Page 256 - Concise Encyclopedia of Robotics

Pressure Sensing
PRESSURE SENSING
Robotic pressure sensing devices detect and measure force, and can in
some instances tell where the force is applied.
In a basic pressure sensor, a pressure-sensitive transducer tells a robot
when it collides with something.Two metal plates are separated by a layer of
nonconductive foam. This forms a capacitor. The capacitor is combined
with a coil (inductor). The coil/capacitor circuit sets the frequency of an
oscillator. The transducer is coated with plastic to keep the metal from
shorting out to anything. If an object hits the sensor, the plate spacing
changes. This changes the capacitance, and therefore the oscillator fre-
quency.When the object moves away from the transducer,the foam springs
back, and the plates return to their original spacing. This device can be
fooled by metallic objects. If a good electrical conductor comes near the
transducer, the capacitance might change even if contact is not made.
Conductive foam, rather than dielectric foam, can be placed between
the plates, so that the resistance changes with pressure. A direct current is
passed through the device. If something bumps the transducer, the current
increases because the resistance drops. This transducer will not react to
nearby conductive objects unless force is actually applied.
The output of a pressure sensor can be converted to digital data using an
analog-to-digital converter. This signal can be used by the robot controller.
Pressure on a transducer in the front of a robot might cause the machine
to back up; pressure on the right side might make the machine turn left.
See also BACK PRESSURE SENSOR, CAPACITIVE PRESSURE SENSOR, CONTACT SENSOR,
ELASTOMER, PROXIMITY SENSING, and TACTILE SENSING.
PRINTED CIRCUIT
A printed circuit is a wiring arrangement made of foil on a circuit board.
Printed circuits can be mass-produced inexpensively and efficiently.
They are compact and reliable. Most electronic devices today are built
using printed-circuit technology.
Printed circuits are fabricated by first drawing an etching pattern. This
is photographed and reproduced on clear plastic. The plastic is placed
over a copper-coated glass-epoxy or phenolic board, and the assembly
undergoes a photochemical process. The copper dissolves in certain
areas, leaving the desired circuit as a pattern of foil runs.
The use of printed circuits has vastly enhanced the ease with which
electronic equipment can be serviced. Printed circuits allow modular con-
struction, so that an entire board can be replaced in the field and repaired
in a fully equipped laboratory.
See also MODULAR CONSTRUCTION.

