Study On Repeated Scanning Technology For Large-scale Projection Stereolithography

In modern industrial manufacturing,the application of large-scale complex parts structure forming is more and more common,and it puts forward higher requirements for parts forming precision,mechanical properties and forming efficiency.Therefore,the research on high-precision,high-quality large-scale complex parts structure forming has become one of the important topics of modern manufacturing technology.Additive manufacturing(AM)technology is not only widely used in mold design,biomedical,aerospace,automotive and other fields,but also provides a good solution for the manufacturing of large complex parts and other structures.As the representative of additive manufacturing technology,the development of stereolithography technology is relatively mature,and has high forming efficiency and precision.However,in the process of large-scale parts stereolithography,its forming precision and mechanical properties of parts are restricted.On the basis of ensuring large-scale stereolithography,further research on how to improving the forming accuracy and mechanical properties of parts has become the focus of the existing technology.Based on galvanometer dynamic scanning projection based Stereolithography(GDSPSL),the repeat scanning projection based stereolithography(RSPSL)process was studied,In order to improve the forming efficiency and quality of large-scale stereolithography,some key technologies,such as distortion correction of plane projection,dynamic segmentation of projection area,partition boundary stitching equalization algorithm and exposure energy compensation algorithm based on the gray level of adjacent pixels,were proposed.The main research contents and achievements are as follows(1)In order to improve the forming accuracy and mechanical properties of the existing large-scale projection stereolithography parts,a large-scale repeated scanning projection stereolithography process was proposed.Firstly,the characteristics of largescale scanning projection stereolithography technology are analyzed,and the theoretical modeling and experimental analysis of scattered stereolithography cumulative molding are carried out.It is verified that in a certain period of time,the stereolithography effect of a certain place in the photosensitive resin material is positively related to the energy absorbed by the place,but has nothing to do with the time continuity of exposure.When the energy accumulation of a certain area is the same,the curing depth of the area is lower The degree is the same.On this basis,a large-scale repeated scanning projection stereolithography process was proposed.By dividing the part forming area into multiple forming sub areas for projection,the single-layer forming time of each forming sub area is divided into multiple exposure time periods in equal proportion,and fast moving the projection area for rapid cross exposure,so that each sub area is alternately exposed in the forming process In order to improve the precision and mechanical properties of the parts,the synchronous solidification is realized,which makes the solidification at the boundary of the part segmentation region more closely.In order to further improve the forming efficiency and forming accuracy of the process,the digital scanning galvanometer is used to deflect the projection of the stereolithography pattern,and its high deflection accuracy and fast speed are used to solve the problem of scanning path growth under the process,so that the forming efficiency in the repeated scanning process is further improved,and the forming efficiency is improved under the premise of improving the forming accuracy and forming quality The efficiency of the system is similar to that of the dynamic scanning projection system.In addition,the process of repeated scanning projection stereolithography is analyzed in detail,and the key technologies and devices used in the system are discussed.The implementation scheme of the system is further proposed,and the feasibility of the system is demonstrated.(2)In this thesis,we studied the area boundary stitching equalization algorithm and the exposure energy compensation algorithm based on the gray level of adjacent pixels in the RSPSL process.Firstly,based on the analysis of the mechanism of stitching error in large-scale stereolithography process,a segmentation region boundary equalization algorithm was proposed to reduce the stitching error caused by unbalanced exposure,material shrinkage and stress concentration In the process of one exposure,the position of the partition boundary changes dynamically,and the phenomenon of exposure imbalance and stress concentration at the partition boundary will not accumulate step by step,so as to improve the overall forming quality of the parts.Furthermore,aiming at the problem of poor forming accuracy caused by uneven energy diffusion at the boundary,a gray compensation algorithm based on adjacent pixels was proposed to further improve the forming accuracy of parts.Through analysis and calculation,the influence of the exposure state of the adjacent point on the energy density of the reference point is obtained,and the gray value of the reference point is adjusted to compensate the energy density of the point.Finally,the experimental results show that the RSPSL process can not only improve the forming accuracy of the parts,but also effectively improve the mechanical properties of the parts.(3)In order to solve the position distortion and projection distortion caused by the deflection of galvanometer and the refraction of lens,an algorithm for correcting the deflection distortion of plane projection in large-scale repeated scanning projection was proposed.Firstly,the causes of the position error of the stereolithography projection in the deflection of the galvanometer are analyzed and summarized,and the path deflection distortion after the galvanometer and lens is analyzed.According to the trajectory formed by the position error distortion is quadratic curve,the distortion correction algorithm of the point deflection was designed.Firstly,the correction coefficient pair(a,b)is introduced to fit the quadratic curve,then the projection plane is meshed and the coefficient pair is calibrated,then the correction coefficient pair of the given point is calculated by bilinear interpolation to improve the accuracy,and then the projection position of the point is corrected by substituting the coefficient pair into the formula.Furthermore,the deformation in the process of face projection deflection is analyzed,and the deflection correction algorithm of face projection is proposed.By calibrating the feature points of the projection plane,the pixel points in the projection plane are corrected point by point,and the calculation is simplified,so that the projection pattern can be corrected in real time according to the dynamic position of the projection.(4)According to the requirements of large-scale repeated scanning projection stereolithography process,the design of stereolithography controller was completed.Firstly,the functions and requirements of the controller are analyzed,and the overall control scheme of the controller is designed,and then each key module is designed in detail.The motion planning of the projection deflection of the digital scanning galvanometer is realized,so that the stereolithography projection can run at the Sshaped curve speed without jitter.The controller realizes the real-time compensation of the deflection position distortion of the digital galvanometer,and sends the compensated position data to the galvanometer through the xy2-100 protocol according to the refresh rate of 100 k Hz to realize the deflection of the optical path.In addition,the dynamic control of the energy of the stereolithography backlight is realized to test and improve the forming effect of the system.Finally,a dual core stereolithography controller based on ARM + FPGA is formed.