Electrical Steering System in the World

7/27/2019 Electrical Steering System in the World

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CONTENTS

ABSTRACT

1. Introduction

2. Steering system

3. Fundamental condition for true rolling and correct steering angle

4. Terms related to the steering system

5. Components of steering system

6. Power steering

7. Reversible and Irreversible Steering

8. Under-steering and Over-steering

9. Electric power steering system

10. Future steering system

11. Essential components of electric power steering system

12. Advantages of steer-by-wire system

13. System structure of safe electrical steering system14. Conclusion

REFERENCES


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ABSTRACT

Additional future requirements for automobiles such as improved vehicle

dynamics control; enhanced comfort, increased safety and compact packaging are

met by modern electrical steering systems. Based on these requirements the new

functionality is realized by various additional electrical components for measuring,

signal processing and actuator control.

However, the reliability of these new systems has to meet the standard of

today's automotive steering products. To achieve the demands of the respective

components (e.g. sensors, bus systems, electronic control units, power units,

actuators) the systems have to be fault-tolerant and/or fail-silent.

The realization of the derived safety structures requires both expertise andexperience in design and mass production of safety relevant electrical systems.

Beside system safety and system availability the redundant electrical systems also

have to meet economic and market requirements. Within this scope the paper starts

by tracing the history of developments in the steering system for automobiles and

then discusses three different realizations of electrical steering systems

Electrical power steering system (mechanical system with electrical boosting)

Steer-by-wire system with hydraulic back-up and

Full steer-by-wire system

The paper presents solutions for these systems and discusses the various

advantages and disadvantages, respectively. Furthermore strategies for failure

detection, failure localization and failure treatment are presented. Finally the various

specifications for the components used are discussed.


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1. INTRODUCTION

In this paper we try to understand the detailed working of steering system, power

steering system and the technical solutions and safety aspects of various

electrical steering systems. We will show that for car manufacturers and end

customers the use of new electrical steering systems offers many advantages

concerning flexibility, enhancement of familiar steering functions and the

introduction of innovative steering functions. New steering functions, which are

even coupled with automatic steering interventions, call for an adaptation of

regulations concerning the approval of steering equipment. Development and

production of the next generations of electrical steering systems up to purely

electrical steering systems create high safety demands on components and

systems. Reliable and safe electrical steering systems can be realized by using

appropriate safety techniques for these new systems and their components

combined with the know-how of safety relevant vehicle systems. At the same time

the transition to purely electrical steering systems will take place step by step via

systems with mechanical or hydraulic backup.


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2. STEERING SYSTEM

Steering system: It is the system which provides directional change in the

performance of an automobile. This system converts rotary movement of the

steering wheel into angular movement of the front wheels. It multiplies drivers effort

by mechanical advantage, enabling him to turn the wheels easily.


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2.1 Steering system requirements and functions

For proper and smooth operation and performance of the system, the steering

system of any vehicle should fulfil the following requirements:

It should multiply the turning effort applied on the steering wheel by the driver.

It should be to a certain extent irreversible. In other words, the shocks of the

road surface encountered by the wheels should not be transmitted to the

drivers hands.

The mechanism should have self rightening effect i.e., when the driver

releases the steering wheel after negotiating the turn, the wheel should try toachieve straight ahead position.

Functions of the steering system are as follows:

It helps in swinging the wheels to the left or right.

It helps in turning the vehicle at the will of the driver.

It provides directional stability. It helps in controlling wear and tear of tyres.

It helps in achieving the self-rightening effect.

It converts the rotary movement of the steering wheel into an angular turn of

the front wheels.

It multiplies the effort of the driver by leverage in order to make it fairly easy

to turn the wheels.

It absorbs a major part of the road shocks thereby preventing them to gettransmitted to the hands of the driver.

The complete steering system which performs the above functions, can be divided

into two portions, namely, steering gear provided at the end of the steering column

and the linkage between the steering gear and the wheels.


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2.2 Types of steering system:

Steering system is of the following types

(a) Fifth wheel steering system, (b) Side pivot steering system.

Fifth wheel steering system:

It is single pivot steering system in which the front axle along with the wheels, moves

to right or left. The movement to the whole axle and wheel assembly is affected by

means of a steering and a wheel which is placed between chassis frame and axle.The fifth wheel acts as a turntable. The axle assembly is connected with the frame

by means of a pin which serves as a pivot around which the axle assembly moves.

The fifth wheel contains a ring gear mounted at its rim and is moved by means of a

steering. Movement of the steering wheel tends the front axle and wheel assembly to

move away.


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Side pivot steering mechanism:

There are two types of steering gear mechanisms:

Davis steering gear mechanism

Ackermann steering gear mechanism

The main difference between the two steering gear mechanisms is that the Davis

steering has sliding pairs, whereas the Ackermann steering has only turning pairs.

The sliding pair has more friction than the turning pair; therefore the Davis steering

gear will wear out earlier and become inaccurate after certain time. The Ackermann

steering gear is not mathematically accurate except in three positions, contrary to the

Davis steering gear which is mathematically correct in all positions. However, theAckermann steering gear is preferred to the Davis steering gear.

Davis Steering Gear:

The Davis gear mechanism consists of a cross link KL sliding parallel to another link

AB and is connected to the stub axles of the two front wheels by means of two

similar bell crank levers CAK and DBL pivoted at A and B respectively. The cross

link KL slides in slides in the bearing and carries pins at its end K and L. The slideblocks are pivoted on these pins and move with the turning of bell crank levers as

the steering wheel is. When the vehicle is running straight, the gear said to in its mid-

position. The short arms AK and BL are inclined an angle 90+ to their stub axles

AC and BD. The correct steering depends upon a suitab1e selection of cross-arm

angle , and is given by

tan = b / 2l

Where,

b=AB=distance between the pivots of front axles.

l=wheel base.

