A. James Clark School of Engineering
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The collections in this community comprise faculty research works, as well as graduate theses and dissertations.
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Item Microfabricated Bulk Piezoelectric Transformers(2017) Barham, Oliver M.; DeVoe, Don L; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Piezoelectric voltage transformers (PTs) can be used to transform an input voltage into a different, required output voltage needed in electronic and electro- mechanical systems, among other varied uses. On the macro scale, they have been commercialized in electronics powering consumer laptop liquid crystal displays, and compete with an older, more prevalent technology, inductive electromagnetic volt- age transformers (EMTs). The present work investigates PTs on smaller size scales that are currently in the academic research sphere, with an eye towards applications including micro-robotics and other small-scale electronic and electromechanical sys- tems. PTs and EMTs are compared on the basis of power and energy density, with PTs trending towards higher values of power and energy density, comparatively, indicating their suitability for small-scale systems. Among PT topologies, bulk disc-type PTs, operating in their fundamental radial extension mode, and free-free beam PTs, operating in their fundamental length extensional mode, are good can- didates for microfabrication and are considered here. Analytical modeling based on the Extended Hamilton Method is used to predict device performance and integrate mechanical tethering as a boundary condition. This model differs from previous PT models in that the electric enthalpy is used to derive constituent equations of motion with Hamilton’s Method, and therefore this approach is also more generally applica- ble to other piezoelectric systems outside of the present work. Prototype devices are microfabricated using a two mask process consisting of traditional photolithography combined with micropowder blasting, and are tested with various output electri- cal loads. 4mm diameter tethered disc PTs on the order of .002cm^3 , two orders smaller than the bulk PT literature, had the following performance: a prototype with electrode area ratio (input area / output area) = 1 had peak gain of 2.3 (± 0.1), efficiency of 33 (± 0.1)% and output power density of 51.3 (± 4.0)W cm^-3 (for output power of 80 (± 6)mW) at 1MΩ load, for an input voltage range of 3V-6V (± one standard deviation). The gain results are similar to those of several much larger bulk devices in the literature, but the efficiencies of the present devices are lower. Rectangular topology, free-free beam devices were also microfabricated across 3 or- ders of scale by volume, with the smallest device on the order of .00002cm^3 . These devices exhibited higher quality factors and efficiencies, in some cases, compared to circular devices, but lower peak gain (by roughly 1/2 ). Limitations of the microfab- rication process are determined, and future work is proposed. Overall, the devices fabricated in the present work show promise for integration into small-scale engi- neered systems, but improvements can be made in efficiency, and potentially voltage gain, depending on the applicationItem Miniaturized Thin-Film Piezoelectric Traveling Wave Ultrasonic Motor(2014) Rudy, Ryan Quenon; DeVoe, Don L; Polcawich, Ronald G; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)For many small-scale systems, compact rotary actuators are highly attractive. Many current millimeter-scale motor technologies, such as electrostatic motors and electromagnetic motors, operate at high speeds (on the order of 105 RPM) but low torque, usually pico- or nano-newton-meters. In order to drive large loads at speeds closer to 100 to 1000 RPM, gearing would be required, which drastically increases system complexity and size. Electromagnetic motors, which are effective at the macro-scale, become less practical at the millimeter-scale due to unfavorable scaling of energy density and complex fabrication. Electrostatic micro-motors require approximately 100 V for operation and produce limited torque. Traveling wave ultrasonic motors (TWUM) can provide micro- to milli-newton-meters of torque at low speeds and fill a necessary place within the millimeter-scale rotary motor landscape. Using recent developments in high quality piezoelectric film deposition and microfabrication techniques, TWUM can be made an order of magnitude smaller than currently possible. The fabrication process for the TWUM is described within, with a focus on stator fabrication and the enabling fabrication methods developed for the manufacture of TWUM, including backside vapor-HF release, deep reactive ion etch footing release, and photoresist deep-trench refill. Design and characterization of the traveling wave stator component, both disc and ring are described. Disc stators, 1 to 3 mm in diameter, exhibited traveling waves up to 1 μm in out-of-plane amplitude with quality factors in