Frequency domain processing of digital holograms applied to high-accurate vision-based position control in micro-robotics (Conference Presentation)

We propose a vision-based position sensor based on Digital Holography (DH) for in-plane and out-of-plane displacements measurement of a patterned plate with sub-pixel resolutions. DH is a lensless imaging principle using solid-state camera and/or spatial light modulators (SLM). Object scenes are generated or reconstructed numerically through wave propagation computations applied to a diffracted optical field recorded as an interferogram. The application of visual positioning to manipulation tasks in micro-robotics requires high accuracy and wide ranges of displacements that, unfortunately, are limited by finite depth-of-focus and fixed working distance of refractive imaging systems. Recently, we demonstrated that DH allows in-plane positioning of mobile targets ensuring nanometer resolutions at diverse working distances within a continuous range of more than 15 centimeters. By recording a set of digital holograms of a pseudo-periodic pattern fixed onto a moving target, images in phase and in intensity are restored by numerical reconstruction using Angular Spectrum Propagation methods by adjusting the reconstruction distance. A last step consists in performing a direct phase measurements of periodic pattern to reach nanometer resolutions. Three 2DFFT are required at minimum to extract the pattern position, which is time consuming if several hundred of holograms are recorded. We explore a new approach that consists to restore in-plane / out-of-plane position directly from the 2DFFT of the digital hologram without any need for image restitution. The proposed vision-based position sensor combines a 10 Mp CMOS camera and a SLM in order to perform a fine control of the interferometer reference arm.