KEYWORDS: Aerodynamics, Control systems, Cesium, Cerium, Actuators, Process control, Patents, Smart structures, System integration, Current controlled current source
Active twist control of airfoils by means of embedded actuators has been widely studied during the last decade
(N.A.S.A., University of Maryland, D.L.R., O.N.E.R.A., . . . ) Here, we propose a method which is to our
knowledge new and which makes it possible to control the twist by modifications of the internal structure of the profile inducing displacement of the shear center and, therefore, modifications of the torsional moment and angle. This method is only operating if the profile is submitted to an external force. This is why this method is called "reactive" by opposition to the method mentionned above. Experiment has been done to verify the efficiency of the method proposed.
ONERA - The French Aerospace Lab - has launched an internal program on biologically-inspired Micro Air
Vehicles (MAVs), covering many research topics such as unsteady aerodynamics, actuation, structural dynamics
or control. The aim is to better understand the flapping flight performed in nature by insects, and to control state
of the art technologies and applications in this field. For that purpose, a flight-dynamics oriented simulation
model of a flapping-wing concept has been developed. This model, called OSCAB, features a body and two
wings along which the aerodynamics efforts are integrated, so as to determine the global motion of the MAV.
The model has been improved by taking into account the flexibility of the wings (flexion of the leading edge
and passive torsion of the wings, induced by the flapping motion itself under wing inertia). Thus, it becomes
possible to estimate the coupling between flexibility and the aerodynamic forces. Furthermore, the model shows
that using elastic properties of the wings allows a diminution of the mechanical energy needed for wings motion,
and a reduction of the number of actuators to be implanted into the MAV.
An array of piezoelectric ultrasonic exciters/sensors and fiber Bragg grating sensors is embedded between an Aluminum
plate and a composite patch. Using Lamb waves, the array is shown to be capable of detecting a developing damage in
the aluminum plate, as well as locating it.
The Micro Aerial Vehicle (MAV) represents a strategic and industrial goal. The challenge is truly technical, as the needs are very specific in terms of mission and efficiency. The aerospace French agency (Onera ) has launched an internal program on that purpose including many research topics, essential to understand how to reach the goals. Thus, aerodynamic (generally unsteady with low Reynolds number), structural dynamics, propulsion, actuation, control,.. are being studied in this field. On a structural and mechanical actuation point of view, presently our main interest, the problem is already very large. So before analyzing and formulating, we found not so meaningless to look how nature proceeds and to start a design study with a "biologically-inspired" approach (dragonfly).
Piezoelectric plates have been widely used for the vibration reduction and noise control of structures. Due to power forces considerations, electrostrictive patches present a growing interest. It is thus the purpose of the present research to contribute to modeling aspects of thin structures integrating such actuators. Several three-dimensional finite elements have been elaborated to simulate these structures behavior. Using the constitutive relationships analyzed by the first author in a previous paper, an original two-dimensional plate theory have been presented at SPIE'01 with a focus on modeling aspects and corresponding appropriate variational formulations. Let us recall that the element here developed has the particular property of reducing the initial electromechanical problem to a purely mechanical problem based on a modified elastic constitutive law. The electrical unknowns are then explicitly derived from the mechanical displacements. This theory thus proved that there is no need to develop new plate finite elements but that one could use classical elements for laminated plates. The previous paper, moreover underlined that using current as actuators driving input leads in simplifications of the behavior modeling. The purpose of this presentation is then to validate the current driven and voltage driven plate models, previously established, through numerical finite element results and experimental correlations.
Electrostrictive materials have been widely studied for the last decade with the view of integrating them in smart structures. Many three-dimensional finite element models have then been elaborated to simulate these structures behavior but no two-dimensional models have been presented up to now. The aim of this article is thus to set out the elaboration of a thin plate electrostrictive finite element for PMN-PT type ceramics used as actuators. This element is developed for dynamic purposes and thus takes into account phenomena induced by applying to the patch a cycling electric field. The finite element formulation is based on electromechanical constitutive equations derived in a previous paper, mechanical and electrical considerations and direct a priori plate assumptions. The electrostrictive finite element is here derived using techniques inspired from a piezoelectric finite element. This method has the particular property of reducing the initial electromechanical problem to a purely mechanical problem based on a modified elastic constitutive law. The electrical unknowns are then explicitly derived from the mechanical displacements. This method considerably simplifies the resolution of the problem since classical finite elements for laminated plates can be used to model the electrostrictive plate with a modified constitutive law.
