Regarding the man-machine interface, the influence of the oversteer characteristic when braking and turning on driver’s steering operation, the influence of driver reaction in system fai
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June 12-15, 2000, Seoul, Korea
Development of the Active Front Steering Control System
AISIN SEIKI CO., LTD., 2-1 Asahi-Machi, Kariya, Aichi 448-8650, Japan For active front steering control systems that intervene in driver’s operation to assist in the control of the vehicle’s motion, the effect of the man-machine interface is much larger than for other conventional control systems This paper focuses on human factors The results of analysis regarding control effects and system design concerns are also described The user benefits of this control system are improved vehicle stability and reduced driving workload Both theoretical and experimental evaluations are described Regarding the man-machine interface, the influence of the oversteer characteristic when braking and turning on driver’s steering operation, the influence of driver reaction in system failure and steering wheel reaction torque when driving with the actuator are also analyzed
Keywords: front steering control, human factor, variable gear ratio, vehicle motion control, failure analysis
INTRODUCTION
Recently, front steering control systems [1]-[4] such as
the steer-by-wire system have been proposed to respond to
the demand for improved handling for better safety and to
aid our aging population Front steering is most closely
connected to the driver as the point of contact between the
man and the vehicle Therefore, large user benefits can be
obtained by control intervention in this area At the same
time, there are some new challenges These items, which
are related to human factors, were analyzed theoretically
and experimentally
SYSTEM CONFIGURATION
A pure steer-by-wire system, which has no mechanical
linkage between the tires and the steering wheel, is the
ultimate steering control system However, at the present
time, a control system that is added to the conventional
steering system and superimposes a controlled steering
angle on the driver’s operation, is reasonable in terms of
cost, reliability and compatibility with current vehicles
Thus, we developed the control system in Fig 1 and
installed it in an experimental vehicle The test subjects
were analyzed using this configuration
USER BENEFITS
The functions of the front steering control system can be classified in two groups One is the passive control such
as variable gear ratio or lead steer The other is active control such as yaw rate feedback control or cooperative control with the brake system The latter can improve vehicle dynamics performance However, it is not directly related to human factors, but to the vehicle motion characteristics Thus, we will not described it here and concentrate on the results of analyzing and evaluating the passive control functions in this paper
VARIABLE GEAR RATIO
EFFECT OF VEHICLE STABILITY IMPROVEMENT
The effect of varying the steering gear ratio to improve vehicle stability is theoretically analyzed Using the simple yaw rate feedback driver model in Fig 2, the performance was analyzed by yaw rate convergence
Driver model:
s Td 1
Kd ) s ( Gd
⋅ +
=
Actuator model: Ga(s) = Ka
s 2 T s 1 T 1
s Tr 1 0 Grdf ) s ( Gv
⋅ +
⋅ +
⋅ +
⋅
=
The 1st order delay model for yaw rate feedback was adopted as a simplified driver model The actuator was represented as a steering angle amplification model with a constant gain and no delay The bicycle model was used for the vehicle The transfer function of this closed loop system is calculated as follows:
* Corresponding author e-mail: akita@rd.aisin.co.jp
Driver’s inputs
● Steering wheel angle
● Steering wheel torque
● Master cylinder pressure
● Throttle angle
Vehicle motion
● Wheel’s velocity
● Yaw rate
● Lateral acceleration
● Longitudinal acceleration
Tire
Brush-less
DC motor
Steering Planetary
gear sets
ECU
Fig.1 AFS control system configuration
Vehicle :Gv(s)
Driver :Gd(s)
AFS Actuator :Ga(s)
target yaw rate+ yaw rate
-Vehicle :Gv(s)
Driver :Gd(s)
AFS Actuator :Ga(s)
target yaw rate+ yaw rate
-Fig.2 Stability analysis model including a driver
