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A nonlinear active vibration absorber to control the vibrations of plates is investigated. The absorber is based on the saturation phenomenon associated with dynamical systems with quadratic nonlinearities and a two-to-one internal resonance. The technique is implemented by coupling a second-order controller with the plate's response through a sensor and an actuator. Energy is exchanged between the primary structure and the controller and, near resonance, the plate's response saturates to a small value. Numerical as well as experimental results are presented for a cantilever rectangular plate. For numerical studies, finite-element methods as well as modal analysis are implemented. The commercially available software ABAQUS is used in the finite-element analysis together with a user-provided subroutine to model the controller. For the experimental studies, the plate is excited using a dynamic shaker. Strain gages are used as sensors, while piezoelectric ceramic patches are used as actuators. The control technique is implemented using a dSPACE digital signal processing board and a modeling software (SIMULINK). Both numerical and experimental results show that the control strategy is very efficient. A numerical study is conducted to optimize the location of the actuators on the structure to maximize its controllability. In this regard, the control gain is maximized for the PZT actuators. Furthermore, a more general method is introduced that is based on a global measure of controllability for linear systems. Finally, the control strategy is made adaptive by incorporating an efficient frequency-measurement technique. This is validated by successfully testing the control strategy for a non-conventional problem, where nonlinear effects hinder the application of the non-adaptive controller. / Ph. D.

We tackle the problem of suppressing high-amplitude vibrations of cantilever beams when subjected to either primary external or principal parametric resonances. Guided by results of previous investigations into the nonlinear dynamics of single- and multi-degree-of-freedom structures, we design mechatronic systems of sensors, actuators, and electronic devices and implement nonlinear active feedback control. In the case of external excitation, we devise two vibration absorbers based on either quadratic or cubic feedback. We conduct theoretical analyses and demonstrate that when a two-to-one (one-to-one) internal resonance condition is imposed between the plant and the quadratic (cubic) absorber, there exists a saturation phenomenon. When the plant is forced near its resonant frequency and the forcing amplitude exceeds a certain small threshold, the nonlinear coupling creates an energy-transfer mechanism that limits (saturates) the response of the plant. Our theoretical studies reveal that the cubic absorber creates regimes of high-amplitude quasiperiodic and chaotic responses, thereby limiting its utility. However, we show that superior results can be achieved when the natural frequency of the quadratic absorber is set equal to one-half the excitation frequency. Consequently, we apply the quadratic technique through a variety of linear and nonlinear actuators, sensors, and electronic devices. We design and build second-order analog circuits that emulate the quadratic absorber. Using a DC motor, piezoelectric ceramics, and Terfenol-D struts as actuators and potentiometers, strain gages, and accelerometers as sensors, we demonstrate successful single- and multi-mode vibration control. In order to realize a more versatile implementation of the control strategy, we resort to a digital signal processing (DSP) board. We compose a code in C and design a digital absorber by developing algorithms that, in addition to replacing the analog circuit, automatically detect the amplitude and frequency of oscillation of the plant and fine-tune the absorber parameters. We take advantage of the digital realization, implement a linear absorber, and compare the performance of the quadratic absorber with that of its linear counterpart. In the case of parametric excitation, we investigate two techniques. First, we explore application of the quadratic absorber. We prove theoretically and demonstrate experimentally that this control scheme is not reliable. Then, we propose an alternate approach. We devise a control law based on cubic velocity feedback. We conduct theoretical and experimental investigations and show that the latter strategy leads to effective vibration suppression and bifurcation control. / Ph. D.

In this study, a small-scale point absorber wave energy converter has been designed contemplating a full-scaled point absorber in the Mediterranean Sea state. The scaling factor for the small-scaled version has been determined by the damping coefficient calculation of the power take-off in 1:10, 1:15, and 1:20 scaling factors. Here a rotational power take-off has been designed instead of the linear one. The rotational power take-off will follow a similar principle as the Eddy current brake. The effect of change in the radius of the translator and magnetic flux in the damping coefficient had been calculated as well. The calculation for the damping coefficient has been conducted in COMSOL Multiphysics. The design for the point absorber was assembled in SolidWorks. In this article, specific attention is given to a variety of aspects affecting the damping coefficient and the way it can aid to determining the scaling factor parameters for a small-scale wave energy converter.

