82,853 research outputs found

    Infinite non-causality in active cancellation of random noise

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    Active cancellation of broadband random noise requires the detection of the incoming noise with some time advance. In an duct for example this advance must be larger than the delays in the secondary path from the control source to the error sensor. In this paper it is shown that, in some cases, the advance required for perfect noise cancellation is theoretically infinite because the inverse of the secondary path, which is required for control, can include an infinite non-causal response. This is shown to be the result of two mechanisms: in the single-channel case (one control source and one error sensor), this can arise because of strong echoes in the control path. In the multi-channel case this can arise even in free field simply because of an unfortunate placing of sensors and actuators. In the present paper optimal feedforward control is derived through analytical and numerical computations, in the time and frequency domains. It is shown that, in practice, the advance required for significant noise attenuation can be much larger than the secondary path delays. Practical rules are also suggested in order to prevent infinite non-causality from appearing

    Cherenkov luminescence measurements with digital silicon photomultipliers: a feasibility study.

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    BackgroundA feasibility study was done to assess the capability of digital silicon photomultipliers to measure the Cherenkov luminescence emitted by a β source. Cherenkov luminescence imaging (CLI) is possible with a charge coupled device (CCD) based technology, but a stand-alone technique for quantitative activity measurements based on Cherenkov luminescence has not yet been developed. Silicon photomultipliers (SiPMs) are photon counting devices with a fast impulse response and can potentially be used to quantify β-emitting radiotracer distributions by CLI.MethodsIn this study, a Philips digital photon counting (PDPC) silicon photomultiplier detector was evaluated for measuring Cherenkov luminescence. The PDPC detector is a matrix of avalanche photodiodes, which were read one at a time in a dark count map (DCM) measurement mode (much like a CCD). This reduces the device active area but allows the information from a single avalanche photodiode to be preserved, which is not possible with analog SiPMs. An algorithm to reject the noisiest photodiodes and to correct the measured count rate for the dark current was developed.ResultsThe results show that, in DCM mode and at (10-13) °C, the PDPC has a dynamic response to different levels of Cherenkov luminescence emitted by a β source and transmitted through an opaque medium. This suggests the potential for this approach to provide quantitative activity measurements. Interestingly, the potential use of the PDPC in DCM mode for direct imaging of Cherenkov luminescence, as a opposed to a scalar measurement device, was also apparent.ConclusionsWe showed that a PDPC tile in DCM mode is able to detect and image a β source through its Cherenkov radiation emission. The detector's dynamic response to different levels of radiation suggests its potential quantitative capabilities, and the DCM mode allows imaging with a better spatial resolution than the conventional event-triggered mode. Finally, the same acquisition procedure and data processing could be employed also for other low light levels applications, such as bioluminescence

    Fourth Aircraft Interior Noise Workshop

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    The fourth in a series of NASA/SAE Interior Noise Workshops was held on May 19 and 20, 1992. The theme of the workshop was new technology and applications for aircraft noise with emphasis on source noise prediction; cabin noise prediction; cabin noise control, including active and passive methods; and cabin interior noise procedures. This report is a compilation of the presentations made at the meeting which addressed the above issues

    Feedback control architecture & the bacterial chemotaxis network

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    Bacteria move towards favourable and away from toxic environments by changing their swimming pattern. This response is regulated by the chemotaxis signalling pathway, which has an important feature: it uses feedback to ‘reset’ (adapt) the bacterial sensing ability, which allows the bacteria to sense a range of background environmental changes. The role of this feedback has been studied extensively in the simple chemotaxis pathway of Escherichia coli. However it has been recently found that the majority of bacteria have multiple chemotaxis homologues of the E. coli proteins, resulting in more complex pathways. In this paper we investigate the configuration and role of feedback in Rhodobacter sphaeroides, a bacterium containing multiple homologues of the chemotaxis proteins found in E. coli. Multiple proteins could produce different possible feedback configurations, each having different chemotactic performance qualities and levels of robustness to variations and uncertainties in biological parameters and to intracellular noise. We develop four models corresponding to different feedback configurations. Using a series of carefully designed experiments we discriminate between these models and invalidate three of them. When these models are examined in terms of robustness to noise and parametric uncertainties, we find that the non-invalidated model is superior to the others. Moreover, it has a ‘cascade control’ feedback architecture which is used extensively in engineering to improve system performance, including robustness. Given that the majority of bacteria are known to have multiple chemotaxis pathways, in this paper we show that some feedback architectures allow them to have better performance than others. In particular, cascade control may be an important feature in achieving robust functionality in more complex signalling pathways and in improving their performance

    Feedback control architecture and the bacterial chemotaxis network.

