In fringe projection profilometry (FPP), non-sinusoidal fringes due to the gamma effect of a projector will cause measurement errors. To solve this problem, a binary defocusing technique has been introduced in recent years, yet the appropriate defocusing is hard to evaluate quantitatively. An innovative approach to quantify binary defocusing in real-time is proposed. No matter how the projector and camera are arranged in FPP system, the fringe period in a captured defocusing fringe image is strictly varying; therefore, unlike previous methods, the proposed method uses a limited window for defocusing evaluation. This not only improves the accuracy of defocusing evaluation, but also greatly improves the evaluation speed, thus in turn enabling real-time evaluation. In addition, numerical differentiation is introduced on the window using a modified five-interval-point algorithm to effectively improve the sensitivity to improper (inadequate or excessive) defocusing. The difference between the numerical differentiation and its fundamental harmonic extracted by Levenberg–Marquardt iterative is taken as an evaluation value of binary defocusing. The smaller the difference, the more appropriate is the defocusing. By minimizing this difference, the most appropriate defocusing can be obtained quantitatively. Both numerical simulations and experiments validate the high sensitivity and high speed of the proposed method.
Multiple digital image correlation (DIC) systems can enlarge the measurement field without losing effective resolution in the area of interest (AOI). However, the results calculated in substereo DIC systems are located in its local coordinate system in most cases. To stitch the data obtained by each individual system, a data merging algorithm is presented in this paper for global measurement of multiple stereo DIC systems. A set of encoded targets is employed to assist the extrinsic calibration, of which the three-dimensional (3-D) coordinates are reconstructed via digital close range photogrammetry. Combining the 3-D targets with precalibrated intrinsic parameters of all cameras, the extrinsic calibration is significantly simplified. After calculating in substereo DIC systems, all data can be merged into a universal coordinate system based on the extrinsic calibration. Four stereo DIC systems are applied to a four point bending experiment of a steel reinforced concrete beam structure. Results demonstrate high accuracy for the displacement data merging in the overlapping field of views (FOVs) and show feasibility for the distributed FOVs measurement.
This study presents the electromechanical response of a piezoelectric laminated micro plate under the excitation of an ultrasonic wave. The laminated plate consists of a piezoelectric layer (AlN), an elastic layer (SiO2) and two electrode layers (Au and Pt). Since the whole thickness of the plate is in micro scale, the size dependence of the dynamic behavior of the laminated plate is evaluated using the couple stress theory. The results show that the bending rigidity of the micro plate increases when the size effect is considered and the amplitudes of output of electric charge and voltage are reduced accordingly when the plate is excited by ultrasonic wave. Also the resonant frequency of the laminated plate increase because of the enhancement of the bending rigidity of the plate. The analysis results can provide a reference for the design of micromachined piezoelectric sensors.
In order to have an efficient design methodology and achieve high-performance inertial instruments, it is important to
analyze the dynamic characteristics of micromechanical vibratory gyroscopes. In this paper, a novel optical method
based on a high-speed CMOS camera and different image recognition methods is developed to measure the translational
motion of the sensitive elements. Experimental results show that digital image correlation (DIC) in the spatial domain is
more accurate for determination of small displacements while quite time-consuming, and the mode of addition,
subtraction or multiplication of spectrum images is much faster and provide more accurate results for determination of
large displacements. Due to the edge features of images, the edge detection method based on wavelet is also proposed to
calculate the displacements. This technique is quite fast and its precision is moderate. Accordingly, the combination of
different image recognition methods will be more suitable for dynamic displacement measurements which require both
high accuracy and high speed.
Micro beams are frequently used as executive components in MEMS such as pressure and acceleration sensors, micro
switches, and micro resonators. In this paper, the influence of air viscous damping on the dynamic characteristics of a
micro beam that oscillates in the air is evaluated analytically. A closed-form solution that can be used to predict the
dynamic response of a micro beam is obtained based on a beam model used for evaluating the air viscous damping. The
resonant frequencies versus the geometry parameters of a micro beam undergoing the air viscous damping are analyzed.
The analysis results show that the influence of the air viscous damping on the resonant frequencies of a micro beam is
notable and hence it should be take into account in the design process of MEMS. Moreover, as we know, there are
various excitation methods for a micro beam in dynamics such as the electrostatic and PZT methods. The analytical
results in our work also show that the influence of air viscosity on the dynamic characteristics of a micro beam will be
diverse under different excitation methods.
An effective method for reducing speckle noise in digital holography is proposed in this paper. The proposed method does not require classical filtering technique; instead it utilizes multiple holograms of an object generated by rotating an illuminating light continuously. The reconstructed hologram intensity fields possess different speckle patterns, and by properly averaging the intensity fields, speckle noise in the reconstructed images is reduced significantly. A merlion sculpture is evaluated and experimental results show that the proposed method is simple and effective in reducing speckle noise in digital holography.
In this paper, optical methods for the surface profile and deformation measurement of MEMS materials and structures are introduced. The methods are based on the digital image correlation, interferometry and fringe projection. The deformations of a micro-mirror and a micro-beam are measured by phase shifting fringe projection and interferometry. The surface profile of an electrode pad was used for the demonstration of the microscopic surface contouring. For the MEMS material test, using a long working distance microscope and digital speckle correlation method the tensile strain of a thin copper wire is measured.
A new phase unwrapping algorithm is proposed in this paper. This algorithm, based on the global continuity of the physical information being measured, is shown to be reliable by experimental results. Unlike the brunch-cut method, this new algorithm needs not placing brunch cuts but ratifying the errors in the initial unwrapped phase map obtained by the conventional method using the least difference of phase-cross-section. It is suitable for unwrapping phase map in the presence of errors such as noise, phase discontinuity more than (pi) and insufficient sampling. In addition, the new method is simple and requires little computing time.
The reliability and lifetime of microelectromechanical systems (MEMS) are strongly depending on the material properties. It is important to develop methods for the measurement of MEMS material properties. In this paper an optical technique for measuring tensile strain of micro-specimen is proposed. The technique, which is based on digital speckle correlation method works by determining the deformation of specimen between two neighboring loading step by step. The problem of non-correlation is solved when the deformation is too large. The experimental results of a copper wire are given in this paper. The proposed technique is also applicable for measuring other MEMS materials that may be even smaller or thinner than that used in this paper.
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