Experimental modal testing using pressurized air excitation

Non-fixed excitation methods are often needed in modal analysis (for instance in quality control applications). Several non-fixed methods exist to perform this task: impact hammer, electro-magnetic, acoustic and laser excitation. In this article the use of pneumatic excitation using an impinging air jet is proposed as an alternative. Through a detailed experimental study it is shown that forces up to 0.6 N can be attained, while with the current valve technology one can excite up to 1 kHz. Two application cases are considered: a modal analysis of a cantilevered beam and a vibration test of a hard disk.     

Application of golay complementary coded excitation schemes for non-destructive testing of sandwich structures

In recent years, InfraRed Thermography (IRT) has become a widely accepted non-destructive testing technique to evaluate the structural integrity of composite sandwich structures due to its full-field, remote, fast and in-service inspection capabilities. This paper presents a novel infrared thermographic approach named as Golay complementary coded thermal wave imaging is presented to detect disbonds in a sandwich structure having face sheets from Glass/Carbon Fibre Reinforced (GFR/CFR) laminates and core of the wooden block.     

Self-adaptive modal control for time-varying structures

For the past several years, modal controllers are widely studied and used in the field of vibration or vibro-acoustics control. They are efficient but not robust, because these methods involve a reconstructor based on a modal truncation. When the dynamic behavior of the structure change, the controller and reconstructor must be updated to cope with the changes in the structure behavior, in order to maintain both performance and robustness. A solution is adaptive control but this approach needs some specific information not generally available particularly in the case of undergone modifications. This paper deals with a self-adaptive modal control based on a real-time identifier, which avoid the need of specific information. The identifier permits to update the controller and the reconstructor according to the changes of modal characteristics of time-varying structures. A classical algorithm of identification is used to obtain a state space model with an unspecified state vector. Then, based on this model, a well adapted transformation is carried out to get the modal characteristics from the expression of complex modes, including the mode shapes. As a criterion of running identification, the value of “variance-accounted for” (VAF) is employed to carry out the identifier only when the initial or previous model is not enough exact. A Linear Quadratic Gaussian Algorithm is employed in such a way that the controller and observer can be optimized according to the updated modal model. By this way, a self-adaptive modal control is completed and can demonstrate some smart properties. The proposed methodology is carried out on a simple but representative time-varying mechanical discrete structure. An inertia modification leads not only to low modal frequency shifts but also to inversion of a mode shape which is shown to lead to unstable configuration when control system is not updated. The overall procedure will be described through simulations and performed for different operating conditions, which will prove that mode shapes have to be precisely determined and updated in the controller and observer to guarantee a robust modal control with high performance in spite of the changes of structure.     

Independent modal control for nonlinear flexible structures: An experimental test rig

This paper investigates the use of independent modal control to suppress the vibration of nonlinear flexible structures. In recent years technological improvements in the mechanical field have led to high-performance systems with low weight and, as a consequence, high flexibility and low damping. Here active control quickly bettered the traditional passive damping systems. The structure investigated in this paper is a multi-body flexible boom moved by hydraulic actuators. The nonlinear system dynamic was numerically reproduced and a control strategy, based on the use of the same actuators, was developed. Finally a test rig was created to validate the proposed approach experimentally.     

Application of magnetomechanical sensors for modal testing

A new modal testing technique using magnetomechanical sensors is proposed in this paper. To list some advantages of this technique, sensors are cost-effective and require no direct physical contact with a structure. The specific application made in this paper is the modal testing for the bending vibration of a solid circular beam. The theoretical analysis explaining the principle of the magnetostrictive sensor-based modal testing is presented for beam bending. The present results are compared with those obtained by the use of standard accelerometers. Although the application of this technique is made to relatively simple problems, the potential of magnetomechanical sensors for modal testing has been revealed.     

A method for modal identification of lightly damped structures

In many cases modal tests are conducted on individual components of complex engineering structures where interest is confined to deriving an undamped model of the structure. A method is proposed for this task which demands a minimum of input data and which, in particular, does not require accurate measurements around resonance. The method is simple to program and its application to various practical structures is described.     

Hybrid holographic-numerical method for modal analysis of complex structures

This paper presents a hybrid holographic-numerical method for modal analysis of complex structures. A continuous structure is first lumped into a number of discrete elements to form an elastically connected lumped linear system. The matrix of influence coefficients of the lumped linear system are then determined by exerting a static load to the element centers and measuring the corresponding whole-field displacement using digital holographic interferometry. The eigenvalues and eigenvectors of the influence coefficients, which in a physical sense represent the natural frequencies and mode shapes of the structure, are then calculated using the numerical method. A major advantage of the proposed hybrid method is that it is not necessary to know the Young's modulus, the Poisson's ratio of the material and the boundary conditions, as the displacement field measured by the optical method has automatically reflected the real boundary conditions and the material properties, which makes this method particularly useful for studying objects made from anisotropic materials such as composites. Another advantage of the proposed method is that structures of any complex and irregular shape will not increase the complexity of the characterization process. The proposed is also suitable for experimentally validating the modal analysis results from finite element method models.     

