MIMO technique for simultaneous measurement of translational and rotational mobilities

Abstract

It is more and more usual that mobilities, including the rotational degrees of freedom, are needed in order to improve the models in experimental analyses such as mechanical source identification, transfer path analyses, dynamic coupling of structures, etc. The most difficult mobilities to measure are those involving moment excitations. The standard method to measure moment mobility requires a pure moment excitation, which in this case means just a moment and zero force applied and vice versa. Unfortunately a pure moment excitation is often difficult to achieve. In practise to overcome the problem of pure excitations of moment, a multiple input multiple output (MIMO) technique can be used where both force and moment are excited simultaneously. This technique is examined for mobility measurements on a beam structure. The MIMO results are compared with measurements made using a conventional twin shaker configuration, in which two shakers are driven 180° out of phase, and with theoretical calculations. The normalised random errors are calculated for the two methods and they are compared. Bias errors are also discussed. It is found that the MIMO technique for simultaneous force and moment excitation is very useful and easy to use as an alternative to the conventional method. Another advantage of the method is that it can give a consistent mobility matrix (reciprocity is achieved), since the measurements are performed with the same loading for both force and moment excitation since they are simultaneous and since purity of excitations are not needed.