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The characteristics of this system were theoretically
analyzed by the pole location and step response of the
above transfer function for various steering gain and driver
characteristics This steering gain is the reciprocal of the
steering gear ratio, thus increasing the gain is the same as
decreasing the gear ratio
In the case of the standard driver, shown in Fig 3, yaw
rate convergence is improved by increasing the steering
gain, which means the stability of the vehicle can be
improved
In Example 2 (Fig.4), the driver steers too quickly with
the controller set at high gain The imaginary parts of the
poles get bigger, thus yaw rate overshoots more Since
this is a linear model, there is no divergence The actual
system tends to become unstable This means that it’s
more difficult for a driver who cannot adapt to different
vehicle characteristics to operate the vehicle with an
excessively large steering gain This analysis illustrates
the basic effect of the variable gear ratio function on the
vehicle stability characteristics
The experimental results using the prototype system to
verify these effects are shown in Figs 6 to 10 A double
lane change maneuver, shown in Fig.5, run on a low
friction road, was used for these tests
The vehicle characteristics with steering gear ratios of
17 (Conventional), 10 and 6 were evaluated by a test driver The experimental results are shown in Figs 6 to 8, including Lissajous’s figures of steering angle and yaw rate The average yaw rates for each gear ratio in the evaluation are shown in Fig 9 The peak yaw rate when passing the last gate is shown in Fig.10 These results show that with a smaller gear ratio, the yaw rate is slower, thus vehicle stability can be improved
The system was also evaluated with a driver who was not familiar with driving on a low friction road The results are shown in Figs 11 to 13
Closed loop transfer function:
3
2 T2 Td s s
) Td 1 T 2 T ( s ) Tr Ka Kd 0 Grdf Td 1 T ( Ka Kd 0 Grdf 1
s Tr 1 Kd
Ka
)
s
(
G
⋅
⋅ +
⋅
⋅ + +
⋅
⋅
⋅
⋅ + + +
⋅
⋅ +
⋅ +
⋅
⋅
=
γ
Fig.3 Ex.1: Yaw rate stability for standard driver
(Td=0.5 sec, Kd=0.245 rad/(rad/sec), Ka=1,2,4,8)
Fig.4 Ex.2: Yaw rate stability for quick and high gain driver
(Td=0.2 sec, Kd=0.49 rad/(rad/sec), Ka=1,2,4,8)
•μ: 0.17
60 km/h
Fig.5 Double lane change course on low friction road
0 1 2 3 4 5 6 -100
-50 0 50 100
0 1 2 3 4 5 6 -20
-10 0 10 20
tim e (sec)
-100 -50 0 50 100 -20
-15 10 -10 -5 0 5 10 15 20
Steering angle (deg)
Fig.6 Yaw rate and steering angle at gear ratio 17
0 1 2 3 4 5 6 -100
-50 0 50 100
0 1 2 3 4 5 6 -20
-10 0 10 20
tim e (sec)
-100 -50 0 50 100 -20
-15 10 -10 -5 0 5 10 15 20
Steering angle (deg)
Fig.7 Yaw rate and steering angle at gear ratio 10
0 1 2 3 4 5 6 -100
-50 0 50 100
0 1 2 3 4 5 6 -20
-10 0 10 20
tim e (sec)
-100 -50 0 50 100 -20
-15 10 -10 -5 0 5 10 15 20
Steering angle (deg)
Fig.8 Yaw rate and steering angle at gear ratio 6
Real Axis
-20
-15
-10
-5
0
5
10
15
20
4
4
8
8
Real Axis
-20
-15
-10
-5
0
5
10
15
20
4
4
8
8
0 0.5 1 1.5
time (sec)
2 Ka=1 4 8
0 0.5 1 1.5
time (sec)
2 Ka=1 4 8
Real Axis
-50
0
50
Ka=1
4
8
2
Ka=1
4
8
2
Real Axis
-50
0
50
Ka=1
4
8
2
Ka=1
4
8
2
0 0.5 1 1.5
time (sec)
Ka=1 4 8
2
0 0.5 1 1.5
time (sec)
Ka=1 4 8
2
3.5 m
5 m
40 m
3.5 m
25 m
2.2 m
5 m
5 m
40 m
3.5 m
25 m
2.2 m
5 m IN
Steering angle
Tire angle
Steering angle
Tire angle
6.47
0 2 4 6
(deg/sec)
Steering gear ratio
6.47
0 2 4 6
(deg/sec)
Steering gear ratio
20.1
0 5 10 15 20 25
Steering gear ratio
(deg/sec) 20.1
0 5 10 15 20 25
Steering gear ratio (deg/sec)
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When the gear ratio was large, the vehicle spun off the
course due to the delay in the steering operation as in Fig
11 In contrast, when the gear ratio was too small, the
vehicle ran off the course since the driver steered too much
and too quickly, as in Fig 13 Only when the proper gear
ratio was selected did the driver successfully maneuver the
course These results match the aforementioned analysis
The appropriate gear ratio must be selected considering
the compatibility with conventional vehicles for various
drivers
EFFECT OF MITIGATION FOR OPERATION LOAD
