激光全场模态测量技术研究

详细阐述了电子散斑时间平均法振动测量技术的原理,通过对影响测量精度的诸因素进行理论和实验分析,提出通过应用连续相位扫描技术克服相移系统相移不准确所带来的误差.通过散斑平均技术的实施,消除散斑对测量精度的影响.在实验中得到了满意的测量结果.通过线性近似法,解决了振幅低于30nm的振动测量问题,从而使系统分辨率达到亚纳米级,测量精度达到纳米级.     

The system of double-optical-path ESPI for the vibration measurement

In this paper, a system of double-optical-path electronic speckle pattern interferometry (ESPI) for measuring larger vibration is proposed. This system combines the conventional ESPI optical path for measuring out-of plane vibration and the de-sensitized optical path. It can be used to observe qualitatively and analyze quantitatively the vibration mode. At the same time, the phase unwrapping is avoided. The anti-disturbance capability of this system is high, so it can be used to analyze the vibration characteristics of complex and discontinuous structure in actual engineering.     

The separation of out-of-plane displacement from in-plane components by fringe carrier method based on large image-shearing ESPI

A fringe carrier method for separating out-of-plane displacement from in-plane components based on large image-shearing electronic speckle pattern interferometry (ESPI) is presented. If the test object is respectively illuminated by two expanded symmetric illuminations in large image-shearing ESPI, two interferometers are formed. Carrier fringe patterns can be introduced by tilting reference surface a small angle. The carrier fringe patterns are demodulated after deformation of the object. Two phase maps, which include out-of-plane and in-plane displacement, can be obtained by using Fourier transform. Then out-of-plane displacement can be easily separated from in-plane displacement by simple operation between two unwrapped phase distributions. The principle of spatial carrier frequency modulation in large image-shearing ESPI is discussed. A typical three-point-bending experiment is completed. Experimental results are offered. The results show that the method offers high visibility of carrier fringes. And the system presented does not need a special beam as a reference light and has simple optical setup.     

ESPI solution for non-contacting MEMS-on-wafer testing

Rapid progress in the field of micro-electro-mechanical systems (MEMS) makes the development of appropriate measuring and testing means timely. Characterizing the mechanical properties of MEMS structures at a very early stage of manufacturing is a challenging task for quality assurance in this field. The paper describes a new solution that is based upon the vibration analysis of the microparts. The nanometer amplitudes are detected by advanced electronic speckle pattern interferometry (ESPI). A specific signal processing technique has been applied to make the solution robust. Comprehensive numerical simulations provide the theoretical base for the HNDT concept. A laboratory system for 4″ wafer has been built, and extensive tests show that such key properties as e.g. the thickness of springs or membranes can be determined exactly. Automated frequency scanning and corresponding digital image processing open the way to reliable and fast industrial systems for MEMS testing on wafer level.     

Analysis of an internal crack of pressure pipeline using ESPI and shearography

In this study, shearography and ESPI have been used for quantitative analysis of an internal crack of pipeline and both of them have proved to be suitable to qualitatively detect inside crack. However, shearography needs several critical? factors including the amount of shearing, shearing direction and induced load for the quantitative evaluation of the inside crack. In this study, the factors were optimized for the quantitative analysis and the size of cracks has been determined. Although the critical? factors in shearography have been optimized, it is difficult to determine the factors exactly because they are related to the details of cracks. On the other hand, ESPI is independent of the details of a crack and only the induced load plays an important role. The out-of-plane displacement was measured under the optimized load and the measured results were numerically differentiated, which resulted in an equivalent to the shearogram. The size of cracks can be determined quantitatively without any detail of a crack.     

Matched data storage in ESPI by combination of spatial phase shifting with temporal phase unwrapping

We combine the spatial phase-shifting technique with the real-time fringe counting capability of temporal phase unwrapping to provide simple solutions for some practical tasks in ESPI. First, we develop a method for automatically matched data storage intervals and apply this technique to a long-term observation of a biological object with strongly varying deformation rate. Second, we easily obtain on-line displacement and deformation data during the observation of a complexly structured discontinuous object.     

