Measurement and Application of Multidirectional Mobilities

Doctoral thesis, 1996

The direct measurement of moment mobility, employing a twin exciter technique, is theoretically and experimentally examined. This is continued by a theoretical and experimental analysis of an improved technique using source signal filtering, for two degrees of freedom, for the measurement of multidirectional mobilities. Finally some application utilizing mobilities are presented. Special attention is given to moment mobility as this is the most difficult to measure and two measurement set-ups are studied, the I-like configuration and the T-like configuration. It is shown theoretically and experimentally that the mass of the measuring equipment can give large errors in moment mobility measurements and an approximate correction is given to improve measured results. One measure of excitation precision is found by forming a quotient between the two exciting forces. By theoretical examination, that includes the mass of exciter coils, the deviation of this force quotient from unity is explained. It is shown that the I-like configuration, due to its orientation, gives better results for beam-like structures. A sophisticated technique to measure multidirectional mobilities in two degrees of freedom, using source signal filtering to counteract mass loading and rotational inertia loading caused by the measuring equipment, is theoretically derived and experimentally tested on a freely suspended beam. The source signal filtering effects the force quotient to tend to unity thus ensuring proper force and moment excitations. The technique is shown to function in this initial work and can form the basis for a continued application to all six degrees of freedom of motion. Some examples in the application of multidirectional mobilities are presented for vibration isolation prediction and measurement, transfer path analysis, and energy flow propagation in a T-shaped beam. Focus is placed on including the rotational degrees of freedom in an analysis and the estimation of the resulting error with their omission.