Micro-vision System Research Articles

A novel calibration method for the zoom micro-vision system containing electrically tunable lens

For zoom micro-vision system containing electrically tunable lens (ETL), traditional calibration methods are usually ineffective or inefficient due to the limited field of view (FOV) and shallow depth of field (DOF). This article proposes a novel calibration method that can overcome these problems. Firstly, the geometric imaging model of the zoom micro-vision system considering the input focal power is established. Subsequently, a unique image acquisition scheme and the corresponding feature extraction algorithm developed in frequency domain are given, which can greatly reduce the calibration workload. In addition, how to calculate the initial value of the parameters in the imaging model and how to optimize these parameters globally are presented. Finally, calibration experiments including the comparisons with traditional multi-focus and fitting methods are designed and implemented. Experimental results show that the proposed method has good consistency and accuracy when calibrating at different focal intervals, and the average reprojection error is less than 0.25 pixels. Comparative experiments show that the traditional multi-focus and fitting methods not only need more calibration images and the auxiliary high-precision positioning stage, but also have poor applicability for zoom micro-vision systems in practical.

Design and Test of a Spatial Nanopositioner for Evaluating the Out-of-Focus-Plane Performance of Micro-Vision

Micro-vision possesses high in-focus-plane motion tracking accuracy. Unfortunately, out-of-focus-plane displacements cannot be avoided, decreasing the in-focus-plane tracking accuracy of micro-vision. In this paper, a spatial nanopositioner is proposed to evaluate the out-of-focus-plane performance of a micro-vision system. A piezoelectric-actuated spatial multi-degree-of-freedom (multi-DOF) nanopositioner is introduced. Three in-plane Revolute-Revolute-Revolute-Revolute (RRRR) compliant parallel branched chains produce in-focus-plane motions. Three out-of-plane RRRR chains generate out-of-focus-plane motions. A typical micro-vision motion tracking algorithm is presented. A general grayscale template matching (GTM) approach is combined with the region of interest (ROI) method. The in-focus-plane motion tracking accuracy of the micro-vision system is tested. Different out-of-focus-plane displacements are generated using the proposed nanopositioner. The accuracy degradation of the in-focus-plane motion tracking is evaluated. The experimental results verify the evaluation ability of the proposed nanopositioner.

A contour-guided pose alignment method based on Gaussian mixture model for precision assembly

Purpose The purpose of this study was to solve the problem of pose measurement of various parts for a precision assembly system. Design/methodology/approach A novel alignment method which can achieve high-precision pose measurement of microparts based on monocular microvision system was developed. To obtain the precise pose of parts, an area-based contour point set extraction algorithm and a point set registration algorithm were developed. First, the part positioning problem was transformed into a probability-based two-dimensional point set rigid registration problem. Then, a Gaussian mixture model was fitted to the template point set, and the contour point set is represented by hierarchical data. The maximum likelihood estimate and expectation-maximization algorithm were used to estimate the transformation parameters of the two point sets. Findings The method has been validated for accelerometer assembly on a customized assembly platform through experiments. The results reveal that the proposed method can complete letter-pedestal assembly and the swing piece-basal part assembly with a minimum gap of 10 µm. In addition, the experiments reveal that the proposed method has better robustness to noise and disturbance. Originality/value Owing to its good accuracy and robustness for the pose measurement of complex parts, this method can be easily deployed to assembly system.

Motion measurement system of compliant mechanisms using computer micro-vision

Position sensing is essential to testify the validity of the mechanical design and verify the performance in micromanipulation. A practical system for non-contact micro-motion measurement of compliant nanopositioning stages and micromanipulators is proposed using computer micro-vision. The micro-motion measurement method integrates optical microscopy and an optical flow-based technique, in which the motions of complaint mechanisms are precisely detected and measured. Simulations are carried out to validate the robustness of the proposed method, while the micro-vision system and a laser interferometer measurement system are also built up for a series of experiments. The experimental results demonstrate that the proposed measurement system possesses high stability, extensibility, and precision with 0.06 µm absolute accuracy and 0.05 µm standard deviation.

