VTechWorks Repository :: Browsing by Author "Robertshaw, Harry H."

Browsing by Author "Robertshaw, Harry H."

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Active control of acoustic radiation due to discontinuities on thin beams
Frampton, Kenneth D. (Virginia Tech, 1991-04-15)
Two experiments were conducted to study the active control of acoustic radiation due to discontinuities on thin beams. One experiment investigated the radiation from a clamped end condition and the other investigated the radiation from a blocking mass. The beams were excited by subsonic flexural traveling waves which "scattered" (or produced reflected and transmitted traveling and near-field waves) when they encountered the discontinuity. This "scattering" produced supersonic wave number components in the beam vibrational response which were responsible for the acoustic radiation. The main purpose of these experiments was to control the acoustic radiation from discontinuities on beams by actively changing the characteristics of the "scattered" waves with control actuators. In each experiment the system was disturbed by a harmonic, subsonic input from a point force shaker. Control actuator( s) (in the form of shakers and piezoelectric actuators) were attached to the beam near the discontinuity. Error microphone(s) were positioned in the acoustic field which supplied an error signal to the digital controller. The digital controller employed was the filtered-x version of the adaptive LMS algorithm programmed on a dedicated signal processing board in a personal computer. An array of accelerometers was attached to the beam which were used to decompose the complex amplitudes of an assumed displacement equation. By applying a spatial Fourier transform to the displacement equation the wavenumber components present in the beam displacement were calculated. This aided in the investigation of the mechanism by which control of the acoustic field was affected. Results from these experiments showed that large attenuations at the error microphones were possible (as much as 50dB) along with global attenuation of the acoustic field. The mechanism by which the control of the acoustic far-field was achieved was demonstrated as a decrease in the supersonic wavenumber components in the beam vibrational response.
Active control of automobile cabin noise with conventional and advanced speakers
Couche, Jerome Christophe (Virginia Tech, 1999-03-24)
Recently much research has focused on the control of enclosed sound fields, particularly in automobiles. Both Active Noise Control (ANC) and Active Structural Acoustic Control (ASAC) techniques are being applied to problems stemming from power train noise and road noise (noise due to the interaction of the tires with the surface of the road). Due to the low frequency characteristics of these noise problems, large acoustic sources are required to obtain efficient control of the sound field. This creates demand in the automobile industry for compact lightweight sources. This work is concerned with the application of active control to power train noise, as well as road noise in the interior cabin of a sport utility vehicle using advanced, compact lightweight piezoelectric acoustic sources. First, a test structure approximately the same size as the automobile was built to study the principles of active noise control in a cavity. A finite element model of the cavity was created in order to optimize the positions of the error sensors and the control sources. Experimental work was performed with the optimized actuator and sensor locations in order to validate the model, and draw conclusions regarding the conditions to obtain global control of the sound field. Second, a broad-band feedforward filtered-X LMS algorithm was used to control power train noise. Preliminary power train noise tests were conducted using arrangements of four microphones and up to four commercially available speakers for control. Attenuation of seven decibel (dB) at the error sensors was measured in the 40-500 Hz frequency band. The dimensions of the zone of quiet generated by the control were measured, and show that noise reductions were obtained for a large volume surrounding the error sensors. Next, advanced speakers were implemented for active control of power train noise. The results obtained with different arrangements of these speakers were very similar to those obtained with the commercially-available speakers. These advanced speakers use piezoelectric devices to induce the displacement of a speaker membrane, which radiates sound. Their lighter weight and compact dimensions are a significant advantage over conventional speakers, for their application in automobile. Third, preliminary results were obtained for active control of road noise. The controller used an optimized set of four reference signals to control the noise at one error sensor using one control source. Two sets of tests were conducted. The first set of tests was performed on a dynamometer, which simulates the effects of the road on the tires. The second set of tests was performed on a rough road. Reduction of two to four decibel of the sound pressure level at the error sensor was obtained between 100 and 200 Hz.