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[pt] Este trabalho apresenta a idealização, caracterização e medições de um absorvedor metamaterial ultrafino com uma banda ultra larga, baseado em espiras quadradas arranjadas periodicamente. O circuito equivalente dessa estrutura é proposto para fornecer uma maior compreensão teórica. Os parâmetros desse circuito são extraídos utilizando um algoritmo baseado no método dos mínimos quadrados, usando uma abordagem direta, que pode ser aplicada aos absorvedores com banda larga em geral. O mecanismo de absorção da estrutura proposta é discutido e detalhado. Um dos resultados da simulação mostrou uma absortividade acima de 90 porcento de 11.4 até 20 GHz, cobrindo a banda Ku inteiramente tanto para a polarização TM quanto para a TE, e essa banda larga é confirmada pelas medições experimentais. Além disso, a espessura do absorvedor metamaterial proposto é lambda/16.4, considerando o maior comprimento de onda da banda de operação, e apresenta estabilidade angular até 50 graus Celsius. Ademais, o uso do absorvedor proposto para redução da seção transversal de radar em antenas de microfita é investigado e reduções até 12.8 dB são obtidas, e uma parede absorvedora é apresentada para diminuição do acoplamento mútuo em conjuntos de antenas. / [en] This work presents the design, characterization, and measurement of an ultrathin and ultrawideband metamaterial absorber based on periodicallyarranged metallic square spiral and its equivalent circuit is proposed to provide a theoretical insight. The parameters of the equivalent circuit are extracted using an algorithm based on the least-square method and using a straightforward approach, which can be applied to broadband absorbers in general. The mechanism of absorption of the proposed structure is discussed and detailed. One of the simulated results showed an absorptivity of more than 90 percent from 11.4 to 20.0 GHz, covering the Ku band for TM as well as TE polarization, and this broadband feature is confirmed by the experimental measurement. Furthermore, the proposed metamaterial absorber is lambda/16.4 thick at the lowest frequency of absorption and presents angle stability up to 50 Celsius degrees. In addition, the usage of the proposed absorber to the radar crosssection reduction in microstrip antennas is investigated and reductions up to 12.8 dB are achieved, and an absorber wall is presented to decrease the mutual coupling in antenna arrays.

Successful race teams rely heavily on track testing to search for the ideal suspension setup. As more restrictions are placed on the amount of on-track testing by major racing sanctioning bodies, such as NASCAR, teams have increased their attention to alternate testing methods to augment their track data and better understand the dynamics of their racecars. One popular alternate to track testing is 8-post dynamic shaker rig testing. Eight-post rig testing gives the team a better understanding of the vehicle's dynamics before they arrive at the race track, allowing them to use their limited track testing time more efficiently. While 8-post rig testing certainly is an attractive option, an extensive test matrix is often required to find the best suspension setups. To take full advantage of 8-post rig tests, more efficient experimental methods are needed. Since investigating shock absorber selection is often the most time-consuming task, this study focuses on developing more efficient methods to select the best shock absorber setups. This study develops a novel method that applies dynamic substructuring and system identification to generate a mathematical model that predicts the results of future tests as both command inputs and components are changed. This method is used to predict the results of 8-post rig tests as actuator commands and shock absorber forces are varied. The resulting model can then be coupled with shock absorber models to simulate how the vehicle response changes with shock absorber selection. This model can then be applied to experimental design. First, a physically-motivated nonlinear dynamic shock absorber model is developed, suitable for quickly fitting experimental data and implementing in simulation studies. Next, a system identification method to identify a vehicle model using experimental data is developed. The vehicle model is then used to predict response trends as shock absorber selection is varied. Comparison of simulation and experimental results show that this model can be used to predict the response levels for 8-post rig tests and aid in streamlining 8-post rig testing experimental designs. / Ph. D.