The range of b / l is 0.4 to 0.5

hence angle lies between 11.3 and 14.10.


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Ackermann Steering Gear:

The Ackermann steering gear mechanism consists of a cross link KL connected to

the short axles AC and BD of the two front wheels through the short arms AK and

BL, forming bell crank levers CAK and DBL respectively. When the vehicle is runningstraight, the crosslink KL is parallel to AB, the short arm AK and BL both make angle

to the horizontal axis of chassis. In order to satisfy the fundamental equation for

correct steering, the links

AK and KL are suitably proportioned and angle is suitably selected.

For correct steering

cot cot = b / l


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The angles and are shown in the Figure.

The value of b / l is between 0.4 and 0.5, generally 0.455.

The value of cot cot corresponds to the positions when the steering is

correct.

In fact there are three values of angle which give correct steering of the

vehicle:

first while it is turning to right,

second while it is turning to left and

third while it is running straight.


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3. Fundamental condition for true rolling and correct

steering angle

The perfect steering is achieved when all the four wheels are rolling perfectly under

all conditions of running. While taking turns, the condition of perfect rolling is satisfied

if the axes of the front wheels when produced meet the rear wheel axis at one point.

Then this point is the instantaneous centre of the vehicle. It is seen that the inside

wheel is required to turn though a greater angle than the outer wheel. The larger the

steering angle, the smaller

is the turning circle. There is, however, a maximum to which we can go as regards

the steering angle. It has been found that steering angle (of the inner wheel) can

have a maximum value of about 44 degree. The extreme positions on either side are

called lock positions. The diameter of the smallest circle which the outer front wheel

of the car can traverse and obtained when the wheels arc at their extreme positions

is known as the turning circle.

Referring to Fig, for correct steering,

Equation represents the basic condition for the steering mechanism for perfect rolling

of all wheels.


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4. Terms related to the steering system

Camber angle is the angle made by the wheels of a vehicle; specifically, it is the

angle between the vertical axis of the wheels used for steering and the vertical axis

of the vehicle when viewed from the front or rear. It is used in the design

of steering and suspension. If the top of the wheel is farther out than the bottom (that

is, away from the axle), it is called positive camber; if the bottom of the wheel is

farther out than the top, it is called negative camber.

Casterangle orcastor angleis the angular displacement from the vertical axis

of the suspension of a steered wheel in a car, bicycle or other vehicle, measured in

the longitudinal direction. It is the angle between the pivot line (in a car - an

imaginary line that runs through the center of the upper ball joint to the center of the

lower ball joint) and vertical. Car racers sometimes adjust caster angle to optimize

their car's handling characteristics in particular driving situations.


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Toe, also known as tracking, is the symmetric angle that each wheel makes with

the longitudinal axis of the vehicle, as a function of static geometry, and kinematic

and compliant effects. This can be contrasted with steer, which is the antisymmetric

angle, i.e. both wheels point to the left or right, in parallel (roughly). Positive toe,

ortoe in, is the front of the wheel pointing in towards the centerline of the vehicle.

Negative toe, ortoe out, is the front of the wheel pointing away from the centerline

of the vehicle. Toe can be measured in linear units, at the front of the tire, or as an

angular deflection.

KING-PIN INCLINATION is set at an angle relative to the true vertical line, as

viewed from the front or back of the vehicle. This is the kingpin inclination or KPI

(also called steering axis inclination, or SAI).SAI is non-adjustable, since it would

change only if the wheel spindle or steering knuckles are bent. This has an important


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effect on the steering, making it tend to return to the straight ahead or centre

position.

Scrub radius is the distance in front view between the king pin axis and the centre

of the contact patch of the wheel, where both would theoretically touch the road. The

kingpin axis is the line between the upper and lower ball joints of the hub. There are

two types of scrub radius: negative and positive.

Large positive values of scrub radius, 4 inches/100 mm or so, were used in cars for

many years. The advantage of this is that the tire rolls as the wheel is steered, which

reduces the effort when parking. This also allows greater width in the engine bay,

which is very important in some compact sports cars. But advantage of a negative

scrub radius is that the geometry naturally compensates for split mu braking, or

failure in one of the brake circuits. It also provides centre point steering in the event

of a tire deflation, which provides greater stability and steering control in this

emergency situation.


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5. COMPONENTS OF STEERING SYSTEM:

Steering wheel

Steering is effected by the steering wheel. The steering wheels of commercial

vehicles have a metal armature comprised of a screw machined hub with metal

spokes and rim. The hub, spokes and rim are all fabricated into one piece by

welding. The armature serves as the load bearing structure of the wheel. The

armature is surrounded by a moulded rubber or plastic material. Rubber wheels are

painted, and plastic wheels utilize impregnated colours. The steering wheel is of

large diameter. This helps to convert the available driver rim pull into maximum input

torque. However, the size of the wheel is limited by the following:

(1) The comfort of the driver when using the steering wheel.

(2) The space available for the steering in the interior of the cabin.

(3) The ease of performing manoeuvres requiring more than an eighth of a turn of

the steering wheel.

The diameter of the steering wheel lies between 42 and 45 cm the case of motor

cars, whereas it is 50 to 55 cm in the case of commercial vehicles.

