Simulating Balance Recovery Responses to Trips Based on Biomechanical Principles

1. Simulating Balance Recovery Responses to Trips Based on Biomechanical Principles Brooke Coley3 Jessica K. Hodgins1,2 Takaaki Shiratori1,2 Rakié Cham3 1 3 2

2. Physical Simulation for Human Characters 1 Steady-state behaviors. Interaction Yin et al., 2007 Muico et al., 2009 Reactive responses required. 1 http://lh6.ggpht.com/_UAku2WOHdSE/SlP6lTodsMI/AAAAAAAADOU/BFQRfrvySDM/

3. Reactive Response to Trips Biomechanical Principles Obstacle Collision with obstacle. Clear obstacle. Recover balance. Controller Motion capture data Simulation

4. Prior Work • Synthesize reactive responses. Simulation-based method Kudoh et al., 2002 Komura et al., 2004 Zordan et al., 2002 Macchietto et al., 2009 Biomechanics-based method Not applicable to tripping. Ye et al., 2008 • Trip and slip for bipedal robots. Boone et al., 1997 Not for human characters.

5. Strategies Push-off reaction Elevating strategy Collision in early swing (5-50% of entire swing) Touchdown Flight phase or double support Clearance with tripped leg Push-off reaction Strategy selection Lowering strategy Collision in late swing (40-75% of entire swing) Clearance with non-tripped leg Leg swap Flight phase or double support Touchdown [Eng et al. 1994, Schillings et al. 2000, Pijnappels et al. 2004, 2005]

6. Push-off Reaction Ground reaction force vector passes anteriorly to the COM. [Pijnappels et al. 2005] Reduce forward angular velocity.

7. Arm Motions • Increase moment of inertia to reduce angular acceleration. Arm ipsilateral to tripped leg moves in sideward direction. Arm contralateral to tripped leg moves in forward direction. • Protect head/chest. [Roos et al. 2008, Pijnappels et al. 2008]

8. Capturing Tripping Motion Subjects must not know when/where tripping occurs. Harness Trip machine Trip slide Semi-rigid shoe Look here

9. Motion Capture Dataset Elevating Lowering Faster walking speeds tend to lead to Flight Phase. (DS: double support FL: flight phase)

10. Human Model Create a 3D skin model from ~400 optical markers.  Calculate mass and moment of inertia from volume. 42 DOFs in total 96 contact points per foot. [Park and Hodgins 2006]

11. Controller Overview Finite state machine with Proportional Derivative (PD) servo. Strategy? Flight phase? Single Support After Trip Passive Flight Reaction Trailing leg leaves ground. Leg Swap Elevating Leading leg contacts ground. Yes Muscle activities start. Double Fall No Support Clearance Collision Lowering Leading leg contacts ground. Tripped leg touches ground. COM starts falling.

12. Controller for Tripping Reactions Elevating Strategy? Flight phase? Leading leg contacts ground. Yes Muscle activities start. Single Support After Trip Passive Flight Reaction No Clearance Collision Lowering Trailing leg leaves ground. Leg Swap Simulation initialization Leading leg contacts ground. Tripped leg touches ground. COM starts falling. Double Fall Support • Baseline walking • Playback of motion capture data. • Simulation • Initialized with tripping forces just before trip occurs.

13. Controller for Tripping Reactions Elevating Strategy? Flight phase? Leading leg contacts ground. Yes Muscle activities start. Single Support After Trip Passive Flight Reaction No Clearance Collision Lowering Trailing leg leaves ground. Leg Swap Simulation initialization Leading leg contacts ground. Tripped leg touches ground. COM starts falling. Double Fall Fore-aft (x) Force [N] 100 z Support Vertical (z) 50 x 0 -50 -100 -150 Time [sec] -200 0 0.02 0.04 0.06 0.08 0.10 Vertical: sine function Fore-aft: Gaussian function Observed tripping forces. [Pijnappels, et al., 2004]

14. Controller for Tripping Reactions Elevating Strategy? Flight phase? Leading leg contacts ground. Yes Muscle activities start. Single Support After Trip Passive Passive Flight Reaction Reaction No Clearance Collision Lowering Trailing leg leaves ground. Leg Swap Leading leg contacts ground. Tripped leg touches ground. COM starts falling. Double Fall Support • Support leg Control attitude of upper body. • Swing leg Moving forward like walking. Target angles: motion capture data of walking.

