Active Driver Assistance for Vehicle Lanekeeping Eric J. Rossetter October 30, 2003 D D L ynamic esign aboratory
Motivation In 2001, 43% of all vehicle fatalities in the U.S. were caused by a collision with a fixed obstacle (NHTSA) This accounted for over 18,000 fatalities in 2001 Stanford University Active Driver Assistance for Vehicle Lanekeeping - 2 Dynamic Design Lab
Spectrum of Lanekeeping Assistance PASSIVE ACTIVE Stanford University Active Driver Assistance for Vehicle Lanekeeping - 3 Dynamic Design Lab
Spectrum of Lanekeeping Assistance PASSIVE ACTIVE Lane Departure Warning These systems warn the driver of an imminent lane departure (Suzuki and Jansson (2003), Feng et al. (2000), Lee et al. (1999)) Visual, Auditory, or Haptic Lanekeeping is still the drivers responsibility Stanford University Active Driver Assistance for Vehicle Lanekeeping - 4 Dynamic Design Lab
Spectrum of Lanekeeping Assistance PASSIVE ACTIVE Lane Departure Warning Fully Autonomous Completely autonomous systems remove the driver entirely from the lanekeeping task (Fenton and Mayhon (1991), Shladover et al. (1991)) Requires an extremely high level of robustness Stanford University Active Driver Assistance for Vehicle Lanekeeping - 5 Dynamic Design Lab
Spectrum of Lanekeeping Assistance PASSIVE ACTIVE Lane Departure Warning Fully Autonomous Active Assistance Active assistance combines driver and controller commands to aid in lanekeeping Stanford University Active Driver Assistance for Vehicle Lanekeeping - 6 Dynamic Design Lab
Design Challenge for Active Assistance Design objectives for a lanekeeping assistance system: Provide lanekeeping ability without driver inputs Must not hinder normal driving These are often competing objectives How do we design the lanekeeping controller to satisfy both these objectives? Stanford University Active Driver Assistance for Vehicle Lanekeeping - 7 Dynamic Design Lab
Talk Outline Enabling Technologies Spring Analogy for Lanekeeping Stability Conditions Spring attachment point Sensing location Sizing the Spring Experimental Results Conclusions Stanford University Active Driver Assistance for Vehicle Lanekeeping - 8 Dynamic Design Lab
Enabling Technologies The ability to determine lane position Vision Systems (Gehrig et al. (2002), Franke et al. (1997)) GPS combined with Precision Road Maps (Omae & Fujioka (1999), Farrell & Barth (2000), Alban (2002)) Control of steering, braking, and throttle Brake- and Throttle-by-Wire Steer-by-Wire Source: Delphi Automotive Stanford University Active Driver Assistance for Vehicle Lanekeeping - 9 Dynamic Design Lab
Steer-by-Wire Corvette 1997 Corvette was modified to a steer-by-wire format No mechanical connection between the steering wheel and front road wheels handwheel steering column intermediate shaft belt drive handwheel angle sensor handwheel feedback motor steering actuator universal joints power assist unit pinion angle sensor gear assembly pinion rack Conventional Steering System Steer-by-Wire System Stanford University Active Driver Assistance for Vehicle Lanekeeping - 10 Dynamic Design Lab
Global Positioning System The Global Positioning System (GPS) is a worldwide positioning system consisting of over 24 satellites that broadcast ranging information Stanford University Active Driver Assistance for Vehicle Lanekeeping - 11 Dynamic Design Lab
Position Using GPS Distance from each satellite is determined by the time of flight of the satellite signal ρ = c t Satellite 2 Satellite 1 Satellite 3 ρ 1 Stanford University Active Driver Assistance for Vehicle Lanekeeping - 12 Dynamic Design Lab
Position Using GPS Distance from each satellite is determined by the time of flight of the satellite signal ρ = c t Satellite 2 Satellite 1 ρ Satellite 3 2 ρ 1 Stanford University Active Driver Assistance for Vehicle Lanekeeping - 13 Dynamic Design Lab
Position Using GPS Distance from each satellite is determined by the time of flight of the satellite signal ρ = c t Satellite 2 Satellite 1 ρ Satellite 3 2 ρ 1 ρ 3 Stanford University Active Driver Assistance for Vehicle Lanekeeping - 14 Dynamic Design Lab
