M.S Ramaiah School of Advanced Studies - Bangalore

Transcription

1 Wheel Alignment, Steering Systems and Steering Control Lecture delivered by: Prof. Ashok C.Meti MSRSAS-Bangalore 1 Session Objectives To study various wheel alignment related issues To understand the steering system in an automobile To study the components of steering systems and their configurations To identify the need for the electronic control in power steering and its importance. To study working of Hydraulic power steering and EPS 2 1

2 Topics Wheel alignment Steering systems Power assisted steering systems Hydraulic Power Assisted Steering EPS 3 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 Reduction in fuel consumption Less strain on steering system and suspension components 4 2

3 The alignment geometry The proper alignment f a suspension/ steering system depends on the accuracy of the following: Caster Camber Toe Thrust line alignment Steering Axle Inclination (SAI) Turning radius Tracking There are host of gauges and electronic equipment to check and adjust these parameters. 5 Caster Is the angle of the steering axis of a wheel from the vertical, as viewed from the side of the vehicle It designed to provide steering stability Caster is not related to tire wear Camber AND Castor Gauge 6 3

4 Camber It is the angle represented by thetilt of either the front 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, and vehicle weight Camber and Castor Gauge 7 Toe It is the distance comparison between the leading 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 straight ahead have to drag as they travel forward 8 4

5 Thrust Line Alignment The vehicle must run straight down the road with the 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 If not, the vehicle tends to travel in the direction of the thrust line, rather than straight ahead 9 Steering Axle Inclination (SAI) It locates the inside or outside 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 steering system return to straight-ahead ahead after a turn SAI angles FWD degrees RWD 6-8 degrees 10 5

6 Turning Radius Is the amount of toe-out present in turns It is also called toe-out on turns 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 the steering geometry to turn the inside wheel sharper than the outside 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 11 Tracking All vehicles are built around a geometric centerline that runs through the center of the chassis from the back to the front An ideal alignment has all 4 wheels parallel l to the centerline making the thrust line parallel to the centerline (tracking) 12 6

7 Wheel alignment equipment 13 Steering System The purpose of the steering system is to turn the front wheels(normally) (normally). The wheels constantly change direction, while switching lanes, rounding sharp turns, and when avoiding roadway obstacles. The manual steering is consisting of Steering linkages Steering gear Steering column and wheel Working: As the steering wheel is turned by the driver, the steering gear transfers this motion to the steering linkages. The steering linkage turns the wheels to control the vehicle s direction. 14 7

8 Steering Linkage: It is the system of pivots and connecting parts placed between the steering gear and the steering arms that are attached to the front or rear wheels that control the direction of the vehicle travel. It transfers motion of the steering gear output shaft to the steering arms, 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- Pitman Arm Idler Arm Links (Center, drag, or steering links) Tie-rods

9 Pitman arm Connects the linkage to the steering column through a steering gear located at the base of the column 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 ypermits sideways movement of the linkage 17 Links Depending on the design application, can be referred to as Center link Drag 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- 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 18 9

10 19 Steering Gear box The purpose of the steering gear is to change the rotational motion of the steering wheel to a reciprocating motion to move steering linkage. Different style of steering gear boxes used- Recirculating ball steering gear Worm and roller type steering gear box Rack and pinion type A recirculating ball type is generally found in large cars

11 Rack and Pinion It is lighter in weight and has few 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 21 Steering input is received from the pinion gear attached to the steering column. This gear moves a toothed rack that is attached to the tie- rods. The movement of the rack pushes or pulls 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 points. (reduced isolation and vibrations damping) 22 11

12 Steering wheel and column The role of the steering wheel and column is to produce the necessary force to turn the steering ggear. The steering wheel is used to produce the turning effort. Types of steering columns: Fixed column Telescopic column Tilt column, and so on The steering column is made collapsible that allows the column to fold into itself, upon impact. The driver s side Air bag assembly is contained in the center portion of the steering wheel. 23 Power Assisted Steering It is designed to reduce the amount of effort required to turn the steering wheel. It also reduces the driver fatigue on long drives and makes it easier to steer the vehicle at low road speeds, particularly while parking. At higher steering efforts are required at higher speeds to provide the improved down-to-road feel. Two 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 motor and an electronic control are used to provide the assistance

