Experimental Modal Analysis of Angle Signals Based on the Stochastic Subspace Identification Method

This paper aims to verify the extraction of modal parameters from angle signals using the stochastic subspace identification (SSI) method. The use of angle signal-based mode shapes can reduce the loss of node information and enhance the robustness in curva-ture-based damage detection. In this regard, the system identification of angle signals should be first conducted prior to the damage detection. For large structures, an out-put-only system identification method should be considered for the modal analysis of an-gle signals, because artificial shaking excitation or impact excitation is practically impos-sible. In order to achieve this, the SSI method is used; it is one of the most powerful tools among the output-only system identification methods because it does not cover nonlinear problems. In order to demonstrate the system identification process of angle signals using the SSI method, the transformation matrix is assumed to represent the relationship be-tween the angular displacement and the normal displacement. Next, the modified block Hankel matrix that consists of angle signals, which can be expressed as a multiplication between the transformation matrix and displacement series vector, is constructed. The observability matrix can be estimated using the singular value decomposition for the pro-jection of the future part onto the past part of the modified block Hankel matrix. Finally, the natural frequencies and angle signal-based mode shapes are calculated using the state and observation matrices. In order to verify the results of the analytical studies, the modal properties estimated from the numerical simulation and the SSI method using angu-lar-velocity measurements are compared.