1,721,064 research outputs found
An active viscoelastic metamaterial for isolation applications
No full textMetamaterials are of interest due to their ability to produce novel acoustic behaviour beyond that seen in naturally occurring media. Of particular interest is the appearance of band gaps which lead to very high levels of attenuation within narrow frequency ranges. Resonant elements within metamaterials allow band gaps to form within the long wavelength limit at low frequencies where traditional passive isolation solutions suffer poor performance. Hence metamaterials may provide a path to high performance, low frequency isolation. Two metamaterials are presented here. An acoustic material consisting of an array of split hollow spheres is developed, and its performance is validated experimentally. The application of an acoustic/mechanical analogy allows the development of an elastodynamic metamaterial that could be employed as a high performance vibration isolator at low frequencies. A prototype isolator is manufactured, and its performance is measured. The passively occurring band gap is enhanced using an active control architecture. The use of the active control system in conjunction with the natural passive behaviour of the metamaterial enables high levels of isolation across a broad frequency range. An eventual goal of the work is to produce such materials on a small scale, and as such the metamaterials developed are designed for, and produced using, additive layer manufacturing technique
A geometric approach to the design of remotely located vibration control systems
No full textOver the past three decades, a wide variety of active control methods have been proposed for controlling problematic vibration. The vast majority of approaches make the implicit assumption that sensors can be located in the region where vibration attenuation is required. For many large scale structures or where the system environment is harsh, this is either not feasible or it is prohibitively expensive. As a result, the optimal control of local vibration may lead to enhancement at remote locations. Motivated by such problems in marine system environments, this paper describes a simple geometric methodology that provides an approach for defining the design freedom available for reducing vibration both at local and remote locations. The results can be used to develop design procedures for both discrete frequency and broad-band control. Robustness to modelling error can also be treated in the same geometric framework. Validation of the approach is carried out using an experimental facility that has been developed to replicate the problems associated with rotor blade vibration.<br/
A method of adaptation between steepest-descent and Newton's algorithm for multichannel active control of tonal noise and vibration
No full textActive control methods have been applied to a number of practical problems in which it is necessary to control a tonal disturbance. For example, the control of engine noise and vibration in vehicles, propeller noise in aircraft and the vibration produced by reciprocating machinery in many industrial systems. In such applications the steepest-descent algorithm has been widely employed, in part due to its robustness to variations in the plant response. This robustness, however, comes at the expense of a potentially slow convergence speed and this may limit the performance in applications where the disturbance is non-stationary. To improve the speed of convergence, an iterative least-squares algorithm can be employed, such as Newton’s algorithm. The convergence of these algorithms is less dependent on the potentially large eigenvalue spread of a multichannel plant matrix and, therefore, can theoretically achieve more rapid convergence. However, these algorithms are significantly less robust to plant response variations and, therefore, their practical performance can be somewhat limited. Generalised algorithms have been presented which combine steepest-descent and Newton’s method in or- der to provide a fixed compromise between convergence and robustness. This paper presents a method of adaptively combining steepest-descent and Newton’s method in order to achieve both rapid convergence and robustness to plant response variations. The two algorithms are combined into a single update equation in which a single mixing parameter facilitates a trade-off between the two algorithms. A method of adapting this parameter to minimise the cost function is presented and the performance of the proposed algorithm is assessed through a series of simulations. The proposed combination algorithm is shown to improve the control performance in the presence of plant response variations compared to both the steepest-descent and Newton’s algorithms
Active structural acoustic control using the remote sensor method
No full textActive structural acoustic control (ASAC) is an effective method of reducing the sound radiation from vibrating structures. In order to implement ASAC systems using only structural actuators and sensors, it is necessary to employ a model of the sound radiation from the structure. Such models have been presented in the literature for simple structures, such as baffled rectangular plates, and methods of determining the radiation modes of more complex practical structures using experimental data have also been explored. A similar problem arises in the context of active noise control, where cancellation of a disturbance is required at positions in space where it is not possible to locate a physical error microphone. In this case the signals at the cancellation points can be estimated from the outputs of remotely located measurement sensors using the “remote microphone method”. This remote microphone method is extended here to the ASAC problem, in which the pressures at a number of microphone locations must be estimated from measurements on the structure of the radiating system. The control and estimation strategies are described and the performance is assessed for a typical structural radiation problem
An investigation of delayless subband adaptive filtering for multi-input multi-output active noise control applications
No full textThe broadband control of noise and vibration using multi-input, multi-output (MIMO) active control systems has a potentially wide variety of applications. However, the performance of MIMO systems is often limited in practice by high computational demand and slow convergence speeds. In the somewhat simpler context of single-input, single- output broadband control, these problems have been overcome through a variety of methods including subband adaptive filtering. This paper presents an extension of the subband adaptive filtering technique to the MIMO active control problem and presents a comprehensive study of both the computational requirements and control performance. The implementation of the MIMO filtered-x LMS algorithm using subband adaptive filtering is described and the details of two specific implementations are presented. The computational demands of the two MIMO subband active control algorithms are then compared to that of the standard full-band algorithm. This comparison shows that as the number of subbands employed in the subband algorithms is increased, the computational demand is significantly reduced compared to the full-band implementation provided that a restructured analysis filter-bank is employed. An analysis of the convergence of the MIMO subband adaptive algorithm is then presented and this demonstrates that although the convergence of the control filter coefficients is dependent on the eigenvalue spread of the subband Hessian matrix, which reduces as the number of subbands is increased, the convergence of the cost function is limited for large numbers of subbands due to the simultaneous increase in the weight stacking distortion. The performance of the two MIMO subband algorithms and the standard full-band algorithm has then been assessed through a series of time-domain simulations of a practical active control system and it has been shown that the subband algorithms are able to achieve a significant increase in the convergence speed compared to the full-band implementatio