In Situ Measurement of 3D Crystal Size Distribution by Double-View Image Analysis with Case Study onl-Glutamic Acid Crystallization

An in situ measurement method is proposed for monitoring three-dimensional (3D) crystal size distribution (CSD) during a crystallization process, based on a binocular microvision system. The stereo particle shape is reconstructed from double-view images captured by two microscopic cameras fixed at different angles outside the crystallizer. To overcome the influence from solution turbulence and uneven illumination background involved with in situ imaging, a microscopic double-view image analysis method is established to identify the key corners of each particle shape in the captured images, including corner detection and corner matching. Two fast algorithms are therefore given for online detection of two typical crystal morphologies of prismatic and needle-like shapes, such as α- and β-forms of l-glutamic acid (LGA) crystals, respectively. On the basis of the identified key corners for different particle shapes, a 3D geometry model is established to approximately reconstruct the 3D shape for each imaged particle, such that 3D sizes of each particle could be quantitatively estimated, along with the particle volume. Experiments on the LGA cooling crystallization are performed to demonstrate the effectiveness of the proposed method.

Micro-vision image measurement for spot arrays on silicon wafer ablated by femtosecond laser

When using femtosecond laser micromachining center to ablate spot array on silicon wafers, it is difficult to measure the spot spacing directly by physical method because of the small geometric size between the spots. In order to quickly acquire the geometric dimensions between the spot arrays, the micro vision image measurement method is used to measure the distance of spot arrays, and a high precision measurement result is obtained. In the experiment, femtosecond laser power with 20 mW was used to ablate the spot on the surface of silicon wafer and visual measurement for the obtained spot array is realized. Firstly, a telecentric micro-vision system with a field of view of 2.528 × 1.896 mm is constructed to realize image acquisition of spot arrays. Secondly, the micro-vision system is calibrated by using the geometrical optical resolution plate, and the measurement ratio constant Kw is obtained; in the meantime, in order to improve the calibration accuracy, the resolution plate image was processed by vertical correction and pseudo-color enhancement, and the separation lines between the line pairs were extracted by Hough transform. Thirdly, in order to accurately extract the geometric center of the spot image and improve the measurement accuracy of the spacing between the spot, the R-component partitioning image of spot array is analyzed; its three-dimensional image presents a complete stereoscopic morphology of spot, which can be used as an input image for image measurement. Finally, the spot arrays were measured by image measurement method and compared with the size measured under a microscope. The results show that the maximum measurement error of micro vision measurement method is 3%, which meets the requirement of measurement accuracy.

Micro-motion detection of the 3-DOF precision positioning stage based on iterative optimized template matching.

This study presents a method for micro-motion detection of the three-degrees-of-freedom (3-DOF; x, y, θz) precision positioning stage (PPS) based on iterative optimized template matching (IOTM). In this method, a micro-vision system (MVS) is constructed and employed to capture magnified images of the measured PPS's surface with high quality. In addition, an efficient and accurate IOTM algorithm, which includes a pyramid hierarchical matching step for generating the initial guess and an iterative searching step for 3-DOF fine matching, is proposed to detect the micro-motion of the 3-DOF PPS. The simulation results show that the locating accuracy of the translation component (TC) and rotation component of this algorithm can respectively reach 0.01 pixels and 0.01deg when the image quality is high and the initial guess is close to the real location. Measurement tests of a nano-PPS verify that the proposed method is practical and effective for 3-DOF micro-motion detection and the absolute accuracy of the TC of the MVS can easily reach the nanometer level.

A fast autofocus sharpness function of microvision system based on the Robert function and Gauss fitting.

For the microvision system, a new autofocus evaluation function based on the Robert function is proposed by increasing the threshold value. Compared with the traditional evaluation function, the new focus function reduces the local extreme value and increases the steepness of the focusing curve. According to the characteristics of the focusing evaluation function, the focus curve can be divided into two stages: the gentle area and the steep area. In the gentle area, there will be set a large step-length to realize the fast search. In the steep area, the data will be fitted by Gauss method, and on the basis of the fitting results, the motor of microvision system was directly driven to achieve the focal plane and this method has been improved in real-time and accuracy.

Research on auto-focusing technology for micro vision system

The paper presents a passive auto-focusing technology by combining the discrete cosine transform(DCT) algorithm and small univalve segment assimilating nucleus(SUSAN) algorithm with evaluation function. The new method has the advantage of unimodal, local extremum and sensitivity of the focus curve. For the focusing search algorithm, the improved mountain climbing method which combined the large step and small step has been proposed, it makes the micro-vision system could find the focal plane more quickly and accurately.

