L. Gammaitoni

Tuning in to Noise

Two sweeping generalizations can be made about most natural systems: They are intrinsically nonlinear and they operate in noisy environments. Examples abound, ranging from weather systems to oscillating chemical reactions to the movements of an eel. The most complex example is arguably the human central nervous system, flooded as it is with the “noise” of modern life.

Nonlinear oscillators for vibration energy harvesting

Vibration to electricity energy conversion strategies are discussed by using nonlinear stochastic dynamics. General principles for the exploitation of nonlinear oscillators in energy harvesting that provide useful leads for the realization of micropower generators of practical interest are presented.

Piezoelectric buckled beams for random vibration energy harvesting

Among the main vibration-to-electricity conversion systems, resonant harvesters suffer from a series of strong limits like their narrow frequency response and poor output power at small scale. Most of all, realistic vibration sources are variable in time and abundant at relatively low frequencies. Nonlinear vibration harvesters, on the other hand, are more attractive, thanks to their large bandwidth response and flexibility to convert kinetic energy of the natural frequency of the sources. In particular, bistable oscillators have been proven to show higher global performances when excited by random vibrations. In this paper, such an approach is investigated for piezoelectric beams by exerting an increasing axial compression. An advantage of this technique is the absence of magnetic forces to create bistable dynamics. A thin piezoelectric axially loaded beam is theoretically modelled and experimentally investigated under wideband random vibrations. In the buckled configuration, the device exhibits superior power generation over a large interval of resistive load, with gains up to more than a factor of ten compared to the unbuckled state. The numerical model and experimental results are in good qualitative agreement. (Some figures may appear in colour only in the online journal)

The Virgo interferometer

The Virgo gravitational wave detector is an interferometer with 3 km long arms in construction near Pisa to be commissioned in the year 2000. Virgo has been designed to achieve a strain sensitivity of a few times at 200 Hz. A large effort has gone into the conception of the mirror suspension system, which is expected to reduce noise to the level of at 10 Hz. The expected signals and main sources of noise are briefly discussed; the choices made are illustrated together with the present status of the experiment.

Low-frequency internal friction in clamped-free thin wires

We present a series of internal friction measurements for the normal modes of circular fibres made of different materials, that can suspend the test masses of an interferometric gravity wave detector. For metallic wires, the frequency independent loss angle ranges between 10 y3 and 10 y4 . The losses in fused silica are two orders of magnitude lower than those in metals.

A real vibration database for kinetic energy harvesting application

In this article, we discuss the project of a vibration signal database for energy harvesting purpose. After a brief description where we present the technologies used to create the database and the procedures to acquire the signals, we show some results obtained using selected environmental noises from the database to characterize nonlinear energy harvesters.

Stochastic resonance in multi-threshold systems

We discuss the dynamical behaviour of multi-threshold systems in the presence of noise and periodic inputs. Here, the stochastic resonance phenomenon displays some peculiarities such as a clear dependence on the noise statistics and the presence of a multi-peaked characteristic curve, which are not observed in simple bistable systems. This phenomenon is described without reference to any frequency matching condition as a special case of the well-known dithering effect.

THE CREEP PROBLEM IN THE VIRGO SUSPENSIONS : A POSSIBLE SOLUTION USING MARAGING STEEL

Abstract Each optical component of the interferometric gravitational wave detector VIRGO is suspended from a cascade of mechanical filters designed to suppress the transmission of seismic vibrations. Each mechanical filter supports the weight of the filters below it by means of a set of steel cantilever blade springs. The stress from the load acting on the blades was found to induce a drooping of the blade tips of several microns per day due to a series of microscopic yielding events (micro-creep). This process induces a mechanical displacement shot-noise on the optical component which can dominate the small displacements produced by gravitational waves. The use of a special precipitation hardened steel (Maraging C250), instead of common spring steel, allows the construction of blades that show an acceptable stability under stress.

Universal and reconfigurable logic gates in a compact three-terminal resonant tunneling diode

Submicron-sized mesas of resonant tunneling diodes (RTDs) with split drain contacts have been realized and the current-voltage characteristics have been studied in the bistable regime at room temperature. Dynamically biased, the RTDs show noise-triggered firing of spikelike signals and can act as reconfigurable universal logic gates for small voltage changes of a few millivolt at the input branches. These observations are interpreted in terms of a stochastic nonlinear processes. The logic gate operation shows gain for the fired-signal bursts with transconductance slopes exceeding the thermal limit. The RTD junction can be easily integrated to arrays of multiple inputs and have thus the potential to mimic neurons in nanoelectronic circuits.

The Virgo Project

The Virgo project is a Italian-French collaboration aiming at the construction of a long baseline interferometric antenna for the detection of gravitational radiation signals of cosmic origin. We describe the principles of the system, and high-light the technical challenges we need to overcome for reaching a sensitiity as low as 10−23Hz−1/2.The gravitational clustering of collisionless particles in an expanding universe is modelled using some simple physical ideas. I show that it is possible to understand the nonlinear clustering in terms of three well defined regimes: (1) linear regime; (2) quasilinear regime which is dominated by scale-invariant radial infall and (3) nonlinear regime dominated by nonradial motions and mergers. Modelling each of these regimes separately I show how the nonlinear two point correlation function can be related to the linear correlation function in hierarchical models. This analysis leads to results which are in good agreement with numerical simulations thereby providing an explanation for numerical results. Using this model and some simple extensions, it is possible to understand the transfer of power from large to small scales and the behaviour of higher order correlation functions. The ideas presented here will also serve as a powerful analytical tool to investigate nonlinear clustering in different models.