Differential GPS A base station and vehicle see the same atmospheric errors Centimeter level accuracy is obtained with differential corrections Ionosphere ~20,000 km Troposphere ~1 km Stanford University Active Driver Assistance for Vehicle Lanekeeping - 15 Dynamic Design Lab
Control Concept Make vehicle behave as though it is attached to the road with a mechanical spring Road Centerline Force = k e Spring does not have to be extremely stiff Control force smoothly assists the driver Energy interpretation is useful for analysis Lateral Error: e Stanford University Active Driver Assistance for Vehicle Lanekeeping - 16 Dynamic Design Lab
Realizing this Control Concept With by-wire technology the steering, braking, and throttle are controlled to create the equivalent spring force F yl (δ) F yr (δ) F x F xlf F xrf F x x cf F y M F y F xlr F xrr Physical System Equivalent Control Force Stanford University Active Driver Assistance for Vehicle Lanekeeping - 17 Dynamic Design Lab
Realizing this Control Concept With control of only the front steering the control force is constrained at the front axle F yl (δ) F yr (δ) F x F y F x a M F y Physical System Equivalent Control Force Stanford University Active Driver Assistance for Vehicle Lanekeeping - 18 Dynamic Design Lab
Vehicle Behavior How does the vehicle behave under this control concept? Perform a linear analysis of the system dynamics: Model the vehicle dynamics with the lanekeeping controller Linearize the vehicle dynamics Look at eigenvalues (tell us about the system behavior) Lanekeeping behavior is related to the spring attachment point Stanford University Active Driver Assistance for Vehicle Lanekeeping - 19 Dynamic Design Lab
Attachment Point Condition Intuitively, creating the effect of a spring attached near the front of the vehicle will rotate the vehicle in the direction of the applied force Desired Location Stanford University Active Driver Assistance for Vehicle Lanekeeping - 20 Dynamic Design Lab
Attachment Point Condition Intuitively, creating the effect of a spring attached near the front of the vehicle will rotate the vehicle in the direction of the applied force Desired Location Stanford University Active Driver Assistance for Vehicle Lanekeeping - 21 Dynamic Design Lab
Attachment Point Condition Intuitively, creating the effect of a spring attached near the front of the vehicle will rotate the vehicle in the direction of the applied force Desired Location Stanford University Active Driver Assistance for Vehicle Lanekeeping - 22 Dynamic Design Lab
Attachment Point Condition What if the attachment point moves too far back? Desired Location Stanford University Active Driver Assistance for Vehicle Lanekeeping - 23 Dynamic Design Lab
Attachment Point Condition What if the attachment point moves too far back? Desired Location Stanford University Active Driver Assistance for Vehicle Lanekeeping - 24 Dynamic Design Lab
Attachment Point Condition What if the attachment point moves too far back? Desired Location Vehicle moves away from desired location! Stanford University Active Driver Assistance for Vehicle Lanekeeping - 25 Dynamic Design Lab
Neutral Steer Point Critical point is called the neutral steer point Neutral steer point is the location on a vehicle where an external force creates no rotation (yaw rate) Neutral Steer Point UNSTABLE STABLE Location of the neutral steer point depends on vehicle properties Tires and weight distribution Stanford University Active Driver Assistance for Vehicle Lanekeeping - 26 Dynamic Design Lab
Neutral Steer Point This point is used in vehicle design to ensure appropriate responses to physical disturbances Side-wind or gravity force on banked roads What is the implication for the design of a lanekeeping control system? Must coordinate the steering, braking, and throttle commands to ensure that the resultant force is in front of the neutral steer point With only front steering the control force is always in front of the neutral steer point Stanford University Active Driver Assistance for Vehicle Lanekeeping - 27 Dynamic Design Lab
