KEYWORDS: Capacitance, Ferroelectric polymers, Microsoft Foundation Class Library, Cesium, Capacitors, Circuit switching, Polymers, Composites, Energy transfer, Energy harvesting
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.
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.
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.
Space structures would benefit greatly from an ability to tune the dissipation and stiffness of the structural
element. This would provide a compromise between large passive systems, and complex, real-time, active control
implementations. Different elements of a structure could be altered based on the loads that they experience.
This study will focus on thin piezoelectric film strips connected in parallel with an electronic circuit which
provides a "negative capacitance," and an electrical load consisting of a resistor and a capacitor. Due to
the inverse piezoelectric effect, each film forms an electromechanical system in conjunction with the parallel
circuit. The overall impedance of this system can be controlled by correctly varying gain parameters within the
circuit. This work models the PVDF strips of non-vanishing thickness and stretched under a constant, boundary
applied tension. Both flexural stiffness and in-plane tension are accounted for in setting up the partial differential
equation of motion. Harmonic excitation was provided with an acoustic speaker driven by a wave form generator.
Measurements of out-of-plane deflection at a chosen point were taken using an LED/photodiode pair, which was
calibrated experimentally. The voltage developed between the electrodes was also measured. Theoretical and
experimental results are analyzed and compared.
The following is an investigation into the dynamic behavior of small deformable mirrors based on thin, metallized
membranes. Focusing on providing a predetermined focus/defocus correction to a beam, as well as producing
specified angular deflections of the beam in the vertical and horizontal planes. Directing the mirrored surface
is accomplished using electrostatic actuation. Current designs are comprised of 3 actuator pads fabricated on a
fiber reinforced plastic substrate that drive a metallized kapton membrane, which is separated from the substrate
by spacers that provide a known air gap.
A previous paper[3] consisted of a variable area actuation strategy that would only allow membrane deflection
of 1/3 the total gap size before incurring instability due to "snap down". Addressed here is the proingblem of
extend the control strategy into the deflection regime where the full nonlinear model must be used for the
actuation force. A solution to this problem is an extended controller that can handle the full deflection range
of the 40m air gap between the charged electrodes on the fixed substrate and the movable metallized reflective
membrane.
The observer for the control system operates in continuous time mode. Although the discrete area approximation
is also shown alongside, only the continuous-area approximation is studied here. From the continuous-area
approximation it is easily seen that the open loop system would be unstable, while the closed loop system closely
follows the desired reference specification (maximum deflection approaching 40m, and bandwidth approaching
500 Hz).
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.
This paper discusses small deformable mirrors based on thin metallized membranes and designed for steering and
focusing of beams as well as providing correction for a small number of optical aberration modes. We consider
circular membranes driven electrostatically by four electrodes deposited on or milled into a light weight substrate.
The metallized surface on the membrane, while acting as a mirror also provides the ground electrode to enable
actuation. Here we discuss prototype testing of the mirrors based on different production techniques, and report
on closed loop control studies using two formulations: (1) an approximate lumped-parameter model based on
a single-input-single-output linearized treatment, and (2) a linearized, continuous parameter model leading to
a multiple-input-multiple-output state space system via modal expansion. A quadrant photo-detector ("quad cell") is used as an output sensor.
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.
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