Measuring and Modeling 3D Mode Shapes of FalconSAT-5 Structural Engineering Model

Abstract

As part of the FalconSAT student-built small satellite program at the United States Air Force Academy, a structural engineering model (SEM) was constructed to simulate the mass and stiffness properties of the flight model. The SEM was dynamically characterized using a scanning laser velocimeter that quickly collected data from several thousand closely-spaced points on the surface of the satellite. Orthogonal mode shape vectors were then extracted from the measured frequency response functions and used to evaluate an initial a priori finite element (FE) model created from the solid model geometry files from which the SEM was machined. Visual and Modal Assurance Criteria comparisons of the measured and modeled mode shapes and frequencies indicated a need to update the FE model. An optimization was then run in which an objective cost function consisting of the error in first three natural frequencies and second mode shape was minimized, resulting in a very accurate FE model of the first three modes (less than 1% frequency errors, greater than 0.95 MAC values) – the modes that experience the greatest deflection under launch loads and are therefore potentially damaging to the satellite and booster. The high-density experimental data collected here is vital to the extension of the FE model tuning beyond the first three modes to create a model that is accurate over a wide range of frequencies and a large number of modes.