Accurate compensation method for probe pre-travel errors in on-machine inspections

Abstract

On-machine inspection (OMI) systems have been widely used for the automatic setting of workpieces and the determination of the kinematic errors of a machine tool. Touch-trigger probes are commonly used in OMI systems due to their high reliability and low cost. However, the associated pre-travel error, which is inherent by nature, often severely affects the measurement accuracy and must be compensated during the measuring process in order to ensure the required measurement precision. This paper proposes a compensation method for probe errors using a 3D error map. To construct the 3D error map of the probe, a weighted least squares (WLS) method based on the probe’s mechanical model was first employed to accurately fit the reference sphere center; subsequently, the errors of the calibrated directions could be obtained. Finally, the bicubic Coons patch (BCP) interpolation method was used to calculate the errors of the uncalibrated directions. To validate the proposed method, calibration tests were conducted on an OMI system mounted with a strain gauge probe. The compensation results indicated an increase of 57.40% in the accuracy compared to that of the existing method.