Avec04 FF

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					Handwheel Force Feedback
for Lanekeeping Assistance:
  Combined Dynamics and
          Stability
                    Josh Switkes

                  Eric J. Rossetter
                     Ian A. Coe
                 J. Christian Gerdes

                  Stanford University
                                              D    ynamic
                                              D    esign
     AVEC 2004                    24-Aug-04
                                              L   aboratory
 Why Lanekeeping Assistance?
• Over 40% of accident fatalities in the U.S.
  in 2001 were from collisions with fixed
  objects
  – This accounted for 19,000 deaths in 2001
• Lanekeeping assistance
  seeks to keep the car in
  the lane in the absence
  of driver input
  – Medical problem
  – Distraction
                                               D    ynamic
                                               D    esign
            AVEC 2004      24-Aug-04
                                               L   aboratory
      Why Force Feedback?
• One way to implement lanekeeping
  assistance is with steer-by-wire
• Advantage: Handwheel and roadwheels
  decoupled
   – Complete freedom in roadwheel command
• Challenge: Handwheel and roadwheels
  decoupled
   – No natural force feedback
• Need to recreate force feedback artificially

                                                 D    ynamic
                                                 D    esign
            AVEC 2004            24-Aug-04
                                                 L   aboratory
           Force Feedback
• Can be function of vehicle states, handwheel
  states and lanekeeping information
  – Change mechanical feel
  – Reproduce feel of tire forces
  – Add lanekeeping signal




                                            D     ynamic
                                            D     esign
            AVEC 2004          24-Aug-04
                                            L    aboratory
         Coupled System
• Force feedback and vehicle motion are
  coupled
• Need to ensure entire system is stable




                                       D    ynamic
                                       D    esign
          AVEC 2004     24-Aug-04
                                       L   aboratory
                        Outline
• Potential field lanekeeping control
  – Specific goal of this assistance
• Force feedback system
  – Types of force feedback and modeling
• Combined dynamics and stability
  – How do we stabilize the system
• Experimental results
  – Model successfully predicts stability
                                            D    ynamic
                                            D    esign
            AVEC 2004        24-Aug-04
                                            L   aboratory
          Potential Field Control
• Seeks to guarantee vehicle will not leave lane
  in the absence of driver input
• Generalized PD controller:
  – applies forces derived from
    gradient of potential field
• Zero assistance at
  lane center, with increasing
  assistance towards lane edges


                                                   D    ynamic
                                                   D    esign
               AVEC 2004          24-Aug-04
                                                   L   aboratory
           Potential Field Control
• Through steer-by-wire, we can add steering on top of
  that commanded by the driver
• Steering addition is function of lateral error and
  heading error
   – Heading error multiplied by lookahead distance
   – Lookahead distance can be chosen from a range of stable
     values




• Modeled with linear bicycle model
                                                               D    ynamic
                                                               D    esign
                AVEC 2004            24-Aug-04
                                                               L   aboratory
Possible Sources of Force Feedback
• Aligning moment
   – Function of states including slip angle
• Potential field force
   – Replicates feel of driving in a physical potential well
• Damping on wheel
   – Approximates feel of friction in rack, bushings and hydraulic
     system
• Inertia on wheel
   – Difficult to control wheel position with low inertia
   – Low inertia allows sudden motion of wheel never needed for
     normal driving
• Not all of these may be necessary
• Model as second order linear system:
                                                                     D    ynamic
                                                                     D    esign
                   AVEC 2004               24-Aug-04
                                                                     L   aboratory
        Three Driving Situations

• Driver holding steering wheel straight
  – Performance of system depends on potential field
    characteristics
  – Past work
• Driver is steering normally
  – User feel
  – Requires user testing
• Driver has no influence on steering wheel
  – Depends on coupling between vehicle and handwheel
  – Stability not automatic
                                                   D    ynamic
                                                   D    esign
             AVEC 2004          24-Aug-04
                                                   L   aboratory
      Combined Linear System
• Linear model allows root locus analysis
• Can plot 6 system poles as a function of:
   – Force feedback gains
   – Controller Parameters
   – Speed
• This allows identification of stable force feedback gains
  and intuition about effect of changing gains
• Begin with no force feedback, add sources one by one




