VTechWorks Repository :: Browsing by Author "Inman, Daniel J."

Browsing by Author "Inman, Daniel J."

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Acoustic Characterization and Preliminary Noise Control of Pneumatic Percussion Tools
Schwartz, Kyle Wayne (Virginia Tech, 2006-08-04)
Pneumatic percussion tools are extensively used in the construction industry. They are one of the noisiest machines in the construction industry generating noise levels above 110 dBA which are well beyond the permissible exposure limit (PEL) of 85 dBA. This work presents a comprehensive methodology for the acoustic characterization and noise source identification of these percussion tools. The methodology is applied to a representative pneumatic tool and the characterization results are described in detail. A mechanical analysis was performed on a chipping hammer finding mode shapes and natural frequencies of individual components. The mechanical analysis included modal hammer measurements and creating FE models. Fluid measurements were performed on the chipping hammer to find the velocity of the exhaust and pressure in the upper and lower chambers. The fluid tests found that the velocity of the exhaust is approximately Mach 1.0 or greater. Noise measurements were carried out on the chipping hammer to determine the spectral characteristics, overall sound power level, and spatial source strength maps of the tool. A spherical array of microphones was used to obtain an accurate estimate of the overall sound power levels and the directivity. The overall sound power radiation was found to be in the range of 110-115dBA. An advanced 63 microphone phased array was used to successfully locate and identify the major sources of noise from this tool via the use of beam-forming maps. This thesis also presents a preliminary noise control method employing commercial-off-the-shelf pneumatic silencers. The outcome of the tests is illustrated in detail in this thesis.
Active Dynamic Analysis and Vibration Control of Gossamer Structures Using Smart Materials
Ruggiero, Eric John (Virginia Tech, 2002-05-07)
Increasing costs for space shuttle missions translate to smaller, lighter, and more flexible satellites that maintain or improve current dynamic requirements. This is especially true for optical systems and surfaces. Lightweight, inflatable structures, otherwise known as gossamer structures, are smaller, lighter, and more flexible than current satellite technology. Unfortunately, little research has been performed investigating cost effective and feasible methods of dynamic analysis and control of these structures due to their inherent, non-linear dynamic properties. Gossamer spacecraft have the potential of introducing lenses and membrane arrays in orbit on the order of 25 m in diameter. With such huge structures in space, imaging resolution and communication transmissibility will correspondingly increase in orders of magnitude. A daunting problem facing gossamer spacecraft is their highly flexible nature. Previous attempts at ground testing have produced only localized deformation of the structure's skin rather than excitation of the global (entire structure's) modes. Unfortunately, the global modes are necessary for model parameter verification. The motivation of this research is to find an effective and repeatable methodology for obtaining the dynamic response characteristics of a flexible, inflatable structure. By obtaining the dynamic response characteristics, a suitable control technique may be developed to effectively control the structure's vibration. Smart materials can be used for both active dynamic analysis as well as active control. In particular, piezoelectric materials, which demonstrate electro-mechanical coupling, are able to sense vibration and consequently can be integrated into a control scheme to reduce such vibration. Using smart materials to develop a vibration analysis and control algorithm for a gossamer space structure will fulfill the current requirements of space satellite systems. Smart materials will help spawn the next generation of space satellite technology.
Active Rigidization of Carbon Fiber Reinforced Composites via Internal Resistive Heating
Sarles, Stephen Andrew (Virginia Tech, 2006-02-20)
The use of inflatable, rigidizable structures in solar arrays and other space structures has the potential to drastically reduce the weight, volume, and cost of placing payloads into orbit. Inflatable components consist of ultra-lightweight, flexible materials that enable compact packaging prior to launch. These structures are then transformed from their initially flexible state to one that offers permanent shape-holding and structural integrity through a tailored rigidization process. Inflatable spacecraft must be impervious to the environmental conditions in space--such as ionizing radiation, UV and particle radiation, atomic oxygen, and impacts from space debris and meteoroids. They must also exhibit stable operation over a useful storage and mission life. Methods for causing rigidization in inflatable spacecraft include both passive and active techniques. Passive techniques rely on an uncontrolled, unprovoked reaction between the rigidizable materials in the structure and the surrounding space environment. The benefits of a passive system are offset by their inherent lack of control, which can lead to long curing times and weak spots due to uneven curing. This work presents internal resistive heating as an alternative approach for inducing matrix consolidation and curing of thermoset-coated carbon fiber tows. The ability to dictate this physical transformation through temperature-controlled resistive heating highlights the responsive nature of thermoset polymer composites and demonstrates the advantages of active rigidization. Feedback temperature control is implemented so as to provide a reliable, robust heating method for prescribing material-specific curing profiles. Resistive heating curing schedules developed from previous thermal analysis on two resins, U-Nyte Set 201A and 201B, are prescribed for samples of carbon fiber tow coated with each resin. The rigidization success of each curing profile is then evaluated with respect to both the increase in mechanical stiffness and the cure completion. These experiments indicate that rigidizing the coated fiber tow results in a composite material that is 20 times stronger in bending than prior to curing. The stiffening process requires roughly 1W-hr of energy with 5W peak power over the course of a 24-minute curing schedule. Curing temperature, curing time, and heating rate are also individually varied to determine their effect on rigidization as well as develop methods for reducing curing time and energy. The rigidization of an inflatable structure culminates this work and demonstrates the ability to achieve real strengthening through temperature-controlled internal resistive heating.
