Gait Disc Presentation

8/8/2019 Gait Disc Presentation

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Gait discA compact locomotion device for the virtualenvironment

gioved 23 dicembre 2010


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Virtual reality

Virtual reality is not an invention of a new technology, but theintegration of existing ones

3-D computer graphics

image processing

sound control

network infrastructures

real-time control

With the help of existing technologies, virtual reality creates afriendly envinoment in fields such as surgery training, merchandisingand entertainment



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Locomotion devices

A locomotion device is an hardware interface, in the form of a platform,allowing to reproduce users walking in virtual reality

Many locomotion devices have been designed in the past, but all of themhad limitations regarding movement possibilities

Main features of a locomotion device:

no motion tracking devices to be worn (feature 1)

walking an infinite distance in a limited area (feature 2)

the user can walk infinitely without leaving the platorm

for example, treadmills allowed walking on only one direction



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Omni-direction Ball-bearing Disc Platform (OBDP)

Its a locomotion device granting a natural gait in the virtualenvironment

no 3D trackers, but only sphere sensors on platform to detectusers pace

no sensors on users body, apart from head tracker

slipping of users feet always to platforms center, to obtain aninterface more conformed to human walking



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OBDP and feature 1

The orbiting frame+gait disc sensors

structure doesnt require any tracker

wearing

The nature of ball sensors doesnt hinder

users movements, assuring a low roughness

and friction under feet



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OBDP and feature 1: ball sensors

perceives users pace and provides a slick

surface

the six ballsand the shim provide an anti-

slippery action during walking

in total, on disc surface, there are 975sensors, able to detect paces ranging from

5 to 50 cms



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Feature 2: common solutions

The main difficulty in obtaining feature 2 lies in finding a way to

force user to the center of platform during motion

A treadmill could be a solution to the problem, but it has manyproblems and limitations

it must be started by signals from a tracker attached to user andperceiving his/her movements

its difficult to control its velocity, specially in the event ofsudden stops



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Feature 2: OBDPs solution

The goal of keeping the user in center area is

achieved by the mean of ball sensors

The sensors are disposed in rings, so that 2D

human walking can be accomplished

The arc design of disc surface is based on

human swing angle, i.e. the angle composed

by swing leg during walking

the user is slipped to center during

locomotion

the user can walk with natural posture

and without the needing of specific

training



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OBDP and safety

Even if the platform produces no stumbles, discs

curvature may cause unbalances in case of novices

training

The orbiting frame wards off unbalance danger,

surrounding users waists

The frame can even be used to counteract force

feedback produced by slipping on platform



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OBDPs interactive response

Ball sensor 1 Ball sensor n...

D/A-A/D

converterD/A-A/D

converter...

Computer

The amount of data exchanged by sensors and

computer is huge and is translated on both sides

by converters

The 975 sensors are organized in 19 circlesand logically represented in a 28x36 matrix

with this organization, the system can

detect pace in10 ms and reproducequickly users posture



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Gait model: study of the gait

WALKINGsequence of swing and stance phases,

exchanged by the two legs

step length: measured as the distance between feet when they are not aligned and

both touching the ground

distance: can be derived by the step length

speed: step length divided by time interval

its not necessary to distinguish rightfoot from left one

CONSIDER:

a) right leg forth and user facing Northb) left leg backward and user facing

South

changes in walking posture are detected

through changes in users gravityposition



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Gait model: state diagram

feet aligned on the

ground

feet on the ground,

but not aligned

gaits state diagram

feet both in central areaa foot in central area, a

foot in sliding area

gaits diagram for OBDP

when system detects that feet are not both in

platforms central area, the sliding area is scannedto search for the swing foot

(single stance or double stances phase)



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Gait sensing algorithm(GSA)|gait state reasoning

phase 1: measuring foots size

size is extimated through a bit-cluster, set of bitsderiving from sensors activated by feet

size is stored to determine the so called foot andfeet clusters (15% error tolerance in measuring)

clusters are used to detect the state transition

phase 2: feet cluster monitoring

continously check center areauntil bit-clusterchanges from feet cluster to foot cluster

inspect sliding area to distinguish between doublestances and single stance phases

when the bit-cluster on sliding area decreases up to85% of foot cluster, then the return to stand stillphase is detected (a cycle of walking posture iscompleted)



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GSA | geometrical center of footstep

walking posture and gravity shifting are directlycorrelated

change of gravity can be used to representstate of walking

human gravity is always balanced between the

two legs

its worth calculating gravity point to study walking

be X the mean value ofxcoordinates of bits ina foot cluster

be Y the mean value ofycoordinates of bits ina foot cluster

the point C = is called geometric centerof the foot cluster

calculate C1 and C2 as the geometric centers offeet; the middle point of segment uniting themis defined center of gravity



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GSA | speeding up state recognition

when passing from double stance to singlestance face, the GSA detection can befastened analizing only one of four platformsquadrants

draw a line between the cluster center onsliding area and OBDPs center

the drawn line is the middle line of onequadrant, that is the only one to be takeninto consideration for detection

on getting back to center, the foot in thesliding area will move only in the quadrantof interest



