Fast modal rotation measurement using a dual sinusoidal-scan continuously scanning laser Doppler vibrometer

Rotational degrees of freedom (DOF) are usually indirectly derived from the measured translational DOF in structural dynamic tests by using the finite difference method. However, the noise of translational DOF measurement is easily propagated and further amplified when the finite difference method is employed to obtain the rotational DOF. Therefore, the accuracy of the obtained rotational DOF was rather poor. In order to overcome this disadvantage, a novel approach of using a dual sinusoidal-scan continuously scanning laser Doppler vibrometer (CSLDV) with the objective of obtaining the structural modal rotational DOFs is put forward in this paper. For plate structures, the operational deformation shape of the normal translational DOFs can be measured and described with polynomial functions through the modulation of CSLDV output signals. Then, rotational out-of-plane DOFs can be achieved with respect to the first derivatives of the polynomial functions along orthogonal directions. In such a way, the measurement noise that considerably affects the rotational out-of-plane DOFs derived from normal translational DOF can be significantly eliminated. The approach is validated experimentally with a cantilever plate structure, and a comparison with the results obtained from discrete point measurement is given. The results demonstrate the noticeable improvement in accuracy and spatial resolution of the proposed technique. (paper)