Browsing by Author "Clark, Robert L."

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    Characterization of multiple piezoelectric actuators for structural excitation
    Clark, Robert L.; Fuller, Chris R.; Wicks, Alfred L. (Acoustical Society of America, 1991-07-01)
    The thrust of the present work is to analytically and experimentally study the response of a simply supported beam driven by multiple piezoelectric actuators in an effort to understand distributed excitation of the structure. The results indicate that the theoretical model provides the basis for a viable means of determining appropriate locations for piezoelectric actuators for exciting desired modal distributions in the structural response.
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    Control of sound transmission into payload fairings using distributed vibration absorbers and Helmholtz resonators
    Estève, Simon J. (Virginia Tech, 2004-04-22)
    A new passive treatment to reduce sound transmission into payload fairing at low frequency is investigated. This new solution is composed of optimally damped vibration absorbers (DVA) and optimally damped Helmholtz resonators (HR). A fully coupled structural-acoustic model of a composite cylinder excited by an external plane wave is developed as a first approximation of the system. A modal expansion method is used to describe the behavior of the cylindrical shell and the acoustic cavity; the noise reduction devices are modeled as surface impedances. All the elements are then fully coupled using an impedance matching method. This model is then refined using the digitized mode shapes and natural frequencies obtained from a fairing finite element model. For both models, the noise transmission mechanisms are highlighted and the noise reduction mechanisms are explained. Procedures to design the structural and acoustic absorbers based on single degree of freedom system are modified for the multi-mode framework. The optimization of the overall treatment parameters namely location, tuning frequency, and damping of each device is also investigated using genetic algorithm. Noise reduction of up to 9dB from 50Hz to 160Hz using 4% of the cylinder mass for the DVA and 5% of the cavity volume for the HR can be achieved. The robustness of the treatment performance to changes in the excitation, system and devices characteristics is also addressed. The model is validated by experiments done outdoors on a 10-foot long, 8-foot diameter composite cylinder. The excitation level reached 136dB at the cylinder surface comparable to real launch acoustic environment. With HRs representing 2% of the cylinder volume, the noise transmission from 50Hz to160Hz is reduced by 3dB and the addition of DVAs representing 6.5% of the cylinder mass enhances this performance to 4.3dB. Using the fairing model, a HR+DVA treatment is designed under flight constraints and is implemented in a real Boeing fairing. The treatment is composed of 220 HRs and 60 DVAs representing 1.1% and 2.5% of the fairing volume and mass respectively. Noise reduction of 3.2dB from 30Hz to 90Hz is obtained experimentally. As a natural extension, a new type of adaptive Helmholtz resonator is developed. A tuning law commonly used to track single frequency disturbance is newly applied to track modes driven by broadband excitation. This tuning law only requires information local to the resonator simplifying greatly its implementation in a fairing where it can adapt to shifts in acoustic natural frequencies caused by varying payload fills. A time domain model of adaptive resonators coupled to a cylinder is developed. Simulations demonstrate that multiple adaptive HRs lead to broadband noise reductions similar to the ones obtained with genetic optimization. Experiments conducted on the cylinder confirmed the ability of adaptive HRs to converge to a near optimal solution in a frequency band including multiple resonances.
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    An experimental study implementing model reference active structural acoustic control
    Clark, Robert L.; Gibbs, Gary P.; Fuller, Chris R. (Acoustical Society of America, 1993-06-01)
    Model reference active structural acoustic control is experimentally investigated in this study for physical systems characterized by stationary, narrow-band disturbances in which the acoustic field remains relatively unchanged. The filtered-x version of the multichannel adaptive least-mean-square (LMS) algorithm was implemented on a TMS320C25 digital signal processing board to achieve the desired control approach, and control inputs were generated with piezoelectric actuators. Model reference control provides the designer with a method of replacing acoustic error sensors such as microphones located in the far-field with error sensors such as accelerometers located on the surface of the structure. As opposed to driving the response of the structure to zero at the coordinates of the accelerometers, the response is driven to some predetermined ''reference'' value corresponding to the desired far-field acoustic directivity pattern. In essence, the uncontrolled structure is adaptively modified to behave like the reference structure under acoustic control conditions. Results from this study indicate that the same acoustic directivity pattern can be achieved with model reference control, and the only requirement is that the number of structural sensors used in the control approach is at least equal to the number of control actuators required to achieve the desired acoustic directivity pattern.
