1. D D L ynamic aboratory esign 5-Nov-04Group Meeting Accelerometer Based Handwheel State Estimation For Force Feedback in Steer-By-Wire Vehicles Joshua P. Switkes Ian A. Coe J. Christian Gerdes

2. D D L ynamic aboratory esign 5-Nov-04Group Meeting What is Steer-By-Wire? Future automotive technology No mechanical connection between handwheel and roadwheels

3. D D L ynamic aboratory esign 5-Nov-04Group Meeting Steer-By-Wire Steer-By-Wire Advantages –Reduced packaging constraints –Increased front impact safety –New possibilities for vehicle control Lanekeeping assistance Change handling dynamics Challenge: Handwheel and roadwheels decoupled –No natural force feedback No mechanical transmission of force feedback Need to recreate force feedback artificially

4. D D L ynamic aboratory esign 5-Nov-04Group Meeting Outline Sensing needs Accelerometer setup concept Data processing Simple force feedback implementation Experimental results

5. D D L ynamic aboratory esign 5-Nov-04Group Meeting Sensing Needs Steer-By-Wire creates new possibilities for force feedback –Complete freedom to tune force feedback –Remove unwanted high frequency vibration Resulting signal has low bandwidth –Modify mechanical feel Damping: need angular velocity of handwheel Inertia: need angular acceleration of handwheel –Tire forces Position of handwheel –Add other force feedback sources

6. D D L ynamic aboratory esign 5-Nov-04Group Meeting Sensing Needs cont. Steer-by-wire control –Precise control of roadwheels to track handwheel requires feedforward control Position, velocity and acceleration of handwheel command

7. D D L ynamic aboratory esign 5-Nov-04Group Meeting Automotive Restrictions/Current Solutions Cost of sensing must be extremely low –High resolution encoder cost prohibitive Current Handwheel Sensor Solutions –Potentiometer –Low resolution encoder Both position based –Requires double differentiation for acceleration Each derivative increases noise in signal

8. D D L ynamic aboratory esign 5-Nov-04Group Meeting Possible Solution: Accelerometers for Force Feedback MEMS Accelerometers and gyros very attractive –Small (5mm x 5mm) –Cheap (appr. $1 in quantity) –Reliable (no moving parts) Directly measure velocity or acceleration –No differentiation needed –Should result in cleaner signal for same cost Still need absolute measurement of position –Accelerometers have bias which must be removed –Use cheap low resolution encoder or potentiometer

9. D D L ynamic aboratory esign 5-Nov-04Group Meeting Accelerometer Setup Two dual-axis 1.7g units Opposed direction of measurement allows cancellation of accelerations common to two sensors (gravity or vehicle accelerations)

10. D D L ynamic aboratory esign 5-Nov-04Group Meeting Concept: –Potentiometer removes bias from accelerometer –Accelerometer gives clean acceleration signal and is integrated for velocity and position Implementation –Kalman filter combines measurement sources based on knowledge of noise Data Processing

11. D D L ynamic aboratory esign 5-Nov-04Group Meeting Kalman Filtering Standard sensor fusion with Kalman filter Three states: Handwheel position, velocity, and accelerometer bias (constant offset) Filter dynamics: Propagate Kinematics Correct for bias Using Position Sensor Propagate states From Accelerometer

12. D D L ynamic aboratory esign 5-Nov-04Group Meeting Kalman Filter Tuning Filter gain chosen using knowledge of system noise –Filter is optimal given noise assumptions Accelerometer and potentiometer noise measured in stationary vehicle, with engine running, handwheel not moving Bias variance not measured: tuning parameter

13. D D L ynamic aboratory esign 5-Nov-04Group Meeting Experimental Implementation 1997 Corvette converted to steer-by- wire Belt drive force feedback on steering wheel

14. D D L ynamic aboratory esign 5-Nov-04Group Meeting Simple Force Feedback More complicated feedback possible –Simple case to evaluate state estimates Inertia and Damping recreated on handwheel –Inertia decreased by conversion to steer-by-wire setup due to disconnection form massive steering rack and wheels –Damping decreased due to disconnection from power steering, bushings –Both returned to approximately stock values for Corvette No model involved: direct feedback of state estimates

15. D D L ynamic aboratory esign 5-Nov-04Group Meeting Experimental Results Noise sources isolated through testing –Isolate algorithmic or mounting errors Stationary vehicle with steering –Isolate errors due to road excitation Moving vehicle, no steering –Isolate errors due to vehicle motion Roadwheels steering without handwheel motion –Fully realistic implementation Moving vehicle with driver

16. D D L ynamic aboratory esign 5-Nov-04Group Meeting Stationary Vehicle Bias variation due to alignment error –Error is easily estimated and cancelled –Industrial implementation sufficiently precise

17. D D L ynamic aboratory esign 5-Nov-04Group Meeting Moving Vehicle, Not Steering Straight Over PotholesAutonomous Turns Potholes produce noise in acceleration –Velocity is clean Autonomous turns produce small error in bias –Small compared to normal force feedback signal –Easily cancelled

18. D D L ynamic aboratory esign 5-Nov-04Group Meeting Moving Vehicle Velocity and position very clean Acceleration cleaner than a twice differentiated potentiometer

19. D D L ynamic aboratory esign 5-Nov-04Group Meeting Conclusions Accelerometers can be used to effectively estimate handwheel states in a moving vehicle Measurement scheme cancels common mode noise, allowing use in noisy environment Simple low cost way to estimate states for force feedback