wheel steering control

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PEMP

AEL2501

Wheel Alignment, Steering Systems and Wheel Alignment, Steering Systems and Steering ControlSteering Control

Lecture delivered by:Lecture delivered by:

M.S Ramaiah School of Advanced Studies - Bangalore1

Prof. Ashok Prof. Ashok C.MetiC.MetiMSRSASMSRSAS--BangaloreBangalore

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AEL2501

Session Objectives

• To study various wheel alignment related issues

• To understand the steering system in an automobileautomobile

• To study the components of steering systems and their configurations

• To identify the need for the electronic control in power steering and its importance.

T d ki f H d li i d


• To study working of Hydraulic power steering and EPS

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Topics

• Wheel alignment

• Steering systems

• Power assisted steering systems

• Hydraulic Power Assisted Steering

• EPS


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Wheel alignment

• At today’s increased driving speeds, it most important to have swift and positive response from the steering system.

• To accomplish this, the wheels must be in alignment.

• It allows the wheels to roll without scuffing, dragging, or slipping on different types of road conditions.

• Proper alignment in both front and rear wheels ensures-– Greater safety in driving

– Easier steering

– Longer tire life


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– Reduction in fuel consumption

– Less strain on steering system and suspension components

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The alignment geometry• The proper alignment f a

suspension/ steering system depends on the accuracy of thedepends on the accuracy of the following:

– Caster

– Camber

– Toe

– Thrust line alignment

– Steering Axle Inclination (SAI)


(SAI)

– Turning radius

– Tracking

– There are host of gauges and electronic equipment to check and adjust these parameters.

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AEL2501

• Caster– Is the angle of the

steering axis of a wheel gfrom the vertical, as viewed from the side of the vehicle

– It designed to provide steering stability

– Caster is not related to


Caster is not related to tire wear

Camber AND Castor Gauge

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• Camber– It is the angle represented

by thetilt of either the front or rear wheelsfront or rear wheels inward outward from the vertical as viewed from the front of the car

– It is designed to compensate for road crown passenger weight


crown, passenger weight, and vehicle weight

Camber and Castor Gauge

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AEL2501

• Toe– It is the distance

comparison between the leading edge and trailingleading edge and trailing edge of the front tires

– If the leading edge distance is less, then there is toe-in

– If greater, it is toe-out

– Toe is critical as a tire wearing angle


–– Wheels that do not track Wheels that do not track straight ahead have to drag straight ahead have to drag as they travel forwardas they travel forward

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• Thrust Line Alignment– The vehicle must run

straight down the road with the rear tires trackingthe rear tires tracking directly behind the front tires when steering wheel is in the straight-ahead position

– The geometric centerline of the vehicle should parallel the road direction


the road direction

– If not, the vehicle tends to the vehicle tends to travel in the direction of the travel in the direction of the thrust line, rather than thrust line, rather than straight aheadstraight ahead

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• Steering Axle Inclination (SAI)– It locates the inside or

outside of the verticaloutside of the vertical centerline of the tire

– Is the angle between true vertical and a line drawn between the steering pivots as viewed from the front of the vehicle

– The SAI helps the vehicle’s SAI helps the vehicle’s steering system return tosteering system return to


steering system return to steering system return to straightstraight--ahead after a turnahead after a turn

– SAI angles• FWD 12-18 degrees• RWD 6-8 degrees

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• Turning Radius– Is the amount of toe-out present in

turns– It is also called ‘toe-out on turns’

‘ i l ’or ‘turning angle’– As the car goes round the corner,

the inside tire must travel in a smaller radius circle than the outside tire

–– This is accomplished by designing This is accomplished by designing the steering geometry to turn the the steering geometry to turn the inside wheel sharper than the inside wheel sharper than the outside wheeloutside wheel


– This toe-out eliminates the tire scrubbing on the road surface by keeping the tires pointed in the direction they have to move

Turning radius is affected in turns

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• Tracking– All vehicles are built around a geometric centerline that runs

through the center of the chassis from the back to the front

A id l li h ll 4 h l ll l h li– An ideal alignment has all 4 wheels parallel to the centerline making the thrust line parallel to the centerline (tracking)


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Wheel alignment equipment


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Steering System• The purpose of the steering system is to turn the front turn the front

wheelswheels(normally)(normally). . • The wheels constantly change direction, while switching

l di h t d h idi dlanes, rounding sharp turns, and when avoiding roadway obstacles.

