Biomedical Engineering Reference
In-Depth Information
FIGURE 7-10
Photographs of
handheld ultrasonic
prostheses. (a) Ray
electronic mobility
aid. (b) Nottingham
obstacle detector.
(c) K-Sonar.
7.5.1 Some Existing Systems
In the past 3 decades a large number of sonar-based visual prostheses have been introduced,
with the aim of improving mobility in terms of safety and speed. Some of the more
successful devices are discussed in the following sections.
7.5.1.1 Pathsounder
One of the earliest ultrasonic travel aids, the pathsounder consists of two ultrasonic trans-
ducers mounted on a board that the user wears around the neck, at chest height. This
unit provides only three discrete levels of feedback (series of clicks), coarsely indicating
distances to an object.
7.5.1.2 Mowat Sensor and Derivatives
Shown in Figure 7-10 are three of a number of commercially available handheld ultrasonic-
based devices that inform the user of the distance to detected objects by means of tactile
vibrations or sound. The frequency of the vibration is typically inversely proportional to
the distance between the sensor and the object.
The Nottingham obstacle detector (NOD) is an early handheld sonar device that
provides an auditory feedback indicating eight discrete levels of distance by different
musical tones. The NOD has been commercially available since 1980 (Bissitt and Heyes,
1980).
Other sensors in this genre include the K-Sonar, shown attached to a cane in Fig-
ure 7-10. This newer prosthesis uses a wideband signal (one octave), probably based on
frequency-modulated continuous wave (FMCW) techniques, to improve resolution and
sensitivity. The multiple echoes are converted into “tone-complex” sounds, which the user
can learn to interpret for improved obstacle classification (K-Sonar, 2008).
7.5.1.3 Sonic Pathfinder
This is a head-mounted pulse-echo sonar system comprising three receivers and two
transmitters, controlled by a microprocessor. Developed in 1984 by Tony Heyes as an
improvement to the NOD, it does not give information about surface texture, and normally
the auditory display indicates only the nearest object, which is why it should be classified
as an obstacle detector rather than as an imaging device. Heyes's approach is rather
different from Kay's in that the Sonic Pathfinder deliberately supplies only the most
