Hong Miao

Wavelet analysis of speckle patterns with a temporal carrier

A novel temporal phase-analysis technique that is based on wavelet analysis and a temporal carrier is presented. To measure displacement on a vibrating object by using electronic speckle pattern interferometry, one captures a series of speckle patterns, using a high-speed CCD camera. To avoid ambiguity in phase estimation, a temporal carrier is generated by a piezoelectric transducer stage in the reference beam of the interferometer. The intensity variation of each pixel on recorded images is then analyzed along the time axis by a robust mathematical tool, i.e., a complex Morlet wavelet transform. After the temporal carrier is removed, the absolute displacement of a vibrating object is obtained without the need for temporal or spatial phase unwrapping. The results obtained by a wavelet transform are compared with those from a temporal Fourier transform.

Vibration measurement of a miniature component by high-speed image-plane digital holographic microscopy.

Measuring deformation of vibrating specimens whose dimensions are in the submillimeter range introduces a number of difficulties using laser interferometry. Normal interferometry is not suitable because of a phase ambiguity problem. In addition, the noise effect is much more serious in the measurement of small objects because a high-magnification lens is used. We present a method for full-field measurement of displacement, velocity, and acceleration of a vibrating miniature object based on image-plane digital holographic microscopy. A miniature cantilever beam is excited by a piezoelectric transducer stage with a sinusoidal configuration. A sequence of digital holograms is captured using a high-speed digital holographic microscope. Windowed Fourier analysis is applied in the spatial and spatiotemporal domains to extract the displacement, velocity and acceleration. The result shows that a combination of image-plane digital holographic microscopy and windowed Fourier analyses can be used to study vibration without encountering a phase ambiguity problem, and one can obtain instantaneous kinematic parameters on each point.

A multi-point laser Doppler vibrometer with fiber-based configuration

Laser Doppler vibrometer (LDV) is a non-contact optical interferometric system to measure vibrations of structures and machines with a high precision. Normal LDV can only offer a single-point measurement. Scanning LDV is usually impractical to do measurement on transient events. In this paper, a fiber-based self-synchronized multi-point LDV is proposed. The multiple laser beams with different frequency shifts are generated from one laser source. The beams are projected onto a vibrating object, reflected and interfered with a common reference beam. The signal including vibration information of multiple spatial points is captured by one single-pixel photodetector. The optical system is mainly integrated by fiber components for flexibility in measurement. Two experiments are conducted to measure a steady-state simple harmonic vibration of a cantilever beam and a transient vibration of a beam clamped at both ends. In the first measurement, a numerical interpolation is applied to reconstruct the mode shape with increased number of data points. The vibration mode obtained is compared with that from FEM simulation. In transient vibration measurement, the first five resonant frequencies are obtained. The results show the new-reported fiber-based multipoint LDV can offer a vibration measurement on various spatial points simultaneously. With the flexibility of fiber configuration, it becomes more practical for dynamic structural evaluation in industrial areas.

Phase extraction algorithm considering high-order harmonics in fringe image processing.

A phase calculation method using discrete Fourier series (DFS) is proposed to eliminate the effects of nonsinusoidal characteristics. In this method, the fundamental coefficients are extracted from continuous N samples in one cycle by DFS, with which four images with π/2 intervals are reconstructed, and then more accurate phase distribution can be further obtained. This method is applicable for improving the precision of the traditional phase-shifting algorithm. Its effectiveness and accuracy are verified by computer simulations and moiré fringe and projecting fringe experiments with about 85% of the phase error reduced compared with a four-step phase-shifting algorithm, about 70% reduction compared with a 16-step phase-shifting algorithm.

In situ investigation of the silicon carbide particles sintering

A real-time observation of the microstructure evolution of irregularly shaped silicon carbide powders during solid state sintering is realized by using synchrotron radiation computerized topography (SR-CT) technique. The process of sintering neck growth and material migration during sintering are clearly distinguished from 2D and 3D reconstructed images. The sintering neck size of the sample is presented for quantitative analysis of the sintering kinetics during solid state sintering. The neck size-time curve is obtained. Compared with traditional sintering theories, the neck growth exponent (7.87) obtained by SR-CT experiment is larger than that of the two-spheremodel. Such condition is discussed and shown in terms of sintering neck growth, in which the sintering process slows down when the particle shape is irregular rather than spherical.