• The manual steering is consisting of –– Steering linkages– Steering gear– Steering column and wheel

Working:Working:


Wo g:Wo g:• As the steering wheelsteering wheel is turned by the driver, the steering steering

geargear transfers this motion to the steering linkagessteering linkages.• The steering linkagesteering linkage turns the wheelswheels to control the

vehicle’s direction.

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• Steering Linkage: – It is the system of pivotspivots and connecting partsconnecting parts placed between the

steering gearsteering gear and the steering armssteering arms that are attached to the frontfront or rear wheelsrear wheels that control the directiondirection of the vehicle travel.

– It transfers motion of the steering gear output shaftoutput shaft to the steering steering armsarms, turning the wheels to maneuver the vehicle.

– Parallelogram Steering linkage:• Provides good steering and suspension geometry• Used in conjunction with IFS• Can be configured for placing behind or ahead of the front-wheel

suspension• Uses components like


• Uses components like-– Pitman Arm– Idler Arm– Links (Center, drag, or steering links)– Tie-rods

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• Pitman arm– Connects the linkage to the steering column through a steering gear

located at the base of the column

i h i i i f h li k i– Transmits the motion it receives from gear to the linkage, causing the linkage to move left or right to turn the wheels

• Idler arm– It is normally attached on the opposite side of the center link, from

pitman arm and the car frame, supporting the center link at the correct height

– A pivot built into the arm or assembly permits sideways movement


p y p yof the linkage

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• Links– Depending on the design application, can be referred to as –

• Center link• Drag linkDrag link• Steering links

– Their purpose is to control sideways linkage movement, which changes the wheel directions

• Tie rods– Are usually assemblies that make final connections between the

steering linkage and steering knuckle– They consists of-


They consists of• Inner tie-rod ends, which are connected to the opposite sides of the

center linkage• Outer tie-rods, which are connected to the steering knuckle• They also have provision for varying lengths for correct settings

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• Steering Gear box– The purpose of the steering gear is

to change the rotational motionrotational motion of the steering wheelsteering wheel to athe steering wheelsteering wheel to a reciprocating motionreciprocating motion to move steering linkage.

– Different style of steering gear boxes used-

• Recirculating ball steering gear

• Worm and roller type steering gear box


gear box

• Rack and pinion type

– A recirculatingrecirculating ballball type is generally found in large cars.

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• Rack and Pinion– It is lighterlighter in weight and has fewfew

components

– Rack • Is a toothed bar contained in a metal

housing

• The rack is similar to the parallelogram center link in that its sideways movement in the housing is what pulls or pushes the tie-rods to change wheel directions

– Pinion


• It is a toothed or worm gear mounted at the base of the steering column assembly where it is moved by steering wheel

• The pinion gear meshes with the teeth in the rack so that the rack is propelled sideways in response to the turning of the pinion

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– Steering inputinput is received from the pinion gearpinion gearattached to the steering column. This gear moves a toothed racktoothed rack that is attached to the tietie--moves a toothed racktoothed rack that is attached to the tietierodsrods.

– The movement of the rack pushespushes or pullspulls the tie-rods to change the wheel’s direction.

– The driver gets greater feeling of the road with rack & pinion as this assembly has less friction


rack & pinion as this assembly has less friction points. (reduced isolation and vibrations damping)

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• Steering wheel and column– The role of the steering wheel and column is to produce

the necessary forceforce to turn the steering gear.y g g

– The steering wheel is used to produce the turning effortturning effort.

– Types of steering columns: • Fixed column

• Telescopic column

• Tilt column, and so on…

The steering column is made collapsiblecollapsible that allows the


– The steering column is made collapsiblecollapsible that allows the column to fold into itself , upon impactimpact.

– The driver’s side Air bag assemblyAir bag assembly is contained in the center portion of the steering wheel.

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Power Assisted Steering– It is designed to reducereduce the amount of efforteffort required to turn the

steering wheel.– It also reduces the driver fatiguedriver fatigue on long drives and makes it easiereasier

to steer the vehicle at low road speeds, particularly while parkingparking..– At higher steering efforts are required at higher speeds to provide

the improved down-to-road feel.–– Two design arrangementsTwo design arrangements:

• Conventional– Hydraulic power is used to assist the driver. Some of the variants:

» Integral piston linkage» Power assisted rack and pinion » External piston linkage (older version)


• Electronically controlled– An electrical motorelectrical motor and an electronic controlelectronic control are used to provide the

assistance.

