Structured Light Based Small Field Of View Three Dimensional Measurement Method And System Development

With the development of precision manufacturing technology,the precise threedimensional inspection of the complex parts surface and their tiny structures characteristics has become a common requirement in the industry.These parts have complex surface structure features and high manufacturing precision,and it is a major challenge for them to perform high-efficiency and high-precision three-dimensional topography measurement.Domestic and foreign scholars have proposed a variety of noncontact optical measurement methods for this problem,such as confocal microscopy,white phase shift interference,phase shift microstriping,passive vision,space-time stereo vision,and structured light technology.However,there are still problems with measurement accuracy and efficiency.The structured light measurement technology has the characteristics of simple structure,large field of view and fast speed.However,a large number of fringe patterns need to be projected to improve the measurement accuracy,which will induce lower overall measurement speed and other issues,and limit its on-line measurement applications in automated production lines.By focusing on the problems associated with structured light measurement technology,this paper conducts in-depth research on key techniques such as structured light pattern coding,fringe projection acceleration,and system calibration algorithms.A new fast and accurate 3D measurement method is proposed,which include a novel multi-wavelength phase expansion algorithm with variable magnification,a fringe projection acceleration method based on a programmable projector,an search algorithm with improved calibration plate pattern and feature points,and a system calibration algorithm based on the feature points mapping.The proposed method can improve the efficiency of the three-dimensional measurement while ensuring the measurement accuracy of the system,and solve the problem of precise measurement of the characteristics of the minute structure.The main contents of this article are summarized as follows:1.An in-depth investigation of the non-contact three-dimensional topography measurement methods both at home and abroad is carried out.The current methods are compared and analyzed from the three aspects of principle,structure and applicable objects,and at the same time the difficulties that still exist in this field are pointed out and the research contents of this paper are clarified.2.For the problem of the measurement speed effected by the increasing the projection fringe of high-precision measurement,the phase shift method and phase unwrapping algorithm are thoroughly studied,and the codec principle and phase error compensation method of the triple-frequency heterodyne algorithm and multi-wavelength algorithm are analyzed.A new multi-wavelength phase expansion algorithm with variable magnification is proposed,which can greatly reduce the number of projection stripe while guaranteeing the measurement accuracy.The effectiveness of the algorithm is verified by experiments.3.When a commercial projector is used for high-precision measurement,if an 8-bit phase shift pattern is projected,the measurement rate is very low(only 1 Hz).This study is based on a programmable projector,which achieves simultaneous triggering and measurement of projection and camera,and can significantly increase its rate(up to 10Hz);and on this basis,it proposes a grayscale fringe projection acceleration method,which further increases the projection rate to 20Hz,to achieve a significant increase in three-dimensional measurement speed.4.The aperture imaging model of the camera and the projector is studied and the calibration principle is deduced in detail.An improved calibration plate pattern is proposed to ensure the accurate coordinates of the feature points and the absolute phase values on the feature points.Based on the system calibration algorithm of the feature point mapping,the code value of the coded image acquired by the camera collection is converted to the image plane of the DLP projector by combing with the multi-wavelength phase encoding method.Thereby,the transforming the calibration of the DLP projector can be transformed into a mature camera calibration,and the stereo vision calibration can be used by the structure light 3D measurement system to achieve the complete system calibration.5.This party develops and implements the above algorithms,and establishes a set of fast,high-precision structural light three-dimensional topography measurement system.The measurement range of the system is 51.6×32.2×1 mm,the measurement accuracy is 10 um/15 um,and the scan time is 0.9 s/1.8 s.Through the measurement experiment of the example parts,the various functions of the developed small-field three-dimensional measurement system are verified,and the three-dimensional precision measurement of precision parts is realized.