Line-structured Light Research Articles

A novel method to calibrate the rotation axis of a line-structured light 3-dimensional measurement system

Rotation axis calibration is the key to line-structured light 3-dimensional (3D) measurement. The traditional calibration method based on circle fitting only uses the position information of the calibration target and ignores the orientation information, which is easily affected by noise. This paper proposes a new calibration method combining the orientation and position information to improve the calibration accuracy. Compared with the circle fitting method, the added orientation information makes the description of rotation more complete. On this basis, an improved extrinsic parameters estimation method is proposed to obtain more accurate position and orientation information of the target, which further improves the calibration accuracy. The experiment results show that, the radius error is reduced by about 90% and the standard deviation of radius error is reduced by about 40% compared with the traditional circle fitting method.

A Scanning Direction Calibration Method of Line-Structured Light Three-Dimensional Sensors

A Scanning Direction Calibration Method of Line-Structured Light Three-Dimensional Sensors

Line-Structured Light Fillet Weld Positioning Method to Overcome Weld Instability Due to High Specular Reflection

Fillet welds of highly reflective materials are common in industrial production. It is a great challenge to accurately locate the fillet welds of highly reflective materials. Therefore, this paper proposes a fillet weld identification and location method that can overcome the negative effects of high reflectivity. The proposed method is based on improving the semantic segmentation performance of the DeeplabV3+ network for structural light and reflective noise, and, with MobilnetV2, replaces the main trunk network to improve the detection efficiency of the model. To solve the problem of the irregular and discontinuous shapes of the structural light skeleton extracted by traditional methods, an improved closing operation using dilation in a combined Zhang-suen algorithm was proposed for structural light skeleton extraction. Then, a three-dimensional reconstruction as a mathematical model of the system was established to obtain the coordinates of the weld feature points and the welding-torch angle. Finally, many experiments on highly reflective stainless steel fillet welds were carried out. The experimental results show that the average detection errors of the system in the Y-axis and Z-axis are 0.3347 mm and 0.3135 mm, respectively, and the average detection error of the welding torch angle is 0.1836° in the test of a stainless steel irregular fillet weld. The method is robust, universal, and accurate for highly reflective irregular fillet welds.

Fuzzy Membership Grade-Based Binocular Line-Structured Light Parameter Calibration Algorithm

Fuzzy Membership Grade-Based Binocular Line-Structured Light Parameter Calibration Algorithm

Research on Measurement of Tooth Profile Parameters of Synchronous Belt Based on Point Cloud Data.

Accurately detecting the tooth profile parameters of the synchronous belt is crucial for the transmission’s load distribution and service life. However, the existing detection methods have low efficiency, are greatly affected by the manual experience, and cannot realize automatic detection. A measurement method based on point cloud data is proposed to solve this issue. The surface space points of the synchronous belt are acquired by a line-structured light sensor, and the raw point clouds are preprocessed to remove outliers and reduce the number of points. Then, the point clouds are divided into plane and arc regions, and different methods are used for fitting. Finally, the parameters of each tooth are calculated. The experimental results show that the method has high measurement accuracy and reliable stability and can replace the original detection method to realize automatic detection.

A method to reduce the systematic error of line-structured light sensors based on light plane correction

Line-structured light sensors are widely used in 3-dimentional (3D) reconstruction, however, there are few studies on the systematic error and the correction method. To further improve the measurement accuracy, this paper establishes the systematic error model of the line-structured light sensor, and the distribution function of the systematic error caused by the inaccurate light plane equation is derived. Then, the measurement method of the systematic error is proposed, the distance error and the orientation error of the light plane equation are corrected in two steps according to the distribution characteristics of the systematic error. The experimental results show that the systematic error of the sensor is reduced by at least 80% after correction. This method effectively reduces the systematic error of the line-structured light sensor and has good robustness.

Vision occlusion solution for line-structured light measurement system based on a plane mirror

To solve the problem of the measurement information loss caused by occlusion of the field of view in line-structured light measurement, a vision occlusion solution of the line-structured light measurement system based on a plane mirror is proposed in this work. In the system, a symmetrical structure between real and virtual images was created by the mirror. A checkerboard with a light stripe and its mirror image were captured by the camera in an image. The centers of the light stripe and its mirror image were corrected by the vanishing point constraint, and a calibration method of the free plane target was used to calibrate the real and virtual light planes. The real and virtual point clouds of the object were obtained by scanning with a linear guide. According to the ideal reflection model of the mirror, the real and virtual point clouds of the object were transformed into a common coordinate system, and an iterative closest point (ICP) algorithm was used to realize fine registration. As a result, the occluded parts can be compensated for each other. Compared with a standard ball, the mean/maximum absolute error of the measurement was 0.2553/0.3276 mm. The experimental results revealed that the proposed method can obtain the complete surface of the object, thus effectively solving the occlusion problem in the line-structured light measurement, and reducing the loss of the measurement information, which can meet the industrial measurement requirements.