If a smaller gear ratio is selected, the driver’s workload
can be reduced [1] Steering operation with various gear
ratios is shown in Fig 15 when driving on the evaluation
circuit course in Fig 14 The steering operation energy is
plotted in Fig 16 The energy reduction corresponds to
the lower gear ratio However, if an excessively small
gear ratio is selected, the energy reduction effect
corresponding to the lower gear ratio cannot be obtained
due to frequent steering corrections, as showing in Fig 16
Furthermore, if a smaller gear ratio is selected, the high
frequency part of the steering operation is increased, as in
Fig 17 This means the operation is not as smooth and the
driver’s mental workload increases It is important to
select the appropriate gear ratio from this point of view as
well
HUMAN FACTORS CONCERNS
This system is closely connected to the driver’s operation compared to other control systems, so it is important to consider human factors issues The main concerns regarding human factors were determined through our vehicle experiments, and then analyzed
OVERSTEER WHILE BRAKING AND TURNING
For the variable gear ratio function, the gear ratio changes as a function of vehicle velocity Thus, the vehicle quickly steers toward the inside of a turn when rapidly braking and turning due to the gear ratio change
A similar phenomenon occurs when accelerating and turning However, the amount of under-steer is smaller than when braking and was not a problem in the vehicle tests So the accelerating and turning condition was not analyzed and the braking while turning problem is the focus in this paper
By considering the required steering speed at maximum deceleration in a small, constant radius turn, it can be determined whether general drivers can easily operate the system This value can be calculated by the following equation
Required steering angular velocity:
dt / } fvgr(v) R
l ) v K 1 {(
st = + ⋅ ⋅ ⋅
0 1 2 3 4 5 6
-200
-100
-50
0
50
100
0 1 2 3 4 5 6
-20
-10
0
10
20
tim e (sec)
-200 -100 0 100 200 -20
-15 10 -10 -5 0 5 10 15
Steering angle (deg)
Fig.11 Result with a beginner driver at gear ratio 17
0 1 2 3 4 5 6
-200
-100
-50
0
50
100
200
0 1 2 3 4 5 6
-20
-10
0
10
20
tim e (sec)
-200 -100 0 100 200 -20
-15 10 -10 -5 0 5 10 15 20
Steering angle (deg)
Fig.12 Result with a beginner driver at gear ratio 10
0 1 2 3 4 5 6
-200
-100
-50
0
50
100
200
0 1 2 3 4 5 6
-20
-10
0
10
20
tim e (sec)
-200 -100 0 100 200 -20
-15 10 -10 -5 0 5 10 15 20
Steering angle (deg)
Fig.13 Result with a beginner driver at gear ratio 6
Fig.14 Composite circuit course
Fig.15 Steering operation with each gear ratio
Fig.16 Operation energy Fig.17 Power spectrum
of steering angle
-540 -450 -360 -270 -180 -90 0 90 180 270 360 450 540
0 10 20 30 40 50
time (sec)
gear ratio : 17 (normal) 6 3
-80 -70 -60 -50 -40 -30 -20 -10 0
frequency (Hz)
gear ratio : 3 6
17 (normal)
17 6 3 gear ratio 0
200 400 600 800 1000 1200 1400 1600
Additional energy due to correction
5m 25R
15R
15R
15 R
15R Start line
160m
White line Pylon
5m 25R
15R
15R
15 R
15R Start line
160m
White line Pylon
Off the course
Steering angle
course
Steering angle
Tire angle
Spin out
Spin out
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When using the variable gear ratio characteristic shown
in Fig 19 [1] based on a limited steering wheel operation
range, the required velocity is 260 deg/sec Since the
general driver’s fastest steering speed is about 200 to 1600
deg/sec, almost any driver should be able to operate the
system However, this maneuver is supposed to be
difficult An experimental evaluation using this gear ratio
characteristic was done under the condition in Fig 18
Fig 20 shows the experimental results indicating that a
rapid return steering operation is necessary even at a
deceleration of 0.5G
For the modified gear ratio characteristic in Fig 21, the
gear ratio change gradient (∂fvgr(v)/∂v) is around 0.1
/(km/h) Using the above equation, the required steering
operation speed is 50 deg/sec, which general drivers
should easily be able to do In the experiments, the driver
could smoothly maneuver through the course as in Fig 22
This is also a requirement for the variable gear ratio set up
EFFECT OF SYSTEM FAILURE
