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In this paper the energy harvesting problem involving typical MEMS technology is reduced to an optimal control problem, where the objective function is the absorption of the maximum amount of energy in a given time interval from a vibrating environment. The interest here is to identify a physical upper bound for this energy storage. The mathematical tool is a new optimal control called Krotov’s method, that has not yet been applied to engineering problems, except in quantum dynamics. This approach leads to identify new maximum bounds to the energy harvesting performance. Novel MEMS-based device control configurations for vibration energy harvesting are proposed with particular emphasis to piezoelectric, electromagnetic and capacitive circuits. [ABSTRACT FROM AUTHOR]

Active and semi-active control of oscillating devices and structures is a challenging field and this paper proposes an original investigation based on variational controls that can be successfully applied to mechanical systems. The method produces a general class of new controllers, named VFC – Variational Feedback Controllers, that is the main theoretical contribution of the paper. The value of the theory relies on using a reformulation of the Variational Optimal Control Theory, that has in general the limit of producing control program strategies and not directly feedback control methods. The difficulties are in fact related to the intrinsic nature of the variational optimal control, that must solve initial and final boundary conditions. A special definition of the class of the considered objective functions, permits to skip this difficulty, producing a pure feedback control. The presented theory goes beyond with respect to the most acknowledged LQR variational-based techniques, in that VFC can be applied to more general nonlinear dynamical systems, even with finite time horizon. To test the effectiveness of the novel approach in real engineering problems, a deep investigation on nonlinear suspension systems treated by VFC is proposed in this paper. To this aim, VFC is systematically compared with the most recent methods available in this field and suitable to deal with nonlinear system control of car suspensions. In particular, the comparative analysis is made in terms of both comfort and handling key performance indexes, that permits to easily and significantly compare different control logics, such as the Sky-hook and Ground-hook control families, the Acceleration and Power Driven Dampers. The results of this comparison are collected in a performance plane, having comfort and handling indexes as coordinate axes, showing that VFC controllers completely cover the regions reached by the other mentioned control logics in this plane, but reveal to have access to new valuable areas of unexplored different performance combinations. [ABSTRACT FROM AUTHOR]

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 campaign is performed 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 a roughly maximum speeds of 90 km/h. The measurements were continuatively performed for over 6 months, with a sampling frequency of about 400 Hz. The large amount of data/sensors allows a rather accurate statistical treatment of the measurement data and permits, 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 localized imperfections. [ABSTRACT FROM AUTHOR]

Abstract: The present paper considers the problem of realizing an effective targeted energy pumping from a linear oscillator to a set of ungrounded linear resonators attached to it. Theoretical as well as numerical results demonstrate the efficacy of using a complex attachment as a passive absorber of broadband energy injected into the primary structure. The paper unveils also the existence of an instantaneous frequency associated with the master response characterized by intermittency: a rather surprising result for a linear autonomous system. Comparison with nonlinear energy sinks demonstrates that the two systems have some analogies in this respect and that the linear complex attachment is a very efficient energy trap. [Copyright &y& Elsevier]

Abstract: A novel technique for the identification of the tire–road grip conditions is presented. This is based on the use of strain information inside the tire, from which relevant characteristics of the tire–road contact can be extracted also through a factor named area slip ratio. This process forms the basis of a technology for grip identification that requires a new model of the tire dynamics. The model permits to determine closed form analytical relationships between the measured strain and the area slip ratio. On this basis, a procedure that can extract the contact kinematic parameter from the time history of the internal strain of the rolling tire is presented. Numerical simulations offer the chance to validate the identification algorithm. [Copyright &y& Elsevier]

Abstract: The introduction of vibration-based Nano Electro-Mechanical Transistors (NEMT) opens a new horizon for mechanics in computer science. NEMT working principle is based on an electrical charge shuttle between two electrodes operated by a vibrating conductor body. Advantages of these novel devices would be very low power dissipation, limited influence of external electromagnetic disturbances, and improved thermal resistance. The paper introduces an analytical model for such a device, in which the matching of a mechanical resonator and an electric circuit is studied: the coupling is provided by capacitance effects, electrostatic force and the quantum tunneling. The approach is quasi-classical, describing the quantum phenomena through a non-linear conductance and using a continuous variable for the charges. Through suitably introduced simplifications, the model is reduced to a set of two differential equations in terms of pillar position and charge. These equations represent the simplest model still preserving the basic phenomenology of the investigated system. Numerical simulations show different possible motion regimes, both in the single- and multiple-module configurations, the latter able to reproduce the conventional transistor functionality. This opens the way to mechanical voltage-driven switches or amplifiers. [Copyright &y& Elsevier]

Abstract: Why it is difficult to solve a vibroacoustic problem; what are the hidden pitfalls that one is called to overcome; are there methods or techniques suggested in other fields of physics that can help to formulate an efficient vibroacoustic prediction model? The authors provide personal answers to these questions and suggest also a classification of the numerous prediction methods proposed in the literature. [Copyright &y& Elsevier]

