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Locomotion Interfaces

Locomotion Interfaces

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Locomotion Interfaces

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  1. Locomotion Interfaces John M. Hollerbach School of Computing University of Utah http://www.cs.utah.edu/~jmh/Locomotion.html

  2. Types of Motion Interfaces 1. Passive motion interfaces • Non-inertial systems (e.g., joysticks) • Inertial systems (e.g., Stewart platforms) 2. Active motion interfaces • Normal rooms with CAVE or HMD displays • Locomotion interfaces (e.g., exercise machines)

  3. Features of Motion Interfaces 1. Passive motion interfaces • Rate control is used. • User is seated and does not expend energy. 2. Locomotion interfaces • Cyclic proportional control is used. (gait) • User expends energy to move through VE. • Sensorimotor integration for geometry.

  4. Proposed Applications • Training and mission rehearsal. • Architectural walkthroughs. • Education. • Mobile robot interface (virtual tourist). • Entertainment: arcades and exercise. • Health rehabilitation. • Psychological research.

  5. Types of Locomotion Interfaces • Pedaling devices • Walking-in-place systems • Programmable foot platforms • Treadmills

  6. Pedaling Devices Hodgins, Georgia Tech Sarcos Uniport

  7. Room-Size Environments

  8. Room Instrumentation

  9. Walking-in-place systems Templeman’s Gaiter system, NRL

  10. Programmable Foot Platforms Sarcos Biport Iwata’s GaitMaster

  11. Linear Treadmill Devices Sarcos Treadport ATR ATLAS ATR GSS (ground surface simulator)

  12. Planar Treadmill Devices Omni-Directional Treadmill Torus Treadmill

  13. Treadport II Specifications • 6x10 foot white belt • 12 mph belt speed, 1 g belt acceleration • 5% belt slowdown at 750 lb normal force • +/- 20 degrees tilt in 1 second • 70 lbs tether force at 3 Hz

  14. Locomotion Display Issues • Unilateral constraints • Linear motion • Turning • Slope and uneven terrain

  15. Active Mechanical Tether Uses • Centering force f = k x • Unilateral constraint force f = k x - b v • Gravity force f = m g sin q • Inertial force f = m a

  16. Unilateral Constraints • We should not be able to walk through objects. • Braking the locomotion interface is not enough --- the user stumbles forward. • Tether wall force f = k x - b v

  17. Linear Motion Display The Treadport allows • Normal human gait • Varied movements and postures • Inertial force display

  18. Missing Inertial Force • Running on a treadmill requires much less energy than running on the ground. - 35% less energy when replaying 100y dash. - The body is stationary w.r.t. the ground. • Modulating treadmill belt speed can only partially compensate. • Active mechanical tether can supply missing inertial force.

  19. Inertial Force Feedback f = m a

  20. Evaluation of Tether Force Strategies 3. No tether force: f = 0 - Unstable running. 2. Spring force: f = k x - k fixed for all users. 1. Inertial force f = 0.8 m a - Universally preferred. - Partial force preference.

  21. Treadport Tilt Capability

  22. Slope Display • Tilt mechanism is rather slow. • Fast slope transients cannot be displayed by tilt. • Tilting complicates ground projection and CAVE display. • Tether force can simulate gravity and slope.

  23. Slope Walking g m g sin q m q mg q

  24. Gravity Feedback F = m g sin q

  25. Psychological Slope Experiments • Subjects walked on treadmill tilted at angle q. • Subjects then walked on the leveled treadmill. • Tether force f was adjusted until the subjects judged “equivalence.”

  26. F = 0.65 m g sin q

  27. Biomechanical Measurements • Optotrak markers • Rigid bars on leg segments • Comfortable but firm mountings • Joint angles calculated from vector relations

  28. Hip-Knee Cyclograms during Slope Walking

  29. Hip-Knee Cyclograms with Tether Force

  30. Properties during Slope Walking

  31. Tether Force Walking

  32. Derived Force versus Slope Hip range vs. force: HR = a f + b User 1 2 3 4 5 6 (c/a) -437 -289 -494 -444 -576 -480 (d-b)/a 2.0 -15.5 -19.8 -17.9 32.0 3.6 Hip range vs. slope: HR = c q + d Force vs. slope: f = (c/a) q + (d-b)/a

  33. Force/slope normalized by mass User 1 2 3 4 5 6 (c/a/m) 0.647 0.520 0.756 0.526 0.730 0.663 Psychophysical result: f = 0.65 m g q

  34. Tether Force Realistically Simulates Slope • Psychologically equivalent. • Biomechanically equivalent. • A platform tilt mechanism is not needed.

  35. Partial Force Preference • Slope rendering: f = 0.65 m g sin q • Inertia rendering: f = 0.8 m a Possible Explanations • Non-distributed force application to body • Oversimplified locomotion dynamics model

  36. Belt Motor Sizing • Load is predominantly friction force from impact • Impact forces are 3-6 times body weight • Design specification is slowdown < 5% • A 5 Hp motor handles a 90 kg with mu=0.15 • An 8 Hp motor chosen

  37. Tether Force Requirements • Inertial force display • Suppose m = 90 kg, a = 10 m/s^2 f = m a = 900 N • Safety concerns with 900 N force applied to the back • Reduce required tether force by allowing some actual acceleration on the belt

  38. Running model: x = (v /k) (exp(-kt) + kt - 1)(Hill’s equation)

  39. Simulation of Required Tether Force • Controller was tuned to allow • 0.8m forward movement • Tether force reduced to 350N • 80% preference = 280N • Design set at 315N • Acceptable load on back

  40. Harness Version 1

  41. Harness Design Issues • Mechanical coupling for pushing is good (metal plate against the backbone) • Mechanical coupling for pulling is a problem • Backlash due to straps and soft tissue • Metal plate lifts off • Better strapping must also consider different people sizes, sexes, and comfort

  42. Harness Version 2

  43. Harness Version 3

  44. Visual Display CAVEs • High resolution and brightness (no stereo yet) • Seeing your body increases immersion (and safety) • Panorama views not convenient with treadmills • Objects are mostly at a distance HMDs • Panorama and stereo viewing • Low optical quality and resolution • Safety, encumbrance, and body image

  45. CAVE Layout

  46. System Safety • Vertical motion restraint • Mechanical limit stops on tether • Limit switches on tether • Hardware springs at base and endpoint for twist, software spring for forward motion • Kill switches by user and operator • Watchdog timer • Software bounding box, and limits on velocity, acceleration, force, and force rate

  47. System Safety (cont) • Human subjects committee approval • Consent form • Tourist versus expert settings • No minors

  48. What are the Design Tradeoffs? We can’t display completely natural locomotion yet because of device limitations. What aspects are most important and how are they best implemented? • Platform motions (planar, tilt, deformed belt) vs. or with Whole-body force feedback • CAVEs vs. HMDs for visual display • Platform motions may interfere with CAVEs.