Patent Publication Number: US-11390036-B2

Title: Method of aligning pixelated light engines

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This non-provisional patent application claims priority to U.S. Provisional Application Ser. No. 62/842,565, Entitled “Method of Aligning Pixelated Light Engines” by Kirt Winter, filed on May 3, 2019, incorporated herein by reference under the benefit of U.S.C. 119(e). 
    
    
     FIELD OF THE INVENTION 
     The present disclosure concerns an apparatus and method for the digital fabrication of three dimensional articles of manufacture through the solidification of liquid photon-curable (photocure) resins using plural light engines. More particularly, the present disclosure concerns an accurate and efficient method of aligning plural light engines to provide large, high quality articles of manufacture. 
     BACKGROUND 
     Three dimensional (3D) printers are in rapidly increasing use. One class of 3D printers includes stereolithography printers having a general principle of operation including the selective curing and hardening of radiation curable (photocurable) liquid resins. A typical stereolithography system includes a resin vessel holding the photocurable resin, a movement mechanism coupled to a support surface, and a controllable light engine. The stereolithography system forms a three dimensional (3D) article of manufacture by selectively curing layers of the photocurable resin onto a “support fixture.” Each selectively cured layer is formed at a “build plane” or “build field” within the resin. 
     One class of stereolithography systems utilizes light engines based on spatial light modulators such as arrays of micromirrors. Such systems are generally limited by the pixel count of the spatial light modulator. There is a desire to provide systems having larger numbers of pixels to form larger and higher resolution articles of manufacture. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         FIG. 1  is a schematic diagram of an embodiment of a three-dimensional printing system for manufacturing or fabricating a three-dimensional article. 
         FIG. 2  is a schematic diagram depicting an embodiment of a target overlaying a build field. 
         FIG. 3  is a diagram of a spatial sequence of pixel columns that are projected onto a portion of a target. 
         FIG. 4  is a graph of sensed intensity versus pixel column location. 
         FIG. 5  is a flowchart of an embodiment of a method of aligning a projection module to a target along one axis. 
         FIG. 6  is a diagram of a spatial sequence of pixel columns that are projected onto a portion of a target. The spatial sequence of pixel columns have an exaggerated theta-z angular alignment error with respect to the target. 
         FIG. 7A  is a graph of sensed intensity versus pixel array location in which the pixel columns have a relatively small theta-z angular alignment error with respect to the target. 
         FIG. 7B  is a graph of sensed intensity versus pixel array location in which the pixel columns have a relatively large theta-z angular alignment error with respect to the target. 
         FIG. 8  is a flowchart depicting an embodiment of a method of angularly aligning a projection module to a target with respect to theta-Z. 
         FIG. 9  is a flowchart depicting an embodiment of an overall process sequence for aligning a plurality of projection modules of a three-dimensional printing system. 
     
    
    
     SUMMARY 
     In a first aspect of the disclosure, a three-dimensional printing system is for fabricating or manufacturing a three-dimensional article. The three-dimensional printing system includes a substrate, a light engine, a radiation sensor, and a controller. The substrate has a surface positioned proximate to a build field. The build field is for hardening layers of a build material during fabrication of the three-dimensional article. The surface supports a calibration target which includes or defines elongate light modulating bars disposed at two different orientations and including a Y-bar aligned with a Y-axis and an X-bar aligned with an X-axis. The light engine includes a plurality of projection modules including at least a first projection module and a second projection module. The first projection module is configured to project an array of pixels onto a first image field. The second projection module is configured to project an array of pixels onto a second image field. The image fields of the light engine cover the build field including at least one overlap field between the first image field and the second image field. The radiation sensor receives light from the calibration target that is reflected, emitted, or transmitted. The controller is configured to: (1) operate the first projection module to project a first sequence of columns of pixels onto the target; the columns are individually approximately angularly aligned with the Y-axis; the first temporal sequence of columns are separated from each other along the X-axis; (2) operate the radiation sensor to measure an intensity of light from the target during the first sequence; (3) store first information indicative of the measured intensity versus column axial position; (4) analyze the stored first information to align the first projection module to the calibration target along the X-axis. The projected columns of light are spatially and temporally separated from each other so that the radiation sensor receives light originating from one projected column at a time. The target can be either a light field or dark field target. The system can further include a resin vessel for containing photocurable resin to be selectively cured by the light engine and a support plate for alternately supporting the resin vessel and the substrate. 
     In one implementation the stored information defines an intensity received by the sensor versus position of column of pixels. The intensity versus position includes a perturbation caused by the Y-bar. Analyzing includes finding the center of the perturbation to find the center of the Y-bar. 
