The main emphasis of the paper is put on the experimental verification and comparison of classical modal analysis techniques and recurrence plots sensitivity to damage size. Identification experiments were carried out for the laboratory object subjected to random and chirp excitations, respectively. In the course of carried out experiments, the process of damage propagation was simulated by the successive drilling into one of the object elements. Measured time histories of system responses were analyzed with the application of the classical modal analysis, recurrence plots (RP), cross recurrence plots (CRP) and joint recurrence plots (JRP) methods. Obtained results proved that the RP, CRP and JRP methods are much more sensitive to changes in dynamical system properties resulting from damage initialization and propagation than classical modal analysis methods and can be successfully applied to damage detection and tracking changes in the system natural frequencies.
In the paper, the procedure for the estimation of modal filter coefficients from output-only data is presented. The basic concept of the procedure consists in frequency response functions synthesis based on the knowledge of an operational modal model. A method of operational mode shapes scaling is described. The method is then compared with the classical modal filter and with modal filtration of responses spectra, which is sometimes used as a solution for modal filtration based on the output-only data. Each solution is applied to load identification and damage detection. The study shows the method verification on data obtained from laboratory experiment.
The paper presents an attempt to identify forces acting on the control unit of a helicopter model during flight. The helicopter model will, according to the project assumptions, operate as an inspection robot i.e. it will autonomously fly in selected regions and monitor them with an embedded camera. It could be used for visual inspection of high masts or chimneys. Autonomous flying requires a sophisticated control system. The control unit is placed in an additional box hung under the fuselage. One of the stages of the unit development was examination of in-flight forces acting on the box and if it was necessary, a vibroinsulation selection. For this reason, an experiment was conducted. During the test, forces in the box suspension and accelerations in selected locations of the object were recorded. Having both, forces and responses in the form of vibrations accelerations, enabled to verify a force identification procedure. It is a procedure based on regressive parametric models inversion. The paper presents this method’s description and an attempt to apply it to the described forces identification.
Modal models are commonly encountered forms of dynamic characterization of
mechanical structures. They are applied in machine-condition diagnosing as well as in monitoring
processes. In research practice the most frequently applied method is Experimental Modal Analysis,
and in exploitation - Operational Modal Analysis. Difficulties associated with the EMA and OMA
have contributed to the increase of OMAX model applications, especially in case of energetic
machines.
Theoretical assumptions related with modal analysis are very strong. In reality when big and heavy
structures are considered often those assumptions are not fulfilled. In the paper such situations are
considered and practical solutions are showed.
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