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    PMCID: PMC3088647This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.Bacteria move towards favourable and away from toxic environments by changing their swimming pattern. This response is regulated by the chemotaxis signalling pathway, which has an important feature: it uses feedback to 'reset' (adapt) the bacterial sensing ability, which allows the bacteria to sense a range of background environmental changes. The role of this feedback has been studied extensively in the simple chemotaxis pathway of Escherichia coli. However it has been recently found that the majority of bacteria have multiple chemotaxis homologues of the E. coli proteins, resulting in more complex pathways. In this paper we investigate the configuration and role of feedback in Rhodobacter sphaeroides, a bacterium containing multiple homologues of the chemotaxis proteins found in E. coli. Multiple proteins could produce different possible feedback configurations, each having different chemotactic performance qualities and levels of robustness to variations and uncertainties in biological parameters and to intracellular noise. We develop four models corresponding to different feedback configurations. Using a series of carefully designed experiments we discriminate between these models and invalidate three of them. When these models are examined in terms of robustness to noise and parametric uncertainties, we find that the non-invalidated model is superior to the others. Moreover, it has a 'cascade control' feedback architecture which is used extensively in engineering to improve system performance, including robustness. Given that the majority of bacteria are known to have multiple chemotaxis pathways, in this paper we show that some feedback architectures allow them to have better performance than others. In particular, cascade control may be an important feature in achieving robust functionality in more complex signalling pathways and in improving their performance

