Rockwell Automation 5370-CVIM2 Module - Manual (Page 283)

5 Chapter Chapter 7
Inspection Tools
7–45
Contour Measurement Functions
The contour measurement functions described earlier are mostly self
explanatory. Thus, the height, length, perimeter, area, X and Y center (of
centroid, not midpoint), and minimum and maximum radius measurement
functions are relatively straightforward in concept and implementation.
Roundness, circularity, inertia, and theta are less intuitive, however, and are
therefore discussed here in more detail.
Roundness –– Roundness is a measure of how close an object is to being a
perfect circle when the perimeter is smooth and crisp. The measurement is
expressed in percentages, with 100 percent being a theoretically perfect
circle. (A straight line will yield a “roundness” of about 1 or 2 percent.) The
formula that the CVIM2 system uses to calculate roundness in percent is:
. . . where A is the area value and P is the perimeter value.
Figure 7.40 (A) (page 7–46) compares the roundness of two figures, a circle
(1) and an ellipse (2), whose area measurements are very close (11,992.0
pixels for the circle and 12,052.0 pixels for the ellipse), but whose roundness
values are noticeably different (96.6% for the circle and 83.5% for the
ellipse). Thus, the roundness measurement function could be used to
differentiate two objects such as these.
Figure 7.40 (B) compares the “roundness” of two rectangular figures, one
with sharply squared corners (2) and the other with rounded corners (1),
whose area measurements are very close (18,536.0 pixels for the squared
rectangle and 18,695.0 pixels for the rounded rectangle), but whose
roundness values are noticeably different (75.9% for the squared rectangle
and 86.9% for the rounded–corner rectangle). Thus, the roundness
measurement function could also be used to differentiate two non–round
objects such as these.
The roundness measurement function works best when the perimeter is
smooth and regular and conforms to the basic shape of the object. When the
perimeter is fuzzy or convoluted, however, the roundness value can be
lowered considerably compared to an object of the same basic shape, but
with smooth, regular perimeter. In such cases, the circularity measurement
function may be required.
ǒ
4
pA
P 2
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100