Stephen Daley

Active vibration control for marine applications

Abstract A major problem, in isolating large marine machinery rafts, is how best to mitigate against the effects of excited resonances. These generate large forces on the hull and produce undesirable characteristic signatures. The paper describes the development of a new hybrid active/passive mounting system that has the potential to eradicate such signatures by making use of digitally controlled actuators that ignore local displacements while controlling the response of the structures rigid body modes. Also described in the paper is a method that has been developed for damping flexural modes of vibration in the receiving structure. This further reduces the levels of vibration by directly tackling the mechanism by which the effect of small forces is amplified. The active damping method has recently been successfully demonstrated on a full-scale vessel and a number of key results obtained in the development process are presented.

Active vibration isolation in a “smart spring” mount using a repetitive control approach

In a variety of different engineering systems there is a requirement to isolate sensitive equipment from foundation vibration or alternatively, isolate the foundation from machinery vibration. Passive solutions to this problem provide some isolation but performance is significantly degraded in the presence of structural compliance. A recently proposed hybrid active/passive solution known as the “Smart Spring” mounting system specifically addresses this problem of compliance. In earlier work on this system the required local controller was based on LQG design on the assumption that the vibration sources are random. The work reported here investigates the application of a repetitive control approach to deal with periodic vibration sources. The industrial potential of the approach has been shown using an experimental facility where isolation results in the region of 50dB have been achieved. Copyright

Harmonic control of a ‘smart spring’ machinery vibration isolation system

A major problem for isolating vibration from large marine machinery rafts is how best to deal with excited resonances. These generate large forces on the hull that create a significant vibration problem. The passive design of such mounts typically represents a compromise between providing good vibration isolation and good machinery alignment under seaway motion. The ‘Smart Spring’ isolation system, a new hybrid passive—active approach to solving this problem that is being developed by BAE Systems, has been described in a series of earlier papers. The fundamental concept utilizes digitally controlled actuators to apply forces that are independent of local displacement while controlling the response of the structures rigid body modes. The current paper describes recent work to extend the functionality of the isolation system to deal specifically with discrete-frequency vibration sources. Two novel harmonic control strategies are introduced and evaluated using a six-degrees-of-freedom experimental active mount. It is shown that a strategy that employs recursive least-squares estimation provides both exceptionally high isolation performance and rapid convergence.

Performance and stability analysis of active elastic metamaterials with a tunable double negative response

Metamaterials can possess material parameters which do not exist in conventional materials. Consequently metamaterials provide a novel way to control wave propagation within a structure. These characteristics are achieved by designing a material with a particular sub-wavelength structure which leads to negative constitutive parameters in the long wavelength limit. For elastic materials these parameters are the density and modulus. In a previous study, a theoretical design for a novel active elastic metamaterial (AEM) was proposed. In this material control forces are applied to an array of resonant units in order to achieve a simultaneously negative effective density and modulus over a prescribed frequency band. This design potentially overcomes some of the restrictions imposed by previous passive designs. In this paper a new design of AEM is proposed which compensates for actuator dynamics and provides a basis for practical implementation. This design is shown to have a stable response with a tunable double negative frequency band.

An active viscoelastic metamaterial for isolation applications

Metamaterials 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 techniques.

Application of optimal iterative learning control to the dynamic testing of mechanical structures

Abstract Dynamic testing of mechanical components is carried out across a variety of industries to assess fatigue life or to facilitate optimal design. Such machines are predominantly hydraulically actuated and systems range from single-channel testing of small components to multichannel testing of motor vehicles and airframes. A typical requirement is that in-service loads or displacements are replicated as closely as possible on the tested structure. Such a control problem cannot normally be solved with standard feedback control and it is common to employ iterative methods whereby the actuator command signals are modified in a series of repetitive trials. The industry standard approach, the so-called inverse algorithm, does not always converge to a suitable solution. In this paper an alternative iterative approach, derived using optimization methods, is presented and shown to have superior robustness properties. This is demonstrated through the use of simulation studies and application to both laboratoryscale and full industrial-scale test rigs. Some key theoretical results are also presented that provide important support for the experimental results. In practice, the robustness properties of the new algorithm can enable the required accuracy to be obtained in a greatly reduced number of iterations in relation to the conventional approach, thereby significantly accelerating the test commissioning process.

Design of stable proportional-integral-plus controllers

There are a number of reasons why, in practical situations, implementation of an unstable dynamic controller may be undesirable. In particular, sensor failure can result in a forward path which is unstable and might severely damage the plant. As with many dynamic controller design techniques, the PIP design philosophy may produce controllers which stabilize the closed-loop system, but are themselves unstable. This paper builds upon the existing PIP design philosophy, introducing a design step which stabilizes the PIP controller. This design strategy not only stabilizes the controller, but also ensures that the equivalent closed-loop transfer function of the system remains identical (in the absence of model mismatch) to that of the original PIP closed-loop system.

Enhancing the band gap of an active metamaterial

Metamaterials have been the subject of significant interest over the past decade due to their ability to produce novel acoustic behaviour beyond that seen in naturally occurring media. As well as their potential in acoustic cloaks and lenses, of particular interest is the appearance of band gaps which lead to very high levels of attenuation across the material within narrow frequency ranges. Unlike traditional periodic materials which have been employed at high frequencies, the resonant elements within metamaterials allow band gaps to form within the long wavelength limit; at low frequencies where it is most difficult to design satisfactory passive isolation solutions. Hence, metamaterials may provide a path to high-performance isolation at low frequencies. Passively these band gaps occur over a narrow bandwidth, however the inclusion of active elements provide a method for enhancing this behaviour and producing attenuation over a broad band. A new type of active viscoelastic metamaterial is presented that achieves double negativity and could be employed as a high-performance vibration isolator at low frequencies. A mathematical method for manipulating the band gap profile is developed and a prototype is produced. The passive band gap is confirmed in the laboratory, and then by applying active control using optimised feedback filters it is shown that the region at which attenuation occurs around the band gap could be greatly enhanced whilst retaining the peak passive band gap performance. The active metamaterial demonstrates that a unified design philosophy matching the best features of active and passive functionality can achieve high levels of attenuation over wide frequency bands.

An Investigation of Delayless Subband Adaptive Filtering for Multi-Input Multi-Output Active Noise Control Applications

The 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 implementation.

The online optimisation of stator vane settings in multi-stage axial compressors

Axial 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 VGVs 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 VGVs 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.