77 research outputs found
A dynamical approach to generate chaos in a micromechanical resonator
Chaotic systems, presenting complex and non-reproducible dynamics, may be
found in nature from the interaction between planets to the evolution of the
weather, but can also be tailored using current technologies for advanced
signal processing. However, the realization of chaotic signal generators
remains challenging, due to the involved dynamics of the underlying physics. In
this paper, we experimentally and numerically present a disruptive approach to
generate a chaotic signal from a micromechanical resonator. This technique
overcomes the long-established complexity of controlling the buckling in
micro/nano-mechanical structures by modulating either the amplitude or the
frequency of the driving force applied to the resonator in the nonlinear
regime. The experimental characteristic parameters of the chaotic regime,
namely the Poincar\'e sections and Lyapunov exponents, are directly comparable
to simulations for different configurations. These results confirm that this
dynamical approach is transposable to any kind of micro/nano-mechanical
resonators, from accelerometers to microphones. We demonstrate a direct
application exploiting the mixing properties of the chaotic regime by
transforming an off-the-shelf microdiaphragm into a true random number
generator conformed to the National Institute of Standards and Technology
specifications. The versatility of this original method opens new paths to
combine chaos' unique properties with microstructures' exceptional sensitivity
leading to emergent microsystems
Experiment and simulation validated analytical equivalent circuit model for piezoelectric micromachined ultrasonic transducers
An analytical Mason equivalent circuit is derived for a circular, clamped plate piezoelectric micromachined ultrasonic transducer (pMUT) design in 31 mode, considering an arbitrary electrode configuration at any axisymmetric vibration mode. The explicit definition of lumped parameters based entirely on geometry, material properties, and defined constants enables straightforward and wide-ranging model implementation for future pMUT design and optimization. Beyond pMUTs, the acoustic impedance model is developed for universal application to any clamped, circular plate system, and operating regimes including relevant simplifications are identified via the wave number-radius product ka. For the single-electrode fundamental vibration mode case, sol-gel Pb(Zr[subscript 0.52])Ti[subscript 0.48]O[subscript 3] (PZT) pMUT cells are microfabricated with varying electrode size to confirm the derived circuit model with electrical impedance measurements. For the first time, experimental and finite element simulation results are successfully applied to validate extensive electrical, mechanical, and acoustic analytical modeling of a pMUT cell for wide-ranging applications including medical ultrasound, nondestructive testing, and range finding.Masdar Institute of Science and Technology (Massachusetts Institute of Technology Cooperative Agreement Grant 6923443)National Science Foundation (U.S.). Graduate Research Fellowshi
A reconfigurable tactile display based on polymer MEMS technology
This research focuses on the development of polymer microfabrication technologies for the realization of two major components of a pneumatic tactile display: a microactuator array and a complementary microvalve (control) array. The concept, fabrication, and characterization of a kinematically-stabilized polymeric microbubble actuator (¡°endoskeletal microbubble actuator¡±) were presented. A systematic design and modeling procedure was carried out to generate an optimized geometry of the corrugated diaphragm to satisfy membrane deflection, force, and stability requirements set forth by the tactile display goals.
A refreshable Braille cell as a tactile display prototype has been developed based on a 2x3 endoskeletal microbubble array and an array of commercial valves. The prototype can provide both a static display (which meets the displacement and force requirement of a Braille display) and vibratory tactile sensations. Along with the above capabilities, the device was designed to meet the criteria of lightness and compactness to permit portable operation. The design is scalable with respect to the number of tactile actuators while still being simple to fabricate.