Threshold peak structures in the vibrational excitation of HCl by low-energy electron impact

A new model which describes dissociative electron attachment and vibrational excitation for the HCl molecule is proposed. The model is based on ab initio scattering data calculated in the fixed nuclei approximation by Morgan et al. [J. Phys. B 23 (1990) 99]. The cross sections for vibrational excitation exhibit interesting oscillatory structures in the threshold peak region.     

Role of vortex structures in excitation of condensed system self-oscillatory burning

The effect of free convection and vortex structures arising near the “singing” flame of a gasoline blow torch on excitation of thermal self-oscillations in a resonator tube is studied experimentally. A technique for measuring the oscillation amplitude of the gas column is suggested. It is found that the excitation of acoustic oscillations decreases the height of the singing flame and the mass velocity of burning but raises the gasoline combustion efficiency. The variation of the temperature field of the singing flame over an oscillation cycle is studied by digital photometry. Hysteretic dependences of the acoustic oscillation amplitude on the thermal power of the gasoline diffusion flame are obtained. A mechanism explaining the influence of vortex structures on the self-oscillatory mode of burning in condensed systems is discussed.     

Response of periodic structures by modal analysis

A periodic structure is a structure consisting of identical substructures, coupled together in identical ways to form the complete system. The undamped response of such a system is derived by using a modal analysis technique. The procedure allows for arbitrary loads and takes full advantage of the periodic properties of the structure. The algorithm is based on a technique previously developed by the authors.     

Electron impact excitation cross sections for phosphorus

An analytic atomic independent-particle-model is used to generate wave functions for the valence and excited states of the neutral phosphorus atom. These wave functions are used to calculate generalized oscillator strengths, and from these quantities the cross sections are obtained in Born approximation. Various excitations from the ground state are considered, and results are presented for electron impact energies up to 5 keV. Received: 6 May 1997 / Revised: 21 October 1997 / Accepted: 5 December 1997     

小型光电编码器自动检测系统

设计了小型光电编码器自动检测系统,用于准确快速地检测小型光电轴角编码器的误差。采用21位高精度光电编码器作为角度基准来检测低精度的小型光电编码器。用控制系统转动编码器,此时高精度编码器与被检编码器之差即为小型编码器在该位置的误差。检测完毕,根据需要将编码器的误差通过USB接口传输给上位机保存并进行深度分析。采用该系统检测某型号的15位编码器,得到的均方差为29.1″,而传统方法检测的误差均方差为28.5″,表明设计的检测系统满足检测精度要求。     

An analytical model of two-dimensional impact/sliding response to harmonic excitation

An analytical method for the generation of periodic solutions for impact/sliding response to harmonic excitation of a two-dimensional linear oscillator is outlined and applied to generate a simple symmetric solution. The method yields impact reaction forces and sliding distances and hence enables wear rate calculations to be performed.     

Electron impact excitation rate coefficients for hydrogen,helium and alkali atoms

The rates of electron impact excitation of bound electronic states are calculated by interpolating the existing quantum-mechanical theories and applying an empirical correction. The calculation is done for hydrogen, helium, lithium, sodium, potassium, rubidium and cesium. The resulting rate coefficients are expressed by two parameters, the values of which are presented in tables. The error of the present calculation is estimated by comparing with available experimental data to be within a factor of approximately 2.     

振动系统的模态分析实验设计

利用模态分析原理和简单试验仪器，得到了直观的悬臂梁和圆盘前4阶模态振型，使学生熟悉了模态分析原理和测试方法，有利于学生对振动规律的理解．     

Electron impact excitation rate coefficients for Ni-like gadolinium

A detailed and large-scaled calculation is performed on the total electron impact excitation rate coefficients from the ground state to the 106 fine-structure levels in 3l^-14l^′ of Ni-like Gd³⁶⁺ employing the relativistic configuration-interaction distorted-wave approximation. The resonance contributions from 3l¹⁷4l^′n^′′l^′′ and 3l¹⁷5l^′n^′′l^′′ doubly-excited states of Cu-like Gd³⁵⁺ are taken into account using the isolated process and isolated resonances approximation. The effects of the radiative decays from the resonances are investigated carefully and are found to be significant. The present rate coefficients, as well as the collision strength, are compared extensively with the previously published results. We believe our results should be more accurate and reliable.