Jae-Hung Han

Optimal placement of piezoelectric sensors and actuators for vibration control of a composite plate using genetic algorithms

In this paper, the placement of piezoelectric sensors and actuators has been studied. Genetic algorithms have been used to find efficient locations of piezoelectric sensors and actuators of a smart composite place. Locations of both sensors and actuators have been determined with consideration of controllability, observability and spillover prevention. The composite specimen with piezoelectric sensors and actuators has been prepared according to the optimization result. The experimental vibration control shows significant vibration reduction for the controlled modes with little effect on the residual modes. In addition, the closed loop system is observed to be robust with respect to system parameter variations.

An experimental study of active vibration control of composite structures with a piezo-ceramic actuator and a piezo-film sensor

In order to reduce the vibrational level of lightweight composite structures, active vibration control methods have been applied both numerically and experimentally. Using the classical laminated beam theory and Ritz method, an analytical model of the laminated composite beam with piezoelectric sensors and actuators has been developed. Smart composite beams and plates with surface-bonded piezoelectric sensors and actuators were manufactured and tested. It is found that the developed analytical model predicts the dynamic characteristics of smart composite plates very well. Utilizing a linear quadratic Gaussian (LQG) control algorithm as well as well known classical control methods, a feedback control system was designed and implemented. A personal computer (PC) was used as a controller with an analogue - digital conversion card. For a cantilevered beam the first and second bending modes are successfully controlled, and for cantilevered plates the simultaneous control of the bending and twisting modes gives a significant reduction in the vibration level. LQG has shown advantages in robustness to noise and control efficiency compared with classical control methods. In this study examples of control spillover are demonstrated via the instantaneous power spectrum of the sensor output.

Analysis of composite plates with piezoelectric actuators for vibration control using layerwise displacement theory

In order to evaluate closed loop performances of the composite plates with distributed piezoelectric actuators, it is essential to obtain more exact system parameters such as natural modes, damping ratios, and modal actuation forces. In this paper a refined analysis of composite plates with distributed piezoelectric actuators for vibration control has been performed. The in-plane displacements through the thickness have been modeled using the layerwise theory. This layerwise model can describe more refined strain distributions and has the capability of more realistic modeling of boundary conditions. The finite element method based on the developed mechanics has been formulated. The constitutive equations for piezoelectric materials have been used to determine piezoelectric actuation forces, and the modal strain energy method has been applied to analyze the damping capacity of the structures. Through the comparison of present results with those available, the accuracy of the present method was verified. The closed loop performances have been evaluated using the simple control algorithms. Through the comparison of present results with those based on shear deformation plate theory, it is concluded that the developed model can describe more realistic smart composite plates with distributed piezoelectric actuators.

Multi-Modal Vibration Control Using Adaptive Positive Position Feedback

An adaptive controller, Adaptive Positive Position Feedback (APPF) is proposed for the multi-modal vibration control of frequency varying structures. Spillover phenomena and real-time system identification have been obviously difficult obstacles for the multi-modal adaptive vibration control. To overcome these problems, a fast and powerful algorithm is proposed to identify the frequencies of time-varying structures. Variable PPF controllers are adjusted with estimated natural frequencies at every time step. A composite plate with a bonded piezoelectric sensor and an actuator was prepared as an experimental model, and the natural frequencies of the model are changed by attaching masses. The experimental results show that natural frequencies are estimated quite accurately and that the vibration of controlled modes is significantly reduced. No significant performance reduction has been observed with respect to approximately 10% frequency changes of the corresponding modes. On the contrary, the performance of the conventional LQG controller is significantly degraded due to frequency variations.

Displacement field estimation for a two-dimensional structure using fiber Bragg grating sensors

The structure shape itself is of great interest for many aerospace applications. For example, the stability of the surface shape of large, high precision or space reflectors is essential for the communication performance. The knowledge of static and dynamic displacements of these structures would provide the possibility to enhance their performance by appropriate countermeasures. During operation, however, the direct measurement of displacements of the whole structure is often difficult. This study investigates whole displacement field estimation using strain measurement and a displacement–strain-transformation approach. The use of fiber Bragg gratings (FBGs) as strain sensors for this application offers the possible implementation of an integrated sensor network including many measurement points within only a few optical fibers. This paper discusses many issues related to the displacement field estimation of a dynamically excited plate using a transformation matrix based on a modal approach. In order to reduce systematic displacement estimation errors due to aliasing, a parametric study was performed and the sensor locations were optimized. Experimental validation was also conducted using a cantilever plate equipped with 16 FBG sensors in an optimized configuration. The estimated displacements showed good agreements with those measured directly from laser displacement sensors.

