Advantages of Multiple-Input Multiple-Output Testing

Multiple-input, multiple-output (MIMO) measurement techniques are a well-proven and well-established method for collecting FRF (frequency response function) data sets. Despite that, the idea of using multi-references and multiple shakers can be intimidating for an inexperienced modal test engineer. MIMO methods offer some distinct advantages for measuring and extracting basic modal parameters especially while testing larger structures. Compared to single-input, single-output (SISO) techniques, MIMO allows for more uniformly distributed energy across the structure, force levels can be kept lower, modal test data can be taken in one single shot, while nonlinearities are less likely to be excited. Experimental data were taken using single and multiple shakers on an aeronautical structure and results were compared to illustrate the basic principles behind the MIMO technique. Practical aspects of shaker setup, reciprocity checks and the capability of MIMO to resolve closely spaced modes and repeated roots are highlighted throughout the example.