defense v5
TRANSCRIPT
-
8/8/2019 Defense v5
1/63
Steer-by-Wire: Implications for Vehicle Handling
and Safety
Paul Yih
May 27, 2004
-
8/8/2019 Defense v5
2/63
What is by-wire?
Replace mechanical and hydraulic control mechanisms with an
electronic system.
Technology first appeared in aviation: NASAs 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: NASASource: NASA
Source: BoeingSource: USAF
-
8/8/2019 Defense v5
3/63
By-wire technology lateradapted to automobiles:throttle-by-wire and brake-by-wire.
Steer-by-wire poses a moresignificant leap fromconventional automotivesystems and is still severalyears away.
Just as fly-by-wire did to
aircraft, steer-by-wire
promises to significantlyimprove vehicle handling and
driving safety.
Automotive applications for by-wire
Source: Motorola
-
8/8/2019 Defense v5
4/63
Introduction
Car as a dynamic system
Tire properties
Basic handling characteristics and stability
Vehicle control
Estimation
Conclusion and future work
Outline
introduction steering system vehicle control estimation conclusion
-
8/8/2019 Defense v5
5/63
42% of fatal crashes result from
loss of control (European
Accident Causation Survey,
2001).
In most conditions, a vehicleunder proper control is very safe.
However, every vehicle has
thresholds beyond which control
becomes extremely difficult.
Why do accidents occur?
introduction steering system vehicle control estimation conclusion
-
8/8/2019 Defense v5
6/63
Assume constant
longitudinal speed, V,
so only lateral forces.
Yaw rate, r, and sideslip
angle,F, completelydescribe vehicle motionin plane.
Force and mass
balance:
The car as a dynamic system
introduction steering system vehicle control estimation conclusion
ryfyz
ryfyy
FbFarI
FFam
,,
,,
cos
cos
!!H
H
-
8/8/2019 Defense v5
7/63
Lateral forces aregenerated by tire slip.
CE is called tire corneringstiffness.
At large slip angles, lateralforce approaches frictionlimits.
Relation to slip angle
becomes nonlinear nearthis limit.
Linear and nonlinear tire characteristics
EECFy !
introduction steering system vehicle control estimation conclusion
-
8/8/2019 Defense v5
8/63
Equations of motion:
Valid even when tires
operating in nonlinear
region by approximating
nonlinear effects of the tire
curve.
Linearized vehicle model
introduction steering system vehicle control estimation conclusion
H
FFE
E
EEEE
EEEE
-
-
-
!
-
z
f
f
z
rf
z
fr
frrf
I
aC
V
C
VI
bCaC
I
aCbC
V
aCbC V
CC
rr,
,
2,
2,,,
2
,,, 1
-
8/8/2019 Defense v5
9/63
Define understeer gradient:
A car can have one of three characteristics:
Handling characteristics determined by physical
properties
r
r
f
f
usC
W
C
WK
,, EE
!
introduction steering system vehicle control estimation conclusion
Kus
less responsive more responsive-+
understeering oversteeringneutral steering
-
8/8/2019 Defense v5
10/63
Negative real roots at low
speed.
As speed increases, poles
move off real axis.
Understeering vehicle isalways stable, but yaw
becomes oscillatory at higher
speed.
Understeering
introduction steering system vehicle control estimation conclusion
-
8/8/2019 Defense v5
11/63
Negative real roots at low
speed.
As speed increases, one pole
moves into right half plane.
At higher speed, oversteeringvehicle becomes unstable!
Analogy to unstable aircraft: the
more oversteering a vehicle is,
the more responsive it will be.
Oversteering
introduction steering system vehicle control estimation conclusion
-
8/8/2019 Defense v5
12/63
Single negative real root due
to pole zero cancellation.
Always stable with first order
response.
This is the ideal handlingcase.
Not practical to design this
way: small changes in
operating conditions
(passengers or cargo, tirewear) can make it
oversteering.
Neutral steering
introduction steering system vehicle control estimation conclusion
-
8/8/2019 Defense v5
13/63
Full load of passengers shifts weight distribution rearward.
Vehicle becomes oversteering, unstable while still in linearhandling region.
Full load also raised center of gravity height, contributing torollover.
Real world example: 15 passenger van
rollovers
introduction steering system vehicle control estimation conclusion
-
8/8/2019 Defense v5
14/63
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? Dont need such a wide safety margin.
