Locomotion Interfaces John M. Hollerbach School of Computing University of Utah

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

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

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

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

Pedaling Devices Sarcos UniportHodgins, Georgia Tech

Room-Size Environments

Room Instrumentation

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

Programmable Foot Platforms Sarcos BiportIwata’s GaitMaster

Linear Treadmill Devices ATR GSS (ground surface simulator) ATR ATLASSarcos Treadport

Planar Treadmill Devices Torus TreadmillOmni-Directional Treadmill

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

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

Active Mechanical Tether Uses Centering forcef = k x Unilateral constraint forcef = k x - b v Gravity forcef = m g sin  Inertial forcef = m a

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

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

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.

Inertial Force Feedback f = m a

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 forcef = 0.8 m a - Universally preferred. - Partial force preference.

Treadport Tilt Capability

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.

Slope Walking m g sin   m g  mg

Gravity Feedback F = m g sin 

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

F = 0.65 m g sin 

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

Hip-Knee Cyclograms during Slope Walking

Hip-Knee Cyclograms with Tether Force

Properties during Slope Walking

Tether Force Walking

Derived Force versus Slope Hip range vs. force: HR = a f + b Hip range vs. slope: HR = c  d Force vs. slope: f = (c/a)  + (d-b)/a User (c/a) (d-b)/a

Force/slope normalized by mass User (c/a/m) Psychophysical result: f = 0.65 m g 

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

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

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

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

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

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

Harness Version 1

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

Harness Version 2

Harness Version 3

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

CAVE Layout

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

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

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.

The Next Steps 3D active tether –Sideways pull/push Side slope walking –Vertical pull/push Uneven terrain and steps Walking under different gravity conditions Sound simulation (Dinesh Pai) –Footfall sounds –Ambient sounds

Possible Future Steps Olfactory display Wind display Portable haptic interface –Carried by 3D tether