Self-calibration Method and Pose Domain Determination of Light-Pen in a 3D Vision Coordinate Measurement System

Light pen 3D vision coordinate measurement systems are increasingly widely used due to their advantages, such as small size, convenient carrying and wide applicability. The posture of the light pen is an important factor affecting accuracy. The pose domain of the pen needs to be given so that the measurement system has a suitable measurement range to obtain more qualified parameters. The advantage of the self-calibration method is that the entire self-calibration process can be completed at the measurement site without any auxiliary equipment. After the system camera calibration is completed, we take several pictures of the same measurement point with different poses to obtain the conversion matrix of the picture, and then use spherical fitting, the generalized inverse method of least squares, and the principle of position invariance within the pose domain range. The combined stylus tip center self-calibration method calculates the actual position of the light pen probe. The experimental results show that the absolute error is stable below 0.0737 mm and that the relative error is stable below 0.0025 mm. The experimental results verify the effectiveness of the method; the measurement accuracy of the system can meet the basic industrial measurement requirements.

A CMM-Based Method of Control Point Position Calibration for Light Pen Coordinate Measuring System

A light pen coordinate measuring system (LPCMS) is a kind of vision-based portable coordinate measuring technique. It implements coordinate measurement by analyzing the image of a light pen, which has several control points and a probe. The relative positions of control points need to be determined before measurement and serve as the measuring basis in LPCMS. How to accurately calibrate the relative positions of control points is the most important issue in system calibration. In this paper, a new method of control point position calibration based on a traditional coordinate measuring machine (CMM) is proposed. A light pen is fastened to the measuring arm of a CMM and performs accurate translational movement driven by the CMM. A camera is used to capture the images of control points at different positions, and the corresponding readings of the CMM are recorded at the same time. By establishing a separate coordinate system for each control point, the relative positions of the control points can be transformed to the differences of a series of translation vectors. Experiments show that the calibration repeatability of control point positions can reach 10 μm and the standard deviation of measurement of the whole LPCMS can reach 30 μm. A CMM is used to generate accurate translation, which provides a high accuracy basis of calibration. Through certain mathematical treatment, tremendous data acquired by moving the light pen to tens of thousands of different positions can be processed in a simple way, which can reduce the influence of random error. Therefore, the proposed method provides a high-accuracy solution of control point position calibration for LPCMS.

Blue/red light-triggered reversible color switching based on CeO2−x nanodots for constructing rewritable smart fabrics

Rewritable smart fabrics are constructed by coating with CeO2−x/dye/hydroxyethyl cellulose-based inks, and figures can be rapidly and remotely printed by blue-light pen and then erased by red light with the presence of O2.

Control Point Position Calibration for Light Pen Coordinate Measuring System

Light pen coordinate measuring system is a kind of vision-based portable coordinate measuring technique. It implements coordinate measuring by analyzing the image of light pen, which has several control points. With the relative positons of the control points determined and the pixel coordinates of their image points acquired by image processing, the posture of light pen can be obtained by solving perspective- n-point problem. Therefore, the control point positons serve as the measuring basis of the whole system. However, the accuracy of existing calibration methods is not satisfactory. In this paper, a new method of control point positon calibration, which utilizes traditional coordinate measuring machine, is proposed. The light pen is fasten to the measuring arm of a coordinate measuring machine and moves along a three dimensional grid route driven by the machine. A stationary camera is located in front of the machine to capture the image of light pen at each node of the route. For each control point, an individual coordinate system is established. Then the relative positions of control points can be transformed into the differences of a series of translation vectors. Experiments show that, with the proposed method, the accuracy of light pen coordinate measuring system can reach 30m within 10 meters.