Research On 3D Measurement Technology Of Phase-Coding Structured Light

Structured light(SL)technology has been widely developed for three-dimensional(3D)shape measurement due to its advantages of non-contact,high resolution,high speed,and full field automation.Fringe Projection Profilometry(FPP)is one of the most widely used structured light technologies in 3D shape measurement,such as reverse engineering,industrial inspection,manufacturing,and robot navigation.The FPP system projects the fringe pattern onto the measured object,and records the deformed fringe pattern modulated by the object.The modulation phase is then calculated from the recorded image using processing/analysis system using a particular fringe analysis method.The accuracy of extracted phase directly affects the 3D reconstruction result of the measured object.The purpose of this dissertation is to improve the performance of 3D shape measurement system based on phase coding structured light,and to lay a foundation for widening the practical range of FPP.The following research work is carried out on key issues such as projector nonlinear gamma calibration and correction,segmentation quantization phase coding method and a 3D measurement method based on N-step phase-shifting:1?The traditional phase coding method and a series of improved methods are studied.Code words are of great importance in phase coding based phase unwrapping methods.Each phase coding fringe is marked with a unique code word and then used to determine the fringe order.However,the difference between adjacent code words in traditional phase coding method is 1,which is too small.Due to the nonlinear and defocusing effects of the system,the calculation of the fringe order in the case of high-frequency fringe is prone to errors,resulting in phase unwrapping errors.The traditional phase coding method is limited by the number of code words,resulting in the fringe order cannot be too large,which limits its measurement accuracy.2?The sinusoidality of digital fringe is a main factor affecting the measurement accuracy.In the digital projection phase-shifting method,the phase error is mainly derived from the non-sinusoidality of the fringe image,and the non-sinusoidality of the fringe image is the result of the nonlinear gamma effect of the commercial projector.To this end,based on the comparative study and analysis of various methods,the projection and imaging system is modeled by seventh-order polynomial to determine the inverse function of the projector's nonlinear gamma.Based on this,the pre-corrected sinusoidal fringe is generated and finally the imaging system can capture the ideal sinusoidal fringe.Computer simulation and experiments verify the effectiveness of the proposed method.3?In order to calculate the fringe order more accurately and improve measurement accuracy of the phase coding method,a 3D measurement method based on segmented quantization phase coding is proposed,which can achieve absolute phase retrieval without reducing the difference between adjacent quantized phase values or increasing the quantization levels.The quantized phase is modulated by a specific code sequence S "135246" and then embedded into the phase coding fringe pattern,thereby greatly improving the accuracy of the decoding.The coding creates multiple segments of the whole region and each segment embeds the special coding sequence S.Through the corresponding decoding algorithm,the fringe order can be obtained,and following this,the absolute phase is obtained.Computer simulation and experimental results demonstrate the feasibility and superiority of the proposed method to measure both a single object and multiple isolated objects.4?Reducing the number of fringe patterns for FPP has been a hot topic in the field.Phase coding method or other traditional temporal phase unwrapping algorithm generally requires additional fringe information to determine the fringe order,which leads to the excessive number of fringe patterns.From the perspective of reducing the number of fringe patterns,in this paper,a 3D measurement method based on N-step phase-shifting is proposed.Only N-step standard phase-shifting sinusoidal fringe patterns are needed to calculate the absolute phase.Firstly,the standard phase-shifting algorithm is used to calculate the wrapped phase and the Mask to eliminate the background.Then,the wrapped phase and Mask are used to solve the fringe order according to the connected component labeling theorem,and then the absolute phase is obtained.The proposed method needs at least three fringe patterns to complete the 3D measurement,and the data processing speed is fast.Computer simulation and experimental results verify the effectiveness and robustness of the proposed method.