Computer-Controlled Normal Mode Testing

Abstract

Normal mode testing is a long-standing test method of experimental modal analysis. Typically, the harmonic responses of a harmonically-excited structure are investigated during normal mode testing with regard to their phase relation to the excitation. While classical phase resonance methods are suitable for checking the linearity of the structural dynamic behaviour of aerospace structures, a significant amount of time is required to perform normal mode tuning, consequently resulting in considerable costs incurred during the prototype development period. However, improvements in computer technologies and measurement techniques enable improvements in the classical phase resonance procedure, especially with regard to minimizing the test schedule without loss of accuracy of the results. This paper focuses on new methods for automated tuning and on-line data evaluation procedures for normal mode testing by investigating band-limited frequency responses for analytical and experimental vibration systems during the on-line tuning process. Three different approaches for accelerating the tuning process are investigated with respect to their on-line capabilities. In general, these methods deliver estimations about eigenfrequency, modal damping, and generalized mass, but differ in the amount of modal information delivered and, consequently, in the computational effort required. The three different methods of computer-controlled normal mode testing were experimentally verified via ground testing using the Piezoelectric Aeroelastic Response Tailoring Investigation (PARTI) testbed at NASA Langley's Transonic Dynamic Tunnel, as well as the 20m spanwidth sailplane FS-33 located at DLR. Experimental results obtained from these applications are described and initial experiences are discussed.