Environmental Engineering Reference
In-Depth Information
other pieces of large computer equipment. In addition, the detector must be within a
restricted range from the source or it will not be able to send back accurate information
(Sowizral, 1995), so the user has a limited working volume.
Ultrasonic tracking devices consist of three high frequency sound wave emitters in a rigid
formation that form the source for three receivers that are also in a rigid arrangement on the
user. There are two ways to calculate position and orientation using acoustic trackers. The
first is called “phase coherence”. Position and orientation is detected by computing the
difference in the phases of the sound waves that reach the receivers from the emitters as
compared to sound waves produced by the receiver. The second method is “time-of-flight”,
which measures the time for sound, emitted by the transmitters at known moments, to reach
the sensors. Only one transmitter is needed to calculate position, but the calculation of
orientation requires finding the differences between three sensors (Baratoff & Blanksteen,
1993). Unlike electromagnetic trackers that are affected by large amounts of metal, ultrasonic
trackers do not suffer from this problem. However, ultrasonic trackers also have a restricted
workspace volume and, worse, must have a direct line-of-sight from the emitter to the
detector. Time-of-flight trackers usually have a low update rate, and phase-coherence
trackers are subject to error accumulation over time (Baratoff & Blanksteen, 1993).
Additionally, both types are affected by temperature and pressure changes (Sowizral, 1995),
and the humidity level of the work environment (Baratoff & Blanksteen, 1993).
Infrared (optical) trackers utilize several emitters fixed in a rigid arrangement while cameras
or “quad cells” receive the IR light. To fix the position of the tracker, a computer must
triangulate a position based on the data from the cameras. This type of tracker is not affected
by large amounts of metal, has a high update rate, and low latency (Baratoff & Blanksteen,
1993). However, the emitters must be directly in the line-of-sight of the cameras or quad
cells. In addition, any other sources of infrared light, high-intensity light, or other glare will
affect the correctness of the measurement (Sowizral, 1995).
Inertial trackers apply the principle of conservation of angular momentum (Baratoff &
Blanksteen, 1993). Inertial trackers allow the user to move about in a comparatively large
working space where there is no hardware or cabling between a computer and the tracker.
Miniature gyroscopes can be attached to HMDs, but they tend to drift (up to 10 degrees per
minute) and to be sensitive to vibration. Yaw, pitch, and roll are calculated by measuring the
resistance of the gyroscope to a change in orientation. If tracking of position is desired, an
additional type of tracker must be used (Baratoff & Blanksteen, 1993). Accelerometers are
another option, but they also drift and their output is distorted by the gravity field
(Sowizral, 1995).
4.5 Input (interaction) devices
They are used to interact with the virtual environment and objects within the virtual
environment. Examples are joystick (wand), instrumented glove, keyboard, voice
recognition etc.
For interaction with small objects in a virtual world, the user can use one of several gloves
designed to give feedback on the characteristics of the object. This can be done with
pneumatic pistons, which are mounted on the palm of the glove, as in the Rutgers Master II
(Gomez, Burdea, & Langrana, 1995). Gloves are used for sensing the flexion of the fingers.
