Steer-by-Wire: Implications for Vehicle Handling and Safety Paul Yih May 27, 2004

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: NASA Source: Boeing Source: USAF

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. Automotive applications for by-wire Source: Motorola

Introduction –Car as a dynamic system –Tire properties –Basic handling characteristics and stability Vehicle control Estimation Conclusion and future work Outline introductionsteering systemvehicle controlestimationconclusion

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. Why do accidents occur? introductionsteering systemvehicle controlestimationconclusion

Assume constant longitudinal speed, V, so only lateral forces. Yaw rate, r, and sideslip angle, , completely describe vehicle motion in plane. Force and mass balance: The car as a dynamic system introductionsteering systemvehicle controlestimationconclusion

Lateral forces are generated by tire “slip.” C  is called tire cornering stiffness. At large slip angles, lateral force approaches friction limits. Relation to slip angle becomes nonlinear near this limit. Linear and nonlinear tire characteristics introductionsteering systemvehicle controlestimationconclusion

Equations of motion: Valid even when tires operating in nonlinear region by approximating nonlinear effects of the tire curve. Linearized vehicle model introductionsteering systemvehicle controlestimationconclusion

Define understeer gradient: A car can have one of three characteristics: Handling characteristics determined by physical properties introductionsteering systemvehicle controlestimationconclusion K us less responsivemore responsive - + understeeringoversteeringneutral steering

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. Understeering introductionsteering systemvehicle controlestimationconclusion

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. Oversteering introductionsteering systemvehicle controlestimationconclusion

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. Neutral steering introductionsteering systemvehicle controlestimationconclusion

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. Real world example: 15 passenger van rollovers introductionsteering systemvehicle controlestimationconclusion

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! How are vehicles designed? introductionsteering systemestimationvehicle controlconclusion

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. Prior art introductionsteering systemvehicle controlestimationconclusion

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. Research contributions introductionsteering systemvehicle controlestimationconclusion

Steering system: precise steering control –Conversion to steer-by-wire –System identification –Steering control design Vehicle control Estimation Conclusion and future work Outline introductionsteering systemestimationvehicle controlconclusion

Conventional steering system introductionsteering systemestimationvehicle controlconclusion

Conversion to steer-by-wire introductionsteering systemestimationvehicle controlconclusion

Steer-by-wire actuator introductionsteering systemestimationvehicle controlconclusion

Steer-by-wire sensors introductionsteering systemestimationvehicle controlconclusion

Force feedback system introductionsteering systemestimationvehicle controlconclusion

System identification Open loop transfer function. Closed loop transfer function. introductionsteering systemestimationvehicle controlconclusion

Closed loop experimental response test_11_13_pb introductionsteering systemestimationvehicle controlconclusion

Bode plot fitted to ETFE test_11_13_pb introductionsteering systemestimationvehicle controlconclusion

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 introductionsteering systemestimationvehicle controlconclusion

Identified response with friction test_11_13_pb Not perfect, but we have feedback. introductionsteering systemestimationvehicle controlconclusion

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 introductionsteering systemestimationvehicle controlconclusion

Feedback control only test_12_3_b0_j0 introductionsteering systemestimationvehicle controlconclusion

Feedback with feedforward compensation test_12_3_b0_j0 introductionsteering systemestimationvehicle controlconclusion

Feedforward and friction compensation test_12_3_b0_j0 introductionsteering systemestimationvehicle controlconclusion

Vehicle on ground test_12_3_b0_j0 (Same controller as before) introductionsteering systemestimationvehicle controlconclusion

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). Aligning moment due to mechanical trail introductionsteering systemestimationvehicle controlconclusion

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 introductionsteering systemestimationvehicle controlconclusion

Controller with aligning moment correction test_12_3_b0_j0 introductionsteering systemestimationvehicle controlconclusion

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? From steering to vehicle control introductionsteering systemestimationvehicle controlconclusion

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 introductionsteering systemestimationvehicle controlconclusion

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 Active steering concept introductionsteering systemestimationvehicle controlconclusion

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. Active steering concept drivervehicle environment command angle vehicle states controller active system steer angle introductionsteering systemestimationvehicle controlconclusion

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). Physically motivated handling modification introductionsteering systemestimationvehicle controlconclusion

Define new cornering stiffness as: Choose feedback gains as: Vehicle state equation is now: Physically motivated handling modification introductionsteering systemestimationvehicle controlconclusion

Experimental testing at Moffett Field introductionsteering systemestimationvehicle controlconclusion

Unmodified handling: model vs. experiment introductionsteering systemestimationvehicle controlconclusion Confirms model parameters match vehicle parameters. mo_1_3_eta0_d

Experiment: normal vs. reduced front cornering stiffness introductionsteering systemestimationvehicle controlconclusion Difference between normal and reduced cornering stiffness. mo_1_3_a05u_b

Reduced front cornering stiffness: model vs. experiment introductionsteering systemestimationvehicle controlconclusion Understeer characteristic in yaw exactly as predicted. mo_1_3_a05u_b

introductionsteering systemestimationvehicle controlconclusion Verifies sideslip estimation is working. mo_1_3_eta0_d Unmodified handling: model vs. experiment

introductionsteering systemestimationvehicle controlconclusion Understeer characteristic in sideslip as predicted. mo_1_3_a05u_b Reduced front cornering stiffness: model vs. experiment

Reducing front cornering stiffness returns vehicle to unloaded characteristic. Modified handling: unloaded vs. rear weight bias mo_2_3_eta02u_w_b introductionsteering systemestimationvehicle controlconclusion

We need accurate, clean feedback of sideslip angle to smoothly modify a vehicle’s handling characteristics. Can we do this without GPS? From control to estimation introductionsteering systemestimationvehicle controlconclusion

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 introductionsteering systemestimationvehicle controlconclusion

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. Sideslip estimation introductionsteering systemestimationvehicle controlconclusion

Steering system dynamics road wheel angle moment of inertia damping constant Coulomb friction aligning moment motor torque motor constant motor current introductionsteering systemestimationvehicle controlconclusion

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. introductionsteering systemestimationvehicle controlconclusion

Link between aligning moment and sideslip angle Aligning moment can be expressed as function of the vehicle states,  and r, and the input, . introductionsteering systemestimationvehicle controlconclusion

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

Aligning moment and state estimation Choose disturbance observer gain T so that A-TC is stable and x err =x-x est approaches zero. introductionsteering systemestimationvehicle controlconclusion

Not exact, but doesn’t need to be. Estimated aligning moment data_012504b introductionsteering systemestimationvehicle controlconclusion

Sideslip estimate from observer is comparable to estimate from GPS. Estimated sideslip and yaw rate data_012504b introductionsteering systemestimationvehicle controlconclusion

State feedback from observer: yaw results comparable to using GPS. Experiment: normal vs. reduced front cornering stiffness mo_041104_stetam3_a introductionsteering systemestimationvehicle controlconclusion

Experiment: normal vs. reduced front cornering stiffness mo_041104_stetam3_a introductionsteering systemestimationvehicle controlconclusion Sideslip results also comparable to using GPS.

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. Conclusion introductionsteering systemestimationvehicle controlconclusion

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 introductionsteering systemestimationvehicle controlconclusion

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 introductionsteering systemestimationvehicle controlconclusion