Local and Global Calibration of Coordinate Measuring Machines

Abstract

In the last years, we assisted to the development of new high precision machine tools: the hexapod machines. We tried to use the hexapod machine as an artefact for CMM's calibration. However, given their cost and their resolution, they are not the best solution for a transportable artefact. By modifying the hexapod structure, we develop two different artefacts: one for local CMM's calibration and another one for global calibration. Local calibration allows us the determination of the transfer function characterizing the sensor displacement of the CMM. This local calibration is based on the measurement of a rigid artefact of a known geometry, derived from hexapod geometry, which allows us to determine the errors of displacement of the sensor of the CMM. The artefact for global calibration uses a self-calibrated method, based on measurements from three miniature laser interferometers, measuring the position of a sphere in the volume of the CMM. The paper describes the patented artefacts for local and global calibration, as well as the referring mathematical problems resulting from self-calibration of global artefact and the method of interpretation of the measurement results of the artefact used in the measure of local errors.

Estimation of Precision and Uncertainty of a Calibration Artefact for CMMS

“…In addition, laser interferometers are also used in order to perform the measurement of linear accuracy, straightness and angularity checks. Additionally, there are uncalibrated devices, like the artefact presented in [1], which is the object of the main study described in this text.…”

“…In order to solve those problems, a new kind of artefact, based on a modification of the known structure of the hexapod machine, has been introduced in [1]. The proposed artefact was similar to a tripod system constituted by three links connecting three fixed spheres to a mobile sphere, with the mobile sphere acting as the reference point for CMM calibration (see figure 6, from [1]). This kind of artefact needs to be validated, for the purpose of verifying its adequacy.…”

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A new kind of artefact, based on a modification of the hexapod machine's well-known structure, has been introduced by Antunes, S. D. et al in [1], in order to determine the global errors of coordinate measuring machines. Here we are presenting results from validation of the technique: using a self-calibrated method and modeling the reference value for calibration based on laser trilateration.

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A New Artefact, Hexapod Based, for Local Calibration of Coordinate Measuring Machines

“…In addition, laser interferometers are also used in order to perform the measurement of linear accuracy, straightness and angularity checks (for example, in [5], Umetsu et al present a laser tracking system to perform the geometric calibration of CMMs). Additionally, there are uncalibrated devices, like the artefact presented in [6], which is the object of the main study described in this text.…”