Breaking reciprocity in wave propagation problems is of great interest within the research community, given the opportunity to design new devices for one-way communication with unprecedent performances. In the context of mechanics and phonon transport, directional wave manipulation can be achieved exploiting space-time periodic materials. Namely, structures whose elastic or physical properties are functions of space and time.
In this work we experimentally study nonreciprocal wave propagation in a space-time modulated beam based on piezoelectric actuation. The system is made of an aluminum beam with an array of piezoelectric patches bonded on its top and bottom surface. Each patch is attached to a negative capacitance (NC) shunt driven by switching circuit, which is able to provide a square-wave stiffness profile in time to the layered material. Spatiotemporal modulation is induced by phase shifting the temporal modulation law of three consecutive piezo-elements, generating a traveling Young’s modulus that mimics the propagation of a wave along the beam dimension. As a result, we achieved 1kHz directional bandgaps, i.e. bandgaps at different frequencies for counter propagating waves, which can be moved in a range spanning 8-11 kHz.
Vibration energy harvesters have been proposed as an autonomous power source for meeting the limited power requirements of present-day sensors and electronics that find extensive usage in structural health monitoring systems. Recent research reveals that nonlinear energy harvesters outperform their linear counterparts, designed to operate on the principle of resonance, owing to their wide frequency bandwidth which allows for better performance in realistic operational environments. Particularly, bi-stable energy harvesters designed to exploit piezoelectricity to achieve the mechanical to electrical energy conversion have been widely investigated in literature. Additionally, several investigations have been also proposed to enhance power conversion in linear harvesters by introducing nonlinear circuits, e.g. based on synchronized switching (SS). In this respect, unveiling the effects on the bandwidth and coexisting solutions in the response of strongly nonlinear electrical SS shunts interacting with multi-stable structures requires further investigation. In particular, synchronized switch harvesting on inductor (SSHI) circuits, when connected in parallel with bi-stable energy harvesters, facilitate an increase in the harvested voltage, thus allowing for higher power generation as compared to a standard shunt load. This paper investigates the effects of utilizing a SSHI circuit for enhancing the power output in the different dynamical regimes of a bi-stable energy harvester. In particular, we present a comprehensive study of the efficiency of the SSHI circuit when the response configuration of the system is shifted between coexisting dynamical states of the bi-stable harvester. The herein presented semi-analytical and numerical results show that the qualitative performance of the SSHI circuit is quite robust in terms of superiority over the standard load circuit. However, the quantitative nature of the increase in power harvested by the SSHI circuit is sensitive to the optimal load in the circuit, which in turn varies with the dynamical state of the system.
Spatiotemporal periodic structures are systems whose properties are periodically modulated both in space and time, able to support waves only in one direction, so breaking the so called reciprocity principle. Studies till now focused mainly on continuous systems, where properties are modulated in a continuous manner both in space and time. However, this is not the case of real mechanical systems: if on the one hand it is possible to think of a continuous temporal modulation by the means of an actively controlled smart material, on the other hand it is really hard to imagine a system that can be controlled punctually in space. For this reason discretely modulated structures are studied in this work, putting in evidence the differences with the continuous ones and providing some basis for the actual design of such a system.
Periodic structures have received large interest due to their peculiar behavior: they have band gaps, that is portions of the frequency response along with any wave incoming in the structure is reflected. Numerous are the applications, like metamaterials and locally resonant structures. Nowadays, new possibilities could come from mechanical periodic structures that are connected to an electrical transmission line, periodic in turn. Starting from this idea, this paper analyses ideal a mono-atomic spring-mass chain, considering the springs connected to a periodic electric network, composed by inductances (and resistors): these simple examples will show how the frequency response is affected. In particular, the mutual influence between the electric and mechanical domain is highlighted, and the contribution of parameters on band gap positioning and design is explored. Details are provided about vibration modes and wave transmission.