Calibration of line-structured light vision sensors based on simultaneous polarization imaging

Stray light from line-structured light vision sensors easily interferes with the center extraction accuracy of a structured light strip. In this paper, a high-accuracy calibration technique using polarized beam splitting simultaneous imaging is proposed. First, two polarization component images with different polarization directions are obtained through a single measurement. The method overcomes problems related to stray light and errors caused by the shaking of the instruments. Therefore, false polarization information can be avoided in this way. Second, an image fusion algorithm is used to improve the imaging quality of the light bar. Experiments are performed to show that the light strips no longer have local overexposure, and the calibration accuracy of the system is improved.

Line-Structured Light Rut Detection of Asphalt Pavement with Pavement Markings Interference under Strong Light

Line-Structured Light Rut Detection of Asphalt Pavement with Pavement Markings Interference under Strong Light

A novel method to correct the bias of curvilinear light stripe center

Light stripe center extraction is an important operation for the line-structured light measurement system. Common methods use Gaussian kernels to smooth the image before extracting, but it changes the grayscale distribution of curvilinear light stripe, which causes the bias towards the center of curvature. A novel method is proposed to correct the bias of curvilinear light stripe center. The method involves two steps: first, the new images similar in nature to the linear light stripe are formed by the gray values of the light stripe center before correction and its surrounding pixels. Second, the first derivative zero-crossing detection is applied to the new images to obtain the corrected center points. Simulations and experiments show that the bias of curvilinear light stripe center caused by the Gaussian smoothing can be effectively corrected by this method.

Metal surface 3D imaging based on convolutional autoencoder and light plane calibration

Convolutional autoencoders play an important role in computer vision and image processing. In this paper, an optical stripe extraction method based on convolutional autoencoder is proposed to reconstruct reflective metal surfaces. A six-layer convolutional autoencoder network is constructed to extract the center line of the light stripe. In addition, combined with the center line extraction algorithm and line-structured light calibration algorithm, a good reconstruction effect under reflective conditions is obtained. Experimental results show that the relative error of the 3D imaging method based on the convolutional autoencoder is decreased from 6.233% to 2.183% compared with the traditional line-structured laser 3D imaging method.

Calibration Method for Line-Structured Light Three-Dimensional Measurement Based on a Simple Target

Line-structured light three-dimensional (3D) measurement is apt for measuring the 3D profile of objects in complex industrial environments. The light-plane calibration, which is a crucial link, greatly affects the accuracy of measurement results. This paper proposes a novel calibration method for line-structured light 3D measurement based on a simple target, i.e., a single cylindrical target (SCT). First, the line-structured light intersects with the target to generate a light stripe. Meanwhile, the camera captures the calibration images, extracts the refined subpixel center points of the light stripe (RSCP) and detects the elliptic profiles of the two ends of the cylindrical target (EPCT). Second, two elliptical cones defined by the EPCT and the camera optical center are determined. Combining the two defined elliptical cones and the known radius of the SCT, we can uniquely solve the axis equation of the SCT. Finally, because the coordinates of RSCP in the camera coordinate system fulfill both the camera model and the cylindrical equation, these coordinates can be solved and used to obtain the optimal solution of the light-plane equation. The results of simulations and experiments verify that our proposed method has higher accuracy and effectiveness.

A virtual binocular line-structured light measurement method based on a plane mirror

A virtual binocular line-structured light measurement method based on a plane mirror is proposed. A checkerboard target and its virtual image in a front coating plane mirror are captured by a camera in a single picture. The target is imaged in several positions to determine the spatial relationship between the camera and plane mirror. The centers of light stripes are matched with its mirror images by the vanishing point constraint between the checkerboard coplanar point and its corresponding mirrored point. By scanning the object on a moving stage, 3D point cloud data can be obtained. Compared with the traditional binocular line-structured light measurement system, the proposed method just needs a single camera, and two images can be obtained in one procedure. It improves the measurement speed, reduces the cost of measurement, and avoids measurement errors caused by non-strict synchronization between the left and right camera in binocular vision. Experimental results demonstrate good performance with the ability to reconstruction surfaces, and measurement accuracy (RMSE = 0.032 mm) can meet the industrial requirement.

