Any piezoelectric generator structure can be modeled close to its resonance by an equivalent circuit derived from the well known Mason equivalent circuit. This equivalent circuit can therefore be used in order to optimize the harvested power using usual electrical impedance matching. The objective of this paper is to illustrate the full process leading to the definition of the proper passive load allowing the optimization of the harvested energy of any harvesting device. First, the electric equivalent circuit of the generator is derived from the Mason equivalent circuit of a seismic harvester. Theoretical ideal impedance matching and optimal load analyze is then given emphasizing the fact that for a given acceleration a constant optimal output power is achievable for any frequency as long as the optimal load is feasible. Identification of the equivalent circuit of an experimental seismic harvester is then derived and matched impedance is defined both theoretically and experimentally. Results demonstrate that an optimal load can always be obtained and that the corresponding output power is constant. However, it is very sensitive to this impedance, and that even if impedance matching is a longtime well known technique, it is not really experimentally and practically achievable.
An alternative switching technique for piezoelectric energy harvesting is presented. The energy harvester based on piezoelectric elements is a promising method to scavenge ambient energy. Several non-linear techniques such as SSHI have been implemented to improve the global harvested energy. However, these techniques are sensitive to load and should be tuned to obtain optimal power output. This technique, called Series Synchronized Switch Harvesting (S3H), has both the advantage of easy implementation and independence of the harvested power with the load impedance. The harvesting circuit simply consists of a switch in series with the piezoelement and the load. The switch is nearly always open and is triggered-on each time the piezoelectric voltage reaches an extremum. It is opened back after an arbitrary on-time t_{0}. The energy scavenging process happens when switch is closed. Based on linear motion assumption, the harvester structure is modeled as a “Mass-Spring-Damper” system. The analysis of S3H technique is considered with harmonic excitation. An analytical model of S3H is presented and discussed. The main advantage of this approach compared with the usual standard technique is that the extracted power is independent of the load within a wide range of load impedance, and that the useful impedance range is simply related to the defined switch on-time. For constant displacement excitation condition, the optimal power output is more than twice the power extracted by the standard technique as long as the on-time interval is small comparatively with the vibration period. For constant force excitation, an optimal on-time can be defined resulting in an optimally wide load bandwidth. Keywords: piezoelectric; energy harvesting; non-linear harvesting techniques; switching techniques.
Growing demands in self-powered, wireless Structural Health Monitoring (SHM) systems has placed a particular attention on energy harvesting products. While most of works done in this domain considered directly coupled active materials, it may be preferential to use seismic (or indirect-coupled) harvesters for maintenance issues. With a seismic type harvester, a model considering constant vibration magnitude excitation is no longer valid as electrical energy extraction from mechanical vibration leads to a reduction of the vibration magnitude of the harvester because of electromechanical coupling effect. This paper extends a Single Degree of Freedom (SDOF) model with a constant force or acceleration excitation to a Two Degree of Freedom (TDOF) approach to describe the tradeoff between the damping effect on the host structure and the harvested power due to the mechanical to
mechanical coupling effect. When the harvester mass to host structure mass ratio is around 10^{-3}, the maximal
power is obtained and the host structure has then a sudden displacement reduction due to the strong mechanical to mechanical coupling. Its application to self-powered SHM will be also introduced in the paper.
A new global approach for improved vibration damping of smart structure, based on global energy redistribution by means of a network of piezoelectric elements is proposed. It is basically using semi-active Synchronized Switch Damping technique. SSD technique relies on a cumulative build-up of the voltage resulting from the continuous switching and it was shown that the performance is strongly related to this voltage. The increase of the piezoelectric voltage results in improvement of the damping performance. External voltage sources or improved switching sequences were previously designed to increase this voltage in the case of single piezoelectric element structure configurations. This paper deals with extended structure with many embedded piezoelectric elements. The proposed strategy consist of using an electric network made with non-linear component and switches in order to set up and control a low-loss energy transfer from source piezoelements extracting the vibration energy of the structure and oriented toward a given piezoelement in order to increase its operative energy for improving a given mode damping. This paper presents simulation of a clamped plate with four piezoelectric elements implemented in the Matlab/Simulink^{TM} environment and Simscape^{TM} library. The various simulation cases show the relationship between the damping performance on a given targeted mode and the established power flow. SSDD and SSDT are two proposed original networks. Performances are compared to the SSDI baseline. A damping increase of 18dB can be obtained even with a weakly coupled piezoelectric element in the multi-sine excitation case. This result proves the importance of new global non-linear multi-actuator strategies for improved vibration damping of extended smart structure.
