This Master Degree Thesis aims to analyse the current characteristics of a 3-DOF shaker, describing its dynamic behaviour in order to suggest the appropriate modifications, with the purpose of increasing the useful frequency bandwidth and reducing the harmonic distortion of the resulting signal. Starting from the structure’s description, the finite element model of the shaker has been developed and a modal analysis is conducted. Natural frequencies and mode shapes of the model allowed to identify specific tests and setups for the EMA on the structure, until 130 Hz. Results of this EMA are reported as FRF, highlighting in this way the mode shapes and the natural frequencies. Comparing the results from EMA and from FEA, the modal behaviour of the structure has been understood. In this way it has been possible to know that the useful frequency band is 0-20 Hz, free of resonances. Further dynamic tests are conducted, with the platform in motion. From these tests the non-linear response of the system has been evidenced, including the secondary harmonics, multiples of the fundamental one. These multiples stimulates resonances (in particular the translational mode): when platform motion in x direction is at 5 Hz, the multiple 5x at 25 Hz stimulates the resonance at that frequency. This contribution enters in the resulting signal and it has amplitude comparable with the one of the fundamental harmonic. By means of these test it has been possible to understand the cause of the harmonic distortion. So, in order to reduce the harmonic distortion and increase the useful bandwidth, it is suggested to increase the stiffness of the ground fixing system, lighten the movable heavy components where possible and modify guides to reduce their compliance.
Lo scopo della tesi è analizzare le attuali caratteristiche di uno shaker a 3 GDL, descrivendone il comportamento dinamico per suggerire le opportune modifiche atte ad aumentare la banda utile di frequenze e migliorare la distorsione armonica del segnale ottenuto. Partendo dalla descrizione della struttura, si è sviluppato il modello a elementi finiti dello shaker per svolgere un’analisi modale. Frequenze e modi di vibrare ottenuti dal modello hanno consentito di individuare appositi test e setup per l’EMA sulla struttura, svolta fino a 130 Hz. I risultati di tale EMA vengono riportati come FRF, evidenziando così l’aspetto dei modi e la frequenza propria. Confrontando i risultati relativi al modello e quelli relativi all’EMA, si è ricostruito il comportamento modale della struttura. Si è dunque compreso che la conseguente banda utile di frequenze, priva di risonanze, è 0-20 Hz. Ulteriore analisi viene svolta mediante test dinamici, con la piattaforma in movimento. A seguito di tali test si è evidenziata la risposta non lineare del sistema comprendente anche le armoniche secondarie multiple dell’armonica fondamentale. Tali multiple sollecitano le risonanze (in particolare quelle dei modi di traslazione sui supporti): quando la piattaforma si muove con un segnale sinusoidale in direzione x a 5 Hz, la multipla 5x sollecita la risonanza a 25 Hz. Tale contributo si somma all’interno del segnale e risulta avere ampiezza comparabile con quella della forzante. Mediante questi test dinamici è stato dunque possibile comprendere la causa della distorsione del segnale armonico. Per migliorare il comportamento dinamico e incrementare la banda utile di frequenze vengono pertanto suggerite modifiche riguardanti l’incremento della rigidezza del sistema di fissaggio dello shaker a terra, l’alleggerimento dei componenti mobili più pesanti e la modifica delle guide per ridurne la cedevolezza.
Model and test of a 3-DOF shaker for the study of human response to multiaxial vibrations
ALBANETTI LUCIANO, ALESSANDRO
2017/2018
Abstract
This Master Degree Thesis aims to analyse the current characteristics of a 3-DOF shaker, describing its dynamic behaviour in order to suggest the appropriate modifications, with the purpose of increasing the useful frequency bandwidth and reducing the harmonic distortion of the resulting signal. Starting from the structure’s description, the finite element model of the shaker has been developed and a modal analysis is conducted. Natural frequencies and mode shapes of the model allowed to identify specific tests and setups for the EMA on the structure, until 130 Hz. Results of this EMA are reported as FRF, highlighting in this way the mode shapes and the natural frequencies. Comparing the results from EMA and from FEA, the modal behaviour of the structure has been understood. In this way it has been possible to know that the useful frequency band is 0-20 Hz, free of resonances. Further dynamic tests are conducted, with the platform in motion. From these tests the non-linear response of the system has been evidenced, including the secondary harmonics, multiples of the fundamental one. These multiples stimulates resonances (in particular the translational mode): when platform motion in x direction is at 5 Hz, the multiple 5x at 25 Hz stimulates the resonance at that frequency. This contribution enters in the resulting signal and it has amplitude comparable with the one of the fundamental harmonic. By means of these test it has been possible to understand the cause of the harmonic distortion. So, in order to reduce the harmonic distortion and increase the useful bandwidth, it is suggested to increase the stiffness of the ground fixing system, lighten the movable heavy components where possible and modify guides to reduce their compliance.File | Dimensione | Formato | |
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Descrizione: Model and Test of a 3 DOF Shaker for the Study of Human Response to Multiaxial Vibrations
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https://hdl.handle.net/10589/143868