200. comparison of scanning laser vibrometer and accelerometers for modal testing of large flexible structures

Department: Structural Engineering
Faculty Advisor(s): John B. Kosmatka

Primary Student
Name: Andrew Ming Fann
Email: amfann@ucsd.edu
Phone: 818-635-1187
Grad Year: 2018

Abstract
Vibration testing is often performed with accelerometers mounted on the test unit to measure accelerations at the mounting location. However, scanning laser vibrometers provide an alternative non-contacting sensing option by measuring surface velocities of the test unit at the location where the laser is directed. For small-scale structures requiring few measurement points, the differences between accelerometer and vibrometer for instrumentation may seem insignificant. However, as the number of measurement points increase with structure size, the advantages of laser vibrometers over traditional accelerometers become apparent, as the accelerometers require more time to mount and manage cables, as well as increase mass to the structure. These issues are non-trivial particularly when testing large space structures, such as solar arrays and the supporting structures. The current project explores the differences in measurements collected by a laser vibrometer and those collected by accelerometers, relative to a finite element model of the test unit. The test unit is a three-dimensional aluminum truss structure representative of the International Space Station (ISS) longerons, which support and actuate its large solar arrays. Vibration testing is performed for the baseline structure, then a modified configuration in which a concentrated mass is attached to cause a slight shift in natural frequencies and mode shapes. The results show that the laser vibrometer is better capable of measuring the slight changes in mode shapes than are the accelerometers. The laser vibrometer provides promising applications for modal survey testing of large flexible structures, for which accurate and precise measurements of natural frequencies and mode shapes are required for successful deployment in space.

Industry Application Area(s)
Aerospace, Defense, Security

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