The rim of the steering wheel is elliptical in cross section with the finger indentations

on the under surface. The section of the rim is so designed and dimensioned to


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provide the driver a good grip both with and without heavy gloves. The lower portion

of the steering wheel hub fits into the upper flange of the steering column. Within the

steering column, the steering shaft exists and is free to rotate. On the inside

diameter of the steering wheel hub, female serrations are provided. On the upper

end of the steering shaft matching male serrations and a locking taper are provided.

The steering wheel is mated to the upper end of the steering shaft by means of the

locking taper and the serrations. The steering wheel is held on the steering column

and fixed to the steering shaft by a nut. The nut mates with the male threads on the

upper end of the steering shaft. Modern steering wheels have spring spokes. These

spokes are generally so arranged to give an unrestricted view of the instrument

panel. Some cars have a telescoping steering wheel. This wheel can be moved out

of or into the veering column to suit the drivers convenience. In some designs, the

hub of the steering wheel also houses part of the turn signal, horn, and vehicle

hazard flasher mechanism.

Steering column:

Steering column positions the steering wheel in the drivers cabin in relation to the

drivers seat and pedal controls. The steering column is made either as fixed or

adjustable. In the fixed type, the location of the steering wheel cannot be changed.

The steering wheel position is then decided taking into account the range of seat

position and driver size. On the other hand, if the steering column is made

adjustable, then the steering wheel movement can take care of the optimum wheel to

driver relationship in all seat positions. The fixed steering column is attached to the

cabin by brackets on the instrument panel and firewall. In some cases, a bracket on

the toe board is used for structural integrity. The fixed steering column is usually

tubular in construction. It has a stamped flange welded on to the upper end. A

bearing in the upper part of the steering column serves for cantering the steering

shaft. The type of bearing, bush or ball bearing or roller bearing provided for the

shaft affects the

steering effort. Usually no bearing is placed in the lower end of the steering column,

if the steering shaft has adequate support at the steering gear. However, when the

lower end of the steering shaft is terminated by a double cardan or constant velocity

joint, a bearing is provided at the lower end of the steering column. This bearing


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becomes essential for the proper functioning of the joint. The adjustable steering

column can be subdivided into adjustment by rotation and adjustment by sliding. The

former type is called tilt steering wheel column assembly while the latter type is

called collapsible steering column assembly.

Tilt steering wheel column assembly

In this unit, the steering wheel is pivoted about a point in the steering column. The

wheel can now be moved in a circular arc, forward and up or rearward and down.

The tilting wheel permits the driver to change the steering wheel angle to the

horizontal to suit his build. He can also change the position of the steering wheel to

the horizontal during a long drive to suit his driving posture. The tilting and

telescoping steering column can be seen in figure. All the above devices, wherein

the position of the steering wheel can be altered with respect to the driver have

locking mechanisms, which lock the steering wheel into the position selected. In one

design, the steering wheel and the column can be swung to one side. This makes

the driver to get into or out of the car easily. The tilt steering column has an interlock

to the transmission operating lever. This interlock is a safety feature. This

mechanism locks the transmission system, until the steering column is reset in the

driving position. It also prevents the steering column from being accidentally moved

when the car is in motion.


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Collapsible steering column assembly

This unit is a safety device. In the front end crash, the driver will be thrown forward

and on to the steering wheel. The steering column will now collapse in its length due

to this impact and absorb the possibility of the driver getting injured. The collapsiblesteering column is therefore called the energy absorbing steering column. The

Collapsible steering column assembly permits to maintain the optimum steering

wheel in all positions of adjustment.

There are two types of collapsible steering column assembly

the Japanese lantern type and

the tube and ball type.

In the Japanese lantern type, the flexible portion of the steering column folds up on

impact.

The tube and ball type has a stationary outer tube and a sliding inner tube. Grooves

are there in these tubes; these grooves form ball races for ball bearings. The outer

tube is attached to the fireball and instrument panel. The inner tube supports the

steering shaft and the steering wheel. On impact, the inner tube is forced into the

outer tube. Now the balls plough furrows in the tubes to permit the relative motion.

The movement of the steering column absorbs the shock. The ball and tube type is

claimed to give a more uniform collapse rate than the Japanese lantern type.

Steering shaft

The steering shaft assembly performs two important functions:

It transmits the drivers turning effort or torque from the steering wheel to the

steering gear.

It absorbs the angular and/or length changes in the relationship between the

steering wheel (chassis mounted) and steering gear (cab mounted) for the

following operating

Conditions: cab to chassis movement during driving, length change for adjustable

columns and cab on tilt cab models.


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On the non-adjustable column installations, the axial length displacement of the

steering shaft is usually achieved through the use of displacement characteristics of

a flexible coupling, pot joint or splined section in the shaft with cardan or constant

velocity joint.

Steering gear

The heart of the steering system is the steering gear. This unit is also called steering

mechanism. This unit is ordinarily fixed to the bottom of the steering column. This

unit is located between the steering shaft and the steerable stub axles which carry

the road wheels. The input shaft of the steering gear is operated by the steering

shaft.

The steering gear performs two functions:

It converts the rotary motion of the steering wheel into linear motion of the

steering linkage which moves the front wheels.

It introduces leverage between the steering wheel and the stub axles. This

leverage reduces the effort that has to be applied by the driver to the steering

wheel in order to overcome the frictional forces opposing the turning of the

stub axles and the road wheels.

In order to have the above leverage, the steering wheel has to be turned through

larger angles than the stub axles. In the case of automotive vehicles, normally the

road wheels are deflectable to about 500 on each side of the straight ahead position.

