Made available in DSpace on 2014-06-11T19:28:34Z (GMT). No. of bitstreams: 0 Previous issue date: 2006-12Bitstream added on 2014-06-13T18:57:56Z : No. of bitstreams: 1 almeida_fe_me_guara.pdf: 3463520 bytes, checksum: 687003c1392e31477b62d4db52bb211e (MD5) / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) / Equipamentos aeronáuticos embarcados podem ser submetidos a elevados níveis de vibração durante o vôo. Normas internacionais especificam os níveis de vibração que os componentes de uso civil e militar devem ser capazes de suportar de acordo com o emprego e com o tipo de aeronave ao qual serão integrados. Em alguns equipamentos aeronáuticos considerações de geometria e massa limitam severamente as opções de projeto no sentido de evitar ressonâncias na faixa de freqüência de qualificação do equipamento, o que pode fazer com que componentes eletrônicos internos sofram níveis muito elevados de vibração. O presente trabalho visa à redução das cargas dinâmicas com a introdução de dispositivos do tipo passivo de isolamento de vibração. Para redução e amortecimento de vibrações foram utilizados materiais com baixa rigidez, mas resistência mecânica suficiente para suportar os esforços dinâmicos. Diversas configurações de isoladores fabricadas com vários materiais (poliuretano, silicone, polietileno e uma combinação de polietileno e silicone) foram ensaiadas. A solução adotada foi analisada também por simulações numéricas pelo método dos elementos finitos, obtendo-se respostas em freqüência (acelerações), deslocamentos e tensões. Os deslocamentos e os valores máximos de tensão calculados apresentaram valores inferiores aos máximos admissíveis. Os resultados numéricos e experimentais apresentaram boa correlação entre si. A combinação de polietileno e silicone obteve o melhor desempenho permitindo uma redução de aproximadamente 85% do nível de vibração RMS sobre o equipamento e os componentes eletrônicos. / Aeronautical systems can be submitted to high levels of vibration during flight. International standards specify the vibration levels that this kind of systems must withstand according to its function and the aircraft where it will be integrated. For some of these systems, geometry and mass properties are strictly defined by some design issues and therefore cannot be changed in order to achieve better dynamical properties and avoid resonance peaks or high vibrations level in internal electronic components. The main goal of this work is to reduce the dynamic loads acting on sensible electronic equipment using passive vibration isolators. Materials witch provide low suspension resonance frequencies but enough mechanical strength and a medium loss factor are used to build vibration isolators. Some simple vibration isolators devices made by polymers are evaluated by dynamic tests performed with an airborne equipment. The devices tested are made by polyurethane, silica, polyethylene and a combination of the two formers. The dynamical behavior of the configuration with better performance is also analyzed through numerical simulation by finite element method. The numerical results for the dynamic responses are compared with the experimental results and shown good agreement. The combination of silica and polyethylene have show the better performance supporting appropriately the dynamic loads and reducing the RMS vibration level measured in the electronic components in 85%.