13 Variable-assist steering Application of electronic control technology to vehicle steering system is still in the development stage. Several techniques have been suggested by several designers and implemented on vehicles by manufacturers. 25 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- Reduce the steering effort when driving at low speeds Supply feedback for the appropriate steering reaction force when driving at high speeds. The vehicle speed sensors are used to sense the vehicle speed in order to make smooth and continuous changes in the steering assist rate under conditions ranging from steering maneuvers at zero speed to those at high speeds

14 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 flow and pressure of the hydraulic fluid is controlled by means of the operation of the solenoid valves. 27 Flow Control Method: Solenoid valve is located at the pump discharge port. Controller regulates the solenoid valve opening at high speeds to reduce the pump discharge volume, thus increasing the required steering effort. Vehicle Speed-responsive pump discharge flow volume control type 28 14

15 Cylinder Bypass Control Method: A solenoid valve and a bypass line are located between both chambers of the power cylinder. The opening of the valve is extended by electronic control equipment in accordance with increase in vehicle speed, thus reducing the hydraulic pressure in the power cylinder and increasing the steering effort. 29 Hybrid System: Utilize a flow control method in which the hydraulic power steering pump is driven by an electric motor. The steering effort is controlled by controlling the rotating speed 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 loss is lower than that of the engine pump when driving at high speeds. There is large degree of freedom in mounting location selection

16 Driving mode responsive-type hybrid power steering Driving Mode Response method: Control system consists of Vehicle speed sensor Steering angular velocity sensor 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 the appropriate steering effort for the driving conditions. Driving mode and pump flow rate 31 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 velocity of the steering wheel and vehicle speed The discharge flow volume of the pump is reduced and the steering response drops when the vehicle is driven at high speeds In this system, therefore, the speed of the motor is increased in accordance with the detected angular velocity of the steering wheel in order to increase the discharge flow volume to solve the problem

17 Steering Speed-responsive-type hybrid power steering 33 Basic Control 34 17

18 Electric Power Steering (EPS): Electric power steering is a fully electric system which reduces the amount of steering effort by directly applying the output from an electric motor to the steering system. The system consists of: Vehicle speed sensor A steering sensor (torque, angular velocity) ECU Drive unit Motor 35 Structure of EPS System (Rack assist-type ball screw drive) 36 18

19 The signals outputs from each of the sensors are input to the ECU where necessary steering assistance is calculated and applied by the drive unit to control the operation of the motor. In this system, setting range for the steering 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. 37 Column shaft drive method Pinion shaft drive method 38 19

20 Rack-and-pinion steering mechanisms, the EPS system applies the motor power to the pinion gear shaft or to the rack shaft. Several reduction gears are employed to increase the torque of the motor. 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 amount of assist from the rack assist method than from pinion assist method. 39 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 40 20

21 Torque sensors: Pinion shaft in the rack-and-pinion steering mechanism is divided into two sections. Input shaft Pinion gear It consists of a torsion bar that connects the two sections. A differential transformer is used to to generate an electrical signal. 41 Angular velocity sensor: It consists of a gear train which is located around the 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

22 Torque Sensor and Angular Velocity Sensor 43 Torque sensor operation 44 22

23 Electronic Control Unit: The ECU consists of an interface circuit that coordinates the signals from the various sensors, an A/D converter and a PWM unit all built into one 8-bit one-chip microprocessor, a watchdog timer(wdt) and a PWM drive circuit. The ECU conducts a search for data according to a table lookup method based on the signals input from each sensor and carries out a prescribed calculation using this data to obtain the assist force. Trouble diagnosis for the sensors and the microprocessor is also carried out and when problem is detected, power to motor is interrupted, an indicator lamp is illuminates, and the DTC is stored in memory. 45 Power unit: The power unit consists of a power MOSFET bridge circuit which drives the motor in a forward or reverse 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 ECU and power unit, while other systems have each section separate

24 ECU and Drive Unit 47 Motor Control Methods: Current control method: 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

25 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: Introduction of fuzzy logic Adaptive power steering Active reaction power steering ESP concepts 49 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 Working of EPS has been discussed 50 25