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Fixture design and calibration process |
Table of Contents
Within the JoeScan-provided JSDiag software, the Calibration Editor is used to calibrate the scanner's position and angle. Whenever a scanner is replaced or physically moved, it will need to be recalibrated. In general, it is wise to periodically perform a calibration on all the scanners in a system to ensure they are providing highly accurate measurements.
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Standoff distance,
Minimum and maximum log diameters,
Accuracy requirements,
Transport speed, and
Mechanical considerations.
Figure 1. Example schematic of a lineal system
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- With a fixture that has a mirror symmetry, there are two exact locations at which the head can consolidate the measured points with the fixture description. Both solutions are equally valid, but only one of them corresponds to the physical location of the head. Even worse, a rotationally symmetrical fixture (e.g., a single cylinder) will not work at all – there simply isn’t enough data to resolve the positional ambiguity.
- Non-symmetrical fixtures make recognizing calibration orientation errors much easier. In addition to calculating the position and rotation for a head, the JSDiag software will also calculate an error measure for the quality of each calibration. With the example fixture as shown below, the JSDiag software would display a large error measure if the user tried to calibrate with the fixture “flipped.”
Figure 2. Example calibration fixtures
When making the fixture, it is important that the fixture is designed such that it can be placed in the same position when re-calibrating the system.
Figure 3. Example calibration fixture dimensions (in inches)
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For best calibration results, the fixture should be designed such that it covers a majority of the desired scan envelope. For example, if the largest log diameter to be scanned is 13 inches, the fixture should extend nearly to the edges as shown in Figure 4.
Figure 4. Circles fixture nearly fills the desired scan envelope
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The way the calibration fixture is defined in the parameters file. Choose an origin on the fixture and measure all features from that point.
Where you place the fixture in the laser plane. It is important that the fixture is stable and that its position in the system is repeatable.
Figure 5. Calibration fixture in the laser plane of a JoeScan lineal scanning system
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It is important that the fixture is physically measured and the actual measurements are used to describe the features in the param.dat file, for the example radius and X,Y position of the circles shown in Figure 6. The measured radius of the top left circle is 2.001 inches, centered at the coordinates (-3.010", 2.990"). The radius of the top right is 1.755 inches, centered at the coordinates (3.005", 3.011"). The radius of the bottom circle is 1.508 inches, centered at the coordinates (0.0", -3.030"). |
Figure 6. JSDiag screenshot – Circles calibration fixture
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Rigidly position the calibration fixture in the laser plane where all scanners see two or more features, as demonstrated in Figure 5.
Switch to the “Calibration Editor” (F7) window in JSDiag.
Select the first scanner to calibrate from the Scanner.Laser: drop-down box.
Select the appropriate calibration fixture from the Calibration Fixture: drop-down box.
Click Update Profile to show the latest data point profile from that scanner.
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In Figure 7, the red data points are from the top scanner from Figure 5.
Figure 7. Data points visible from scanner 0.0 (scanner.laser) not aligned.
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- Click the Calibrate button repeatedly until the Error doesn't get noticeably better (the lower Error the better). JSDiag will automatically fine-tune the calibration to achieve the best fit.
- Tip: "Nudging" the rotation can help JSDiag get a better calibration.
Figure 8. Data points visible from scanner 0.0 (scanner.laser) aligned
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