     In another implementation the controller is configured to: (a) operate the second projection module to project a second sequence of columns of pixels onto the target; the second sequence of columns of pixels are individually approximately angularly aligned with the Y-axis; the second temporal sequence of columns are separated from each other along the X-axis; (b) operate the radiation sensor to measure an intensity of light from the target during the second sequence; (c) store second information indicative of the measured intensity versus column axial position; (d) analyze the stored second information to align the second projection module to the calibration target along the X-axis. The first sequence of columns of pixels are aligned to a first Y-bar within the first image field and outside of the second image field. The second sequence of pixels are aligned to a second Y-bar that is located within the second image field and outside of the first image field. 
     In yet another implementation the controller is configured to: (a) operate the second projection module to project a second sequence of columns of pixels onto the target; the second sequence of columns of pixels are individually approximately angularly aligned with the Y-axis; the second temporal sequence of columns are separated from each other along the X-axis; (b) operate the radiation sensor to measure an intensity of light from the target during the second sequence; (c) store second information indicative of the measured intensity versus column axial position; (d) analyze the stored second information to align the second projection module to the calibration target along the X-axis. The first and second sequence of pixels are aligned to the same Y-bar within an overlap field between the first image field and the second image field. 
     In a further implementation the controller is configured to: (a) operate the first projection module to generate a third sequence of rows of pixels onto the target, the rows are individually approximately angularly aligned with the X-axis; the third sequence of rows are displaced from each other along the Y-axis; (b) operate the radiation sensor to measure an intensity of light from the target during the third sequence; (c) store third information indicative of the measured intensity versus column position; (d) analyze the stored third information to align the first projection module to the calibration target along the Y-axis. 
     In a yet further implementation the controller is configured to: (a) operate the first projection module to generate a plurality of sequences of columns of pixels onto the target having a varying theta-Z orientation with respect to a vertical Z-axis; each column within a sequence of columns being displaced from each other along the X-axis; (b) operate the radiation sensor to measure an intensity of light from the target during the plurality of sequences; (c) store fourth information indicative of the measured intensity versus column position for a plurality of intensity versus position curves that each correspond to one of the sequences; (d) analyze the fourth information to angularly align the first projection module to the calibration target with respect to theta-Z. The analyzing can include selecting an orientation corresponding to one or more of: (1) a maximized slope of intensity versus position for a transition at the edge of a Y-bar, (2) minimize a width of a perturbation from a field intensity, and (3) maximize a width of an intensity extremum corresponding to complete overlap between a pixel column and a Y-bar. 
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 1  is a schematic diagram of a three-dimensional printing system  2 . In describing the three dimensional printing system  2 , mutually perpendicular axes X, Y and Z will be used. Axes X and Y are lateral axes. In some embodiments X and Y are also horizontal axes. In some embodiments Z is a vertical axis. In some embodiments the direction +Z is generally upward and the direction −Z is generally downward. In addition, a rotational coordinate theta-Z or θ z , will be used. Theta-Z refers to rotation about the vertical Z-axis. 
     System  2  includes a support plate  4  for supporting a resin vessel  6  above a light engine  8  during manufacture or fabrication of a three-dimensional article. The resin vessel  6  is for containing a photocurable resin  10 . The photocurable resin  10  is selectively cured by the light engine  8  in a layer-by-layer manner during manufacture of the three-dimensional article. The selective curing occurs across a lateral build field  20  that is at a certain height above the light engine  8 . 
     In the illustrated embodiment a transparent substrate  12  is shown installed upon the support plate  4 . The transparent substrate  12  has an upper surface  14  that supports a target  16 . The target  16  defines a pattern  18  that is used for aligning portions of light engine  8 . In some embodiments, the target  16  can be formed directly onto the substrate  12 . In an illustrative embodiment, the target  16  is a material sheet with a printed pattern  18 . The printed pattern  18  is positioned at the build field  20 . 
     The light engine  8  includes two or more projection modules  9  including at least a first projection module  9 A and a second projection module  9 B. The first projection module  9 A and the second projection module  9 B separately image different portions of the build field  20  but they also image an overlapping portion as will be discussed in more detail with respect to  FIG. 2 . 
     A radiation sensor  22  is positioned either above or below the build field  20 . Sensor  22  is configured to sense radiation that is either transmitted by, re-emitted by, or reflected by the target  16 . 
     A controller  24  is coupled to the light engine  8  and the sensor  22 . The controller  24  includes a processor  26  coupled to an information storage device  28 . The information storage device  28  includes a non-transitory computer readable storage medium that stores software instructions. In response to execution by the processor, the software instructions operate and monitor the light engine  8 , the sensor  22 , and other portions of system  2 . 
