ERROR MINIMIZATION IN GROUND VIBRATION TESTING

Recent government-funded ground vibration testing (GVT) of a U. S. Army AH-1G helicopter at Kaman resulted in the development of new methods and techniques for vibration testing and modal analysis. The reader will be exposed to the vibration testing research which was conducted on this vehicle with the express purpose of technology transfer. This paper is directed especially to the test engineer who will be performing a helicopter GVT in the near future. The intent is to highlight many of the potential sources of error and the methods available to minimize those errors. Items discussed are: determination of force levels corresponding to the true linear range of the structure, coarse identification of structural modes with broadband random excitation, global parameter estimation with high resolution narrow band log sine sweeps, selection of driving points, the local mode effect as a source of error, the use of strain mobilities for global parameter estimation, and a discussion on ensemble averaging. Emphasis is placed on the use of a digital signal analyzer as a "working tool" to obtain reasonably accurate frequency response functions. The reader is introduced to the "Power Method" which uses the analyzer's rapid computational capability for natural frequency determination.