Structured Light 3D Measurement System Research Articles

Phase-aided online self-correction method for high-accuracy three-dimensional measurement.

The binocular structured light 3D measurement system is widely used in situ industrial inspection and shape measurement, where the system structure is generally unstable due to mechanical loosening or environmental disturbance. Timely corrections to the changing structural parameters thus is an essential task for online high-accuracy measurement, which is difficult for traditional unidirectional fringe projection methods to self-correct the structural change. To this end, we propose an online self-correction method based on the investigation that orthogonal fringe projection can intrinsically relax the constraint on the epipolar geometry relationship and provide bidirectional phases for accurate corresponding point searching. Since orthogonal fringe projection may sacrifice the measurement efficiency, we further design a searching strategy by locally unwrapping one directional phase to reduce the number of projection patterns. Experimental results demonstrate that the proposed method is effective for online self-correction of unstable system structure to achieve high-accuracy 3D measurement under complex measurement environments.

Research on the center extraction algorithm of structured light fringe based on an improved gray gravity center method

Abstract In this study, we proposed a fast line-structured light stripe center extraction algorithm based on an improved barycenter algorithm to address the problem that the conventional strip center extraction algorithm cannot meet the requirements of a structured light 3D measurement system in terms of speed and accuracy. First, the algorithm performs pretreatment of the structured light image and obtains the approximate position of the stripe center through skeleton extraction. Next, the normal direction of each pixel on the skeleton is solved using the gray gradient method. Then, the weighted gray center of the gravity method is used to solve the stripe center coordinates along the normal direction. Finally, a smooth strip centerline is fitted using the least squares method. The experimental results show that the improved algorithm achieved significant improvement in speed, sub-pixel level accuracy, and a good structured light stripe center extraction effect, as well as the repeated measurement accuracy of the improved algorithm is within 0.01 mm, and the algorithm has good repeatability.

A high-accuracy online calibration method for structured light 3D measurement

Traditional offline calibration of a structured light 3D measurement system is cumbersome, ineffective, and dependent on calibration targets, whereas state-of-the-art (SoTA) online calibration methods are often haunted by low-precision matching points and thus exhibit poor accuracy and reliability. To this end, we present an automated and high-precision online calibration approach for structured light probes from a multi-camera-projector structured light system(MCPSLS). At the core of the approach, a new criterion for matching points filtration is constructed based on local point set topological similarity, which demonstrates higher accuracy and more robustness than typical RANSAC to outliers. Moreover, the fundamental matrix is optimized based on trinocular epipolar constraints, which further prevents the extrinsic parameter solution from being inaccurate and unreliable due to the small number of filtered matching points. Besides, to achieve a simultaneous and automated online calibration, the MCPSLS’s registration objects are served as spatial constraint fields to solve scale factors. Finally, all calibration parameters are nonlinearly optimized for their global optimal solutions. Experimental results show that the proposed approach outperforms SoTA online methods in robustness and accuracy and achieves comparable performance as the traditional offline calibration method.

Analysis and compensation of lateral chromatic aberration of structured light 3D measurement system

Color coding greatly improves the measurement efficiency of structured light three-dimensional (3D) measurement system. However, chromatic aberration (CA) reduces the measurement accuracy of the system. In this study, the lateral chromatic aberration (LCA) of camera and projector is analyzed by experiments, and the optimization model of LCA is studied according to the analysis results. Based on the study, a compensation method of system LCA is proposed. The simulation results show that the compensation method is feasible, and the experimental results prove that the proposed method can effectively reduce the measurement error caused by LCA.

Self-calibration for linear structured light 3D measurement system based on quantum genetic algorithm and feature matching

In this paper, a self-calibration method for a linear structured light 3D measurement system and its advantages are presented. According to the mathematical model of the linear-structured light 3D measurement system, the calibration problem is a highdimensional optimization problem. The principle of this self-calibration method can be drawn from a traditional calibration method. In this method, quantum genetic algorithm and feature matching are applied to self-calibration. Feature matching is used to derive two points with fixed spatial relationships, and an optimal solution of system parameters can be obtained by quantum genetic algorithm. Finally, experimental results are given. The measurement error is 0.05 mm and the ratio of the error to scanning distance is 1.54e-4.In this paper, a self-calibration method for a linear structured light 3D measurement system and its advantages are presented. According to the mathematical model of the linear-structured light 3D measurement system, the calibration problem is a high-dimensional optimization problem. The principle of this self-calibration method can be drawn from a traditional calibration method. In this method, quantum genetic algorithm and feature matching are applied to selfcalibration. Feature matching is used to derive two points with fixed spatial relationships, and an optimal solution of system parameters can be obtained by quantum genetic algorithm. Finally, experimental results are given. The measurement error is 0.05 mm and the ratio of the error to scanning distance is 1.54e-4.

Calibration method for structured light 3D measurement system having single camera and single projector

Calibration method for structured light 3D measurement system having single camera and single projector

Analysis and Compensation for Lateral Chromatic Aberration in a Color Coding Structured Light 3D Measurement System.

While color-coding methods have improved the measuring efficiency of a structured light three-dimensional (3D) measurement system, they decreased the measuring accuracy significantly due to lateral chromatic aberration (LCA). In this study, the LCA in a structured light measurement system is analyzed, and a method is proposed to compensate the error caused by the LCA. Firstly, based on the projective transformation, a 3D error map of LCA is constructed in the projector images by using a flat board and comparing the image coordinates of red, green and blue circles with the coordinates of white circles at preselected sample points within the measurement volume. The 3D map consists of the errors, which are the equivalent errors caused by LCA of the camera and projector. Then in measurements, error values of LCA are calculated and compensated to correct the projector image coordinates through the 3D error map and a tri-linear interpolation method. Eventually, 3D coordinates with higher accuracy are re-calculated according to the compensated image coordinates. The effectiveness of the proposed method is verified in the following experiments.

A flexible and rapid micro-adjustment algorithm for structured light 3D measurement system with camera–projector

The structured light 3D measurement system with camera–projector has been widely studied and applied because of its characteristics of simple structure. In practice, we find that slight shock, vibration, or long-term applying may slightly reduce its level of measurement accuracy; thus, correcting the system parameters is occasionally necessary. However, most calibration algorithms are complicated and time-consuming procedures, not suitable for flexible and rapid precise calibration operation. To address this limitation, a flexible and rapid system parameter micro-optimization algorithm based on an implicit projection model is proposed. With a flexible measurement of an arbitrary 3D gauge, the system parameters are optimized rapidly by Levenberg-Marquardt (L-M) algorithm; thus, the high degree of accuracy of the system can be restored. Experiments were performed to validate the availability and reliability of the adjustment method. The algorithm can meet the convenient and real-time accuracy maintenance in practical application for structured light 3D measurement system.