Correlated Solutions Vic-Gauge 3D User manual

Vic-Gauge 3D
T e s t i n g G u i d e
www.correlatedsolutions.com

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Safety and Risk Management
Safety
Some mechanical assemblies contain pinch points - use proper care and tools
when assembling to avoid injury.
Incandescent lights can get very hot. Use caution when moving or working
around these lights, and beware of excessive heating of the specimen, cameras,
lenses, or other surfaces. Leave lights off when possible.
When mounting cameras it may be possible to create an unstable or unbalanced
situation. Be careful not to cantilever cameras excessively, especially heavier or
high-speed cameras.
Personnel should be in a safe location whenever a dangerous or destructive test is
being performed. Various solutions are available for extending cabling/controls to
allow the system to be operated in a safe, remote location –please contact
Correlated Solutions for more details.
Various paints, chemicals, and solvents may be used in preparing specimens for
testing. Always observe label precautions.
If any cables become frayed or damaged, replace immediately to avoid risk of
shock.
Do not attempt to repair or disassemble the computer or any electronic parts. Risk
of shock or damage may result.
Risk Management
Always fully support cameras when moving, assembling, or adjusting, until all
fasteners are completely tight. For larger high-speed cameras, two people may be
required. Cameras can suffer severe damage if dropped.
Always support lenses when installing or removing. In addition, for certain lenses,
during focusing operation, the front must be supported to prevent drops.
While most power and data connections are keyed, it may be possible to
incorrectly plug in or force a USB, 1394a, or 1394b connection. Always check
orientation and compatibility before making any connection.
For any test where excessive heat, shock, or flying debris may be present, take
steps to protect the cameras and equipment. Various shielding solutions are
available –please contact Correlated Solutions for more details.

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Introduction
This guide explains the basic steps in running a Vic-Gauge 3D test, from start to finish. A
typical sequence will be:
Preparing the specimen
Setting up, pointing, and focusing the cameras on the specimen
Calibrating the camera system
Placing gauges
Setting the reference image
Analog connection and setup
Running the test
Preparing the specimen
Begin by preparing the region of interest on your specimen with a speckle pattern. For
more information on techniques and guidelines, please see the application note AN525,
Speckle Pattern Fundamentals. In this example, our specimen is prepared with a laser-
printed speckle pattern on adhesive paper.

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Setting up the cameras
Pointing the cameras
To begin, set your prepared specimen in its testing location. Be aware of the orientation
and potential camera placement; for example, for a dog-bone specimen in a test frame,
the prepared face of the specimen should face outwards from the test frame rather than
facing the frame‟s columns.
Our test specimen for this example is a small demonstration fixture which is designed to
strain a small low-modulus material such as rubber (used here).
Make sure two cameras are connected, and start Vic-Gauge. You will initially be
prompted for a file name and location:
The prefix will determine the name of the output data files as well as any saved images.
All files and images will be placed in the selected path. You can click the …button to
select a new path, or type the path in directly. If you type a path that does not yet exist,
Vic-Gauge will prompt you and allow creating the path automatically.
Click OK, and the main window will appear; the two cameras will come up in a live
view.

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The cameras are shown side by side; you can also view an individual image by selecting
the tab for a given camera. The red areas in each image indicate overdrive/saturation,
where the pixel is driven to its highest possible brightness value.
The images can be zoomed by placing the mouse over an image and using the scroll
wheel. This is useful for checking focus or examining details but affects only the display
and not the saved image or analysis.
To adjust the exposure time, use the large slider at the bottom of each window. The
typical default range is 0-50ms –to select a higher maximum, or a smaller range (to
allow finer adjustments), you can right-click on the slider. Select a preset, or “Custom” to
specify.
After starting Vic-Gauge, check the position of your camera rig. The distance between
the camera system and the specimen will be determined by your available lenses; when
multiple lenses are available, you should use the shortest one that works for your setup.
In some cases, test or room setup may require you to place the cameras farther away and
compensate with longer lenses; here, there are no such restrictions, so we use 8mm
lenses. The short lenses are generally easier to work with and can give somewhat better
results.

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The distance is set so that the specimen fully fits in the field of view, while being close
enough to get a good image and use much of the camera‟s resolution. Note that the field
of view should be set with consideration to the final size and position of the specimen.
Here, because we expect axial tension, we‟ve allowed room for the specimen to strain.
Once a lens and approximate distance is selected, the cameras can be pointed. The
cameras should be positioned somewhat symmetrically about the specimen; this will keep
the magnification level consistent. The exact angle included between the cameras is not
critical but selecting a correct stereo angle will give best results: the angle should be at
least 25º for short lenses (8mm, 12mm), at least 20º for medium lenses (35mm), and at
least 15º for longer lenses (70mm). The angle should be kept below approximately 60º.
In the image below, we‟ve pointed both cameras correctly at the specimen. Note that the
image is well centered in both cameras, and that there is plenty of room for the specimen
to strain in tension:

