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Introduction
Most conventional optical comparators are difficult to
use and do not yield accurate results unless meticulous care is exercised. MPM has developed a digital optical
comparator measurement system that is user friendly and yields both accurate
and precise results. The system can be used to measure the dimensions of any
part or piece of equipment that can be imaged with the optics. The camera and lens can be mounted on the
supplied table top rack and pinion stand, or they can be used as a portable
measurement unit. This system can also be conveniently used for
percent shear
determination and for Brinell hardness measurement.
System Features
|
Figure 1 - Digital Optical Comparator System |
The MPM digital imaging system consists of a camera, a
telecentric lens, an opaque lighting source, an infrared back lighting source,
a rack and pinion stand, data acquisition software, and image analysis
software. The 7200 system can be provided with fixed zoom detents (that is,
pre-defined magnification points) or can be upgraded to the 7300 system which
incorporates a motorized system for continuous
zoom adjustment. It is desirable to use the detents so that the calibration at each
magnification can be used. These calibrations only need to be updated
annually. For the 7300 system, the software returns to precisely the
magnification specified by the user.
In a typical measurement, the system will automatically
capture the image and use the system tools to make the user-specified
measurements. The user can also make various measurements interactively
using, for example, length measurement tools, radius tools, angle tools,
pattern tools, polygon tools, and geometry tools.
Calibration
The software has the capability to calibrate distances
and areas in real-world coordinates. The calibration can be performed in one
dimension (1D) or in two dimensions (2D). The 1D calibration is achieved
using a precision reticle as shown in Figure 2.
While the telecentric lens greatly reduces parallax
distortion and other optics problems such as lens axis perpendicularity, a
recently developed two-dimensional calibration technology eliminates residual
errors in the optics. With this technology, it is possible to re-map the
pixel grid to yield a nearly undistorted image. An example 2D calibration
grid is shown in Figure3. The 2D calibration approach is the most accurate
and is recommended by MPM.
Figure 2 - Example 1D Calibration using a Precision Reticle |
Figure 3 - Example 2D Dot-Matrix Distance Calibration
using Precision Grid
|
Example Measurements
An
example measurement would be the verification that a series of machined
notches meet the specified tolerances on notch radius, notch depth, and notch
angle. As shown in Figure 4, the software can be configured to automatically
acquire the image, make the measurements, and store the data in a file. The
user can also interact and use the measurement tools manually. Other
potential applications are shown in Figures 5 through 7.
Figure 4 - All Three Desired Measurements are Made
Automatically and Stored for Reference |
 
Figure 5a - Auto Threshold and Rotate Image 180
Unchecked |
 
Figure 5b - Auto Threshold and Rotate Image 180
Checked |
Figure 6 - Diameter Measurement Tool |
Figure 7 - Radius Measurement Tool |
Example System
Verification Data
Precision reticles traceable to NIST can be used to
verify the system accuracy and precision. Example verification results
for both a 1D and a 2D calibration are shown in Tables 1 and 2. MPM has
determined that the small errors are primarily due to inaccuracies in the
operator's trace of the length or area to be measured.
Table 1 - One-Dimensional System
Verification Data
|
Detent |
Known Length |
Measured Length |
Known Area |
Measured Area |
Length |
Area |
|
Magnification |
(inches) |
(inches) |
(square inches) |
(square inches) |
% Error |
% Error |
|
0.6 |
0.3000 |
0.3003 |
0.1600 |
0.1601 |
0.10 |
0.06 |
|
1.0 |
0.2000 |
0.2002 |
0.04000 |
0.03997 |
0.10 |
0.08 |
|
1.5 |
0.1000 |
0.0999 |
0.02000 |
0.02000 |
0.10 |
0.00 |
|
2.0 |
0.0900 |
0.0901 |
0.00500 |
0.004995 |
0.11 |
0.10 |
|
2.5 |
0.0700 |
0.0700 |
0.00500 |
0.004997 |
0.00 |
0.06 |
|
3.0 |
0.0500 |
0.0500 |
0.00320 |
0.003199 |
0.00 |
0.03 |
|
3.5 |
0.0500 |
0.0500 |
0.00180 |
0.001799 |
0.00 |
0.06 |
|
4.0 |
0.0400 |
0.0400 |
0.00180 |
0.001798 |
0.00 |
0.11 |
|
4.5 |
0.0400 |
0.0400 |
0.000800 |
0.0008000 |
0.00 |
0.00 |
|
5.0 |
0.0300 |
0.0300 |
0.000800 |
0.0007994 |
0.07 |
0.08 |
|
5.5 |
0.0300 |
0.0300 |
0.000800 |
0.0008007 |
0.07 |
0.09 |
|
6.0 |
0.0300 |
0.0300 |
0.000800 |
0.0007997 |
0.10 |
0.04 |
|
6.5 |
0.0300 |
0.0300 |
0.000800 |
0.0008006 |
0.10 |
0.08 |
|
7.0 |
0.0200 |
0.0200 |
0.000400 |
0.0003996 |
0.05 |
0.10 |
Table 2 - Two-Dimensional System
Verification Data
|
Detent |
Known Length |
Measured Length |
Known Area |
Measured Area |
Length |
Area |
|
Magnification |
(inches) |
(inches) |
(square inches) |
(square inches) |
% Error |
% Error |
|
0.6 |
0.3000000 |
0.3000 |
0.1600 |
0.1601 |
0.00 |
0.04 |
|
1.0 |
0.2000000 |
0.2001 |
0.04000 |
0.04000 |
0.05 |
0.00 |
|
1.5 |
0.1000000 |
0.1000 |
0.02000 |
0.02001 |
0.00 |
0.05 |
|
2.0 |
0.0900000 |
0.0900 |
0.00500 |
0.00500 |
0.02 |
0.06 |
|
2.5 |
0.0700000 |
0.0701 |
0.00500 |
0.00500 |
0.07 |
0.06 |
|
3.0 |
0.0600000 |
0.0600 |
0.00320 |
0.00320 |
0.00 |
0.00 |
|
3.5 |
0.0500000 |
0.0500 |
0.00180 |
0.00180 |
0.04 |
0.00 |
|
4.0 |
0.0400000 |
0.0400 |
0.00180 |
0.00180 |
0.00 |
0.00 |
|
4.5 |
0.0400000 |
0.0400 |
0.000800 |
0.000800 |
0.08 |
0.00 |
|
5.0 |
0.0300000 |
0.0300 |
0.000800 |
0.000801 |
0.00 |
0.09 |
|
5.5 |
0.0300000 |
0.0300 |
0.000800 |
0.000800 |
0.00 |
0.04 |
|
6.0 |
0.0300000 |
0.0300 |
0.000800 |
0.000801 |
0.00 |
0.08 |
|
6.5 |
0.0300000 |
0.0300 |
0.000800 |
0.000800 |
0.00 |
0.01 |
|
7.0 |
0.0200000 |
0.0200 |
0.000400 |
0.000400 |
0.00 |
0.08 |
Other Measurements
Please refer to the
Percent Shear, Lateral Expansion,
Notch Verification and the
Brinell
Hardness Measurements pages for a description of other features that
are included with these systems.
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