Modal Behavior Monitoring of a Discrete Evolutive Structure

The proposed paper takes part in a general project of behavior monitoring of structures or rotating machines, particularly with the characterization of modal changes. The study presented here focuses on implementation of an integrated modal identification method in time domain allowing the monitoring of mode shapes. A three degrees of freedom system is designed as three pendulums, one of them offering the possibility to move its mass. Changes in position of this moving mass introduce small changes on natural frequencies but significant changes in modal shapes under controlled operating conditions. A corresponding analytical model is achieved for simulation and characterization of the proposed identification method. The latter is based on classical identification tools (N4SID) in association with modal reconstruction methodology. An experimental validation is performed by comparing the results obtained with a nonparametric traditional identification (swept sine and characterization of the modal displacements by harmonic excitation at the resonance frequencies). Then, with the validation of the model by the experimental setup, it is shown that the modal reconstruction remains effective for rapid changes in behavior of the studied system.