Measurement along Multiple Portions of an Image Large features that do not fit on one screen have to be measured by precisely controlling the position of the sensor and stage so as to locate each reference point within individual images. By this means, the system can measure even a large circle, as shown below, by detecting the edge while moving the stage across various parts of the periphery.
Variation in Contrast Depending on the Focus Condition Edge contrast is low due to out-of-focus edges. Edge contrast is high due to sharp, in-focus edges.
High
High
Low
Low
Contrast in the scanning direction
Contrast in the scanning direction
Overview of ISO 10360-7:2011
Composite Coordinates of a Point
ISO 10360-7:2011 (Geometrical product specifications (GPS) -- Acceptance and reverification tests for coordinate measuring machines (CMM) -- Part 7: CMMs equipped with imaging probing systems) was published on June 1, 2011. Some inspection items are listed in ISO 10360-7:2011. The following summarizes the test method for determining length measurement error (E) and probing error (P F2D ). Length measurement error, E Five test lengths in seven different directions within the measuring volume, each length measured three times, for a total of 105 measurements. Four directions are the space diagonal. Remaining three directions are user specified; default locations are parallel to the VMM axes. When CTE (coefficient of thermal expansion) of the test-length artifact is < 2×10 -6 /K, additional measurement using an artifact with a normal CTE (8 to 13×10 -6 /K) is performed.
Vision coordinate system
Machine coordinate system
M
Mz
Vx
V
My
Vy
Mx
Measuring machine stage position M = (Mx, My, Mz)
Detected edge position (from the center of vision) V = (Vx, Vy)
Actual coordinates are given by X=(Mx+Vx), Y=(My+Vy), and Z=Mz, respectively.
Since measurement is performed while individual measured positions are stored, the system can measure dimensions that cannot be included in one screen, without problems.
Principle of Auto Focusing The system can perform XY-plane measurement, but cannot perform height measurement using only the camera image. The system is commonly provided with the Auto Focus (AF) mechanism for height measurement. The following explains the AF mechanism that uses a common image, although some systems may use a laser AF.
4 positions (space diagonals)
3 positions (user specified)
Normal CTE additional inspection
Probing error, P F2D Measure 25 points distributed evenly around the test circle (14.4º pitch). Each of the 25 points shall be measured using the specified 25 areas of the field of view. Calculate probing error as the range of the 25 radial distances (Rmax - Rmin) from the center of the least-square circle.
Z coordinate
The AF system analyzes an image while moving the camera up and down in the Z-axis. In the analysis of image contrast, an image in sharp focus will show a peak contrast and one out of focus will show a low contrast. Therefore, the height at which the image contrast peaks is the just-in-focus height.
Camera
In-focus height
M
Field of view
Contrast
M-16
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