Abstract: Hilbert–Huang transform (HHT) is an innovative data-processing technique for analyzing nonstationary and nonlinear signals. A novel HHT-based method for damage detection of bridge structures under a traveling load is proposed. The technique uses a single point measurement and is able to identify the presence and the location of the damage along the beam. The measured data are processed by the HHT technique, and none a priori information is needed about the response of the undamaged structure. Damage location is revealed by direct inspection of the first instantaneous frequency, which presents a sharp crest in correspondence of the damaged section. The identification capabilities of the proposed technique are studied varying the damage locations, crack depths and velocity of the moving load. The effect of ambient noise is also taken into account. Theoretical as well as numerical results show the identification is rather accurate, results are not very sensitive to the crack depth and ambient noise, while they are sensibly affected by the damage location and by the speed of the moving load as well. Theoretical analysis identifies a characteristic load velocity interval, depending both on the first natural frequency of the bridge and the damage location, within which the HHT can be successfully applied. [Copyright &y& Elsevier]

Abstract: This paper presents the theory of a novel mechanism of energy absorption and induced damping in structural systems and its application to aerospace industry. The underlying principles of the physical phenomena have been addressed in several earlier publications, which focused on prototypical systems of absorbers that consist of a set of single-degree-of-freedom resonators. This paper generalizes those theoretical developments to the case of a cluster of beams attached to a continuous primary structure, to develop predictive methods for the expected performance of this new type of absorber, with particular emphasis on its optimal design. An embodiment of the conceived device is illustrated for an aerospace structure, a satellite, with the purpose of reducing the vibration of the electronic components on board during lift-off. Experimental results illustrate the feasibility and the attractiveness of this new absorption technique. [Copyright &y& Elsevier]

Abstract: The question of energy sharing among complex engineering vibrating systems is still an open problem. On the basis of some recent investigations, this paper is addressed to the prediction of the equilibrium energies of interacting conservative resonators and to a better understanding of the principle of energy equipartition for large undamped systems. The analysis explores the field of linear and nonlinear vibrations, the effects of inhomogeneity, the weak or the strong coupling as well as the effect of the initial energy distribution among the subsystems. The principle of energy equipartition is shown to react in a different fashion to these factors. Some general arguments, supported by the results of numerical experiments, enlightening promoting or inhibiting factors to the reaching of energy equipartition conditions, are given. [Copyright &y& Elsevier]

This paper is concerned with emergence of novel effects in wave propagation in one-dimensional waveguides, when integrated with periodic nonlocalities. The nonlocalities are introduced by a connectivity superimposed to a conventional waveguide and depicted as a graph with trees and leaves, each with its own periodicity. Merging nonlocality and periodicity notions induces a distinction between homogenous and non-homogenous periodic configurations. Specifically, various unconventional phenomena linked to the presence of nonlocalities result in disruption of the energy transmission in such systems, disclosing new opportunities for vibration isolation applications. To demonstrate these effects, simple models of propagation of plane extension/compression waves in a uniform infinite rod equipped with co-axial spring-like elements is used. The homogenous case is analysed by a direct double, space and time, Fourier transform, leading to discussion of unusual dispersion effects, including vanishing and negative group velocity. In the non-homogeneous case, the canonical Floquet theory is used to identify stopbands and control their positions in the frequency domain. The results are compared with eigenfrequency analysis of unit periodicity cells and finite structures. Next, the forcing problem is considered and the insertion losses in a semi-infinite rod with nonlocal spring effects are computed to corroborate predictions of Floquet theory, providing physical explanations of the obtained results. Finally, possibilities to employ the non-local interaction forces in an active control format to generate stopbands at arbitrarily low frequencies are highlighted. [ABSTRACT FROM AUTHOR]

Two statistical techniques are developed to predict the statistical moments of the horizontal motion of a floating moored dock, known as catenary anchor leg mooring (CALM), loaded by hydrodynamic random forces. The dock is represented by a lumped mass, the mooring cables by equivalent nonlinear springs and the hydrodynamic forces are modelled by a modified Morison equation. The model of the floating dock leads to a nonlinear ordinary differential equation. Although the problem could be approached by a direct numerical integration, e.g. by Monte-Carlo simulations, because of the stochastic nature of the excitation, this would imply a large number of runs to produce results of some statistical significance. In the present paper an alternative solution is based on the development of two more efficient techniques to predict the relevant statistical moments of the dock response. The first method, called CPSP (conventional perturbation–statistical perturbation), is based on the application of two subsequent perturbation techniques, the first relying on a classical perturbation method, the second on a statistical perturbation approach. The second method, called SLSP (statistical linearization–statistical perturbation), combines indeed a statistical linearization approach together with a statistical perturbation approach. The procedures allow the linearization of the cables restoring forces as well as of the hydrodynamic load and they can be easily generalized to be applied to different dock configurations or to systems of different physical nature. The results, compared with those obtained by a Monte-Carlo simulations, show, in terms of statistical moments of the dock response, a satisfactory agreement. [Copyright &y& Elsevier]

