Page 145 - Concise Encyclopedia of Robotics

Gross Motion Planning
GRAVITY LOADING
Gravity loading is a phenomenon that introduces positioning error into
robot arms as a result of the force of gravity.
All robot arms are comprised of materials that bend or stretch to some
extent; no known substance is perfectly rigid. In addition, all materials
have some mass; thus, in a gravitational field they also have weight. The
weight of the robot arm and end effector always causes some bending
and/or stretching of the materials from which the assembly is made. The
effect can be exceedingly small, as in a telescoping, vertically oriented
robot arm; or it can be larger, as in a long, jointed robotic arm. However,
the effect is never entirely absent.
The error caused by gravity loading is not always significant.In situations
where gravity loading causes significant positioning errors, a scheme for
correction is necessary.
See also ERROR CORRECTION.
GRAYSCALE
Grayscale is a method of creating and displaying digital video images. As
its name implies, a grayscale vision system is color-blind.
Each image is made up of pixels. One pixel is a single picture (pix)
element. The pixels are tiny squares, each with a shade of gray that is
assigned a digital code. There are three commonly used schemes for
rendering pixels in grayscale: percentage-of-black, 16 shades of gray, and
256 shades of gray.
In the percentage scheme, there are usually 11 levels according to the
following sequence: {black, 90 percent black, 80 percent black, …, 20 per-
cent black, 10 percent black, white}. Sometimes the brightness is broken
down further, into increments of 5 percent or even 1 percent rather than
10 percent; such gradations tend to be imprecise because computer digital
codes are binary (power-of-2), not decimal (power-of-10).
In the 16-shade scheme, four binary digits, or bits, are needed to repre-
sent each level of brightness from black = 0000 to white = 1111. In the
256-shade scheme, eight binary digits are used, from black = 00000000 to
white = 11111111.
See also COLOR SENSING and VISION SYSTEM.
GRIPPER
See ROBOT GRIPPER.

GROSS MOTION PLANNING
Gross motion planning is the scheme a mobile robot employs to navigate
in its work environment without bumping into things, falling down
stairs, or tipping over. The term can also refer to the general, programmed

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