                                                       D      ynamic
                                                       D      esign
                AVEC 2004         24-Aug-04
                                                       L   aboratory
 Increasing Assistance Based FF


• With any
  significant
  assistance-based
  force feedback,
  system is unstable




                                     D    ynamic
                                     D    esign
             AVEC 2004   24-Aug-04
                                     L   aboratory
  Increasing Handwheel Damping

• With reasonable
  amount of
  assistance based
  force feedback
  system can be
  stabilized with
  handwheel
  damping
• Too much damping
  may not be
  desirable to driver
                                      D    ynamic
                                      D    esign
              AVEC 2004   24-Aug-04
                                      L   aboratory
      Add Aligning Moment

• Another way
  to stabilize
  the system:
  aligning
  moment
• Effect is
  similar to
  damping
                                 D    ynamic
                                 D    esign
         AVEC 2004   24-Aug-04
                                 L   aboratory
       Increasing Lookahead


• Stability is also
  increased with
  increasing
  lookahead
• Tends to
  prevent large
  yaw
                                    D    ynamic
                                    D    esign
            AVEC 2004   24-Aug-04
                                    L   aboratory
       Increasing Vehicle Speed
• Increasing speed
  initially increases
  stability due to
  tighter coupling
  between
  handwheel and
  vehicle motion
• At higher speeds
  underlying vehicle
  damping
  decreases
  (understeering
  vehicle)
                                      D    ynamic
                                      D    esign
              AVEC 2004   24-Aug-04
                                      L   aboratory
 Linear Analysis Conclusions
• Handwheel and vehicle are very
  coupled
• Stable system possible with appropriate
  choice of:
  – Handwheel damping
  – Aligning Moment
  – Lookahead
• Must verify model experimentally
                                       D    ynamic
                                       D    esign
         AVEC 2004      24-Aug-04
                                       L   aboratory
    Experimental Implementation
• Verifies utility of model
• Steer-by-wire Corvette C5
  – Steering shaft removed, brushless motor connected
    to input of power steering
    unit
• Force Feedback System
  – Speed: 700 deg/s
  – Torque: 20 Nm maximum
            5 Nm continuous
  – Designed for good user feel
     • No gearbox
     • Minimum torque ripple

                                                    D    ynamic
                                                    D    esign
                 AVEC 2004        24-Aug-04
                                                    L   aboratory
            Experimental Setup
• Straight smooth taxiway at Moffett airfield
• Constant speed of 20m/s
• Vehicle starts with offset into potential and
  nonzero handwheel angle
• No driver input to hand
  wheel (hands-off)
• Types of feedback
   – Potential field force
   – Damping
   – Inertia
                                                  D    ynamic
                                                  D    esign
               AVEC 2004      24-Aug-04
                                                  L   aboratory
Unstable Experimental Result




                              D    ynamic
                              D    esign
      AVEC 2004   24-Aug-04
                              L   aboratory
     Stable Experimental Result
• Similar gains
• More hand-
  wheel damping
• Similar response
  to system with
  no force
  feedback


                                     D    ynamic
                                     D    esign
             AVEC 2004   24-Aug-04
                                     L   aboratory
                     Conclusions
• This work provides a framework for the simultaneous
  design of force feedback and assistance systems.
• This analysis shows that within a range of reasonable
  values changes to the lanekeeping controller or force
  feedback can have marked affects on the response
  of the vehicle.
• A system designed for good driver feel can very
  easily be unstable in the absence of driver inputs.
• This work finds the range of values which will result in
  a stable system
   – parameters must be chosen from this range to ensure
     stability
                                                           D    ynamic
                                                           D    esign
              AVEC 2004            24-Aug-04
                                                           L   aboratory

				
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posted:9/29/2012
language:English
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