Active, Passive and Active/Passive Control Techniques For Reduction of Vibrational Power Flow in Fluid Filled Pipes
Kartha, Satish Chandrashekhar (Virginia Tech, 2000-02-04)
The coupled nature of vibrational energy flow in fluid filled piping systems makes its control and subsequent reduction a difficult problem. This work experimentally explores the potential of different active, passive and active/passive control methodologies for control of vibrational power flow in fluid filled pipes. Circumferential modal decomposition and measurements of vibrational power carried by individual wave types were carried out experimentally. The importance of dominant structural bending waves and the need to eliminate them in order to obtain meaningful experimental results has been demonstrated. The effectiveness of the rubber isolator in reducing structural waves has been demonstrated. Improved performance of the quarter wavelength tube and Helmholtz resonator was obtained on implementation of the rubber isolator on the experimental rig. Active control experiments using the side-branch actuator and 1/3 piezoelectric composite yielded significant dB reductions revealing their potential for practical applications. A combined active/passive approach was also implemented as part of this work. This approach yielded promising results, which proved that combining advantages of both active and passive approaches was a feasible alternative.
Actuator-Work Concepts Applied to Morphing and Conventional Aerodynamic Control Devices
Johnston, Christopher Owen (Virginia Tech, 2003-11-14)
The research presented in this thesis examines the use of an estimated "actuator work" value as a performance parameter for the comparison of various aerodynamic control device configurations. This estimated "actuator work," or practical work as it will be referred to as in this thesis, is based on the aerodynamic and structural resistance to a control surface deflection. It is meant to represent the actuator energy cost required to deflect a general configuration of conventional or unconventional control surface. Thin airfoil theory is used to predict the aerodynamic load distribution required for this work calculation. The details of applying thin airfoil theory to many different types of control surface arrangements are presented. Convenient equations for the aerodynamic load distributions and aerodynamic coefficients are obtained. Using the developed practical work equations, and considering only the aerodynamic load component, the practical work required for a given change in lift is compared between different control surface arrangements. For single control surface cases, it is found that a quadratic (morphing) trailing edge flap requires less practical work than a linear flap of the same size. As the angle of attack at which the change in lift occurs increases, the benefit of the quadratic flap becomes greater. For multiple control surface cases, it is necessary to determine the set of control deflections that require the minimum practical work for a given change in lift. For small values of the initial angle of attack, it is found that a two-segment quadratic trailing edge flap (MTE) requires more work than a two-segment linear flap (TETAB). But, above a small value of angle of attack, the MTE case becomes superior to the TETAB case. Similar results are found when a 1-DOF static aeroelastic model is included in the calculation. The minimum work control deflections for the aeroelastic cases are shown to be strongly dependent on the dynamic pressure.
Adaptation of Nontraditional Control Techniques to Nonlinear Micro and Macro Mechanical Systems
Daqaq, Mohammed F. (Virginia Tech, 2006-07-28)
We investigate the implementation of nontraditional open-loop and closed-loop control techniques to systems at the micro and macro scales. At the macro level, we consider a quay-side container crane. It is known that the United States relies on ocean transportation for 95% of cargo tonnage that moves in and out of the country. Each year over six million loaded marine containers enter U.S. ports. Current growth predictions indicate that container cargo will quadruple in the next twenty years. To cope with this rapid growth, we develop a novel open-loop input-shaping control technique to mitigate payload oscillations on quay-side container cranes. The proposed approach is suitable for automated crane operations, does not require any alterations to the existing crane structure, uses the maximum crane capabilities, and is based on an accurate two-dimensional four-bar-mechanism model of a container crane. The shaped commands are based on a nonlinear approximation of the two-dimensional model frequency and, unlike traditional input-shaping techniques, our approach can account for large hoisting operations. For operator-in-the-loop crane operations, we develop a closed-loop nonlinear delayed-position feedback controller. Key features of this controller are that it: does not require major modifications to the existing crane structure, accounts