The extreme wheel positions are called full lock positions of the wheels. To effect

this extent of turning of the road wheels, the steering wheel may have to be turned

through from 4 to 9 or 10 times that angle. This relationship is called steering ratio.

The term steering ratio is the ratio of the 6 number of degrees of movement at the

hand wheel (steering wheel) which will produce one degree of movement of the front

wheels. The amount of leverage provided by the steering gear depends upon several

factors. The most important among them are the weight of the vehicle and the type

of tyre used. The typical gear ratios are 14:1 or higher. In heavy duty vehicles this

ratio is sometime as high as 30:1 to 35: 1. The greater the ratio, steering gear ratio,

the easier the steering wheel turns. Trucks are provided with higher leverage than


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cars. The steering gear incorporates another important feature called back locking.

The steering gear is so constructed that it is easy to turn the vehicle by the steering

wheel but it is difficult to turn the steering wheel by turning the front wheels. This

irreversible character of the steering gear prevents the bumps and shocks

experienced by the wheel at the road surface from being transmitted to the steering

wheel, but still give the driver the feel of the road. The steering gear is mounted to

the vehicle frame by bolts in the mounting pad of the steering gear.

Steering linkage

The steering linkage consist of pitman arm, ball joints, drag link, steering arm,

spindle, tie rod and king pin assembly.

Pitman arm It is also called the drop arm. It converts the output torque from the

steering gear into a force to the drag link. It is attached to the sector shaft of the

steering gear by a split joint. In this construction either full serrations or partial spline

is used to transmit the torque from the sector shaft to the pitman arm. The split arm

is tightened around the sector shaft by the clamping bolt to mate the male and

female serrations or splines. The end of the pitman arm which connects with the

drag link has a taper hole in it. The ball stud on the drag link is fitted into this hole.

Ball joints - are used on both ends of the drag link and the tie rod. These take care

of the angular displacement and rotational movement of the drag link and the tie rod,

which are caused by the front wheel rotation and suspension articulation.

Drag link - connects the pitman arm and the steering arm. In some cases, it is a one

piece forging with a ball joint socket formed in the end.

Steering arm - is usually a forged component and is attached to the steering

knuckle. It converts the drag link force into a turning moment about the left king pin.

The steering arm is attached to the spindle by a keyway, a locking taper and a nut.

The arm extends either to the front or rear of the spindle, depending upon the

package constraints and then bends to locate the steering arm ball joint at the


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correct geometric location. The end of the steering arm which connects with the drag

link has a tapered hole in it to accept the ball stud on the drag link.

Left spindle and king pin - The torque from the steering arm rotates the left

spindle, wheel and tyre about the king pin.

Left tie rod arm - The left tie rod arm is attached to the spindle in the same manner

as the steering arm, that is, key, taper and clamping nut. This converts the torque

available to turn the right wheel into a force in the tie rod. The tie rod of this link has

a tapered hole to accept the tie rod ball stud.

Tie rod - The tie rod is a tubular member which connects the left and right tie rod

arms. As such it transmits the force between these two components. The tie rod

ends have female threads. The ball joint shafts have mating male threads. The

threaded connections can be held together firmly by the locking clamps after the

proper length has been set. The length of the tie rod has to be adjusted so that the

front axle toe in will be to the specified amount.

Right tie rod arm, spindle and kingpin - The right tie rod arm is a mirror image of

the left. This converts the force from the tie rod into a moment to turn through the

knuckle arm, the right spindle wheel and the tyre about the king pin. The right spindle

and the king pin assembly is similar to the assembly on the left side except that it

has no steering arm attached to it.

Steering stops - In order to limit the angular deflections of the front wheels, stops

must be provided. The purpose of these stops is to avoid rubbing of tyres against the

frame or against the fenders which would cause undue wear and tear of the tyres.

These steering stops can be provided at two different places. First, they may be

placed in the path of motion of the steering arm or drop arm. Secondly, they may be

placed in the path of motion of the steering knuckle.


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Steering gears:

If the steering wheel is connected directly to the steering linkage it would require a

great effort to move the front wheels. Therefore, to assist the driver, a reduction

system is used having a movement ratio between 10:1 to 24:1 the actual valuedepending upon the type and weight of the vehicle. But the power steering reduces

the ratio on an average by 20 percent. The low gear ratios produce fast steering,

while the high ratios produce slow steering. When the mechanical advantage of the

linkage between cross shaft and stub axles is considered then this ratio is increased

from 15 to 20 percent and is called overall steering ratio. The steering gear is a

device for converting the rotary motion of the steering wheel into straight line motion

of the linkage with a mechanical advantage. The steering gears are enclosed in a

box, called the steering gear box. There are many different designs of steering gear

box. They are as follows:

a) Worm and wheel steering gear

b) Worm and sector steering gear

c) Cam and lever / peg steering gear

d) Cam and roller or worm and roller steering gear

e) Worm and nut or screw and nut steering gear

f) Recirculating ball steering gear

g) Rack and pinion steering gear.


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Worm and wheel steering gear:

The system consists of worm wheel which is carried in bearings in a cast iron case.

The case is made in halves. The outer end of the worm wheel is fixed to a drop arm

which is having ball end to connect the side rod. The side rod is connected to thesteering arm which is fixed to the stub axles. The worm which is keyed on to a

steering shaft has a mesh with the worm wheel. The steering wheel is mounted at

the upper end of the steering shaft.

When driver rotates the steering wheel then drop arm moves either backward or

forward direction. This motion results in motion of the stub axles.

Worm and sector steering gear:

This is the modified form of steering wheel type, in which the wheel is being replaced

with sector of wheel. In actual case, the worm wheel is not essential as it is having

only partial rotation. Hence in this type only a sector of wheel is used instead of

worm wheel.


