Automatic geometric calibration using laser scanning reflectometry

Related application

This application claims the priority benefit under US Patent Law of U.S. Provisional Patent Application No. 60 / 957,576 filed August 23, 2007, which is incorporated herein by reference. ..

The present invention relates to a method and apparatus for calibrating a solid imaging apparatus.

Solid imaging equipment is used for high-speed trial production of models for product development, and more recently for production activities. A solid imaging device produces a three-dimensional object by exposure to radiation, typically in response to computer control, from a meltable powder or photocurable liquid. The data representing the transverse layers of a three-dimensional object typically give the computer layer-by-layer control parameters for a program for automatic construction of the object. A laser or other light source for chemical beam emission suitable for solid imaging continuously irradiates individual thin layers of construction material, and in response to that irradiation the material changes to a solid layer by layer and is solid. -The imaging product is generated. Exemplary stereolithography devices are described in Patent Documents 1 and 2, which are incorporated herein by reference.

Solid imaging, sometimes referred to as "fast prototyping or manufacturing," involves a variety of techniques such as stereolithography, laser sintering, inkjet printing, and more. Powders, liquids, ejectable phase change materials and other materials for solid imaging are sometimes referred to as construction materials. The 3D objects produced by solid imaging techniques are sometimes referred to as "constructions," "parts," "objects," and "solid imaging products," which can be produced in a variety of shapes and dimensions.

The constructs are made on surfaces called "construction pads" or "construction platforms", which expose the surface of the constructs to chemical lines and construction materials, "working surfaces", "construction planes" or "images". It is raised or lowered to make contact with the "plane".

<p><patcit num="1"><text>U.S. Pat. No. 4,575,330</text></patcit><patcit num="2"><text>U.S. Pat. No. 5,495,328</text></patcit></p>

<p> Despite the various devices or methods developed for solid imaging, many shortcomings must still be overcome in order for the process to be more efficient and cost effective. This includes, for example, a complex and time-consuming alignment step of aligning the radiation source with the image plane to properly create the object.</p>

Means to solve problems

The present invention is directed to methods and devices for calibrating solid imaging devices that form three-dimensional objects, especially calibrating laser scans in such devices on a flat surface. Calibration is performed to calculate local and overall shape errors so that the laser beam is guided accurately and accurately when forming a 3D object.

One aspect of the method comprises obtaining a sufficient number of position measurements to provide an iterative solution to an unknown parameter of a given nonlinear model that governs the motion of the laser scan. Positional measurements are generated by a laser scanning a flat, horizontal calibration plate with a substantially non-scattering surface and a periodic array of reference marks, and the reference marks are on or on the non-scattering surface. It is formed inside and scatters chemical rays. As the laser scans over the calibration plate, a detector located on top of the calibration plate receives the scattered light from the reference mark.

Another aspect of the invention is a method of forming a three-dimensional object and calibrating a solid imaging system comprising a mirror-based optical system to generate and guide a light beam having a chemical ray wavelength. The solid imaging system has an elevating system that movably supports a building platform for building objects. The method involves the step of operatively placing the calibration plate on the construction platform. The calibration plate has a periodic array of reference marks formed on a substantially non-scattering background, the reference marks being configured to scatter chemical rays. In that method, the reference marks that are orthogonal to each other to establish the first center position or the first coordinate system used to establish the "theoretical" center position of the other reference marks in the reference mark array. Performs a first scan that scans the light beam on the first and second rows of the light beam, from which scattered light is detected. The first and second columns that are orthogonal to each other are preferably the central X column and the central Y column. The method further performs a second scan, scanning at least a portion of the reference mark array, using the first coordinate system to measure the corresponding center position scanned by the second scan. , The scattered light beam is detected from there. The method also uses interpolation of the measured center position and the angular position of the mirror to establish a calibrated relationship between the measured center position and the (x, y) position of the construction plate.

The calibration method is substantially unaffected by temperature environment variables and the calibration process can typically be completed in less than an hour.

The methods described above meet the calibration criteria of being quick, not involving the operation of solid imaging systems (other than scanning mirrors), and being relatively inexpensive. Preferably, a computer controller in a solid imaging system is used for the calibrator, which matches the detection signal associated with scattered light from the reference mark to the known dimensions of the reference mark for knowledge pattern matching search. Through the algorithm performed, it receives a command (eg, software) stored on a computer-readable medium that recognizes when the laser beam finds the center position of each reference mark. Any change in laser power during the scan has little effect, especially in the exemplary method of employing multiple scans of the reference mark that are subsequently averaged.

In order to model as close to the system error as possible so that the calibration is extremely accurate and accurate, all unknown parts of the system are repeatedly discovered and redistributed so that each series of errors can be identified as smaller. Need to be introduced. A single detector located on top of the calibration plate provides a central position for receiving data from the calibration plate and allows the calibration plate to be completely scanned in minutes rather than hours. .. This time reduction makes it possible to obtain a sufficient amount of position measurement information to perform the required number of iterative calculations to provide calibration near the limits of the calibration device's capabilities.

