Biomedical Engineering Reference
In-Depth Information
6.4.1 Potential Implications for CyberWalk
The above-described large scale navigational studies demonstrate the need for a
platform like the CyberWalk, more than to offer constraints on its design. Specifically,
they demonstrate the real need for a laboratory setup that allows a walker to go
in circles or to walk along meandering paths. Nevertheless, they show that more
controlled environments are essential in studying human navigation. For instance, the
forest experiment revealed that one apparently major factor in being able to stay on a
straight trajectory was whether the sky was overcast or not. The CyberWalk achieves
environmental control through the use of VR technologies which allow us to create
large scale visual environments with high fidelity and control over environmental
factors that are normally beyond control, such as the presence and position of the sun.
6.5 Putting it All Together: The CyberWalk Platform
The CyberWalk treadmill (Fig.
6.7
) consists of 25 segmented belts each 5m long
and 0.5m wide, which are mounted on two large chains in the shape of a torus.
The entire setup is embedded in a raised floor. The belts constitute one direction of
motion, while the chains form the perpendicular direction. The chains are capable
of speeds up to 2m/s, while the belts can run at 3m/s. The chains are driven by four
powerful motors placed at the corners of the platform and each belt segment has
its own smaller motor. The drives are controlled such that they provide a constant
speed independent of belt load. The walking surface is large enough to accommodate
several steps without large changes in treadmill speed. This size allows for changes
in treadmill speed which are low enough to maintain postural stability of the user,
but makes it unavoidable that these accelerations will sometimes be noticeable to
the user. To what extent this affects self-motion perception needs to be determined
more closely, although Souman et al. [
95
] found that walking behavior and spatial
updating on the CyberWalk treadmill approached that of overground walking.
The high-level control system determines how the treadmill responds to changes
in walking speed and direction of the user in such a way that it allows the user
to execute natural walking movements in any direction. It tries to keep the user
as close to the center of the platform as possible, while at the same time taking
into account perceptual thresholds for sensed acceleration and speed of the moving
surface. The control law has been designed at the acceleration level to take into
account the limitations of both the platform and the human user, while ensuring a
smoothly changing velocity input to the platform (see [
29
]). The treadmill velocity
is controlled using the head position of the user. The control scheme includes a
dead-zone in the center of the treadmill where changes in the position of the user are
not used when the user is standing still. This makes it much more comfortable for
users to look around in the VE while standing still [
95
]. Users wear a safety harness
