Grid Calibration - Allen-Bradley Bulletin 5370 CVIM User Manual

Chapter 5 Camera and Lighting Parameters
5-25
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Using Object Calibration
andlGrid Calibration
NOTE: Object and grid calibration applies only to gages
configured for linear or angular gaging measurements,
Ideally, a ratio of 1 to 1 should exist for a measurement of the
same physical distance along the X and Y axis of the screen
image. But, because of image resolution, pixel shape, and
optical and other distortions, the actual ratio may not be 1 to
1.
Thus, an uncalibrated gage will return different values for
the same physical distance along the X and Y axes, and for
angles in between. The reason is that the gage "measures"
the number of pixels along its length, and a gage of a specific
length will encounter a different number of pixels on the X
and Y axes.
Object and grid calibration can compensate for these
distortions and, at the same time, convert the calibrated
measurements into inches or centimeters. The main
difference between object and grid calibration is this:
Object calibration uses four reference points - two on each
axis - as the basis for its calibration calculation. It is most
accurate when the inspected parts appear in the same part of
the screen image where the calibration is performed.
Grid calibration uses multiple reference points as the basis
for its calibration calculation. It is more accurate than object
calibration, and can compensate for distortions in al2 areas of
the screen image.
In performing object and grid calibration, the CVIM system
measures a calibration object or calibration grid of known
dimensions, then calculates a calibration factor on the basis
of these measurements. The CVIM system will then apply
the calibration factor to all subsequent linear and angular
gaging measurements.
The basic steps in object and grid calibration are these:
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Object calibration: Position the calibration object in the
part of the screen image where your application will be
performing the gage measurements.
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Grid calibration: Position the calibration grid to fill as
much of the screen image as your application needs to
perform the gage measurements, then align the grid with
the X and Y axes of the screen image.
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Position the X and Y axes of the calibration "window" so
that they lie properly over the calibration object or grid.
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Define the edges where the window's X and Y axes
intersect the calibration object or grid.
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Calibrate the CVIM system to the calibration object or
grid.
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Enter the actual X and Y "edge-to-edge" dimensions.
The following steps show you the details of performing the
object and grid calibration functions.