Distance Learning's Sample Lesson - page 3
Video Clips Identify Mode Shapes
Let’s use a small electrodynamic (ED) shaker and repeat/extend the experiment of Figure 10-3, shaking just one solid (no extra damping) cantilever beam clamped to the shaker table.
In Video Clip 10-1, forcing frequency ff is far below the beam’s first mode bending natural frequency fn, so that magnification or transmissibility (ratio of response to input) ≈ 1.
Click on the image above to watch video clip 10-1
Low Forcing Frequency
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Real Player to watch the video above, just click here to download it for free.)
You didn’t see much, did you? You weren’t supposed to. That’s an important point. Away from resonance, response is very small. But wait till the next video clip!
Off camera, we’ve now adjusted the shaker’s forcing frequency to excite the beam’s first bending mode at 7.4 Hz. See Video Clip 10-2 in which ff = fn. We have resonance! The beam is bending in its first mode.
Click on the image above to watch video clip 10-2
First Mode Resonance
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Without changing the shaker, we’ve changed the illumination, adding to daylight the light from a stroboscopic light or "strobe". It is firing at about 6.4 Hz, so that the bright "slow motion" reflection off the beam appears to be about 1 Hz.