Orientador: Fernando de Azevedo Silva / Banca: José Elias Tomazini / Banca: Carlos de Andrade Souto / Resumo: Equipamentos aeronáuticos embarcados podem ser submetidos a elevados níveis de vibração durante o vôo. Normas internacionais especificam os níveis de vibração que os componentes de uso civil e militar devem ser capazes de suportar de acordo com o emprego e com o tipo de aeronave ao qual serão integrados. Em alguns equipamentos aeronáuticos considerações de geometria e massa limitam severamente as opções de projeto no sentido de evitar ressonâncias na faixa de freqüência de qualificação do equipamento, o que pode fazer com que componentes eletrônicos internos sofram níveis muito elevados de vibração. O presente trabalho visa à redução das cargas dinâmicas com a introdução de dispositivos do tipo passivo de isolamento de vibração. Para redução e amortecimento de vibrações foram utilizados materiais com baixa rigidez, mas resistência mecânica suficiente para suportar os esforços dinâmicos. Diversas configurações de isoladores fabricadas com vários materiais (poliuretano, silicone, polietileno e uma combinação de polietileno e silicone) foram ensaiadas. A solução adotada foi analisada também por simulações numéricas pelo método dos elementos finitos, obtendo-se respostas em freqüência (acelerações), deslocamentos e tensões. Os deslocamentos e os valores máximos de tensão calculados apresentaram valores inferiores aos máximos admissíveis. Os resultados numéricos e experimentais apresentaram boa correlação entre si. A combinação de polietileno e silicone obteve o melhor desempenho permitindo uma redução de aproximadamente 85% do nível de vibração RMS sobre o equipamento e os componentes eletrônicos. / Abstract: Aeronautical systems can be submitted to high levels of vibration during flight. International standards specify the vibration levels that this kind of systems must withstand according to its function and the aircraft where it will be integrated. For some of these systems, geometry and mass properties are strictly defined by some design issues and therefore cannot be changed in order to achieve better dynamical properties and avoid resonance peaks or high vibrations level in internal electronic components. The main goal of this work is to reduce the dynamic loads acting on sensible electronic equipment using passive vibration isolators. Materials witch provide low suspension resonance frequencies but enough mechanical strength and a medium loss factor are used to build vibration isolators. Some simple vibration isolators devices made by polymers are evaluated by dynamic tests performed with an airborne equipment. The devices tested are made by polyurethane, silica, polyethylene and a combination of the two formers. The dynamical behavior of the configuration with better performance is also analyzed through numerical simulation by finite element method. The numerical results for the dynamic responses are compared with the experimental results and shown good agreement. The combination of silica and polyethylene have show the better performance supporting appropriately the dynamic loads and reducing the RMS vibration level measured in the electronic components in 85%. / Mestre

Utilizing flywheels to store and reuse energy from regenerative braking on locomotives is a new technology being developed in the Vibration Control and Electromechanics Lab at Texas A&M. This thesis focuses on the motion analysis of a locomotive mounted energy storage flywheel system for a variety of support motion inputs. Two input cases, sinusoidal floor input and ramp input, are analyzed in different sections. Simulation results and methods of ensuring the operating success of the flywheel system are provided at the end of each section. Section 1 introduces the problem and method being used to study the vibration under different circumstances. Section 2 analyzes the response of the flywheel system to sinusoidal floor input given by Ahmadian and Venezia 2000. Natural frequency and transmissibility of the system are utilized to explain the simulation results carried out in the frequency domain. It is found that the motion differences between flywheels(rotors) and magnetic bearings(stators) are guaranteed to be small. Section 3 emulates the locomotive traversing a bump with 1:150 slope. Simulation shows that catcher(backup) bearings are needed to limit the vibration of rotors through a bump. It is also found that gyroscopic effect causes problems in vibration isolation. Section 4 explores de-levitation method and installation of gimbals as possible remedies to this problem. Finally, a summary of simulation results from different input cases is made.

In this study, the prediction and control of the acoustic radiation from a Bristol H25A refrigeration compressor are investigated. For the acoustic prediction, a modal decomposition approach is used. To this end, a boundary element model of the shell is created, and it is used to compute the modal radiation efficiency curves of the shell. These radiation efficiencies are then used in conjunction with the experimentally measured spring forces to obtain the acoustic power radiated by the compressor. Of twenty-three structural modes included in the analysis, it is found that eight have high radiation efficiency and six contribute significantly to the total radiated power. The analytically predicted overall radiated sound power of 82.2 dBA agrees very well with the 82 dBA experimentally measured. For the noise control of the compressor, three approaches are investigated to reduce the forces transmitted to the shell and thus the radiation. (a) The spring mounts are moved to various locations on the shell, (b) dynamic vibration absorbers (DVAs) are added to the mounts, and (c) low modulus materials are inserted between the mounts and the springs to create an impedance mismatch. For all three approaches, efficient analytical methods to compute the radiated acoustic power upon the system modifications are developed. The most promising approach is the insertion of the low modulus materials, which yields a reduction of 6.4 dBA on the total radiated acoustic power. The addition of DVAs and the relocation of the mounts yield a reduction of 5.5 dBA and 1.7 dBA in the total radiated acoustic power, respectively. / Master of Science