    Active control of noise transmitted from barriers

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    Active noise cancellation is a unique approach that helps passive noise control in reducing sound levels at low frequencies; nevertheless, successful use of active noise cancellation necessitates performing numerous and tedious experiments together with defining several parameters properly. The locations and quantity of active control system transducers are among these parameters. The present research provides a comprehensive framework for placing control sources and error microphones near a noise barrier in order to improve its efficiency in both narrowband and broadband noise spectra. To accomplish this, the appropriate locations for the control sources are first determined using a repetitive computation method, and then the optimizations are completed by determining the best position for the error microphone. Several alternative transducer locations near the barrier are incorporated in the repetitive computation, and the optimal sites for the control sources and error microphones are found using two-step optimization methods as well as the genetic algorithms approach. The findings reveal that the best places to put the control sources are on the incident side, below the barrier's edge, and the best locations to place the error microphones are on the shadow side, as close as possible to the target area. The effect of ground reflection on the efficiency of the active noise control system is also investigated, and it is discovered that while ground reflection has no significant effect on the performance of the active noise control system for broadband frequency ranges, it does reduce the control system's efficiency at tonal noises. In order to optimize more parameters, further calculations are performed based on the genetic optimizer. The output of the GA calculations found new configurations for the control units that result in higher noise level reduction at the target area. In addition to the active noise barrier, the application of active noise cancellation for open windows as a particular case of the barrier is explored as a particular case of the barrier. Different arrangements are studied for the control units close to the open windows, including linear, boundary, and planar control arrangements. The effect of several parameters such as the incident angle of noise waves, the distance between error microphones and the opening, and the number of control units are investigated. The results demonstrate that the active noise control system with obliqued linear placements of transducers have higher performance than the other arrangements. Furthermore, when the frequency and incident angle increase, the effectiveness of active noise reduction decreases.La cancelación activa de ruido es un enfoque único que ayuda al control pasivo del ruido a reducir los niveles de sonido a bajas frecuencias; sin embargo, el uso exitoso de la cancelación activa de ruido requiere la realización de numerosos y tediosos experimentos junto con la definición adecuada de varios parámetros. La ubicación y la cantidad de transductores del sistema de control activo se encuentran entre estos parámetros. La presente investigación proporciona un marco completo para colocar fuentes de control y micrófonos de error cerca de una barrera de ruido con el fin de mejorar su eficiencia en espectros de ruido de banda estrecha y banda ancha. Para lograr esto, primero se determinan las ubicaciones apropiadas para las fuentes de control usando un método de cálculo repetitivo, y luego se completan las optimizaciones determinando la mejor posición para el micrófono de error. Varias ubicaciones de transductores alternativas cerca de la barrera se incorporan en el cálculo repetitivo, y los sitios óptimos para las fuentes de control y los micrófonos de error se encuentran utilizando métodos de optimización de dos pasos, así como el enfoque de algoritmos genéticos. Los hallazgos revelan que los mejores lugares para colocar las fuentes de control están en el lado del incidente, debajo del borde de la barrera, y los mejores lugares para colocar los micrófonos de error están en el lado de la sombra, lo más cerca posible del área objetivo. También se investiga el efecto de la reflexión del suelo sobre la eficiencia del sistema de control de ruido activo, y se descubre que si bien la reflexión del suelo no tiene un efecto significativo en el rendimiento del sistema de control de ruido activo para rangos de frecuencia de banda ancha, sí reduce el rendimiento del sistema de control. eficiencia en ruidos tonales. Para optimizar más parámetros, se realizan más cálculos basadosen el optimizador genético. El resultado de los cálculos de GA encontró nuevas configuraciones para las unidades de control que dan como resultado una mayor reducción del nivel de ruido en el área objetivo. Además de la barrera de ruido activa, se explora la aplicación de la cancelación de ruido activa para ventanas abiertas como un caso particular de la barrera. Se estudian cuatro disposiciones para las unidades de control cercanas a las ventanas abiertas. Las unidades de control en una configuración de límite se colocan en el borde de la abertura, mientras que en el control plano, se ubican en la superficie de la abertura. En una configuración de contorno, las unidades de control se colocan en el borde de la abertura, mientras que en un diseño plano, se colocan en la superficie de la abertura. Se investiga el efecto de varios parámetros como el ángulo de incidencia de las ondas de ruido, la distancia entre los micrófonos de error y la apertura, y el número de unidades de control. Los resultados demuestran que el sistema de control de ruido activo con configuración plana tiene un rendimiento más alto que el control de límites. Además, cuando la frecuencia y el ángulo de incidencia aumentan, la eficacia de la reducción activa del ruido disminuye.Postprint (published version

    Aeronautical Engineering: A continuing bibliography, supplement 120

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    This bibliography contains abstracts for 297 reports, articles, and other documents introduced into the NASA scientific and technical information system in February 1980

    Smart double panel with decentralised active dampers for control of sound transmission

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    This report presents the results of a theoretical study of active sound transmission control through a double panel. The double panel material and geometrical properties have been chosen so as to emulate section of an aircraft fuselage, or bodywork of a vehicle. It consists of two plates: an aluminium plate simply supported along all the edges and a honeycomb plate with all the edges free. The two plates, having the same length and width, are connected using elastic mounts, so that a double panel with a thin rectangular cavity between the plates is formed. Since the two plates are linked by the mounting system, and since the air is confined in the cavity between them, they form a structurally and acoustically coupled system. The sound transmission properties of the system are studied in such a way that the aluminium plate (“source panel”) is excited using a plane acoustic wave, while the honeycomb plate (“radiating panel”) radiates sound into free field.The aim of the active control is to reduce the sound transmitted in a broad frequency band, but with a particular focus on the reduction of the sound transmission at lower frequencies of the band. Decentralised velocity feedback control systems (applying active damping) are implemented, with purpose of reducing sound transmission at resonance frequencies. Control sensors and actuators are embedded into the double plate system as a regular array, so that a smart double panel is created. The theoretical study includes analysis of the passive sound transmission in terms of a parametric study, implementation of the active control using skyhook velocity sensors and skyhook force actuators, and the performance/stability analysis in case when reactive actuators and skyhook velocity sensors are used. In the latter case the actuating force is obtained using actuators located in the air cavity which can react off the two plate
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