Calibration of line-structured light vision sensor based on free-placed single cylindrical target

This paper proposes a novel calibration method for the Line-structured Light Vision Sensor (L-LVS) based on a straight cylindrical target that can be free placed. The cylindrical target is placed at least once in the measurement space. The camera captures the image of the cylindrical target and the light stripe. Based on the geometric properties of the cylinder and vanishing points, we calculate the 3D centers of the cylindrical end circles and reconstruct the cylindrical quadric surface. Then, multiple 3D points on the cylindrical surface are obtained according to the perspective projection relationship, which finally allows for the reconstruction of the light plane. Both simulation and real experiments show that the proposed method is effective. In the real experiments, the measurement accuracy of a L-LVS based on the proposed calibration method achieves 0.056 mm, when the measuring distance is about 400 mm. Due to the small-size and pose-unconstrainted target, our calibration method is superior to be applied in the confined measurement space in the wild.

Calibration Method for Line-Structured Light Three-Dimensional Measurements Based on a Single Circular Target

Single circular targets are widely used as calibration objects during line-structured light three-dimensional (3D) measurements because they are versatile and easy to manufacture. This paper proposes a new calibration method for line-structured light 3D measurements based on a single circular target. First, the target is placed in several positions and illuminated by a light beam emitted from a laser projector. A camera captures the resulting images and extracts an elliptic fitting profile of the target and the laser stripe. Second, an elliptical cone equation defined by the elliptic fitting profile and optical center of the camera is established based on the projective geometry. By combining the obtained elliptical cone and the known diameter of the circular target, two possible positions and orientations of the circular target are determined and two groups of 3D intersection points between the light plane and the circular target are identified. Finally, the correct group of 3D intersection points is filtered and the light plane is progressively fitted. The accuracy and effectiveness of the proposed method are verified both theoretically and experimentally. The obtained results indicate that a calibration accuracy of 0.05 mm can be achieved for an 80 mm × 80 mm planar target.

High-Precision 3D Reconstruction for Small-to-Medium-Sized Objects Utilizing Line-Structured Light Scanning: A Review

Three-dimensional reconstruction technology has demonstrated broad application potential in the industrial, construction, medical, forestry, agricultural, and pastural sectors in the last few years. High-quality digital point cloud information exists to help researchers to understand objects and environments. However, current research mainly focuses on making adaptive adjustments to various scenarios and related issues in the application of this technology rather than looking for further improvements and enhancements based on technical principles. Meanwhile, a review of approaches, algorithms, and techniques for high-precision 3D reconstruction utilizing line-structured light scanning, which is analyzed from a deeper perspective of elementary details, is lacking. This paper takes the technological path as the logical sequence to provide a detailed summary of the latest development status of each key technology, which will serve potential users and new researchers in this field. The focus is placed on exploring studies reconstructing small-to-medium-sized objects, as opposed to performing large-scale reconstructions in the field.

On-Site Calibration Method for Line-Structured Light Sensor-Based Railway Wheel Size Measurement System.

Line-structured light has been widely used in the field of railway measurement, owing to its high capability of anti-interference, fast scanning speed and high accuracy. Traditional calibration methods of line-structured light sensors have the disadvantages of long calibration time and complicated calibration process, which is not suitable for railway field application. In this paper, a fast calibration method based on a self-developed calibration device was proposed. Compared with traditional methods, the calibration process is simplified and the calibration time is greatly shortened. This method does not need to extract light strips; thus, the influence of ambient light on the measurement is reduced. In addition, the calibration error resulting from the misalignment was corrected by epipolar constraint, and the calibration accuracy was improved. Calibration experiments in laboratory and field tests were conducted to verify the effectiveness of this method, and the results showed that the proposed method can achieve a better calibration accuracy compared to a traditional calibration method based on Zhang’s method.

Defect inspection for underwater structures based on line-structured light and binocular vision.

Surface defect inspection for underwater structures is important. However, the inspection technologies based on passive vision cannot meet accuracy requirements. In this paper, we propose a two-stage method based on structured light images for defect detection. In the first stage, light stripes are extracted based on the analysis of hue, saturation, value (HSV) space and gray space. Then a hole-filling method is applied to ensure stripe integrity. In the second stage, depth information for all light stripes is calculated to synthesize a depth map, which is segmented for defect localization and measurement. Experimental results have verified the feasibility and effectiveness of our method.

Real-time line-structured light measurement system based on Scheimpflug principle

Real-time line-structured light measurement system based on Scheimpflug principle

Solution for vision occlusion based on binocular line-structured light

Solution for vision occlusion based on binocular line-structured light