Active control of coupled wave propagation in fluid-filled elastic cylindrical shells
Brevart, Bertrand J. (Virginia Tech, 1994-12-10)
The vibrational energy propagating in straight fluid-filled elastic pipes is carried by the structure as well as by the internal fluid. Part of the energy in the system may also transfer from one medium to the other as propagation occurs. For various types of harmonic disturbance, this study demonstrates that, whether the propagating energy is predominantly conveyed in the shell or in the fluid, large attenuations of the total power flow may be achieved by using an active control approach. As the shell and fluid motions are fully coupled, the implementation of intrusive sources/sensors in the acoustic field can be also avoided. The approach is based on using radial control forces applied to the outer shell wall and error sensors observing the structural motion. A broad analytical study gives insight into the control mechanisms. The cylindrical shell is assumed to be infinite, in vacuo or filled with water. The first disturbance source investigated is a propagating free wave of circumferential order n=0 or n= 1. The control forces are appropriate harmonic line forces radially applied to the structure. The radial displacement of the shell wall at discrete locations downstream of the control forces is minimized using linear quadratic optimal control theory. The attenuation of the total power flow in the system after control is used to study the impact of the fluid on the performance of the control approach. Results for the shell in vacuo are presented for comparison. Considering the breathing mode (n=O), the fluid decreases the control performance when the disturbance is a structural-type incident wave. Significant reductions of the transmitted power flow can be achieved when the disturbance is a fluid-type of wave. Regarding the beam mode (n=1), the fluid increases the control performance below the first acoustic cut-off frequency and decreases it above this frequency.
Active control of floor vibrations
Hanagan, Linda M. (Virginia Tech, 1994-12-15)
The active control of structures is a diverse field of study, with new applications being developed continually. One structural system, which is often not considered a dynamic system, is the floor of a building. In many cases the dynamics of a floor system are neglected in the design phase of a building structure. Occasionally, this omission results in a floor which has dynamic characteristics found to be unacceptable for the intended use of the building. Floor motion of very small amplitudes, often caused by pedestrian movement, is sometimes found objectionable by occupants of the building space. Improving an unacceptable floor system's dynamic characteristics after construction can be disruptive, difficult and costly. In search of alternative repair measures, analytical and experimental research implementing active control techniques was conducted to improve the vibration characteristics of problem floors. Specifically, a control scheme was developed utilizing the measured movement of the floor to compute the input signal to an electromagnetic actuator which, by the movement of the actuator reaction mass, supplies a force that reduces the transient and resonant vibration levels. Included in the analytical component of this research is the development of a mathematical model for a full scale experimental test floor. This model is studied, using a matrix computation software, to evaluate the effectiveness of the control scheme. The experimental component of the research serves two purposes. The first is the verification of the system behavior assumed in the analytical component of the research. The second is the verification of control system effectiveness for various excitations, control gains, and actuator locations on the experimental test floor and six additional floors.
Active control of sound radiation from fluid loaded plates
Gu, Yi (Virginia Tech, 1992-10-15)
Active control of sound radiation due to subsonic wave scattering from an infinite or a finite fluid-loaded plate excited below the critical frequency is analytically studied. The disturbance is caused by a flexural wave in an infinite plate, or by a point force on a finite plate at subsonic frequencies. The wave scattering is caused by discontinuities on the plate or by the boundary conditions. A feed-forward control approach is applied by implementing either point/line forces or piezoelectric actuators on the plate. The amplitude and phase of control forces are determined by the optimal solution of a cost function which minimizes the far-field radiated acoustic power over a prescribed surface in the half space of the fluid field. The results show that for subsonic excitations, high global reduction in radiated pressure is possible with properly located active control forces. The number and location of control forces employed in order to obtain high control performance are related to the excitation frequency. The far-field sound radiation directivity pattern, the modal amplitudes of the plate vibration, the plate vibration autospectrum in the wave number domain, and the near-field intensity distribution are extensively studied in order to uncover the mechanisms of control.