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Cam and lever/ peg steering gear:

The example of the cam steering gear is shown in the simplified sketch. In this

system helical groove is formed at the bottom end of the steering wheel shaft. The

helical groove engages the projected pin of the drop arm spindle lever. The drop-armis made rigid with the lever (sometimes referred as peg) by

a splined spindle. The to and fro motion is obtained at the drop-arm when the

steering wheel shaft is turned. This motion results the turning of the stub axles. The

end play of the steering wheel shaft can be adjusted by putting a suitable washer at

the lock nut. The meshing of the projected pin in helical groove is also adjusted by a

screw provided at the end of the lever spindle. In the recent models, the projected

pin is made in the form of a roller. The projected pin may be one or two in number,

accordingly they are referred as cam and single lever or double lever steering gear

mechanism.


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Warm and roller steering gear

The type of steering gear is shown in the sketch where a two toothed roller is

fastened to the cross-shaft so that it meshes with the threads of the warm gear. The

worm gear is formed on the bottom end of the steering wheel shaft. Worm isfastened between the two ball bearings in the casing. Play of the bearings can be

adjusted by an adjuster provided at the end of the casing. The outer end of the

cross-shaft is formed in the spindle to fix the drop arm.

When the worm gear is turned by the steering wheel shaft, it causes the roller to

move in an arc so as to rotate the cross shaft and at the same time turn on the roller

pin connecting it to the cross-shaft. The casing of the system is fixed with the column

and generally bolted to the frame. Similarly there can be other design of steering

gears which may use one or- three-tooth rollers.


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Screw and nut type steering gear or worm and nut type steering

gear

In this system a screw or worm is formed at the lower end of the steering shaft and

the upper end is fixed to the steering wheel. The nut consists of integral

trunnions which pivot in the holes of the arms of the fork. The fork is connected to

the drop arm by a splined shaft. The upper end of the steering shaft is supported in

the steering column by a ball and socket joint so that the shaft may swung slightly.

The swing of the shaft is essential because the trunnions of the nut move in arc

when the nut moves along the axis of the shaft. Sometimes instead of ball and

socket joint an ordinary journal bearing supported in a rubber bush is used at the

upper end of the steering shaft because the rubber accommodates the rocking of thesteering shaft. This mechanism is very cheap and reduces the number of the

bearings required.


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Recirculating ball type Steering Gear

It consists of a worm at the end of steering rod. A nut is mounted on the worm with

two sets of balls is the grooves of the worm, in between the nut and, the worm. The

balls reduce the friction during the movement of the nut on the worm. The nut has anumber of with on the outside, which mesh with the teeth on a worm wheel sector,

on which is further mounted the drop arm, which steers the road wheels through the

link rod and the steering arms. When the steering wheel is turned, the balls in the

worm roll in the grooves and cause the nut to travel along the length of the worm.

The balls, which are in 2 sets, are recirculated through the guides, as shown in the

figure. The movement of the nut causes the wheel sector to turn at an angle and

actuate the link rod through the drop arm, resulting in the desired steering of the

wheels. The end play of the worm can be adjusted by means of the adjuster nut

provided. To compensate for the wear of the teeth on the nut and the worm, the two

have to be brought nearer bodily. To achieve this, the teeth on the nut are made

tapered in the plane perpendicular to the plane of Figure. A screw is also provided by

means of which the drop arm, aid hence, the wheel sector can be positioned along

its axis. When the wheel sector has to be moved bodily closer to the nut to eliminate

backlash due to wear, the screw is turned which slides the wheel sector in a direction

in which the tapered teeth on the nut are narrower, till the required adjustment is

achieved.


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Rack and pinion steering mechanism

It is very simple and common type mechanism, the system is shown in simplified

sketch. This type is very well suitable in an independent suspension system. The

system consists of a rack housed in a tubular casing. The casing is supported on theframe near its ends. The ends of the rack are connected to the track rods with the

help of ball and socket joints. The pinion shaft is carried in the plain bearings housed

in casing. The pinion is meshed with the rack and the clearance is adjusted with the

adjusting screw. When the pinion is given rotary motion with the steering wheel, then

the rack slides in either sides. This sliding motion of the rack is used through the

track rods to turn the wheels in desired side.


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6. POWER STEERING

Principles of the Power steering

Power steering has two types of device for steering effort one type is a hydraulic

device utilizing engine power. The other type utilizes an electric motor. For the

former, the engine is used to drive a pump. For the latter, an independent electric

motor in the front luggage compartment is used the pump. Both develop fluid

pressure, and this pressure acts on a piston within the power cylinder so that the

pinion assists the rack effort. The amount of this assistance depends on the extent of

pressure acting on the piston. Therefore, if more steering force is required, the

pressure must be raised. The variation in the fluid pressure is accomplished by a

control valve which is linked to the steering main shaft.

Neutral (Straight-ahead) position:

Fluid from the pump is sent to the control valve. If the control valve is in the neutral

position, all the fluid will flow pass through the control valve into the relief port and

back to the pump. At this time, hardly any pressure is created and because the

pressure on the cylinder piston is equal on both sides, the piston will not move in

either direction.

While turning:

When the steering main shaft is turned in either direction, the control valve also

moves, closing one of the fluid passages. The other passage then opens wider,

causing a change in fluid flow volume and, at the same time, pressure is created.

Consequently, a pressure difference occurs between both sides of the piston and the

piston moves in the direction of the lower pressure so that the fluid in that cylinder is

forced back to the pump through the control valve.


