Harmonic components extraction influence on resulting modal parameters of vibrating structures

It is possible to estimate condition of static structures, like bridges and buildings, by monitoring how their modal parameters (frequency, shape and damping) change in time, based on modal parameters relation to mechanical properties. Operational modal analysis (OMA) provides estimation of modal parameters. OMA basic assumption is that excitation forces are random and their amplitude and frequency nature is similar to white noise. For structures with periodic dynamic excitation, like wind turbines or helicopter blades, this assumption is violated, which makes OMA application complicated and unreliable. There is a challenge of extracting periodic components before modal parameter estimation, which otherwise contaminate signals and mask modal information. This paper considers a signal processing tool for extraction of periodic components from signals aiming to perform OMA and to estimate modal parameters. The tool is based on time synchronous averaging and utilizes tachometer signals. The influence of signal processing technique on modal parameters is studied by comparing these parameters with and without given signal processing tool. The paper also considers the effectiveness of the tool and discusses its possible uses.