Stereo Vision Measurement Research Articles

Development of a Stereo Vision Measurement System for a 3D Three-Axial Pneumatic Parallel Mechanism Robot Arm

In this paper, a stereo vision 3D position measurement system for a three-axial pneumatic parallel mechanism robot arm is presented. The stereo vision 3D position measurement system aims to measure the 3D trajectories of the end-effector of the robot arm. To track the end-effector of the robot arm, the circle detection algorithm is used to detect the desired target and the SAD algorithm is used to track the moving target and to search the corresponding target location along the conjugate epipolar line in the stereo pair. After camera calibration, both intrinsic and extrinsic parameters of the stereo rig can be obtained, so images can be rectified according to the camera parameters. Thus, through the epipolar rectification, the stereo matching process is reduced to a horizontal search along the conjugate epipolar line. Finally, 3D trajectories of the end-effector are computed by stereo triangulation. The experimental results show that the stereo vision 3D position measurement system proposed in this paper can successfully track and measure the fifth-order polynomial trajectory and sinusoidal trajectory of the end-effector of the three- axial pneumatic parallel mechanism robot arm.

A Novel Point Matching Method for Stereovision Measurement Using RANSAC Affine Transformation

A binocular point matching method using affine transformation is presented in this paper to deal with matching ambiguities. The epipolar geometry is used to determine all the possible matching pairs to form an initial correspondence data set. Then, an affine registration model with four parameters that is invariant to scaling, rotation and translation is built using the Random Sample Consensus (RANSAC) method to describe the coordinate transformation between the two members of correspondences. Finally, correspondences are picked out using minimal nearest neighbor distances based on the geometric similarity between the right image points and the transformed left ones. The proposed method is applied to measure the profile of a 3.5m parabolic reflector of an inflatable antenna and proved to be able to handle the extra or missing point matching problems coursed by occlusion and sheltering. Satisfactory results are obtained with high correct rate for matching most image pairs despite the significantly different viewpoints, which indicates its validity and application feasibility in automatic stereovision measurement field.

Modeling and Error Analysis of Monocular Stereo Vision System for Large-Scale Workpiece On-Machine Measurement

A novel movable monocular stereo vision measurement system was designed considering the application objectives and on-machine measurement situation. The physical model and mathematical model for measurement accuracy was established. The appropriate structure parameters scope was obtained after simulation analysis. Finally, an application example of the measurement system is introduced.

Evaluating 3D position and velocity of subject in parabolic flight experiment by use of the binocular stereo vision measurement

A practical method for evaluating the three-dimensional (3D) position and velocity of a moving object used in the parabolic flight experiment is developed by using the binocular stereo vision measurement theory. The camera calibration mathematic model without considering the lens distortion is introduced. The direct linear transformation (DLT) algorithm is improved to accomplish the camera calibration. The camera calibration result and optimization algorithm are used to calculate the object's world coordinate from image coordinate. The 3D position and the velocity of the moving object are obtained. The standard uncertainty in estimating the velocity is 0.0024 m/s, which corresponds to 1% level of the velocity of the object in the experiment. The results show that this method is very useful for the parabolic flight experiments.

Simultaneous perimeter measurement for 3D object with a binocular stereo vision measurement system

A simultaneous measurement scheme for multiple three-dimensional (3D) objects’ surface boundary perimeters is proposed. This scheme consists of three steps. First, a binocular stereo vision measurement system with two CCD cameras is devised to obtain the two images of the detected objects’ 3D surface boundaries. Second, two geodesic active contours are applied to converge to the objects’ contour edges simultaneously in the two CCD images to perform the stereo matching. Finally, the multiple spatial contours are reconstructed using the cubic B-spline curve interpolation. The true contour length of every spatial contour is computed as the true boundary perimeter of every 3D object. An experiment on the bent surface's perimeter measurement for the four 3D objects indicates that this scheme's measurement repetition error decreases to 0.7 mm.

Precise perimeter measurement for 3D object with a binocular stereo vision measurement system

A novel measurement scheme for a three dimensional (3D) object's surface boundary perimeter is proposed. This scheme consists of three steps. First, a binocular stereo vision measurement system with two CCD cameras is devised to obtain the two images of a detected object's 3D surface boundary. Second, two B-spline active contours are applied to converge to the object's contour edges accurately in the two CCD images to perform the stereo matching. Finally, for the reconstructed 3D active contour, its true contour length is computed as the detected object's true boundary perimeter. An experiment on a bent surface's perimeter measurement indicates that this scheme's measurement repetition error decreases to 0.6%.

A Point Matching Method for Stereovision Measurement

Many vision tasks such as 3D measurement, scene reconstruction, object recognition, etc., rely on feature correspondence among images. This paper presents a point matching method for 3D surface measurement. The procedure of the method is as follows: (1) rectification for stereo image pairs; (2) computation of epipolar lines; (3) sequential matching in vertical direction; (4) sequential matching in horizontal direction. The fourth step is performed to deal with the ambiguity in dense areas where points have closer vertical coordinates. In the fourth step a threshold limit of vertical coordinate difference is designed to determine those points potential to cause ambiguity. This method was applied to the 3D surface measurement for an inflatable parabolic reflector with validity of point matching up to 100%. Experiment results show that this method is feasible in application of sparse point matching for continuous surface measurements.

