A Method of Modifying Modal Test Boundary Conditions based on Experimental Reaction-Force Measurement

摘要

A method of modifying boundary conditions of a modal tested structure, independent of analytical modeling is presented. It is referred to as the Multiple Reaction-Force Technique (MRFT). There are two obvious applications for such a technique, one in the aviation industry, the other in the space industry. Both of these industries perform modal tests on the ground, but have need for dynamic characteristics in either flight or on-orbit configurations. For an airplane, the modal testing is often performed with the aircraft supported by its landing gear. For a large deployable space structure, the modal testing is done with ground support equipment to offload the structure for gravity loads. It would seem a trivial problem to validate a structure using the finite-element model and then to update it to remove boundary-condition effects. This is not a trivial problem. One of the most complex problems remaining in linear modal analysis is the update of finite-element models based on test data. There are several approaches that have been developed to eliminate the boundary-condition issues associated with modal testing. Some of them rely partially on the associated finite-element model. At least one relies on developing a support system to eliminate the boundary-condition constraint effects by using an active control system as part of the support structure. The approach presented in this paper relies solely on the experimental data. It requires that force be measured at the boundary condition to be modified. Force measurement is still a challenging problem in itself. It is only within the past few years that stiff 3 degrees-of-freedom, force transducers with wide dynamic ranges have become available commercially. The extension of these force transducers to measure 6 degrees-of-freedom force, over a continuous frequency range including static loads has not yet been achieved. The measurement of the reaction force data provides a significant amount of new and powerful information about the structure being tested. It gives insight into a more global characteristic of the structure than that provided by a single accelerometer. The MRFT method exploits the capability force measurement provides.