Steer-by-Wire: Implications for Vehicle Handling and Safety

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11 Δεκ 2013 (πριν από 3 χρόνια και 7 μήνες)

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Steer
-
by
-
Wire: Implications for Vehicle Handling and
Safety




What is by
-
wire?



Replace mechanical and hydraulic control mechanisms with an
electronic system.


Technology first appeared in aviation: NASA’s digital fly
-
by
-
wire
aircraft (1972).


Today many civil and most military aircraft rely on fly
-
by
-
wire.


Revolutionized aircraft design due to improved performance and
safety over conventional flight control systems.

Source: NASA

Source: NASA

Source: Boeing

Source: USAF

Automotive applications for by
-
wire



By
-
wire technology later
adapted to automobiles:
throttle
-
by
-
wire and brake
-
by
-
wire.


Steer
-
by
-
wire poses a more
significant leap from
conventional automotive
systems and is still several
years away.


Just as fly
-
by
-
wire did to
aircraft, steer
-
by
-
wire promises
to significantly improve vehicle
handling and driving safety.


Source: Motorola

Outline



Introduction


Car as a dynamic system


Tire properties


Basic handling characteristics and stability


Vehicle control


Estimation


Conclusion and future work


introduction

steering system

vehicle control

estimation

conclusion

Why do accidents occur?



42% of fatal crashes result from
loss of control (European
Accident Causation Survey,
2001).


In most conditions, a vehicle
under proper control is very safe.


However, every vehicle has
thresholds beyond which control
becomes extremely difficult.

introduction

steering system

vehicle control

estimation

conclusion

The car as a dynamic system



Assume constant
longitudinal speed,
V
, so
only lateral forces.


Yaw rate,
r
, and sideslip
angle,
b
, completely
describe vehicle motion
in plane.


Force and mass
balance:

introduction

steering system

vehicle control

estimation

conclusion

r
y
f
y
z
r
y
f
y
y
F
b
F
a
r
I
F
F
a
m
,
,
,
,
cos
cos













Linear and nonlinear tire characteristics



Lateral forces are
generated by tire “slip.”





C
a

is called tire cornering
stiffness.


At large slip angles, lateral
force approaches friction
limits.


Relation to slip angle
becomes nonlinear near
this limit.

a
a
C
F
y


introduction

steering system

vehicle control

estimation

conclusion

Linearized vehicle model



Equations of motion:






Valid even when tires
operating in nonlinear region
by approximating nonlinear
effects of the tire curve.

introduction

steering system

vehicle control

estimation

conclusion




b
b
a
a
a
a
a
a
a
a
a
a






































z
f
f
z
r
f
z
f
r
f
r
r
f
I
a
C
mV
C
V
I
b
C
a
C
I
a
C
b
C
mV
a
C
b
C
mV
C
C
r
r
,
,
2
,
2
,
,
,
2
,
,
,
1


Handling characteristics determined by physical
properties



Define understeer gradient:





A car can have one of three characteristics:

r
r
f
f
us
C
W
C
W
K
,
,
a
a


introduction

steering system

vehicle control

estimation

conclusion

K
us

less responsive

more responsive

-

+

understeering

oversteering

neutral steering

Understeering



Negative real roots at low
speed.


As speed increases, poles
move off real axis.


Understeering vehicle is always
stable, but yaw becomes
oscillatory at higher speed.


introduction

steering system

vehicle control

estimation

conclusion

Oversteering



Negative real roots at low speed.


As speed increases, one pole
moves into right half plane.


At higher speed, oversteering
vehicle becomes unstable!


Analogy to unstable aircraft: the
more oversteering a vehicle is,
the more responsive it will be.

introduction

steering system

vehicle control

estimation

conclusion

Neutral steering



Single negative real root due
to pole zero cancellation.


Always stable with first order
response.


This is the ideal handling
case.


Not practical to design this
way: small changes in
operating conditions
(passengers or cargo, tire
wear) can make it
oversteering.


introduction

steering system

vehicle control

estimation

conclusion

Real world example: 15 passenger van rollovers



Full load of passengers shifts weight distribution rearward.


Vehicle becomes oversteering, unstable while still in linear handling
region.


Full load also raised center of gravity height, contributing to rollover.


introduction

steering system

vehicle control

estimation

conclusion

How are vehicles designed?



Most vehicles designed to be understeering (by tire selection,
weight distribution, suspension kinematics).


Provides safety margin.


Compromises responsiveness.



What if we could arbitrarily change handling characteristics?