15. Controller for Tripping Reactions Elevating Strategy? Flight phase? Leading leg contacts ground. Yes Muscle activities start. Muscle activities start. Single Support After Trip Passive Passive Flight Reaction Reaction No Clearance Collision Lowering Trailing leg leaves ground. Leg Swap Leading leg contacts ground. Tripped leg touches ground. COM starts falling. Double Fall Support Support knee Transit: touch sensor  brain  muscle Rectus femoris Vastus lateralis 0: tripping instant Muscle recruitment (40 msec) Knee torque [Ralston et al. 1976] [Schillings et al. 2000] [Pijnappels et al. 2005]

16. Controller for Elevating Strategy Elevating Strategy? Flight phase? Leading leg contacts ground. Yes Muscle activities start. Muscle activities start. Single Support After Trip Passive Passive Flight Reaction Reaction No Clearance Collision Lowering Trailing leg leaves ground. Leg Swap Elevating strategy? Leading leg contacts ground. Tripped leg touches ground. COM starts falling. Double Fall Support • Support leg • Push-off reaction: • Extend all joints. • Compensation torque to ankle for COM.

17. Controller for Elevating Strategy Elevating Strategy? Flight phase? Leading leg contacts ground. Yes Muscle activities start. Muscle activities start. Single Support After Trip Passive Flight Reaction No Clearance Collision Lowering Trailing leg leaves ground. Leg Swap Elevating strategy? Leading leg contacts ground. Tripped leg touches ground. COM starts falling. Double Fall Support • Swing leg • Clear the obstacle. • Target angles: • motion capture data. Motion capture Simulation

18. Controller for Elevating Strategy Elevating Strategy? Flight phase? Flight phase? Leading leg contacts ground. Yes Muscle activities start. Single Support After Trip Passive Flight Flight Reaction No Clearance Collision Lowering Trailing leg leaves ground. Leg Swap Leading leg contacts ground. Tripped leg touches ground. COM starts falling. Double Fall Support • Leading leg • Extended for touchdown. • Target angles: • motion capture data. • Trailing leg Start flexion. Motion capture Simulation

19. Controller for Elevating Strategy Elevating Strategy? Flight phase? Leading leg contacts ground. Leading leg contacts ground. Yes Muscle activities start. Single Support After Trip Single Support After Trip Passive Flight Flight Reaction No Clearance Collision Lowering Trailing leg leaves ground. Leg Swap Leading leg contacts ground. Tripped leg touches ground. COM starts falling. Double Fall Support • Leading leg • Control attitude of upper body. • Trailing leg Move forward for the next step.

20. Controller for Elevating Strategy Elevating Strategy? Flight phase? Flight phase? Leading leg contacts ground. Yes Muscle activities start. Single Support After Trip Passive Flight Reaction No Clearance Collision Lowering Trailing leg leaves ground. Leg Swap Leading leg contacts ground. Tripped leg touches ground. COM starts falling. COM starts falling. Double Fall Support • Leading leg • Extended for the next step • Target angles: • motion capture data. • Trailing leg • Control attitude of upper body. • Keep extension. Motion capture Simulation

21. Controller for Elevating Strategy Elevating Strategy? Flight phase? Leading leg contacts ground. Yes Muscle activities start. Single Support After Trip Passive Flight Reaction No Clearance Collision Lowering Trailing leg leaves ground. Leg Swap Leading leg contacts ground. Leading leg contacts ground. Tripped leg touches ground. COM starts falling. Double Double Fall Support Support • Leading leg • Control attitude of upper body and extend knee. • Trailing leg • Control attitude of upper body . • Plantar-flex ankle for the next step.

22. Controller for Elevating Strategy Elevating Strategy? Flight phase? Leading leg contacts ground. Yes Muscle activities start. Single Support After Trip Single Support After Trip Passive Flight Reaction No Clearance Collision Lowering Trailing leg leaves ground. Trailing leg leaves ground. Leg Swap Leading leg contacts ground. Tripped leg touches ground. COM starts falling. Double Double Fall Support Support • Leading leg • Control attitude of upper body. • Trailing leg Move forward for the next step.