Active control of sound transmission/radiation from elastic plates using multiple piezoelectric actuators
Wang, Bor-Tsuen (Virginia Tech, 1991)
This thesis presents a theoretical analysis of active control of sound radiation from elastic plates with the use of piezoelectric transducers as actuators. A strain-energy model (SEM), based upon the conservation of strain energy, for a laminate beam with attached or embedded finite-length spatially distributed induced strain actuators was first developed to determine the induced strain distribution. The equivalent axial force and bending moment induced by the embedded or surface bonded actuators were also calculated. The one-dimensional SEM was then extended to a two-dimensional model by employing the classical laminate plate theory and utilizing Heaviside functions to integrate the actuator influence on the substructure. The mechanics model can determine the structural coupling effect and predict the structural response as a result of piezoelectric actuation. A baffled simply-supported rectangular plate subjected to harmonic disturbances was considered as the plant. Piezoceramic materials bonded to the surfaces of the plate or point force shakers were applied as control actuators. Both microphones in the radiated far-field and accelerometers located on the plate were considered as error sensors. In addition, distributed sensors for pressure and structural motion were modelled. The cost function was formulated as the modulus squared of the error signal. Linear quadratic optimal control theory was then applied to minimize the cost function to obtain the optimal input voltages to the actuators. Both near-field and far-field pressure and intensity responses as well as plate displacement distributions were presented to show the effectiveness and mechanisms of control for various configurations of the actuators and sensors. Plate wavenumber analysis was also shown to provide a further insight into control technique. The results show that piezoelectric actuators perform very well as control sources, and that pressure sensors have many advantages over acceleration sensors while distributed sensors are superior to discrete sensors. The optimal placement of multiple fixed size piezoelectric actuators in sound radiation control is also presented. A solution strategy is proposed to calculate the applied voltages to piezoelectric actuators with the use of linear quadratic optimal control theory. The location of piezoelectric actuator is then determined by minimizing an objective function, which is defined as the sum of the mean square sound pressure measured by a number of error microphones. The optimal location of piezoelectric actuators for sound radiation control is found so as to minimize the objective function and shown to be dependent on the excitation frequency. In particular, the optimal placement of multiple piezoelectric actuators for on-resonance and off-resonance excitation is presented. Results show that the optimally placed piezoelectric actuators perform far better in sound radiation control than arbitrarily selected. This work leads to a design methodology for adaptive or intelligent material systems with highly integrated actuators and sensors. The optimization procedure also leads to a reduction in the number of control transducers.
Active damage control using artificial intelligence: initial studies into identification and mitigation
Kiel, David H. (Virginia Tech, 1993-06-05)
This thesis presents an initial investigation into Active Damage Control (AD C) using Artificial Intelligence (AI). AI can alleviate the sometimes complicated task of modelling the system and also provides an adaptable solution process. The two research areas of ADC, damage identification and damage control, are studied in separate investigations. An AI technique called "rule induction" is used for the damage identification study. Velocity data from three plates (one without damage, one with damage at the center, and one with damage at the edge) are acquired using a laser data acquisition system. A set of rules is then induced from these data which accurately identifies which plates have damage and where the damage is located. With regard to the damage control, a real-time, machine-learning technique called "BOXES" is used to locally control the vibration of various systems by identifying their vibrational patterns. Using this technique, it is shown that the computer successfully learns an effective control law for various simulations using its trials and failures as the only learning information. It is also seen that the learning algorithm is somewhat less effective when experimentally applying this method to a pin-pin, aluminum beam. A discussion of possible improvements are presented in the future work section.
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 dynamic response tuning of adaptive composites utilizing embedded nitinol actuators
Barker, Daniel Keith (Virginia Tech, 1989-07-04)
Adaptive composites utilizing embedded nitinol fibers have the unique ability to change their material properties, induce large internal distributed forces in a structure, and can modify the stress and strain distribution within a structure in a controlled manner. In this study, nitinol fibers are embedded in graphite-epoxy and are used as distributed actuators to actively tune the dynamic response of clamped-clamped beams. The natural frequencies of clamped-clamped nitinol composite beams are shown, experimentally. to increase linearly as a function of temperature. Beams with nitinol volume fractions of 5% 10%, and 15% can increase their first natural frequency by factors of 1.7, 2.5, and 3.0 respectively. Classical lamination theory is used to formulate a mathematical model of the dynamic response which includes the adaptive properties of the embedded nitinol fibers as a function of temperature, as well as the thermal aspects of the matrix material. Experimental characterization of nitinol for use as constrained thermosets is performed and the results are used in the mathematical model. The mathematical model is used to calculate the natural frequencies of clamped-clamped nitinol composite beams and the results are compared to experimental results. It is clear that adaptive composites represent a new concept in active control of structural responses and may act as a catalyst for future developments in both material and structures technology. Demonstrating, experimentally and computationally, the ability to alter the dynamic response using unique adaptive qualities will hopefully inspire new material/structural interaction paradigms.