Sandwich structures we are interested in (glass fibers skins foam core) are very sensitive to low velocity impacts. These impacts induce debonding between the skins and the core and a crush of the foam, which are not visible from the outside. We propose a health monitoring system to localize these damages and estimate their size. This system is based on thin piezoelectric discs bonded on the skins and used as transducers, making it possible to generate and detect Lamb waves propagating in the sandwich. Experiments show that it is possible to choose waves having interaction with the particular damages of this sandwich. In order to analyze the interaction of Lamb waves with the defect, and define and optimize identification procedures for these defects, we have done f.e.m. computations which make it possible to obtain theoretical background to the analysis of the experimental data and propose an identification procedure of defects. When applied to numerically simulated experimental data, the identification procedure for defects give quite good results.
The feasibility of an integrated system for permanent detection and estimation of damaging impacts on composite plates has been evaluated. This system is based on the existence, during a damaging impact, of an intense acoustic emission in the high frequency range. This acoustic emission is registered by a network of piezoelectric sensors and is used to obtain estimations of the location of damaging impacts and estimations of damage areas. Experiments have been carried out with carbon-epoxy plates equipped with four small and thin disc-shaped piezoelectric sensors. Each plate has been impacted using a weight drop machine equipped with a Boeing window. The impact energy varies from 2 J to 6 J for 16 plies coupons and from 4 J to 8 J for 32 plies coupons. The procedure used to identify the damage as a noise source from the signals received by the sensors allows to localize them with an accuracy of a few centimeters. It is also possible to obtain a nearly linear relation between the amplitude of the identified noise source and the projected area of the damage. It is then possible to estimate the damage area in a plate from the amplitude of the noise source after a calibration of the sensors of a reference plate.
In composite materials, delaminations are discontinuities producing mode conversion processes generating various out-going modes. The Discrete Wavelet Transform allows isolating various propagation modes and extracting them in order to measure the time delay between the arrivals of the main burst and a specific out-going mode, for various propagation paths. This process permits, with a good accuracy, to localize a damage and to estimate its extension. An active health monitoring system composed of integrated disc-shaped, 100 (mu) m-thick and 5 mm-dia PZT transducers working sequentially as actuators and receives is presented. The diagnostic is based on multiresolution process by wavelet transform applied on recorded Lamb wave signals obtained before and after damage. The robustness and portability of this technique is demonstrated by the fact that, after validation in our laboratory it was successfully applied to data coming from an experiment conducted in an other Laboratory using its own Health Monitoring system.
Health and usage monitoring systems for composite structures based on Lamb wave propagation are one of the most promising systems presently evaluated. Generally, they are constituted of a network of piezoelectric transducers. The various transducers generate and receive the Lamb waves. The diagnostic is based on the perturbation in the propagation of the waves due to the presence of the damage. In view to develop less intrusive actuators/sensors, the possibility of using a fiber optic delivery system for laser generation of Lamb waves is evaluated. 125 μm-dia. optical fibers are embedded in the composite during the process. The energy of the pulses are lower than a mJ allowing a good functioning of the system even after several hundred of thousands pulses. The out-of-plane displacement field on the surface of a carbon-epoxy structure equipped by such a system is reconstructed (B-scan) using point measurement from a very sensitive and wide-band (20 kHz - 30 MHz) interferometric probe. The analysis of typical B-scans by 2-D Fourier transform clearly demonstrates the generation of Lamb waves. Various modes are excited, from the fundamental ones to high order ones. Comparison to theoretical dispersion curves is presented.
The feasibility of an integrated system for permanent detection of damaging impacts on composite structures has been evaluated. This system is based on the existence, during the damaging impact, of an intense acoustic emission in the high frequency range. This acoustic emission is registered by a network of piezoelectric sensors. Experiments are carried out with carbon-epoxy plates equipped with four miniaturized disc- shaped piezoelectric sensors. Both fully embedded and surface positioned sensors are used. Each plate is impacted using a weight drop machine equipped with a Boeing window. The impact energy varies from 2 J to 6 J for 16 ply coupons and from 4 J to 8 J for 32 ply coupons. The procedure used to identify the damaged area as a noise source from the signals received by the sensors allows localizing them with an accuracy of a few centimeters. It also allows to correlate the <<amplitude>> of the identified noise source to the area of the damage. It is then possible to estimate the damaged area from the amplitude of the noise source.
We present shearographic imaging of the interaction of AO waves with defects in plates. The images are good when the plates are composite plates, due to some attenuation of the waves which prevents reflections. Those images contain information on the interaction between waves and defects. Nevertheless, this information is very difficult to extract and this extraction need numerical models. Here we show some examples of interaction. It is expected that this images will help to better understand the wave-defect interaction and will help to define and optimize health monitoring systems for carbon-epoxy plates and sandwich plates.
We propose to adapt the asymptotic model of plate developed by Ciarlet to take into account specific effects as to the behavior of a piezoelectric device. Then we show how to distribute them in the thickness of a stratified composite structure, so as to detect or activate its strains optimal manner.
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