Biomedical Engineering Reference
In-Depth Information
Fig. 7.13
Left
a typical Intersense IS900 sensor bar installation.
Right
Intersense IS9000 MicroTrax
Wand. Courtesy of Intersense, LLC
There are many ways of comparing tracking systems and we refer the interested
reader to the previously mentioned reviews [
21
,
41
,
44
]. One specific point of com-
parison that is often overlooked but which is especially important for travel applica-
tions is how well the system scales. We can mention at the outset that certain designs
do not scale because of the nature of the tracking technology. Magnetic tracking for
example does not scale because multiple systems can interfere with each other.
The first wide area tracker of note was the UNC HiBall [
42
,
43
] which is an
optical tracker, but which, unlike the optical tracking systems that are now common, is
“inside-out,” in that the cameras are mounted on the user pointing towards the ceiling.
The ceiling contains strips of LEDs that flash in sequence, and from recognizing
points on the ceiling, the position of the camera can be calculated. This technology
scales well because there is a single set of cameras, and the LED strips can be mounted
over a wide area. Essentially, in the HiBall tracker, the environment is changed to
include easily tracked points. In a similar manner, camera-based tracking systems
based on markers, such as the InterSense IS1200, can also work over a large area.
However the camera arrays are currently bulky, and these inside-out systems can
suffer from problems of occlusions of the camera by the body.
Thus, although they are more expensive, large motion capture systems are often
used to track large spaces. Motion capture systems such as Vicon are designed to scale
up to large numbers of cameras: as the tracked volume increases more cameras are
required to retain precision. An impressive system of this type is the VIRTSIM system
from Motion Reality Inc. This can track a simple skeleton on 13 users simultaneously
over an area of 50
by 100
.Italsodrives13wirelessHMDstogiveusersthe
impression they are training in a squad with 12 others.
The next class of device is by far the most diverse: hand-held devices. The space
of physical and virtual devices that could be used to control travel is vast. It ranges
from joysticks through props such as steering wheels, to versatile devices such as
smartphones. Many variants have been tried and we will only mention a sampling.
