Aldo Sestieri

Experimental analysis of friction-induced vibrations at the finger contact surface:

Abstract When a finger moves to scan the surface of an object, the sliding contact generates vibrations that propagate in the finger skin and transmit the information about the object characteristics to mechanoreceptors. Mechanoreceptors convert vibrations into electric impulses sent to the brain. In this context, by appropriate experiments, a frequency analysis of the signal characterizing the surface scanning can be carried out to investigate the vibration spectrum measured on the finger and to highlight how the tactile sense is connected to the measured frequency spectra. Although the correlation between the surface roughness with respect to the tactile sensation is deeply analysed in the literature, the vibration spectra induced by the finger—surface scanning and the consequent activation of mechanoreceptors on the skin were rarely investigated. In particular, in this paper, interests will be focused on the changes shown in the vibration spectra, caused by variations in the characteristic contact parameters such as scanning velocity and roughness.

Space average and wave interference in vibration conductivity

Plane wave interference and space averages play a significant role in the derivation of some vibration conductivity equations that are becoming more and more popular in modelling vibroacoustic problems. Particularly, the thermal approach and the modified vibration conductivity equations are here considered with the aim of establishing similarities and/or differences between them and stating relevant consequences. It is shown by formal developments that the thermal equation is obtained under the assumption of performing appropriate space averages, while the modified vibration conductivity equation does not need, in some cases, this condition. It is discussed, however, that in practical applications the conditions of validity of both approaches are quite similar.

Reducing scatter from derived rotational data to determine the frequency response function of connected structures

Abstract When two structures are connected together, the frequency response function (FRF) of the coupled system cannot be determined from the translational FRF of each substructure. Rotational FRFs are also needed. These quantities are not usually measured because of the lack of accurate rotational transducers, they can be obtained from the translational measured FRF through a finite difference computation. However errors in measurements and noise problems seriously affect the derived quantities and lead to inefficient estimates. Suggested improvements to the accuracy of rotational parameters derived from experimental translational data include mass correction, smoothing of data, estimates of low and high frequency residuals and appropriate selection of spacing between measurement locations. The results obtained for the assembled system following implementation of these actions are quite satisfactory.

Regularisation Techniques for Dynamic Model Updating using Input Residual

The updating process to correct finite element models using experimental data is generally an ill conditioned problem. The method here presented is based on the use of the input residual, that presents some important advantages over other proposed techniques. The actions required to limit the solution instability due to ill conditioning are discussed. The importance of using regularisation techniques to minimise the influence of experimental errors is demonstrated. Several procedures are analysed through simulated and experimental tests, finally showing that the most reliable results are obtained using a priori information and the singular values truncation technique based on the minimisation of the output residual.

Estimation of Rotational Degrees of Freedom by EMA and FEM Mode Shapes

In this paper a new technique is presented to estimate the rotational degrees of freedom of a flexural structure, using only a limited number of sensors that measure the translational DoFs of the system. A set of flexural mode shapes in a limited number of nodes is obtained by modal testing, while a different set of approximated mode is calculated by a Finite Element Model (FEM) at all the nodes and degrees of freedom of the structure. The technique is based on the classical assumption that the response can be determined by a linear combination of the structure’s mode shapes. The structure’s mode shapes are approximated by using the local correspondence principle for mode shapes, i.e. by using an optimally selected set of finite element mode shapes as Ritz vectors for the true mode shapes. This allows to obtain the rotational response at unmeasured DoFs. The technique is validated by comparing predicted and experimental results.

Circumventing Space Sampling Limitations in Mechanical Vibrations

The aim of this paper is to review the main techniques used to provide a significant solution to high frequency vibrations and/or structural-acoustic coupling. The characteristics of the different approaches are illustrated, focusing similarities and differences among them. A detailed description of the complex envelope displacement analysis is then presented, because of the promising developments of this approach. Finally some results are shown and discussed to emphasize the informative character of the different solutions.SommarioScopo di questo articolo è quello di fornire un panorama critico delle tecniche che si sono o si stanno attualmente sviluppando per lo studio delle vibrazioni nel campo delle alte frequenze e per problemi di accoppiamento acustico-strutturale. Vengono, in particolare, messe in evidenza similitudini e differenze tra i vari metodi esaminati. Successivamente viene descritto in dettaglio un modello, chimato inviluppo complesso di spostamento, che appare molto promettente e ricco di sviluppi futuri. Vengono infine presentati alcuni risultati per evidenziare il diverso contenuto informativo delle soluzioni ottenute con le varie tecniche analizzate.

