Robotic near optical coaxial phase measuring stitching deflectometry for form measurement of specular surfaces

Form measurement of specular surfaces is critical for advanced manufacturing in various engineering fields. Near optical coaxial phase measuring deflectometry (NCPMD) has been proposed for in-situ form metrology with the advantages of good accuracy, small volume, lightweight, and minimal measurement errors caused by surface structure shadows. Accurate scanning of large freeform specular surfaces remains a challenge because there are no features on the surfaces that can be used for sub-surface stitching. In this paper, an accurate and automatic area scanning method is studied based on robotic near optical coaxial phase measuring stitching deflectometry (NCPMSD), which consists of an NCPMD system fixed on a robot arm. The NCPMSD scans the measured surface through robot arm control and reconstructs the form data of each sub-surface of the entire surface. Through the proposed robotic NCPMSD calibration method, the relationship between the NCPMD system and the robot arm is calibrated, and the coarse stitching is completed according to the coordinate transformation relationship and robot arm movement data. The stitching accuracy is then improved based on the gradient feature of the measured surfaces. The accuracy and effectiveness of the proposed method are verified through experiments by measuring a ring calibration mirror target and a freeform mirror. The method shows significant advancements in increasing the measurement field and flexibility of the in-situ metrology of the NCPMD system.