Adaptive kernel cross-time-frequency transformations for the identification of structural systems

The identification of structural systems using time-frequency(11) analysis has been recently proposed to detect possible damages under normal serviceability conditions'. Accelerometric signals are recorded at different points of the structure(7,8) They consist of the superposition of vibration modes (that identify the system) and residual components. Each vibration mode is represented by its frequency, its amplitude at different points of the structure, and the damping factor that determines its amplitude modulation. We have lately proposed a Cohen Class(11) cross-time-frequency based technique to identify the vibration modes(4,5,6). In this paper we show how the technique may be developed in a fully automatic procedure and we discuss how the use of adaptive kernels Ina improve the reliability of the identification. The automatic procedure is based on two properties that characterize the vibration modes: 1) the ratio between die amplitude of the same modal component at different points of the structure is constant; and 2) the phase difference between the signals corresponding to the same modal component at different point of the structure is constant. These properties enable the vibration modes and residual components to be discriminated.