The following 2 kinds of system failures can arise
1 Sudden, unintended steering
The failure analysis and countermeasures study that
examined various failure detection mechanisms, sudden,
unintended steering can never arise as long as the dual
failure does not occur Thus, the analysis for the system
failure 1 is not considered in this paper For this
configuration, when the system locks up, the steering
characteristic just returns to that of conventional system
However, if the change is large, it may affect to drive The
following analysis focuses on the variable gear ratio function due to the large characteristic change
To understand the phenomena with concrete value, the simple model in Fig 23 was set up under the following assumptions
[Assumption]
1 At the beginning, the driver steers with a small gear ratio in the variable gear ratio function
2 The system locks up and the gear ratio returns to the normal one that is large
3 The driver notices the failure after some constant lag time and starts steering using a normal steering gain to return to the planned trajectory
For the driver model, a 1st order linear prediction model was used where the driver steers corresponding to the deviation between the planned trajectory and a point projected on straight line a set distance ahead of the car This “preview distance” is a function of the vehicle speed Then the corrective steering operation speed is limited to match the actual environment The linear bicycle model with the Pacejka tire model is used as the vehicle model The maximum deviation from the planned trajectory was used as the evaluation parameter
The simulation results for these rather severe conditions are shown in Fig 24 The permissible value depends on the driving environment The deviation is not so large with small steering gains, even under this severe condition
The vehicle experiments were done to verify these results A severe double lane change course on a dry asphalt road in Fig 25 was set up to evaluate the failure effect The Active Front Steering Control System was locked up during the 1st or 2nd turn The driver did not know in advance when the failure was to occur Gear ratio values of 10 and 6 were chosen for the Active Front Steering Control When the system locked up, it reverted
to a gear ratio of 17 4 people including professional and beginner drivers were evaluated
[Calculation parameters]
•R: 15
•Gmax: 1 G
•v: 5 to 150 km/h
Fig.18 "Braking and turning" evaluation method
and calculation parameters
Fig.21 Adjusted gear ratio Fig.22 Driver’s operation
Fig.19 Proposed gear ratio Fig.20 Driver’s operation
Fig.23 System fail model
•Vehicle velocity: 40km/h
•Maximum corrective steering speed: 200 deg/sec
•Planned turning radius: 20 m
•Driver preview time: 2 sec Fig.24 Maximum deviation in system failure
Steering gain
Recognition time=0.5 sec
0.2 sec
0 1 2 3 4
1 1.5 2 2.5 3
0.1 sec
Steering gain
Recognition time=0.5 sec
0.2 sec
0 1 2 3 4
1 1.5 2 2.5 3
0.1 sec
60km/h
Braking Pyron
4 m
15R
stop line
0
5
10
15
20
25
0 50 100 150
velocity (km/h)
0
5
10
15
20
25
0 50 100 150
velocity (km/h)
-200 -150 -100 -50 0 50
100
velocity (km/h)
yaw rate (deg/sec) steering angle (deg)
time (sec)
0 1 2 3 4 -200
-150 -100 -50 0 50
100
velocity (km/h)
yaw rate (deg/sec) steering angle (deg)
time (sec)
0 1 2 3 4
10
20
30
40
50
velocity (km/h)
10
20
30
40
50
velocity (km/h)
-200 -150 -100 -50 0 50 100
0 1 2 3 4
velocity (km/h) yaw rate (deg/sec)
steering angle (deg)
time (sec) -200
-150 -100 -50 0 50 100
0 1 2 3 4
velocity (km/h) yaw rate (deg/sec)
steering angle (deg)
time (sec)
Trajectory with AFS
Recognize failure, then start correcting Preview point Maximum deviation
Trajectory
in failure
Bicycle model with Pacejka tire model
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The test results showed that each driver could
successfully pass through the course in all cases The
recognition lag time was 0.1 to 0.3 sec and the maximum
steering speed was about 300 deg/sec in most cases Figs
26 to 28 show the results when using a gear ratio of 6
These simulations and experiments show that the driver
can manage a lock-up failure under these conditions when
using the gear ratio characteristic in Fig.21 A problem