In the mammalian auditory system, specifically in the cochlea of the inner ear, the Basilar Membrane (BM) and hair cells are responsible for transducing incoming acoustic waves into electrical signals. These acoustic signals are carried as traveling waves by the BM and propagate from the base of the cochlea toward its apex where the helicotrema is located. An impressive feature of the mammalian auditory system is to prevent the propagated waves from reflecting which allows mammals to hear sounds without any reflection or overlap. This extraordinary characteristic of the inner ear is the main inspiration for this work. In the present study, the dynamic behavior of a beam structure with one or more attached spring-damper (SD) systems as passive absorbers is studied when excited by a harmonic force. The location of the spring-damper system divides the beam into two dynamic regions: one which exhibits non-reflecting traveling waves and the other with standing waves. In this work, the separation of traveling and standing waves is studied analytically, numerically, and experimentally. To the best of the author's knowledge, this is the first time in the literature that traveling and standing wave separation in a beam is realized experimentally using a single-point excitation and a spring-damper. Experimental results are used to validate the models of the system. Moreover, a parametric study is performed to gain a better understanding of the effect of different parameters on the quality of the generated waves in the structure. Furthermore, the effect of attaching the second spring-damper to the system is presented. Adding the secondary SD system results in increasing the excitation frequency range so that wave separation can be achieved. The results of this work can be used in various applications such as vibration suppression, energy absorption, particle transportation, and in exploring possible explanations for the BM and helicotrema functions in the cochlea. / Doctor of Philosophy / In the inner ear of the mammalian auditory system, the sound waves travel inside the cochlea where they are converted to electrical signals sent to the brain. A fascinating characteristic of the mammalian auditory system is that the sound waves traveling in the cochlea do not reflect when they reach its apex where the helicotrema is located. Therefore, we are able to hear sounds without any reflection or overlap. This work is inspired by the biological behavior of the inner ear and studies the dynamic behavior of a simple structure such as a beam with one (or two) attached spring-damper(s). In this work, the attached spring-damper system(s) prevents the waves traveling from the source to the beam's boundary from reflecting. This is similar to what happens in the inner ear. The location of the spring-damper divides the beam into two dynamic regions, one which exhibits non-reflecting traveling waves and the other with standing waves. The wave separation and parameters affecting the wave quality and its reflective or non-reflective features are studied analytically, numerically, and experimentally. To the best of the author's knowledge, the experiments carried out to generate the aforementioned wave types coexisting with each other on the beam are one of a kind. Furthermore, the results of this study showed a very good agreement between the experimental and theoretical results. The outcomes of this work can potentially be used in exploring possible explanations for the function of the cochlea and helicotrema and various applications such as particle transportation and suppression of unwanted vibrations.

Natamycin is an antimycotic compound that is widely used in the cheese industry to increase the shelf life of cheeses, especially shredded cheeses, by inhibiting the growth of molds. Natamycin is applied to the surface of cheese as an aqueous suspension or as a powder. However, natamycin is not readily water soluble making it harder to distribute evenly over shredded cheese Natamycin is degraded by ultraviolet (UV) light at wavelengths of 350 nm and below. Typical packaging applications do not provide adequate UV protection causing natamycin to degrade. This work was undertaken to determine the efficacy of UV absorber film to prevent UV light degradation of natamycin on the surface of shredded cheese. Current accepted methods to determine concentration of natamycin were evaluated for appropriateness in natamycin degradation studIes. The use of cyclodextrins to increase water solubility was tested to see if a uniform distribution of natamycin over the shredded cheese could be done effectively. Furthermore, a known application of mold was performed to see how well natamycin and each of its applications could prevent visible mold growth from occurring. The International Dairy Federation recognizes two methods to quantify natamycin on shredded cheese: high performance liquid chromatography (HPLC) and spectrophotometry. Concentrations of natamycin in aqueous suspensions were determined using both methods. Results show that spectrophotometry is flawed when quantifying the amount of active natamycin because the method gives erroneously high results. The amount of active natamycin is not accurately quantified using spectrophotometric techniques because it cannot separate the active form from the inactive form of natamycin. Polymer packages containing a UV absorber (11.4% light transmission at 350 nm) allow significantly less UV-associated degradation of natamycin than those packages that lacked a UV protectant (90.0% light transmission at 350 nm) (p<0.05). Incorporating a UV absorber into a package helps protect natamycin and its various complexes from UV light degradation, which can increase the shelf life of shredded cheese. However, even with a UV absorber, natamycin is still able to degrade. Natamycin was complexed with different cyclodextrins to help better solubilize natamycin â β-cyclodextrin, hydroxy-propyl β-cyclodextrin and γ-cyclodextrin. Using cyclodextrins to apply natamycin more uniformly onto shredded cheese did not significantly increase the consistency of distribution (p<0.05). Variability was uniform throughout all treatments with the exception of HPBCD complex. After 27 days, all of the UV packages treated with each of the cyclodextrin treatments containing shredded cheese began to show visible mold growth. Those packages stored in total darkness remained mold free through the duration of the experiment ending on day 62. When untreated with natamycin and an initial concentration of 101-102 spores/gram of Penicillium roqueforti, shredded cheese remained free from visible mold growth for 24 days in total darkness at 4°C. Samples treated with one of the natamycin treatments were able to remain mold free for at least 9 more days, showing visible signs of mold growth at day 33. There was no statistical difference between the treatments of dry natamycin, aqueous suspension natamycin, β-cyclodextrin-natamycin complex, and γ-cyclodextrin-natamycin complex (p<0.05). However, there was a difference with the use of hydroxy-propyl β-cyclodextrin-natamycin complex. Hydroxy-propyl β-cyclodextrin-natamycin complex allowed the shredded cheese to last for 41 days, 17 days longer than the control sample. / Master of Science