Don’t need such a wide safety margin.


Can make vehicle responsive without crossing over to
instability.



Can in fact do this with combination of steer
-
by
-
wire and state
feedback!

introduction

steering system

estimation

vehicle control

conclusion

Prior art



Active steering has been demonstrated using yaw rate and
lateral acceleration feedback (Ackermann et al. 1999, Segawa et
al. 2000).


Yaw rate alone not always enough (vehicle can have safe yaw
rate but be skidding sideways).


Many have proposed sideslip feedback for active steering in
theory (Higuchi et al. 1992, Nagai et al. 1996, Lee 1997, Ono et
al. 1998).


Electronic stability control uses sideslip rate feedback to
intervene with braking when vehicle near the limits (van Zanten
2002).


No published results for smooth, continuous handling control
during normal driving.

introduction

steering system

vehicle control

estimation

conclusion

Research contributions



An approach for precise by
-
wire steering control taking into account
steering system dynamics and tire forces.


Techniques apply to steer
-
by
-
wire design in general.



The application of active steering capability and full state feedback
to virtually and fundamentally modify a vehicle’s handling
characteristics.


Never done before due to difficulty in obtaining accurate sideslip
measurement, and


There just aren’t that many steer
-
by
-
wire cars around.



The development and implementation of a vehicle sideslip observer
based on steering forces.


Two
-
observer structure combines steering system and vehicle dynamics
the way they are naturally linked.


Solve the problem of sideslip estimation.


introduction

steering system

vehicle control

estimation

conclusion

Outline



Steering system: precise steering control


Conversion to steer
-
by
-
wire


System identification


Steering control design


Vehicle control


Estimation


Conclusion and future work


introduction

steering system

estimation

vehicle control

conclusion

Conventional steering system


introduction

steering system

estimation

vehicle control

conclusion

Conversion to steer
-
by
-
wire


introduction

steering system

estimation

vehicle control

conclusion

Steer
-
by
-
wire actuator


introduction

steering system

estimation

vehicle control

conclusion

Steer
-
by
-
wire sensors


introduction

steering system

estimation

vehicle control

conclusion

Force feedback system


introduction

steering system

estimation

vehicle control

conclusion

System identification



Open loop transfer function.





Closed loop transfer function.


s
b
s
J
s
s
s
G
s
s
M





2
1
)
(
)
(
)
(
)
(
1
)
(
)
(
)
(
s
KG
s
KG
s
s
d




introduction

steering system

estimation

vehicle control

conclusion

Closed loop experimental response


test_11_13_pb

introduction

steering system

estimation

vehicle control

conclusion

Bode plot fitted to ETFE


test_11_13_pb

introduction

steering system

estimation

vehicle control

conclusion

System identification



Bode plot confirms system to be second order.


Obtain natural frequency and damping ratio from Bode plot.


Solve for moment of inertia and damping constant.






Adjust for Coulomb friction.


2
2
2
2
2
)
(
)
(
n
n
n
s
s
d
s
s
K
s
b
s
J
K
s
s











M
s
s
s
F
b
J











sgn
introduction

steering system

estimation

vehicle control

conclusion

Identified response with friction













Not perfect, but we have feedback.


test_11_13_pb

introduction

steering system

estimation

vehicle control

conclusion

What do you need in a controller?



Actual steer angle should
track commanded angle with
minimal error.


Initially consider no tire
-
to
-
ground contact.


M

actuator torque

commanded angle (at handwheel)

actual angle (at pinion)

effective moment of inertia

effective damping

d


s
J
s
b
introduction

steering system

estimation

vehicle control

conclusion

Feedback control only

















d
d
d
p
feedback
K
K
feedback
M



test_12_3_b0_j0

introduction

steering system

estimation

vehicle control

conclusion

Feedback with feedforward compensation


test_12_3_b0_j0

d
d
d
feedforwar
b
J








d
feedforwar
feedback
M





introduction

steering system

estimation

vehicle control

conclusion

Feedforward and friction compensation


test_12_3_b0_j0



d
c
friction
F



sgn

friction
d
feedforwar
feedback
M







introduction

steering system

estimation

vehicle control

conclusion

Vehicle on ground


test_12_3_b0_j0

friction
d
feedforwar
feedback
M







(Same controller as before)

introduction

steering system

estimation

vehicle control

conclusion

Aligning moment due to mechanical trail



Part of aligning moment from the wheel caster angle.


Offset between intersection of steering axis with ground and
center of tire contact patch.


