Eugenio Brusa

On the role of nonsynchronous rotating damping in rotordynamics

Nonsynchronous rotating damping, i.e. energy dissipations occurring in elements rotating at a speed different from the spin speed of a rotor, can have substantial effects on the dynamic behaviour and above all on the stability of rotating systems.

Electromechanical Tuning of Self-Sensing Piezoelectric Transducers:

The application of a self-sensing piezoelectric transducer included in a bridge readout network is here investigated taking into account the electromechanical interaction. The parametric uncertainties and the piezoelectric device losses as well as the tolerances of the electric components make the balancing of the bridge difficult to achieve in practice. A loss and uncertainties compensation, based on a real-time software implementation of the bridge reference arm is here presented and validated. The standard electrical and the proposed electromechanical balancing are both theoretically and experimentally compared in the case of a simple beam. The bridge output signal that is proportional to the strain rate is then fed back by means of a sharp phase second order low pass filter which is aimed at increasing the damping of the first bending mode of the beam.

Analytical characterization and experimental validation of performances of piezoelectric vibration energy scavengers

One of the main requirements in wireless sensor operation is the availability of autonomous power sources sufficiently compact to be embedded in the same housing and, when the application involves living people, wearable. A possible technological solution satisfying these needs is energy harvesting from the environment. Vibration energy scavenging is one of the most studied approaches in this frame. In this work the conversion of kinetic into electric energy via piezoelectric coupling in resonant beams is studied. Various design approaches are analyzed and relevant parameters are identified. Numerical methods are applied to stress and strain analyses as well as to evaluate the voltage and charge generated by electromechanical coupling. The aim of the work is increasing the specific power generated per unit of scavenger volume by optimizing its shape. Besides the conventional rectangular geometry proposed in literature, two trapezoidal shapes, namely the direct and the reversed trapezoidal configuration, are analyzed. They are modeled to predict their dynamic behavior and energy conversion performance. Analytical and FEM models are compared and resulting figures of merit are drawn. Results of a preliminary experimental validation are also given. A systematic validation of characteristic specimens via an experimental campaign is ongoing.

Second approximation structural assessment of the Aurora solar sail spacecraft

Abstract Aurora spacecraft is a scientific probe propelled by a “fast” solar sail whose first goal is to perform a technology assessment mission. A key point of the experiment is the possibility of building a very light structure in order to achieve a high speed with the very low force due to the pressure of solar light. This in turn implies the need to perform a very accurate stress analysis of the whole structure. A first approximation study allowed one to verify that the structural goals that were stated in the preliminary design can be met. The aim of the present paper is to build a complete mathematical model based on the finite element method (FEM) of the whole spacecraft allowing to perform a detailed stress analysis of the various components and to compute the inflected shape of the sail. The code that has been developed can take into account the effects of the thermal expansion of the sail and of the spacecraft acceleration and can be adapted to solar sail spacecraft with different geometrical configurations.

Design and Structural Optimization of the Electric Arc Furnace Through a Mechatronic-Integrated Modeling Activity

Vibration of electrodes in operation strongly affects the efficiency of the electric arc furnace (EAF) fed by ac current during the steel smelting process. Therefore, an effective control of the structural dynamics through an active system is a current goal of the “intelligent manufacturing” approach. A vertical position control applied to each electrode allows keeping the arc length almost constant and reduces the effects of some electromechanical actions due to the mutual magnetic induction among the three electric phases. Nevertheless, control action needs for a detailed model of the whole system dynamic behavior. A new method for modeling the equipment behavior and somehow the process was implemented. A key issue was including into the model all the electromechanical coupling effects occurring in this system and suitably linking to the structural dynamics. Modeling activity was performed by resorting to the multibody dynamics and the finite-element method, while some analytical formulations were used to describe both the electric arc behavior and the control. A preliminary validation on a real plant was performed as far as the huge size of the system allowed and an assessment of the mechanical design of the EAF was completed.

Evaluating the electrical discharge machining (EDM) parameters with using carbon nanotubes

Electrical discharge machining (EDM) is one of the most accurate non traditional manufacturing processes available for creating tiny apertures, complex or simple shapes and geometries within parts and assemblies. Performance of the EDM process is usually evaluated in terms of surface roughness, existence of cracks, voids and recast layer on the surface of product, after machining. Unfortunately, the high heat generated on the electrically discharged material during the EDM process decreases the quality of products. Carbon nanotubes display unexpected strength and unique electrical and thermal properties. Multi-wall carbon nanotubes are therefore on purpose added to the dielectric used in the EDM process to improve its performance when machining the AISI H13 tool steel, by means of copper electrodes. Some EDM parameters such as material removal rate, electrode wear rate, surface roughness and recast layer are here first evaluated, then compared to the outcome of EDM performed without using nanotubes mixed to the dielectric. Independent variables investigated are pulse on time, peak current and interval time. Experimental evidences show that EDM process operated by mixing multi-wall carbon nanotubes within the dielectric looks more efficient, particularly if machining parameters are set at low pulse of energy.