       FIG. 2  is a diagram depicting an embodiment of the target  16  and the build field  20  superposed. The embodiment of  FIG. 2  is a result of a system  2  having four projection modules  9 A-D. The build field  20  is a composite of four image fields  21 A,  21 B,  21 C, and  21 D that correspond to the projection modules  9 A-D. 
     The image fields  21  individually include a non-overlapping zone  30  that is imaged by a single projection module  9 . For example, non-overlapping zone  30 A of image field  21 A is imaged by only the projection module  9 A. The build field  20  also has overlap zones  32  within which two or more image fields  21  overlap. For example, overlap zone  32 AB is a rectangular zone over which image fields  21 A and  21 B overlap. The overlap zone  32 ABCD is a rectangular zone over which all four image fields  21 A-D overlap. 
     In an illustrative embodiment, the target  16  includes a sheet of material that either reflects, transmits, and/or fluoresces in response to radiation from the light engine  8 . The target includes a plurality of elongate printed light modulating lines or bars of varying width including X-bars ( 34 ,  38 ) and Y-bars ( 36 ,  40 ) which are further referred to as wide X-bars  34 , wide Y-bars  36 , narrow X-bars  38 , and narrow Y-bars  40 . In  FIG. 2 , element numbers also include letter indicia to indicate which image fields  21  are overlaid. For example, the wide X-bar  34 AC passes through the image fields  21 A and  21 C. Narrow Y-bar bar  40 CD passes through image fields  21 C and  21 D. Narrow X-bar bar  38 ABCD passes through all four image fields  21 A-D. 
     In the illustrative embodiment there can be four sensors  22 A-D corresponding to the four image fields  21 A-D. The four sensors  22 A-D can be individually positioned directly above or below an approximate center of the corresponding image field  21 A-D which is approximately where a wide X-bar  34  crosses a wide Y-bar  36 . For example, sensor  22 A is placed above or below the intersection of the X-bar  34 AC and the Y-bar  36 AB. 
     The controller  24  is configured to operate the projection modules  9 , capture information from sensors  22 , and to analyze the information to align the image fields  21  of the projection modules  9  relative to the target  16  and to each other. In doing so, the wide X-bars  34  are used to individually align the projection modules  9  to the target  16  along the Y-axis. The wide Y-bars  36  are used to individually align the projection modules  9  to the target along the X-axis. Inner narrow X-bar  38 ABCD and Y-bar  40 ABCD can be used to fine tune alignment of the projectors with respect to each other. Outer narrow X-bars  38  and Y-bars  40  can be used to compensate for distortion such as barrel distortion and keystone distortion. 
       FIG. 3  is a diagram superposing a spatial sequence  42  of pixel  44  columns  46  onto a portion of the target  16  which is superposed with a non-overlapping portion  30 A of image field  21 A. Target  16  includes a portion of wide Y-bar  36 AB. The columns of pixels are generated at different times so that useful sensor data can be captured. 
     The arrows labeled x 1 , x 2 , and so on indicate pixel columns  46  that are projected onto the target  16  at different axial locations (generally along the X-axis) by the projection module  9 A. At axial location x 1 , the column above the arrow x 1  is projected. At axial location x 2 , the column above the arrow x 2  is projected. The axial location indicators x 1 , x 2 , x 3 , etc., also correspond to different times. In other words, at a particular time, a single one of the columns are projected to avoid confounding a signal captured by sensor  22 A. The columns  46  can be projected in any temporal order. 
     When a pixel column  46  is projected onto the light (white) area of the target, a relatively maximum intensity of radiation is received by the sensor  22 A because the radiation is either reflected, re-emitted (as a longer wavelength), or transmitted to the sensor  22 A. Thus, the pixel column  46  displayed at X-axis locations x 1 , x 2 , x 8 , and x 9  will tend to result in a maximum radiation signal for the sensor  22 A. 
     On the other hand, when a pixel column is projected fully onto the dark area (Y-bar  36 AB) of the target, a relatively minimum intensity of radiation is received by sensor  22 A. Thus, the pixel columns  46  displayed at X coordinates x 4 , x 5 , and x 6  will tend to result in a minimum radiation signal. 
     The pixel columns  46  for x 3  and x 7  partially overlap the Y-bar  36 AB and so an intermediate intensity of radiation is received by sensor  22 A. Thus, the pixel columns  46  displayed at x 3  and x 7  will tend to result in an intermediate radiation signal. 
       FIG. 4  is an approximate graph of intensity received by sensor  22 A versus position x which corresponds to  FIG. 3 . Some of the x-values including x 1 , x 3 , and x 5  from  FIG. 3  are indicated. As shown, the intensity is maximized whenever the pixel column  46  is away from a Y-bar  36  as at value x 1 . When the pixel column  46  partially overlaps the Y-bar, the intensity is at an intermediate level as at x 3 . When the pixel column  46  is fully within the Y-bar  36 , the intensity is minimized as at x 5 . 
