Defense v5

Embed Size (px)

Citation preview

  • 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