The single-degree-of-freedom (SDOF) system is the most widely used model for vibration isolation systems. The SDOF system is a simple but worthy model because it quantifies many results of an isolation system. For instance, a SDOF model predicts that the high frequency transmissibility increases when the isolator has passive damping although this does not occur for an isolator implementing active damping. A severe limitation of this system is that it cannot be used when the base and/or equipment are flexible. System flexibility has been considered in previous literature but the flexibility has always been approximated which leads to truncation errors. The analysis used in this work is more sophisticated in that it can model the system flexibility without the use of any approximations. Therefore, the true effects of system flexibility can be analyzed analytically. Current literature has not fully explored the choice of mount frequency or actuator placement for flexible systems either. It is commonly suggested that isolators should be designed with a low-frequency mount. That is, the isolator frequency should be much lower than any of the system frequencies. It is shown that these isolators tend to perform best in an overall sense; however, mount frequencies designed between system modes tend to have a coupling effect. That is, the lower frequencies have such a strong interaction between each other that when isolator damping is present, multiple system modes are attenuated. Also, when the base and equipment are flexible, isolator placement becomes a critical issue. For low-frequency mount designs, the first natural frequency can shift as much as 15.6% for various isolator placements. For a mid-frequency mount design, the shift of the first three modes can be as high as 34.9%, 26.6% and 11.3%, respectively, for varying isolator placements. NOTE: (03/2011) An updated copy of this ETD was added after there were patron reports of problems with the file. / Ph. D.

Combining shock and vibration isolation into a single isolation system package is explored through the use of an air spring in parallel with a controlled magnetorheological fluid damper. The benefits of combining shock and vibration isolation into a single package is discussed. Modeling and control issues are investigated and test and simulation results are discussed. It is shown that this hybrid isolation system provides significantly increased performance over current state-of-the-art passive systems. Also explored is the feasibility of scavenging and storing ambient shipboard vibration energy for use in powering the isolation system. To date the literature has not adequately explored the direct design of a combined shock and vibration isolation system. As shock and vibration isolation are typically conflicting goals, the traditional approach has been to design separate shock and vibration isolation systems and operate them in parallel. This approach invariably leads to compromises in terms of the performance of both systems. Additionally, while considerable research has been performed on magnetorheological fluids and devices based on these fluids, there has been little research performed on the use of these fluids in devices that are subjected to high velocities such as the velocity seen by a ship exposed to underwater near-miss explosive events. Also missing from the literature is any research involving the scavenging and storage of ambient shipboard vibration energy. While the focus of this work is on the use of this scavenged energy to power the subject isolation system, many other uses for this energy can be envisioned. Experimental and analytical results from this research clearly show the advantages of this hybrid isolation system. Drop tests show that inputs as great as 167 g's were reduced to 3.42 g's above mount at 1.11 inches of deflection using a Velocity Feedback controller suggested by the author. When contrasted with typical test results with similar inputs, the subject isolation system achieved reductions in above mount accelerations of 300% and reductions in mount deflections of 200% over current state-of-the-art passive shipboard isolation systems. Furthermore, simulations using a validated model of the isolation system suggest that this performance improvement can be achieved in multi-degree-of-freedom isolation systems as well. It was shown that above mount accelerations in the vertical and athwartship directions could be effectively limited to a predefined value, while achieving the absolute minimum mount defections, using an Acceleration Limiting Bang-Bang controller suggested by the author. Further experimentation suggests that the subject isolation system could be entirely self-powered from scavenged ambient shipboard vibration energy. An experiment using an energy scavenging and storage system consisting of a Piezoelectric Stack Generator and a bank of ultracapacitors showed that enough energy could be harvested to power the isolation system though several shock events. / Ph. D.