Active Flow Control For Reduction of Unsteady Stator-Rotor Interaction In a Turbofan Simulator
Feng, Jinwei (Virginia Tech, 2000-10-10)
The research effort presented in this dissertation consists of employing active trailing edge blowing control to reduce the unsteady stator-rotor interaction in a turbofan simulator. Two active flow control systems with different wake sensing approaches are successfully implemented on the engine simulator. The first flow control system utilizes Pitot probes as flow sensors. Use of Pitot probes as sensors is appropriate as a first step toward a more in depth investigation of active trailing edge blowing control. An upper performance limit in terms of wake-filling can be obtained and serves as the baseline in evaluating other control systems with indirect wake sensors. The ability of the system to achieve effective wake filling when subjected to a change in inlet flow conditions demonstrates the feasibility and advantage of active flow control. Significant tonal noise reductions in the far field are also obtained. The second control system involves using microphones as indirect wake sensors. The significance of these acoustic sensing approaches is to provide a practical TEB approach for realistic engines implementations. Microphones are flush mounted on the inlet case to sense the tonal noise at the blade passing frequency. The first sensing approach only uses the tone magnitude while the second novel sensing approach utilizes both the tone magnitude and phase as error information. The convergence rate of the second sensing approach is comparable with that of the Pitot-probe based experiments. The acoustic results obtained from both sensing approaches agree well with those obtained using Pitot probes as sensors. In addition to the experimental part of this research, analytical studies are also conducted on the trailing edge blowing modeling using an aeroacoustic code. An analytical model for trailing edge blowing is first proposed. This model is then introduced into the two-dimensional aeroacoustic code to investigate effect of various trailing edge blowing managements in the tonal sound generation.
Active structural acoustic control of broadband disturbances
Baumann, William T.; Ho, Fu-Sheng; Robertshaw, Harry H. (Acoustical Society of America, 1992-09-01)
A control design technique is developed to actively suppress the acoustic power radiated from a structure, with negligible fluid loading, that is persistently excited by narrow-band or broadband disturbances. The problem is constrained by the assumption that the far-field pressure cannot be measured directly. A method for estimating the total radiated power from measurements on the structure is developed. Using this estimate as a cost function and assuming knowledge of the spectrum of the disturbance, a controller is designed using the linear-quadratic-Gaussian (LQG) theory to minimize the cost. Computer simulations of a clamped-clamped beam show that there is a significant difference in the total radiated power between a system with a vibration-suppression controller and a system with an acoustic controller that accounts for the coupling of these vibrations to the surrounding fluid. In some cases, the acoustic controller increases the system vibration in order to minimize the radiated power.
Active structural acoustic control of double panel systems including hierarchical control approaches
Carneal, James P. (Virginia Tech, 1996-06-06)
The general trends and principles of active structural acoustic control when applied to double panel systems are investigated to determine the respective advantages and limitations of this approach. Included is the application of a novel hierarchical control approach which may reduce the controller complexity and the collinearity issue for large order controllers. This research was initiated by an interest in studying the noise transmission path from the noise field generated by an advanced turboprop engine through the aircraft fuselage and the interior trim into the interior acoustic field which can be modeled as a double panel system. The system studied was a double panel model consisting of two rectangular, uniform, flat plates separated by a sealed air cavity, mounted in a transmission loss test facility and excited by an oblique acoustic plane wave. Piezoelectric control inputs were mounted directly on the double panel system incident or radiating plates. Error sensors were microphones placed in the acoustic free field. The cost function was defined as the total radiated sound power from the double panel system. The investigation was carried out analytically with experimental verification. Results of active structural acoustic control (ASAC) applied to double panel systems indicated that the best control performance was exhibited by a double panel system controlled by PZT control actuators mounted on a sandwich board radiating plate. The sandwich board radiating plate double panel system exhibits a decreased coupling of the incident and radiating plates and a lower modal density which results in increased uncontrolled and controlled transmission loss. Piezoelectric (PZT) control actuators should be mounted on the radiating plate of a double panel system which can couple into the radiating acoustic field better than actuators mounted on the incident plate. As expected, better control is achieved with more control actuators since a more distributed forcing function can be attained. However, for on-resonance excitation, the increased number of actuators decreases performance due to collinearity of the actuators which results in spillover. Results of the biologically inspired hierarchical (BIO) control algorithm indicated that significant performance increases over a one output channel controller were attained for all of the BIO methods while performance lagged compared to a full order controller with the same number of control channels. One advantage of the hierarchical control structure was the ability to avoid the collinearity issue when the degrees of freedom excited in the double panel system was less than the number of control channels. In this instance, the hierarchical structure exhibited less spillover than a fully adaptive LQOCT controller.
