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Four wheel Active Steering

Artin SpiridonoffGoodarz Mehr

Automotive Chassis Design & Vehicle Dynamics – Spring 2015

Dr. M. SaadatSharif University of Technology, School of Mechanical Engineering

Background (and some other) images are a result of a Google® search and are copyrighted material of their respective owners.

Outline

Introduction

Historical Backgrounds

Mechanisms

Control Methods

Modeling and Simulation

Conclusions

Introduction

Video courtesy of Acura.

Introduction

How it works During Braking, rear wheels toe-in to assist braking

and vehicle stability.In low speed turning, rear wheels turn in the opposite

direction of front wheels to decrease Steering Wheel rotation and increase vehicle maneuverability through tight corners and small spaces.

Introduction

How it works In high speed lane change, rear wheels turn in the

direction of front wheels to decrease Lateral Acceleration, Roll, Yaw Rate and assist in vehicle stability.

Image courtesy of 4 Wheel Active Steer, Nissan Motor Co., LTD.

Historical Backgrounds

Among the earliest vehicles to incorporate four wheel steering was this 1937 Mercedes-Benz Type G 5.

Image courtesy of Wikipedia, the free encyclopedia.

Historical Backgrounds

Before 2000, Honda and Mazda offered this technology in a selected few of their models.

GM offered Delphi’s Quadrasteer in Silverado/Sierra and Suburban/Yukon, but due to low demand for these models, the technology was discontinued.

Since then, it has again attracted attention and at the present is used in various models of Audi, Acura, BMW, Honda, Infiniti, Mazda, Nissan, Mitsubishi, Toyota and Porsche.

Mechanisms

Mechanical 4wsMechanical 4WS uses two separate steering gears to

control the front and rear wheels.

Hydraulic 4wsHydraulic 4WS uses a two-way hydraulic cylinder to

turn both wheels in the same direction, hence it is not possible to turn them in the opposite direction. In Hydraulic systems the rear wheels turn only in the same direction as the front wheels and the system only activates at speeds above 30 mph (50 kmph).

Mechanisms

Mechanisms

Mechanisms

Mechanisms

Electro /Hydraulic 4wsThe electro/hydraulic 4WS combines computer

electronic controls with hydraulics to make the system sensitive to both steering angle and vehicle speed.

Mechanisms

Mechanisms

Advantages and DisadvantagesMechanical 4ws

HeavyNot sensitive to vehicle speed and Yaw rate

Hydraulic 4WSOnly in the same direction as the front wheelsNot sensitive to vehicle speed and Yaw rateOil pump high power consumption

Electro / Hydraulic 4WSSensitive to vehicle speed and Yaw rateRear wheels turn independently

Control MethodsTypically, the following three methods are used to

model Four wheel steering:• Steering Angle Sensing Type 4WS• Vehicle Speed Sensing Type 4WS• Speed Sensing and Yaw Rate Feedback Type 4WS

We will take a brief moment to review these three control methods.

This part is mostly taken from Woongsang Jeong et al., Modeling & Dynamic Analysis of Four Wheel Steering Vehicle.

Control Methods

Steering Angle Sensing Type 4WSRear wheels are rotated only in response to front

wheels’ rotation according to a characteristic curve.This method is genuinely mechanical without control

logic and is a method which is most easily applied.On the other hand, it is not carried out by sensing

vehicle’s dynamic state, so in a situation where vehicle characteristics drastically change, the performance can’t always be guaranteed.

Control Methods

Steering Angle Sensing Type 4WS

Characteristic curve of Steering Angle Sensing Type.

Control Methods

Vehicle Speed Sensing Type 4WSSteering angle of rear wheels change according to

vehicle speed.Researched by Sano et al., the steering equation

between front and rear wheels can be obtained from,2

2

r rs

f

f

Mbc V

C Lk

Mcb V

C L

Control Methods

Vehicle Speed Sensing Type 4WS

Characteristic curve of Steering Angle Sensing Type.

Control Methods

Speed Sensing and Yaw Rate Feedback Type 4WSThis method combines speed sensing and yaw rate

feedback to enhance the operation stability.The mathematical expression for this method can be

given as, r f s yk YR k

Modeling and Simulation

The following five cases were modeled and analyzed:• Double Lane Change at 80 kmph• Low Speed Turning at 20 kmph• Brake Test from 100 kmph to 0 kmph• Tight Double Lane Change at 120 kmph on a low mu road• Tight Double Lane Change at 120 kmph on a high mu road

These models are created using CarSim vehicle simulation software, and MATLAB Simulink was used to create the required controllers.

Modeling and Simulation

Vehicle configuration used was D-class sedan with default properties and default shifting, braking and steering algorithms.

Except for the Brake Test, the steering wheel angle, vehicle speed, sideslip and yaw rate were the input signals and the rear wheels’ steering angles were the output signals.

Four controllers were designed, one for the steering angle sensing type 4WS, one for the vehicle speed sensing type 4WS, and two for the speed sensing and yaw rate feedback type 4WS, incorporating slightly different algorithms.