for motion inversion delays, rejects external disturbances, and is superimposed on the crane operator commands. To validate the controllers, we construct a 1:10 scale model of a 65-ton quay-side container crane. The facility consists of a 7-meter track, 3.5-meter hoisting cables, a trolley, a traverse motor, two hoisting motors, and a 50-pound payload. Using this setup, we demonstrated the effectiveness of the controllers in mitigating payload oscillations in both of the open-loop and closed-loop modes of operation. At the micro level, we consider a micro optical device known as the torsional micromirror. This device has a tremendous number of industrial and consumer market applications including optical switching, light scanning, digital displays, etc. To analyze this device, we develop a comprehensive model of an electrically actuated torsional mirror. Using a Galerkin expansion, we develop a reduced-order model of the mirror and verify it against experimental data. We investigate the accuracy of representing the mirror using a two-degrees-of-freedom lumped-mass model. We conclude that, under normal operating conditions, the statics and dynamics of the mirror can be accurately represented by the simplified lumped-mass system. We utilize the lumped-mass model to study and analyze the nonlinear dynamics of torsional micromirrors subjected to combined DC and resonant AC excitations. The analysis is aimed at enhancing the performance of micromirrors used for scanning applications by providing better insight into the effects of system parameters on the microscanner's optimal design and performance. Examining the characteristics of the mirror response, we found that, for a certain DC voltage range, a two-to-one internal resonance might be activated between the first two modes. Due to this internal resonance, the mirror exhibits complex dynamic behavior. This behavior results in undesirable vibrations that can be detrimental to the scanner performance. Torsional micromirrors are currently being implemented to provide all-optical switching in fiber optic networks. Traditional switching techniques are based on converting the optical signal into electrical signal and back into optical signal before it can be switched into another fiber. This reduces the rate of data transfer substantially. To realize fast all-optical switching, we enhance the transient dynamic characteristics and performance of torsional micromirrors by developing a novel technique for preshaping the voltage commands applied to activate the mirror. This new approach is the first to effectively account for inherent nonlinearities, damping effects, and the energy of the significant higher modes. Using this technique, we are able to realize very fast switching operations with minimal settling time and almost zero overshoot.
Adaptive Collocated Feedback for Noise Absorption in Acoustic Enclosures
Creasy, Miles Austin (Virginia Tech, 2006-10-06)
This thesis focuses on adaptive feedback control for low frequency acoustic energy absorption in acoustic enclosures. The specific application chosen for this work is the reduction of high interior sound pressure levels (SPL) experienced during launch within launch vehicle payload fairings. Two acoustic enclosures are used in the research: the first being a symmetric cylindrical duct and the other being a full scale model of a payload fairing. The symmetric cylindrical duct is used to validate the ability of the adaptive controller to compensate for large changes in the interior acoustical properties. The payload fairing is used to validate that feedback control, for a large geometry, does absorb acoustic energy. The feedback controller studied in this work is positive position feedback (PPF) used in conjunction with high and low pass Butterworth filters. An algorithm is formed from control experiments for setting the filter parameters of the PPF and Butterworth filters from non-adaptive control simulations and tests of the duct and payload fairing. This non-adaptive control shows internal SPL reductions of 2.2 dB in the cylindrical duct for the frequency range from 100 to 500 Hz and internal SPL reductions of 4.2 dB in the full scale fairing model for the frequency range from 50 to 250 Hz. The experimentally formed control algorithm is then used as the basis for an adaptive controller that uses the collocated feedback signal to actively tune the control parameters. The cylindrical duct enclosure with a movable end cap is used to test the adaptation properties of the controller. The movable end cap allows the frequencies of the acoustic modes to vary by more than 20 percent. Experiments show that a 10 percent change in the frequencies of the acoustic modes cause the closed-loop system to go unstable with a non-adaptive controller. The closed-loop system with the adaptive controller maintains stability and reduces the SPL throughout the 20 percent change of the acoustic modes' frequencies with a 2.3 dB SPL reduction before change and a 1.7 dB SPL reduction after the 20 percent change.