     Various metrics can be computed using the data from the graph of  FIG. 4 . A time XE (width of extremum) is a width of the graph at which the intensity is at minimum. A width XW is a width of the overall perturbation caused by the Y-bar  36 . Yet another metric would be a magnitude of a slope (m) of the graph at time x 3  and/or time x 7  in a transition between a high intensity and a low intensity signal. A further metric would be a ratio of XE to XW. 
       FIG. 5  is a flowchart of a method of aligning a projection module  9  to the target  16 . According to 50, a sequence of pixel arrays is generated and projected onto the target  16 . Each of the sequence of pixel arrays is displayed at a different axial location.  FIG. 3  previously illustrated the pixel arrays as pixel columns  46  displayed at axial locations x 1 , x 2 , and so on. 
     According to 52, concurrent with the sequence generation, a sensor  22  receives the radiation and outputs a signal to the controller  24 . According to 54, the signal is analyzed to align the projection module  9  to the target  16 . 
     In one embodiment of  FIG. 4  a center of the perturbation (deflection from maximum intensity) is computed to be a center of the Y-bar along the X-axis. This center can be aligned to a corresponding pixel column  46  or an interpolation between two pixel columns  46 . 
       FIG. 3  depicts the pixel column  46  as being very nearly parallel to the Y-bar  36 . However, the Y-bar may not be perfectly parallel.  FIG. 6  depicts a situation in which the pixel row  46  is angularly misaligned about the Z-axis (in Theta-Z) relative to the Y-bar  36 . The angular skew of  FIG. 6  is exaggerated for illustrative purposes to facilitate explanation. 
       FIGS. 7A and 7B  show the comparison of intensity versus X sequences (like  FIG. 4 ) that are obtained when the angular misalignment is relatively small ( FIG. 7A ) and large ( FIG. 7B ). Therefore,  FIG. 7A  corresponds to the pixel column sequence of  FIG. 3  and  FIG. 7B  corresponds to the pixel column sequence of  FIG. 6 . As the angular alignment improves, several changes occur. First, the slope m of the graph between minimum intensity and maximum intensity increases. At the same time, the width XW (see  FIG. 4 ) decreases. Third, the width XE increases. Fourth, a ratio of XE to XW converges on unity. 
       FIG. 8  is a flowchart depicting a method  58  of angularly aligning a projection module  9  to a target  16 . According to  60 , a sequence of pixel arrays is generated and projected onto the target  16 . According to  62 , concurrent with the sequence generation, a sensor  22  receives the radiation and outputs a signal to the controller  24 . According to  64 , a data set characterizing the signal is stored. The data set can define a graph such as that depicted in  FIG. 4 . Steps  60 - 64  are repeated for varying orientations of the pixel arrays and the characterizing data stored. 
     According to  66 , the data sets are analyzed to align the projection module  9  to the target  16 . This can be done by analyzing metrics such as the slope m, width XE, or width XW. The angular orientation of a pixel array for which slope m is maximized, XE is maximized, and/or XW is minimized would be the closest angular alignment to the Y-bar  36 . 
       FIG. 9  is a flowchart depicting an overall process sequence  68  for aligning the projection modules  9  of a three-dimensional printing system  2 . According to  70 , the individual projection modules are aligned in X and Y to the target  16 . For a single projection module, this includes performing the method  48  twice—once aligning to the Y-bar and then aligning to the X-bar. 
     According to  72 , the projection modules  9  are individually aligned to the target  16  in theta-Z. This can include performing method  58  for each projection module  9 . 
     According to  74 , the alignment can be performed in the overlapping regions  32  using thin X-bar  38  and thin Y-bar  40 . The method of step  74  is essentially the same as method  48 , and serves to refine alignment accuracy between the projection modules  9 . 
     According to  FIG. 76 , further processes can be performed using the outer thin X-bars  38  and thin Y-bars  40  to provide data to correct for distortions such as barrel distortions and keystone errors. Generating data sets for such corrections can employ methods similar or related to those discussed with respect to  FIGS. 3-8 . 
     Methods have been described with respect to  FIGS. 2-9  using a light field target (white or clear target with dark bars). However, very similar methods can be performed using dark field targets in which the indicia are clear or white and the field is dark or black. 
     Although the above disclosure has been described in terms of aligning plural projectors, some of the apparatus and techniques above may be applicable to correcting distortions for a system having a single projector. The specific embodiments and applications thereof described above are for illustrative purposes only and do not preclude modifications and variations encompassed by the scope of the following claims.