A Low-Cost Camera-Based Tracking Theodolite For Large-Scale Metrology Applications

A wide spectrum of Large-Scale Metrology instruments for measuring 3D positions of mobile targets with different levels of accuracy, operational ranges and corresponding cost exist. However, future assembly paradigms and further industrial applications would benefit from a system with ultra low-cost targets and an accuracy in the order of magnitude of 1 mm, which is currently not available. Therefor the authors propose a camera-based tracking theodolite as a novel instrument for measuring 3D positions of printed visual markers. The approach is implemented as a prototype composed of two rotation stages with built-in encoders, a state-of-the-art industrial CMOS-camera and lens with fixed focal length in conjunction with printed chessboard targets. According mathematical models for the calculation of the targets' position, kinematic error compensation and measurement uncertainty estimation are elaborated. Competitive experiments against a laser tracker show that the position of the marker can be measured with an absolute error below 1 mm and a statistical uncertainty close to 1 mm in a radius of 5 m, confirming the applicability of the developed concept for a novel Large-Scale Metrology instrument.