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    Experiments on active control of structurally radiated sound using multiple piezoceramic actuators
    Clark, Robert L.; Fuller, Chris R. (Acoustical Society of America, 1992-06-01)
    Active control of sound radiation from a vibrating rectangular plate excited by a steady-state harmonic point force disturbance is experimentally studied. Control structural inputs are achieved by three piezoceramic actuators bonded to the surface of the panel. Microphones were implemented as error sensors in the radiated field, while the control approach was based upon a filtered-X version of the adaptive least-mean-squares (lms)algorithm. Both position and number of piezoceramic actuators were varied during the test to determine the effects on control authority. A variety of test cases were studied for controlling sound radiation due to a disturbance both on and off resonance. Results from these experiments indicate that piezoceramic elements provide an efficient method for distributed modification of structural response to attenuate sound radiation. In addition, the adaptive lms algorithm is shown to be an effective narrow-band controller, which in contrast to feedback approaches, requires little system modeling.
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    Identifying protein interaction subnetworks by a bagging Markov random field-based method
    Chen, Li; Xuan, Jianhua; Riggins, Rebecca B.; Wang, Yue; Clark, Robert L. (Nucleic Acids Research, 2013)
    Identification of differentially expressed subnetworks from protein-protein interaction (PPI) networks has become increasingly important to our global understanding of the molecular mechanisms that drive cancer. Several methods have been proposed for PPI subnetwork identification, but the dependency among network member genes is not explicitly considered, leaving many important hub genes largely unidentified. We present a new method, based on a bagging Markov random field (BMRF) framework, to improve subnetwork identification for mechanistic studies of breast cancer. The method follows a maximum a posteriori principle to form a novel network score that explicitly considers pairwise gene interactions in PPI networks, and it searches for subnetworks with maximal network scores. To improve their robustness across data sets, a bagging scheme based on bootstrapping samples is implemented to statistically select high confidence subnetworks. We first compared the BMRF-based method with existing methods on simulation data to demonstrate its improved performance. We then applied our method to breast cancer data to identify PPI subnetworks associated with breast cancer progression and/or tamoxifen resistance. The experimental results show that not only an improved prediction performance can be achieved by the BMRF approach when tested on independent data sets, but biologically meaningful subnetworks can also be revealed that are relevant to breast cancer and tamoxifen resistance.
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    Implications of noise on the reference signal in feedforward control of harmonic disturbances
    Clark, Robert L.; Gibbs, Gary P. (Acoustical Society of America, 1994-10-01)
    The purpose of this Letter is to detail the effect of random (zero-mean) noise on the reference input of an adaptive feedforward filter in the control of single-frequency disturbance inputs. Results from this study indicate that if system identification is required to execute the adaptive algorithm (as is the case with the filtered-x LMS algorithm), then the sample rate should be set at 4 samples per period to minimize the number of finite impulse response filter coefficients needed to compute an accurate estimate of the filtered reference signal for multi-channel control. However, results from this study demonstrate that the sample rate has very little effect on the minimum mean-squared error that can be achieved with the adaptive control filter, owing to the reverberant characteristics of the dynamics system.
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    Modal sensing of efficient acoustic radiators with polyvinylidene fluoride distributed sensors in active structural acoustic control approaches
    Clark, Robert L.; Fuller, Chris R. (Acoustical Society of America, 1992-06-01)
    An experimental investigation was performed to determine the feasibility of implementing polyvinylidene fluoride (PVDF) piezoelectric distributed sensors on the surface of a structure as error sensors in an adaptive least-mean-squares (lms) control approach to minimize acoustic radiation. While much research has been devoted to controlling vibration of structures with these sensors, they have yet to be implemented in structural acoustic control. To this end, two narrow strip PVDF sensors were positioned on a simply supported plate such that the dominant observed response was due to the odd-odd modes of the plate (i.e., the more efficient acoustic radiators). The error sensors in effect act as spatial wave-number filters and only observe those components that contribute significantly to far-field sound radiation. A variety of test cases were studied for controlling sound radiation due to a disturbance both on and off resonance. Results from these experiments indicate that PVDF sensors and piezoceramic actuators show much promise for controlling acoustic radiation from structures, to a large degree overcoming the need for error microphones in the far field.
