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Feasibility Analysis of Steering Control in Collisions as a Driver-Assistance or an Automated Function Ching-Yao Chan, Han-Shue Tan 1999 PATH Annual Meeting October 14-15, 1999, Richmond, California. Presentation Outlines.
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Feasibility Analysis of Steering Control in Collisions as a Driver-Assistance or an Automated FunctionChing-Yao Chan, Han-Shue Tan1999 PATH Annual MeetingOctober 14-15, 1999, Richmond, California
PresentationOutlines 1. Background2. Problem Descriptions3. Modeling, Analysis and Controller Design4. Cases Studies of Lane-Tracking Control5. Concluding Remarks
Motivations • Safety is a primary concern for ground vehicle and highway operations. The work presented here deals with safety problems in the post-impact phase. • Without corrective maneuvers, vehicle trajectories after an initial collision can become erratic within a few seconds. Lane departure occurs within 1-3 seconds in typical highway vehicle-following conditions. • Drivers may lose control due to the physical and mental shocks from the collision. Serious consequences may result from such hazardous conditions. • If vehicles can be slowed down or brought to a stop and vehicle trajectories can be stabilized after an initial condition, accident consequences may be mitigated significantly.
Motivations (cont.) • With developments in AVCSS (advanced vehicle control and safety systems), applications of sensing and control systems are promising for damage control in either manual or automated modes. • Previous studies at PATH on collision analysis showed that follow-up maneuvers could be effective in maintaining vehicle trajectories. • PATH experimental groups have successfully demonstrated the use of lateral control in lane-keeping and lane-changing maneuvers.The current study represents a systematic evaluation of lateral controllers in vehicle-following collisions.
Problem Definition: Feasibility of applying steering control in vehicle- following collisions
Vehicle Dynamics: Collision versus Driving • A collision causes sudden changes in the state variables of the involving vehicles, such as longitudinal speeds and yaw rates. The changes are “impulsive” in nature when compared to the vehicle dynamics under normal driving conditions. • The collision phase of a vehicle-to-vehicle collision typically lasts for about 80-150 milliseconds for passenger cars. • The magnitude of the peak acceleration experienced during the collision phase is about 10-30 g’s in 10-30 m/sec (closing speed) vehicle-to-vehicle collisions.
Vehicle Dynamics: Collision versus Driving • Since the acceleration magnitude during the collision is much greater than those generated by tires, it is unrealistic to expect effective steering control during the collision phase. • The question is whether steering control to stabilize vehicle trajectories after the collision phase can be effective. • Therefore the problem of interest is whether control efforts can bring the condition of the vehicles back to the desirable states after the impulsive disturbances.
Collision Conditions and Assumptions • The leading vehicle is initially traveling at a speed slower than the trailing vehicle. • The trailing vehicle rear-ends the leading vehicle. • The vehicles may be laterally offset from the center of the lane. • The two vehicles do not come into contact with other vehicles or obstacles. • The steering systems on both vehicles remain functional after the collision; however, steering angle offset due to collision damage maybe introduced into individual wheels.
Accident Scenarios • The described collision may occur under the following operating scenarios:(a)A vehicle fails to detect a stationary vehicle or obstacle.(b)A vehicle in front is slowing but the trailing vehicle fails to slow down.(c)A vehicle impacts another vehicle in a lane-changing maneuver. • Despite the seemingly diverse situations of these accident types, the nature of the problem remains the same. • The feasibility of performing lane-tracking control is bounded by the physical limitations of exercising control and the ability to acquire necessary sensor information.
Multiple-Vehicle Accident Scenarios • Another group of accidents related to the problem of interest is potential multiple-car accidents on a highway. The associated questions in these cases are:(a) Whether a vehicle suffering multiple impacts can be controlled effectively?(b)What is the appropriate strategy of vehicle control to optimize the outcome or minimize the total risks if the movements of all vehicles can be coordinated? • The second question is more closely associated with an automated highway system, such as a platoon operation, which are addressed in other PATH studies (Tongue, O’Reilly, Swaroop). • The answer to the questions still resides within the same boundary existent for other accidents discussed above and it still requires the same assessment of disturbance magnitude and physical integrity of on-board systems.