Can make vehicle responsive without crossing over toinstability.
Can in fact do this with combination of steer-by-wire and state
feedback!
How are vehicles designed?
introduction steering system estimationvehicle control conclusion
-
8/8/2019 Defense v5
15/63
Active steering has been demonstrated using yaw rate andlateral acceleration feedback (Ackermann et al. 1999, Segawaet al. 2000).
Yaw rate alone not always enough (vehicle can have safe yawrate but be skidding sideways).
Many have proposed sideslip feedback for active steering intheory (Higuchi et al. 1992, Nagai et al. 1996, Lee 1997, Ono etal. 1998).
Electronic stability control uses sideslip rate feedback tointervene with braking when vehicle near the limits (van Zanten2002).
No published results for smooth, continuous handling controlduring normal driving.
Prior art
introduction steering system vehicle control estimation conclusion
-
8/8/2019 Defense v5
16/63
An approach for precise by-wire steering control taking intoaccount 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 vehicleshandling characteristics.
Never done before due to difficulty in obtaining accurate sideslipmeasurement, and
There just arent that many steer-by-wire cars around.
The development and implementation of a vehicle sideslipobserver based on steering forces.
Two-observer structure combines steering system and vehicledynamics the way they are naturally linked.
Solve the problem of sideslip estimation.
Research contributions
introduction steering system vehicle control estimation conclusion
-
8/8/2019 Defense v5
17/63
Steering system: precise steering control
Conversion to steer-by-wire
System identification
Steering control design
Vehicle control
Estimation
Conclusion and future work
Outline
introduction steering system estimationvehicle control conclusion
-
8/8/2019 Defense v5
18/63
Conventional steering system
introduction steering system estimationvehicle control conclusion
-
8/8/2019 Defense v5
19/63
Conversion to steer-by-wire
introduction steering system estimationvehicle control conclusion
-
8/8/2019 Defense v5
20/63
Steer-by-wire actuator
introduction steering system estimationvehicle control conclusion
-
8/8/2019 Defense v5
21/63
Steer-by-wire sensors
introduction steering system estimationvehicle control conclusion
-
8/8/2019 Defense v5
22/63
Force feedback system
introduction steering system estimationvehicle control conclusion
-
8/8/2019 Defense v5
23/63
System identification
Open loop transfer function.
Closed loop transfer function.
sbsJs
ssG
ssM !
85
!2
1
)(
)()(
)(1
)(
)(
)(
sKG
sKG
s
s
d !
5
5
introduction steering system estimationvehicle control conclusion
-
8/8/2019 Defense v5
24/63
Closed loop experimental response
test_11_13_pb
introduction steering system estimationvehicle control conclusion
-
8/8/2019 Defense v5
25/63
Bode plot fitted to ETFE
test_11_13_pb
introduction steering system estimationvehicle control conclusion
-
8/8/2019 Defense v5
26/63
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.
System identification
22
2
22)(
)(
nn
n
ssd ssKsbsJK
ss
[\[[
!
!
55
Msss bJ XUUU ! sgn
introduction steering system estimationvehicle control conclusion
-
8/8/2019 Defense v5
27/63
Identified response with friction
test_11_13_pb
Not perfect, but we have feedback.
introduction steering system estimationvehicle control conclusion
-
8/8/2019 Defense v5
28/63
What do you need in a controller?
Actual steer angle should
track commanded angle with
minimal error.
Initially consider no tire-to-
ground contact.
MX actuator torque
commanded angle (at handwheel)
actual angle (at pinion)effective moment of inertia
effective damping
dU
UsJ
sb
introduction steering system estimationvehicle control conclusion
-
8/8/2019 Defense v5
29/63
Feedback control only
UUUUX ! dddpfeedback KKfeedbackM XX !
test_12_3_b0_j0
introduction steering system estimationvehicle control conclusion
-
8/8/2019 Defense v5
30/63
Feedback with feedforward compensation
test_12_3_b0_j0
dddfeedforwar bJ UUX !
d feedfor arfeedbackM XXX !
introduction steering system estimationvehicle control conclusion
-
8/8/2019 Defense v5
31/63
Feedforward and friction compensation
test_12_3_b0_j0
dcfriction
F UX sgn!
frictiondfeedforwarfeedbackM XXXX !
introduction steering system estimationvehicle control conclusion
-
8/8/2019 Defense v5
32/63
Vehicle on ground
test_12_3_b0_j0
frictiond feedfor arfeedbackM XXXX !(Same controller as before)
introduction steering system estimationvehicle control conclusion
-
8/8/2019 Defense v5
33/63
Part of aligning moment from the wheel caster angle.