In order to further reduce the size and cost of the tactile display, a microvalve array can be integrated into the tactile display system to control the pneumatic fluid that actuates the microbubble actuator. A piezoelectrically-driven and hydraulically-amplified polymer microvalve has been designed, fabricated, and tested. An incompressible elastomer was used as a solid hydraulic medium to convert the small axial displacement of a piezoelectric actuator into a large valve head stroke while maintaining a large blocking force. The function of the microvalve as an on-off switch for a pneumatic microbubble tactile actuator was demonstrated. To further reduce the cost of the microvalve, a laterally-stacked multilayer PZT actuator has been fabricated using diced PZT multilayer, high aspect ratio SU-8 photolithography, and molding of electrically conductive polymer composite electrodes.Ph.D.Committee Chair: Allen,Mark; Committee Member: Bucknall,David; Committee Member: Book,Wayne; Committee Member: Griffin,Anselm; Committee Member: Yao,Donggan
Acoustically and electrokinetically driven transport in microfluidic devices
Electrokinetically driven flows are widely employed as a primary method for liquid pumping in micro-electromechanical systems. Mixing of analytes and reagents is limited in microfluidic devices due to the low Reynolds number of the flows. Acoustic excitations have recently been suggested to promote mixing in the microscale flow systems.Electrokinetic flows through straight microchannels were investigated using the Poisson-Boltzmann and Nernst-Planck models. The acoustic wave/fluid flow interactions in a microchannel were investigated via the development of two and three-dimensional dynamic predictive models for flows with field couplings of the electrical, mechanical and fluid flow quantities. The effectiveness and applicability of electrokinetic augmentation in flexural plate wave micropumps for enhanced capabilities were explored. The proposed concept can be exploited to integrate micropumps into complex microfluidic chips improving the portability of micro-total-analysis systems along with the capabilities of actively controlling acoustics and electrokinetics for micro-mixer applications.Acoustically excited flows in microchannels consisting of flexural plate wave devices and thin film resonators were considered. Compressible flow fields were considered to accommodate the acoustic excitations produced by a vibrating wall. The velocity and pressure profiles for different parameters including frequency, channel height, wave amplitude and length were investigated. Coupled electrokinetics and acoustics cases were investigated while the electric field intensity of the electrokinetic body forces and actuation frequency of acoustic excitations were varied. Multifield analysis of a piezoelectrically actuated valveless micropump was also presented. The effect of voltage and frequency on membrane deflection and flow rate were investigated. Detailed fluid/solid deformation coupled simulations of piezoelectric valveless micropump have been conducted to predict the generated time averaged flow rates. Developed coupled solid and fluid mechanics models can be utilized to integrate flow-through sensors with microfluidic chips.Ph.D., Mechanical Engineering -- Drexel University, 201
Design, fabrication and characterisation of silicon carbide resonators
Micro-electro-mechanical systems (MEMS) are integrated mechanical and electrical
elements realised with micro-fabrication technology and employed as sensors and actuators. The integration of reliable MEMS switches and resonators into transceiver
devices is a challenging and attractive solution to increase the efficiency and reduce the
power consumption. Silicon carbide (SiC) is an excellent candidate for developing robust and reliable high frequency MEMS for transceivers applications due to its unique
mechanical properties.This thesis presents the design, fabrication and characterisation of 3C-SiC micromechanical vertical resonators. New device architectures have been developed for the
study of the electro-mechanical behaviour of the devices with the aim of optimising
the actuation efficiency, increasing the resonant frequency and obtaining new device
functions.A process for the fabrication of single or poly-crystalline 3C-SiC cantilevers, bridges
and rings has been developed with the option of integrating top electrodes made of
aluminium (Al) or lead zirconium titanate (PZT). The crystal structure and quality
of the SiC layers have been evaluated with X-ray diffraction and Raman spectroscopy.
A Young's Modulus of ~ 440 GPa has been calculated for the single crystalline SiC
from the mechanical resonant frequency of the fabricated single material cantilevers.