Abstract: This paper attempts to introduce a new point of view on energy analysis in structural dynamics with particular emphasis to its link with uncertainty and complexity. A linear, elastic system undergoing free vibrations, is considered. The system is subdivided into two subsystems and their respective energies together with the shared energy flow are analysed. First, the ensemble energy average of the two subsystems, assuming uncertain the natural frequencies, is investigated. It is shown how the energy averages follow a simple law when observing the long-term response of the system, obtained by a suitable asymptotic expansion. The second part of the analysis shows how the ensemble energy average of a set of random samples is representative even of the single case if the system is complex enough. The two previous points, combined, produce a result that applies to the energy sharing between two subsystems even independently of uncertainty: for complex systems, a simple energy sharing law is indeed stated. Moreover, in the case of absence of damping, a nonlinear relation between the energy flow and the energy (weighted) difference between the two subsystems is derived; on the other hand, when damping is present, this relationship becomes linear, including two terms: one is proportional to the energy (weighted) difference between the two subsystems, the other being proportional to its time derivative. Therefore, the approach suggests a way for deriving a general approach to energy sharing in vibration with results that, in some cases, are reminiscent of those met in Statistical Energy Analysis. Finally, computational experiments, performed on systems of increasing complexity, validate the theoretical results. [Copyright &y& Elsevier]

Abstract: The problem of an elastic lifting hydrofoil in a randomly perturbed flow is considered. It appears that in these conditions the phenomenon of hydroelastic-induced vibrations is controlled by a stochastic differential operator. By using the theory of stochastic perturbation, a technique of solution for this class of problems is proposed, leading to an effective numerical solution. The fundamentals of the method are given and it is applied to the general problem of hydroelastic vibrations. A numerical application to the case of an elastic control surface for a prototype-high- speed marine vehicle is presented. Comparisons between the results obtained by the Stochastic Perturbation Method (SPM) and those provided by standard Monte Carlo simulations (MCS) show the accuracy of the proposed method and a useful saving in computational time. A method is given for comparing the computational time required by the two methods, for a given statistical accuracy. [Copyright &y& Elsevier]

The aim of this paper is to provide a theoretical and experimental analysis of the response of an elastic system carried on board a wedge-shaped body impacting the water surface. The wedge entering the water has a sudden deceleration with shock characteristics, resulting in a short-time duration and a sharp peak value. On the other hand, the carried system undergoes an oscillatory motion induced by the inertial load generated by the impact. The study of this problem reveals the occurrence of special conditions in the response of the on-board oscillator, depending on the parameters associated with the water entry problem, that lead to large elastic forces. The experiments show that when varying the characteristic natural frequency of the on-board oscillator, critical impact conditions occur characterized by very large amplitudes of the structural response that confirm the theoretical predictions. The analysis, based on the water shock spectral response developed by the authors in some previous papers, is generalized here and validated experimentally. [Copyright &y& Elsevier]

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. [Copyright &y& Elsevier]

Several energy-based methods to approach noise and vibro-acoustic problems are actually under development. These techniques provide a chance of describing the vibro-acoustic behaviour of complex systems by the energies of a limited number of sub-components. This process is the base of one of the most acknowledged methods in this field, i.e. the statistical energy analysis (SEA). However, SEA invokes only the first law of thermodynamics, i.e. the energy conservation principle. On the contrary, it seems that the formulation of a complete theory of energy transmission among oscillators would claim also for the second principle of thermodynamics. Such direction of investigation, via the entropy concept, is developed in this paper leading to a theoretical energy flow analysis to predict the energy exchanged among complex systems. Some classical SEA results are obtained as a special case of a more general approach. [Copyright &y& Elsevier]

The present paper contains the description of a new technology for optical strain measurements in rolling tires. The apparatus is based on the use of Fiber Bragg Gratings sensors and light spectrum analyzer. With the help of a new analytical model, describing the tire strain during its revolution, the experiments permit, for the first time, to provide evidence of the transition along the contact patch between the grip and the slip regions. Moreover, the contact patch is identified and the experimental evidence suggests that the existing methods provide underestimated lengths. [ABSTRACT FROM AUTHOR]

A method for evaluating the energy flow confidence level in vibrating systems with randomly perturbed parameters is presented. The energy flow is predicted in terms of the mobilities of resonant subsystems or by the solution of the velocity wave field for non-resonant subsystems. The statistical moments of the energy flow are calculated by a perturbation technique and a confidence factor is defined as the ratio between mean and standard deviation. The properties of the confidence factor are investigated by a theoretical analysis as a function of frequency. Three cases are studied to compare the confidence factor obtained theoretically with a prediction provided by a Monte-Carlo simulation. [Copyright &y& Elsevier]