Steer-by-Wire: Implications for Vehicle Handling and Safety What is by-wire? Replace mechanical and hydraulic control mechanisms with anelectronic system. Technology first appeared in aviation: NASAs digital fly-by-wireaircraft (1972). Today many civil and most military aircraft rely on fly-by-wire. Revolutionized aircraft design due to improved performance andsafety over conventional flight control systems. Source: USAF Source: Boeing Source: NASA Source: NASA Automotive applications for by-wire
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 toaircraft, steer-by-wire promisesto significantly improve vehiclehandling and driving safety. Source: Motorola 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 introduction steering system vehicle control estimation conclusion Why do accidents occur? 42% of fatal crashes result fromloss of control (EuropeanAccident Causation Survey,2001). In most conditions, a vehicleunder proper control is very safe. However, every vehicle hasthresholds beyond which controlbecomes extremely difficult. The car as a dynamic system
introduction steering system vehicle control estimation conclusion The car as a dynamic system Assume constantlongitudinal speed, V, soonly lateral forces. Yaw rate, r, and sideslipangle, b, completelydescribe vehicle motionin plane. Force and massbalance: Linear and nonlinear tire characteristics
introduction steering system vehicle control estimation conclusion Linear and nonlinear tire characteristics Lateral forces aregenerated by tire slip. Ca is called tire corneringstiffness. At large slip angles, lateralforce approaches frictionlimits. Relation to slip anglebecomes nonlinear nearthis limit. Linearized vehicle model
introduction steering system vehicle control estimation conclusion Linearized vehicle model Equations of motion: Valid even when tiresoperating in nonlinear regionby approximating nonlineareffects of the tire curve. 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 Understeering Negative real roots at low speed.
introduction steering system vehicle control estimation conclusion Understeering Negative real roots at lowspeed. As speed increases, polesmove off real axis. Understeering vehicle is alwaysstable, but yaw becomesoscillatory at higher speed. 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 polemoves into right half plane. At higher speed, oversteeringvehicle becomes unstable! Analogy to unstable aircraft: themore oversteering a vehicle is,the more responsive it will be. introduction steering system vehicle control estimation conclusion Neutral steering Single negative real root dueto pole zero cancellation. Always stable with first orderresponse. This is the ideal handlingcase. Not practical to design thisway: small changes inoperating conditions(passengers or cargo, tirewear) can make itoversteering. 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 handlingregion. Full load also raised center of gravity height, contributing to rollover. 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? 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 statefeedback! introduction steering system vehicle control estimation conclusion Prior art Active steering has been demonstrated using yaw rate andlateral acceleration feedback (Ackermann et al. 1999, Segawa etal. 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. Research contributions
introduction steering system vehicle control estimation conclusion Research contributions An approach for precise by-wire steering control taking into accountsteering system dynamics and tire forces. Techniques apply to steer-by-wire design in general. The application of active steering capability and full state feedbackto virtually and fundamentally modify a vehicles handlingcharacteristics. 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 sideslip observerbased on steering forces. Two-observer structure combines steering system and vehicle dynamicsthe way they are naturally linked. Solve the problem of sideslip estimation. 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 Conventional steering system
introduction steering system estimation vehicle control conclusion Conventional steering system Conversion to steer-by-wire
introduction steering system estimation vehicle control conclusion Conversion to steer-by-wire Steer-by-wire actuator
introduction steering system estimation vehicle control conclusion Steer-by-wire actuator Steer-by-wire sensors
introduction steering system estimation vehicle control conclusion Steer-by-wire sensors Force feedback system introduction steering system estimation
vehicle control conclusion Force feedback system System identification
introduction steering system estimation vehicle control conclusion System identification Open loop transfer function. Closed loop transfer function. Closed loop experimental response
introduction steering system estimation vehicle control conclusion Closed loop experimental response test_11_13_pb Bode plot fitted to ETFE
introduction steering system estimation vehicle control conclusion Bode plot fitted to ETFE test_11_13_pb 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. 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 What do you need in a controller?
introduction steering system estimation vehicle control conclusion What do you need in a controller? Actual steer angle shouldtrack commanded angle withminimal error. Initially consider no tire-to- ground contact. actuator torque commanded angle (at handwheel) actual angle (at pinion) effective moment of inertia effective damping Feedback control only introduction steering system estimation
vehicle control conclusion Feedback control only test_12_3_b0_j0 Feedback with feedforward compensation
introduction steering system estimation vehicle control conclusion Feedback with feedforward compensation test_12_3_b0_j0 Feedforward and friction compensation
introduction steering system estimation vehicle control conclusion Feedforward and friction compensation test_12_3_b0_j0 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 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 andcenter of tire contact patch. Lateral force acting on contact patch generates moment aboutsteer axis (against direction of steering). 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 ofcontact patch. Pneumatic trail also contributes to moment about steer axis(usually against direction of steering). Controller with aligning moment correction
introduction steering system estimation vehicle control conclusion Controller with aligning moment correction test_12_3_b0_j0 From steering to vehicle control
introduction steering system estimation vehicle control conclusion From steering to vehicle control 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? 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 Active steering concept
introduction steering system estimation vehicle control conclusion Active steering concept One of the main benefits of steer-by-wire over conventionalsteering mechanisms is active steering capability. For a conventional steering system, road wheel angle has adirect correspondence to driver command at the steering wheel. driver conventional steering system vehicle environment steer angle vehicle states command angle Active steering concept
introduction steering system estimation vehicle control conclusion Active steering concept For an active steering system, actual steer angle can be differentfrom driver command angle to either alter drivers perception ofvehicle handling or to maintain control during extrememaneuvers. driver vehicle environment command angle vehicle states controller active system steer angle Physically motivated handling modification
introduction steering system estimation vehicle control conclusion Physically motivated handling modification Automotive racing example: driver makes pit stop to changetires. Virtual tire change: effectively alter front cornering stiffnessthrough 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
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: Experimental testing at Moffett Field
introduction steering system estimation vehicle control conclusion Experimental testing at Moffett Field 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 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 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 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 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 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 From control to estimation
introduction steering system estimation vehicle control conclusion From control to estimation We need accurate, clean feedback of sideslip angle to smoothlymodify a vehicles handling characteristics. Can we do this without GPS? 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 introduction steering system estimation vehicle control conclusion Sideslip estimation 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-wireactuator. 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 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 isdisturbance to be estimated. 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 vehiclestates, and r, and the input, d. Vehicle state observer
introduction steering system estimation vehicle control conclusion Vehicle state observer Express in state space form.Steering angle is input.Yaw rateand aligning moment (from the disturbance observer) are outputs(measurements). 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 andxerr=x-xest approaches zero. Estimated aligning moment
introduction steering system estimation vehicle control conclusion Estimated aligning moment Not exact, but doesnt need to be. data_012504b 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 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 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 introduction steering system estimation vehicle control conclusion Conclusion 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. introduction steering system estimation vehicle control conclusion Future work Adaptive modeling to accommodate nonlinear handlingcharacteristics. Apply knowledge of tire forces to determine where the limits areand stay below them. Bounding uncertainty in observer-based sideslip estimation. Apply control and estimation techniques to a dedicated by-wirevehicle (Nissan project).