Synchronized Switch Damping control is one of the most interesting solutions for vibration suppression proposed in the
last years. It is based on the electromechanical coupling provided by piezoelectric actuators. By connecting the
piezoelectric actuator to a shunting circuit (typically a RL one) and properly switching between open and closed circuit,
an equivalent hysteresis cycle is created and the structure energy is dissipated.
It is known that, in order to maximize the damping effect the resistance of the circuit must be reduced. Anyway, as
shown in literature, this effect is limited by the presence of a beating effect when the resistance is lower than a certain
The aim of this paper is to analyse the beating phenomenon in order to propose new solutions to cancel it. The paper will
show some innovative solutions modifying the shunting circuit to avoid the beating phenomenon and thus increase the
control performance. These solutions will be described from a theoretical point of view and then tested to demonstrate
Helicopters are among the most complex machines ever made. While ensuring high performance from the aeronautical
point of view, they are not very comfortable due to vibration mainly created by the main rotor. Traditionally this problem
is solved by mounting several TMD inside the helicopter, each tuned to the frequency of the disturbance the main
associated with the main rotor. In particular, this frequency is equal to the angular speed of the main rotor times the
number of blades. Despite the angular speed of the main rotor is kept fixed and constant during the flight, it happens that
some changes may be needed in particular situations.
Therefore it happens that the mass dampers are no more tuned and thus ineffective. This leads to a significant increase of
the amplitude of vibration. This work proposes to replace the purely passive systems with semi-active systems that are
able to change their own natural frequency in order to be effective at each angular speed of the main rotor.
The paper deals with the preliminary analysis of the project to numerically and experimentally evaluate the feasibility of
The semi-active Synchronized Switching Damping (SSD) family is based on a nonlinear shunting circuit applied to piezoelectric actuators, where the circuit characteristics are switched along the vibration cycles of the structure. SSD offers many advantages with respect to other vibration suppression techniques using piezoelectric actuators. Indeed, multiple modes can be suppressed with a relatively simple system and with very low power consumption. This allows the realization of self-powered control systems, without the need of wiring and external power supply. Moreover, the characteristics of this control strategy make it very robust to the variation of the dynamic characteristics of the structure, outperforming the classic passive linear shunts.
Different SSD techniques have been developed, varying the circuit characteristics and the switching logic. Although this control family has been studied for many years, all the works are limited to the single actuator case, losing in generality with respect to many practical cases. For this reason, the aim of this work is to apply SSD control with multiple actuators and to study the interaction of the actuators and their shunting circuits in order to optimize the damping performance.
The study will be performed numerically and then an experimental setup will be realized to test the proposed solutions.
Self-powered switching control has been proved to be very effective for vibration damping in case it is not possible to power up a fully active system and/or you have stringent requirements from an added weight point of view. However, the capability of harvesting the required energy for switching as well as the authority of the piezo actuator are greatly influenced by the actual position of the patches along the beam as well as by the circuit parameters. Indeed, varying them both control performance and energy requirement strongly change, making feasible or compromising the possibility to obtain a self-powered control system. Moreover, a difference of system or circuit parameters from ideality can affect the behavior of the damping layout.
In this paper the influence of all these parameters on control behavior are deeply studied. Aim of the paper is to provide a handbook to the choice of circuit parameters depending on the control system requirements.
Simple piezo benders are able to harvest energy almost only at their first natural frequency. Thus, their efficiency is good only in presence of harmonic excitation having frequency equal to their first natural frequency. In case of wide band excitation, instead, more complex solutions have to be sought for. In particular, various forms of nonlinearity have been investigated that allow to “bend” resonance peaks and thus convert kinetic energy into electric energy over a wider frequency band. Also harvester with more benders have been analyzed.
In the present paper a novel periodic substructure is proposed that allows to obtain several eigen-frequencies in a given frequency range. Moreover, the position where the piezo bender is placed has the same mode of vibration for all these eigen-frequencies. The bender is therefore able to harvest energy at all these eigen-frequencies thus greatly improving the overall efficiency. The next step will be to introduce nonlinearities in the substructure to even further extend the range of frequencies at which energy is harvested.
