Determination of Excitation Locations in Modal Survey Testing - Final Report - ESTEC Contract No. 7509/87/NL/PH.

Present modal survey test techniques suffer under the absence of a criterion for optimal exciter placement. Based on combinatorical optimization in combination with the condition number of the modal matrix of potential exciter points, a technique for optimal exciter point selection was developed. The a priori information on the normal mode parameters, required for these calculations, is determined by means of indicator function information and a difference criterion for the structural deformations. Tests on an analytical eleven-degree-of-freedom system, as well as a modal survey test on the real satellite model SIMOD, verify the performance of the proposed technique

Real and Complex Exciter Forces for Experimental Modal Analysis - A Simulation Study -.

Most modal analysis testing of aerospace structures is performed by applying the phase resonance test technique, and the experiences gained with this procedure are based on longtime applications. Most -fforts to improve this test technique were focused on the determination of the real exciter force distribution to isolate the individual normal modes. Nevertheless, in the absence of efficient force tuning and force placement procedures, many normal modes have been identified as complex normal modes only. Strong damping coupling effects were assumed to explain this dynamic behavior. Therefore, some publications treat the relationship between the real and complex normal modes and offer numerical algorithms for the calculation of the real normal modes. In the recent past the exclusive isolation of the complex normal modes was proposed, assuming that the excitation of these normal modes is simpler and more reliable. Based on these publications, the real and complex exciter force formulations were applied on an eleven-degree-of-freedom model. The mean error of the normal mode shapes is presented as a comparison of the test results. These results document advantages and shortcomings of the individual real and complex exciter force tuning formulations. Also taking into consideration the neighbouring normal modes, the discrepancies and the accuracy of most test results can be improved significantly. This parameter variation documents the requirement of consistent control of the dynamic responses of the test structure and shows that "complex normal modes" may also be a result of incomplete -xcitation. Taking into account the test philosophies of some popular phase separation techniques, most published "complex normal modes" merely seem to be the result of incomplete excitation, simultaneously regarding the small damping factors of aerospace structures

Static and Dynamic Test Data Combined with Analytical Models.

Practical applications of updating techniques using measured normal mode parameters or mobility measurements document the advantages and future prospects of the updating technique, but also show the difficulties and problems for practical applications. These insufficiencies are mainly based on the limited number of measured normal modes and measured deformations compared to the large amount of stiffness and mass data of the finite element model. Moreover, the measured normal modes must not show elastic deformations on all substructures or coupling elements. However, modal synthesis technique requires accurate coupling stiffnesses to obtain reliable results. Interface loading may be one way to simulate dynamic loads at the coupling points, but this also has disadvantages caused by the interface loading structure. Another possible method of obtaining the required stiffness information of critical structural components is to perform aimed static deformation measurements. The combination of both static and dynamic test data enlarges the test information. This paper describes the formulation of an updating procedure which uses static and dynamic test data simultaneously. The updating results of a simulated eleven degree of freedom system and of a simple real structure (sandwich plate) show the performance of the proposed technique

On the Information Content of Normal Modes for Updating Applications.

The updating technique aims for the reconciliation of analytical structural models with static and dynamic test data. A finite element model which produces correct static and dynamic deformations and normal frequencies is expected as the result of this updating process. Therefore, the finite elements that contribute to the discrepancies between the analysis and test have to be identified. The kinetic and potential energy distribution in combination with thecondition number of the updating equations is proposed as a powerful and reliable criterion to solve the task