William S. Oates

Photomechanical bending mechanics of polydomain azobenzene liquid crystal polymer network films

Glassy, polydomain azobenzene liquid crystal polymer networks (azo-LCNs) have been synthesized, characterized, and modeled to understand composition dependence on large amplitude, bidirectional bending, and twisting deformation upon irradiation with linearly polarized blue-green (440–514 nm) light. These materials exhibit interesting properties for adaptive structure applications in which the shape of the photoresponsive material can be rapidly reconfigured with light. The basis for the photomechanical output observed in these materials is absorption of actinic light by azobenzene, which upon photoisomerization dictates an internal stress within the local polymer network. The photoinduced evolution of the underlying liquid crystal microstructure is manifested as macroscopic deformation of the glassy polymer film. Accordingly, this work examines the polarization-controlled bidirectional bending of highly concentrated azo-LCN materials and correlates the macroscopic output (observed as bending) to measured ...

Piezoelectric Hydraulic Pump System Dynamic Model

A system dynamic model was developed for use in the design of piezohydraulic pumps. The model was implemented in MATLAB to predict the electrical/mechanical/fluid coupled behavior of the piezoelectric pump system. Model results were used in a performance assessment of a pumping system that was comprised of a stack actuator driven pump, accumulator, power supply, four-way valve, and hydraulic actuator. The model provided reasonable predictions of performance. Rate effects at higher voltage and frequency reduced pressure and flow rate performance. The heat generation of the piezoelectric stack actuator, flow resistance of the check valves, and compressibility of the fluid contributed to bandwidth limitations.

Nonlinear Optimal Control Techniques for Vibration Attenuation Using Magnetostrictive Actuators

This article addresses the development of a nonlinear control design for attenuating structural vibrations using magnetostrictive transducers operating in nonlinear and highly hysteretic operating regimes. We consider as a prototype a thin plate subjected to exogenous pressure waves and controlled via Terfenol-D transducers at the plate edges; however, the methodology is sufficiently general to encompass a wide range of structures and magnetic transducer designs. Hysteresis inherent to the transducer materials is quantified using a homogenized energy framework and the resulting nonlinear constitutive relations are used to construct a PDE representation and corresponding finite dimensional model of the structural system. We employ optimal control theory to construct nonlinear open loop control inputs which accommodate the hysteresis inherent to the transducers but are not robust with regard to unmodeled dynamics or disturbances. Robustness is incorporated by employing perturbation techniques to provide linear feedback laws acting on measured disturbances. As illustrated via numerical examples, the resulting hybrid control design provides excellent control authority and robustness for transducers operating in hysteretic and nonlinear regimes.

Experimental Implementation of a Hybrid Nonlinear Control Design for Magnetostrictive Actuators

Abstract : A hybrid nonlinear optimal control design is experimentally implemented on a ferromagnetic Terfenol-D actuator to illustrate enhanced tracking control at relatively high speed. The control design employs a homogenized energy model to quantify rate-dependent nonlinear and hysteretic ferromagnetic switching behavior. The homogenized energy model is incorporated into a finite-dimensional nonlinear optimal control design to directly compensate for the nonlinear and hysteretic ferromagnetic constitutive behavior of the Terfenol-D actuator. Additionally, robustness to operating uncertainties is addressed by incorporating proportional-integral (PI) perturbation feedback around the optimal open loop response. Experimental results illustrate significant improvements in tracking control in comparison to PI control. Accurate displacement tracking is achieved for sinusoidal reference displacements at frequencies up to 1 kHz using the hybrid nonlinear control design whereas tracking errors become significant for the PI controller for frequencies equal to or greater than 500 Hz.

Aerodynamic control of micro air vehicle wings using electroactive membranes

Dielectric elastomer materials are ideal candidates for developing high-agility micro air vehicles due to their electric field–induced deformation. Consequently, the aero-structural response and control authority of the dielectric elastomer material, VHB 4910, are characterized on an elliptical membrane wing. An experimental membrane wing platform was constructed by stretching VHB 4910 over a rigid elliptical wing-frame. The low Reynolds number (chord Reynolds number < 106) and aerodynamics of the elliptical wing were characterized when different electrostatic fields were applied to the membrane. We observe an overall increase in lift with maximum gains of 20% at an applied voltage of 4.5 kV and demonstrate the ability to delay stall. The time-averaged aerodynamic surface pressure is also investigated by comparing sting balance data and membrane deformation measured using visual image correlation. The experimental results are compared to a nonlinear finite element membrane model to further understand the effects of aerodynamic load and electric fields on membrane displacements. Model predictions of surface pressure provide insight into how the electrostrictive constitutive relations influence the fluid–structure interactions of the membrane. This is validated by comparing lift predictions from the model with time-averaged wind tunnel lift measurements near stall.