Lateral force acting on contact patch generates moment about
steer axis (against direction of steering).

introduction

steering system

estimation

vehicle control

conclusion

Aligning moment due to pneumatic trail



Other part from tire deformation during cornering.


Point of application of resultant force occurs behind center of
contact patch.


Pneumatic trail also contributes to moment about steer axis
(usually against direction of steering).

introduction

steering system

estimation

vehicle control

conclusion

Controller with aligning moment correction


test_12_3_b0_j0

aligning
friction
d
feedforwar
feedback
M









a
a
aligning
K


ˆ

introduction

steering system

estimation

vehicle control

conclusion

From steering to vehicle control



Disturbance force acting on steering system causes tracking
error.


Simply increasing feedback gains may result in instability.


Since we have an idea where the disturbance comes from, we
can cancel it out.








We now have precise active steering control via steer
-
by
-
wire
system…what can we do with it?


introduction

steering system

estimation

vehicle control

conclusion

Outline



Steering system: precise steering control


Conversion to steer
-
by
-
wire


System identification


Steering control design


Vehicle control: infinitely variable handling characteristics


Handling modification


Experimental results


Estimation


Conclusion and future work


introduction

steering system

estimation

vehicle control

conclusion

Active steering concept



One of the main benefits of steer
-
by
-
wire over conventional
steering mechanisms is active steering capability.


For a conventional steering system, road wheel angle has a
direct correspondence to driver command at the steering wheel.


driver

conventional
steering system

vehicle

environment

steer angle

vehicle states

command angle

introduction

steering system

estimation

vehicle control

conclusion

Active steering concept



For an active steering system, actual steer angle can be different
from driver command angle to either alter driver’s perception of
vehicle handling or to maintain control during extreme
maneuvers.

driver

vehicle

environment

command angle

vehicle states

controller

active
system

steer angle

introduction

steering system

estimation

vehicle control

conclusion

Physically motivated handling modification



Automotive racing example: driver makes pit stop to change
tires.


Virtual tire change: effectively alter front cornering stiffness
through feedback.


Full state feedback control law: steer angle is linear combination
of states and driver command angle.




Obtain sideslip from GPS/INS system (Ryu’s PhD work).


d
d
r
K
K
r
K

b

b



introduction

steering system

estimation

vehicle control

conclusion

Physically motivated handling modification



Define new cornering stiffness as:




Choose feedback gains as:




Vehicle state equation is now:

)
1
(



b






d
r
K
V
a
K
K


d
I
a
C
mV
C
CG
V
I
b
C
a
C
I
a
C
b
C
mV
a
C
b
C
mV
C
C
CG
z
f
f
z
r
f
z
f
r
f
r
r
f
r
r

b
b
a
a
a
a
a
a
a
a
a
a






































ˆ
ˆ
ˆ
ˆ
ˆ
ˆ
2
2
2
1





a
a


1
ˆ
f
f
C
C
introduction

steering system

estimation

vehicle control

conclusion

Experimental testing at Moffett Field


introduction

steering system

estimation

vehicle control

conclusion

Unmodified handling: model vs. experiment


introduction

steering system

estimation

vehicle control

conclusion












Confirms model parameters match vehicle parameters.


mo_1_3_eta0_d

Experiment: normal vs. reduced front cornering
stiffness


introduction

steering system

estimation

vehicle control

conclusion












Difference between normal and reduced cornering stiffness.


mo_1_3_a05u_b

Reduced front cornering stiffness: model vs.
experiment


introduction

steering system

estimation

vehicle control

conclusion












Understeer characteristic in yaw exactly as predicted.


mo_1_3_a05u_b

introduction

steering system

estimation

vehicle control

conclusion












Verifies sideslip estimation is working.


mo_1_3_eta0_d

Unmodified handling: model vs. experiment


introduction

steering system

estimation

vehicle control

conclusion












Understeer characteristic in sideslip as predicted.


mo_1_3_a05u_b

Reduced front cornering stiffness: model vs.
experiment













Reducing front cornering stiffness returns vehicle to unloaded
characteristic.


Modified handling: unloaded vs. rear weight bias


mo_2_3_eta02u_w_b

introduction

steering system

estimation

vehicle control

conclusion

From control to estimation



We need accurate, clean feedback of sideslip angle to smoothly
modify a vehicle’s handling characteristics.