Contrast and sensitivity of the vibration fringes in time-averaged electronic speckle-pattern interferometry: effect of variations of force level

The time averaged frame subtraction technique is improved by subtracting two Bessel fringe patterns at two different force levels. The technique enables significant enhancement of fringe contrast and increased measurement sensitivity. The contrast and sensitivity of the fringes obtained at different force ratios are investigated. Both theoretical and experimental results are presented.     

Measurement of out-of-plane static and dynamic deformations by processing digital speckle pattern interferometry fringes using wavelet transform

In this paper, we are presenting measurement of static and dynamic deformations in rectangular plate fixed at one end using digital speckle pattern interferometry (DSPI). To improve the measurement accuracy, we have developed a new filtering scheme based on combination of average/median filtering and Symlet wavelet filtering which enhances the signal-to-noise ratio (SNR) in the speckle interferogram obtained from DSPI. Experimental results show that our filtering scheme is quite effective in improving SNR of the speckle interferogram. The measurements by DSPI and calculations from beam theory in the case of out-of-plane static deformation and the measurements by DSPI and accelerometer in the case of dynamic deformation are in good agreement.     

Full field and microregion deformation measurement of thin films using electronic speckle pattern interferometry and array microindentation marker method

Thin films now are widely used in micro devices and structures, such as MEMS, electronic packaging, micro sensors, and so on. Their performances highly affect the reliability of the devices. Therefore, it is important to investigate the deformation and the failure mechanism of thin films. In this paper, we present two experimental methods to measure the mechanical properties of thin films. In the first method, a double-field-of-view electronic speckle pattern interferometry system (ESPI) and an integrated deformation and load measurement system are employed, which allows in situ and real-time measurements of full-field deformations of the thin films and microforces under uniaxial tensile test. In the second method, the array microindentation markers were indented on the surface of the thin film using a nanoindenter and the microregion deformations of the tested thin films were measured. In the proposed methods, the tested thin films can be made of metals, oxide ceramics, and multi-layer composites of thickness from several tens micrometers to less than a micron, and the tensile loads from 88 μN to 15 N for the first method or up to 100 N to the second one. The underlying principle of the methods and the experimental set-ups are presented. The deformations of Au and Au/Cr multi-layer films, and the pure Ni films are measured. The performance of the methods and the testing systems are also discussed.     

Measurement of transverse vibrations/visualization of mode shapes in square plate by using digital speckle pattern interferometry and wavelet transform

New filtering scheme is investigated and implemented on digital speckle pattern interferometric fringes to enhance the signal-to-noise ratio (SNR) in the speckle interferograms. To establish the potential of new filtering scheme the experiment was conducted on the vibrating cantilever beam. Experimental results revealed that the new filtering scheme is more powerful than other known filtering schemes (Kumar et al. Opt Laser Eng 2004;41:81–93. Kumar et al. Opt Laser Technol 2001;33:567–571. Shakher et al. Opt Eng 2002;41:176–180. Shakher et al. Proceedings ICICS, Singapore, 9–12 September 1997, p. 953–956. Bowler et al. Proceedings of IEE second international conference on image processing, London, 1986, p. 24–26. Lim. Opt Eng 1981;20:670–678. Devila et al. J Mod Opt 1995;42:1795–1804; Kaufmann et al. Opt Eng 1996;35:9–14). The new scheme reduced the speckle noise and improved SNR in speckle interferograms. Further, the new investigated filtering scheme is implemented to study the mode shapes of square plates under two different boundary conditions. In the first condition all the edges of the square plate were fixed while in the second condition two adjacent edges were fixed and the remaining two edges were free. From the experimental results it is clear that the mode shapes are more distinctly visible with the implementation of the new filtering scheme as compared to other known schemes. Under both the boundary conditions the recorded resonance frequencies were compared with the calculated values of resonance frequencies based on classical theory. The results obtained form DSPI show good agreement with classical theory.