Page 240 - Concise Encyclopedia of Robotics

Photoelectric Proximity Sensor
A voice can be displayed on an oscilloscope screen. The hardware is sim-
ple: a microphone, audio amplifier, and oscilloscope.When a person speaks
into the microphone, a jumble dances across the screen. Phonemes look
simpler than ordinary speech. Any waveform, no matter how complex, can
be recognized or generated by electronic circuits. A speech synthesizer
can,in theory,be made to sound exactly like anyone’s voice,saying anything,
with any inflection desired. The output of such a machine has precisely the
same waveform, as seen on an oscilloscope, as the particular speaker’s voice.
See also SPEECH RECOGNITION and SPEECH SYNTHESIS.
PHOTOELECTRIC PROXIMITY SENSOR
Reflected light can provide a way for a robot to tell if it is approaching
something. A photoelectric proximity sensor uses a modulated light-beam
generator, a photodetector, a frequency-sensitive amplifier, and a micro-
computer. The illustration shows the principle of this device.
The light beam reflects from the object and is picked up by the photo-
detector. The light beam is modulated at a certain frequency, say 1000 Hz
(hertz), and the detector has an amplifier that responds only to light
modulated at that frequency. This prevents false imaging that might other-
wise be caused by stray illumination such as lamps or sunlight. If the robot
is approaching an object, the microcomputer senses that the reflected
beam is getting stronger. The robot can then steer clear of the object.
This method of proximity sensing does not work for black or very
dark objects, or for flat windows or mirrors approached at a sharp angle.
These sorts of objects fool this system, because the light beam is not
reflected back toward the photodetector.
See also ELECTRIC EYE and PROXIMITY SENSING.
PICTURE SIGNAL
See COMPOSITE VIDEO SIGNAL.
PIEZOELECTRIC TRANSDUCER
A piezoelectric transducer is a device that can convert acoustic waves to
electrical impulses, or vice versa. It consists of a crystal, such as quartz or
ceramic material, sandwiched between two metal plates, as shown in the
illustration on page 226.
When an acoustic wave strikes one or both of the plates, the metal
vibrates. This vibration is transferred to the crystal. The crystal generates
weak electric currents when it is subjected to mechanical stress. There-
fore, an alternating-current (AC) voltage develops between the two metal
plates, with a waveform similar to that of the sound waves.

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