Can we do this without GPS?


introduction

steering system

estimation

vehicle control

conclusion

Outline



Steering system: precise steering control


Conversion to steer
-
by
-
wire


System identification


Steering control design


Vehicle control: infinitely variable handling characteristics


Handling modification


Experimental results


Estimation: steer
-
by
-
wire as an observer


Steering disturbance observer


Vehicle state observer


Conclusion and future work

introduction

steering system

estimation

vehicle control

conclusion

Sideslip estimation



Yaw rate easily measured, but sideslip angle much more difficult
to measure directly.



Current approaches:


GPS: loses signal under adverse conditions


optical ground sensor: very expensive



Steer
-
by
-
wire approach:


Aligning moment transmits information about the vehicle’s
motion

we canceled it out, remember?


Can be determined from current applied to the steer
-
by
-
wire
actuator.


introduction

steering system

estimation

vehicle control

conclusion

Steering system dynamics


w
w
w
a
M
M
M
J
b
F
k
i



road wheel angle

moment of inertia

damping constant

Coulomb friction

aligning moment

motor torque

motor constant

motor current

w w w a s M
M M M
J b F r
k i
   

   

introduction

steering system

estimation

vehicle control

conclusion

Steering system as a disturbance observer



Express in state space form. Choose steering angle as output
(measured state). Motor current is input. Aligning moment is
disturbance to be estimated.




0 1 0 0
1
0
0 0 0 0
1 0 0
w s M
M
w w w
a a
a
b r k
i
J J J
 
 
 

 

   
 
 
   
 
 
   
   
 
 
   
 
 
 
   
 
   
 
 

 
 
 
introduction

steering system

estimation

vehicle control

conclusion

Link between aligning moment and sideslip
angle



Aligning moment can be expressed as function of the vehicle
states,
b

and
r
, and the input,

.



f
y
m
p
a
F
t
t
,














b

b
a
a
a
a
a
a
f
m
p
f
m
p
f
m
p
f
m
p
f
f
m
p
C
t
t
r
V
C
t
t
a
C
t
t
V
ar
C
t
t
C
t
t





















introduction

steering system

estimation

vehicle control

conclusion

Vehicle state observer



Express in state space form. Steering angle is input. Yaw rate
and aligning moment (from the disturbance observer) are outputs
(measurements).

















b


b
b
a
a
a
a
a
a
a
a
a
a
a
a
a



































































f
m
p
CG
V
C
t
t
a
f
m
p
a
I
a
C
mV
C
CG
V
I
b
C
a
C
I
a
C
b
C
mV
a
C
b
C
mV
C
C
CG
C
t
t
r
C
t
t
r
r
r
f
m
p
z
f
f
z
r
f
z
f
r
f
r
r
f
0
1
0
1
2
2
2


introduction

steering system

estimation

vehicle control

conclusion

Aligning moment and state estimation









Choose disturbance observer gain
T

so that
A
-
TC

is stable and
x
err
=x
-
x
est

approaches zero.



est
est
est
y
y
T
Bu
Ax
x







err
err
x
TC
A
x





Ty
Bu
x
TC
A
est




introduction

steering system

estimation

vehicle control

conclusion












Not exact, but doesn’t need to be.


Estimated aligning moment


data_012504b

introduction

steering system

estimation

vehicle control

conclusion












Sideslip estimate from observer is comparable to estimate from
GPS.


Estimated sideslip and yaw rate


data_012504b

introduction

steering system

estimation

vehicle control

conclusion












State feedback from observer: yaw results comparable to using
GPS.


Experiment: normal vs. reduced front cornering
stiffness


mo_041104_stetam3_a

introduction

steering system

estimation

vehicle control

conclusion

Experiment: normal vs. reduced front cornering
stiffness


mo_041104_stetam3_a

introduction

steering system

estimation

vehicle control

conclusion












Sideslip results also comparable to using GPS.


Conclusion



Driving safety depends on a vehicle’s underlying handling
characteristics.



Can make handling characteristics anything we want provided
we have:


Precise active steering capability


Full knowledge of vehicle states



Precise steering control requires understanding of interaction
between tire and road.


Treated as disturbance to be canceled out.



Vehicle state estimation uses interaction between tire and road
as source of information.


Seen by observer as force that govern vehicle’s motion.


introduction

steering system

estimation

vehicle control

conclusion

Future work



Adaptive modeling to accommodate nonlinear handling
characteristics.


Apply knowledge of tire forces to determine where the limits are
and stay below them.


Bounding uncertainty in observer
-
based sideslip estimation.


Apply control and estimation techniques to a dedicated by
-
wire
vehicle (Nissan project).


introduction

steering system

estimation

vehicle control

conclusion