Determining the extrinsic parameters of a network of Large-Volume Metrology sensors of different types

Large-Volume Metrology (LVM) instruments – such as laser trackers, photogrammetric systems, rotary-laser automatic theodolites, etc. – generally include several sensors, which measure the distances and/or angles subtended by some targets. This measurements, combined with the spatial position/orientation of sensors (i.e., the so-called extrinsic parameters), can be used to locate targets in the measurement volume. Extrinsic parameters of sensors are generally determined through dedicated sensor calibration methods, which are based on repeated measurements of specific artefacts. The combined use of multiple LVM instruments enables exploitation of available equipment but may require multiple instrument-dedicated sensor calibrations, which inevitably increase set-up time/cost. This document presents a novel calibration method – called global calibration – which allows the extrinsic parameters of all sensors to be determined in a single process. The proposed method uses a special artefact – i.e., a hand-held probe with assorted types of targets and inertial sensors – and includes a data-acquisition stage, in which the probe is repositioned in different areas of the measurement volume, followed by a data-processing stage, in which an ad hoc mathematical/statistical model is used to determine the extrinsic parameters of sensors. Additionally, the proposed method includes the formulation of a system of linearized equations, which are weighed considering the uncertainty of input variables.