Design and analysis of an intelligent vibration isolation platform for reaction/momentum wheel assemblies

This study focuses on design and analysis of an intelligent vibration isolation platform for reaction wheel assemblies (RWAs) and momentum wheel assemblies (MWAs). A passive platform consisting of four folded beams is designed and analysed for MWAs. A simple and effective mathematical model is developed for the system consisting of the platform and MWAs, and this model is used to investigate the passive vibration isolation performance. Further development is performed to produce an intelligent platform for RWAs, with piezoelectric sensors and actuators bonded to the vertical beams. The flywheel imbalance and impulse load are assumed to be input disturbances for the investigation of the active vibration isolation performance by the finite element method (FEM). The simulation results show that the passive vibration isolation platform is particularly effective for the suppression of a high frequency range vibration for MWAs, and the intelligent platform using velocity feedback control effectively attenuates the dynamic amplification of amplitude at resonance for RWAs. Thus, it is concluded that the passive platform can be used as a vibration isolation platform for MWAs and that the intelligent one can be used for RWAs.     

Active isolation of a flexible structure from base vibration

This paper presents a theoretical and experimental investigation into an active vibration isolation system. Electromagnetic actuators are installed in parallel with each of four passive mounts, which are placed between a flexible equipment structure and a base structure which is either flexible or rigid. Isolation of low-frequency vibration is studied, so that the passive mounts can be modelled as lumped parameter springs and dampers. Decentralized velocity feedback control is employed, where each actuator is operated independently by feeding back the absolute equipment velocity at the same location. Good control and robust stability have been obtained both theoretically and experimentally for the multichannel control systems. This is to be expected if the base structure is rigid, in which case the actuator and sensor are, in principle, collocated and the control system implements a skyhook damper. With a flexible base structure, however, collocation is lost due to the reactive actuator force acting on the base structure, but the control system is still found to be robustly stable and to perform well. Attenuations of 20 dB are obtained in the sum of squared velocities on the equipment structure at the rigid-body mounted resonance frequencies. In addition, attenuations of up to 15 dB are obtained at the resonance frequencies of both the low order flexible modes of the base structure and the equipment structure.     

A vibration isolation system in low frequency excitation region using negative stiffness structure for vehicle seat

This paper designs and fabricates a vibration isolation model for improving vibration isolation effectiveness of the vehicle seat under low excitation frequencies. The feature of the proposed system is to use two symmetric negative stiffness structures (NSS) in parallel to a positive stiffness structure. Here, theoretical analysis of the proposed system is clearly presented. Then, the design procedure is derived so that the resonance peak of frequency-response curve drifts to the left, the load support capacity of the system is maintained, the total size of the system is reduced for easy practical application and especially, the bending of the frequency-response curve is minimized. Next the dynamic equation of the proposed system is set up. Then, the harmonic balance (HB) method is employed to seek the characteristic of the motion transmissibility of the proposed system at the steady state for each of the excitation frequency. From this characteristic, the curves of the motion transmission are predicted according to the various values of the configurative parameters of the system. Then, the time responses to the sinusoidal, multi frequency and random excitations are also investigated by simulation and experiment. In addition, the isolation performance comparison between the system with NSS and system without NSS is realized. The simulation results reveal that the proposed system has larger frequency region of isolation than that of the system without NSS. The experimental results confirm also that with a random excitation mainly spreading from 0.1 to 10 Hz, the isolation performance of the system with NSS is greatly improved, where the RMS values of the mass displacement may be reduced to 67.2%, whereas the isolation performance of the system without NSS is bad. Besides, the stability of the steady-state response is also studied. Finally, some conclusions are given.     

Free vibration of a wheel

The paper is concerned with the spectral densities (direct and cross) of the multiple response records of a linear system, such as a structure, subjected to multiple random excitation. The various spectral densities of a multivariate random process are in general independent, but the response characteristics of a linear structure impose certain relationships on the response spectral densities, and these are elucidated here. The relationships depend essentially on the numbers of excitation and response components. Special relationships are shown to apply in the common enough situation where the responses of a vibratory system may be taken to derive from the contributions of a finite number of normal modes.     