23. Controller for Lowering Strategy Elevating Strategy? Flight phase? Leading leg contacts ground. Yes Muscle activities start. Muscle activities start. Single Support After Trip Passive Passive Flight Reaction Reaction No Clearance Collision Lowering Trailing leg leaves ground. Leg Swap Leg Swap Elevating strategy? Leading leg contacts ground. Tripped leg touches ground. COM starts falling. Double Fall Support • Swing leg (tripped) • Extended for touchdown immediately.

24. Controller for Lowering Strategy Elevating Strategy? Flight phase? Leading leg contacts ground. Yes Muscle activities start. Single Support After Trip Passive Flight Reaction No Clearance Clearance Collision Lowering Trailing leg leaves ground. Leg Swap Leg Swap Leading leg contacts ground. Tripped leg touches ground. Tripped leg touches ground. COM starts falling. Double Fall Support • Swing leg (tripped) • Extended for touchdown immediately. • Support leg (non-tripped) • Leaves ground after swing leg touchdown. • Clear the obstacle.

25. Control of Arm Motion Start reaction. Timing: 100 msec Target angles: motion capture data. Elevating Strategy? Flight phase? Leading leg contacts ground. Yes Muscle activities start. Single Support After Trip Passive Flight Reaction No Clearance Clearance Collision Lowering Trailing leg leaves ground. Leg Swap Leading leg contacts ground. Tripped leg touches ground. COM starts falling. Double Fall Support [Pijnappels et al. 2008] Motion capture Simulation

26. Control of Arm Motion Elevating Strategy? Flight phase? Leading leg contacts ground. Yes Muscle activities start. Single Support After Trip Single Support After Trip Passive Flight Reaction No Clearance Collision Lowering Trailing leg leaves ground. Leg Swap Leading leg contacts ground. Tripped leg touches ground. COM starts falling. Double Fall Support • Back to motion in normal walking. Motion capture (walking) Simulation

27. Simulation Result Elevating strategy with double support. Input walking speed: 1.0 m/s Elevating Lowering DS FL DS FL

28. Simulation Result Elevating strategy with flight phase. Input walking speed: 1.4 m/s Elevating Lowering DS FL DS FL

29. Simulation Result Lowering strategy with double support. Input walking speed: 0.75 m/s Elevating Lowering DS FL DS FL

30. Simulation Result Lowering strategy with flight phase. Input walking speed: 1.1 m/s Elevating Lowering DS FL DS FL

31. Comparison with Motion Capture Data Elevating strategy with flight phase. 20 120 Pitch [deg] Pitch [deg] 0 100 -20 80 60 -40 40 -60 20 Time [sec] Time [sec] -0.5 0 0.5 1.0 0 -80 Hip Knee -20 : tripping instant : simulation result : motion capture data -100 -0.5 0 0.5 1.0

32. Comparison with Motion Capture Data 45 Elevating strategy with flight phase. 40 35 Height [m] 30 0.5 Pitch [deg] 25 0.4 20 15 0.3 10 0.2 5 0 0.1 Foot trajectory Pelvis Length [m] -5 Time [sec] : tripping instant : simulation result : motion capture data 0 -10 0 0.5 1.0 -0.5 0 0.5 1.0

33. Quantitative Comparison Root mean square errors Unit: [deg/frame] (frame rate = 120 Hz)

34. Discussion Recovery with multiple steps.

35. Discussion Better contact model • Many force plates. • Larger marker set for feet. • More precise model of tripping forces. Push-off reaction Tripping forces.

36. Summary • Controllers for strategies of balance recovery responses to trips. • Graphics Integrate walking controllers. Other reactive responses. • Biomechanics application Answer “what if” questions. Improve training and rehabilitation systems. 1 2 3 1 http://www.youtube.com/watch?v=LVStmLCoH30 2 http://www.yamakai.org/profiles/marriott.html 3 http://www.treadmilladviser.com/landice-l7-rehabilitation-treadmill.html

37. Acknowledgements • Adam Bargteil for help with calculating mass and moment of inertia. • Moshe Mahler for rendering animation. • Justin Macey for the human model. • Subjects for participation in the experiments. • NSF -0540865 Quality of Life Technology Engineering Research Center • F31 AG025684-03 NIH Ruth L. Kirschstein Award • Autodesk for Maya donation.