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    A model reference approach for implementing active structural acoustic control
    Clark, Robert L.; Fuller, Chris R. (Acoustical Society of America, 1992-09-01)
    A model reference active structural acoustic control (ASAC) approach is investigated as a method for replacing microphone error sensors located in the acoustic field with accelerometers located on the structure. The suggested controller is the multi-input/multi-output adaptive LMS algorithm. As opposed to driving the response of the sensor located on the structure to zero with the control inputs, the response is driven to some pre-determined "reference" value corresponding to an a priori training cost function, which is originally chosen to minimize either the far-field pressure at a number of acoustic field points, or the supersonic region of the wave-number transform. In effect, the uncontrolled structure is adaptively modified to behave like the reference structure. Results indicate that the identical optimal control solution can be obtained when implementing the structural sensors, and the required number of sensors must simply equal the number of control actuators. In addition, for the example studied here, significant reduction in far-field sound radiation can be achieved even when errors of +/- 5% are present in the reference model. A considerable level of attenuation in sound can also be achieved for a band of frequencies within +/- 5% of the reference frequency used to obtain the models for creating the disturbance. Thus it appears that the model reference approach is reasonably robust.
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    A novel approach to feedforward higher-harmonic control
    Clark, Robert L.; Gibbs, Gary P. (Acoustical Society of America, 1994-09-01)
    A novel adaptive filter structure is proposed for the control of systems characterized by higher harmonic response. The control approach has been designated the higher harmonic least-mean squares (HLMS) algorithm to differentiate it from the standard multifrequency filtered-x version of the least-mean squares approach. In the HLMS algorithm, a single frequency reference at that of the fundamental is all that is required to implement the controller. The remaining harmonics are generated internally based upon simple trigonometric relationships. The filtered-x LMS algorithm is implemented in parallel for each frequency to be controlled, minimizing or totally eliminating the contribution of the time-varying terms during the convergence process and increasing the rate of convergence for the higher harmonic control application. Results from the simulation demonstrate that the HLMS approach is far superior to the standard multifrequency, filtered-x LMS algorithm in adaptive, feedforward, higher-harmonic control.
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    Optimal placement of piezoelectric actuators and polyvinylidene fluoride error sensors in active structural acoustic control approaches
    Clark, Robert L.; Fuller, Chris R. (Acoustical Society of America, 1992-09-01)
    Optimization of the location of a rectangular piezoelectric actuator and both the size and location of a rectangular surface strain error sensor constructed from polyvinylidene fluoride (PVDF) for active structural acoustic control (ASAC) is studied in this work. An algorithm is proposed for choosing the optimal actuator/sensor configuration for controlling sound from a baffled simply supported plate excited harmonically, and the resulting acoustic response is predicted from analytical models. These results are compared to those measured in the lab on a test rig duplicating the appropriate boundary conditions and situated in an anechoic chamber. Results from a single optimally located control actuator are compared to those from control with a nonoptimally positioned actuator as well as multiple control actuators. In addition, either microphones are used to provide error information in the test cases or a single optimally located and dimensioned PVDF error sensor is implemented as the cost function. Results from this study indicate that optimization of control actuators and error sensors provides a method for realizing adaptive structures for active structural acoustic control (ASAC), rivaling in importance the performance increases gained when acoustic control is achieved with microphone error sensors and multiple control actuators.
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    A study of the large-scale structure in a supersonic slot injected flow field
    Clark, Robert L. (Virginia Tech, 1988-06-15)
    Large-scale structures were studied in a supersonic stream of air (M = 3) with tangential supersonic slot injection of air (M = 1.7). A dual constant temperature hot-wire probe was used to determine the average structure angles and the characteristic length of the turbulent structures. A zero pressure gradient supersonic boundary layer was studied upstream of the slot injection, and results were compared with previously published data. Structure angles on the order of 50° were obtained throughout the majority of the boundary layer, which was consistent with previously published data. The slot injected flowfield was studied at three axial locations (X/H of 4, 10, 20). Two distinct regions were apparent at each station. A region dominated by the upstream supersonic boundary layer resulted in structure angles on the order of 50°. The mixing region between the slot injected flow (M = 1.7) and the tunnel flow (M = 3) resulted in structure angles on the order of 65°. A compression ramp was used to generate a shock between X/H of 10 and X/H of 20. Structure angles obtained at X/H of 20 appeared unaffected by the streamwise pressure gradient. The characteristic length of the turbulent structures in the supersonic boundary layer and the mixing region of the slot injected flowfield were less than 3.5 mm; however, the characteristic length could not be resolved in this region due to limitations imposed by the frequency response of the hot-wire anemometer systems.