Proposed noncryogenic, nondrag-free test of the equivalence principle in space

Ever since Galileo scientists have known that all bodies fall with the same acceleration regardless of their mass and composition. Known as the Universality of Free Fall, this is the most direct experimental evidence of the Weak Equivalence Principle, a founding pillar of General Relativity according to which the gravitational (passive) mass m and the inertial g mass m are always in the same positive ratio in all test bodies. A space experiment offers two main advantages: a signal i

Experimental Characterization of Electro–Thermo–Mechanical Coupling in Gold RF Microswitches

This paper analyses the multiple coupling occurring in some metallic microstructures electrostatically actuated in presence of thermal effect. Some relevant issues of basic procedures implemented to identify the relevant design parameters and the material properties of micro-electro-mechanical systems (MEMS) are preliminarily discussed. Temperature effects on the MEMS strain distribution in operating conditions are then investigated. Thermo-mechanical coupling is studied through some experiments performed on microbridge prototypes electrostatically actuated by comparing their behavior in case of superposed electro-thermal actuation and of thermal loading without electrostatic coupling. A detailed analysis is here performed to evaluate the role of the thermal stress upon the buckling of the microstructure and its effect on the deformed shape of the microbeam. Electro-thermo-mechanical coupling is therefore modeled and experimentally analyzed. Pull-in tests are used to detect some effects of mutual interaction between temperature and electrical actuation. The methodologies, here proposed, apply to several materials used in microfabrication, although experimental evidence described herein concerns some gold microbridges to be included in RF devices.

OPTIMISATION OF A HYBRID ENERGY SCAVENGER WITH PIEZOELECTRIC/MAGNETIC COUPLING FOR SENSOR-BEARING UNITS

Vibration monitoring based on wireless distributed sensors is currently used in steelmaking plants to early detect structural damage occurring in the rolling mill components. This approach allows overcoming some severe limitations of access to those industrial equipments, but sensors need a local power supply. Vibration energy harvesting based on piezoelectric materials is therefore proposed for this purpose. Nevertheless, very often it happens that dimensions of the energy scavenger are incompatible with the size of the system, thus not allowing a perfect tuning of its resonance upon the frequency of the dynamic excitation. Moreover, sometimes the amplitude of vibration is too low to induce a sufficient amount of energy conversion. Those problems motivated a previous work of the author, about the feasibility of plucking the flexible structure through either a relative motion or rotation of the harvested system and the energy scavenger, respectively. To avoid the drawbacks due to the wear in plucking the material, a contactless electromechanical coupling was proposed. The interaction between two permanent magnets, being one applied to the scavenger tip and the other fixed, was used to excite the vibration and the electromechanical conversion through the piezoelectric layer. The effectiveness of such hybrid system composed by the structure with surface bonded piezoelectric layers and the couple of magnets was investigated and compared to the power requirements of some sensors currently used to measure the dynamic response of the backup roll bearings located at the outer crown of the rolling mill. An optimisation of the whole device to increase the overall performance is proposed by following some approaches assessed in the literature and tested on some specimens of energy scavenger. The optimisation activity was based on a suitable selection of the piezoelectric material aimed at reaching the highest electromechanical coupling with a good mechanical strength and on a suitable shaping of the electrode surface aimed at assuring the largest efficiency in the energy conversion.

ELECTROMECHANICAL COUPLED RESPONSE OF THE AC ELECTRIC ARC FURNACE STRUCTURES DURING THE SCRAP MELTING PROCESS

Prediction of structural dynamics of the Electric Arc Furnace (EAF) is rather difficult, because of a number of phenomena occurring during the scrap melting process. Three large electrodes, corresponding to each phase of a AC circuit, are lowered by the main mast towards the scrap to activate the melting process, induced by the electric arc. Electric current fed to each electrode produces a strong magnetic field and applies an electromechanical force on the other electrodes. Arc voltage looks irregular upon time, even because of the scrap motion within the vessel and temperature growth. The vertical position of the mast is controlled by an hydraulic actuator. Nevertheless, a heavy vibration of the structures affects the regularity of the melting process. A fully coupled model of the whole system is herein proposed, through a multi-physics approach. A first analytical approach, describing the electric circuit of the whole system, is implemented into a Multi Body Dynamics (MBD) model of the EAF, while a reduced Finite Element Method (FEM) model of the flexible structures is used for a preliminary optimization of the design parameters. Electromechanical forces due to the mutual induction among the electrodes are computed and the dynamic response of the system is investigated. Proposed model allows a first refinement of the EAF design, although a complete experimental validation on the real machine has to be performed, in spite of problems due the extremely difficult accessibility of structures during the melting process.Copyright