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Variable-assist steering

• Application of electronic control technologyelectronic control technologyto vehicle steering systemvehicle steering system is still in the d l t tdevelopment stage.

• Several techniques have been suggested by several designers and implemented on vehicles by manufacturers.


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Fundamentals of Electronically controlled power steering:• Electronically controlled steering:

– Improves steering feel – Improves power-saving effectiveness – Increases steering performance.

• The intent of an electronic control is to-–– ReduceReduce the steering effort when driving at low speedslow speeds– Supply feedback for the appropriate steering reaction force when

driving at high speedshigh speeds.


g g pg p

• The vehicle speed sensorsvehicle speed sensors are used to sense the vehicle speed in order to make smoothsmooth and continuous changescontinuous changes in the steering assist ratesteering assist rate under conditions ranging from steering maneuvers at zero speedzero speed to those at high speeds.high speeds.

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Electronically controlled Hydraulic System:Electronically controlled Hydraulic System:• The system consists of –

– A linear solenoid valve– A vehicle speed sensor– Other electronic devices as part of hydraulic system

• Based on the signals from the speed sensor, the flowflow and pressure pressure of the hydraulic fluid is controlled by means of the operation of the solenoid valvessolenoid valves.


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AEL2501

Flow Control Method:

• Solenoid valve is located at the pump discharge pump discharge port.port.

• Controller regulates the solenoid valve openingopening at high speeds to reducereduce the pump dischargepump discharge volume, thus increasing the


required steering effortsteering effort.

Vehicle Speed-responsive pump discharge flow volume control type

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Cylinder Bypass Control Method:

• A solenoid valvesolenoid valve and a bypass linebypass line are located between both chambers of the power cylinderpower cylinderbetween both chambers of the power cylinderpower cylinder.

• The opening of the valve is extended by electronic electronic controlcontrol equipment in accordance with increase in vehicle speed, thus reducing reducing the hydraulic pressure in the power cylinder and increasing the steering effortsteering effort.


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AEL2501

Hybrid System:• Utilize a flow control methodflow control method in which the hydraulic power hydraulic power

steering pumpsteering pump is driven by an electric motorelectric motor.• The steering effort is controlled by controlling the rotatingrotating

speed of the pumpspeed of the pump.• The drive efficiency of the generator and motor are low

compared to that of the hydraulic pump, which is driven by the vehicle engine.

• But because any residual flow is not discharged, the power power lossloss is lowerlower than that of the engine pumpengine pump when driving at


lossloss is lowerlower than that of the engine pumpengine pump when driving at high speedshigh speeds.

• There is large degree of freedomlarge degree of freedom in mounting locationmounting locationselection.

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Driving Mode Response method:

• Control system consists of –– Vehicle speed sensor

– Steering angular velocity sensor

El t i t l it– Electronic control unit

– Motor driven hydraulic pump

• Driving conditions are automatically judged, and the pump flow rates is controlled in accordance with this condition in order to provide

Driving mode responsive-type hybrid power steering


condition in order to provide the appropriate steering effort for the driving conditions.

Driving mode and pump flow rate

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AEL2501

Steering wheel speed responsive Method:• System consists of components such as-

– Vehicle speed sensor– Steering wheel angular velocity sensor– ECU– Motor driven hydraulic pump

• The pump flow is controlled in accordance with the angular angular velocityvelocity of the steering wheelsteering wheel and vehicle speedvehicle speed

• The discharge flow volumeflow volume of the pump is reduced reduced and the steering response dropssteering response drops when the vehicle is driven at high high speedsspeeds


speedsspeeds• In this system, therefore, the speedspeed of the motorof the motor is increasedincreased

in accordance with the detected angular velocityangular velocity of the steering wheel in order to increase the discharge flow flow volumevolume to solve the problem.

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AEL2501Steering Speed-responsive-type hybrid power steering


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Basic Control

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Electric Power Steering (EPS):

• Electric power steering is a fully electricfully electric systemsystem which reducesreduces the amount of steering effortsteering effort by directly applying the outputoutput from an electric motorelectric motor to the steering systemsteering system.

