Determination of Structural Modes of Vibration Using Digital Photogrammetry

Abstract

It is shown that it is possible to recover the three-dimensional modes of vibration of an oscillating structure through an offline digital photogrammetric approach employing only a single video camera. In addition to the camera, the technique requires a high powered synchronized strobe unit and the careful control of fixed sequential delays between the excitation force, the strobe illumination, and the multiple sequence of camera exposures. The dynamic three-dimensional object point triangulation problem is then reduced to a set of static problems by capturing object shape in a number of different phases at a given number of measurement epochs. For each repeatable oscillation cycle, the vibrating object is imaged from a different camera position. The XYZ object coordinates of the target array at each sampling epoch can, thus, be determined by photogrammetric triangulation using all images corresponding to the same instant of time within the oscillation cycle. The dynamic mode shape is then determined from the triangulated object points at the various phases. The process is illustrated for the determination of modes of vibration for an aircraft wing section. Note that the advantages of using only a single camera are multifold; the system is cheaper, the system is simpler and, thus, more robust in that multiple cameras do not have to be synchronized, and finally the system has the potential to deliver greater accuracy without having an excessively large number of cameras. © 2002 Journal of Aircraft.