Quantitative evaluation of powder spray effects on stereovision measurements

The effects of powder spray on surface topography and gauge repeatability and reproducibility (R&R) of a stereovision measurement system are investigated for specular machined components. Results obtained in the micro-scale laser profilometer and macro-scale stereovision measurement of powder sprayed surfaces are correlated. Three artifacts, two step gauges and a gauge ball, all with specular surfaces, are utilized to evaluate this optical, non-contact measurement method. While the use of powder spray has demonstrated the reduction of the measurement noise of the stereovision system, as shown in this study, the surface topography of powder spray is not suitable for µm-level precision measurements with the powder droplets as high as 98.4 µm. The accuracy and gauge R&R of the system for a 32 µm step height are 9.4 and 4.6 µm, respectively. A case study of flatness measurement of an automotive engine head combustion deck surface is conducted.

基于光学参考棒的立体视觉测量系统现场标定技术

基于光学参考棒的立体视觉测量系统现场标定技术

Three-Dimensional Optical Measurements of Porous Foams

The optical, noncontact stereovision system and data analysis procedure are developed for the measurement of porous foams. The stereovision measurement system has demonstrated the capability to capture both the micro-scale features and the macro-scale shape of both the open-cell and closed-cell porous foams. A computational procedure, denoted as the grid method, is developed to identify representative planes on the porous foam surface using the stereovision measured data points. The relative positions between planes can be used to calculate the angles and distances between porous foam surfaces. A SiC open-cell and an aluminum closed-cell foams are used as examples to validate the grid method and demonstrate its computational efficiency. This research enables the form measurements and geometrical dimensioning and tolerancing of porous foams for quality control and assembly and contact analysis.

Online Calibration for a Stereo Vision Measurement System

A new method for calibrating stereo vision measurement system is presented. By moving a rigid optical reference bar in a pattern in and around the measurement volume, the relative orientation and position of the two cameras can be determined by using the epipolar constraint, the linear normalization eight-point algorithm and the M-estimator method. The scale factor of the translation between the two cameras can be calculated by the distances between the marks on the reference bar. With automatically controlled infrared LED marks, the imaging feature points have a uniform intensity profile and a high contrast with background. The calibration accuracy can be improved. The simulation and experiments have shown that the calibration accuracy can be comparable with the complex off-line calibration.

Improving Measurement Accuracy Using Image Super-Resolution

A super resolution measurement technique is proposed to improve the accuracy of the automated stereovision measurement systems. Image super resolution is useful to reconstruct a visually enhanced high resolution image from a set of low resolution images. Due to the ill conditioning problem of the super resolution model, a-priori information is augmented into the model. We examined different a-priori and concluded that the Solution norm is the most suitable apriori to be used with the optimization technique described. Experiment also showed that the super resolution technique could perform measurement on small images, which are not possible without the technique. An increase in measurement accuracy from 99.73% to 99.91% is obtained.

Automated image matching with coded points in stereovision measurement

A coding-based method to solve the image matching problems in stereovision measurement is presented. The solution is to add and append an identity ID to the retro-reflect point, so it can be identified efficiently under the complicated circumstances and has the characteristics of rotation, zooming, and deformation independence. Its design architecture and implementation process in details based on the theory of stereovision measurement are described. The method is effective on reducing processing data time, improving accuracy of image matching and automation of measuring system through experiments.

Deformations in wide, center-notched, thin panels, part I: three-dimensional shape and deformation measurements by computer vision

The response of wide, thin, center-notched, 2024-T3 aluminum panels undergoing far-field tensile load is investigated. Three panels with a notch length to panel width of 0.33 and widths of 305, 610, and 1016 mm were subjected to far-field tensile loading. As part of the experimental program, two pairs of cameras were configured into separate stereovision systems and used to simultaneously capture both the global response of the sheet and the local response near a notch tip. Global areas, ranging in size from 250x 250 mm to 550x 550 mm, were imaged for each panel. A second stereovison system recorded images of a small area, 10×20 mm, ahead of one notch tip. Postprocessing of the stereovision measurement data from global and local systems using three-dimensional digital image correlation was used to obtain the complete displacement field at each point in the region of interest. In general, results demonstrate that the combination of stereovision and three-dimensional digital image correlation is capable of accurately measuring true, three-dimensional structural deformations in regions undergoing both large out-of-plane displacements and large displacement gradients. Furthermore, 3-D measurements on the panel specimen near the grip location are shown to provide an independent assessment of the true boundary conditions, with specimen slippage clearly noted in the 1016-mm specimen. Results from the extensive notched, wide panel experimental program demonstrate that (a) each panel has an initial shape that deviates up to 3 mm from planarity, with the greatest deviations occurring at the center of the notch, (b) the global load-displacement response is essentially linear for load levels that are well beyond the onset of large, out-of-plane displacements in the notch region, and (c) increasing the size of the notched, thin panel specimen results in distinctly different surface deformations and deformed shapes, with three separate maxima/minima in the out-of-plane component of the largest panel. The region where tensile opening strains are above 2% extends several millimeters ahead of the hole, while compressive strains parallel to the notch direction are contained within a few millimeters of the hole. The in-plane shear strains are concentrated along circular lobes at +/-45 deg from the horizontal direction, a trend which is generally consistent with plane stress conditions.