Shaft diameter measurement using a digital image

• Rather than the whole image, only the measurement region of image is calibrated.
• A measurement method of shaft diameter was proposed using calibration model.
• The real shaft diameter is calculated based on the principle of pinhole imaging.
• Extrinsic parameters were re-calibrated to achieve a high-precision.

In this paper, a non-contact and high-precision method based on the processing of a digital image is presented to measure the diameter of a shaft. The method mainly involves three steps: first, the camera is calibrated by an improved approach, which only uses the feature points in the measurement area of the image to optimise the local camera model; second, with the help of the parameters of the model in the first step, a measurement method for determining the shaft diameter is proposed; finally, to embody the spatial attitude of the shaft accurately, the extrinsic parameters are re-calibrated by measuring a shaft whose diameter is known, and then the precision of measurement is improved by means of the new extrinsic parameters. The experimental data demonstrate that the proposed method exhibits high precision, with relative errors of approximately 0.005 mm.