Based on digital video technology, video image vibration measuring system is designed with the general USB digital camera and PC as its hardware. Meanwhile, the first-order modal parameters of a simply supported beam with concentrated masses applied are identified. Vibration video acquisition and image edge detection procedures are programmed in the MATLAB environment. The displacement-time curve of the beam vibration is obtained. Modal analysis is applied to determine the natural frequencies, the damping ratio and the vibration modes of the simply supported beam. The testing results show that this testing system is feasible for realization of the vibration test for low-frequency structures.

Through the theoretical discussion of the principal methods of loads identification, the effect of measuring noise in the response on the identification is analyzed. The noise effect on the loads identified by differential method, time domain method, frequency domain method and other related hybrid methods is rely on the length of time step. The reason of the noise effect is theoretical analyzed. The de-noising suggestion is proposed for the loads identification.