Suppression of random vibration in flexible structures using a hybrid vibration absorber

A design method is proposed to suppress stationary random vibration in flexible structures using a hybrid vibration absorber (HVA). While the traditional vibration absorber can damp down the vibration mainly at the pre-tuned mode of the primary structure, active damping is generated by the proposed HVA to damp down all resonant modes of interest of the vibrating structure and the spatial average mean square motion of the vibrating structure can be minimized. Only one absorber and one feedback signal are required to achieve global vibration suppression of a flexible structure under stationary random excitation. A special pole-placement controller is designed such that all vibration modes of the flexible structures become critically damped. It is proved analytically that the proposed HVA damps the vibration of the entire structure instead of just the attachment point of the absorber. The proposed optimized HVA is tested on a beam structure and it shows a superior performance on global suppression of broadband vibration in comparison to other published designs of passive and hybrid vibration absorbers.     

Passive reduction of gear mesh vibration using a periodic drive shaft

In this paper, a passive approach to reduce transmitted vibration generated by gear mesh contact dynamics is presented. The approach utilizes the property of periodic structural components that creates stop band and pass band regions in the frequency spectra. The stop band regions can be tailored to correspond to regions of the frequency spectra that contain harmonics and sub-harmonics of the gear mesh frequency, attenuating the response in those regions. A periodic structural component is comprised of a repeating array of cells, which are themselves an assembly of elements. The elements may have differing material properties as well as geometric variations. For the purpose of this research, only geometric variations are considered and each cell is assumed to be identical. A periodic shaft is designed and machined in order to reduce transmitted vibration of a pair of spur gears. Analytical and experimental results indicate that transmitted vibrations from gear mesh contact to the bearing supports are reduced at a variety of operational speeds under static torque preload.     

A low frequency Raman-active vibration of hexagonal boron nitride

Raman spectra of single crystals and powder of hexagonal boron nitride have been measured at room temperature. Two Raman lines due to phonons of 52.5 and 1366 cm^-1 were observed. The 52.5 cm^-1 line, observed for the first time, has been attributed to the shear-type rigid layer mode of the crystal. The force constants related with the vibrations of these modes and their anisotropy are compared with those of graphite.     

车载光学平台双层隔振系统设计与试验研究

分析了冲击、振动对车载精密光学系统的影响,指出了光学平台的变形和振动是影响车载精密光学系统精度和稳定性的主要因素,提出了车载光学平台双层隔振系统的设计方案。采用ANSYS有限元方法分析了光学平台隔振系统的振动模态和减振比,完成了车载状况下的光学平台振动测试。分析与测试结果表明,车载光学平台的双层隔振结构具有良好的稳定性,且对环境强振动有很好的衰减作用。该双层隔振技术方案及其分析测试方法,对于各种车载精密光学系统和设备具有重要的工程应用价值。     

车载光学平台双层隔振系统设计与试验研究

 分析了冲击、振动对车载精密光学系统的影响,指出了光学平台的变形和振动是影响车载精密光学系统精度和稳定性的主要因素,提出了车载光学平台双层隔振系统的设计方案。采用ANSYS有限元方法分析了光学平台隔振系统的振动模态和减振比,完成了车载状况下的光学平台振动测试。分析与测试结果表明,车载光学平台的双层隔振结构具有良好的稳定性,且对环境强振动有很好的衰减作用。该双层隔振技术方案及其分析测试方法,对于各种车载精密光学系统和设备具有重要的工程应用价值。     

Passive acoustic survey of Yangtze finless porpoises using a cargo ship as a moving platform

In order to periodically investigate the population and distribution of the Yangtze finless porpoise (Neophocaena asiaeorientalis asiaeorientalis) in its main distribution range in the Yangtze River, a passive acoustic system deployed on a cargo ship as a moving platform, rather than a dedicated research ship, was developed. A stereo acoustic event data-logger (A-tag) was installed on the cargo ship to passively detect phonating animals. In three surveys carried out in the Yangtze River from Wuhan to Shanghai, an average of 6059 clicks in each survey and 284 porpoises in total were acoustically detected along an 1100-km stretch. The animals were detected frequently in most of the survey range except two "gap sections" with 40 and 60 km lengths, respectively, where no animals were detected in all three surveys. Detected group sizes of the animals in each 120-s time window were not significantly different among the surveys, but the distribution pattern was different and suggested seasonal migration. The cargo ship based passive acoustic survey was effective in detecting phonating animals and can potentially monitor the distribution and population trend over time. Compared to surveys that used dedicated research ships, the present method is more cost effective.     

