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: 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: USAF
Source: Boeing
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
steering system
vehicle control
estimation
conclusion
Outline
Introduction
Car as a dynamic system
Tire properties
Basic handling characteristics and stability
Vehicle control
Estimation
Conclusion and future work

5. 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.

6. The car as a dynamic system
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:

7. Linear and nonlinear tire characteristics
introduction
steering system
vehicle control
estimation
conclusion
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
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.

9. Handling characteristics determined by physical properties
introduction
steering system
vehicle control
estimation
conclusion
Handling characteristics determined by physical properties
Define understeer gradient:
A car can have one of three characteristics:
understeering
neutral steering
oversteering - +
Kus
less responsive
more responsive

10. Understeering Negative real roots at low speed.
introduction
steering system
vehicle control
estimation
conclusion
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.
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.

12. 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.

13. Real world example: 15 passenger van rollovers
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.

14. How are vehicles designed?
introduction
steering system
estimation
vehicle control
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!

15. introduction
steering system
vehicle control
estimation
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.

16. Research contributions
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.

17. Outline Steering system: precise steering control
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
Estimation
Conclusion and future work

18. Conventional steering system
introduction
steering system
estimation
vehicle control
conclusion
Conventional steering system

19. Conversion to steer-by-wire
introduction
steering system
estimation
vehicle control
conclusion
Conversion to steer-by-wire

20. Steer-by-wire actuator
introduction
steering system
estimation
vehicle control
conclusion
Steer-by-wire actuator

21. Steer-by-wire sensors
introduction
steering system
estimation
vehicle control
conclusion
Steer-by-wire sensors

22. Force feedback system introduction steering system estimation
vehicle control
conclusion
Force feedback system

23. System identification
introduction
steering system
estimation
vehicle control
conclusion
System identification
Open loop transfer function.
Closed loop transfer function.

24. Closed loop experimental response
introduction
steering system
estimation
vehicle control
conclusion
Closed loop experimental response
test_11_13_pb

25. Bode plot fitted to ETFE
introduction
steering system
estimation
vehicle control
conclusion
Bode plot fitted to ETFE
test_11_13_pb

26. System identification
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.

27. Identified response with friction
introduction
steering system
estimation
vehicle control
conclusion
Identified response with friction
Not perfect, but we have feedback.
test_11_13_pb

28. What do you need in a controller?
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.
actuator torque
commanded angle (at handwheel)
actual angle (at pinion)
effective moment of inertia
effective damping

29. Feedback control only introduction steering system estimation
vehicle control
conclusion
Feedback control only
test_12_3_b0_j0

30. Feedback with feedforward compensation
introduction
steering system
estimation
vehicle control
conclusion
Feedback with feedforward compensation
test_12_3_b0_j0

31. Feedforward and friction compensation
introduction
steering system
estimation
vehicle control
conclusion
Feedforward and friction compensation
test_12_3_b0_j0

32. Vehicle on ground (Same controller as before) introduction
steering system
estimation
vehicle control
conclusion
Vehicle on ground
(Same controller as before)
test_12_3_b0_j0

33. Aligning moment due to mechanical trail
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).

34. Aligning moment due to pneumatic trail
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).

35. Controller with aligning moment correction
introduction
steering system
estimation
vehicle control
conclusion
Controller with aligning moment correction
test_12_3_b0_j0

36. From steering to vehicle control
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?

37. Outline Steering system: precise steering control
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

38. Active steering concept
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

39. Active steering concept
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

40. Physically motivated handling modification
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).

41. Physically motivated handling modification
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:

42. Experimental testing at Moffett Field
introduction
steering system
estimation
vehicle control
conclusion
Experimental testing at Moffett Field

43. Unmodified handling: model vs. experiment
introduction
steering system
estimation
vehicle control
conclusion
Unmodified handling: model vs. experiment
Confirms model parameters match vehicle parameters.
mo_1_3_eta0_d

44. Experiment: normal vs. reduced front cornering stiffness
introduction
steering system
estimation
vehicle control
conclusion
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
introduction
steering system
estimation
vehicle control
conclusion
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
introduction
steering system
estimation
vehicle control
conclusion
Unmodified handling: model vs. experiment
Verifies sideslip estimation is working.
mo_1_3_eta0_d

47. Reduced front cornering stiffness: model vs. experiment
introduction
steering system
estimation
vehicle control
conclusion
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
introduction
steering system
estimation
vehicle control
conclusion
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
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?

50. Outline Steering system: precise steering control
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

51. 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.

52. Steering system dynamics
introduction
steering system
estimation
vehicle control
conclusion
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
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.

54. Link between aligning moment and sideslip angle
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,  and r, and the input, d.

55. Vehicle state observer
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).

56. Aligning moment and state estimation
introduction
steering system
estimation
vehicle control
conclusion
Aligning moment and state estimation
Choose disturbance observer gain T so that A-TC is stable and xerr=x-xest approaches zero.

57. Estimated aligning moment
introduction
steering system
estimation
vehicle control
conclusion
Estimated aligning moment
Not exact, but doesn’t need to be.
data_012504b

58. Estimated sideslip and yaw rate
introduction
steering system
estimation
vehicle control
conclusion
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
introduction
steering system
estimation
vehicle control
conclusion
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
introduction
steering system
estimation
vehicle control
conclusion
Experiment: normal vs. reduced front cornering stiffness
Sideslip results also comparable to using GPS.
mo_041104_stetam3_a

61. introduction
steering system
estimation
vehicle control
conclusion
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.

62. 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).