Page 17 - Concise Encyclopedia of Robotics

ACOUSTIC PROXIMITY SENSOR A
An acoustic proximity sensor can be used by a robot to detect the presence
of, and determine the distance to, an object or barrier at close range. It
works based on acoustic wave interference. The principle is similar to that
of sonar; but rather than measuring the time delay between the transmis-
sion of a pulse and its echo, the system analyzes the phase relationship
between the transmitted wave and the reflected wave.
When an acoustic signal having a single, well-defined, constant fre-
quency (and therefore a single, well-defined, constant wavelength) reflects
from a nearby object, the reflected wave combines with the incident
wave to form alternating zones at which the acoustic energy adds and
cancels in phase. If the robot and the object are both stationary, these
zones remain fixed. Because of this, the zones are called standing
waves. If the robot moves with respect to the object, the standing waves
change position. Even a tiny shift in the relative position of the robot
and the sensed object can produce a considerable change in the pattern
of standing waves. This effect becomes more pronounced as the acoustic
wave frequency increases, because the wavelength is inversely propor-
tional to the frequency.
The characteristics and effectiveness of an acoustic proximity sensor
depend on how well the object or barrier reflects acoustic waves. A solid
concrete wall is more easily detected than a sofa upholstered with cloth.
The distance between the robot and the obstacle is a factor; in general, an
acoustic proximity sensor works better as the distance decreases, and less
well as the distance increases. The amount of acoustic noise in the robot’s
work environment is also important. The higher the noise level, the more
limited is the range over which the sensor functions, and the more likely
are errors or false positives. Ultrasound waves provide exceptional accuracy
at close range, in some cases less than 1 cm. Audible sound can allow the

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