Advanced Time Domain Sensing For Active Structural Acoustic Control
Maillard, Julien (Virginia Tech, 1997-02-27)
Active control of sound radiation from vibrating structures has been an area of much research in the past decade. In Active Structural Acoustic Control (ASAC), the minimization of sound radiation is achieved by modifying the response of the structure through structural inputs rather than by exciting the acoustic medium (Active Noise Control, ANC). The ASAC technique often produces global far-field sound attenuation with relatively few actuators as compared to ANC. The structural control inputs of ASAC systems are usually constructed adaptively in the time domain based on a number of error signals to be minimized. One of the primary concerns in active control of sound is then to provide the controller with appropriate ``error'' information. Early investigations have implemented far-field microphones, thereby providing the controller with actual radiated pressure information. Most structure-borne sound control approaches now tend to eliminate the use of microphones by developing sensors that are integrated in the structure. This study presents a new sensing technique implementing such an approach. A structural acoustic sensor is developed for estimating radiation information from vibrating structures. This technique referred to as Discrete Structural Acoustic Sensing (DSAS) provides time domain estimates of the radiated sound pressure at prescribed locations in the far field over a broad frequency range. The structural acoustic sensor consists of a set of accelerometers mounted on the radiating structure and arrays of digital filters that process the measured acceleration signals in real time. The impulse response of each filter is constructed from the appropriate radiation Green's function for the source area associated with each accelerometer. Validation of the sensing technique is performed on two different systems: a baffled rectangular plate and a baffled finite cylinder. For both systems, the sensor is first analyzed in terms of prediction accuracy by comparing estimated and actual sound pressure radiated in the far field. The analysis is carried out on a numerical model of the plate and cylinder as well as on the real structures through experimental testing. The sensor is then implemented in a broadband radiation control system. The plate and cylinder are excited by broadband disturbance inputs over a frequency range encompassing several of the first flexural resonances of the structure. Single-sided piezo-electric actuators provide the structural control inputs while the sensor estimates are used as error signals. The controller is based on the filtered-x version of the adaptive LMS algorithm. Results from both analytical and experimental investigations are again presented for the two systems. Additional control results based on error microphones allow a comparison of the two sensing approaches in terms of control performance. The major outcome of this study is the ability of the structural acoustic sensor to effectively replace error microphones in broadband radiation control systems. In particular, both analytical and experimental results show the level of sound attenuation achieved when implementing Discrete Structural Acoustic Sensing rivaled that achieved with far-field error microphones. Finally, the approach presents a significant alternative over other existing structural sensing techniques as it requires very little system modeling.
Advancing Autonomous Structural Health Monitoring
Grisso, Benjamin Luke (Virginia Tech, 2007-11-27)
The focus of this dissertation is aimed at advancing autonomous structural health monitoring. All the research is based on developing the impedance method for monitoring structural health. The impedance technique utilizes piezoelectric patches to interrogate structures of interested with high frequency excitations. These patches are bonded directly to the structure, so information about the health of the structure can be seen in the electrical impedance of the piezoelectric patch. However, traditional impedance techniques require the use of a bulky and expensive impedance analyzer. Research presented here describes efforts to miniaturize the hardware necessary for damage detection. A prototype impedance-based structural health monitoring system, incorporating wireless based communications, is fabricated and validated with experimental testing. The first steps towards a completely autonomous structural health monitoring sensor are also presented. Power harvesting from ambient energy allows a prototype to be operable from a rechargeable power source. Aerospace vehicles are equipped with thermal protection systems to isolate internal components from harsh reentry conditions. While the thermal protection systems are critical to the safety of the vehicle, finding damage in these structures presents a unique challenge. Impedance techniques will be used to detect the standard damage mechanism for one type of thermal protection system. The sensitivity of the impedance method at elevated temperatures is also investigated. Sensors are often affixed to structures as a means of identifying structural defects. However, these sensors are susceptible to damage themselves. Sensor diagnostics is a field of study directed at identifying faulty sensors. The influence of temperature on these techniques is largely unstudied. In this dissertation, a model is generated to identify damaged sensors at any temperature. A sensor diagnostics method is also adapted for use in developed hardware. The prototype used is completely digital, so standard sensor diagnostics techniques are inapplicable. A new method is developed to work with the digital hardware.
Aerodynamic and Electromechanical Design, Modeling and Implementation Of Piezocomposite Airfoils
Bilgen, Onur (Virginia Tech, 2010-08-02)
Piezoelectrics offer high actuation authority and sensing over a wide range of frequencies. A Macro-Fiber Composite is a type of piezoelectric device that offers structural flexibility and high actuation authority. A challenge with piezoelectric actuators is that they require high voltage input; however the low power consumption allows for relatively lightweight electronic components. Another challenge, for piezoelectric actuated aerodynamic surfaces, is found in operating a relatively compliant, thin structure (desirable for piezoceramic actuators) in situations where there are relatively high external (aerodynamic) forces. Establishing an aeroelastic configuration that is stiff enough to prevent flutter and divergence, but compliant enough to allow the range of available motion is the central challenge in developing a piezocomposite airfoil. The research proposed here is to analyze and implement novel electronic circuits and structural concepts that address these two challenges. Here, a detailed theoretical and experimental analysis of the aerodynamic and electromechanical systems that are necessary for a practical