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Straight ahead condition

Fig. While taking turn


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There are two kinds of power steering currently in use

a) Integral power steering and

b) Linkage booster power steering

Integral power steering

The figure shows the arrangement of integral power steering when the vehicle

moves straight ahead on the road. In this system the oil pump is driven by a belt

from the engine crank shaft pulley. The system consists of solid cylinder on whichtwo grooves have been cut, known as valve spool, which slides closely within the

hole in the valve housing. The housing has three internal grooves the central groove

is connected to the pump and two at ends are connected to the reservoir. The two

additional openings from the internal collars are connected to the two sides of the

cylinder as shown in the Figure. When the valve spool is in the position shown in the

Figure, then the pump delivers the oil in the central part of the housing and then

delivers back to the reservoir by the passages shown by the arrows. In this position

there will be no oil pressure in the cylinder and there is no tendency for the piston to


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slide in any direction. Thus there is no steering action and vehicle moves straight

ahead.

Similarly refer the above figure when the valve spool is moved towards right side

then the direct return line from the pump to reservoir is closed. The oil now flows into

the cylinder by the right side passage and pushes the piston to slide left ward as

shown by the arrow in the Figure. The oil on the left side of the piston is discharged

to the reservoir thro the valve housing under this position. This outward move of the

piston rod results to turn the vehicle tow left side on the road. Similarly the vehicle

can be turned to right side by reversing the steering operation.

Linkage-booster power steering

In this type power assistance is applied directly to the steering linkage. The power

cylinder consists of piston and the piston rod is extended out on the right and is fixed

to the frame member. The relay rod is linked with the cylinder housing. In the neutral

position the spool valve is held in the centre position by the centering springs. In this


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position oil from pump flows to both sides of the piston in the power cylinder at equal

pressure and then there will not be displacement in the power cylinder thus there will

no steering action. In this position the vehicle moves straight ahead on the road.

Again when the steering wheel is turned anticlockwise as shown, then the ball of the

pitman arm shifts the valve spool towards right side. Due to this shifting, the oil from

pump flows in the valve section of the unit, through the ports. Then the oil through

feed line flows into the right hand side of the power cylinder. The high pressure oil

inside the cylinder, forces it to move to the right which results to turn the stub axles

to the left side. For the right side of the vehicle this operation of the system is

reversed to it.


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7. Reversible and Irreversible Steering

When deflection of the steered wheels due to road surface is transmitted through the

steering linkage and steering gear box to the steering wheel, the system is said to be

reversible, if every small imperfection of the road surface causes the steering wheel

to rotate, the driver would find much tiring and frustrating. Such reversibility is not

desired. Some degree of reversibility is desired so that the wheels will find to

straighten up after negotiating a bend. Some degree of irreversibility is desired to

stop shocks sustained by the road wheels. Such a steering system is known as

semi-reversible. When the steered wheels do not transfer any deflection to the

steering wheel, the steering system is said to be irreversible. It would not tend tostraighten out after negotiating a turn, and would not easily follow the course of a

gutted road without undue stress on the mechanism. Therefore, in most of the

passenger cars semi-reversible steering gears are used.


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8. Under-steering and Over-steering

While taking a turn, the wheels are not always pointing in direction in which the

vehicle is moving, due to the distortion of tyre tread. The angle between the wheel

inclination and the path taken by the wheel is known as slip angle. When the angle is

greater at the rear than at the front, the vehicle tends to oversteer, that is to turn into

the curve more than the driver intended. When the slip angle is smaller at the rear

than at the front, the vehicle tends to understeer. Of course, the understeer is

opposite to oversteer and is preferred because correction by the driver involves

rotating the steering wheel a little more in the direction of the turn. It is to be noted

that the slip angle is affected by the road camber, side winds, tyre inflation and

variations in the load on either the front or rear axle


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9. ELECTRIC POWER STEERING SYSTEM

The electric power steering system combines a mechanical steering system

with an electronically controlled electric motor to a dry power steering. The hydraulic

system, which so far delivered the steering boost, is substituted by an electrical

system. For this, a torque sensor measures the steering wheel torque and an

electronic control unit calculates the necessary servo torque. This is delivered by an

electric motor in such a way that the desired torque curve at the steering wheel is

created. Depending on the necessary steering forces the electric motor engages by

a worm gear at the steering column or at the pinion and for high forces directly at the

rack by a ball-and-nut gear. In figure 4 the pinion-solution is represented, which is

intended for middle class vehicles. The components involved in the electrical powersteering are besides the mechanical steering components: Electric motor, electronic

control unit, power electronics, steering wheel torque sensor and CAN data bus to

other systems. The electrical power steering system offers large benefits compared

to the hydraulic power steering. Apart from about 80% lower energy consumption the

omission of the hydraulic fluid increases the environmental compatibility. The

electrical power steering is delivered to the car manufacturer as a complete system

module ready-to install. The adaptation of the servo power assistance to certain

vehicle types as well as the modification of the control strategy dependent on

different parameters and vehicle sizes is easily and rapidly feasible.

From the safety point of view as with the other power steering systems

due to failures in electrical components, again the steering boost can be impaired,

here by faults of components of the electrical servo system. The steering systems

unintentional self-activity as well as too strong steering boosts is to be concerned as

new potential safety critical effects, which must be avoided by appropriate

countermeasures.


