Jigui Zhu

A single-station multi-tasking 3D coordinate measurement method for large-scale metrology based on rotary-laser scanning

This paper presents a novel 3D coordinate measurement method based on the rotary-laser scanning technique for large-scale metrology. The method is implemented by a rotary-laser transmitter and a probe integrated with several photoelectric receivers. Measurement is accomplished with the tip of the probe contacting the measured point. The receivers capture the scanning angles of the laser planes emitted by the transmitter to calculate their corresponding equations. Then, we can establish the multi-plane constraint that the receiver points are in the corresponding planes. Subsequently, the coordinates of the measured point can be obtained through an optimization calculation method. In a 480 mm × 480 mm × 480 mm measurement volume that is 6 m away from the transmitter, the distance measurement accuracy of the proposed method is better than 0.40 mm and repeatability remains within 0.17 mm. For coordinate measurement, the accuracy and repeatability exceed 0.46 mm and 0.12 mm respectively. Experimental results show that the method is feasible and valid with good accuracy.

Design and calibration of a single-camera-based stereo vision sensor

A low-cost, single camera model is introduced for the measurement of point coordinates in a 3-D space. By adding two groups of symmetrical reflectors, a single charge-coupled device (CCD) camera can be imaged as a pair of identical virtual cameras, from which a stereo pair of images of an object with parallax can be simultaneously obtained in one image plane and used to retrieve the 3-D coordinates of a point using triangulation. Doing so, the traditional dual-camera system is simplified as a single camera structure. The calibration method is also presented, which is simpler and easier to carry out than that for a dual-camera system. Experimental results show that the single camera method is efficient.

A Vision-Based Self-Calibration Method for Robotic Visual Inspection Systems

A vision-based robot self-calibration method is proposed in this paper to evaluate the kinematic parameter errors of a robot using a visual sensor mounted on its end-effector. This approach could be performed in the industrial field without external, expensive apparatus or an elaborate setup. A robot Tool Center Point (TCP) is defined in the structural model of a line-structured laser sensor, and aligned to a reference point fixed in the robot workspace. A mathematical model is established to formulate the misalignment errors with kinematic parameter errors and TCP position errors. Based on the fixed point constraints, the kinematic parameter errors and TCP position errors are identified with an iterative algorithm. Compared to the conventional methods, this proposed method eliminates the need for a robot-based-frame and hand-to-eye calibrations, shortens the error propagation chain, and makes the calibration process more accurate and convenient. A validation experiment is performed on an ABB IRB2400 robot. An optimal configuration on the number and distribution of fixed points in the robot workspace is obtained based on the experimental results. Comparative experiments reveal that there is a significant improvement of the measuring accuracy of the robotic visual inspection system.

Pulse-to-pulse alignment based on interference fringes and the second-order temporal coherence function of optical frequency combs for distance measurement.

A pulse-to-pulse alignment method based on interference fringes and the second-order temporal coherence function of optical frequency combs is proposed for absolute distance measurement. The second-order temporal coherence function of the pulse train emitted from optical frequency combs is studied. A numerical model of the function is developed with an assumption of Gaussian pulse and has good agreement with experimental measurements taken by an ordinary Michelson interferometer. The experimental results show an improvement of standard deviation of peak finding results from 27.3 nm to 8.5 nm by the method in ordinary laboratory conditions. The absolute distance measurement with the pulse-to-pulse alignment method is also proposed and experimentally proved.

Optimization for calibration of large-scale optical measurement positioning system by using spherical constraint

The measurement accuracy of a large-scale optical measurement positioning system largely depends on the calibration procedure. A more reliable calibration approach for the system by using spherical constraints is presented in this paper, and both the adjustment model based on spherical constraint and the calculation method for the optimization are given. This approach can provide constraint in every direction of the system in the workspace and thereby estimate the orientation parameters more accurately than by using current methods. The experimental data show that by using the proposed method, which improves the accuracy of the depth direction, the average 3D coordinate error of the system compared with the laser tracker is about 0.18 mm in the whole workspace.

Calibration technology in application of robot-laser scanning system

Abstract. A system composed of laser sensor and 6-DOF industrial robot is proposed to obtain complete three-dimensional (3-D) information of the object surface. Suitable for the different combining ways of laser sensor and robot, a new method to calibrate the position and pose between sensor and robot is presented. By using a standard sphere with known radius as a reference tool, the rotation and translation matrices between the laser sensor and robot are computed, respectively in two steps, so that many unstable factors introduced in conventional optimization methods can be avoided. The experimental results show that the accuracy of the proposed calibration method can be achieved up to 0.062 mm. The calibration method is also implemented into the automated robot scanning system to reconstruct a car door panel.