Proc. SPIE. 8688, Active and Passive Smart Structures and Integrated Systems 2013
KEYWORDS: Switches, Switching, Resonators, Electrodes, Chromium, Energy harvesting, Capacitance, Epoxies, Chemical elements, Voltage controlled current source
This paper focuses on the influence of the topology of a network of piezoelectric harvesters using the SSHI (Synchronized Switch Harvesting on Inductor) technology. Generally, an energy harvester is used as a localized and standalone system. In the case of large structure and for large harvested energies, it is usually not easy to increase the size of the piezoelectric patches. In order to harvest energy in the regions of maximum strain of the structure, a networked piezoelectric harvester including many separated piezoelectric patches must be set up with only one output. The main concern is how to connect the piezoelectric elements together and how to implement accurately the SSHI strategy for maximizing the total output power. This paper presents 5 different circuit topologies with or without SSHI enhancement. This work is based upon simulations of a structure with embedded piezoelectric harvesters, made in the Matlab/Simulink environment and using the Simscape library for defining and simulating the electric network. The simulations are done exclusively in pulse mode. For each circuit topology, the total output energy is computed and the optimal harvesting capacitance is defined. The results show the feasibility of grouping various harvesters within a network connected onto a common harvesting capacitance without affecting the extracted energy. The interest of SSHI for networked configuration is confirmed as well as the need for multiple switching units. The effect of the parasitic capacitances due to the bonding of the piezoelectric patches on a metallic structure is also investigated. This capacitance corresponds to the isolation layer between the structure and the bottom electrode of the piezoelectric patches. Results show that an optimal bonding layer thickness can be found that does not affect significantly the coupling coefficient of the piezoelectric patches and which induces parasitic capacitances that do not affect the network functionality.
Smart structures controlled by active algorithms proved their high efficiency. But active control requires external
energy and heavy amplifier which strongly limit the applications in the transportation field. An alternative to
active control is given by semi-active control which does not require operative energy but which is less efficient
than active control. The proposed hybrid control associates the active control with semi-active control in order
to benefit from the respective advantages of both methods. This hybrid control is intended to control vibration
modes with the same performances than active control while reducing significantly the operative energy.
An application on the second mode of clamped-free smart beam is presented. The results show that this new
control approach appears to be able to decrease the required external energy and to reduce the power and
consequently the weight of the active control amplifiers while maintaining the same damping performances. This
control can be used, for example, in the transportation field to improve the lifetime of systems which use smart
structure.
The new proposed method is the hybridization between SSDI techniques and active methods developed for modal active
control such as time sharing control and modal observer in order to control several modes of a structure with a good
performance but without operative energy. It is designed in order to minimize the number of control components. The
principal application field is the transportation.
It is based on several modal SSDI controllers which act on the same actuator voltage. They are synchronized on each
extremum of the corresponding modal displacement. Modal displacements are reconstructed thanks to a modal model of
the smart structure from a modal observer. In order to reduce the number of actuators, the time sharing method is adapted
to SSDI techniques: all the modal SSDI are connected to the same piezoelectric actuator, but only one controller is
selected to control the voltage inversion for each step time. In order to select modal SSDI controller having the most
effective action for damping, a computation of modal energies is realized from the estimated modal state. A controller
selector is used to connect the modal SSDI command, whose corresponding mode has the highest modal energy [6], to
the switch trigger.