Simulation 2 Lateral Error of Vehicle Lateral Position (m) 1.5 1 0.5 Force behind neutral steer point Force in front of neutral steer point 0-0.5 0 1 2 3 4 5 6 7 8 9 10 Time (s) Stanford University Active Driver Assistance for Vehicle Lanekeeping - 28 Dynamic Design Lab
High Speed Oscillations Satisfying the neutral steer point condition is not sufficient for high speed lanekeeping 8 Lateral Error Also need to look at sensing location Drivers look forward while driving Lateral Position (m) 6 4 2 0-2 -4-6 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 Time (s) Stanford University Active Driver Assistance for Vehicle Lanekeeping - 29 Dynamic Design Lab
Sensing Location Controller must use a certain amount of lookahead for high speed stability Base spring compression/extension on the lateral error in front of the vehicle Sensing at C.G. 8 6 No Lookahead 15 m Lookahead 4 Using Lookahead lateral position (m) 2 2 0 4 6 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 time (s) Stanford University Active Driver Assistance for Vehicle Lanekeeping - 30 Dynamic Design Lab
Sizing the Spring Control force must be large enough to keep the vehicle in the lane without driver commands Most important factor for sizing the control force (or spring) is the road curvature Spring force must provide the necessary centripetal force for the turn Stanford University Active Driver Assistance for Vehicle Lanekeeping - 31 Dynamic Design Lab
Sizing the Spring: Basic Concept Centripetal force needed to go around a turn of radius R at a velocity V is: F c = m V 2 R Recall that the spring force is: F s = ke Equate the two forces and solve for the spring stiffness k = m V 2 Re Road Edge R Stanford University Active Driver Assistance for Vehicle Lanekeeping - 32 Dynamic Design Lab
Sizing the Spring In reality it is a little more complicated Curvature is not constant Vehicle dynamics influence motion Transient effects Basic spring analogy and concept of energy storage is used to scale the gain of the controller (or spring) Lyapunov theory Stanford University Active Driver Assistance for Vehicle Lanekeeping - 33 Dynamic Design Lab
Lyapunov Approach Create an energy-like function of the states with the properties: L(x) > 0 L(x) < 0 x(0),x(0) x max Energy Bowl 10 8 Sublevel Sets 100 6 80 4 Energy 60 40 20 Velocity 2 2 0 Initial Condition Max Position 0-10 -5 0 Velocity 5 10 10 5 0 Position -5-10 4 6 8 10 10 5 0 5 10 Position Stanford University Active Driver Assistance for Vehicle Lanekeeping - 34 Dynamic Design Lab
Experimental System Test vehicle is a 1997 Corvette Modified to include steer-by-wire DC-DC Converters GPS Receivers Inertial Sensors Control Computer Motor Amplifier Stanford University Active Driver Assistance for Vehicle Lanekeeping - 35 Dynamic Design Lab
Video Stanford University Active Driver Assistance for Vehicle Lanekeeping - 36 Dynamic Design Lab
Experiment vs. Simulation 100 Map 2 Lateral Error Simulation Experimental 50 1.5 0 1 North (m) 50 100 150 200 Lateral Error (m) 0.5 0-0.5 250-1 300-1.5 350 200 100 0 100 200 300 East (m) -2 20 30 40 50 60 70 80 90 100 110 Time (s) Experimental results show one loop at 11m/s (25mph) without driver steering inputs Guarantee the lanekeeping ability of the system Stanford University Active Driver Assistance for Vehicle Lanekeeping - 37 Dynamic Design Lab
Lanekeeping Bounds 100 Map 2 Lateral Error Simulation Experimental 50 1.5 0 1 North (m) 50 100 150 200 Lateral Error (m) 0.5 0-0.5 Reachable Set 250-1 300-1.5 350 200 100 0 100 200 300 East (m) -2 20 30 40 50 60 70 80 90 100 110 Time (s) Experimental results show one loop at 11m/s (25mph) without driver steering inputs Guarantee the lanekeeping ability of the system Stanford University Active Driver Assistance for Vehicle Lanekeeping - 38 Dynamic Design Lab
Conclusions Lanekeeping system works remarkably well Two important conditions for a well-behaved lanekeeping system Neutral steer point condition Sensing location (lookahead) Quantitative guarantee on the lanekeeping performance Stanford University Active Driver Assistance for Vehicle Lanekeeping - 39 Dynamic Design Lab
The End Questions? Stanford University Active Driver Assistance for Vehicle Lanekeeping - 40 Dynamic Design Lab