Giant Magnetostrictive Actuators (GMA) can be profitably used in application of vibration control on smart structures. In
this field, the use of inertial actuators based on magnetostrictive materials has been consolidate. Such devices turn out to
be very effective in applications of vibration control, since they can be easily paired with sensors able to ensure the
feedback signal necessary to perform the control action.
Unlike most widespread applications, this paper studies the use of patch magnetostrictive actuators. They are made of a
sheet of magnetostrictive material, rigidly constrained to the structure, and wrapped in a solenoid whose purpose is to
change the intensity of the magnetic field within the material itself.
The challenge in the use of such devices resides in the impossibility of having co-located sensors. This limit may be
exceeded by using strain gauge sensors to measure the deformation of the structure at the actuator. This work analyzes
experimentally the opportunity of introducing, inside a composite material structure, both the conventional electric strain
gauges and the less conventional optical sensors based on Bragg’s gratings.
The performance of both solutions are analyzed with particular reference to the signal to noise ratio, the resolution of the
sensors, the sensitivity to variations of the electric and magnetic fields and the temperature change associated with the
operation of the actuator.
The paper discusses the opportunity to use piezoelectric actuators (PZT) and Fiber Bragg Grating sensors (FBGs) to realize a smart structure including in itself both the sensing and the actuating devices. Different control strategies have been implemented on a test rig consisting on a plate made of carbon fiber using two chains with 15 FBG sensors each and 6 PZT actuators. Control forces are designed to increase the damping of the structures, allowing to increase of damping of the first modes of vibration of about 10 times.
Fiber Bragg Gratings (FBG) sensors have a great potential in active vibration control of smart structures thanks to their small transversal size and the possibility to make an array of many sensors. The paper deals with the opportunity to reduce vibration in structures by using distributed sensors embedded in carbon fiber structures through the so called sensors-averaging technique. This method provides a properly weighted average of the outputs of a distributed array of sensors generating spatial filters on a broad range of undesired resonance modes without adversely affecting phase and amplitude. This approach combines the positive sides of decentralized control techniques as the control forces applied to the system are independent of one another, while, as for the centralized controls it has the possibility to exploit the information from all the sensors. The ability to easily manage this information allows to synthesize an efficient modal controller. Furthermore it enables to evaluate the stability of the control, the effects of spillover and the consequent effectiveness in reducing vibration. Theoretical aspects are supported by experimental applications on a large flexible system composed of a thin cantilever beam with 30 longitudinal FBG sensors and 6 piezoelectric actuators (PZT).
The work presented aims at modeling, designing and implementing an energy harvesting system capable of generating electricity from environmental vibrations. Subject of the analysis is a piezoelectric bimorph; this particular transducer, composed of two layers of piezoceramic material, is clamped in a cantilever configuration and is dynamically bent due to vibrations. The resulting deformation ensures enough current to power the electronic circuit of a wireless sensor. An analytical model is adopted, that describes the dynamics of the mechanical system using an electrical duality. In particular the coupling of the variables is represented by an equivalent transformer. The obtainable voltage and power are investigated, focusing on the influence of the electric load on the performance of the conversion process. In addition, to overcome the limitations related to the analytical study, a finite element model is provided, capable of simulating the behavior of the system more accurately. Finally, both models are validated by means of experimental tests, showing the mutual influence between the mechanical and the electrical domain.
In general active vibration control intrinsically implies a fatigue damage reduction. Anyway, this assumption is
not always verified. In these cases it is possible to deeper investigate the fatigue phenomena on smart flexible
structures and their reduction from a control point of view. In this article, to identify the problem main
parameters, a simplified interpretation of fatigue damage is given using the frequency analysis framework. Then,
the active control logic is defined as an optimization problem with a quadratic functional taking into account
the previously cited parameters. Finally, because of non-linearity of fatigue phenomenon, an adaptive approach
is applied and a numerical/experimental validation is carried out.