Measuring Mechanical Properties of the 3D Carbon/Carbon Composite Using Automated Grid Method

A precise measurement, which was based on the automated grid method, was developed to analyze the mechanical properties of a three-dimensional reinforced carbon/carbon composite under tensile and shear loading conditions. Young’s moduli and Poisson’s ratios of the carbon/carbon composites were studied by means of unidirectional tension testing, and the shear modulus was measured using the Iosipescu shear testing technique. The contact measurement method also was applied to test the tensile strain, and a more sophisticated method based on the digital image correlation technique was applied to test the shear strain. All the testing results obtained from the different methods agree well with one another, and the analysis indicates that the automated grid method is appropriate for testing the mechanical properties of carbon/carbon composites. Based on the stress state analysis, the Young’s modulus along the wrap or weft fiber orientations of the carbon/carbon composite can be obtained via the Iosipescu shear test.

Profilometry using optical edge projection

A novel optical edge projection method to measure the profile of an object is proposed. It can be applied to some special field for example to measure a profile of a specimen with a black and soft surface. A structured black-and-white light edge is projected onto an object surface and the distorted optical edge shadow image is captured by a CCD camera. The border line of the optical edge shadow is extracted and compared with a reference line, the distortion modulated by the height of the object surface is then obtained. By calculating the height, and scanning the optical edge shadow along the object, the height information obtained from a series of images can be determined. Combining the border line in these images, the profile of the object can be measured.

Calibration of fringe projection system for surface profile measurement

Abstract. We propose a calibration method of a fringe projection system for surface profile measurement. The calibration method is divided into two parts: (1) phase to z calibration based on measuring the absolute phase distributions of a flat calibration board placed at several different known z positions. (2) Camera calibration based on Zhang’s method of imaging a checkerboard with different poses. Experiments of a flat plane, a sphere, and a Gorky sculpture demonstrate the validity and accuracy of the proposed technique.

Dynamic measurement via laser interferometry: crystal growth monitoring and modal parameter analysis

Generally there are two categories of noncontact laser interferometric methods commonly used in dynamic measurement, camera-based full-field interferometry and photo-sensor-based laser Doppler interferometry. The two methods have different advantages and disadvantages thus are suitable for different applications. The camera-based interferometry enjoys the valuable merit of full-field observation and measurement. In this paper, one typical full-field interferometry, digital holography, is employed to monitor the growth process of aqueous sodium chlorate crystals. The phase proportional to the solution concentration is retrieved from the holograms captured by CCD camera in real time. There exist no phase ambiguity problem in holography compared with other optical interferometric methods. On the other hand, laser Doppler interferometry is usually a point-wise measurement but with a very high temporal sampling rate. A multipoint laser Doppler interferometer is proposed for modal parameter measurement in this paper. The multiple transient vibration signals of spatially separated points on a beam structure subjected to a shock excitation are recorded synchronously. The natural frequencies and mode shapes are extracted in the signal processing stage. This paper shows that laser interferometry is able to contribute more to the practical applications in dynamic measurement related fields.

Phase distortion removing in fringe projection using windowed Fourier transform

The analysis of the phase distortion in fringe projection with phase shifting is described. A phase distortion is induced when phase shifting method is applied to extract the phase values from the projected fringe patterns in surface contouring. The phase distortion will affect the measurement results especially with measurement of micro-components. The cause of phase distortion is analyzed and its influence on the measurement of micro-component is discussed. In order to remove the phase distortion, a Windowed Fourier Transform (WFT) is employed to extract the phase information from contour measurement. The advantage of WFT phase extraction over the conventional phase shifting method is that WFT can overcome the phase distortion in phase extraction. The principle of the proposed WFT phase extraction method is described and experiment is conducted to measure the surface profile of a micro-component. It is shown that by the use of WFT phase extraction method the phase distortion induced in the conventional phase-shifting technique can be completely eliminated.