An application on a smart clamped free beam including one actuator and two sensors is presented. Three modes are
controlled and the modal responses are observed on five modes. The results show that the control reduces significantly
the vibration of targeted modes. Moreover, the method is not subject to stability problems.
This paper presents a combination of the SSD (Synchronized Switch Damping) semi-active control and techniques
developed for active control. The principle of modal SSDI is to synchronize the piezoelectric voltage inversion or
switching with the extremum of the targeted mode modal displacement. This modal displacement is estimated even in
the case of complex, broadband or noisy excitation with a modal observer. The switching process control induces a non
linear processing of the piezoelectric voltage which results in a cumulative self generated control voltage in phase with
the mode speed, thus generating an important damping of the targeted mode. This voltage self building is optimal if the
piezoelectric voltage is maximum when the modal displacement of the targeted mode is extremum. But in the case of
complex excitation or when the targeted mode amplitude is lower than higher modes, the performances are altered. The
proposed method consists in implementing a decision algorithm allowing waiting for the next voltage extremum before
to trig the voltage inversion, the whole process being globally synchronized with the targeted modal displacement.
Indeed, the targeted mode amplitude is reduced by using part of the energy of the higher modes which enhances the build
up of the self generated piezoelectric control voltage. Simulations carried out on a clamped free beam are presented.
Results obtained first with a bimodal excitation then in the case of pulse excitation demonstrates a large increase of the
damping on the targeted mode.
The integration of autonomous wireless elements in health monitoring network increases the reliability by suppressing
power supplies and data transmission wiring. Micro-power piezoelectric generators are an attractive alternative to
primary batteries which are limited by a finite amount of energy, a limited capacity retention and a short shelf life (few
years). Our goal is to implement such an energy harvesting system for powering a single AWT (Autonomous Wireless
Transmitter) using our SSH (Synchronized Switch Harvesting) method. Based on a non linear process of the
piezoelement voltage, this SSH method optimizes the energy extraction from the mechanical vibrations.
This AWT has two main functions : The generation of an identifier code by RF transmission to the central receiver and
the Lamb wave generation for the health monitoring of the host structure. A damage index is derived from the variation
between the transmitted wave spectrum and a reference spectrum.
The same piezoelements are used for the energy harvesting function and the Lamb wave generation, thus reducing mass
and cost. A micro-controller drives the energy balance and synchronizes the functions. Such an autonomous transmitter
has been evaluated on a 300x50x2 mm^{3} composite cantilever beam. Four 33x11x0.3 mm^{3} piezoelements are used for the
energy harvesting and for the wave lamb generation. A piezoelectric sensor is placed at the free end of the beam to track
the transmitted Lamb wave.
In this configuration, the needed energy for the RF emission is 0.1 mJ for a 1 byte-information and the Lamb wave
emission requires less than 0.1mJ. The AWT can harvested an energy quantity of approximately 20 mJ (for a 1.5 Mpa
lateral stress) with a 470 μF storage capacitor. This corresponds to a power density near to 6mW/cm^{3}.
The experimental AWT energy abilities are presented and the damage detection process is discussed. Finally, some
envisaged solutions are introduced for the implementation of the required data processing into an autonomous wireless
receiver, in terms of reduction of the energy and memory costs.
The damping of vibration resonance is a crucial problem for light and elongated structures. Different kinds of solutions have been developed in order to address the problem of volume or mass, or temperature dependence which are common to the passive approach. In the semi-passive technique proposed here, damping is obtained through the use of piezoelectric patches bonded on the structure. These piezoelements are controlled with a very simple approach only requiring switches which are driven periodically and synchronously with the structure motion. The overall control circuit requires a very few amount of energy. Results obtained on a beam and on a plate demonstrate that this self-adaptive technique is able to control simultaneously different modes on a broad frequency range.
A new approach of energy reclamation from mechanical vibrations is presented in this paper. The conversion from mechanical energy into electrical energy is achieved using piezoelectric materials. The originality of the proposed approach is based on a nonlinear treatment of the voltage delivered by a piezoelectric insert embedded in a vibrating structure. This nonlinear processing induces a strong increase of the power conversion capability of the piezoelectric insert. The theoretical principle of the nonlinear treatment is exposed, and the analytical model of an electrical generator is developed. The results given by the model are compared to those of an experimental set-up. Experimental results show that the extracted electrical energy may be increased beyond 400%.