The purpose of this research is to analyze a new type of vibration isolator consisting of two pre-bent struts which are clamped at both ends and intermediately bonded with a viscoelastic filler. The proposed isolation device has the ability to support a relatively large static load with little deflection and offers a low axial resistance under dynamic excitation, making it ideal for isolating vertical vibrations. In this research, four of these vibration isolators are used to support a rigid, square plate. The symmetric case is analyzed first. Then the plate has a center of mass which is located at some distance from the geometric center of the plate. When the system is subjected to vertical harmonic base excitations, this eccentric weight introduces rotational as well as vertical motions of the plate. This research will investigate the effects of various eccentricities on the efficiency of the vibration isolators in the configuration described. The displacement transmissibility will be the measure of the isolators' effectiveness at mitigating vibrations transmitted from the base to the rigid plate. For each case, the nonlinear equilibrium equations and the governing equations of motion for small vibrations about equilibrium are numerically solved, and the transmissibility is calculated and plotted over a wide range of frequencies. These plots are used to recognize ranges of frequencies for which isolation is achieved and frequencies at which resonance occurs in the system. At the resonant frequencies, the physical behavior of the system is analyzed to determine the types of vibration modes which occur in the system. A free vibration analysis is also performed to obtain a better understanding of resonances in the system. / Master of Science

No description available.

Industrial machines during their operation generate vibration due to dynamic forces acting on the machines. This vibration may create noise, abrasion in the machine parts, mechanical fatigue, degrade performance, transfer to other machines via floor or walls and may cause complete shutdown of the machine. To limit the vibration pre-installation, vibration isolation measures are usually employed in workshops and industrial units. However, such vibration isolation may not be sufficient due to varying operating and physical conditions, such as machine ageing, structural changes and new installations etc. Therefore, it is important to assess the quantity of vibration generated and transmitted during true operating conditions. The thesis work is aimed at the estimation of vibrational transmissibility or transfer from industrial machines to floor and to other adjacent installed machines. This study of transmissibility is based on the measurement and analysis of various spectral estimation tools such as Power Spectral Density (PSD), Frequency Response Function (FRF) and Coherence Function. The overall study is divided into three major steps. Firstly, the initial measurements are carried in BTH on simple Single Degree of Freedom (SDOF) systems to gain confidence in measurement and analysis. Then the measurements are performed on a Lathe machine “Quick Turn Nexus 300-II” in a laboratory at BTH. Finally, the measurements are taken on the machines of an Industrial workshop (KOSAB). The analysis results revealed that vibration measurements in industry are challenging and not easy as measurement in labs. Measurements are contaminated by noise from other machines, which degrade the coherence function. However, vibration transferred from one machine to the floor or other machines may be studied using FRF and PSD. Appropriate further isolations may be employed based on the spectral analysis.

Vehicle occupants are typically exposed to unpleasant whole-body vibration (WBV) for extended period of time. It is well known that the transmission of unwanted vibration to the human body can lead to fatigue and discomfort. Moreover, the unwanted vibration normally distributed in the low-frequency range has been found as the main risk factor for lower back pain and lumbago, which seriously affect the health and working performance of occupants. Thus vibration cancellation on seats has attracted considerable interest in recent years. So far, for most vehicle seats, vibration isolation is achieved passively by using seat cushions and conventional energy absorbers, which have very limited performance in the low-frequency range. The work presented in this thesis forms a successful development and experimental study of an active seat and control algorithm for occupants’ WBV reduction under low frequency excitations. Firstly, a modelling study of the seat human subjects (SHS) and an extensive experimental measurement of the vibration transmissibility of a test dummy and vehicle seat are carried out. The biodynamic responses of SHS exposed to uncoupled vertical and fore-and-aft WBV is modelled. A comparison with the existing models is made and the results show that an improved fit with the aggregated experimental data is achieved. Secondly, an active seat is developed based upon the observations and understanding of the SHS and seat system. The characteristics of the active seat dynamics are identified through experimental tests found suitable for the development of an active seat to attenuate the vibration experienced by vehicle occupants. The vibration cancellation performance of the active seat is initially examined by feedforward plus proportional-integral (PI) control tests. Through these tests, the effectiveness of the actuators control authority is verified, but the limitations are also revealed. Because the active seat system is subject to non-linear and time-varying behaviour, a self-tuning fully adaptive algorithm is a prime requirement. The Filtered-x Least-Mean-Square (FXLMS) algorithm with the Fast-block LMS (FBLMS) system identification technique is found suitable for this application and is investigated through experimental tests. Substantial vibration reductions are achieved for a variety of input vibration profiles. An excellent capability of the active seat and control system for efficiently reducing the vibration level of seated occupants under low-frequency WBV is demonstrated.