air of 95. The design process for ring stators with mechanical impedance transformer tethers is presented. The tethers are designed to allow large motion at the stator perimeter, while tethering the stator to the anchored substrate. This mechanical impedance transformer tether allowed for an in increase in standing wave amplitude by over 100% compared to straight tethers. TWUM were demonstrated and characterized, and represent the smallest TWUM currently reported, at 2 to 3 mm in diameter and less than 1 mm thick. Motor performance characteristics are presented, with speeds exceeding 2000 RPM while consuming 4 mW of power at 10 V. These millimeter-scale motors have potential applications in fields such as fuzing, medical imaging, micro-robotics, and sensor steering and calibration.Item Thermoacoustic-Piezoelectric Systems with Dynamic Magnifiers(2013) Nouh, Mostafa Akram; Baz, Amr; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Thermoacoustic energy conversion is an emergent technology with considerable potential for research, development, and innovation. In thermoacoustic resonators, self-excited acoustic oscillations are induced in a working gas by means of a temperature gradient across a porous body and vice versa with no need of moving parts. In the first part of this dissertation, thermoacoustic resonators are integrated with piezoelectric membranes to create a new class of energy harvesters. The incident acoustic waves impinge on a piezo-diaphragm located at one end of the thermoacoustic-piezoelectric (TAP) resonator to generate an electrical power output. The TAP design is enhanced by appending the resonator with an elastic structure aimed at enhancing the strain experienced by the piezo-element to magnify the electric energy produced for the same input acoustic power. An analytical approach to model the thermal, acoustical, mechanical and electrical domains of the developed harvester is introduced and optimized. The performance of the harvesters is compared with experimental data obtained from an in-house built prototype with similar dimensions. In an attempt to further understand the dynamics and transient behavior of the excited waves in the presence of piezoelectric coupling, a novel approach to compute and accurately predict critical temperature gradients that onset the acoustic waves is discussed. The developed model encompasses tools from electric circuit analogy of the lumped acoustical and mechanical components to unify the modeling domain. In the second part of the dissertation, piezo-driven thermoacoustic refrigerators (PDTARs) are presented. The PDTARs rely on the inverse thermoacoustic effect for their operation. A high amplitude pressure wave in a working medium is used to create a temperature gradient across the ends of a porous body located in an acoustic resonator. Finally, PDTARs with dynamic magnifiers are introduced. The developed design is shown, theoretically and experimentally, as capable of potentially enhancing the cooling effect of PDTARs by increasing the temperature gradient created across the porous body.Item Swashplateless Helicopter Experimental Investigation: Primary Control with Trailing Edge Flaps Actuated with Piezobenders(2013) Copp, Peter Andrew Pusey; Chopra, Inderjit; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Helicopter rotor primary control is conventionally carried out using a swashplate with pitch links. Eliminating the swashplate promises to reduce the helicopter's parasitic power in high speed forward flight, as well as may lead to a hydraulic-less vehicle. A Mach-scale swashplateless rotor is designed with integrated piezobender-actuated trailing edge flaps and systematically tested on the benchtop, in the vacuum chamber and on the hoverstand. The blade is nominally based on the UH-60 rotor with a hover tip Mach number of 0.64. The blade diameter is 66 inches requiring 2400 RPM for Mach scale simulation. The rotor hub is modified to reduce the blade fundamental torsional frequency to less than 2.0/rev by replacing the rigid pitch links with linear springs, which results in an increase of the blade pitching response to the trailing edge flaps. Piezoelectric multilayer benders provide the necessary bandwidth, stroke and stiffness to drive the flaps for primary control while fitting inside the blade profile and withstanding the high centrifugal forces. This work focuses on several key issues. A piezobender designed from a soft piezoelectric material, PZT-5K4, is constructed. The new material is used to construct multi-layer benders with increased stroke for the same stiffness relative to hard materials such as PZT-5H2. Each layer has a thickness of 10 mils. The soft material with gold electrodes requires a different bonding method than hard material with nickel electrodes. With this new bonding method, the measured stiffness matches precisely the predicted stiffness for a 12 layer bender with 1.26 inch length and 1.0 inch width with a stiffness of 1.04 lb/mil. The final in-blade bender has a length of 1.38 inches and 1.0 inch width with a stiffness