as shown in Figure 27.1.

The following sections will discuss these systems in more detail. In all these systems, the

objective is to move the road wheels by a certain angle for a given angle rotation of the

hand wheel. The ratio of the road wheel angle rotation to the steering angle rotation is

normally referred to as the steer ratio.

2005 by Taylor & Francis Group, LLC

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27.2.1 MANUAL STEERING

A simple way of controlling the road wheel angle from the steering wheel is by means

of a rack-and-pinion gear mechanism, shown in Figure 27.2. In this case the driver provides

all the energy required for the angular movement of the road wheels. The rack-and-pinion

mechanism converts the rotational input of the driver to a linear motion of the rack. This

conversion also allows reducing the driver effort to turn the steering wheel.

The required driver torque is a function of the rack force and the rack-and-pinion

ratio called C-factor(sometimes C-factoris also referred to as steering ratio). This factor

is defined as the distance traveled by the rack in mm when the steering wheel is rotated

for 360 degrees.

Thus, the relationship between the steering wheel speed and torque to the rack speedand force can be written as:

Figure 27.1 Classification of steering systems.

Figure 27.2 A manual rack-and-pinion arrangement for a steering system (courtesy Delphi Cor-

poration).

Steering System

Manual Assist Power Assist

Rack &

Pinion

Recirculating

ball

Hydraulic

Power

Steering

Electro

Hydraulic

Power

Steering

Electric

Power

Steering

To intermediate shaft,

column and steering wheel

Tie rod

Connection to

road wheel

Rack & pinion interface

(rotary to linear conversion)

To intermediate shaft,

column and steering wheel

Tie rod

Connection to

road wheel

Rack & pinion interface

(rotary to linear conversion)


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(27.1)

(27.2)

where Ts is the steering wheel torque in N-m, s is the steering wheel speed in rad/s, Fris the rack force inN, vris the rack velocity in m/s, andrp is the rack-and-pinion efficiency

[1]. For heavier vehicles, the steering effort torque will be large; therefore, these systems

are only used in smaller vehicles.

Another mechanism to convert the driver torque into road wheel rotation is by means

of a recirculating ball arrangement, also known as an integral gear mechanism [1,2]. Since

this system is not currently used with electric power steering, this is outside the scope of

this book.

27.2.2 HYDRAULICALLY ASSISTED STEERING

Traditionally, the power assist is obtained by hydraulic means. Figure 27.3 shows the

schematic of such a system. In this case, a hydraulic pump is driven from the vehicle

engine through a belt and pulley arrangement. The high-pressure fluid is used to move a

piston in the steering gear assembly to assist the driver. The direction of movement is

controlled by a valve mechanism.

The hydraulically assisted system has an assist characteristic, which is independent

of the vehicle speed. It is preferred to have higher assist (or lower driver effort) at low

Figure 27.3 The schematic of a hydraulically assisted steering system.

TF C

N msr

rp

= 2000

v C m srs

=

2000/

Tie rod

Steering gear

assembly

Tie rod

Pressure

hose

Pump

Return hose

Cooler

Intermediateshaft

Steering

column

Road

wheel

Tie rod

Steering gear

assembly

Tie rod

Pressure

hose

Pump

Return hose

Cooler

Intermediateshaft

Steering

column

Road

wheel


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speed and parking conditions, and lower assist at higher vehicle speed situations such as

on a highway. The variable speed-effort characteristic is obtained by using electronically

controlled valve mechanisms or by electromagnetically controlled systems such as Magna

Steer.

The hydraulically assisted steering system provides exceptionally good steering feelcharacteristics, though it has several disadvantages:

1. The continuously running pump creates constant power loss, thus increasing

the fuel consumption of the vehicle.

2. At the end of life of the vehicle, one has to deal with the hydraulic fluid and

the hoses.

3. Tuning of the vehicle steering characteristics is complicated and time consuming.

4. Assembly of the system in the vehicle is time consuming due to the large number

of components to be assembled.

5. Packaging is difficult, as engine accessories are needed for coupling the pumpto the engine.

6. As the power assist is dependent on the engine speed, if the engine is stalled,

the power assist also will be lost (engine dependency).

27.2.3 ELECTROHYDRAULIC POWER STEERING

Some of these disadvantages could be overcome by running the pump from an electric

motor. Such a system as shown in Figure 27.4 is often referred to as an electrohydraulic

power steering system. This system addresses the issues 4 through 6 mentioned above. It

also provides reduced fuel consumption, as the pump speed is independent of the engine

speed and the speed could be controlled to reduce the losses. These systems draw acontinuous current of the order of few amperes from the battery, mostly to support the

hydraulic losses. During the steering maneuver, depending on the vehicle size, the peak

current drawn may be as high as 100 amperes from a 12 V power system.

Figure 27.4 An electrohydraulic power steering system.

Pump, motor

and control unit


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27.2.4 ELECTRIC POWER STEERING

An electrically assisted steering system addresses most of the disadvantages of the hydrau-

lic system, though it brings in some new issues and challenges. In an electric power steering

system, the assist to the driver is provided by an electric motor. The base system is very

similar to the manual rack-and-pinion arrangement explained with Figure 27.2. An electric

motor with a gear reduction mechanism is coupled to the main steering path to provide

the assist. In Figure 27.5, the assist mechanism is coupled to the steering path at the

column (column assist). The assist could also be provided at the pinion (pinion assist) or

even at the rack (rack assist).