implementation of a piezocomposite airfoil is presented. First, the electromechanical response of Macro-Fiber Composite based unimorph and bimorph structures is analyzed. A distributed parameter electromechanical model is presented for interdigitated piezocomposite unimorph actuators. Necessary structural features that result in large electrically induced deformations are identified theoretically and verified experimentally. A novel, lightweight electrical circuitry is proposed and implemented to enable the peak-to-peak actuation of Macro-Fiber Composite bimorph devices with asymmetric voltage range. Next, two novel concepts of supporting the piezoelectric material are proposed to form two types of variable-camber aerodynamic surfaces. The first concept, a simply-supported thin bimorph airfoil, can take advantage of aerodynamic loads to reduce control input moments and increase control effectiveness. The structural boundary conditions of the design are optimized by solving a coupled fluid-structure interaction problem by using a structural finite element method and a panel method based on the potential flow theory for fluids. The second concept is a variable-camber thick airfoil with two cascading bimorphs and a compliant box mechanism. Using the structural and aerodynamic theoretical analysis, both variable-camber airfoil concepts are fabricated and successfully implemented on an experimental ducted-fan vehicle. A custom, fully automated low-speed wind tunnel and a load balance is designed and fabricated for experimental validation. The airfoils are evaluated in the wind tunnel for their two-dimensional lift and drag coefficients at low Reynolds number flow. The effects of piezoelectric hysteresis are identified. In addition to the shape control application, low Reynolds number flow control is examined using the cascading bimorph variable-camber airfoil. Unimorph type actuators are proposed for flow control in two unique concepts. Several electromechanical excitation modes are identified that result in the delay of laminar separation bubble and improvement of lift. Periodic excitation to the flow near the leading edge of the airfoil is used as the flow control method. The effects of amplitude, frequency and spanwise distribution of excitation are determined experimentally using the wind tunnel setup. Finally, the effects of piezoelectric hysteresis nonlinearity are identified for Macro-Fiber Composite bimorphs. The hysteresis is modeled for open-loop response using a phenomenological classical Preisach model. The classical Preisach model is capable of predicting the hysteresis observed in 1) two cantilevered bimorph beams, 2) the simply-supported thin airfoil, and 3) the cascading bimorph thick airfoil.
Aeroelasticity of Morphing Wings Using Neural Networks
Natarajan, Anand (Virginia Tech, 2002-07-03)
In this dissertation, neural networks are designed to effectively model static non-linear aeroelastic problems in adaptive structures and linear dynamic aeroelastic systems with time varying stiffness. The use of adaptive materials in aircraft wings allows for the change of the contour or the configuration of a wing (morphing) in flight. The use of smart materials, to accomplish these deformations, can imply that the stiffness of the wing with a morphing contour changes as the contour changes. For a rapidly oscillating body in a fluid field, continuously adapting structural parameters may render the wing to behave as a time variant system. Even the internal spars/ribs of the aircraft wing which define the wing stiffness can be made adaptive, that is, their stiffness can be made to vary with time. The immediate effect on the structural dynamics of the wing, is that, the wing motion is governed by a differential equation with time varying coefficients. The study of this concept of a time varying torsional stiffness, made possible by the use of active materials and adaptive spars, in the dynamic aeroelastic behavior of an adaptable airfoil is performed here. A time marching technique is developed for solving linear structural dynamic problems with time-varying parameters. This time-marching technique borrows from the concept of Time-Finite Elements in the sense that for each time interval considered in the time-marching, an analytical solution is obtained. The analytical solution for each time interval is in the form of a matrix exponential and hence this technique is termed as Matrix Exponential time marching. Using this time marching technique, Artificial Neural Networks can be trained to represent the dynamic behavior of any linearly time varying system. In order to extend this methodology to dynamic aeroelasticity, it is also necessary to model the unsteady aerodynamic loads over an airfoil. Accordingly, an unsteady aerodynamic panel method is developed using a distributed set of doublet panels over the surface of the airfoil and along its wake. When the aerodynamic loads predicted by this panel method are made available to the Matrix Exponential time marching scheme for every time interval, a dynamic aeroelastic solver for a time varying aeroelastic system is obtained. This solver is now used to train an array of neural networks to represent the response of this two dimensional aeroelastic system with a time varying torsional stiffness. These neural networks are developed into a control system for flutter suppression. Another type of aeroelastic problem of an adaptive structure that is investigated here is the shape control of an adaptive bump situated on the leading edge of an airfoil. Such a bump is useful in achieving flow separation control for lateral directional maneuverability of the aircraft. Since actuators are being used to create this bump on the wing surface, the energy required to do so needs to be minimized. The adverse pressure drag as a result of this bump needs to be controlled so that the loss in lift over the wing is made minimal. The design of such a "spoiler bump" on the surface of the airfoil is an optimization problem of maximizing pressure drag due to flow separation while minimizing the loss in lift and energy required to deform the bump. One neural network is trained using the CFD code FLUENT to represent the aerodynamic loading over the bump. A second neural network is trained for calculating the actuator loads, bump displacement and lift, drag forces over the airfoil using the finite element solver, ANSYS and the previously trained neural network. This non-linear aeroelastic model of the deforming bump on an airfoil surface using neural networks can serve as a fore-runner for other non-linear aeroelastic problems. This work enhances the traditional aeroelastic modeling by introducing time varying parameters in the differential equations of motion. It investigates the calculation of non-conservative aerodynamic loads on morphing contours and the resulting structural deformation for non-linear aeroelastic problems through the use of neural networks. Geometric modeling of morphing contours is also addressed.
