Technical Study Of Three-dimensional Shape Measurement For Dynamic Process

Optical three-dimensional (3D) non-contact profilometry, with the excellence in high speed and high accuracy, has been widely used for 3D sensing, machine vision, robot simulation, industrial monitoring, dressmaking, biomedicine etc. Among several 3D object profilometry methods, Fourier Transform Profilometry (FTP) is particularly unique for its high-speed measurement. Compared with other profilometry, FTP requires only one frame of the deformed fringe pattern, which makes real-time data acquiring and processing of dynamic process possible.3D shape dynamic measurement is essential to the study of machine vision, hydromechanics, high-speed rotation, deformation of material, stress analysis, deformation in impact, explosion process and biomedicine, therefore it has been targeted for my research on Ph.D. dissertation.In this paper, from the point of view of the basic theoretical analysis, filtering in frequency domain, calculating and unwrapping the 3D wrapped phase, we discuss all the details of the principle and steps of 3D shape measurement for dynamic process completely and integrally. We also present some feasible methods on how to calculate and unwrap the 3D wrapped phase, how to obtain and deal with the 3D shape of dynamic process in different speed of object's motion. As a result, all the matters we prove and discuss in this paper compose an integrated, comprehensive and systemic theory and a practice system of 3D shape measurement for dynamic process.According to the speed of object's motion and the requisite precision of time in measurement, from low to high, we divide the dynamic process into three types namely slow movement process, high movement process, rapid rotation and instantaneous process, and propose three technical projects and measurement systems of 3D shape measurements for them respectively.For 3D shape measurement of slow movement process, we propose a practical project that weuse invariableness structured optical field to illumine the measured object and employ a video recording system to collect the deformed fringe. According to this project, we design a low-cost measurement system and use it to reconstruct the 3D facial shape and movement in chewing. The successful result of the experiment shows that this project is feasible, and it will be used in biomedicine for cosmetology and orthopedics or in security system for face recognition.For high movement process, we propound the second practical project that we also use the invariableness structured optical field to illumine the measured object, but employ a high-speed camera to record the deformed fringe rapidly. According to this project, we design a high-speed measurement system and use it to restore the 3D shape of a vibrating drum membrane and a deflating balloon hull. The successful results of the experiments show that this project is high efficiency and high-speed. It will be used in digitizing 3D shape of those objects in rapid motion.Combining active 3D transducer with stroboscopic technology, we bring forward and design a system of stroboscopic structured illumination, which can 'slow down', or even 'freeze' the motion of object and help the camera to shoot a clear, sharp and instantaneous deformed fringe modulated by the 3D height distribution of a rapid motion object. This system will be used in measuring 3D shape of high-speed or rapid motion object with repeat cycle signal or not, by synchronizing the flash of structured illumination and shoot of the camera with the motion of the object, or with the signal of the manual enactment. The system passes successfully the test of acquiring some sharp, clear and instantaneous deformed fringe images of rotating blade.For rapid rotation and instantaneous process, we present the third practical project that we illumine the motion object using a stroboscopic structured illumination system, and synchronously collect the deformed fringe employing an ordinary CCD camera or high-speed one. According to this project, we design a perfect measurement system and use it to rebuild the 3D shape and deformation of rotating blade with 1112 rpm. The successful result of experiment shows that this project is high efficiency and accurate.Based on the dual-direction nonlinear phase-height mapping technique and Pinhole camera model without lens distortion, and considered the characteristic of Fourier spectrum of a linear structured singal, we propound a calibration method which can complete the calibration of height (Z) and lateral coordinate (XY) simultaneously by orientation filter. We use this method to calibrate the system of rotating blade measurement successfully and obtain its standard deviation, which is less than 0.3 mm in lateral coordinate and 0.0222 mm in height.For measurement of the dynamic objects with large height steps or spatially isolated surface, we propose a novel method, named Temporal Fourier Transform Profilometry (TFTP for short), which obtain the information of 3D shape through illuminating the measured object with a muilt-period phase shifting grating and doing the Fourier-fringe analysis along / axis in the temporal domain.Unlike the conventional FTP, the propsed technique permits the objects with discontinuous height steps or surface spatially isolated from one another. Theoretical analysis, computer simulation and experimental results are presented to demostrate the validity of the priciple.In conclusion, with the theories, projects and measurement systems presented in this article, we can perfectly digitize the 3D shape and movement of dynamic process with different motion speed and different application field.Along with the development of photoelectric device, computer hardware, software techniques, high-speed camera and high resolution image grabber, the projects and measurement methods we propose here will be more prominent in their applications.