Space
Requirements for Wheeled Mobility
An International Workshop
University Inn,
Center for Inclusive Design and Environmental Access
University at
..
Wheelchair Simulation in Virtual Reality
Dr. M. Grant
ABACUS
Department of Architecture
and Building Science
131 Rottenrow
Introduction
A recently
completed project at the
Technical
Description
The
outcome of the project has been the development of a haptic interface which
allows wheelchair users to navigate within VR simulations of buildings through
the use of their own wheelchair and which also provides the user with feedback
that is related to the sense of effort required to propel the wheelchair over
changes in floor surface and slope.
The
Motion Platform
The physical
structure of the wheelchair platform is based around a pair of rollers. These
are mounted on independent shafts so that one roller is under each driving
wheel of the wheelchair. The roller shaft is supported by a pair of single row
radial ball bearings mounted in support pillars, fixed to a solid base plate.
The roller, and space for an inertial mass, is situated between the two
bearings. Outside the lateral ball bearing, the axle is machined to accommodate
a hollow shaft encoder similarly, brakes are rigidly mounted coaxial to the
roller assembly. Motors are geared to each roller shaft using a toothed belt
and coupled through an electromagnetic clutch.
The entire structure is designed to be rigid and yet capable of
accommodating a range of manufactures wheelchairs with adjustable straps and
bars ensuring that the wheelchair is rigidly held in place on the rollers. The
provision of a ramp allows the user to gain access to the facility.
Figure 1. The
electro-mechanical components of the motion platform
Graphics System
The role of the
graphics system is to generate a virtual world that can realistically represent
the built environment both visually and physically. The software used to drive
the virtual environment will be based on the Silicon Graphics Performer API.
This is a high performance 3-D rendering toolkit for interactive applications.
The graphics component is closely coupled to a separate asynchronous module
that interfaces between the incoming data from the motion platform control
system and the rendering software. The rendered scenes support texture mapping,
multiple light sources and are of sufficient geometrical complexity so as to
produce a convincing depiction of the built environment.
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Figure 2. Both powered
and manual wheelchairs can be accommodated on the motion platform
Physical
Simulation
The graphics
application requires the Cartesian co-ordinates of the eye point, plus the yaw,
pitch and roll angles of the direction of view. Given the yaw angle the
remaining two parameters can be calculated based on the wheelchairs attitude on
the floor plane. In the database traversal three rays corresponding to the
contact patch of each of the rear wheels and the midpoint of the front axle,
are intersected with the floor. The normal vector of the ground plane at these
points can then be used to calculate the roll, pitch and altitude of the chair
and hence the corresponding view. The same intersection procedure can also be
used to identify the surface under each wheel, this information then being used
to index material properties, such as rolling resistance and surface texture
which can then be passed back to the control system. Rays aligned with the
current direction of motion detect collisions between the wheelchair and obstructions
in the environment.
Control System
A large number of
environmental features can be identified for which accessory physical feedback
will enhance the visual feedback from the virtual environment. These included
object collisions, slopes and cambers, kerbs, uneven surfaces and different
ground surfaces. As outlined previously, the motion simulator and the graphics
software are a close-coupled system with the motion simulator communicating
with the graphics system over a TCP/IP network. The task of the motion
simulator is to accept incoming data from the wheelchair platform, this data
relating to the individual incremental angular displacement of both wheels on
the motion platform. The basis of the motion control algorithm is the
determination, through an analysis of similar triangles, of any translation and
also, using the location of the centre of rotation along the rear axle of the
virtual wheelchair, the angle through which it is turned. These values are
passed to the graphics system where the transformation of the eye point and
rotation of the view vector can be determined. Feedback from the graphics
system determines whether the brakes, clutch or motors should be actuated to
provide a physical level of feedback to the user.
Platform Control
The platform
control system is hosted within a standard Personal Computer, running purpose
written software, interfacing with the virtual world via a network link using
TCP/IP and also with the platform instrumentation via a General Purpose
Interface Board (GPIB). The control system monitors the user input by taking
incremental readings from the rotary encoders on the motion platform whilst
simultaneously controlling the feedback stimuli to the wheelchair on the basis
of feedback data received from the graphics system. The motors are independently controlled for each
wheel therefore simulation of the wheelchair moving down a slope is provided by
active input into the system, providing by the torque motors. Similarly a
resistive torque is supplied when ascending a gradient. Varying floor surfaces are simulated by
altering the resistance to motion of the rollers, using the brakes. When the
graphics system detects a collision the brakes are temporarily applied thus locking
the rollers.
System Summary
While the
potential for this system is still being explored early results do suggest that
this form of simulation could be a powerful instrument for a wide range of
investigations into the interaction of a wheelchair occupant with their
environment. This field of study not only addresses basic manual and powered
wheelchair navigation but could be extended to architectural design evaluation,
cognitive development for the sensory impaired and more generally to raising
the awareness of those who commission new building designs.
If a wheelchair simulation were to be employed in
such a fashion then experience has indicated that the following preliminary
objectives will be required:
·
The ability to accurately monitor intended wheelchair motion
and have the capability to provide physical and optical feedback to the
wheelchair user on the presence of virtual obstacles or changes in floor
coverings or slope.
·
An interface between the platform and a virtual reality
facility in order to provide an immersive virtual environment within which
navigation is linked to the intended wheelchair motion.
·
The ability to generate virtual representations of a range
of building types in order to test and calibrate the performance of the
platform and perform an evaluation of the system by wheelchair users.
This
concept embodied in this last point has become known as Virtual Reality
within which the ability to derive any form of meaningful interaction with a
computer model can be said to be dependant on three characteristics:
·
The quality of the model, which must be dimensionally
consistent, visually compelling, and should accurately simulate the physical
constraints of the real world.
·
The ability to visualise the graphical output free from the
subjugation of the traditional computer monitor.
·
The ability to interact with the virtual world in a manner
that is free from the constraints and abstraction of an artificial control
metaphor.
The
development and construction of a wheel chair motion platform, based on the
above principles, required the incorporation of several key features which are
outlined below:
·
The platform must accurately detect the rotation of the
driving wheels and use this to provide realistic navigation within the VR
world.
·
Non-visual environmental feedback (haptics) should be
provided that match the altered sense of effort needed to propel a wheelchair
over varying surfaces and slope conditions.
·
Collisions with virtual objects should combine visual and
non-visual simulation directly analogous to that encountered in reality.
·
Communications between platform and VR host should be fast
and provide near real-time interaction.
·
The platform should allow wheelchair users to use their own
wheelchairs while navigating in the virtual environment. The platform must have
the flexibility to accommodate chairs from a range of manufactures.
·
The navigation route through a VR model together with any
collision points and other data should be logged for off-line evaluation
purposes.
·
The user should be able to interact with the virtual
environement not just through the interface provided by their wheelchair but
also through the ability to reach into the world manipulate any objects found
within.
Conclusion
The design and implementation
of the system has been completed. (Harrison 2000). User testing has demonstrated that users find
the system to be a realistic depiction of wheelchair use in the real world.
(Harrison 2004). The integration of the motion platform with virtual reality
can result in a positive experience and would enable performance based metrics
to be investigated. (Grant 2003). Future research is directed towards extending
the capabilities of the system, for instance by allowing for the users
personal interaction with the virtual world, and in investigating the role of
simulation in the architectural desgn decision making process.
References
Grant M ,
Harrison C, Conway B, 2003, "VR in the Service of People with Special
Needs" Include 2003: the practice
of inclusive design, Royal College of
Art,