Current Noise, Vibration and Harshness (NVH) Analysis are largely used to make predictions of structural vibrations and air borne noise that affects driver and passengers experience while seated in a moving car. Dealing with road noise, modern tire models when coupled to a vehicle body model can accurately predict the sound generated by the tire-road contact in a wide frequency range (up to 300 Hz). Since the accuracy of the predictions isn’t guaranteed at low frequency, these models cannot be used to study low-frequent vibrations for comfort analysis. In this master thesis the cause of that limitation is studied and an improvement in the quality of the prediction at low frequency is achieved to enable the use of current tire models also for comfort applications. A pre-study is made to focus on the main source of the problem, which turned out to be the effect of shock absorber non-linearity causing a different suspension behavior in operational condition and static lab-measurement condition. The stick-slip phenomenon is presented as a consequence of the shock absorber dry friction and its effect on the dynamic behavior of the car is described. To reduce the effects of suspension characteristics at low frequency a new measurement setup for Transfer Path Analysis is developed and the classic instrumentation with accelerometers is joined by the use of strain sensors. Besides, two shakers are used while building the FRF model of the car body to check the benefits of breaking the damper dry friction when measuring the FRFs in lab condition. Wheel center forces are identified with Matrix Inversion and seat vibrations are estimated both using acceleration and strain based approach, using the car body FRFs measured with and without shakers, and the quality of the prediction is compared in all these cases. The vehicle body model is then validated in time domain. Finally the car body model is coupled to a test-based tire model using Frequency Based Substructuring to build a full vehicle model. Transfer Path Analysis are performed to predict tire patch displacements and seat vibrations. Results show that thanks to the new setup and the use of strain sensors a more accurate prediction of knuckle forces and patch displacements is achieved, together with a better low-frequent analysis of noise and vibrations. Therefore NVH tire models can be extent for low-frequent comfort applications.
Attualmente le analisi di tipo Noise, Vibration and Harshness (NVH) sono largamente utilizzate per lo studio della propagazione di suono e vibrazioni all’interno delle vetture e degli effetti che esse hanno su passeggeri e guidatore. I modelli di pneumatico disponibili consentono in particolare di predire correttamente il rumore derivante dal contatto pneumatico-strada in un ampio range di frequenza (fino a 300 Hz). Poiché l’accuratezza di questi modelli cala drasticamente a basse frequenze essi non possono però essere utilizzati per analisi inerenti il comfort. In questo lavoro di tesi viene condotto uno studio relativo alle cause di questa limitazione, e viene migliorata la qualità delle predizioni a basse frequenze in modo da estendere l’uso degli attuali modelli NVH di pneumatico ad applicazioni comfort. A seguito di uno studio preliminare la principale fonte di inaccuratezza viene identificata nelle non linearità della sospensione derivanti dall’attrito all’interno dell’ammortizzatore, che provoca un differente comportamento dinamico della sospensione stessa tra condizioni operative e condizioni statiche in laboratorio. Viene presentato il fenomeno “stick-slip” dell’ammortizzatore e suoi effetti sulla dinamica della sospensione e del veicolo. Per limitare gli effetti di queste non linearità si sviluppa un nuovo setup per le Transfer Path Analysis e alla tipica strumentazione accelerometrica viene affiancato l’utilizzo di estensimetri piezo-elettrici. Inoltre, durante le misure in laboratorio, vengono utilizzati due shaker per valutare gli effetti sulle FRF del veicolo del passaggio da condizione stick a slip all’interno dell’ammortizzatore. Vengono identificate le forze al mozzo e calcolate le vibrazioni sui sedili del veicolo, utilizzando sia le misure estensimetriche che quelle accelerometriche, e valutando come esse cambiano se vengono utilizzate le FRF misurate con o senza shaker. Il modello del body viene poi validato nel dominio del tempo. Infine viene creato un modello completo di veicolo accoppiando tramite Frequency Based Substructuring il precedente modello del body con un modello test-based di pneumatico precedentemente costruito. Vengono svolte ulteriori Transfert Path Analysis sul modello completo per stimare gli spostamenti all’orma di contatto dello pneumatico e le vibrazioni dei sedili. I risultati mostrano che grazie al nuovo setup e all’utilizzo di estensimetri si raggiunge una predizione più accurata delle forze al mozzo e degli spostamenti dell’orma di contatto, che si traduce in una migliore analisi del suono e delle vibrazioni a basse frequenze. In questo modo i modelli NVH di pneumatico possono essere utilizzati anche per lo studio del comfort.
Low frequency NVH behavior : the influence of suspension non linearities
LEPORE, ALESSANDRO
2010/2011
Abstract
Current Noise, Vibration and Harshness (NVH) Analysis are largely used to make predictions of structural vibrations and air borne noise that affects driver and passengers experience while seated in a moving car. Dealing with road noise, modern tire models when coupled to a vehicle body model can accurately predict the sound generated by the tire-road contact in a wide frequency range (up to 300 Hz). Since the accuracy of the predictions isn’t guaranteed at low frequency, these models cannot be used to study low-frequent vibrations for comfort analysis. In this master thesis the cause of that limitation is studied and an improvement in the quality of the prediction at low frequency is achieved to enable the use of current tire models also for comfort applications. A pre-study is made to focus on the main source of the problem, which turned out to be the effect of shock absorber non-linearity causing a different suspension behavior in operational condition and static lab-measurement condition. The stick-slip phenomenon is presented as a consequence of the shock absorber dry friction and its effect on the dynamic behavior of the car is described. To reduce the effects of suspension characteristics at low frequency a new measurement setup for Transfer Path Analysis is developed and the classic instrumentation with accelerometers is joined by the use of strain sensors. Besides, two shakers are used while building the FRF model of the car body to check the benefits of breaking the damper dry friction when measuring the FRFs in lab condition. Wheel center forces are identified with Matrix Inversion and seat vibrations are estimated both using acceleration and strain based approach, using the car body FRFs measured with and without shakers, and the quality of the prediction is compared in all these cases. The vehicle body model is then validated in time domain. Finally the car body model is coupled to a test-based tire model using Frequency Based Substructuring to build a full vehicle model. Transfer Path Analysis are performed to predict tire patch displacements and seat vibrations. Results show that thanks to the new setup and the use of strain sensors a more accurate prediction of knuckle forces and patch displacements is achieved, together with a better low-frequent analysis of noise and vibrations. Therefore NVH tire models can be extent for low-frequent comfort applications.File | Dimensione | Formato | |
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https://hdl.handle.net/10589/40064