Ultra high-speed 3D shape measurement technology for specular surfaces based on μPMD

Abstract

Phase measuring deflectometry (PMD) has been extensively applied to measure specular surfaces due to its non-contact, high-precision, full-field measurement capabilities. Liquid crystal display (LCD) screen is the most common structured light source in PMD. However, the response time of liquid crystal molecules limits its frame rate to around 100 frames per second (fps). Therefore, it is quite difficult for traditional PMD to measure rapidly moving surfaces. This paper proposes a 3D dynamic sensing technique, microsecond-PMD (µPMD) based on the high-frame-rate sinusoidal fringe display (HSFD). In the proposed method, the switching time for each fringe pattern display is at a sub-microsecond level, enabling high-speed fringe acquisition with kHz-level area array detection or 100kHz-level line array scanning. The HSFD method uses a specially designed LED array and two-step optical expansion. The high-speed switching characteristic of LED sources is utilized to allow a superfast display rate. Moreover, the superior sinusoidal property can be achieved by the combination of the specially designed discrete sinusoidal LED array, the light-diffracting effect of orthogonal gratings, and the filtering effect of the light diffuser. The mechanism and analytic model of fringe generation are thoroughly analyzed and discussed in this work. Furthermore, the swarm optimization algorithm and corresponding weighted fringe quality evaluation function are presented to obtain the optimal fringes. To the best of our knowledge, the proposed µPMD, for the first time, achieved a superfast fringe acquisition rate of 4000fps with sub-micrometer precision in three-dimensional (3D) reconstruction for specular surfaces. We envision this proposal to be broadly implemented for real-time monitoring in manufacturing.