Micro-vision servo control of a multi-axis alignment system for optical fiber assembly

This paper describes a novel optical fiber assembly system featuring a multi-axis alignment function based on micro-vision feedback control. It consists of an active parallel alignment mechanism, a passive compensation mechanism, a micro-gripper and a micro-vision servo control system. The active parallel alignment part is a parallelogram-based design with remote-center-of-motion (RCM) function to achieve precise rotation without fatal lateral motion. The passive mechanism, with five degrees of freedom (5-DOF), is used to implement passive compensation for multi-axis errors. A specially designed 1-DOF micro-gripper mounted onto the active parallel alignment platform is adopted to grasp and rotate the optical fiber. A micro-vision system equipped with two charge-coupled device (CCD) cameras is introduced to observe the small field of view and obtain multi-axis errors for servo feedback control. The two CCD cameras are installed in an orthogonal arrangement—thus the errors can be easily measured via the captured images. Meanwhile, a series of tracking and measurement algorithms based on specific features of the target objects are developed. Details of the force and displacement sensor information acquisition in the assembly experiment are also provided. An experiment demonstrates the validity of the proposed visual algorithm by achieving the task of eliminating errors and inserting an optical fiber to the U-groove accurately.

Full closed-loop controls of micro/nano positioning system with nonlinear hysteresis using micro-vision system

The design and full closed-loop controls of micro/nano positioning system are presented in this paper. The micro/nano positioning system mainly consists of a 3-DOF compliant mechanism, a micro-vision system and three piezoelectric actuators. In order to compensate for nonlinear hysteresis in the piezoelectric actuators, an adaptive controller using direct inverse modeling approach based on a modified Prandtl–Ishlinskii model is presented. Instead of using traditional displacement sensors, a micro-vision system (MVS) is used to obtain feedback signals online. Based on the micro-vision system, a full closed-loop tracking control is developed to enhance the positioning accuracy of micro/nano positioning system. Taking into account the coupling motions, another full closed-loop tracking control with a decoupling feedforward controller is also proposed. Experimental results demonstrate that the full closed-loop controls can improve the positioning accuracy when compared with an open-loop control. Furthermore, the decoupling feedforward controller can improve the tracking performance of full closed-loop tracking control as well.

High-precision displacement measurement method for three degrees of freedom-compliant mechanisms based on computer micro-vision.

A practical method for the high-precision displacement measurements of the three degrees of freedom-compliant mechanisms based on computer micro-vision is proposed. The method consists of two steps. In the first step, the candidate pixels are selected using a ring projection transform matching approach. In the second step, the exact location is determined by an improved pseudo-Zernike moment method. The setup of the micro-vision system is also introduced. A series of simulations is carried out and the results show that the proposed algorithm enjoys extremely high precision and robustness in the presence of image translations and rotations. Finally, a micro-vision system and a laser interferometer measurement (LIM) system are built up to validate and compare the actual performances of the proposed method. The experimental results demonstrate that the proposed approach can obtain a high accuracy and shows higher operability and stability than the LIM system. Moreover, the measuring accuracy can reach a pixel.

An in-depth study of tool wear monitoring technique based on image segmentation and texture analysis

We present a new micro-vision system for tool wear monitoring, which is essential for intelligent manufacturing. The tool wear area is divided into regions by a watershed transform, then subjected to automatic focusing and segmentation. The individual pixel gray values in each region are then replaced with the corresponding regional mean gray value. A hill climbing algorithm based on the sum modified laplacian (SML) focusing evaluation function is used to search the focal plane. In addition, we implement an adaptive Markov Random Field (MRF) algorithm to segment each region of tool wear. For our MRF model, the connection parameter value is adaptively determined by the connection degree between regions, which improves image acquisition of more integral tool wear areas. Our findings suggest that automatic focusing and segmentation of the tool wear area by region (within the tool wear area) enhance accuracy and robustness, and allow for real time acquisition of tool wear images. We also implement a complementary tool wear assessment procedure based on the surface texture of the workpiece. The optimal texture analysis window is determined using the entropy metric – a texture feature generated using a Gray Level Co-occurrence Matrix (GLCM). In the best texture analysis window, entropy remains monotonic as tool wear increases, demonstrating that entropy can be used effectively to monitor tool wear. Information from combined measurements of tool wear and workpiece texture can reliably be used to monitor tool wear conditions and improve monitoring success rates.

Telecentric stereo micro-vision system: Calibration method and experiments

When a telecentric stereo micro-vision system is used for 3D measurement, the fundamental and key issue is the calibration of the system. However, a telecentric lens generally possesses a small field of view (FOV), which renders the calibration complicated and difficult. In our case, the FOV is 2.4mm×3.2mm. From the existing literature published to date, little attention has been paid to the calibration of visual systems with limited FOVs; therefore, a highly effective calibration method is required. In this paper, a new and accurate calibration method is proposed. First, we present a geometric model of the telecentric camera used in this study, considering lens distortion. Afterward, a method for single-camera calibration is described in detail. With respect to recovering the camera rotation matrix, the problem of sign ambiguity induced by the planar-object-based calibration technique is successfully solved. Based on this technique, a method for calibrating a telecentric stereo micro-vision system is presented. The results of experiments conducted in this study show that the method is quite accurate and reliable.