On the Use of Different Fundamental Solutions for the Interior Acoustic Problem

An alternative form of fundamental solution to the Helmholtz equation is presented, which proves to be very effective when employed in the integral formulation for interior acoustic problems. A BEM code was developed based on this solution and gave satisfactory results in dealing with structural-acoustic coupling in a cavity, either with or without absorption walls. Theoretical arguments supporting the use of the alternative fundamental solution are provided; then the discussion of some numerical results highlights the main differences between the present method and the one using the well known free-space Green’s function as fundamental solution.

Real time monitoring and wear estimation of railway track with FBG sensors

In this work we present the results of a field trial with a FBG sensor array system for the real time monitoring of railway traffic and for the structural health monitoring of both the railway track and train wheels. The test campaigns are performed on the on the 2nd line of Milan metropolitan underground, employing more than 50 FBG sensors along 1,5 km of the rail track, where the trains are tested during daily passenger rail transport, with maximum speeds roughly of 90 km/h. The measurements were continuatively performed for over six months, with a sampling frequency of about 400 Hz. The large amount of data/sensors allow a rather accurate statistical treatment of the measurement data and permit, with dedicated algorithms, the estimation of rail and wheel wear, key traffic parameters such as the number of axles, the train speed and load, and, in the next future, the detection of localised imperfections.

Experimental Analysis of the Transition Between Veering and Crossing on a Two-Beam System

When there is a parameter varying in a system, so that one natural frequency approaches another one, the phenomenon of veering is generally found and highly coupled modes are the emerging characteristic of this dynamic behavior. It is far more difficult to find systems that instead of a veering present crossing between modes, and the crossing phenomenon is almost unreported in the scientific literature, unless the case of uncoupled modes is considered.In this paper, the two-modes interaction is presented. In particular, mode veering and mode crossing are introduced and investigated through a simple analytic 2-dof model that allows for closed-form solution. Then, an experimental setup, appropriately designed to study the two-modes veering and crossing is presented and experimental evidences of both phenomena are measured showing the main characteristics of such modal interaction..Copyright

Application of the Complex Envelope Vectorization to a Boundary Element Formulation

The complex envelope vectorization (CEV) is a recent method that has been successfully applied to structural and internal acoustic problems. Unlike other methods proposed in the last two decades to solve high frequency problems, CEV is not an energy method, although it shares with all the other techniques a variable transformation of the field variable. By such transformation involving a Hilbert transform, CEV allows the representation of a fast oscillating signal through a set of low oscillating signals. Thanks to such transformation it is possible to solve a high frequency dynamic problem at a computational cost that is lower than that required by finite elements. In fact, by using finite elements, a high frequency problem usually implies large matrices. On the contrary the CEV formulation is obtained by solving a set of linear problems of highly reduced dimensions. Although it was proved that CEV is in general a successful procedure, it was shown that it is particularly appropriate when the modes of the system have a negligible role on the solution. Moreover, the numerical advantage of the CEV formulation is much more pronounced when full matrices are used. Thus, for the first time it is applied to a boundary element formulation (BEM). Both external and internal acoustic fields of increasing complexity are considered: the internal and external field generated by a pulsating sphere; the external field of a forced box, where the velocity field is determined by finite elements; a set of 4 plates that form an open cavity. The results are compared with those obtained by a BEM procedure (SYSNOISE), highlighting the good quality of the proposed approach. An estimate of the computational advantage is also provided. Finally it is worthwhile to point out that the reduction of the BE matrices allows for an in-core solution even for large problems.Copyright