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10. FUTURE STEERING SYSTEMS

The main feature of future steering systems is the missing direct mechanical

link between steering wheel and steered wheels. With such a steer-by-wire steering

system Fig. 3.1 the missing steering columns function must be reproduced in both

directions of action. In forward direction the angle set by the driver at the steering

wheel is measured by a steering angle sensor and transferred with the suitable

steering ratio to the wheels. In reverse direction the steering torque occurring at the

wheels is picked up via a torque sensor and attenuated respectively, modified fed

back to the driver as a counter torque on the steering wheel.

Figure 3.1 Principle illustration steer-by-wire

First, steering wheel module and steering module are implemented with

familiar components of mechanical and electrical steering systems, like: Steering

wheel, gearbox, electrical motors, rack. The operational principle is, however, in

principle open for more futuristic designs like side stick operation on the drivers side

and single wheel steering on the wheel side. While in systems with mechanical

connection in the case of electrical errors only the steering boost is concerned,

corresponding measures must be taken with steer-by wire systems that in case of

any electrical failure steering control is always guaranteed.


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11. ESSENTIAL COMPONENETS OF ELECTRIC

POWER STEERING SYSTEM

Details of electric power steering system designs differ amongst manufacturers;

however there are certain components that are intrinsic. These are:

Torque sensor

Electric motor

Rotational angle sensor

Controller

Vehicle speed sensor

Coupling between motor and steering mechanism

The torque sensor is perhaps the most important component; it measures the effort

being applied by the driver to steer the vehicle. The torque sensor output is then

used to drive a motor to reduce the effort, while achieving the desired steering. The

motor may be located at a number of locations to achieve this. The purpose of the

motor controller is essentially to control the torque delivered to the steering

mechanism. The vehicle speed must be used to adjust the sensitivity of the torque

controller. The angle of rotation of the steering wheel must also be used to adjust the

sensitivity and the performance around the null position of the wheel.

Electromechanical specifications of a typical pinion type P-EPAS system from Koyo

are reproduced in following table:


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Items Specifications

Rake force 7747 NRake stroke 144 mm

Stroke ratio 45.335 mm/rev.

Rack & pinion

Module

Number of teeth

2.3

6

Reducer

Type

Reduction gear ratio

Worm & resin wheel

15.1

Motor

Type

Rated voltage

Rated current

Rated torque

Rated rotational speed

Brushed DC motor

12 V

65 A

3.4 Nm

1.180 rev/min

Table: Specifications of a typical EPAS system


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12. ADVANTAGES OF STEER-BY-WIRE SYSTEMS

Steer-by-wire is a universal actuator for automatic steering intervention. For vehicle

dynamic steering intervention a steering angle actuator is needed which does not

affect the steering wheel while rapidly correcting the vehicle wheels. On the other

hand, a torque actuator will be needed for automatic lateral guidance interference

and future steering systems of autonomous driving, thus imparting a superimposed

torque onto the steering wheel and letting the driver with that know the intended

direction, evaluated by the lateral guidance control system.

Steer-by-wire meets both requirements ideally. Along with "drive by wire and

"brake by wireit provides the condition to materialize vehicle dynamics and comfortoriented automatic controls in one system.


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13. SYSTEM STRUCTURES OF SAFE ELECTRICAL

STEERING SYSTEMS

1. SYSTEM STRUCTURE OF ELECTRICAL POWER STEERING

FUNCTIONAL DESCRIPTION

In an electrical power steering system the steering torque initiated by the

driver Fig. 4.1 is measured by a steering wheel torque sensor and is fed into an

electronic control unit. The latter then calculates along with the driving speed a

reference torque for the steering motor, which, however, can optionally also depend

on the steering angle and steering angle velocity. By means of the calculated

reference torque the currents of the steering motor are actuated. Fig. 4.1 shows the

pinion-type realization, where at the pinion the electrical torque is superimposed to

the torque initiated by the driver. In further versions both torques can be

superimposed either on the steering column or on the rack. In case of a failing

electrical component of this steering system the non-boosted mechanical

intervention by the driver is maintained.

SAFETY FEATURES

The systems fail-safe behaviour concerning electrical faults is

accomplished by detecting and evaluating all electrical failures. In case of major

electrical faults the electrical power steering system is switched off. Sensor failures

or failures in the electronic control unit might be considered as an example, resulting

in an unintentional self-activity of the steering or in a too strong steering boost. Risks

of that kind are avoided by an effective monitoring strategy where failures are

detected on time and the power steering system is switched-off. One detection

method for this constitutes checking sensor signals and motor currents for plausible

system conditions on a second path.


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Figure 4.1 System structure of Electric Power Steering


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2. SYSTEM STRUCTURE OF STEER-BY-WIRE SYSTEM

WITH HYDRAULIC BACKUP


The steer-by-wire system with hydraulic backup is shown in Fig.4.2. The

system consists of components at the steering wheel and at the vehicle wheel level,

an electronic control unit and a hydraulic backup. Steering wheel motor and sensors

for the steering wheel angle and the steering wheel torque are arranged at the

steering wheel. These components identify the drivers desire and reproduce the

return forces to the steering wheel, which are transferred to the steering wheel by

conventional steering systems. These feedback forces are important to gain a safe

feeling while driving. At the vehicle wheels side the system consists of an electricmotor directing the mechanically coupled wheels via a gear and a rack, and of

sensors to measure angles and torques. The electronic control unit registers

periodically all sensor values, processes them via efficient control algorithms and

supplies the control signals to actuate the motors. Via a serial data bus, the

electronic control unit communicates with a vehicle guidance unit, which coordinates

the superior steering interventions, e.g. to improve vehicle dynamics. This unit at the

same time constitutes an interface to the driver information system, and to additional

control units for engine and brakes.