of 0.325 lb/mil and stroke of 20.2 mils for an energy output of 66.3 lb-mil. The behavior of piezobenders under very high electric fields is investigated. High field means +18.9 kV/cm (limited by arcing in air) and -3.54kV/cm (limited by depoling). An undocumented phenomenon is found called bender relaxation where the benders lose over half of their initial DC stroke over time. While the bender stiffness is shown not to change with electric field, the DC stroke is significantly less than AC stroke. A two-bladed Mach-scale rotor is constructed with each blade containing 2 flaps each actuated by a single piezobender. Each flap is 26.5% chord and 14% span for a total of 28% span centered at 75% of the blade radius. Flap motion of greater than 10 degrees half peak-peak is obtained for all 4 flaps at 900 RPM on the hoverstand. So, the flaps show promise for the Mach-scale rotor speed of 2400 RPM. A PID loop is implemented for closed loop control of flap amplitude and mean position. On the hoverstand at 900 RPM, the swashplateless concept is demonstrated. The linear springs used to lower the torsional frequency are shown to have minimum friction during rotation. 1/rev blade pitching of ±1 degree is achieved at a torsional frequency of 1.5/rev for each blade. At resonance, the blade pitching for each blade is greater than ±4 degrees. Primary control is demonstrated by measuring hub forces and moments. At resonance state, the flaps in conjunction with the blade pitching provide ±15 lbs of normal force at a mean lift of 15 lbs yielding ±100% lift authority. Significant hub forces and moments are produced as well. For a production swashplateless helicopter, it may be prudent to eliminate the pitch links by reducing the blade structural stiffness. A novel wire sensor system network is proposed in order to measure blade elastic flap bending, lead-lag bending and torsion. The theory for measuring blade twist is rigorously derived. A blade is constructed with the wire sensor network and validated on the benchtop for blade elastic bending and twist. This work is a step forward in achieving a swashplateless rotor system. Not only would this reduce drag in high speed forward flight, but it would lead to a hydraulic-less rotorcraft. This would be a major step in vertical flight aviation.Item Pneumatic Artificial Muscle Driven Trailing Edge Flaps For Active Rotors(2011) Woods, Benjamin King Sutton; Wereley, Norman M; Kothera, Curt S; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This research focuses on the development of an active rotor system capable of primary control and vibration reduction for rotorcraft. The objective is to investigate the feasibility of a novel Trailing Edge Flap (TEF) actuation system driven by Pneumatic Artificial Muscles (PAMs). A significant design effort led to a series of experimental apparatuses which tested various aspects of the performance of the actuators themselves and of TEF systems driven by them. Analytical models were developed in parallel to predict the quasistatic and dynamic behavior of these systems. Initial testing of a prototype blade section with an integrated PAM driven TEF proved the viability of the concept through successful benchtop testing under simulated M = 0.3 loading and open jet wind tunnel tests under airspeeds up to M = 0.13. This prototype showed the ability of PAM actuators to generate significant flap deflections over the bandwidth of interest for primary control and vibration reduction on a rotorcraft. It also identified the importance of high pneumatic system mass flow rate for maintaining performance at higher operating frequencies. Research into the development and improvement of PAM actuators centered around a new manufacturing technique which was invented to directly address the weaknesses of previous designs. Detailed finite element model (FEM) analysis of the design allowed for the mitigation of stress concentrations, leading to increased strength. Tensile testing of the swaged actuators showed a factor of safety over 5, and burst pressure testing showed a factor of safety of 3. Over 120,000,000 load cycles were applied to the actuators without failure. Characterization testing before, during, and after the fatigue tests showed no reduction in PAM performance. Wind tunnel testing of a full scale Bell 407 blade retrofitted with a PAM TEF system showed excellent control authority. At the maximum wind tunnel test speed of M = 0.3 and a derated PAM operating pressure of 28 psi, 18.8° half-peak-to-peak flap deflections were achieved at 1/rev (7 Hz), and 17.1° of half-peak-to-peak flap deflection was still available at 5/rev (35 Hz). A quasistatic system model was developed which combined PAM forces, kinematics and flap aerodynamics to predict flap deflection amplitudes. This model agreed well with experimental data. Whirl testing of a sub-span whirl rig under full scale loading conditions showed the ability of PAM TEF systems to operate under full scale levels of centrifugal (CF), aerodynamic, and inertia loading. A commercial pneumatic rotary union