Hybrid Sets of Merged Data for Modal Model Applications Louis Thibault, Bruce LeBlanc, Peter Avitabile Structural Dynamics and Acoustic Systems Laboratory University of Massachusetts Lowell One University Avenue Lowell, Massachusetts 01854 ABSTRACT Often times, data is collected from a variety of different measurement systems that address different frequency ranges where the particular measurement system is well suited. These data sets may come from traditional accelerometer measurements and laser measurements and need to be merged into one complete data set. This can pose some difficulties when the measurement points are not collocated and reside at different points on the structure. With the advent of the use of optical measuring systems such as digital photogrammetry and digital image correlation techniques, the sets of points become even more complicated and a strategy is needed to merge all the data into one complete, scaled data set. This paper presents an approach to take multiple sets of data collected over different frequency ranges and different sets of points to form one complete set of modal vectors to be used for the description of the modal data set. The use of model expansion approaches and a finite element model are used for this work. A set of data is collected to illustrate the approach. Discussion of the results obtained is presented. INTRODUCTION Experimental modal models are typically developed from a test where a specified set of transducers are used to measure dynamic response. However, in many applications where the frequency range needed to address the dynamic characteristics is very broad, then generally it is very difficult to use one set of transducers for the complete test or frequency range to be covered. This is most often the case where rigid body modes of a structure require a completely different set of transducers with appropriate sensitivities and frequency ranges suitable to the extraction of rigid body mode information [1]. This set of transducers used to measure these critical modes is not appropriate for mid range frequency and definitely not suitable for higher frequency ranges. appropriate frequency range. This implies that there may be one test with transducers used to measure rigid body mode information, a set of transducers used to measure lower frequency flexible modes, and yet another set of transducers that are well suited for higher frequency type response. Figure 1 is a schematic showing the problem described with the different transducers that are best suited to measure the various broad frequency ranges that may be required. On top of this logistical nightmare with several tests being required to address the entire frequency range desired, there may be an assortment of different transducers which are advantageous for these different frequency ranges. Displacement type transducers may be most appropriate for the very low frequency response or specifically designed low frequency accelerometers for the low frequency response to be measured. Normal ICP accelerometers may be adequate for the low frequency flexible modes of the structure but may be inappropriate for higher frequency ranges due to lack of adequate frequency response characteristics, insufficient sensitivity, as well as causing mass loading effects on the structural modes to be acquired. In these cases, non-contacting type measurements such as laser doppler vibrometery may be required to make these high fidelity measurements that are not prone to mass loading the structure [2, 3]. To further add to the array of different transducers available, recently, optical methods have become another viable technique that are well suited for low frequency measurements and are extremely useful for the extraction of rigid body mode information [1]. T. Proulx (ed.), Linking Models and Experiments, Volume 2, Conference Proceedings of the Society for Experimental Mechanics Series 5, 165 Therefore, the modal test must be conducted several times with different transducers that are specifically suited for the DOI 10.1007/978-1-4419-9305-2_11, © The Society for Experimental Mechanics, Inc. 2011
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