with this experiment is that the drivers knew a failure was
supposed to occur, even though they didn’t know when,
and quickly responded without panic It is necessary to
evaluate the performance of general drivers under actual
conditions to improve the reliability of the experiments A
driving simulator would be useful for this purpose This is
one of our future research tasks
EFFECT OF REACTION STEERING TORQUE
Reaction torque is applied by the actuator to not only
the tires, but also the steering wheel Thus, the effect of
the reaction torque for driver’s operation was analyzed
This analysis was also classified into 2 functions One is
the passive function where the system follows driver’s
intension and operation The other is the active function
where the system automatically activates based on the
vehicle condition
Passive function
The fundamental mechanism regarding the reaction
torque is described using a simple model in Fig 29 The
difference between the system with and without the AFS
system can be described as:
Steering torque without AFS:
sw I t I
Steering torque with AFS:
m I sw I t I Tm '
This means that the torque generated by the AFS motor
is not additional torque but same torque that is actuating
the tire Therefore, since the actuator follows the driver’s
steering operation, the driver should not feel any
unexpected steering wheel torque However, the control
system has some delay The effect of the delay and the
level that is acceptable was analyzed
The driver’s haptic evaluation of the steering operation when the vehicle was stationary was done using the Significant Difference Method Various amounts of lag time after the target signal was generated by the steering wheel angle, were added to the AFS actuator command signal As shown in Fig 30, the driver feels uncomfortable with more than 0.1 sec of lag time This is the system delay limit
•ωsw,αsw: Steering velocity, acceleration
•ωm,αm: Motor velocity, acceleration
•ωt,αt: Tire velocity, acceleration
•Tsw’: Steering torque
•Tm: Motor torque Fig.29 Principle model for reaction torque generation mechanism
40 km/h Fig.25 Evaluation course for system failure
-200
-150
-50
0
50
100
200
-20
-10
0
10
20
-300
-200
-100
0
100
200
300
tim e (se )
-200 -100 -50 0 50 100 200
-20 -10 0 10 20
-300 -200 -100 0 100 200 300
tim e (se )
-200 -100 -50 0 50 100 200
-20 -10 0 10 20
-300 -200 -100 0 100 200 300
tim e (se )
•Surface: Dry asphalt
•Vehicle velocity: 0km/h
•Steering input: Random
(Various pattern)
Fig.30 Haptic evaluation for the system lag time
3.5 m
5 m
20 m
3.5 m
15 m
2.2 m
5 m
5 m
20 m
3.5 m
15 m
2.2 m
5 m IN
Steering angle Tire angle
Superimposed angle Failure
1 2 3 4 5
System lag time (sec.)
Standard level
= no incompatibility
Steering angle
Tire angle
Failure
sw
sw,α ω m
m,α ω t
t,α ω
Steering wheel
sw
sw,α ω m
m,α ω t
t,α ω
Steering wheel
Steering angle
Tire angle Superimposed angle
Superimposed angle
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The required response of the system was analyzed It
can be calculated from the driver’s maximum steering
operation speed, operation range at that speed and
permissible lag time using the following equation
Required system functioning speed:
Rnorm: Normal gear ratio
Rvgr: Gear ratio in variable gear ratio function
θmax: Steering operation range
ωswmax: Driver’s maximum steering operation speed
Tlag: Acceptable system lag time
The results from 2 example calculations for severe
driving conditions are shown in Fig 31 One condition is
for high speed steering with a narrow range such as in case
of a collision avoidance maneuver The other is for slow
speed operation with a wide steering angle range like when
parking The haptic evaluation using the experimental
vehicle with gear ratios of 6 and 10 was done The
maximum driving speed of the prototype system was 32
deg/sec at the tire angle As the results of the haptic
evaluation, the drivers didn’t notice the system delay with
a gear ratio of 10, but felt it with a gear ratio of 6 This
experimental result matches the calculation results
Active function
When the system with active functions is triggered
such as when the vehicle starts to become unstable, the
motor angle is superimposed to the driver’s steering