• The system consists of:– Vehicle speed sensor

– A steering sensor (torque, angular velocity)

– ECU

– Drive unit


– Motor

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Structure of EPS System (Rack assist-type ball screw drive)

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• The signals outputs from each of the sensorssensors are input to the ECU ECU where necessary steering assistancesteering assistance is calculated and applied by the drive unit to control the operation of the motormotor.

• In this system, setting range for the steering effortsteering effort is large and also because it is possible to supply only the amount of power that is necessary when the steering wheel is turned, a large reduction in power requirements can be effectively achieved with no power losses.


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Column shaft drive method

Pinion shaft drive method


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•• RackRack--andand--pinionpinion steering mechanisms, the EPS system applies the motor powermotor power to the pinion gear shaftpinion gear shaft or to the rack shaftrack shaft.

• Several reduction gearsreduction gears are employed to increase the torquetorque of the motormotor.

• The maximum amount of assist, the smoothness of steering feel and the degree of noise level during steering are the determined by power transmission systems.

• In general, it is possible to obtain a greater amountgreater amount of assistassist


g , p ggfrom the rack assistrack assist method than from pinion assist pinion assist method.

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AEL2501

Sensors:

• The EPS uses variety of sensors to control the motor.– Torque sensor – detects the steering effort

– Wheel angular sensor – determines angular velocity of the steering wheel

– Battery sensor – determines the battery voltage

– Current sensor – determines the motor current & battery current


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AEL2501

Torque sensors:

• Pinion shaft in the rack-and-pinion steering mechanism is divided into two sectionsmechanism is divided into two sections.– Input shaft

– Pinion gear

• It consists of a torsion bar that connects the two sections.

• A differential transformerdifferential transformer is used to to generate an


• A differential transformerdifferential transformer is used to to generate an electrical signal.

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AEL2501

Angular velocity sensor:

• It consists of a gear train which is located around the input shaft and a DC generator which isthe input shaft and a DC generator, which is driven at an increased speed by the gear train.

• The turning direction and angular velocity of the steering wheel are detected by the turning direction and angular velocity of this DC generator


generator.

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Torque Sensor and Angular Velocity Sensor

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Torque sensor operation


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Electronic Control Unit:• The ECU consists of an interface circuitinterface circuit that coordinates

the signals from the various sensors, an A/D converter and PWM it ll b ilt i t 8 bit hihia PWM unit – all built into one 8-bit oneone--chip chip

microprocessormicroprocessor, a watchdog timerwatchdog timer(WDT) and a PWM PWM drive circuitdrive circuit.

• The ECU conducts a search for data according to a table table lookup methodlookup method based on the signals input from each sensor and carries out a prescribed calculation using this data to obtain the assist force.


•• Trouble diagnosisTrouble diagnosis for the sensors and the microprocessor is also carried out and when problem is detected, power to motor is interrupted, an indicator lampindicator lamp is illuminates, and the DTC DTC is stored in memory.

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Power unit:

• The power unit consists of a power MOSFET bridge circuit which drives the motor in a forwardforward or reverse reverse direction.direction.

• A drive circuit which controls the respective power MOSFET of this bridge circuit, a current sensor and a relay which turns the motor current ON and OFF.

• Depending on the magnitude of the current, some systems are provided with an integrated integrated ECU and power unit, while


p gg p ,other systems have each section separate.

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ECU and Drive Unit

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Motor Control Methods:

• Current control method: The target value for the motor current which– The target value for the motor current, which corresponds to the motor assist torque, is set so that it is equal to the vehicle speed response type derived from the signal of the vehicle speed sensor.

• Voltage control method:– Both the motor torque and the motor speed can be


controlled by the output from the torque sensor and the steering wheel angular velocity sensor.

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Trends in R&D– The demands that are continuously increasing:

• Faster speeds• Higher quality• Reduced power requirement

– In order to respond to these demands, R&D is happening in application of electronic control systems with the aim of further improving functions and performances.

– Proposed feature include:


Proposed feature include:• Introduction of fuzzy logic• Adaptive power steering• Active reaction power steering • ESP concepts

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AEL2501

Summary

• Various wheel alignment related issues have been discussed

• The steering systems in an automobile have been discussed

• The components of steering system including power assisted steering systems and their arrangements have been discussed


and their arrangements have been discussed

• Working of EPS has been discussed

wheel steering control

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