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

1
Steer-by-Wire Implications for Vehicle Handling
and Safety
2
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 Boeing
Source USAF
Source NASA
Source NASA
3
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
4
Outline

• Introduction
• Car as a dynamic system
• Tire properties
• Basic handling characteristics and stability
• Vehicle control
• Estimation
• Conclusion and future work

5
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.

6
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

7
Linear and nonlinear tire characteristics

• Lateral forces are generated by tire slip.
• Ca is called tire cornering stiffness.
• At large slip angles, lateral force approaches
  friction limits.
• Relation to slip angle becomes nonlinear near
  this limit.

8
Linearized vehicle model

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

9
Handling characteristics determined by physical
properties

• Define understeer gradient
• A car can have one of three characteristics

understeering
oversteering
neutral steering
-

Kus
less responsive
more responsive
10
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.

11
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.

12
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.

13
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.

14
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?
• Dont 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!

15
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.

16
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 vehicles handling
  characteristics.
• Never done before due to difficulty in obtaining
  accurate sideslip measurement, and
• There just arent 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.

17
Outline

• Steering system precise steering control
• Conversion to steer-by-wire
• System identification
• Steering control design
• Vehicle control
• Estimation
• Conclusion and future work

18
Conventional steering system
19
Conversion to steer-by-wire
20
Steer-by-wire actuator
21
Steer-by-wire sensors
22
Force feedback system
23
System identification

• Open loop transfer function.
• Closed loop transfer function.

24
Closed loop experimental response
test_11_13_pb
25
Bode plot fitted to ETFE
test_11_13_pb
26
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.

27
Identified response with friction

• Not perfect, but we have feedback.

test_11_13_pb
28
What do you need in a controller?

• Actual steer angle should track commanded angle
  with minimal error.
• Initially consider no tire-to-ground contact.

29
Feedback control only
test_12_3_b0_j0
30
Feedback with feedforward compensation
test_12_3_b0_j0
31
Feedforward and friction compensation
test_12_3_b0_j0
32
Vehicle on ground
(Same controller as before)
test_12_3_b0_j0
33
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).

34
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).

35
Controller with aligning moment correction
test_12_3_b0_j0
36
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 systemwhat can we do with it?

37
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

38
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.

39
Active steering concept

• 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.

40
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 (Ryus PhD
  work).

41
Physically motivated handling modification

• Define new cornering stiffness as
• Choose feedback gains as
• Vehicle state equation is now

42
Experimental testing at Moffett Field
43
Unmodified handling model vs. experiment

• Confirms model parameters match vehicle
  parameters.

mo_1_3_eta0_d
44
Experiment normal vs. reduced front cornering
stiffness

• Difference between normal and reduced cornering
  stiffness.

mo_1_3_a05u_b
45
Reduced front cornering stiffness model vs.
experiment

• Understeer characteristic in yaw exactly as
  predicted.

mo_1_3_a05u_b
46
Unmodified handling model vs. experiment

• Verifies sideslip estimation is working.

mo_1_3_eta0_d
47
Reduced front cornering stiffness model vs.
experiment

• Understeer characteristic in sideslip as
  predicted.

mo_1_3_a05u_b
48
Modified handling unloaded vs. rear weight bias

• Reducing front cornering stiffness returns
  vehicle to unloaded characteristic.

mo_2_3_eta02u_w_b
49
From control to estimation

• We need accurate, clean feedback of sideslip
  angle to smoothly modify a vehicles handling
  characteristics.
• Can we do this without GPS?

50
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

51
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
  vehicles motionwe canceled it out, remember?
• Can be determined from current applied to the
  steer-by-wire actuator.

52
Steering system dynamics
road wheel angle moment of inertia damping
constant Coulomb friction aligning moment motor
torque motor constant motor current
53
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.

54
Link between aligning moment and sideslip angle

• Aligning moment can be expressed as function of
  the vehicle states, ? and r, and the input, d.

55
Vehicle state observer

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

56
Aligning moment and state estimation

• Choose disturbance observer gain T so that A-TC
  is stable and xerrx-xest approaches zero.

57
Estimated aligning moment

• Not exact, but doesnt need to be.

data_012504b
58
Estimated sideslip and yaw rate

• Sideslip estimate from observer is comparable to
  estimate from GPS.

data_012504b
59
Experiment normal vs. reduced front cornering
stiffness

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

mo_041104_stetam3_a
60
Experiment normal vs. reduced front cornering
stiffness

• Sideslip results also comparable to using GPS.

mo_041104_stetam3_a
61
Conclusion

• Driving safety depends on a vehicles 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 vehicles
  motion.

62
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).