Analysis and Compensation of Imperfection Effects in Piezoelectric Vibratory Gyroscopes
Loveday, Philip Wayne (Virginia Tech, 1999-01-29)
Vibratory gyroscopes are inertial sensors, used to measure rotation rates in a number of applications. The performance of these sensors is limited by imperfections that occur during manufacture of the resonators. The effects of resonator imperfections, in piezoelectric vibratory gyroscopes, were studied. Hamilton's principle and the Rayleigh-Ritz method provided an effective approach for modeling the coupled electromechanical dynamics of piezoelectric resonators. This method produced accurate results when applied to an imperfect piezoelectric vibrating cylinder gyroscope. The effects of elastic boundary conditions, on the dynamics of rotating thin-walled cylinders, were analyzed by an exact solution of the FlÃ¼gge shell theory equations of motion. A range of stiffnesses in which the cylinder dynamics was sensitive to boundary stiffness variations was established. The support structure, of a cylinder used in a vibratory gyroscope, should be designed to have stiffness outside of this range. Variations in the piezoelectric material properties were investigated. A figure-of- merit was proposed which could be used to select an existing piezoceramic material or to optimize a new composition for use in vibratory gyroscopes. The effects of displacement and velocity feedback on the resonator dynamics were analyzed. It was shown that displacement feedback could be used to eliminate the natural frequency errors, that occur during manufacture, of a typical piezoelectric vibrating cylinder gyroscope. The problem of designing the control system to reduce the effects of resonator imperfections was investigated. Averaged equations of motion, for a general resonator, were presented. These equations provided useful insight into the dynamics of the imperfect resonator and were used to motivate the control system functions. Two control schemes were investigated numerically and experimentally. It was shown that it is possible to completely suppress the first-order effects of resonator mass/stiffness imperfections. Damping imperfections, are not compensated by the control system and are believed to be the major source of residual error. Experiments performed on a piezoelectric vibrating cylinder gyroscope showed an order of magnitude improvement, in the zero-rate offset variation over a temperature range of 60°C, when the control systems were implemented.
The Analysis and Creation of Track Irregularities Using TRAKVU
Kramp, Kenneth P. (Virginia Tech, 1998-06-30)
The accuracy of the results from a rail vehicle dynamic model is dependent on the realism of the track input to the model. An important part of the track input is the irregularities that exist on actual track. This study analyzes the irregularities inherent in railroad track geometry data, and provides an analytical method for creating track data with the irregularities for use as the input to a dynamic model. Track data, measured from various classes of track, was examined using statistical and frequency analysis techniques to identify any similarities in the characteristics of the irregularities. The results showed that each class of track had a distinctive value for the standard deviation of the alignment and profile data. It was also determined that the frequency content of all the tracks was contained within a common bandwidth. The track irregularities could then be generated with the same characteristics as an actual track. The method for creating the track irregularities was then programmed into TRAKVU. TRAKVU is a track preprocessor used in conjunction with NUCARS, a railcar dynamic modeling program¹. TRAKVU enables users to create track data and apply the appropriate irregularities so that the track will have the characteristics of the desired class of track. A validation was then performed to determine how well track created in TRAKVU simulated actual tracks. The statistical and frequency characteristics of created tracks were compared directly with actual tracks. Created track was also used as the input to a dynamic model. The predicted vehicle response was then compared to the actual vehicle response and the predicted vehicle response using measured track data as the input. The results from the validation showed that the created track performed as well as the measured track in providing the input to the model. Although the predicted response using the created track did not compare as well with the actual vehicle response, the differences could be attributed to inaccuracies in the model. ¹NUCARS and TRAKVU are copyrighted property of the Association of American Railroads.
Analysis of a Rotary Ultrasonic Motor for Application in Force-Feel Systems
Murphy, Devon Patrick (Virginia Tech, 2008-08-25)
A qualitative analysis of a rotary traveling wave-type ultrasonic motor (USM) used to supply feedback forces in force-feel systems is carried out. Prior to simulation, the subsystems and contact mechanics needed to define the motor's equations of motion are discussed along with the pitfalls of modeling a USM. A mathematical model is assembled and simulated in MATLAB Simulink. Accompanying the dynamic model, a new reduced model is presented from which predictions of USM performance can be made without a complicated dynamic model. Outputs from the reduced model are compared with those of the dynamic model to show the differences in the transient solution, agreement in the steady state solution, and above all that it is an efficient tool for approximating a motor's steady state response as a function of varying the motor parameters. In addition, the reduced model provides the means of exploring the USMs response to additive loading, loads acting in the direction of motor motion, where only resistive loads, those opposite to the motor rotation, had been considered previously. Fundamental differences between force-feel systems comprising standard DC brushless motors as the feedback actuators and the proposed system using the USM are explained by referencing the USM contact mechanics. Outputs from USM model simulations are explored, and methods by which the motor can be implemented in the force-feel system are derived and proven through simulation. The results show that USMs, while capable of providing feedback forces in feel systems, are far from ideal for the task. The speed and position of the motor can be controlled through varying stator excitation parameters, but the transient motor output torque cannot; it is solely a function of the motor load, whether additive or resistive.
