Page 254 - Concise Encyclopedia of Robotics

Presence Sensing
they are a cheap and effective method to keep a machine from crashing
into obstructions.
Electric eye
Another simple scheme for presence detection is an electric eye. Beams of
IR or visible light are shone across points of entry such as doorways and
window openings. Photodetectors receive energy from the beams. If any
photodetector stops receiving its beam, a signal is generated. See ELECTRIC EYE.
Optical, IR, or microwave reflection sensor
An optical presence sensor is a device similar to the electric eye, except that
it senses light beams reflected from objects, rather than interrupted by
them. An IR presence sensor uses IR rather than visible light; a microwave
presence sensor employs electromagnetic waves having short wavelengths
(of the order of a few centimeters or less).
Beams of visible, IR, or microwave energy are shone into the work envi-
ronment from various strategic locations. If any new object is introduced,
and if it has significant reflectivity, the photodetectors will sense the
reflected energy and cause a signal to be generated. The visible or IR system
can be fooled by nonreflective objects. A good example is a robot coated
with solid, flat black paint. Microwave systems may not respond to objects
comprised entirely of nonconducting (dielectric) materials such as plastic
or wood.
Optical, IR, or microwave interferometer
An interferometer can be used by a robot to detect the presence of an
object or barrier at close range. It works based on wave interference,
and can operate at any electromagnetic (EM) wavelength. Usually, EM
energy in the radio microwave, IR, or visible range is used. When an
object containing sufficiently reflective material intrudes into the work
space, the reflected wave combines with the incident wave to generate an
interference pattern. This wave interference can be detected, and sent to
the robot controller.
The effectiveness of the interferometer depends on how well the object
or barrier reflects energy at the wavelength used by the device. For ex-
ample, a white-painted wall is more easily detected by an optical interfer-
ometer than a similar wall painted dull black. In general, an interferometer
works better as the distance decreases,and less well as the distance increases.
The amount of radio, IR, or optical noise in the robot’s work environ-
ment is also important. The higher the noise level, the more limited is
the range over which the sensor functions, and the more likely are false
positives or negatives.

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