operation to compensate the vehicle’s motion This
operation generates additional reaction torque This
influence was analyzed through vehicle experiments using
the aforementioned course on an artificially low friction
road with 4 drivers, including professionals and beginners
The following simple yaw rate feedback control algorithm
was adopted
Target tire angle: θt=θt0+Krp⋅(γt−γ)−Krd⋅dγ/dt
θt0: Target tire angle calculated from the steering angle
γ t: Target yaw rate calculated using the bicycle vehicle
model with the Pacejka tire model
Krp, Krd: Proportional gain, Differential gain
All drivers were able to pass through the test course
without any negative influences from the AFS system
Fig 32 shows the results from one of these experiments
Fig.33 shows the relationship between the reaction
torque when the correction control was applied and the
steering angle fluctuation at that time It shows that very
little steering angle fluctuations occurred, even when
rather high reaction torque was applied This is likely due
to the fact that the direction of the correction control
coincides with the driver’s intention, thus the driver didn’t
feel large unintended motions It can be concluded from
these experiments that the system does not generate any
negative effects to the driver
CONCLUSION
The following facts were verified when the appropriate gear ratio was selected
1 User benefits 1-1 Vehicle stability can be improved
1-2 Steering operation workload can be mitigated
2 Concerns for human factor 2-1 The driver can manage the oversteer and understeer characteristic caused by the variable gear ratio 2-2 The driver can control the vehicle, even when the control system suddenly locks up
2-3 The reaction torque doesn’t affect the driver’s operation
These are preliminarily results since it is difficult to generalize these effects over the entire driving public, many of whom have different driving styles and preferences Further research using a wider range of drivers is necessary to refine this system for the market
Fig.32 The influence of reaction torque
Fig.31 Required system functioning speed
0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0
0 5 10 15 20 25 Reaction steering torque (Nm)
Fig.33 Influence of reaction torque
Tlag max sw max
max )
1 Rvgr
Rnorm
(
vsr
+ ω
⋅
−
-50 0 50 100
t (sec)
-20 -10 0 10 20
t (sec)
-20 -10 0 10 20
t (sec)
Steering wheel torque
Reaction torque
Steering angle
Tire angle
Compensation
by the system
10 6
Steering gear ratio
0
10
20
30
40
50
60
70
80
Prototype
performance High speed, narrow range(1600deg/sec, 90deg)
Low speed, wide range (540deg/sec, 1080deg)
10 6
Steering gear ratio
0
10
20
30
40
50
60
70
80
0
10
20
30
40
50
60
70
80
Prototype
performance High speed, narrow range(1600deg/sec, 90deg)
Low speed, wide range (540deg/sec, 1080deg)
Trang 77
NOMENCLATURE
p.1 Kd : Driver’s steering gain
Td : Time constant of driver’s operation
Ka : Actuator gain
Grdf0 : Vehicle yaw rate gain [= v/ ( 1 +K ⋅ v )) ]
Tr : Parameter of Gv(s) [ =m ⋅ lf/ ( Cr ⋅ ) ⋅ v ]
T1 : Parameter of Gv(s) [ = lr/v ]
T2 : Parameter of Gv(s) [ = Iz/ ( Cr ⋅ ) ]
M : Vehicle mass
Iz : Yaw moment of inertia
v : Velocity of vehicle body
Cf, Cr : Cornering power
lf, lr : Length between center of gravity and tire
contact point
l : Wheel base
K : Stability factor
p.3 R : turning radius
Gmax : maximum deceleration
fvgr : VGR steering gear ratio function
vt : vehicle velocity at t sec
γ : yaw rate
ACKNOWLEDGMENT
The advice of Dr Y Amano of TOYOTA CENTRAL
R&D LABS., INC in the ergonomic analysis is gratefully
acknowledged
REFERENCES
[1] Akita, T., Yoshida, T., et al 1999 User Benefits of
Active Front Steering Control System: Steer-by-Wire:
FISITA 99SF013
[2] Shimizu, Y., Kawai, T., Yuzuriha, J 1999
Improvement in driver-vehicle system performance by
varying steering gain with vehicle speed and steering
angle: VGS (Variable Gear-ratio Steering system): SAE
1999-01-0395
[3] Wolfgang, K., Matthias, H 1996 Potential Functions
And Benefits Of Electronic Steering Assistance: FISITA
B0304
[4] Karnopp, D 1992 Active Steering Systems: Report of
Department of Mechanical, Aeronautical and Materials
Engineering, The University of California, Davis