Active suppression of acoustic radiation from impulsively excited structures
Baumann, William T.; Saunders, William R.; Robertshaw, Harry H. (Acoustical Society of America, 1991-12-01)
The objective is to use active control to suppress the acoustic energy that is radiated to the far field from a structure that has been excited by a short-duration pulse. The problem is constrained by the assumption that the far-field pressure cannot be directly measured. Therefore, a method is developed for estimating the total radiated energy from measurements on the structure. Using this estimate as a cost function, a feedback controller is designed using linear quadratic regulator theory to minimize the cost. Computer simulations of a clamped-clamped beam show that there is appreciable difference in the total radiated energy between a system with a controller designed to suppress vibrations of the structure and a system with a controller that takes into account the coupling of these vibrations to the surrounding fluid. The results of this work provide a framework for a general, model-based method for actively suppressing transient structural acoustic radiation that can also be applied to steady, narrow, or broadband disturbances.
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.
Adaptive Arrays and Diversity Antenna Configurations for Handheld Wireless Communication Terminals
Dietrich, Carl B. (Virginia Tech, 2000-02-15)
This dissertation reports results of an investigation into the performance of adaptive beamforming and diversity combining using antenna arrays that can be mounted on handheld radios. Handheld arrays show great promise for improving the coverage, capacity, and power efficiency of wireless communication systems. Diversity experiments using a handheld antenna array testbed (HAAT) are reported here. These experiments indicate that signals received by the antennas in two-element handheld antenna arrays with spacing of 0.15 wavelength or greater can be combined to provide 7-9 dB diversity gain against fading at the 99% reliability level in non line-of-sight multipath channels. Thus, peer-to-peer systems of handheld transceivers that use antenna arrays can achieve reliability comparable to systems of single-antenna handheld units, with only one-fifth the transmitter power, resulting in lower overall power consumption and increased battery life. Similar gains were observed for spatial, polarization, and pattern diversity. Adaptive beamforming with single- and multi-polarized four-element arrays of closely spaced elements was investigated by experiment using the HAAT, and by computer simulation using a polarization-sensitive vector multipath propagation simulator developed for this purpose. Small and handheld adaptive arrays were shown to provide 25 to 40 dB or more of interference rejection in the presence of a single interferer in rural, suburban, and urban channels including line-of-sight and non line-of-sight cases. In multipath channels, these performance levels were achieved even when there was no separation between the transmitters in azimuth angle as seen from the receiver, and no difference in the orientations of the two transmitting antennas. This interference rejection capability potentially allows two separate spatial channels to coexist in the same time/frequency channel, doubling system capacity.