The control unit in Fig. 4.2 is presented as a central control unit. It can also be

divided into two modules arranged close to the steering wheel and steering motor,

and connected to a data bus system for communication. A closed hydraulic unit,

consisting of a hydraulic pump at the steering wheel and a plunger on the vehicle

wheel level, constitutes the backup. Both sides of these components are connected

with each other by hydraulic lines. During normal operation the plunger is bypassed.

In case of failure, the fail-safe switching valve actuated by the electronic control unit

will close the bypass. Thus, via the hydraulic backup, the steering actuator can be

operated by means of the steering wheel. Without electric current, the switching

valve must be closed. In case of failure of the 42V vehicle electrical system thus the

hydraulic bypass is automatically closed and the backup safely activated. If the

steering wheel motor can still be controlled during backup operation it can be

adequately actuated to support power steering. The increased pressure needed to

operate the hydraulic backup is provided by means of a small pressure reservoir with


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check valve. This pressure accumulator compensates the leakage, which occurs

during the vehicle lifetime. The pressure within the backup level is continuously

monitored by a pressure or displacement sensor.

Figure 4.2 System structure of steer-by-wire system with hydraulic backup

SAFETY FEATURES

The systems fail-safe behaviour concerning electrical faults is

accomplished by detecting and evaluating all electrical failures. According to the

respective importance of the fault the functionality of the system is reduced. In case

of major electrical faults the electrical steering system is completely switched off and

the switching valve is safely actuated, establishing a firm hydraulic link between

steering wheel and the vehicle wheels. On the hydraulic backup level vehicle

dynamic intervention is no longer possible.


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3. SYSTEM STRUCTURE OF THE PURELY ELECTRICAL

STEER-BY-WIRE SYSTEM


Fig. 4.3 shows the structure of a purely electrical steering system. The

reduced safety by omitting steering column and hydraulic backup is compensated by

higher demands on the safety structure of electrical and electronic components.

Again, the system consists of components at the steering wheel, on the vehicle

wheel level, and it comprises a control unit and a 42V vehicle electrical system. In

this case this must be implemented as a safe 42V vehicle electrical system

containing additional elements for the diagnosis of charge condition, as well as for

the disconnection of batteries.

Figure 4.3 System structure of purely electrical steer-by-wire system

The steering wheel motor and sensors indicating steering wheel angle and

steering wheel torque are arranged at the steering wheel. These components identify

the drivers desire and reproduce the return forces transferred to the steering wheel.

For a safe acquisition of steering wheel position two redundant steering angle

sensors are used. Power stage and power supply for the steering wheel motor are


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likewise redundant. In order to exert a return force on the steering wheel in case of a

defective steering motor a torsion spring is available to generate the return torque.

Optionally a second steering wheel motor can be used in order to redundantly

generate the return torque. On the vehicle wheel level the system is equipped with a

redundant set of electric motors and redundant sensors measuring angles and

torques.

The electronic control unit is designed fail-safe in terms of redundant

power supply, signal processing and power actuation. Sensor values are identified

periodically and redundantly, further processed via matched control algorithms and

the calculated actuation signals are supplied to the two steering motors as well as

the steering wheel motor. As to the link between the electronic control unit and thevehicle guidance unit as well as dividing the functions of these components to the

decentralized units the explanations are in accordance with what has been described

earlier referring to the steer-by-wire system with hydraulic backup.

SAFETY FEATURES

Failure tolerance is required in these areas: sensors, electronics,

actuators, vehicle electrical system and data transmission. This is accomplished by

appropriate redundant structures. The fail-safe behaviour against electric faults is tobe ensured by a complete detection and locating of all electric failures. Locating a

defective channel during signal detection or signal processing requires majority

decisions. The needed redundancy is achieved either by hardware components or

by including additional processing variables of the same kind. The defective channel

is then switched-off consequently. In spite of electrical faults both steerability and

vehicle dynamic interventions are ensured on account of the redundant system

structure.


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14. CONCLUSION

This paper presents various types of electrical steering systems and their safety

aspects. The electro hydraulic power steering does no longer operate the hydraulic

pump via a V-belt drive from the internal combustion engine. Rather, an electric

motor is used, yielding energy savings and flexibility of installation. Electrical power

steering pursues this trend and offers additional advantages since no hydraulic

system is required. A steer-by-wire system with hydraulic backup and a purely

electrical system were discussed.

Future innovative steering functions, such as vehicle dynamic interventions, collision

avoidance, individual wheel steering, tracking assistance, automatic lateral guidance,

and finally autonomous driving functions will be implemented in a system compound

of various vehicle systems. Future steering systems will thus have to be integrated

into a system compound, in terms of interfaces and functions. The steer-by-wire

principle becomes absolutely necessary when those innovative functions are to be

achieved. The transition to purely electrical steering systems will proceed step by

step, both for safety reasons and acceptance by the customer. The path will lead

from electrical power steering via a steer-by-wire system with a hydraulic or

mechanical backup towards purely electrical steer-by-wire systems.


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REFERENCES

1. Automobile Engineering By Kirpal Singh, Vol 1, Vol 2.

2. Wikipedia

3. SAE TECHNICAL PAPER SERIES

4. A. Badawy, F. Bolourchi, & S. Gaut; ESteer system Redefines Steering

Technology; Automotive Engineering pp. 15-18; September 97.

5. Daimler-Chrysler: Forschung und Technologie: Steer-by-wire, Neuartige

Assistenzsysteme, Mobiler Arbeitsplatz, Internetsite http://

www.daimlerchrysler.de/investor/annual98/ fue1_g.htm 3/99.

Documents

Electrical Steering System in the World