Experimental Modal Analysis (EMA) is the field of measuring and analyzing dynamic response of a structure when it is excited with a stimulus. EMA is the useful procedure in dynamic verifying Finite Element Analysis (FEA) results, using modal analysis to determine modal parameters (natural frequency, damping ratio, mode shapes) of the structure. The simple method for obtaining modal parameters is using roving impact test to identify natural frequencies, damping ratios, and mode shapes of the structure. Thus, the modal testing machine is used during the testing process. To gain accurate data from the test, the machine requires special component to guide the modal hammer to hit onto a center of grid marks on a specimen, it must also prevent the hammer from rebound and hit to the same position twice. Otherwise, double hitting of the hammer will resulted in error of data recording. Other than that, the specimen has to be clamped firmly to avoid other vibrations during the test. Hence, to prevent from many errors during the testing procedure, modal testing machine was designed and built.
This paper describes the configuration, function, and building process of the modal testing machine which is composed of two major components such as: An impact hammer (modal hammer) which is attached to a moveable part of the machine for applying impact forces to a specimen, an accelerometer is attached to the bottom of the specimen for measuring fluctuations of the specimen acceleration. The input-output signals from both components can be measured and transfered from time domain to frequecncy domain respectively in the term of Frequecncy Response Function (FRF). The machine was tested with actual material like aluminum alloy 2024-T3 which was attached with an accelerometer underneath and then it was hit by a Modal hammer respectively. As a result, the mode shape of the vibration created during the machine testing, shows that the machine precisely enough to be used for further study in modal analysis.
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การออกแบบและจัดสร้างเครื่องมือทดสอบหาค่าการสั่นสะเทือนในอากาศยาน : Designing and Building of Aircraft Modal Testing Machine