The fabricated Al/SiC bridges and rings have been actuated and driven into resonance electro-thermally. It has been found that wide Al electrodes applied close to the
beams' anchor can maximise the induced displacement and vibration amplitude thus
improving the actuation efficiency. Resonant frequencies in the MHz range have been
obtained with the ring architectures therefore achieving higher frequencies compared
to beam architectures. In addition, electro-thermal mixing of two input frequencies has
been demonstrated and performed with the fabricated Al/SiC structures. Furthermore,
piezo-electric transduction has been used for actuating the PZT/SiC cantilevers and
for sensing the devices' resonance electrically. The design of the PZT piezo-electric
active layer has been shown to influence strongly the devices' resonant frequency and
has been optimised to enhance the electrical output by decreasing the electrodes length
thus decreasing the feedthrough capacitance.The results obtained in this work can be used for the implementation of SiC MEMS
mixer-filters with electro-thermal actuation and piezo-electric sensing for transceiver
applications
Towards the noise reduction of piezoelectrical-driven synthetic jet actuators
This paper details an experimental investigation aimed at reducing the noise output of piezoelectrical-driven synthetic jet actuators without compromising peak jet velocity. Specifically, the study considers double-chamber ('back-to-back') actuators for anti-phase noise suppression and corrugated-lobed orifices as a method to enhance turbulent mixing of the jets to suppress jet noise. The study involved the design, manufacture and bench test of interchangeable actuator hardware. Hot-wire anemometry and microphone recordings were employed to acquire velocity and noise measurements respectively for each chamber configuration and orifice plate across a range of excitation frequencies and for a fixed input voltage. The data analysis indicated a 32% noise reduction (20 dBA) from operating a singlechamber, circular orifice SJA to a double-chamber, corrugated-lobed orifice SJA at the Helmholtz resonant frequency. Results also showed there was a small reduction in peak jet velocity of 7% (~3 m/s) between these two cases based on orifices of the same discharge area. Finally, the electrical-to-fluidic power conversion efficiency of the double-chamber actuator was found to be 15% across all orifice designs at the resonant frequency; approximately double the efficiency of a single-chamber actuator. This work has thus demonstrated feasible gains in noise reduction and power efficiency through synthetic jet actuator design
Development of graphene-based microelectromechanical systems for acoustic sensing
Graphene has been considered to be a desirable material in the application of
semiconductor devices for the next generation due to its outstanding electrical
and mechanical properties. In this thesis, the research focuses on the realization
of graphene-based acoustic microelectromechanical systems (MEMS). The
applications of acoustic MEMS include microphones, hearing aids and ultrasound
identification and non-contact testing. Apart from acoustic technology, the
graphene-based MEMS designs can be applied in areas for sensing and actuation
purpose, such as pressure detectors, micro-drums and ultrasensitive mass sensors.
The performance of the devices is determined by the structures of devices, materials
properties, dimensions, and the spacing between the membranes and the
substrate.
In this project, for the first time, the resonant frequency of graphene-based
acoustic sensors has been extended to lower ultrasonic frequency range (20 kHz
to 200 kHz). Additionally, a modified dry transfer method with Kapton tape and
a novel graphene transfer method with silicon dioxide sacrificial layer have been
developed for millimetre-size graphene membranes. To be more specific, three
types of devices′ structures, including open cavity, closed cavity and partly open
cavity, have been developed, in order to detect the frequency for both audio and
ultrasound range (from 11 kHz to 200 kHz). 450 nm polymethyl methacrylate
(PMMA) layer has been laminated onto 6-layer graphene to support and form
millimetre-size bi-layer membrane. The open cavity resonator for ultrasound
sensing has been fabricated with graphene wet transfer process. For closed cavity
resonators, a modified dry transfer method with the use of Kapton tape frame
has been developed. Using the modified dry transfer method, it is the first time
that the millimetre-size graphene/PMMA have been transferred and suspended
over the closed cavity. Due to good gas encapsulation of graphene/PMMA closed
cavity devices, the vibration of membrane has been prevented due to the air
damping when the air gap is decreasing. For the purpose of increasing the
capacitance between membrane and substrate and improving the electrical output
signal, the air gap should be optimized and decreased. Thus, the partly open
structure has been designed for the realization of the graphene/PMMA electrostatic
sensors. The graphene/PMMA membrane has been released by etching
silicon dioxide sacrificial layer. The air gap of 2 μm of between the millimetre-size
graphene-based membrane and the substrate has been achieved for the first time
and reported to be minimum among the literature. Furthermore, the dynamic
behaviour of the devices have been characterized with laser Doppler Vibrometer
(LDV), the confirmation of graphene has been detected by Raman spectroscopy.
Finite element analysis has been applied for the simulation of membranes′ dynamic
behaviour. The static deformation of graphene after modified dry transfer
method has been measured by white light interferometry (WLI). The realization
of graphene/PMMA acoustic devices paves the way to the integration of graphene
with MEMS to achieve sensors with high sensitivity
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