Abstract
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.
Highlights
Coordinate measuring is a key technique in manufacturing and reverse engineering [1]
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
The proposed method is a vision-based method, the calibration accuracy is more dependent on the accuracy of the coordinate measuring machine (CMM)
Summary
Coordinate measuring is a key technique in manufacturing and reverse engineering [1]. The fundamental principle of LPCMS is to determine the relative position and orientation between the light pen and the camera, according to the control point positions and the image of the control points. The other kind is the vision-based structure-from-motion (SFM) method, which uses a moving camera to obtain a group of images of control points from different viewpoints [11]. The absolute dimension is determined by a calibration bar that has accurate length This is a nonlinear optimization process and does not need to explicitly point out the luminous center of the control points. With the images of control points at different positions and the readings of the CMM, a nonlinear least squares problem is established to optimize the translation vectors between all the control point coordinate systems and camera coordinate system.
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