Offset between intersection of steering axis with ground andcenter of tire contact patch.
Lateral force acting on contact patch generates moment aboutsteer axis (against direction of steering).
Aligning moment due to mechanical trail
introduction steering system estimationvehicle control conclusion
-
8/8/2019 Defense v5
34/63
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).
Aligning moment due to pneumatic trail
introduction steering system estimationvehicle control conclusion
-
8/8/2019 Defense v5
35/63
Controller with aligning moment correction
test_12_3_b0_j0
aligningfrictiondfeedforwarfeedbackM XXXXX !aaaligning K XX !
introduction steering system estimationvehicle control conclusion
-
8/8/2019 Defense v5
36/63
Disturbance force acting on steering system causes trackingerror.
Simply increasing feedback gains may result in instability.
Since we have an idea where the disturbance comes from, wecan cancel it out.
We now have precise active steering control via steer-by-wiresystemwhat can we do with it?
From steering to vehicle control
introduction steering system estimationvehicle control conclusion
-
8/8/2019 Defense v5
37/63
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
Outline
introduction steering system estimationvehicle control conclusion
-
8/8/2019 Defense v5
38/63
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.
driverconventional
steering system vehicle
environment
steer angle
vehicle states
command angle
Active steering concept
introduction steering system estimationvehicle control conclusion
-
8/8/2019 Defense v5
39/63
For an active steering system, actual steer angle can be
different from driver command angle to either alter drivers
perception of vehicle handling or to maintain control during
extreme maneuvers.
Active steering concept
driver vehicle
environment
command angle
vehicle states
controlleractive
system
steer angle
introduction steering system estimationvehicle control conclusion
-
8/8/2019 Defense v5
40/63
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 combinationof states and driver command angle.
Obtain sideslip from GPS/INS system (Ryus PhD work).
Physically motivated handling modification
ddr KKrK HH !
introduction steering system estimationvehicle control conclusion
-
8/8/2019 Defense v5
41/63
Define new cornering stiffness as:
Choose feedback gains as:
Vehicle state equation is now:
Physically motivated handling modification
)1( LLLF !!! dr KV
aKK
d
I
aC
V
C
CG
VI
bCaC
I
aCbC
V
aCbC
V
CC
CG
z
f
f
z
rf
z
fr
frrf
rr H
FFE
E
EEEE
EEEE
-
-
-
!
-
22
21
LEE ! 1 ff CC
introduction steering system estimationvehicle control conclusion
-
8/8/2019 Defense v5
42/63
Experimental testing at Moffett Field
introduction steering system estimationvehicle control conclusion
-
8/8/2019 Defense v5
43/63
Unmodified handling: model vs. experiment
introduction steering system estimationvehicle control conclusion
Confirms model parameters match vehicle parameters.
mo_1_3_eta0_d
-
8/8/2019 Defense v5
44/63
Experiment: normal vs. reduced front cornering
stiffness
introduction steering system estimationvehicle control conclusion
Difference between normal and reduced cornering stiffness.
mo_1_3_a05u_b
-
8/8/2019 Defense v5
45/63
Reduced front cornering stiffness: model vs.
experiment
introduction steering system estimationvehicle control conclusion
Understeer characteristic in yaw exactly as predicted.
mo_1_3_a05u_b
-
8/8/2019 Defense v5
46/63
introduction steering system estimationvehicle control conclusion
Verifies sideslip estimation is working.
mo_1_3_eta0_d
Unmodified handling: model vs. experiment
-
8/8/2019 Defense v5
47/63
introduction steering system estimationvehicle control conclusion
Understeer characteristic in sideslip as predicted.
mo_1_3_a05u_b
Reduced front cornering stiffness: model vs.
experiment
-
8/8/2019 Defense v5
48/63
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 estimationvehicle control conclusion
-
8/8/2019 Defense v5
49/63
We need accurate, clean feedback of sideslip angle to smoothly
modify a vehicles handling characteristics.
Can we do this without GPS?