Analysis of a Split-Path Gear Train with Fluid-Film Bearings
Wolff, Andrew Vincent (Virginia Tech, 2004-05-06)
In the current literature, split path gear trains are analyzed for use in helicopter transmissions and marine gearboxes. The goal in these systems is to equalize the torque in each path as much as possible. There are other gear trains where the operator intends to hold the torque split unevenly. This allows for control over the gearbox bearing loading which in turn has a direct effect on bearing stiffness and damping characteristics. Having control over these characteristics is a benefit to a designer or operator concerned with suppressing machine vibration. This thesis presents an analytical method for analyzing the torque in split path gear trains. A computer program was developed that computes the bearing loads in various gearbox arrangements using the torque information gathered by the analytical method. A case study is presented that demonstrates the significance of the analytical method in troubleshooting an industrial gearbox that has excessive vibration.
Analysis of the sensing region of a PZT actuator-sensor
Esteban, Jaime (Virginia Tech, 1996-07-15)
A high frequency impedance-based qualitative non-destructive evaluation (NDE) technique has been successfully applied for structural health monitoring at the Center for Intelligent Material Systems and Structures (CIMSS) [1-3]. This new technique uses piezoceramic (PZT) patches as actuator-sensors to provide a low-power driven constant voltage dynamic excitation, and to record the modulated current flow through the structure. Therefore, it relies on tracking the electrical point impedance to identify incipient level damage. The high frequency excitation provided by the PZT, ensures the detection of minor changes in the monitored structure. It also limits the sensing area to a region close to the PZT source, therefore only changes in the near field of the PZT are detected, enhancing the ability of this technique to localize incipient damage. The phenomena of the PZT's sensing region localization has been the driving motivation for this research. More fundamental analytical research should be performed before full application of this technique is possible. Thereby, a wave propagation continuum mechanics based approach has been applied to model the high frequency vibrations of one dimensional structures. Energy dissipation mechanisms, such as bolted connections and internal friction, are considered to have a major role in the attenuation of the PZT's induced wave, therefore these mechanisms has been extensively studied. To analyzed bolted connections, linear and nonlinear joint models have been used to describe the wave interaction with such nonconservative discontinuities. Also, with the use of an impedance based model, the electromechanical coupling of the PZT and the host structure is added into the formulation. The wave interaction and energy dissipated at the bolted discontinuity has been assessed with energy flux computations of the incident, transmitted, and reflected waves. The effect of loosening the bolted joint has been also analyzed by reducing the spring stiffness and increasing the damping in the dash pots for the linear joint model, and reducing the Coulomb stiffness and shearing force at the interface for the nonlinear case. A scheme based on the correspondence principle has been applied to calculate the specific damping capacity of a system, at any given frequency, as a quantification of the energy dissipated through the system. The material damping was added into the formulation assuming the modulus to have a complex representation, and therefore the corresponding loss factors were found with active measurement of the material properties of the specimen via a wave propagation method, that monitories the wave's speed at two locations. Once the bases of the analytical model have been set up and corroborated with experiments, a parametric study has been developed to account for the various factors that can affect the sensing range of the PZT’s induced wave, and therefore to have a “rule of thumb on how to go about” when bonding PZTs to structures to monitor them. Apart from the energy dissipation mechanisms, other parameters responsible for the reflection of the incoming wave, and its consequent attenuation, has also been reconstructed. With the extensive analysis of these parameters, an impedance damage metric, based on the undamaged and damaged impedance, has been developed for various factors that can be the source of incipient damage. An attenuation metric has also been introduced to identify the degree of transmission of the propagating wave at certain discontinuities. The analysis of the case scenarios reproduced in this parametric study will aid in the knowledge about the number of PZTs needed to be placed in the monitored structure, the most critical locations, and when a monitored member in a system need to be replaced.