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GSA | noise filtering

another benefit of dividing the disc intosectors is the possibility of reducing noisefrom the sensors

the noise can be caused by malfunctionsof the sensors, that may send signal tocomputer even when not stimulated

the presence of noise may affect thecomputation of clusters geometricalcenters, shifting their actual position

to avoid noise, only the bits in clusterssector will be considered

all the bits, whose distance from clusters

geometric center is greater than 3/5 offoot length, are removed from bit cluster

after the removal of distant bits,geometric center is computed again



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GSA | moving direction and distance

once the geometric centers of both feet have beencalculated, in addition to gravitys coordinates,moving direction and distance can be computed

let be the coordinates respectivelyof right foots geometric center, left footsgeometrical center and gravity coordinate, allof them computed at time istant T1

let be the new coordinates offeets centers and gravity at time instant T2

we can derive the vector C2C1, that indicatesthe moving direction

the distance can be obtained as D = |C2C1|



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GSA | summary

Record foot size

Scan central area

Doublestance

Compute foot cluster in central area

and its geometrical center

Scan sliding area for the other foot

cluster

Singlestance

1) compute geometrical center of foot cluster insliding area

2) perform sectoring process3) compute gravity position

Scan sector area

Stand

still

1) compute geometrical center of foot cluster insliding area

2) compute gravity position3) compute moving direction, distance and speed

YES

NO

NO YES

YES

NO



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Locomotion system

there are many tasks to be executed to control an OBDPsystem

the tasks are distributed among differentcomputers, forming a cluster

the computers in the cluster communicate through aCommunication Backbone (CB), the kernel of aMultiple User Distributed Simulation (MUDS) system,which provides a trasparent communication among

networked computers

using MUDS, each computer works as a standalonemachine, at its own pace (then exploitingparallelism), ignoring the existence of othermachines

the CB, executed locally by every machine, providescommunication and syncronization with other hosts

an initial configuration, requires every host toinform its own CB about the kind of data it willproduce or receive

every computer behaves, then, as a publisher, asubscriber or both

CBs will take care to link and syncronize a publisherto a subscriber (and viceversa), creating a virtualchannel between each pair

the main advantage of distributing tasks

on different computers lies in obtaining a

modular and cost-effective system



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Application | overhead crane training system

portrage device commonly used inmanufactuing industry

main structure:

H-steel frame

alternator-driven mainbody (running on H

frame)

motor inside mainbody that controls a lifthook to portrage cargo

dangers:

to control the lift hook, the user followsand operate the crane on foot

accidents often due to insufficienttraining

the work place and activities are suitable forsimulation and OBDP can be exploited for thescope



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Application | overhead crane training system

the crane training simulator iscomposed by 6 modules:

crane control module

OBDP module

3D tracker module

sound module

3D scene managment module

3D render module



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Application | overhead crane training simulator

OVERHEAD CRANE CONTROLMODULE

receives commands from controlpannel to change states ofsimulated crane

commands are given through twojoysticks, for the north, south, westand east movements of crane andfor up-down movements of hook

OBDP MODULE

receives signals from ball-bearing

sensors and executes gait sensing

algorithm

computes distance and direction ofmanouverer

3D TRACKER MODULE

allows the user to explore the scene bymoving the head

uses Polhemus tracker

Polhemus tracker uses electromagnetic

wave to detect the six degrees of freedomin trainees motion

SOUND MODULE

reproduces environment sounds

(static sounds) and ones due to

collisions and motor working

(dynamic sounds)



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3D SCENE MANAGEMENT MODULE

cares about manipulating objects in virtual scene

its importance is linked to increasing performances in case of complexinteractions in virtual scene

among its main tasks there are simulations of complex physical phenomenasuch as collision detection and inertia oscillations of the hook

creates bounding objects (spheres or boxes) around every virtual object

when two bounding objects collide, the module investigates and in the eventof real collision, it sends messages to sound and 3D rendering modules, to

correctly reproduce what happened

the oscillation of the hook, due to its inertia, is computed both during cranemovement and when crane stops, simulating cables swinging and sendingdata to 3D rendering module to correctly reproduce movements

the cable is assumed inflexible, because of reduced calculation power



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3D RENDERING MODULE

based on Microsofts Direct3D

library

builds a tree structure containing

all the objects of the scene

anyway, the tree structure makesit difficult to manage a single

object in the scene

an object table is designed as an

interface between this module

and 3D scene management one

the displayed scene depends on

data from head movements, gait

disc and control panel



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Conclusions and future developments

OBDP is the first kind of locomotion interface not requiring motorsto enable user to walk around in virtual environment

the mechanism at the basis of OBDP makes this locomotion interfacethe smallest ever designed

future developments:

new types of sensors, easier to use and mantain

a more performig gait analysis algorithm, capable to detect evenside-walking



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Thanks for attention

Gait Disc Presentation

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