For practical use, the electrical field requirements of Electro Active Polymer (EAP) actuators have to be lowered down.
Recently, we developed nano carbon filled polymeric films which can generate a large strain (30-50%) at moderate
electrical field (less than 20 MV/m). Herein, the electrostrictive strain saturates versus electrical field and that the
maximum strain depends strongly on the sample thickness. Combining polarization saturation effect and heterogeneities
in the polymer thickness lead to a model that describes correctly the strain behavior versus electrical field, polymer
thickness and frequency. A three-layer model was established which assumes that the polymer is not homogeneous along
the thickness. Two outer and one inner layers exist, which must be formed during the polymer curing. It is considered
that these layers have slightly different characteristics, such as permittivity. When the electrical field is input parallel to
the polymer thickness, a different strain would take place in each layer according to the field distribution. Since the
layers are attached together, the strain must be the same in each layer. Consequently stresses appear in the different
layers. Introducing in this model a saturation of the polarization for high field leads to simulation results that fit well the
experimental data.
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 mm3 composite cantilever beam. Four 33x11x0.3 mm3 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/cm3.
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.
KEYWORDS: Control systems, Sensors, Actuators, Ferroelectric materials, Fiber Bragg gratings, Digital signal processing, Finite element methods, Ferroelectric polymers, Composites, Vibration control
A new vibration control system, named 'block-by-block' distributed cluster control system, is presented with a CFRP board with stiffeners. Distributed cluster control system which had been applied for flat simple board is a control system includes 'cluster sensing' which classified numberless vibration modes into some limited number of clusters by using a group of sensors, and 'cluster actuation' which can actuate only specific cluster. It means, this system controls only target clusters. When the system is applied to complex structure such as this CFRP board with stiffeners, it is not possible to applied directly since the forms of vibration modes are not as simple as the one of flat board but there exist three blocks; 2 side blocks and one center block between two stiffeners. In this paper, after a rough explanation of distributed cluster control system, the idea of 'block-by-block' control is explained and verified experimentally with FEM analysis using some kinds of sensors and actuator.
During construction of extra high structures such as a skyscraper or a main tower of a long bridge, just a slight wind can generate low frequency vibration, and the maximum displacement at the top of structure can increase up to a few meters. Occurrence of low frequency vibration causes a fear of operators and it deteriorates awfully safeness. The purpose of this paper is a control of low frequency vibration above-mentioned with a development of small-lightweight actuator which can control a big displacement and that of control system which guarantees a stable control. As a first step, beam structure is estimated. Design of modal sensor using PVDF film sensor is explained. For an actuator, SMA/CFRP hybrid moment actuator is used. This actuator has been developed in the recent studies by authors with special consideration on the interfacial strength between SMA wires and matrix. Basic characteristics of this actuator is presented in this paper. To drive this moment actuator smoothly, punctual temperature control in real time which includes rapid heating, exact current control and some cooling control is required. Adaptive feedforward control system is, therefore, designed here for this actuator aiming to apply to beam structure. As a result, control effect on beam structure is demonstrated experimentally. Verification of performance of this actuator is also shown.
In recent years, pre-strained TiNi shape memory alloys (SMA) have been used for fabricating smart structure with carbon fibers reinforced plastics (CFRP). However, since the curing temperature of CFRP is higher than the reverse transformation temperatures of TiNi SMA, special fixture jigs have to be used for keeping the pre-strain during fabrication, which restricted its practical application. We have developed a new method to control the transformation temperatures of SMA by proper thermo-mechanical treatments and composition adjustment, which is suitable to fabricate SMA/CFRP smart composite with a curing temperature of 130C. Furthermore, we tried to develop a new fabrication technique which is also suitable to fabricate SMA/CFRP smart composite with a curing temperature of 180C. It was found that by using cold drawn ultra-thin TiNi wires, TiNi/CFRP composites with a curing temperature of 180C could be fabricated without special fixture jigs. The damage suppression effect by embedded ultra-thin wires in the smart composite was confirmed.
Low frequency modes of tower structure generated by a strong wind or by an earthquake occur deterioration or a collapse of structure because stress concentration happens at the root of structure. High frequency modes, on the other hand, are often possible to be disregarded because they can be damped immediately. In general, all vibration modes which are generated in the structure are tried to be suppressed when it is said as 'vibration control'. There remind, however, a lot of problems to realize a stable control in this case.
The object of present paper is a pick up and a suppression of specific vibration modes which occur such problems, it means here low frequency modes, among all of generated vibration modes in structure.
First of all, a design of modal sensor made of PVDF film is proposed to pick up only low frequency modes separately by using FEM analysis. Then, an applied method of SMA/CFRP hybrid actuator, which can generate great force in a field of low frequency, is explained. By using these PVDF modal sensor and SMA moment actuator, vibration model can be simplified by means of modification to low dimensions. Consequently, modal control system, which suppresses only low frequency vibration modes, is constructed. At the end of the present paper, effect of this control system is demonstrated experimentally.
Despite its great potentials, having a large displacement and force compared to traditional electro-hydraulic servo mechanical actuators or to PZT actuators, there are not so many studies on SMA active actuator. The main reasons are considered as following; (1) SMA has transformation only in one direction, (2) the response is quite slow, and (3) vibration control requires punctual thermo control in real time. In the study at our laboratory, the vibration can be clearly separated into different modes by distributed cluster system. SMA actuators are, then, proposed to use with PZT actuators for control of low and high frequency modes, respectively, to realize all-round actuation. The purpose of this paper is to realize SMA active actuator for low frequency modes. First of all, actuators using SMA wires, partly embedded in CFRP, were fabricated in consideration of SMA/FRP interfacial strength. Their thermo-mechanical behavior had been studied with cooling system. These lightweight actuators were placed on beam structure made of CFRP. Recovery force of beam structure itself was used as reactive force against force generated by SMA. As a result, actuator which is favorable for low frequency vibration modes control, i.e. having a large displacement and a large force, was obtained.
For the purpose of reducing the cost, a control system for a truss structure with a simplified controller equipped with amplifying function alone is proposed. In order to realize a sensor in consideration of system stability, the sensor is provided with multiplication-addition capability and a distributed modal filter capable of isolating multiple vibration modes. Then, a control system is built up to amplify the sensor output through a power amplifier, by using a moment actuator which can exert actuation comparable to that of the velocity feedback for damping the system. Finally, the control system is incorporated to the truss structure and the vibration control effect through direct feedback is studied.
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