Multivariable control of tonal disturbances using minimization of the maximum error signal through adaptive error signal weighting
No full textIn many multichannel active noise and vibration control systems the controller is adapted to minimize the 2-norm of the error signals. This may, however, lead to a large spatial variance in the residual error. A method of achieving a more uniformly controlled error field using a weighted squared error strategy has previously been proposed, although the presented method of defining the error weighting parameters results in a very slow convergence rate. This convergence rate limitation has been overcome by the minimax algorithm which minimizes, in a least-squares sense, the maximum error signal at each iteration. However, due to the inherent switching in this algorithm, for fast convergence speeds it suffers from significant misadjustment and in a tonal control problem this introduces additional unwanted spectral components. In this paper an alternative method of minimizing the maximum error signal is proposed which uses an adaptive error-weighting matrix that is bounded and so avoids the slow convergence speeds previously reported. It is also shown that the proposed algorithm does not suffer from the same misadjustement problems shown by the minimax algorithm. The details of the proposed method are first outlined and then its performance is compared to the previously proposed methods through a series of time-domain simulations employing measurements of a physical system
The effect of active noise control on the effective material properties
No full textActive noise control has been used in a variety of applications where it is impractical to achieve significant levels of noise control using traditional passive noise control treatments, for example, at low frequencies. In recent years, a similar performance benefit has been gained through the use of acoustic metamaterials, which achieve levels of noise control performance that are not achievable with naturally occurring materials. The high levels of noise control performance are generally attributable to the dispersive properties of these metamaterials and, as such, their behaviour has generally been evaluated in terms of their effective material properties, such as the effective density and bulk modulus. It is well known that the performance of active noise control systems is also frequency dependent, however, the links between these two advanced noise control strategies have not been extensively investigated. Therefore, this paper presents an investigation into how active noise control systems implemented in a duct modify the effective material properties. This work, therefore, begins to make links between the behaviour of acoustic metamaterials and active noise control systems
Broad band controller design for remote vibration using a geometric approach
No full textOver the past three decades, a wide variety of active control methods have been proposed for controlling problematic vibration. The vast majority of approaches make the implicit assumption that sensors or actuators can be located in the region where vibration attenuation is required. However this is either not feasible or prohibitively expensive for many large scale structures or where the system environment is harsh. As a result, optimal control of local vibration may lead to enhancement at remote locations. Controlling remote vibration using only local sensing and actuation is an important concept to resolve this remote vibration control problem. Recently, a geometric methodology that provides an approach for defining the design freedom available for reducing vibrations at both local and remote locations has been proposed by the authors. In an earlier paper, the fundamental results were used to develop design procedures for discrete frequency control; in the current paper, however, the focus is on design procedures for broad band control. A systematic approach is developed that provides an additional design constraint to the geometric methodology to ensure that the resulting compensator provides closed loop stability. The design procedure is illustrated through its application to an active vibration isolation structure.<br/
The online optimisation of stator vane settings in multi-stage axial compressors
No full textAxial compressors for high efficiency industrial gas turbines are required to operate over a wide range of mass flow rates and rotational speeds. However, the useful range of operation of the axial-flow compressor is limited by the onset of two instabilities known as surge and rotating stall. To resolve these problems, variable stator blades or VGV's are considered by optimising the blade setting in order to avoid the stall and subsequent surge. A steady state model of a 15 stage multi-stage axial compressor is utilised here to investigate the performance, particularly for obtaining acceptable optimisation convergence time for practical purposes. For the effective search for an optimum setting, the variation in VGV's with respect to a different combination of objective functions is considered. In this paper, self-tuning extremum control and a particle swarm optimisation method are proposed and implemented to obtain the best value for a normalised objective function. The results demonstrate the relative effectiveness of the two algorithms and the suitability for their use in this proposed application. The study clearly demonstrates that the PSO provides the best performance in seeking the optimum of the chosen objective functions. <br/
An H-infinity design approach to the control of human induced vibration in office floor structures
No full textModern design and manufacturing methods have led to the widespread use of slender beams in the construction of office buildings. Coupled with the increasing use of open-plan areas this has led to an increasing problem of excessive floor vibrations as a result of human walking which causes annoyance and discomfort for occupants. One viable solution is to use active vibration control methods to suppress the levels of floor vibration. A number of research studies have proposed the use of decentralised velocity feedback schemes and some success at full scale has been reported. However, the use of such localised schemes can result in increased levels of vibration at other remote locations on the floor. In this paper this problem is highlighted using a finite element model of a typical office floor structure. In addition it is shown that an H-infinity optimisation framework utilising linear matrix inequalities can be used to provide controllers that reduce the global response of the floor. The framework allows for different controller architectures that include both decentralised collocated velocity feedback at limited locations on the floor and centralised dynamic compensator
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