A shape memory alloy adaptive tuned vibration absorber: design and implementation

In this paper a tuned vibration absorber (TVA) is realized using shape memory alloy (SMA) elements. The elastic modulus of SMA changes with temperature and this effect is exploited to develop a continuously tunable device.A TVA with beam elements is described, a simple two-degree-of-freedom model developed and the TVA characterized experimentally. The behaviour during continuous heating and cooling is examined and the TVA is seen to be continuously tunable. A change in the tuned frequency of 21.4% is observed between the cold, martensite, and hot, austenite, states. This corresponds to a change in the elastic modulus of about 47.5%, somewhat less than expected.The response time of the SMA TVA is long because of its thermal inertia. However, it is mechanically simple and has a reasonably good performance, despite the tuning parameters depending on the current in a strongly nonlinear way

On numerical issues for the wave/finite element method

The waveguide finite element (WFE) method is a numerical method to investigate wave motion in a uniform waveguide. Numerical issues for the WFE method are specifically illustrated in this report. The method starts from finite element mass and stiffness matrices of only one element of the section of the waveguide. The matrices may be derived from commercial FE software such that existing element libraries can be used to model complex general structures. The transfer matrix, and hence the eigenvalue problem, is formed from the dynamic stiffness matrix in conjunction with a periodicity condition. The results of the eigenvalue problem represent the free wave characteristics in the waveguide. This reportconcerns numerical errors occurring in the WFE results and proposing approaches to improve the errors.In the WFE method, numerical errors arise because of (1) the FE discretisation error, (2) round-off errors due to the inertia term and (3) ill-conditioning. The FE discretisation error becomes large when element length becomes large enough compared to the wavelength. However, the round-off error due to the inertia term becomes large for small element lengths when the dynamic stiffness matrix is formed. This tendency is illustrated by numerical examples for one-dimensional structures.Ill-conditioning occurs when the eigenvalue problem is formed and solved and the resulting errors can become large, especially for complex structures. Zhong’s method is used to improve the conditioning of the eigenvalue problem in this report. Errors in the eigenvalue problem are first mathematically discussed and Zhong’s method validated. In addition, singular value decomposition is proposed to reduce errors in numerically determining theeigenvectors. For waveguides with a one-dimensional cross-section, the effect of the aspect ratio of the elements on the conditioning is also illustrated. For general structures, there is a crude trade-off between the conditioning, the FE discretisation error and the round-off error due to the inertia term. To alleviate the trade-off, the model with internal nodes is applied. At low frequencies, the approximate condensation formulation is derived and significant errorreduction in the force eigenvector components is observed.Three approaches to numerically calculate the group velocity are compared and the finite difference and the power and energy relationship are shown to be efficient approaches for general structures

Energy harvesting from train vibrations

In this paper, linear mechanical oscillators are designed to harvest energy from train-induced vibrations. The harvested energy could be used, for example, to charge sensors mounted on the rail track for structural health monitoring. The dominant frequencies due to a passing train are determined for a specific train and speed from a recorded acceleration time-history. Using a simple model of an oscillator, the total energy harvested for the passage of one train is calculated. The stiffness, and hence the tuning frequency of the device, is varied in simulations to determine the optimum frequency at which to tune the device for a constant value of mass and damping in the device. Further simulations are conducted to investigate the power that could be harvested from multiple oscillators tuned at several dominant frequencies, and their performances are analysed and compared. The constraint for maximum relative displacement is considered in the design of each harvester, and this is adopted to assure that the amplitude of the oscillation is finite and does not exceed the physical size of the device. The robustness of the harvester is also analysed for different train speeds

Active vibration control (AVC) of a satellite boom structure using optimally positioned stacked piezoelectric actuators

In this paper, results for active vibration control predicted from experimental measurements on a lightweight structure are compared with purely computational predictions. The structure studied is a 4.5m long satellite boom consisting of 10 identical bays with equilateral triangular cross sections. First, the results from a Fortran code that is based on a receptance analysis are validated against the experimental forced response of the boom structure. Exhaustive searches are then carried out to find the optimum positions for one and two actuators. Finally, a genetic algorithm is employed to find high-quality positions for three actuators on the structure that will achieve the greatest reductions in vibration transmission. Having found these actuator positions, experiments are then carried out to verify the quality of the theoretical predictions. It was found that the attenuation achievable in practice for one, two and three actuators were, respectively, 15.1, 26.1 and 33.5 dB

Active vibration control (AVC) of a satellite boom structure using optimally positioned stacked piezoelectric actuators

In this paper, results for active vibration control predicted from experimental measurements on a lightweight structure are compared with purely computational predictions. The structure studied is a 4.5m long satellite boom consisting of 10 identical bays with equilateral triangular cross sections. First, the results from a Fortran code that is based on a receptance analysis are validated against the experimental forced response of the boom structure. Exhaustive searches are then carried out to find the optimum positions for one and two actuators. Finally, a genetic algorithm is employed to find high-quality positions for three actuators on the structure that will achieve the greatest reductions in vibration transmission. Having found these actuator positions, experiments are then carried out to verify the quality of the theoretical predictions. It was found that the attenuation achievable in practice for one, two and three actuators were, respectively, 15.1, 26.1 and 33.5 dB

Point vibration measurements for the detection of shallow-buried objects

AbstractA major UK initiative, entitled ‘Mapping the Underworld’, is seeking to address the serious social, environmental and economic consequences arising from an inability to locate accurately and comprehensively the buried utility service infrastructure without resorting to extensive excavations. Mapping the Underworld aims to develop and prove the efficacy of a multi-sensor device for accurate remote buried utility service detection, location and, where possible, identification. One of the technologies to be incorporated in the device is low-frequency vibro-acoustics, and application of this technique for detecting buried infrastructure is currently being investigated. Here, the potential for making a number of simple point vibration measurements in order to detect shallow-buried objects, in particular plastic pipes, is explored. Point measurements can be made relatively quickly without the need for arrays of surface sensors, which can be expensive, time-consuming to deploy, and sometimes impractical in congested areas.At low frequencies, the ground behaves as a simple single-degree-of-freedom (mass–spring) system with a well-defined resonance, the frequency of which will depend on the density and elastic properties of the soil locally. This resonance will be altered by the presence of a buried object whose properties differ from the surrounding soil. It is this behavior which can be exploited in order to detect the presence of a buried object, provided it is buried at a sufficiently shallow depth. The theoretical background is described and preliminary measurements are made both on a dedicated buried pipe rig and on the ground over a domestic waste pipe. Preliminary findings suggest that, for shallow-buried pipes, a measurement of this kind could be a quick and useful adjunct to more conventional methods of buried pipe detection