The proposed technique is based on an intermittent switching of piezoelectric elements bonded on the structure to be d amped. As a result of the switching, the global losses coefficient of the structure is increased by a significant factor. From a physical point of view, the damping results from the energy dissipation due to the discharge of the piezoelement capacitance in the switch resistance. The switch has to be controlled and thus requires an electrical power about a few milliwatts for be activated. Consequently, the described approach is considered to be a semi-passive technique. For enhanced effects, the switching sequence has to be optimized. No tuning elements such as inductors or resistor^{1} are required, consequently the switching method can operate at any frequency, in particular in the low frequency regime, and is inherently broadband. Transient or continuous vibrations are damped with a comparable efficiency. A theoretical model is proposed to interpret the experimental results, to give a comprehensive understanding of the underlying physics and to optimize the switching sequence. It is show that, unlike standard passive techniques, the added damping in non-newtonian but, indeed exhibits a dry friction behavior. Numerous experimental results are given for flexural damping of steel cantilever beam and aluminum plate. It is shown that the damping efficiency can be up to 20 dB for the steel beam configuration. Harmonic and transient regimes of the beams are considered and compared. The design of electronic switching board and power requirements of the micro-controller are discussed.
The SSD technique proposed here addresses the problem of resonance damping on a mechanical structure. SSD stands for Synchronized Switch Damping. Apart from active techniques, passive ones consist in connecting a piezoelectric insert attached to the structure to a passive electric network in which the energy generated by the piezoelectric inserts is degraded. In the semi passive approach, the piezoelectric inserts are continuously switched from open circuit to short circuit synchronously to the structure motion. Due to this switching mechanism, a phase shift appears between the piezoelectric strain and the resulting voltage, thus creating energy dissipation. For the new technique proposed here, instead of discharging the piezoelectric inserts during a brief short circuit, they are connected on a small inductor, allowing the inversion of the voltage and then released to open circuit. In this case the voltage amplitude is optimized and is 90 degrees out of phase with the motion then enhancing the damping mechanism. The technique is applicable at any frequency without the need for a large tuned inductor, especially for low frequency applications. There is no need for external power supply unless for the low power circuitry of the switch device. The implementation of the switch drive with a very cheap micro-controller is described. Experimental results measured on cantilever beams made with different materials are proposed. Damping ability ranges from 6 dB on a very viscoelastic epoxy beam to nearly 20 dB on a steel beam. Harmonic excitation and transient results are both proposed and compared. Finally, an electromechanical model is proposed, giving an interpretation of the damping mechanism. Theoretical predictions are in good agreement with the experiments.
Passive damping using a piezoelectric device is a well-known technique. Both resistor and inductor loads connected to the piezoceramic are commonly used to attenuate a given resonance mode on a structure equipped with piezo dampers. The main drawback of this technique is its narrow band behavior and especially in the case of an inductor tuned passive piezo damper. The proposed technique is inherently wide band and does not rely on any tuned electric load. The piezoelectric device is simply continuously switched from open-circuit to short-circuit synchronously to the mechanical strain. It is called semi-passive because of the need of a sensor giving the strain of the piezo device. There is no need for external power supply unless for the low-level circuitry of the switch device. The damping efficiency appears to be twice what is obtained with pure resistive damping and is equivalent to what is achievable with a tuned inductor damper. It can work at any frequency without the need for large inductor especially for low frequency applications. A qualitative model gives an understanding of the damping mechanism.
The measurement of shear stresses in the hydrodynamic boundary layer required high sensitivity sensors. A floating element type sensor using the piezoelectric effect is proposed. It is constituted of a composite material with high shear stress sensitivity. The evolution of the sensor characteristics with the PZT volume fraction is analyzed. The experimental results show a good agreement with the Finite Element Modeling predictions.
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