Page 113 - Concise Encyclopedia of Robotics

Error-Sensing Circuit
a communication circuit. If the signal passes through n nodes, there will
be an average of 3n (3 + 3 + 3 + + 3, n times) errors.
In robotic systems, kinematic errors, or errors in movement, can accu-
mulate over time, resulting in eventual positioning or displacement errors.
See also KINEMATIC ERROR.
ERROR CORRECTION
Error correction is a form of computer programming in which certain types
of mistakes are corrected automatically. An example is a program that
maintains a large dictionary of English words.The operator of a computer
connected to a speech-synthesizing robot might misspell words or make
typographical errors. Running the error-correction program will cause
the computer to single out all peculiar-looking words,bringing them to the
attention of the operator. The operator can then decide whether the word
is correct. With modern computers, huge vocabularies are easily stored.
When robots must keep track of variables such as position and speed,
error correction can be used when an instrument is known to be imprecise,
or when values depart from the reasonable range. A computer can keep
track of error accumulation, checking periodically to be sure that discrep-
ancies are not adding up beyond a certain maximum.
Error correction is important in robotic systems subject to gravity
loading. In order to ensure that the end effector in a robot arm does not
stray from its intended position because of the force of gravity on the
assembly itself, position sensing devices can be used, and a feedback system
employed to counter-move the robot arm until the error signal from the
sensor is zero.
In robotic navigation systems, error correction refers to the set of
processes that keep the device on its intended course. In a servomechanism,
error correction is done by means of feedback.
See also ERROR ACCUMULATION, ERROR SENSING CIRCUIT, ERROR SIGNAL, POSITION SENS-
ING, and SERVOMECHANISM.
ERROR-SENSING CIRCUIT
An error-sensing circuit produces a signal when two inputs are different,
or when a variable deviates from a chosen value. If the two inputs are the
same, or if the variable is at the chosen value, the output is zero. This type
of circuit is also sometimes called a comparator.
Suppose you want a robot to home in on some object. The object has a
radio transmitter that sends out a beacon signal. The robot has radio direc-
tion finding (RDF) equipment built-in. When the robot is heading in the
right direction, the beacon is in the RDF null, and the received signal
strength is zero, as shown in the accompanying polar-coordinate plot. If