Analytical and Computational Tools for the Study of Grazing Bifurcations of Periodic Orbits and Invariant Tori
Thota, Phanikrishna (Virginia Tech, 2007-02-02)
The objective of this dissertation is to develop theoretical and computational tools for the study of qualitative changes in the dynamics of systems with discontinuities, also known as nonsmooth or hybrid dynamical systems, under parameter variations. Accordingly, this dissertation is divided into two parts. The analytical section of this dissertation discusses mathematical tools for the analysis of hybrid dynamical systems and their application to a series of model examples. Specifically, qualitative changes in the system dynamics from a nonimpacting to an impacting motion, referred to as grazing bifurcations, are studied in oscillators where the discontinuities are caused by impacts. Here, the study emphasizes the formulation of conditions for the persistence of a steady state motion in the immediate vicinity of periodic and quasiperiodic grazing trajectories in an impacting mechanical system. A local analysis based on the discontinuity-mapping approach is employed to derive a normal-form description of the dynamics near a grazing trajectory. Also, the results obtained using the discontinuity-mapping approach and direct numerical integration are found to be in good agreement. It is found that the instabilities caused by the presence of the square-root singularity in the normal-form description affect the grazing bifurcation scenario differently depending on the relative dimensionality of the state space and the steady state motion at the grazing contact. The computational section presents the structure and applications of a software program, TC-HAT, developed to study the bifurcation analysis of hybrid dynamical systems. Here, we present a general boundary value problem (BVP) approach to locate periodic trajectories corresponding to a hybrid dynamical system under parameter variations. A methodology to compute the eigenvalues of periodic trajectories when using the BVP formulation is illustrated using a model example. Finally, bifurcation analysis of four model hybrid dynamical systems is performed using TC-HAT.
Analytical Modeling and Equivalent Electromechanical Loading Techniques for Adaptive Laminated Piezoelectric Structures
Smith, Clayton L. (Virginia Tech, 2001-01-23)
Many commercial finite element programs support piezoelectric modeling and composite modeling to some extent. The popular program ABAQUS, however, has piezoelectric modeling capabilities only for continuum and one-dimensional truss elements. In situations where aspect ratio constraints and computational inefficiencies become a significant issue, such as modeling very large thin structures, alternate modeling techniques are sometimes required. Much of the focus of this thesis was to introduce equivalent methods for modeling laminated piezoelectric beams and plates. Techniques are derived based on classical beam and plate theory, classical lamination theory, and the linear theory of piezoelectricity. Finite element approximations are used with the principle of minimum potential energy to derive the static equilibrium equations for piezoelectric laminated structures. Equivalent loading techniques are derived based on the constitutive equations of piezoelectricity to simulate actuation forces within the piezoelectric layers. Finite element models using equivalent modeling techniques as well as equivalent loading techniques for piezoelectric laminated structures are developed and compared to ABAQUS models using piezoelectric elements to evaluate the error in theoretical assumptions. The analysis will prove that equivalent structural models and equivalent loading techniques provide excellent means for simplifying the analysis of thin piezoelectric laminated structures.
Analytical Models to Predict Power Harvesting with Piezoelectric Materials
Eggborn, Timothy (Virginia Tech, 2003-05-07)
With piezoceramic materials, it is possible to harvest power from vibrating structures. It has been proven that micro- to milliwatts of power can be generated from vibrating systems. We develop definitive, analytical models to predict the power generated from a cantilever beam and cantilever plate. Harmonic oscillations and random noise will be the two different forcing functions used to drive each system. The predictive models are validated by being compared to experimental data. A parametric study is also performed in an attempt to optimize the cantilever beam system's power generation capability.
Analytical Solutions for the Deformation of Anisotropic Elastic and Piezothermoelastic Laminated Plates
Vel, Senthil S. (Virginia Tech, 1998-11-30)
The Eshelby-Stroh formalism is used to analyze the generalized plane strain quasistatic deformations of an anisotropic, linear elastic laminated plate.The formulation admits any set of boundary conditions on the edges and long faces of the laminate. Each lamina may be generally anisotropic with as many as 21 independent elastic constants. The three dimensional governing differential equations are satisfied at every point of the body.The boundary conditions and interface continuity conditions are satisfied in the sense of a Fourier series. Results are presented for three sample problems to illustrate the versatility of the method. The solution methodology is generalized to study the deformation of finite rectangular plates subjected to arbitrary boundary conditions. The effect of truncation of the series on the accuracy of the solution is carefully examined. Results are presented for thick plates with two opposite edges simply supported and the other two subjected to eight different boundary conditions. The results are compared with three different plate theories.The solution exhibits boundary layers at the edges except when they are simply supported. Results are presented in tabular form for different sets of edge boundary conditions to facilitate comparisons with predictions from various plate theories and finite element formulations. The Eshelby-Stroh formalism is also extended to study the generalized plane deformations of piezothermoelastic laminated plates. The method is capable of analyzing laminated plates with embedded piezothermoelastic patches. Results are presented for a thermoelastic problem and laminated elastic plates with piezothermoelastic lamina attached to its top surface. When a PZT actuator patch is attached to an elastic cantilever substrate, it is observed that the transverse shear stress and transverse normal stress are very large at the corners of the PZT-substrate interface. This dissertation is organized in the form of three self-contained chapters each of which will be submitted for possible publication in a journal.

Browsing by Author "Inman, Daniel J."

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