Design of a Coaxial Positioning System Based on Flexible Decoupling Beams

To overcome the shortcomings of existing technology and achieve a type of shaft and sleeve assembly in navigate with high coaxial requirements, a novel coaxial positioning system is presented based on the flexible decoupling beams and micro-vision system. In order to meet the adjustments online and maintain posture of the shaft and sleeve, a flexible beams structure with pattern of “square lattice” is designed. Combined with micro motorized X-Y stage and clamps, the structure achieves exact coaxial positioning precision and station keeping function. The mechanics theory and finite element method are used to analysis and simulate the beams. The Hough algorithm is applied in the micro-vision system to realize the precise coaxial positioning of real-time detection and calculation. Experiments show that the coaxial positioning accuracy is better than 8μm, and the positioning device could guarantee the realization of positioning assembly in practical applications.

Research of micro-vision technology for micro-fabrication

The micro-vision system is designed for micro-fabrication. This system with force sensor controls micro-fabrication operation. The focus is calculated by one order moment. Gradient evaluation function and frequency domain evaluation function are combined as evaluation function in auto-focus technology. Image definition evaluation function based on gradient is adopted in large step. Image definition evaluation function based on frequency-domain is adopted in small step. Search strategy adopts threshold method and curve fitting method. Edge invariant moment is to recognize objects. The system can measure the distance of three dimensions between the cutting tool and the work-piece. Then the micro-fabrication operation is completed.

MEMS微变形镜测试技术及算法研究

A computer-controlled stroboscopic interferometer technique for measuring surface shape map and out-of-plane-deflections and static voltage versus displacement curve and resonant frequency is introduced.In-plane displacement of MEMS deformable mirrors is measured using the stroboscopic imaging and the computer microvision system and technique of sub-pixel location based on least-square curved surface fitting algorithm.The techniques of stroboscopic imaging and a five-step phase-shifting interferometer are used to obtain wrapped phase.The phase unwrapping algorithm is established to recover true phase which stands for object surface height.Consequently,the static and dynamic characteristics of MEMS deformable mirrors are obtained.The testing results show that the stroboscopic interferometer technique has the advantages of fast measuring speed and high measuring accuracy and realizing automatic control easily.

Research on Wavelet Based Autofocus Evaluation in Micro-vision

This paper presents the construction of two kinds of focusing measure operators defined in wavelet domain. One mechanism is that the Discrete Wavelet Transform (DWT) coefficients in high frequency subbands of in-focused image are higher than those of defocused one. The other mechanism is that the autocorrelation of an in-focused image filtered through Continuous Wavelet Transform (CWT) gives a sharper profile than blurred one does. Wavelet base, scaling factor and form to get the sum of high frequency energy are the key factors in constructing the operator. Two new focus measure operators are defined through the autofocusing experiments on the micro-vision system of the workcell for micro-alignment. The performances of two operators can be quantificationally evaluated through the comparison with two spatial domain operators Brenner Function (BF) and Squared Gradient Function (SGF). The focus resolution of the optimized DWT-based operators is 14% higher than that of BF and its computational cost is 52% approximately lower than BF's. The focus resolution of the optimized CWT-based operators is 41% lower than that of SGF whereas its computational cost is approximately 36% lower than SGF's. It shows that the wavelet based autofocus measure functions can be practically used in micro-vision applications.

Air damping in laterally oscillating microresonators: A numerical and experimental study

In this paper, we investigate computing air damping in a comb-drive resonator by numerically solving the three-dimensional (3-D) Stokes equation for the entire resonator using the FastStokes Solver. In addition, we used a recently developed computer microvision system to directly measure resonator frequency response. By comparing the measured results to those generated by one dimensional analytic models and by numerical solution of the 3-D Stokes' equation, we demonstrate that numerically solving the Stokes' equation is fast and also generates a model that matches quality the factor to within 10%. We also show that results based on one-dimensional (1-D) models mispredict quality factor by more than a factor of two. In addition, the detailed drag force distribution generated by the FastStokes solver is used to identify sources of errors in the 1-D models.

New Three-Dimensional Wafer Bonding Technology Using the Adhesive Injection Method

A new three-dimensional (3D) wafer bonding technology using the adhesive injection method has been proposed, in order to realize a real-time micro-vision system and a real shared memory. Several key technologies for 3D LSI, such as deep trench formation for buried interconnection, wafer grinding and chemical-mechanical polishing, wafer alignment and wafer bonding using the adhesive injection method, have been developed.