"Novel method for measuring dense 3D strain map of robotic flapping wings," Measurement Science and Technology (2018)

[109] B. Li and S. Zhang, "Novel method for measuring dense 3D strain map of robotic flapping wings," Measurement Science and Technology, 29(4), 045402 (2018);

Abstract

Measuring dense 3D strain map of inextensible membranous flapping wings of robots is of vital importance to the field of bio-inspired engineering. Conventional high-speed 3D videography method typically reconstructs the wing geometries through measuring sparse points with fiducial markers, and thus cannot obtain full-field mechanics of the wings in details.  In this research, we propose a novel system to measure dense strain map of the  inextensible membranous flapping wings by developing a superfast 3D imaging system and a computational framework for strain analysis. Specifically, first, we developed a 5,000 Hz 3D imaging system based on the digital fringe projection technique using the defocused binary patterns to precisely measure the dynamic 3D geometries of rapidly flapping wings. Then, we developed a geometry-based algorithm to perform point tracking on the precisely measured 3D surface data. Finally, we developed a dense strain computational method using the Kirchhoff-Love shell theory. Experiments demonstrate that our method can effectively perform point tracking and measure highly dense strain map of the wings without many fiducial markers.

“Superfast, high-resolution absolute 3D recovery of a stabilized flapping flight process,” Opt. Express, (2017)

B. Li and S. Zhang, “Superfast, high-resolution absolute 3D recovery of a stabilized flapping flight process,” Opt. Express, 25(22), 27270-27282 (2017); doi:10.1364/OE.25.027270

Abstract

Scientific research of a stabilized flapping flight process (e.g. hovering) has been of great interest to a variety of fields including biology, aerodynamics and bio-inspired robotics. Different from the current passive photogrammetry based methods, the digital fringe projection (DFP) technique has the capability of performing dense superfast (e.g. kHz) 3D topological reconstruction with the projection of defocused binary patterns, yet it is still a challenge to measure a flapping flight process with the presence of rapid flapping wings. This paper presents a novel absolute 3D reconstruction method for a stabilized flapping flight process. Essentially, the slow motion parts (e.g. body) and the fast-motion parts (e.g. wings) are segmented and separately reconstructed with phase shifting techniques and Fourier transform, respectively. The topological relations between the wings and the body are utilized to ensure absolute 3D reconstruction. Experiments demonstrate the success of our computational framework by testing a flapping wing robot at different flapping speeds.

"Computer-aided-design (CAD) model assisted absolute three-dimensional shape measurement" Appl. Opt. (2017)

[100] B. Li, T. Bell, and S. Zhang, "Computer-aided-design (CAD) model assisted absolute three-dimensional shape measurement,"  Appl. Opt. 56(24), 6770-6776 (2017); doi: 10.1364/AO.56.006770

Abstract

Conventional  three-dimensional (3D) shape measurement methods are typically generic to all types of objects. Yet, for many measurement conditions, such level of generality is inessential when having the pre-knowledge of object geometry. This paper introduces a novel adaptive algorithm for absolute 3D shape measurement with the assistance of the object CAD model. The proposed algorithm includes the following major steps: 1) export the 3D point cloud data from the CAD model; 2) transform the CAD model into the camera perspective; 3) obtain wrapped phase map from three phase-shifted fringe images; 4) retrieve absolute phase and 3D geometry assisted by CAD model. We demonstrate that if object CAD models are available, such algorithm is efficient in recovering absolute 3D geometries of both simple and complex objects with only three phase-shifted fringe images.  

"Pixel-by-pixel absolute three-dimensional shape measurement with modified Fourier transform profilometry" Appl. Opt., (2017)

[95] H. Yun, B. Li, and S. Zhang, "Pixel-by-pixel absolute three-dimensional shape measurement with modified Fourier transform profilometry", Appl. Opt., 56(5), 1472-1480, (2017); doi: 10.1364/AO.56.001472

Abstract

Single-pattern Fourier transform profilometry (FTP) method and double-pattern modified FTP method have great value on high-speed three-dimensional (3D) shape measurement, yet it is difficult to retrieve absolute phase pixel by pixel. This paper presents a method that can recover absolute phase pixel by pixel for the modified FTP method. The proposed method uses two images with different frequencies, and the recovered low frequency phase is used to temporally unwrap the high-frequency phase pixel by pixel. This paper also presents the computational framework to reduce noise impact for robust phase unwrapping. Experiments demonstrate the success of the proposed absolute phase recovery method using only two fringe patterns.

"Pixel-by-pixel absolute phase retrieval using three phase-shifted fringe patterns without markers," Opt. Laser Eng., (2017)

C. Jiang,  B. Li, S. Zhang, "Pixel-by-pixel absolute phase retrieval using three phase-shifted fringe patterns without markers," Opt. Laser Eng., 91, 232-241 (2017);  doi:10.1016/j.optlaseng.2016.12.002

This paper presents a method that can recover absolute phase pixel by pixel without embedding markers on three phase-shifted fringe patterns, acquiring additional images, or introducing additional hardware component(s). The proposed
three-dimensional (3D) absolute shape measurement technique includes the following major steps: 1) segment the measured object into different regions using rough priori knowledge of surface geometry; 2) artificially create phase maps at different z planes using geometric constraints of structured light system; 3) unwrap the phase pixel by pixel for each region by properly referring to the artificially created phase map; and 4) merge unwrapped phases from all regions into a complete absolute phase map for 3D reconstruction. We demonstrate that conventional three-step phase-shifted fringe patterns can be used to create absolute phase map pixel by pixel even for large depth range objects. We have successfully implemented our proposed computational framework to achieve absolute 3D shape measurement at 40 Hz.