The required motor torque can be written as:

(27.3)

where Td is the driver input torque, is the efficiency of conversion, and n is the gear

ratio between the column and the motor. Ts is the total load as in Equation 27.1.

Also, the motor speed m and the steering wheel speed s are related by:

(27.4)

A curve showing the assist torque vs. steering wheel input torque is shown in Figure

27.6. Due to the shape of this curve, it is sometimes also known as the bathtub curve.

This curve is for a particular steering wheel speed and vehicle speed. The torque-speed

requirements of the motor are derived from the bathtub curve using the gear ratio of

the mechanical gear and the efficiencies of other system components. Figure 27.7shows

the typical power flow and losses in a typical electric power steering system. The power

source for an automotive application is usually the battery with a nominal voltage of 12 V.

The maximum current draw allowed from the battery is usually about 75 to 100 A,

depending on the vehicle type and the manufacturer. This automatically places a limit on

the input power (= 12 75 = 900 W peak power). Based on this input power limitation,

the designer has to allocate the efficiencies to the system components in order to get the

Figure 27.5 Mechanization of an electric power steering system (courtesy Delphi Corporation).

TT T

nm

s d=

( )

m sn=


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required output power, which can vary from 250 W to 550 W, depending on the steering

loads, which in turn depend on the gross vehicle weight (GVW). Usually the overall systemcost and efficiency is balanced with tradeoffs between the different components (i.e., gear

reduction mechanism, electronic controller, motor, and wiring harness).

The electric power steering eliminates the hydraulic fluid. This is an on-demand

system, since the motor does not take in any power if the driver does not steer. This results

in an improved fuel economy to the vehicle. When the vehicle is not in a steering maneuver,

the system takes very little current to support the control electronics. In a steering maneu-

ver, the battery current could reach up to 100 A. Heavier vehicles will require higher

current. This limits the usage of electric power steering to smaller vehicles, as the available

battery current is limited. Also due to the engine-independent nature, the steering assist

will be provided even with the engine stalled as long as the battery power is present. It

also reduces the engine accessories, making it simpler.Another advantage of the electric power steering system is in the ease of tunability

of the steering feel. The assist as a function of the vehicle speed can be easily programmed.

As the assist characteristics can be programmed using software, one does not have to

change the mechanical components, as in a hydraulic system, to tune the steering charac-

teristics. This reduces the tuning time substantially compared to a hydraulic system.

27.3 ADVANCED STEERING SYSTEMS

Steering systems have moved from just providing driver assist, to also providing addedcomfort and enhanced vehicle stability. Recently, four-wheel steering and other systems

that enabled better vehicle controllability were introduced in the market.

Figure 27.6 The assist torque vs. the steering wheel torque of a typical electric power steering system.

Figure 27.7 Typical power flow and losses in an electric power steering system.

Steering wheel torque

Assist torque

Wiring loss Controller loss Motor loss Gear Mech. loss R & Pinion loss

Control MotorGear

MechR & P RoadBattery Wiring


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27.3.1 FOUR-WHEEL STEERING

The introduction of electric power steering also allowed other modes of vehicle control.

The addition of steering capability to the rear wheels provides the vehicle with much more

control options. The four-wheel steering introduced in trucks and sport utility vehicles

allows for better vehicle maneuverability. The advantages are obvious when the vehicle

is used for trailering and during parking. At low vehicle speeds, the rear wheels are in

phase opposition to the front wheels and at higher vehicle speeds they are in phase with

each other. Figure 27.8 shows a low-speed maneuver of a four-wheel steering systemwhere the front and rear wheels are in phase opposition.

27.3.2 FUTURE-GENERATION STEERING SYSTEMS

Future steering systems will integrate other functions such as braking, throttle, and sus-

pension to improve the vehicle stability and control. To accomplish this, there has to be

some level of decoupling between the driver and the road wheels. Active front steering

systems use a differential arrangement so as to be able to control the road wheels either

by the driver or by the motor.

Steer-by-wire systems give complete mechanical decoupling of the driver to the road

wheel by eliminating all mechanical linkages between the steering wheel and the road wheel.The sensors and control along with the drive motors precisely position the road wheels at

the desired position. Such systems will require fault tolerant communication and control

schemes. These systems will require actuators to actuate the road wheels and to provide

feedback to the driver.

REFERENCES

[1] J.W. Post, E.H. Law, Procedure for the characterization of friction in automobile power

steering systems, SAE International Congress and Exposition, Detroit, MI, February 2629,

1996, Document Order Number SP-1136.[2] R.K. Jurgen,Electronic Steering and Suspension Systems, Society of Automotive Engineers

Inc.

Figure 27.8 Four-wheel steering under low vehicle speed condition, where the front and rear

wheels are in phase opposition (courtesy Delphi Corporation).


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[3] C.P. Cho, R.H. Johnston, Electric motors in vehicle applications, Proceedings of the IEEE

International Vehicle Electronics Conference (IVEC99) , Changchun, China, 69 September

1999.

[4] N. Iwama, Y. Inaguma, K. Asano, T. Mori, Y. Hayashi, Independent rear wheel control by

electric motors, 23rd FISITA Congress, Torino, Italy, 1990.

[5] E.A. Bretz, By-wire cars turn the corner,IEEE Spectrum, April 2001, pp. 6873.

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