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Unique tool enables vibration researchers to think big

Matt Allen and Mike Sracic

Assistant Professor Matt Allen (left) and Mike Sracic (large image)

In just one second, Engineering Physics Assistant Professor Matt Allen can acquire vibration samples from thousands of points along a downhill ski. In a matter of several minutes, he could analyze an entire airplane wing. “What we’ve developed is a way to both acquire and interpret hundreds or even thousands of times more information in the same amount of time,” he says.

His advance is a new twist on an instrument called a laser vibrometer, which researchers use to measure vibration. Using the vibrometer, Allen and master’s student Mike Sracic sweep a laser beam continuously over the structure they’re studying. At the end of the measurement, they have collected enough information to enable them to determine how the structure has moved everywhere along the laser’s path.

In contrast, researchers who use the conventional, time-consuming laser vibrometry method leave the laser on a single point for a specific amount of time. “In the time it would take to measure one point using the traditional approach, we can determine what’s happening at every point along the laser’s path,” says Allen. “This is important if we want to apply laser vibrometry to low-frequency structures such as airplanes or civil structures, because tests with conventional laser vibrometry would take too long to be practical.”

The method could accelerate the product development process, enabling test engineers to glean more detailed information in significantly less time, and with less cost. Engineers also can use the technique to verify that a structure’s noise and vibration performance meet design specifications, to calibrate computer models, or to diagnose vibration problems in the field, says Allen.

A key factor in their use of continuous scan laser Doppler vibrometry, or CSLDV, is a method Allen and Sracic have developed for analyzing the mountains of data they acquire. They’ve developed an algorithm that breaks down their measurements into the “point-by-point” responses they would have measured if they had used traditional laser vibrometry. In addition, they can post-process the measurements exactly as they would for data collected via the conventional method.

Although one advantage of the CSDLV is faster measurement times, researchers can use the method to measure the simultaneous response of an entire structure in a situation that is impossible or expensive to replicate—for example, an explosion or impact. It also enables researchers to study structures—such as those made of rubber or composite materials—that change over time or with temperature. “We’ve found that we get an incredibly consistent set of measurements, so the quality can be noticeably better even when testing structures that we would usually not expect to change much with time,” says Allen. “We can also acquire extra spatial information, so if the response is noisy or corrupted at one point, we usually have a few extra points on either side that we can use to reconstruct the trend of the data.”

In addition, the method shows promise for studying large structures, such as airplane wings, that previously weren’t practical to measure with laser vibrometry.

Allen’s and Sracic’s approach has attracted international attention: Researchers in Italy are using CSDLV to study rotating helicopter blades. “They’re interested in using this approach to determine how the blades’ spatial deformation pattern changes as a function of their rotational speed,” says Allen.

Through their work with the laser vibrometer, Allen and his students have advanced their understanding of “time-periodic” systems. “With the laser Doppler vibrometer, we can very easily generate a time-periodic system with any level of complexity, just by changing the way we sweep the laser,” says Allen. “We can apply CSDLV to a simple structure, where we know what the answer should be, and use this to evaluate new test methods.”

Using a similar approach, Allen and Sracic also are studying a much more complex time-periodic system: the human body. They hope to develop mathematical models that describe people as they walk. “You could think of a walking person as a periodic system,” says Allen. “Every time you take a pair of steps, your body returns to the same orientation. Researchers might be able to use these methods to experimentally determine a dynamic model for the body at each orientation in the gait cycle.”

The research may help uncover clues about how the brain coordinates muscles and responds to disturbances to maintain a stable walking pattern.