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: 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: 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. introduction 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. 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. 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. 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. 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. 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. 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. 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. 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!

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. 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. 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

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

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

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

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

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

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

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

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

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

28. actuator torque commanded angle (at handwheel) actual angle (at pinion) effective moment of inertia effective damping introduction steering system vehicle control estimation 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.

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

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

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

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

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

36. introduction steering system vehicle control estimation 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. 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: infinitely variable handling characteristics • Handling modification • Experimental results • Estimation • Conclusion and future work

38. command angle steer angle environment conventional steering system driver vehicle vehicle states introduction steering system vehicle control estimation 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.

39. command angle steer angle environment active system driver controller vehicle vehicle states introduction steering system vehicle control estimation 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.

40. introduction steering system vehicle control estimation 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. introduction steering system vehicle control estimation conclusion Physically motivated handling modification • Define new cornering stiffness as: • Choose feedback gains as: • Vehicle state equation is now:

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

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

44. introduction steering system vehicle control estimation conclusion Experiment: normal vs. reduced front cornering stiffness • Difference between normal and reduced cornering stiffness. mo_1_3_a05u_b

45. introduction steering system vehicle control estimation conclusion Reduced front cornering stiffness: model vs. experiment • Understeer characteristic in yaw exactly as predicted. mo_1_3_a05u_b

46. introduction steering system vehicle control estimation conclusion Unmodified handling: model vs. experiment • Verifies sideslip estimation is working. mo_1_3_eta0_d

47. introduction steering system vehicle control estimation conclusion Reduced front cornering stiffness: model vs. experiment • Understeer characteristic in sideslip as predicted. mo_1_3_a05u_b

48. introduction steering system vehicle control estimation conclusion Modified handling: unloaded vs. rear weight bias • Reducing front cornering stiffness returns vehicle to unloaded characteristic. mo_2_3_eta02u_w_b

49. introduction steering system vehicle control estimation 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. 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: 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