Flow Physics of a Pulsed Microjet Actuator for High-Speed Flow Control

Flow control actuators based on a small-diameter source jet and a cylindrical cavity structure take advantage of the flow resonance within the cylindrical cavity to generate a variable-frequency, pulsed high-momentum microjet issuing through the cavity orifice. The flow-acoustic coupling, which leads to resonance within the cavity of the actuator, is the main driving mechanism behind the pulsed microjet. In the present study, a computational methodology based on high-order numerical techniques is used to simulate a highly unsteady and compressible pulsed actuator flowfield. Simulation generated flowfield results are analyzed to further understand the complex flow physics governing the pulsed actuator operation. The simulation provides significant details about the highly unsteady and complex microscale actuator flowfield, which are not observable from the experiments. Qualitative comparisons made between the simulated flowfield visualizations and the experimental microschlieren images show a reasonable le...

Peculiar effect of mechanical stress on polarization stability in micrometer-scale ferroelectric capacitors

Piezoresponse force microscopy (PFM) has been used to study the polarization stability in micrometer size Pb(Zr,Ti)O3 capacitors. It is shown that the top electrode thickness has a profound effect on the equilibrium polarization state of poled capacitors triggering spontaneous polarization backswitching in the absence of an applied electric field and leading to the formation of an abnormal domain pattern. PFM examination of poled capacitors with thick (250 nm) top electrodes reveals domain patterns with the central regions always oriented in the direction opposite to the applied field. It is suggested that the driving force behind the observed effect is a transient response to the residual shear stress created by the top electrode in the poled capacitors during field-induced polarization switching. The proposed mechanism is quantified using finite element ferroelectric phase field modeling. The observed effect provides valuable insight into the polarization retention behavior in micrometer size ferroelectric capacitors.Piezoresponse force microscopy (PFM) has been used to study the polarization stability in micrometer size Pb(Zr,Ti)O3 capacitors. It is shown that the top electrode thickness has a profound effect on the equilibrium polarization state of poled capacitors triggering spontaneous polarization backswitching in the absence of an applied electric field and leading to the formation of an abnormal domain pattern. PFM examination of poled capacitors with thick (250 nm) top electrodes reveals domain patterns with the central regions always oriented in the direction opposite to the applied field. It is suggested that the driving force behind the observed effect is a transient response to the residual shear stress created by the top electrode in the poled capacitors during field-induced polarization switching. The proposed mechanism is quantified using finite element ferroelectric phase field modeling. The observed effect provides valuable insight into the polarization retention behavior in micrometer size ferroelect...

Piezoelectric hydraulic pump performance

A piezohydraulic pump making use of the step and repeat capability of piezoelectric actuators has been developed for actuation of aircraft control surfaces. The piezohydraulic pump utilizes a piezoelectric stack actuator to drive a piston in a cylinder. The cylinder is fitted with two check valves. On the compression stroke, oil is forced out of the cylinder. On the intake stroke, oil is drawn into the cylinder. The oil is used to drive a linear actuator. The actuator was driven at 7cm/sec with a 271N (61lb) blocking force. To achieve this, the piezoelectric stack actuator was driven at 60Hz with a switching power supply. The system utilizes an accumulator to eliminate cavitation. This work discusses piezohydraulic pumping theory, pump design, and pump performance. Consideration of pump performance includes the effects of varying accumulator pressure, hydraulic oil viscosity, and load imposed on the linear actuator.

A Unified Material Description for Light Induced Deformation in Azobenzene Polymers.

Complex light-matter interactions in azobenzene polymers have limited our understanding of how photoisomerization induces deformation as a function of the underlying polymer network and form of the light excitation. A unified modeling framework is formulated to advance the understanding of surface deformation and bulk deformation of polymer films that are controlled by linear or circularly polarized light or vortex beams. It is shown that dipole forces strongly respond to polarized light in contrast to higher order quadrupole forces that are often used to describe surface relief grating deformation through a field gradient constitutive law. The modeling results and comparisons with a broad range of photomechanical data in the literature suggest that the molecular structure of the azobenzene monomers dramatically influences the photostrictive behavior. The results provide important insight for designing azobenzene monomers within a polymer network to achieve enhanced photo-responsive deformation.

Crack Initiation at Electrode Edges in PZN-4.5%PT Single Crystals

Fringing electric fields emanating from an electrode edge in electromechanically coupled materials can potentially lead to cracking from a strain incompatibility of the active and inactive regions. Partial electrodes and specimen geometry are studied in single crystal PZN-4.5%PT to characterize the fracture behavior near an electrode edge. Crack growth was characterized by varying the specimen thickness (t 1/4 0.7 and 2 mm) and electrode coverage (50-95%). An applied electric field of 3 MV/m was required to initiate cracks in the 0.7-mm specimens, while an electric field of 2 MV/m was required to initiate cracks in the 2 mm thick specimens. Linear elastic finite element modeling was used to determine the field quantities near the electrode edge and evaluate the internal energy density.