Coordinate Transformation Uncertainty Analysis in Large-Scale Metrology

3-D coordinate transformation, which is based on aligning two sets of common reference points, is frequently applied in large-scale combined measurement to unify coordinate frames and tie individual measurement systems together. However, it introduces uncertainty into the final measurement results. This uncertainty must be quantified to make the results complete. This paper presents a novel approach to the uncertainty analysis of 3-D coordinate transformation based on the weighted total least squares adjustment. This approach takes full account of the uncertainty characteristics of measuring instruments and is simple in calculation. In this approach, the transformation uncertainty of a point in a world frame is analyzed carefully. The simulations show that the transformation uncertainty has a distribution of concentric ellipsoids and is affected by the measurement uncertainties and layout of common points. Besides, strategies for minimizing transformation uncertainty are recommended. The experimental results from a laser tracker prove that this proposed approach is valid under normal instrument operating conditions and that these strategies are feasible and efficient.

Accurate 3-D Position and Orientation Method for Indoor Mobile Robot Navigation Based on Photoelectric Scanning

Position and orientation of indoor mobile robots must be obtained real timely during operation in structured industrial environment, so as to ensure the security and efficiency of cargo transportation and assembly precision. But for such a large-scale space, only 2-D coordinates and heading angle of the mobile robot can be measured with a relatively low precision in current major methods. This paper presents a novel method for 3-D position and orientation measurement of indoor mobile robot. In this method, a rotary-laser transmitter is utilized, which is mounted on the indoor mobile robot measuring the scanning angles relative to photoelectric artificial landmarks and obtaining its own 3-D space location information. The landmarks whose coordinates in navigation frame should be precalibrated are distributed at the most appropriate positions of the structured industrial environment. On the basis of that, an algorithm of multiangle intersection was established and in-depth discussed to solve transmitters spatial position and orientation. Experimental results show that, in an 8 m × 6 m × 2.5 m working volume, transmitters position, and orientation measurement accuracy of proposed method were higher than 3.8 mm and 0.104°, respectively. It demonstrates that the proposed method is reliable and flexible for indoor mobile robot navigation tasks and the measurement accuracy can be further improved by increasing layout density of landmarks.

Sensor for In-Motion Continuous 3D Shape Measurement Based on Dual Line-Scan Cameras

The acquisition of three-dimensional surface data plays an increasingly important role in the industrial sector. Numerous 3D shape measurement techniques have been developed. However, there are still limitations and challenges in fast measurement of large-scale objects or high-speed moving objects. The innovative line scan technology opens up new potentialities owing to the ultra-high resolution and line rate. To this end, a sensor for in-motion continuous 3D shape measurement based on dual line-scan cameras is presented. In this paper, the principle and structure of the sensor are investigated. The image matching strategy is addressed and the matching error is analyzed. The sensor has been verified by experiments and high-quality results are obtained.

Validation and mathematical model of workspace Measuring and Positioning System as an integrated metrology system for improving industrial robot positioning

This article aims to improve the absolute accuracy of an individual industrial robot by means of integrating itself with the workspace Measuring and Positioning System, which is currently under development at Tianjin University, China. We found that the absolute positioning error persists in the robot base frame, whereas the errors, both in position and orientation, can be reduced by changing the reference frame from the robot to the integrated metrology system, that is, the workspace Measuring and Positioning System. What it needs more is several additional corrective movements. And this correction work just needs roughly calibrated parameters between the robot frame and the workspace Measuring and Positioning System frame. To validate it, we present the experiment which demonstrates that the absolute error of the industrial robot can be less than 0.2 mm by virtue of the workspace Measuring and Positioning System and the convergent corrective movements. Aiming to explain the results, this study deduces the mathematical model in detail about the integration of the robot with the workspace Measuring and Positioning System. The model explains why the integration of the robot with the workspace Measuring and Positioning System applies. First, the model tells that there exists a low requirement, that is, the tolerable rough level of the parameters between the robot frame and the workspace Measuring and Positioning System frame, for assuring the convergence of the corrective movements. Second, the relationship among the relevant factors in the corrective process is given. Finally, besides the workspace Measuring and Positioning System, this model is of general significance for any available metrology systems with which the industrial robot can integrate, and it may also provide theoretical instructions for the improvement of the robot off-line programming, that is, the robot can work at a higher accuracy provided by the integrated metrology system.