Post on 09-Apr-2018
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Gait discA compact locomotion device for the virtualenvironment
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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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Application | overhead crane training simulator
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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Application | overhead crane training simulator
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