Adaptive Control Methods for Non-Linear Self-Excited Systems
Vaudrey, Michael Allen (Virginia Tech, 2001-08-28)
Self-excited systems are open loop unstable plants having a nonlinearity that prevents an exponentially increasing time response. The resulting limit cycle is induced by any slight disturbance that causes the response of the system to grow to the saturation level of the nonlinearity. Because there is no external disturbance, control of these self-excited systems requires that the open loop system dynamics are altered so that any unstable open loop poles are stabilized in the closed loop. This work examines a variety of adaptive control approaches for controlling a thermoacoustic instability, a physical self-excited system. Initially, a static feedback controller loopshaping design and associated system identification method is presented. This design approach is shown to effectively stabilize an unstable Rijke tube combustor while preventing the creation of additional controller induced instabilities. The loopshaping design method is then used in conjunction with a trained artificial neural network to demonstrate stabilizing control in the presence of changing plant dynamics over a wide variety of operating conditions. However, because the ANN is designed specifically for a single combustor/actuator arrangement, its limited portability is a distinct disadvantage. Filtered-X least mean squares (LMS) adaptive feedback control approaches are examined when applied to both stable and unstable plants. An identification method for approximating the relevant plant dynamics to be modeled is proposed and shown to effectively stabilize the self-excited system in simulations and experiments. The adaptive feedback controller is further analyzed for robust performance when applied to the stable, disturbance rejection control problem. It is shown that robust stability cannot be guaranteed because arbitrarily small errors in the plant model can generate gradient divergence and unstable feedback loops. Finally, a time-averaged-gradient (TAG) algorithm is investigated for use in controlling self-excited systems such as the thermoacoustic instability. The TAG algorithm is shown to be very effective in stabilizing the unstable dynamics using a variety of controller parameterizations, without the need for plant estimation information from the system to be controlled.
Adaptive control of a four-bar linkage
Carlson, Stephen O. (Virginia Tech, 1985-04-05)
Three discrete-time adaptive controllers are developed and applied to Four-bar linkage velocity control to reduce the input link velocity fluctuations without compromising the control system velocity transient response. The successful control techniques use the known mechanism kinematics and the mechanism input link position to control the nonlinear mechanism dynamics. The study shows that the adaptive controls are feasible to implement using current microprocessor technology, and the velocity control performance is improved when compared to an industry-standard analog servomotor control. However, more development is required to realize the full potential of the adaptive control technique. A nonlinear Four-bar dynamic model is developed using Kinematic Influence Coefficients. This model is used to develop the adaptive controls and to computer simulate the control scheme performances. The simulated model velocity response is compared qualitatively to experimental data and shown to be similar to an experimental device.
Adaptive vibration absorber
Moyka, Ana S. (Virginia Tech, 1996)
Rapid changes in the drafting profession have left gaps in skills and resources which hinder the ability of community college drafting instructors to provide students with the advanced skills needed for employment. To minimize these problems and to facilitate the integration of CAD into drafting programs, local and state authorities must plan to allocate adequate resources to each program. They must also provide avenues for instructors to become proficient with CAD and to assess the instructor's perceptions about their needs. The purpose of this study was to identify areas of perceived need of community college drafting instructors in Virginia. A survey was mailed to 36 community college drafting instructors. For 14 categories of CAD skills, instructors were asked to indicate their (1) technical skill, (2) need for technical training, (3) level of CAD integration, (4) integration skill, and (5) need for integration training. The instructors were also asked to identify resource needs. The findings suggest that a high level of instruction exists in CAD categories related to traditional drafting tasks. Fewer instructors have ventured into the CAD categories related to design. Many of the instructors who have attempted to teach these categories feel uncertain about their technical skills and skills in integrating advanced CAD categories into their drafting curriculum. From the instructors' responses, it is concluded that a need for resources exists to help instructors to use and integrate CAD categories related to design and customization.
Advanced sensing techniques for active structural acoustic control
Clark, Robert L. Jr. (Virginia Tech, 1992-02-12)
This study presents a basis for the analytical and experimental procedures as well as design techniques required in achieving adaptive structures for active structural acoustic control (ASAC). Test structures studied in this work included a baffled simply supported beam and a baffled simply supported plate which were subjected to a harmonic input disturbance created physically with a shaker and modelled by a point force input. Structural acoustic control was achieved with piezoelectric actuators bonded to the surface of the test structure. The primary focus of this work was devoted to studying alternative sensing techniques in feed forward control applications. Specifically, shaped distributed structural sensors constructed from polyvinylidene fluoride (PVDF), distributed acoustic near-field sensors constructed from PVDF, and accelerometers were explored as alternatives to microphones which are typically implemented as error sensors in the cost function of the control approach. The chosen control algorithm in this study was the feed forward filtered-x version of the adaptive LMS algorithm. A much lower level of system modelling is required with this method of control in comparison to state feedback control methods. As a result, much of the structural acoustic coupling (i.e. system modelling) must be incorporated into the sensor design.
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.

Browsing by Author "Robertshaw, Harry H."

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