Rockwell Automation 5370-CVIM2 Module - Manual (Page 179)

Chapter 6
Reference Tools
6–20
Gray scale gaging mode –– The maximum background object is the one with
greatest number of consecutive pixels (between the first and last detected
edges) that are darker than the adjacent pixels –– the pixels on both sides of
the object. In example (B), above, the leftmost object has the greatest number
of consecutive pixels that are, in this case, darker than the adjacent pixels.
The choice of feature mode should be based upon which one provides the
most reliable reference point for your application. The example in
Figure 6.18 (page 6–21) illustrates this concept by showing how
Max B. Obj
can be used to “track” the maximum object on a workpiece.
In this example, the objective of the inspection application is to measure a
hole in the widest part of the workpiece (the “maximum object”) in a
situation where the workpiece –– and thus the maximum object –– can
appear in either a left or right orientation within the image field.
In example (A), the hole appears on the left. The reference line correctly
identifies the maximum object and applies shift compensation (if needed) to
the window in order to keep it over the hole.
In example (B), the hole appears on the right. Again, the reference line
correctly identifies the maximum object and applies shift compensation to
the window in order to keep it over the hole.
If the All Edges feature mode had been used in this case, a specific reference
point, such as the leftmost edge, would not have detected the alternating
position of the hole in the image field. This is because it “sees” only the
leftmost edge, regardless of the orientation of the workpiece in the image
field.
Direction
The “direction” is the direction used to search for edges along the length of
the reference line axis. The two available choices are these:
Head to Tail
and Tail to Head .
When you pick the Dir menu field (in the Define Feature menu)
successively, it toggles to the opposite direction. Thus, Head to Tail changes
to Tail to Head , and vice versa.
Head to Tail causes the edge search to begin at the “head” (X1, Y1) of the
reference line axis; Tail to Head causes the edge search to begin at the “tail.”
For X–axis reference lines, the head is, by default, at the left end; for Y–axis
reference lines, the head is, by default, at the upper end. The head and tail
positions, however, can be reversed using the pick and place function.
The choice of search direction should be based upon which one leads most
directly to the specified reference point. Normally, the best search direction is
the one in which a false edge is least likely to be encountered.