Four wheel Active Steering enhances vehicle stability

Four wheel Active Steering
Artin Spiridonoff
Goodarz Mehr
Automotive Chassis Design & Vehicle Dynamics – Spring 2015
Dr. M. Saadat
Sharif 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.

Among the earliest
vehicles to incorporate
four wheel steering was
this 1937 Mercedes-
Benz Type G 5.
Image courtesy of Wikipedia, the free encyclopedia.

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 4ws
Mechanical 4WS uses two separate steering gears to control the
front and rear wheels.
Hydraulic 4ws
Hydraulic 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
Electro /Hydraulic 4ws
The electro/hydraulic 4WS combines computer electronic
controls with hydraulics to make the system sensitive to both
steering angle and vehicle speed.

Mechanisms
Advantages and Disadvantages
Mechanical 4ws
Heavy
Not sensitive to vehicle speed and Yaw rate
Hydraulic 4WS
Only in the same direction as the front wheels
Not sensitive to vehicle speed and Yaw rate
Oil pump high power consumption
Electro / Hydraulic 4WS
Sensitive to vehicle speed and Yaw rate
Rear wheels turn independently

Control Methods
Typically, 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 4WS
Rear 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 4WS
Steering 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 r
s
f
f
Mb
c V
C L
k
Mc
b 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 4WS
This 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

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.

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.

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

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.

Designed Yaw Controller
Dimensionless Steering
Wheel Angle
Dimensionless Lateral
Acceleration
Dimensionless Roll
Dimensionless Yaw
Rate
Gain Overall Score
1 1 1 1 0 1
1.080635512 1.01187977 1.01459144 0.981303116 0.04 1.004009351
1.157648076 1.020746376 1.02652989 0.962322946 0.08 1.006381534
1.231353453 1.027612438 1.03634595 0.942766761 0.12 1.007310403
1.300571694 1.033046012 1.04439335 0.922757318 0.16 1.006923701
1.365435751 1.037491664 1.051644853 0.902549575 0.2 1.005645666
1.428671143 1.041171676 1.057923594 0.882285175 0.24 1.003828756
1.488964967 1.044283633 1.063406438 0.86210576 0.28 1.001487391
1.546117796 1.047148608 1.068358684 0.842256846 0.32 0.998841661
1.599315296 1.049741905 1.072957198 0.822625118 0.36 0.995783909
1.648806754 1.051643656 1.076936682 0.803220019 0.4 0.992206752
1.672372532 1.051273185 1.076848249 0.812823418 0.44 0.999273249
1.685335372 1.037689249 1.065617262 0.840717658 0.48 1.009553668
1.695805358 1.024945047 1.049787761 0.864778093 0.52 1.016916144
1.70394868 1.004766727 1.031747435 0.884419263 0.56 1.019907332
1.7104301 0.983674578 1.013176512 0.900613787 0.6 1.020720121
60.172 0.40489 1.1308 10.59
0.99
0.995
1
1.005
1.01
1.015
1.02
1.025
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7
OverallAverage
Gain
Overall average Vs. Gain

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.

Double Lane Change
The ISO Standard test to evaluate vehicle stability is a Double
Lane Change test at 80 kmph.

Double Lane Change

Double Lane Change
The 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).

Low Speed Turn

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

Brake Test

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

To 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).

Tight Double Lane Change, Low Mu Road

Tight Double Lane Change, Low Mu Road
 The Two vehicles with 4WS system stabilize faster.

Tight Double Lane Change, High Mu Road

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!

Four wheel Active Steering enhances vehicle stability

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