TEST SETUP
Setting up for a vibration test begins with pointing and focusing the cameras, as with a quasi-static test. There is only one
major additional concern – lighting. It will be necessary to calculate, or establish empirically, the exposure time required to
freeze your motion.
In general, since Vic-3D is typically accurate to ~0.02 pixel, we would like to see motion of 0.02 pixel or less contained
within the exposure time. For example, suppose we have a tire that is rotating at 120rpm, or 2Hz. The diameter of the tire is
635mm, and we want to image the whole tire using a 1m x 1m field of view, using 1024x1024 cameras.
Since we are imaging 1000mm with 1024 pixels, we have a magnification of about 1 mm/pixel. This means that we need to
keep our motion below
0.02 pixel * 1 mm/pixel = .02mm.
Our tire has a circumference of
635mm * π= 1995mm
So our maximum velocity, at the rim, is
1995mm * 2Hz = 3990mm/s
This means our maximum exposure time is
0.02mm / 3990mm/s = .000005s = 5µs
We could accomplish this with a strobe light with a duration of <5µs. In practice, using a steady light and a camera with an
exposure time of 20µs would give us very useful results; motions of .1 pixel or even .5 pixel can be accommodated at some
loss of accuracy, given a properly coarse speckle pattern. Still, we would require a very strong light source in order to give a
bright image with only 20µs of integration.
These calculations will be different for each test. Also, for a test where the motion is mainly out-of-plane, more motion can
be tolerated because the pixel motions are relatively much smaller, compared to a test where the specimen is moving
directly across the field of view.
You can check the blur visually to some extent by examining the live image, but even a barely visible blur can greatly reduce
Vic-3D accuracy. The actual effect can be quantified in the Error score listed during correlation.
A final note - some cameras can streak at these very short exposure times, and some cameras will streak if light is present
during their readout. For more information about the limits and capabilities of your specific setup, please contact Technical
Support.
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