Miles A. Wickersham

Active control of stiffness and damping of piezoelectric polymer films

This paper investigates a method for actively controlling the stiffness and damping provided by piezoelectric films such as may be used to construct biomimetic skins on small aerial vehicles. The method being investigated is based on the idea of elasticity control via piezoelectric coupling, and uses a tunable electronic circuit in parallel with a polyvinilidene fluoride (PVDF) film. The focus of the current work is on a fundamental-level understanding of the elasticity control method, and in particular, on theoretically and experimentally characterizing the degree of dissipation control possible with this method. The paper discusses the theoretical and experimental work so far which shows encouraging improvements in the dissipation in response to structural loads. Particular emphasis here is on modeling of the impulse response of a PVDF membrane. Work so far shows reasonable agreement between analytical and experimental results. Finally, a control circuit based on a low-power operational amplifier is seen to be effective in significantly improving the dissipation rate available with the PVDF membrane.

Altering the natural frequencies of cantilevered piezoelectric membranes using negative capacitance circuits

A negative capacitance circuit has been designed to change the effective natural frequency of a fixed-free piezoelectric strip attached to a non piezoelectric substrate. Experiments have investigated the extent to which resonances can be shifted using a redesigned negative capacitance circuit. The design replaces the resistive element in the feedback loop with a capacitor, effectively causing the behavior of the circuit to become frequency independent. A PVDF film was mechanically excited and the voltage generated from the piezoelectric effect fed to the circuit. This paper summarizes the theoretical model and describes ongoing experimental work.

Membrane Mirrors with Boundary Located Electrostatic Actuators for excitation of Multiple Modes

This paper investigates electrostatic boundary actuation of a circular deformable mirror for use in beam shaping and adaptive optics. The circumferential electrostatic actuator electrode pattern used here is designed to produce multiple-mode mirror deformations. The design, analysis, and fabrication processes are described herein. Closed loop control to be applied is illustrated by means of an example involving a similar mirror with different actuation placement. A summarized discussion of previous closed loop results is also presented. It is shown that open loop control of this electrostatic actuation application is not feasible, and that closed loop actuation is necessary to meet design deflection requirements. This design enables several more aberration correction modes than any of our existing mirror designs, while making it possible to apply our current closed loop control approach with appropriate extensions.

The effect of a negative capacitance circuit on the out-of-plane dissipation and stiffness of a piezoelectric membrane

This paper investigates the out-of-plane dynamics of a piezoelectric membrane with a negative capacitance circuit connected in parallel. The theoretically possible large changes in stiffness and dissipation could, at full implementation, enable the design of selectively electroded piezoelectric skins that are spatially and temporally highly adaptive. A tunable negative capacitance circuit is here connected in parallel with a polyvinylidene fluoride (PVDF) membrane (Date et al 2000 J. Appl. Phys. 87 863–8). The goal of this paper is to investigate the extent to which such a circuit may influence the out-of-plane stiffness and dissipation characteristics and the associated vibration response of a PVDF membrane strip. We consider strips with a non-vanishing thickness and under a constant boundary-applied tension. As our analytical and experimental results for harmonic loading show, successful application of the method on out-of-plane dynamics is possible with a correct choice of the circuit parameters, and the out-of-plane stiffness and dissipation and the associated vibration response near the first resonance can be altered noticeably by adjusting the circuit settings.

Active Control of Material Damping in a Piezoelectric Membrane

This paper presents ongoing work related to active tuning of certain elastic properties of thin piezoelectric membranes, which could find application in aerospace situations where lightweight adaptive structures are required. The method being investigated here is based on the idea of elasticity control (i.e., stiness and dissipation) via piezoelectric coupling, and uses a pre-tunable electronic circuit in parallel with a polyvinilidene fluoride (PVDF) film. In a practical implementation, PVDF film actuators would be strategically located over a large membrane structure, or located exclusively near the boundary for small mirror type devices to provide a degree of elastic tunability to the whole structure. The focus of the current work is on a fundamental-level understanding of the elasticity control method, and in particular, on theoretically and experimentally characterizing the degree of dissipation control possible with this method. The paper discusses the theoretical and experimental work so far which shows encouraging improvements in the dissipation in response to both structural and acoustic loads. Particular emphasis here is on analytical modeling of the response of a PVDF membrane to structural impulsive loads and acoustic harmonic loads. Work so far shows reasonable agreement between analytical and experimental results for the structural response. Finally, a control circuit based on a low-power operational amplifier is seen to be eective in significantly improving the dissipation rate available with the PVDF membrane.

