Hornsen Tzou

Actuation of Parabolic Cylindrical Shell Panels With Light-Activated Shape Memory Actuators

Parabolic cylindrical shell panels are used in optical and aerospace structures. Light-activated shape memory polymer (LaSMP) is a novel smart material and it is capable of offering a non-contact actuation and control in room temperature. In this study, the parabolic cylindrical shell panels laminated with LaSMP actuators are analyzed. Firstly the dynamic equations of the parabolic cylindrical shell panels coupled with the LaSMP actuators are established; the modal control force of LaSMP actuators is derived with the modal expansion method. Then the strain variation of the LaSMP actuators are modeled based on the chemical kinetics. Further, the shape-memory recovery effect of an LaSMP actuator with initial strains is measured in laboratory. The experiment data of strain variation are used to validate the established strain model. Finally, in the case study the modal control forces of LaSMP actuators for the first four shell modes, i.e., the (1,3), (1,4), (2,4) and (2,5) modes are analyzed. The study shows that LaSMP actuators can induce strains not only in the x, Ψ directions but also in the xΨ direction (induced by the warping effect). The reason is that LaSMP actuators are easy to be cut in any shapes and be deformed in any directions. Thus, LaSMP actuators have potential applications for the non-contact vibration control of double-curvature shells.Copyright

Analytical and Experimental Studies of Flexoelectric Beam Control

Flexoelectricity includes two effects: the direct flexoelectric effect and the converse flexoelectric effect, which can be respectively applied to flexoelectric sensors and actuators to monitor structural dynamic behaviors or to control structural vibrations. This study focuses on the converse flexoelectric effect and its application to dynamic control of cantilever beams analytically and experimentally. In the mathematical model, a conductive atomic force microscope (AFM) probe with an external voltage is used to generate an inhomogeneous electric field driving the flexoelectric beam. The electric field gradient leads to an actuation stress in the longitudinal direction due to the converse flexoelectric effect. The actuation stress results in a bending control moment to the flexoelectric beam since the stress in the thickness is inhomogeneous. In order to evaluate the actuation effect of the flexoelectric actuator, the flexoelectric induced tip displacement is evaluated when the mechanical force is assumed zero. With the induced control moment, vibration control of the cantilever beam is discussed and the control effect is evaluated. Flexoelectric control effects with different design parameters, such as AFM probe location, AFM probe radius and flexoelectric beam thickness, are evaluated. Analytical results show that the optimal AFM probe location for all beam modes is close to the fixed end. Besides, thinner AFM probe radius and thinner flexoelectric beam enhance the control effects. Laboratory experiments are also conducted with different probe locations to validate the analytical predictions. Experimental results show that the induced tip displacement decreases when the input location moves away from the fixed end, which is consistent with the analytical prediction. The studies provide design guidelines of flexoelectric actuations in engineering applications.Copyright

Experiments on Quasi-Static and Dynamic Control of a PVDF Laminated Membrane-Like Mirror

Due to their merits of low areal density, in-orbit deploying and low manufacturing cost properties, membrane-like deformable mirrors meet the ultra-lightweight and large aperture demand of space optical mirror systems well. Membrane-like mirrors has therefore become a hot spot in the field of space science research now. Surface of these in-orbit membrane-like structures are susceptible to external environment variation on account of their low stiffness and high flexibility properties. Once excited by external excitations, the membrane vibration endures and this would not only degrade its performance, but also lead to system or structural failures. So dynamic vibration control of the membrane-like mirror is indispensable. When the orbiting mirror passes from Earth’s shadow into sunlight, the surface temperature of the mirror changes and this will render membrane thermal distortion. Hence, quasi-static surface error adjustment of the mirror is also requisite. In this paper, a 0.2m diameter scaled-down Kapton membrane-like mirror is taken as the experiment subject. Polyvinylidene Fluoride (PVDF) patches are laminated on the non-reflective side of the mirror as in-plane actuators. High precision laser displacement sensors are used to measure the surface deviation of the mirror. A modal vibration control system based on the positive position feedback (PPF) algorithm and a quasi-static surface control system based on the proportional error feedback algorithm are established respectively. Several mirror control experiments are performed and the results indicate that the methods proposed could suppress the membrane vibration and alleviate the membrane thermal deformation effectively.Copyright

Frequency Control of Light-Activated Shape Memory Polymer Laminated Beams: Characterization and Experiments

Light-activated shape memory polymers (LaSMPs) exhibit stiffness variations when exposed to ultraviolet (UV) lights. Thus, LaSMP could manipulate structural natural frequencies with UV light exposures when laminated on structures. This study aims to experimentally demonstrate the effectiveness of LaSMP frequency control of a flexible beam. The natural frequency of a three-layered Euler-Bernoulli beam composed of LaSMP, adhesive tape and the flexible beam is analyzed and its frequency formulation exhibits the LaSMP stiffness influence. As the LaSMP adopted in this study is a new spiropyran based composition, a generic Young’s modulus model is proposed and then simplified to model the present LaSMP composition. To make sure the experiment is carried out in a homogenous light field, the light intensities of the UV surface light source at different positions are tested. The temperature change of the LaSMP sample under UV exposures is also measured. The time constant of the reverse reaction and the threshold intensity of the reverse reaction are measured. LaSMP Young’s modulus variation is tested with a uniaxial tension experiment. The constitutive model of LaSMP’s Young’s modulus is validated by experimental data. With these preparations, the LaSMP laminated flexible beam model is exposed to the UV lights and its natural frequencies are identified with a data acquisition and analysis system. The maximum natural frequency variation ratio achieves 5.67%. Comparing both theoretical and experimental data of natural frequency control, this study also validates the LaSMP Young’s modulus constitutive model.Copyright