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Photogrammetry What is modal analysis, and photogrammetry? Let's take a look All structures experience vibration. From atoms to stars, everything around us vibrates. Most vibration is perfectly acceptable. In some cases however, it is annoying or even destructive. Structures vibrate most readily at natural frequencies. The natural frequency depends on the mass and stiffness found within the structure. If a structure receives energy at the same rate as the natural frequency, then a condition called resonance occurs. One of the most famous structural failures due to resonance was the Tacoma Narrows Bridge in 1940. The source of energy was a steady wind. To understand, and remedy vibration and resonance problems, it is important to understand how structures vibrate. The motion of a structure due to vibration is called the mode shape. We use modal analysis techniques to understand, and solve structural vibration problems. MODAL ANALYSIS This example illustrates two mode shapes for a simple object. Knowledge of the mode shape allows one to properly address the actual problem. Note how a rib stiffener down the center of the upper and lower plates would help in one case, but in the other case, it wouldn't help at all! In most cases, the correct fix to vibration problems depends on a good understanding of the mode shape. PHOTOGRAMMETRY Photogrammetry is the art of taking measurements from photographs. Using a calibrated camera, we take several photographs of a structure from multiple angles. Photomodeler Pro software (EOS systems, Inc.) allows us to quickly create a scaled 3-D model from the photographs. We can rotate the model within the software and observe it from any angle. The 3-D model is exported to ME'Scope VES software (Vibrant Technology, Inc.) to be animated with actual recorded vibration data. We use the animated model to observe the mode shapes. The example shown here is from an actual study performed by Ball Spin Engineering In this particular case, before Ball Spin Engineering arrived, a lot of effort was spent investigating foundation resonance. The client drilled holes in the base and added bolts (stiffness modification), they loaded sandbags around the base (mass modification) and tried various other isolation techniques to isolate the problem. Nothing they did seemed to affect the vibration. The modal analysis took only a few hours to complete. The resulting animation revealed the problem wasn't in the base after all. The problem was actually a lack of stiffness in the motor mounting plate, located nearly 3 meters above the base. When the motor mount was re-designed with heavier components, the resonance problem disappeared and the pump operated safely within specified vibration limits.