Study of a tuned vibration absorber using piezoelectric patches with active shunt circuits

A patch of piezoelectric material driving a negative impedance shunt circuit can be attached to a flexible structure for vibration damping as well as altering the effective stiffness of the overall structure and shift its resonant frequency. This work uses a truly coupled mechanical/electrical analysis where the negative impedance converter (NIC) circuit is modeled using fundamental operational analysis modeling technique, enabling a straightforward analysis of circuit stability, while clarifying the effect of each parameter in the NIC circuit on the overall circuit impedance, and ultimately, the mechanical response of the structure. Two types of piezoelectric materials are considered, a piezoelectric polymer and a macrofiber composite. Also examined in this work is an alternative approach to load impedance tuning which seeks circuit parameter settings that equate the load impedance to the complex conjugate of the mechanical impedance of the piezoelectric for a particular out-of-plane vibration mode. Additionally, the effects of circuit stability and variations of the reference capacitor are investigated. Both theoretical simulations and experimental results are presented.

Experiments on a Twelve Mode Membrane Mirror with Boundary Located Electrostatic Actuators

This paper continues the investigation of the multiple-mode electrostatic boundary actuation technique and reports on the ongoing mathematical model development, surface metrology testing as well as the continued updating of the system materials and construction techniques. Static testing has shown that the mirror deforms in the desired convex and concave mirror pattern with the actuation of inside and outside ring of electrostatic actuation pads. Design upgrades of the mirror stage as well as the deformable mirror itself are underway and will supply the system with more stability and a higher reflective quality. An extension of our current closed loop control method and its use on our new mirror design will enable the aberration correction of more modes than our existing mirror design.

Electrostatic control with discrete area variation for beam steering and focusing using membrane mirrors

This paper reports on the development and testing of electrostatically actuated deformable mirrors for optical correction. The system considered here is limited to the lower modes of aberration; namely, focus/defocus and tip/tilt. The main problem with using electrostatics is due to the nonlinear relationship between force and distance in such a system. Accordingly, this work uses a nonlinear control system in order to obtain greater deflection for a given voltage. The paper describes recent experimental results with closed loop control.

Electronic Control of Stiffness and Dissipation of Piezoelectric Polymer Membranes

This paper discusses a method of stiffness and dissipation control based on piezoelectric coupling with applications in vibration damping. The study focuses on thin piezoelectric film strips modeled with a non-vanishing thickness and a constant boundary tension. Both flexural stiffness and in-plane tension are accounted for in setting up the partial differential equation of motion. Simulation results are presented and compared with experimental results based on harmonic excitation. Nomenclature T e ⎡⎤ = 3 × 3 matrix of electric permittivity coefficients at constant stress

On semi-active enhancement of the dissipation provided by piezoelectric films

Light weight flexible structures designed for space application may be well served by an ability locally to tune the dissipation and stiffness of the structural element. The method investigated here is based on a combination of a piezoelectric strip and an operational amplifier based active circuit which enables control of the effective impedance over a wide range. In this paper, we discuss an analytical model substantially reformulated from our previous work to capture the direct link between membrane tension and voltage across the circuit. It is observed that when tuned for negative impedance, the circuit enables significantly enhanced dissipation of vibrations due to external loads. Theoretical and experimental results are discussed here for the response non-laminated films to line-impact loads. The analytical results presented here account for dissipation and added mass effects of air. A laser doppler vibrometer is used to provide a comparison for the voltage measurements across the piezoelectric strip electrodes.