Action of low frequency vibration on liquid droplets and particles

Liquids handling is an important issue in biomedical analysis. Two different devices for acoustic manipulation of droplets have already been tested. The first one, more classical, uses a high frequency travelling wave and acoustic streaming. The second one uses low frequency flexural standing waves in a plate. This means of liquid handling is original and easy to implement but the physical principle is not obvious. In order to understand more precisely the phenomena involved we present new observations on droplet displacement between two planes and on the behaviour of a droplet on an inclined vibrating plane with this method. The physical principle involved is discussed. The common acoustic radiation pressure formulation is expressed via the non-linear theory of sound propagation, but in our case the acoustic wavelength is much smaller than the height of a water droplet. To get a better understanding of the phenomenon, further experiments on the internal liquid flow and behaviour of particles in the droplet have been performed. These will be compared with results obtained with particles in a thin water-filled vibrating glass tube. The general conclusion is that the phenomenon is practical to use for droplet displacement even if its complex mechanism is not completely understood.     

Decentralized harmonic active vibration control of a flexible plate using piezoelectric actuator-sensor pairs

We have investigated decentralized active control of periodic panel vibration using multiple pairs combining PZT actuators and PVDF sensors distributed on the panel. By contrast with centralized MIMO controllers used to actively control the vibrations or the sound radiation of extended structures, decentralized control using independent local control loops only requires identification of the diagonal terms in the plant matrix. However, it is difficult to a priori predict the global stability of such decentralized control. In this study, the general situation of noncollocated actuator-sensor pairs was considered. Frequency domain gradient and Newton-Raphson adaptation of decentralized control were analyzed, both in terms of performance and stability conditions. The stability conditions are especially derived in terms of the adaptation coefficient and a control effort weighting coefficient. Simulations and experimental results are presented in the case of a simply supported panel with four PZT-PVDF pairs distributed on it. Decentralized vibration control is shown to be highly dependent on the frequency, but can be as effective as a fully centralized control even when the plant matrix is not diagonal-dominant or is not strictly positive real (not dissipative).     

Passive vibration control of a satellite boom structure by geometric optimization using genetic algorithm

In this paper, the superior mid-frequency vibration isolation of a geometrically optimized lightweight structure is demonstrated. The initial structure under test here was a 4.5 m long satellite boom consisting of 10 identical bays with equilateral triangular cross-sections. An unusual geometric variant of this, with inherent isolation characteristics, has been designed by the use of genetic algorithm (GA) methods. In order to obtain the best design, the joints in the boom were allowed to move around by 20% of the length of each bay (i.e., ±9 cm in all three translational directions). This work is based on results from a Fortran code (which was derived from receptance analysis) that are fully validated against detailed finite element (FE) models of the structure. The experimental forced response of the regular boom structure has been measured and compared with predicted curves. Finally, having obtained the geometrically optimized boom structure, its experimental response is compared with the theoretical results predicted by the receptance method. It is shown that the average of 30 dB isolation in the vibration energy transfer between the ends of the network of beams, over a 100 Hz bandwidth predicted in the design process, is achieved experimentally in an essentially undamped structure.     

Active pneumatic vibration isolation system using negative stiffness structures for a vehicle seat