Performance Specifications of Control Systems The following questions are posed to judge the effectiveness of steering control in the defined collision scenarios: • Can the vehicles stay within the designated lane after the collision? This condition is measured by checking the lateral displacement of the center of gravity of each vehicle. • Does the yaw angle of a vehicle deviates from the desired angle and diverges continuously without being corrected? The given condition typically indicates a spinout. • After a vehicle undergoes a collision, can the deviations in lateral position and heading angle converge to a desired range? The allowable errors in positions and heading angles depend on the damage of the vehicles.
Estimation of Force and Momentum Average impact force Fi can be approximated by is the velocity difference of either vehicle before and after the impact, and is the impact duration, Since only the scenario of the vehicle following collision is studied in this paper, qi, the angle of Dvi with respect to the original vehicle traveling direction is usually small. Therefore, one can approximate and where di is the moment arm from the impact point to the vehicle CG
Steering Control Analysis Schematic diagram of the vehicle steering control system immediately after the impact phase of the collision.
Generic “Look-Ahead” Steering Controller The corresponding initial conditions are transferred into where ys is the vehicle lateral displacement at the distance L, the look-ahead distance, in front of the vehicle CG. The look-ahead displacement ys is also the input to the controller G(s).
Steering Control Analysis Modified schematic diagram of the corresponding steering control problem
Performance and Stability Requirements Performance: Analysis conclusion: For most passenger cars, control gains (G) of 0.01-0.2 and Look-ahead (L) of 5-20 meters are satisfactory for and Stability where
Simulation Model • SMAC (Simulation Model of Automobile Collisions), used in this study, was developed by NHTSA and Calspan Corporation in 1970s, and widely used by vehicular accident investigators. • SMAC is capable of simulating planar motions and collisions between two vehicles. • A copy of SMAC source codes was obtained from University of Michigan Transportation Research Institute (UMTRI). • Additional features were added into the source codes to allow the implementation of a feedback controller.
Controller Design Methodology • Use vehicle state immediately after impact as initial conditions for the controller. • Investigate controllers that bring the vehicle back with sufficient stability margins. • Satisfy performance/stability requirements with moderately high gain. • Generic look-ahead Controller • U = - G ( Y + L* )U: steering input G: control gain L: look-ahead distance Y: lateral deviation : heading angle deviation
Controller (cont.) • Controller with Gain Compensation • U = - G ( Yp + L* ) • Yp = C(s) Y • C(s) = ( s + 2f1 )/( s + 2f2 ) • C(s) is a compensation filter, which amplify gains in a low frequency range while maintaining a nominal gain in higher frequency ranges. • The frequencies f1 and f2are selected at 0.1 and 0.02 Hz in the scenarios presented. • The compensator effectively cuts down the steady state error.
Assessment of Controller • Variables: • Control Gain • Look Ahead Distance • Time Delay • Compensator • Steering Angle Offset Due to Collision Damage • Control Constraints: • Steering Angle Magnitude • Change Rate of Steering Angle
Case Studies of Vehicle-Following Collisions • The leading vehicle is initially traveling at a speed of 20 m/sec with the trailing vehicle at 30 m/sec. • The leading vehicle is laterally offset from the center of the lane by 0.45 meters. • Both straight and curved scenarios were simulated. The curved road has a radius of curvature of 400 meters. • The two vehicles have parameters similar to those of a PATH experimental vehicle.
Observations from Simulation Results • The appropriate values of control parameters vary with vehicle steering characteristics. • A look-ahead distance of 10-15 meters was sufficient in various scenarios. • The control gains were consistent with those values used in real-world experimentation under normal driving conditions. • A time delay in the order of 0.1 second was well tolerated by the system, thus implying potential control activation by the crash event. • The controller was found to be effective and the proposed system presented a promising strategy of post-accident maneuvers.
Limitation and Implementation Issues Operating Conditions• Limitations in capacity and operating environment• Evaluation of consequences in unfavorable traffic conditions•Identification of proper conditions for control deploymentEquipment Requirements• Crash worthiness and structural integrity of steering mechanism• Steering actuator specifications• System activation by environment identification and crash triggering• Position and orientation sensing for feedback controlSensing Information for Feedback Control• Integrity in a collision• Backup strategies and information reconstruction alternatives
Concluding Remarks • The feasibility of lateral control in vehicle-following collisions was investigated with various operating scenarios. • Mathematical models were constructed and analysis was conducted to validate the selection of controllers. • A simple yet robust controller was proven effective in performing lane-tracking functions. • A lateral control system can potentially become a driver-assistance or an automated function in highway applications. The implementation of suggested systems should be further evaluated by the exploration of operating limits and integrity of sensing and control systems.
