On Electronic Travel Aid Design
In order to navigate, we must first know "where" things are, in
other words, the spatial positions of objects and other features in the
physical world around us. Thus, we can reframe the problem of traveling
as initially a problem in spatial perception. Man is well endowed with a
refined set of spatial sensing systems, and listed in order of decreasing
range of operation, they include binocular vision, binaural
hearing, and active touch. To compensate for the loss of binocular vision,
a blind traveler desires some sort of electronic travel aid (ETA) that can
spatially sense what is out there in the environment out to a reasonable
distance, and then display this information in an easily understood format
via the remaining senses of hearing and touch. So first, let's consider
the basic problem of spatial sensing and how it is normally done.
Spatial Sensing
a) First is the basic problem of "sensing" itself. Generally,
in order for me to detect an object, it must be a source of energy, either
generated by the object itself or reflected. For example, a sighted individual
usually sees objects by reflected light, such as sunlight or the light generated
by a lamp. Or I can see something that is generating light, such as this
computer monitor that I'm writing on. One exception to the general rule
cited above, is that an object can be detected even though it is not a source
of energy, as when I detect the waving my hand in front of my monitor because
it blocks a source of energy known to be present.
b) Second, we now consider the problem of "spatial" sensing. Our
senses, such as the eyes or ears, can infer the direction of an object because
we assume that the energy being radiated, travels in straight lines. In
the case of the eye, energy arriving from each angular direction, be it
azimuth or elevation, is directed by the lens to a two dimensional array
of photosensors on the retina, each direction being mapped to a unique point
on the array. However, we live in a three dimensional space. This means
that three coordinates are required to specify spatial position, not only
azimuth and elevation, but also range. Thus, nature provides us with another
sensor a few inches away, in other words a second eye, so we can calculate
range using binocular vision. Generally speaking, all three quantities need
to be known, or your movement will be impaired. In case you don't believe
me, try rock climbing while wearing an eye patch. Without going into all
the details, some of which are still not yet known, range appears to be
calculated via a process akin to triangulation, (since I know the distance
between my two eyes, and the two angles specifying direction from each eye
to the object).
In the case of electronic travel aids for the blind, spatial sensing is
to be done electronically. Past approaches include ultrasonic air sonar,
rangefinding via laser triangulation, and most recently GPS (global positioning
system) using radio triangulation of timing signals from simultaneously
viewed satellites.
Display parameters
Presuming that we can satisfactorily do electronic spatial sensing, the
next problem is how to display the spatial information to the blind user.
Since hearing and touch are the two remaining entry points for information
to the blind traveler, let's consider how these two systems operate in man.
We perceive sound as having the following attributes: 1) loudness (intensity),
2) pitch (frequency, repetition rate), 3) phase (with respect to a reference),
4) timbre (spectrum or equivalently the waveform shape as a function of
time), and 5) spatial location (direction and range). Typically, information
gets encoded upon sound via by passing it through a physical space of a
particular shape. For example, when I talk, my vocal cords open and close
generating a train of impulsive-like broadband sounds which passes through
the vocal tract, whose shape changes as a function of time depending upon
what words I wish to say. Similarly, with musical instruments, the shape
of the cavity will determine the pitch and timbre typical of the particular
instrument. In spatial hearing, the asymmetrical shape of the external ear
or pinna will cause a different transformation to be applied to incoming
sound, depending on its direction of arrival and range. In each case, the
auditory system can determine these spatial transformations, so that we
can understand the speech or music, or perceive a sound's location.
In terms of tactual perception, the skin has some similarity to the ear,
and can sense vibrational stimuli (changing pressures) with attributes including:
1) intensity, 2) frequency, 3) phase (when two or more points are stimulated),
4) waveform shape as a function of time and 5) location (and shape) of stimulation
on the skin's surface. In addition to vibration, we can perceive other kinds
of stimuli including 6) temperature (heat or cold), 7) pain or tickling,
8) electrical stimulation and 9) chemical attributes, such as irritation
or taste. Moreover, combining the above sensations with proprioceptive cues
yields "active touch," which means we can discover the shape of
an object by actively moving the skin's sensory surface over it, and feel
the changing sensations as a function of position and orientation.
In the case of ETAs, a wide variety of electronically generated sound and
tactual cues have been used to signal the blind user. In the case of hand
held narrow beam devices (ultrasound or laser ranging), azimuth and elevation
cues are typically conveyed proprioceptively via hand orientation. The coding
of range information is the most difficult for the blind person to understand.
Most often, ETAs convey range information either by a vibration or sound
that increases/decreases in frequency and/or intensity as the detected object
gets closer. More recently, some experimental ETAs may inform the blind
user the exact range, or even location via synthesized speech.
Head mounted devices use similar cues, except that it is head orientation
that conveys azimuth and elevation information. For the devices that have
a wider angle of view, an additional cue can be used to signal azimuth,
such as the binaural interaural intensity difference. Even TV cameras have
been hooked up to large area tactual displays, which convey range information
in a somewhat surprising manner: as an object comes closer in range, the
area of tactual stimulation becomes larger. I have designed systems that
use more naturalistic sounds such as the pinna tranformational cues of spatial
hearing and the human echolocation cue of time separation pitch.
For more details on individual devices, and links to other organizations
presently doing research in ETAs, see my web pages:
Introduction
Currently available
ETAs
Some ETA Patents
Other interesting links in this field:
ASMONC: Autonomous
System for Mobility Orientation, Navigation and Communication
Blind Mobility Research
Unit
Seeing Sounds:
Echolocation by Blind Humans, by Christine Uy
Sonification of Range
for 3-D Space Perception
Multipath Systems, Inc.
Perceptual Alternatives,
by Tony Heyes
External
links by Peter Meijer
Tips for Inventors
of ETAs
Some links to other organizations that work with the blind and visually
impaired:
American Council for the Blind
American Foundation for
the Blind
American
Printing House for the Blind
Helen Keller National
Center
National Federation for the Blind
Smith-Kettlewell Eye Research Institute
References:
Foundations of Orientation
and Mobility
last updated 1 November 1998
Copyright ©
1996 -8 by Duen
Hsi Yen, All rights reserved.
E-mail: yen@noogenesis.com