1. Handwheel Force Feedback for Lanekeeping Assistance: Combined Dynamics and Stability
Josh Switkes
Eric J. Rossetter
Ian A. Coe
J. Christian Gerdes
Stanford University
AVEC 2004
24-Aug-04

2. 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
AVEC 2004
24-Aug-04

3. 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
AVEC 2004
24-Aug-04

4. Force Feedback
Can be function of vehicle states, handwheel states and lanekeeping information
Change mechanical feel
Reproduce feel of tire forces
Add lanekeeping signal
AVEC 2004
24-Aug-04

5. Coupled System Force feedback and vehicle motion are coupled
Need to ensure entire system is stable
AVEC 2004
24-Aug-04

6. Outline Potential field lanekeeping control Force feedback system
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
AVEC 2004
24-Aug-04

7. 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
AVEC 2004
24-Aug-04

8. 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
AVEC 2004
24-Aug-04

9. 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:
AVEC 2004
24-Aug-04

10. 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
AVEC 2004
24-Aug-04

11. 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
AVEC 2004
24-Aug-04

12. Increasing Assistance Based FF
With any significant assistance-based force feedback, system is unstable
AVEC 2004
24-Aug-04

13. 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
AVEC 2004
24-Aug-04

14. Add Aligning Moment
Another way to stabilize the system: aligning moment
Effect is similar to damping
AVEC 2004
24-Aug-04

15. Increasing Lookahead
Stability is also increased with increasing lookahead
Tends to prevent large yaw
AVEC 2004
24-Aug-04

16. 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)
AVEC 2004
24-Aug-04

17. 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
AVEC 2004
24-Aug-04

18. 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
AVEC 2004
24-Aug-04

19. 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
AVEC 2004
24-Aug-04

20. Unstable Experimental Result
AVEC 2004
24-Aug-04

21. Stable Experimental Result
Similar gains
More hand-
wheel damping
Similar response
to system with
no force
feedback
AVEC 2004
24-Aug-04

22. 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
AVEC 2004
24-Aug-04