Page 91 - Concise Encyclopedia of Robotics

Displacement Error
by comparing the orientation of the transducer with some known standard
such as the reading of a magnetic compass. The same is done for the other
source.A computer uses triangulation to figure out the precise location of
the robot.
Radio direction finding (RDF)
A radio receiver, equipped with a signal-strength indicator and connected
to a rotatable, directional antenna, can be used to determine the direction
from which signals are coming. Radio direction finding (RDF) equipment
aboard a mobile robot facilitates determining the location of a transmitter.
RDF equipment can be used to find the location of a robot with respect
to two or more transmitters operating on different frequencies.
In an RDF receiver,a loop antenna is generally used.It is shielded against
the electric component of radio waves, so it picks up only the magnetic
flux. The circumference is less than 0.1 wavelength. The loop is rotated
until a dip occurs in the received signal strength. When the dip is found,
the axis of the loop lies along a line toward the transmitter.When readings
are taken from two or more locations separated by a sufficient distance,
the transmitter can be pinpointed by finding the intersection point of the
azimuth bearing lines on a map.
At frequencies above approximately 300 MHz, a directional transmit-
ting/receiving antenna, such as a Yagi, quad, dish, or helical type, gives
better results than a small loop. When such an antenna is employed for
RDF, the azimuth bearing is indicated by a signal peak rather than by a dip.
See also DIRECTION RESOLUTION and TRIANGULATION.
DIRECTION RESOLUTION
Direction resolution refers to the ability of a robot to separate two objects
that appear, from the robot’s point of view, to lie in almost the same direc-
tion. Direction resolution on the Earth’s surface is also called azimuth
resolution. Quantitatively, it is specified in degrees, minutes, or seconds
of arc.
Two objects might be so nearly in the same direction that a robot
“sees” them as being one and the same object, but if they are at different
radial distances, the robot can tell them apart by distance measurement.
See also DIRECTION FINDING, DISTANCE MEASUREMENT, RADAR, and SONAR.

DISPLACEMENT ERROR
Displacement error refers to an imprecision in robot position that takes
place over time. Displacement error can be measured in absolute terms,
such as linear units or degrees of arc. It can also be measured in terms of
a percentage of the total displacement or rotation.

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