From control to estimation
introduction steering system estimationvehicle control conclusion
-
8/8/2019 Defense v5
50/63
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
Outline
introduction steering system estimationvehicle control conclusion
-
8/8/2019 Defense v5
51/63
Yaw rate easily measured, but sideslip angle much more difficultto 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 vehiclesmotionwe canceled it out, remember?
Can be determined from current applied to the steer-by-wire
actuator.
Sideslip estimation
introduction steering system estimationvehicle control conclusion
-
8/8/2019 Defense v5
52/63
Steering system dynamics
w
w
w
a
M
M
M
J
b
F
k
i
H
X
X
road wheel angle
moment of inertia
damping constant
Coulomb friction
aligning moment
motor torque
motor constantmotor current
w w w a s M
M M M
J b F r
k i
H H X X
X
!
!
&& &
introduction steering system estimationvehicle control conclusion
-
8/8/2019 Defense v5
53/63
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.
? A
0 1 0 0
10
0 0 0 0
1 0 0
w s MM
w w w
a a
a
b r ki
J J J
H HH H
X X
H
H HX
! - - - -
! -
&
&& &
&
&
introduction steering system estimationvehicle control conclusion
-
8/8/2019 Defense v5
54/63
Link between aligning moment and sideslip
angle
Aligning moment can be expressed as function of the vehicle
states,F and r, and the input, H.
fympa Ftt ,!X
HF
HF
E
EE
E
E
E
fp
fp
fp
fp
ffp
CttrV
CttaCtt
V
arCtt
Ctt
!
!
!
introduction steering system estimationvehicle control conclusion
-
8/8/2019 Defense v5
55/63
Vehicle state observer
Express in state space form. Steering angle is input. Yaw rateand aligning moment (from the disturbance observer) are outputs
(measurements).
HF
X
H
FF
EE
E
E
E
EEEE
EEEE
-
-
-
!
-
-
-
-
!
-
fmp
CG
V
Ctta
fmpa
IaC
mV
C
CG
VI
bCaC
I
aCbC
mV
aCbC
mV
CC
CG
CttrCtt
r
rr
fmp
z
f
f
z
rf
z
fr
frrf
010
1
22
2
introduction steering system estimationvehicle control conclusion
-
8/8/2019 Defense v5
56/63
Aligning moment and state estimation
Choose disturbance observer gain Tso that A-TC is stable and
xerr=x-xest approaches zero.
estestest yyTBuAxx !
errerr xCx ! TyBuxTCA est !
introduction steering system estimationvehicle control conclusion
-
8/8/2019 Defense v5
57/63
Not exact, but doesnt need to be.
Estimated aligning moment
data_012504b
introduction steering system estimationvehicle control conclusion
-
8/8/2019 Defense v5
58/63
Sideslip estimate from observer is comparable to estimate from
GPS.
Estimated sideslip and yaw rate
data_012504b
introduction steering system estimationvehicle control conclusion
-
8/8/2019 Defense v5
59/63
State feedback from observer: yaw results comparable to using
GPS.
Experiment: normal vs. reduced front cornering
stiffness
mo_041104_stetam3_a
introduction steering system estimationvehicle control conclusion
-
8/8/2019 Defense v5
60/63
Experiment: normal vs. reduced front cornering
stiffness
mo_041104_stetam3_a
introduction steering system estimationvehicle control conclusion
Sideslip results also comparable to using GPS.
-
8/8/2019 Defense v5
61/63
Driving safety depends on a vehicles underlying handlingcharacteristics.
Can make handling characteristics anything we want providedwe have:
Precise active steering capability
Full knowledge of vehicle states
Precise steering control requires understanding of interactionbetween tire and road.
Treated as disturbance to be canceled out.
Vehicle state estimation uses interaction between tire and roadas source of information.
Seen by observer as force that govern vehicles motion.
Conclusion
introduction steering system estimationvehicle control conclusion
-
8/8/2019 Defense v5
62/63
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).
Future work
introduction steering system estimationvehicle control conclusion
-
8/8/2019 Defense v5
63/63
Advisor, Chris Gerdes
Committee members: Prof. Rock, Prof. Waldron, Prof.
Niemeyer, Chair Enge
Fellow members of the DDL!
Stanford Graduate Fellowship Staff at Moffett Airfield
General Motors Corp.
Nissan Motor Co.
Acknowledgements