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Adjusting focus
When the cameras have been positioned, the next step will be to set focus. Use the focus
control on your lenses to achieve a sharp focus on the entire specimen. Often, it will be
helpful to zoom in on the image to check fine focus; with high resolution cameras, a
slight defocus will not be visible with the image zoomed out to fit on screen. While
zoomed in, look closely at both the far and near edge of the specimen to ensure that the
entire surface is in focus, before proceeding.
Tip: to aid in focusing, open the lens‟s aperture all the way. This will reduce the depth of
field and make any focus issues very obvious. Then return the aperture to the appropriate
setting for the test. (You will need to temporarily reduce the exposure time to compensate
–see following section.)
Focusing with large aperture –small DOF
After closing aperture –focal plane is well
centered
Depth-of-field
Depth-of-field
Specimen
Focal plane
Camera

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Aperture and exposure time
As you make the image sharp through the focus adjustment, it will also be necessary to
adjust the brightness of the image. There are two controls available for this: the
aperture/iris setting on the lens, and the exposure time setting of the camera.
Aperture: opening the aperture allows more light to fall on the sensor. The
aperture setting is also called the f-number; f-numbers are usually indicated on the
lens‟s aperture ring and typically go from an open setting of F/1.4 or F/2.8 to a
closed setting of F/22 or F/32. Using a bigger aperture (lower f-number) will
make the image brighter. However, it will also decrease the depth of field –the
range over which the focus is sharp. Even for a flat specimen, some depth of field
is necessary because each camera is oblique to the plane of the specimen; also,
poor depth of field may make it difficult to achieve a wide range of calibration
target positions (see following section).
At higher magnifications and with higher resolution cameras, care should be taken
when using very small apertures (high F-numbers). In some cases this can limit
resolution due to diffraction; for example, with a 5 megapixel camera and a 75mm
lens, using apertures of F/11 or higher will result in very blurry images. In these
cases it will be necessary to find the best balance of depth of field (requiring high
F-numbers) and resolution (requiring low F-numbers).
Note that the aperture may not be changed after the system is calibrated.
Exposure time: this is the amount of time the camera sensor gathers light before
reading out a new image. Longer exposure times make the image brighter but can
also create blur if significant motion happens during the exposure times. For
many tests, blur is not a concern for the specimen itself, but can be an issue when
acquiring images of a hand-held calibration grid. A rule of thumb is to keep the
exposure time below 1/f, where f is the focal length of the lens (in mm). So, for a
50mm lens, this would mean a limit of approximately 20ms.
In contrast to aperture, exposure time may be adjusted after the system is
calibrated if lighting conditions change or the specimen becomes brighter/darker.
Controls for focus and aperture differ by lens; two common C-mount styles are shown
below.

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This lens has a focus and
aperture ring, each with locking
knob. The aperture ring is
normally closest to the camera.
Loosen the locking knob (if
present); make any adjustments;
and tighten the lock before
calibrating.
This lens has an aperture ring
with a locking knob. To focus,
loosen the collar (for this
particular lens, a 2mm hex
driver is used), and rotate the
entire body of the lens. Loosen
the lens body
(counterclockwise) to focus
closer; tighten (clockwise) to
focus farther. Tighten the collar
when complete.
Caution –the lens body is not
captive and will fall if screwed
all the way out.
When exposure and aperture settings are complete, the image should be bright and there
should be no overdriven (red) pixels in the area of interest. In many cases, ambient
light will not be enough to achieve this without using unacceptably large exposure times
or aperture settings; for these cases, supplemental light will be required.
Too bright –overdriven
Too dim
Correct brightness

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For many moderately sized specimens, a simple task lamp of 50-100 watts will give
excellent light levels while providing diffuse, even illumination. For very small
specimens at high magnification, a fiber optic illuminator can be used. For very large
specimens, a 300-500 watt halogen light or a specialized machine vision lighting solution
may be required. If the specimen is reflective or highly curved, specialized solutions or
careful positioning may be required to avoid reflections/highlights on the surface.
WARNING: If the light is positioned in front of and below the cameras, or if the
surface begins to heat up from the lighting, refractive heat waves can appear in your
image. These waves may not be visible in a zoomed out image but when zooming in
they can be seen as a swimming/shimmering effect which can both defocus and
distort the image.
If heat waves are present in your image they can easily introduce false strains of
several thousand microstrain so it is critical that they be avoided.
Heat waves can be minimized by either repositioning the light source, or, if
necessary, a small desk or stand fan can be set up to blow across the scene. This will
generally mix the air well enough to completely eliminate the heat waves.
To conclusively check for heat waves, you can simply run the Vic-Gauge analysis
with no load/motion. If heat waves are present, you will see large displacement and
strain fields that change randomly over time.
Once the camera positions have been set and the focus and aperture adjusted, calibration
can begin. After this point, changing any aspect of the camera system will invalidate the
calibration; so, all adjustments should be carefully fixed at this point.
Tighten the tilt and rotation adjustments on the camera mounts.
Lock the focus and aperture adjustments on the lenses, if possible.
Tighten the tripod column.
Tie the camera cables firmly to the tripod or crossbar; this will prevent external
force on the cables from applying leverage to the cameras.