Modeling and Simulation

One of the designed 4WS speed sensing and yaw rate feedback controllers.

Modeling and Simulation

In order to determine yaw rate gain, this constant was varied in an interval with small steps and the four most important parameters, namely Steering Wheel Angle, Lateral Acceleration, Roll and Yaw Rate were monitored. These parameters were made dimensionless, and an overall weighted average was associated to each case. In the end, the best gain constant (i.e. the one with the lowest overall average) was selected.

Modeling and Simulation

Designed Yaw ControllerDimensionless Steering

Wheel AngleDimensionless Lateral

Acceleration Dimensionless Roll Dimensionless Yaw Rate Gain Overall Score

1 1 1 1 0 11.080635512 1.01187977 1.01459144 0.981303116 0.04 1.0040093511.157648076 1.020746376 1.02652989 0.962322946 0.08 1.0063815341.231353453 1.027612438 1.03634595 0.942766761 0.12 1.0073104031.300571694 1.033046012 1.04439335 0.922757318 0.16 1.0069237011.365435751 1.037491664 1.051644853 0.902549575 0.2 1.0056456661.428671143 1.041171676 1.057923594 0.882285175 0.24 1.0038287561.488964967 1.044283633 1.063406438 0.86210576 0.28 1.0014873911.546117796 1.047148608 1.068358684 0.842256846 0.32 0.9988416611.599315296 1.049741905 1.072957198 0.822625118 0.36 0.9957839091.648806754 1.051643656 1.076936682 0.803220019 0.4 0.9922067521.672372532 1.051273185 1.076848249 0.812823418 0.44 0.9992732491.685335372 1.037689249 1.065617262 0.840717658 0.48 1.0095536681.695805358 1.024945047 1.049787761 0.864778093 0.52 1.0169161441.70394868 1.004766727 1.031747435 0.884419263 0.56 1.0199073321.7104301 0.983674578 1.013176512 0.900613787 0.6 1.020720121

60.172 0.40489 1.1308 10.59

0 0.1 0.2 0.3 0.4 0.5 0.6 0.70.975

0.980.985

0.990.995

11.005

1.011.015

1.021.025

Overall average Vs. Gain

Gain

Ove

rall

Ave

rag

e

Modeling and Simulation

For the Brake Test, Master Cylinder pressure was the input signal and the rear wheels’ steering angles were the output signals.

A simple controller was used in conjugation with the ABS controller.

Modeling and Simulation

Double Lane ChangeThe ISO Standard test to evaluate vehicle stability is a

Double Lane Change test at 80 kmph.

Modeling and Simulation

Double Lane Change

Modeling and Simulation

Double Lane ChangeThe Speed Sensing and Yaw Rate Feedback Type

controllers more closely follow the path.Maximum Steering Wheel angle is increased by 159%

(from 38.1 degrees to 98.5 degrees).Maximum Lateral Acceleration is increased by 5.37%

(from 0.404g to 0.426 g).Maximum Roll is increased by 9.67% (from 1.109

degrees to 1.217 degrees).But, Maximum Yaw Rate is decreased by 23.8% (from

11.23 degrees per second to 8.56 degrees per second).

Modeling and Simulation

Double Lane Change

Modeling and Simulation

Double Lane Change

Modeling and Simulation

Double Lane Change

Modeling and Simulation

Double Lane Change

Modeling and Simulation

Double Lane Change

Modeling and Simulation

Low Speed Turn

Modeling and Simulation

Low Speed Turn

Steering Wheel Angle is decreased by 15.16% (from 508 degrees to 431 degrees).

Modeling and Simulation

Brake Test

Modeling and Simulation

Brake test

Breaking distance is reduced by 1.722% (from 58.4 m to 57.4 m).

Modeling and SimulationTo compare this system with other stability systems, a

comparison was made between this system and ESC (Electronic Stability Control) which accommodates vehicle stability by the unbalanced distribution of braking forces.

This comparison was performed in two extreme conditions, high mu road (that can cause vehicle roll-over) and low mu road (that can cause uncontrolled vehicle slippage).

Modeling and Simulation

Tight Double Lane Change, Low Mu Road

Modeling and Simulation

Tight Double Lane Change, Low Mu Road

The Two vehicles with 4WS system stabilize faster.

Modeling and Simulation

Tight Double Lane Change, High Mu Road

Modeling and Simulation

Tight Double Lane Change, High Mu Road

The Two vehicles with 4WS system stabilize faster.

Conclusions

As it was seen before, the Four Wheel Active Steering system accommodates braking, low speed turning and high speed stability and maneuverability.

In comparison to ESC, this system holds the advantages of stabilizing faster in extreme conditions and assisting the low speed turning.

On the other hand, Four Wheel Active Steering system requires a number of physical components and actuators to operate which add up to vehicle’s weight and cost, but all ESC needs is a pair of solenoid hydraulic valves, and of course a controller!

Thanks for your attention !