Thermal post-buckling analysis of shape memory alloy hybrid composite shell panels

This paper was performed for the Smart UAV Development program, one of the 21st Century Frontier R&D Programs funded by the Ministry of Science and Technology of Korea

Thermopiezoelastic Snapping of Piezolaminated Plates Using Layerwise Nonlinear Finite Elements

The thermopiezoelastic snapping phenomena of piezolaminated plates are numerically simulated by applying a cylindrical arc-length scheme to Newton-Raphson method. Based on the layerwise displacement theory and von Karman strain-displacement relationships, nonlinear finite element formulations are derived for thermopiezoelastic composite plates. From the static and dynamic viewpoint, nonlinear thermopiezoelastic behavior and vibration characteristics are investigated for symmetric and eccentric structural models with various piezoelectric actuation modes. Present results show the possibility to enhance the performance of thermal structures using piezoelectric actuators and report new phenomena, namely thermopiezoelastic snapping, induced by the excessive piezoelectric actuation in the active suppression of thermally buckled large deflection of piezolaminated plates.

Wind tunnel tests for a flapping wing model with a changeable camber using macro-fiber composite actuators

In the present study, a biomimetic flexible flapping wing was developed on a real ornithopter scale by using macro-fiber composite (MFC) actuators. With the actuators, the maximum camber of the wing can be linearly changed from −2.6% to +4.4% of the maximum chord length. Aerodynamic tests were carried out in a low-speed wind tunnel to investigate the aerodynamic characteristics, particularly the camber effect, the chordwise flexibility effect and the unsteady effect. Although the chordwise wing flexibility reduces the effective angle of attack, the maximum lift coefficient can be increased by the MFC actuators up to 24.4% in a static condition. Note also that the mean values of the perpendicular force coefficient rise to a value of considerably more than 3 in an unsteady aerodynamic flow region. Additionally, particle image velocimetry (PIV) tests were performed in static and dynamic test conditions to validate the flexibility and unsteady effects. The static PIV results confirm that the effective angle of attack is reduced by the coupling of the chordwise flexibility and the aerodynamic force, resulting in a delay in the stall phenomena. In contrast to the quasi-steady flow condition of a relatively high advance ratio, the unsteady aerodynamic effect due to a leading edge vortex can be found along the wing span in a low advance ratio region. The overall results show that the chordwise wing flexibility can produce a positive effect on flapping aerodynamic characteristics in quasi-steady and unsteady flow regions; thus, wing flexibility should be considered in the design of efficient flapping wings.

Experimental investigation on the aerodynamic characteristics of a bio-mimetic flapping wing with macro-fiber composites

This study describes the development of a bio-mimetic flapping wing and the aerodynamic characteristics of a flexible flapping wing. First, the flapping wing is designed to produce flapping, twisting, and camber motions by using a bio-mimetic design approach. A structural model for a macro-fiber composite (MFC) actuator is established, and structural analysis of a smart flapping wing with the actuator is performed to determine the wing configuration for maximum camber motion. The analysis model is verified with the experimental data of the smart flapping wing. Second, aerodynamic tests are performed for the smart flapping wing in a subsonic wind tunnel, and the aerodynamic forces are measured for various test conditions. Additionally, the effects of camber and chordwise wing flexibility on unsteady and quasi-steady aerodynamic characteristics are discussed. The experimental results demonstrate that the effect of the camber generated by the MFC produces sufficient aerodynamic benefit. It is further found that chordwise wing flexibility is an important parameter in terms of affecting aerodynamic performance, and that lift produced in a quasi-steady flow condition is mostly affected by the forward speed and effective angle of attack.

Longitudinal Flight Dynamics of Bioinspired Ornithopter Considering Fluid-Structure Interaction

This paper addresses the flapping frequency-dependent trim flight characteristics of a bioinspired ornithopter. An integrative ornithopter flight simulator including a modal-based flexible multibody dynamics solver, a semiempirical reduced-order flapping-wing aerodynamic model, and their loosely coupled fluid―structure interaction are used to numerically simulate the ornithopter flight characteristics. The effect of the fluid―structure interaction of the main wing is quantitatively examined by comparing the wing deformations in both spanwise and chordwise directions, with and without aerodynamic loadings, and it shows that the fluid―structure interaction created a particular phase delay between the imposed wing motion and the aeroelastic response of the main wing and tail wing. The trimmed level flight conditions of the ornithopter model are found to satisfy the weak convergence criteria, which signifies that the longitudinal flight state variables of ornithopters need to be bounded and that the mean value of the variables are converged to the finite values. Unlike conventional fixed-wing aerial vehicles, the longitudinal flight state variables, such as forward flight speed, body pitch attitude, and tail-wing angle of attack in trimmed level flight, showed stable limit-cycle oscillatory behaviors with the flapping frequency as the dominant oscillating frequency. The mean body pitch attitude and tail-wing angle, and the root-mean-square value of the body pitch attitude, decreased as the flapping frequency increased. In addition, the mean forward flight speed is found to almost linearly increase with the flapping frequency.