In this paper, an active pneumatic vibration isolation system using negative stiffness structures (NSS) for a vehicle seat in low excitation frequencies is proposed, which is named as an active system with NSS. Here, the negative stiffness structures (NSS) are used to minimize the vibratory attraction of a vehicle seat. Owing to the time-varying and nonlinear behavior of the proposed system, it is not easy to build an accurate dynamic for model-based controller design. Thus, an adaptive intelligent backstepping controller (AIBC) is designed to manage the system operation for high-isolation effectiveness. In addition, an auxiliary control effort is also introduced to eliminate the effect of the unpredictable perturbations. Moreover, a radial basis function neural network (RBFNN) model is utilized to estimate the optimal gain of the auxiliary control effort. Final control input and the adaptive law for updating coefficients of the approximate series can be obtained step by step using a suitable Lyapunov function. Afterward, the isolation performance of the proposed system is assessed experimentally. In addition, the effectiveness of the designed controller for the proposed system is also compared with that of the traditional backstepping controller (BC). The experimental results show that the isolation effectiveness of the proposed system is better than that of the active system without NSS. Furthermore, the undesirable chattering phenomenon in control effort is quite reduced by the estimation mechanism. Finally, some concluding remarks are given at the end of the paper.     

Examination of multi-dimensional vibration isolation measures and their correlation to sound radiation over a broad frequency range

This article examines alternate vibration isolation measures for a multi-dimensional system. The isolator and receiver are modelled by the continuous system theory. The source is assumed to be rigid and both force and moment excitations are considered. Our analysis is limited to a linear time-invariant system, and the mobility synthesis method is adopted to describe the overall system behavior. Inverted ‘L’ beam and plate receivers are employed here to incorporate the contribution of their in-plane motions to vibration powers and radiated sound. Multi-dimensional transmissibilities and effectivenesses are comparatively evaluated along with power-based measures for the inverted ‘L’ beam receiver and selected source configurations. Further, sound pressures radiated from the inverted ‘L’ beam receiver are calculated and correlated with power transmitted to the receiver. Interactions within the ‘L’ beam receiver are also analyzed and measures that could identify dominant transfer paths within a system are examined. Sound measurements and predictions for the inverted ‘L’ plate receiver demonstrate that a rank order based on free field sound pressures, at one or more locations, may be regarded as a measure of isolation performance. Measured insertion losses for sound pressure match well with those based on computed results although further study is needed in relation to some discrepancies shown in the results. Finally, several emerging research topics are identified.     

A low frequency piezoelectric power harvester using a spiral-shaped bimorph

We propose a spiral-shaped piezoelectric bimorph power harvester operating with coupled flexural and extensional vibration modes for applications to low frequency energy sources. A theoretical analysis is performed and the computational results show that the spiral structure has relatively low operating frequency compared to beam power harvesters of the same size. It is found that to optimize the performance of a piezoelectric spiral-shaped harvester careful design is needed.     

Enhancing low frequency sound transmission measurements using a synthesis method

The characterization of low frequency sound transmission between two rooms via a flexible panel is investigated experimentally in this work. Previously, the individual effects of the transmission suite on the measured sound reduction index have been studied analytically, and the results have been compared with the ideal case of having free field radiation conditions on both sides of the panel. A new approach is proposed using a near-field array of loudspeakers driven by a set of optimized signals such that a diffuse pressure field is reproduced on the surface of the partition to be tested. The practical effectiveness of this method is assessed when using a set of 16 acoustic sources located in the source reverberant room in close proximity to an aluminium panel. The experimental results obtained confirm the dependence of the characterized sound reduction index on the particular test chamber considered in the low frequency range. They also validate the proposed synthesis method for providing an estimate that only depends on the properties of the partition itself.     

Azimuthal sound localization using coincidence of timing across frequency on a robotic platform

An algorithm for localizing a sound source with two microphones is introduced and used in real-time situations. This algorithm is inspired by biological computation of interaural time difference as occurring in the barn owl and is a modification of the algorithm proposed by Liu et al. [J. Acoust. Soc. Am. 110, 3218-3231 (2001)] in that it creates a three-dimensional map of coincidence location. This eliminates localization artifacts found during tests with the original algorithm. The source direction is found by determining the azimuth at which the minimum of the response in an azimuth-frequency matrix occurs. The system was tested with a pan-tilt unit in real-time in an office environment with signal types ranging from broadband noise to pure tones. Both open loop (pan-tilt unit stationary) and closed loop experiments (pan-tilt unit moving) were conducted. In real world situations, the algorithm performed well for all signal types except pure tones. Subsequent room simulations showed that localization accuracy decreases with decreasing direct-to-reverberant ratio.