ELECTRONIC TRAVEL AID
ANANDA PRAKASH VERMA (07BMD007)
An Electronic Travel Aid (ETA) is a form of
assistive technology having the purpose of
enhancing mobility for the blind
Perhaps the most widely known device is
the LaserCane, which is a regular long cane
with a built-in laser ranging system. The
Mowat Sensor is an example of a pocket-
sized device containing an ultrasonic air
sonar system. When it detects an obstacle,
the device vibrates, thereby signalling the
The research problem of designing a
better ETA is a tough one. Despite 50
years of effort, no one has been able to
design an electronic device that can
replace the long cane.
Blind individuals find travelling
difficult and hazardous because they
cannot easily determine "where"
things are, a process otherwise known
as "spatial sensing." Thus the problem
of mobility can be reframed as a
problem in spatial sensing.
The techniques for spatial sensing
are well known, radar, sonar, and
optical triangulation methods being
the most common, and the latter two
have been incorporated into a wide
variety of previous ETA designs.
However, there are many problems
with currently available devices. First,
the range-finder technology is
unreliable in its detection of step-
downs or step-ups, such as curbs.
Secondly, blind users find the
sounds of various pitches or tactile
vibrations being used to code the
spatial information to be confusing
and difficult to understand.
Thirdly, most blind users do not find
the slight improvement in mobility
performance to be worth the extra
cost (which can be many thousands of
dollars), and the additional worry of
maintaining a complex, expensive
battery operated system that must be
carried around and kept track of.
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
travelling as initially a problem in spatial
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 traveller 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
convey 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.
a) First is the basic problem of "sensing" itself. Generally, in order for us to detect an
object, it must be a source of energy, either generated by the object itself or
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 photo sensors 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, so we can calculate range using binocular vision. Generally
speaking, all three quantities need to be known, or your movement will be impaired.
In the case of electronic travel aids for the blind, spatial sensing is to be done
electronically. Past approaches include ultrasonic air sonar, range finding via laser
triangulation, and most recently GPS (global positioning system) using radio
triangulation of timing signals from simultaneously viewed satellites.
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 traveller.
We perceive sound as having the following attributes: 1) loudness (intensity), 2) pitch (frequency,
repetition rate), 3) phase (with respect to a reference), 4) spatial location (direction and range).
Typically, information gets encoded upon sound via by passing it through a physical space of a
particular shape. 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
In terms of perception through touch, the skin has some similarity to the ear, and can sense
vibration 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. Moreover, the above sensations with
proprioceptive cues yields "active touch," which means we can discover the shape of an object and
feel the changing sensations as a function of position and orientation.
In 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. Most often,
ETAs convey range information either by a vibration or sound that cause changes in frequency or
intensity as the detected object gets closer.
CONVENTIONAL ELECTRONIC TRAVEL AIDS
In the past three decades several electronic travel aids
(ETAs) were introduced that aimed at improving their
blind users' mobility in terms of safety and speed.
Three fundamental shortcomings can be identified in all
ETAs discussed in the foregoing sections:
The user must actively scan the environment to detect
obstacles (no scanning is needed with the Sonic guide,
but that device doesn't detect obstacles at floor level).
This procedure is time-consuming and requires the
traveller's constant activity and conscious effort.
The traveller must perform additional measurements
when an obstacle is detected, in order to determine the
dimensions of the object. A path must then be planned
around the obstacle. Again, a time-consuming, conscious
effort that reduces the walking speed.
One problem with all ETAs based on acoustic feedback is
their interference (called masking) with the blind person's
ability to pick up environmental cues through hearing.
Some blind travellers can actually detect certain obstacles
through sound reflections from such obstacles.
MOBILE ROBOTS AS GUIDES FOR THE BLIND :
In general terms, one could argue that any mobile
robot with obstacle avoidance (and there are tens
or even hundreds of different mobile robots with
such capabilities) can be used as a guide for the
blind. However, mobile robots are inherently
unsuited to the task of guiding a pedestrian. The
foremost limitation of mobile robots is that they
are large, heavy, and incapable of climbing up or
down stairs or boardwalks. One might then argue
that the blind pedestrian could use ramps and
elevators, which are provided in many locations
for the use of disabled persons with wheelchairs.
However, this approach would actually burden the
blind (but mobility-wise perfectly able-bodied
traveller) with the additional, severe handicap of
The NavBelt : RECENT DESIGN
The NavBelt is a portable device equipped with ultrasonic sensors
and a computer.
The NavBelt provided two modes of operation:
1. In the image mode the NavBelt produced a 120o-wide view of
the obstacles ahead of the user (similar to a radar screen image).
This image was then translated into a series of directional
(stereophonic) audio cues through which the user could determine
which directions were blocked by obstacles and which directions
were free for travel. The problem with this method lay in the fact
that a considerable conscious effort was required to comprehend
the audio cues. Because of the resulting slow response time of the
test subjects could not travel faster than roughly 0.3 m/sec (1
foot/sec). And even this marginal level of performance required
tens of hours of training time.
2. Another mode of operation is called guidance mode. In this
mode it was assumed that the system knew the traveller's
momentary position and the traveller's desired target location.
Under these conditions, the NavBelt needed only generate a single
(thus, low-bandwidth) signal that indicated the recommended
direction of travel. It was much easier to follow this signal, and
walking speeds of 0.6 - 0.9 m/sec (2 - 3 feet/sec) were achieved.
• Foundation of Orientation and Mobility.
• Heyes, D. Anthony. (1984). "The Sonic Pathfinder: A New Electronic Travel
Aid." Journal of Visual Impairment and Blindness, 77, 200-202.
• Kay, Leslie. (1984). Acoustic Coupling to the Ears in Binaural Sensory Aids.
Journal of Visual Impairment and Blindness, 77, 12-16.
• Bissitt, D. and Heyes, A. D., 1980, "An Application of Biofeedback in the
Rehabilitation of the Blind." Applied Ergonomics, Vol. 11, No. 1, pp. 31-33.
• Johann Borenstein and Iwan Ulrich, “ The Guide Cane A Computerized
Travel Aid for the Active Guidance of Blind Pedestrians."