was used to provide air in the rotating frame. Extrapolation of the results to 100% of CF acceleration predicts 15.4° of half-peak-to-peak flap deflection at 1/rev (7 Hz), and 7.7° of half-peak-to-peak flap deflection at 5/rev (35 Hz). A dynamic model was developed which successfully predicted the time domain behavior of the PAM actuators and PAM TEF system. This model includes control valve dynamics, frictional tubing losses, pressure dynamics, PAM forces, mechanism kinematics, aerodynamic hinge moments, system stiffness, damping, and inertia to solve for the rotational dynamics of the flap. Control system development led to a closed loop control system for PAM TEF systems capable of tracking complex, multi-sinusoid flap deflections representative of a combined primary control and vibration reduction flap actuation scheme. This research shows the promise that PAM actuators have as drivers for trailing edge flaps on active helicopter rotors. The robustness, ease of integration, control authority and tracking accuracy of these actuators have been established, thereby motivating further research.Item Piezoelectrically Driven Thermoacoustic Refrigerator(2010) Chinn, Daniel George; Baz, Amr; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Thermoacoustic refrigeration is an emerging refrigeration technology which does not require any moving parts or harmful refrigerants in its operation. This technology uses acoustic waves to pump heat across a temperature gradient. The vast majority of thermoacoustic refrigerators to date have used electromagnetic loudspeakers to generate the acoustic input. In this thesis, the design, construction, operation, and modeling of a piezoelectrically-driven thermoacoustic refrigerator are detailed. This refrigerator demonstrates the effectiveness of piezoelectric actuation in moving 0.3 W of heat across an 18 degree C temperature difference with an input power of 7.6 W. The performance characteristics of this class of thermoacoustic-piezoelectric refrigerators are modeled by using DeltaEC software and the predictions are experimentally validated. The obtained results confirm the validity of the developed model. Furthermore, the potential of piezoelectric actuation as effective means for driving thermoacoustic refrigerators is demonstrated as compared to the conventional electromagnetic loudspeakers which are heavy and require high actuation energy. The developed theoretical and experimental tools can serve as invaluable means for the design and testing of other piezoelectrically-driven thermoacoustic refrigerator configurations.Item Piezoelectric Microbeam Resonators Based on Epitaxial Al0.3Ga0.7As Films(2005-11-22) Li, Lihua; DeVoe, Don; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)In this work, piezoelectric resonators based on single crystal Al0.3Ga0.7As films are implemented. The combination of Si doped Al0.3Ga0.7As as electrode layers and moderate piezoelectric properties of updoped Al0.3Ga0.7As film leads to lattice matched single crystal resonators with high attainable quality factors and capability of integration with high speed circuits. To validate the fabrication process, simple cantilever beam structures are developed and characterized by laser Doppler vibrometry. In order to achieve higher center frequencies, a clamped-clamped (c-c) beam design is explored. Important resonator parameters including resonance frequency, quality factor, and power handling ability are investigated. Measured quality factors of c-c beams were found to be limited by anchor losses to the substrate. A free-free (f-f) beam design is proposed in order to alleviate the energy dissipation due to anchor losses. Fabricated f-f beam devices show increased quality factors compared to the c-c beam design. Another improvement is the adoption of bimorph configuration instead of unimorph configuration. Compared to unimorph cantilever beam design, bimorph cantilevers showed 80% to 120% of increase in displacement with the same driving voltage without significant change in quality factors. The quality factors of flexural mode resonators in atmospheric pressure are low due to the effect of air damping. For this reason, proper working of flexural mode resonators requires a vacuum package which imposes unwanted complexity in packaging. To solve this problem, length-extensional mode resonators (bar resonators) are proposed to take advantage of low air shear damping. Bar resonators with lengths ranging from 1000 micro-m to 100 mico-m have been fabricated and tested. Measured resonant frequencies range from 2.5 MHz to 72 MHz with good matching to theoretical predictions. The quality factors of bar resonators at their first resonant frequency are measured in air and in high vacuum, showing values between 4,300 - 8,900 and 8,000 - 17,000, respectively, with corresponding measured motional resistances of 7.3 kohm - 10.5 kohm and 4.0 kohm - 7.8 kohm, respectively. The developed bar resonators showed excellent power handling ability up to -10 dBm which is much higher than equivalent electrostatic resonators.