Page 439 - Sensors and Control Systems in Manufacturing

Sensors in Flexible Manufacturing Systems
characters are normally presented to the recognition system in a 393
relatively constrained orientation, a measurement of which is not
required.

8.6.3 Multiple Objects
It is a natural requirement that the visual system for an assembly
machine should be able to accommodate a number of components
randomly positioned in the field of view. The corresponding problem
of segmentation in character recognition is eased (for printed characters)
by a priori knowledge of character size and pitch. Such information
has fostered techniques for the segmentation of touching characters.
No attempt is made to distinguish between touching objects. Their
combined image will be treated by the identification procedures as
that of a single, supposedly unknown object.
The essentially unlimited size of the set of objects that must be
accommodated by the recognition system demands that a detailed
description of shapes be extracted for each image. There are, how-
ever, a number of basic parameters which may be derived from an
arbitrary shape to provide valuable classification and position infor-
mation. These include:

• Area
• Perimeter
• Minimum enclosing rectangle
• Center of the area
• Minimum radius vector (length and direction)
• Maximum radius vector (length and direction)
• Holes (number, size, position)

Measurements of area and perimeter provide simple classification
criteria that are both position- and orientation-invariant. The dimen-
sionless shape factor area/perimeter has been used as a parameter in
object recognition. The coordinates of the minimum enclosing rectan-
gle provide some information about the size and shape of the object,
but this information is orientation-dependent. The center of area is a
point that may be readily determined for any object, independent of
orientation, and is thus of considerable importance for recognition
and location purposes. It provides the origin for the radius vector,
defined as a line in the center of the area to a point on the edge of an
object. The radius vectors of maximum and minimum length are
potentially useful parameters for determining both identification and
orientation. Holes are common features of engineering components,
and the number present in a part is a further suitable parameter. The
holes themselves may also be treated as objects, having shape, size,
and position relative to the object in which they are found.