Other features and advantages of the present invention are set forth in the detailed description below, some of which will be immediately apparent to those skilled in the art, or the detailed description, claims and subsequent statements that follow. It will be recognized by practicing the invention described including the accompanying drawings. Both the general description given above and the detailed description set forth below provide exemplary embodiments of the invention and provide an overview or framework for understanding the nature and properties of the claimed invention. Intended to provide. The accompanying drawings are provided for the purpose of providing a further understanding of the present invention and are incorporated herein by reference. The drawings exemplify various embodiments of the present invention, and serve to explain the principles and operations of the present invention together with detailed explanations.

<figref num="1">Schematic of a solid imaging system according to an exemplary embodiment in the form of a stereolithography system shown in a vertical section.</figref><figref num="2">Part of the optical system of the system of FIG. 1 showing an exemplary mirror system showing the relationship between the angular coordinates (θx, θy) and the Cartesian coordinates (x, y, z) associated with the construction plane. Enlarged perspective view of.</figref><figref num="3">The schematic diagram of the stereolithographic system of FIG. 1, further comprising a calibration apparatus according to an exemplary embodiment of the invention, which makes it possible to carry out the calibration method of the present invention.</figref><figref num="4">Top view of a calibration plate according to an exemplary embodiment of the invention, including an enlarged view (insertion view) of an exemplary calibration plate having a periodic arrangement of circular reference marks.</figref><figref num="5">A cross-sectional view taken along lines 5-5 of an exemplary embodiment of the calibration plate of FIG. 4, shown with an enlarged view (insertion view) of the calibration plate surface.</figref><figref num="6">An exemplary embodiment of a 1-foot x 1-foot calibration plate with reference marks every 0.25 inches and 48 x 48 = 2,304 reference marks. A plan view showing the calibration plate and the reference mark in reverse contrast for easy display.</figref><figref num="7">FIG. 7A is a schematic plan view showing two adjacent circular reference marks, FIG. 7B showing two adjacent hexagonal reference marks, and FIG. 7C showing two adjacent square reference marks.</figref><figref num="8">FIG. 5 is a schematic side view of an exemplary embodiment of a calibration plate similar to FIG. 5, wherein the calibration plate has a relatively thick support plate that supports a relatively thin "target plate" with a reference mark. The figure which shows.</figref><figref num="9">The flowchart which shows the whole calibration method which concerns on the exemplary embodiment of this invention.</figref><figref num="10">10A-10D are enlarged plan views of an exemplary circular reference mark, each showing how the light beam is two-dimensionally raster-scanned over the reference mark to determine the center position of the reference mark.</figref><figref num="11">11A-11C are enlarged cross-sectional views of the exemplary reference mark and surrounding non-scattering layer showing the light beams before, during, and after passing through the reference mark during scanning of the calibration plate, respectively. ..</figref><figref num="12">Schematic diagram of a dual stereolithography system that performs dual system calibration using two calibration plates.</figref>

Hereinafter, the present invention will be described in detail with reference to the accompanying drawings. The drawings show some, but not all, embodiments of the present invention. In fact, the invention can be embodied in various forms and should not be considered limited to the embodiments presented herein, but rather these embodiments are applicable to the present disclosure. It is shown to meet legal requirements. The present invention is directed to methods and devices for calibrating solid imaging devices. In the following, an exemplary solid imaging device in the form of a stereolithography system will be considered for exemplary purposes, followed by calibration criteria. Following these descriptions are the methods and systems of the invention used to calibrate stereolithography systems.

<u style="single">Illustrative stereolithography system</u> FIG. 1 is a schematic diagram of a solid imaging system in the form of a stereolithography system 10 shown in vertical section. A right-handed Cartesian coordinate system is provided for reference. The system 10 is filled with UV curable resin or the like and has an internal region 21 (referred to as a construction chamber) that provides a designated working surface or construction plane defined by the liquid level. As used herein, the term "construction plane" refers to the location within the container 20 where, if liquid 22, the construction plane 23 is present or can be present.

The optical system 25 having a mirror as a main element is configured to supply a mirror system MS and a chemical ray (ultraviolet) light beam 26 that generates a spot (laser spot) 27 in the plane of the construction plane 23. It has a light source LS or other beam generator (including but not limited to laser diodes, light emitting diodes, etc.). As used herein, "chemical radiation" includes any electromagnetic radiation that causes a photochemical reaction in a material that absorbs electromagnetic radiation, as well as all electromagnetic radiation. Such chemical rays include, but are not limited to, any radiation that absorbs radiation and causes cross-linking of cross-linkable materials. The optical system 25 is configured to move a light beam, such as a laser spot 27, on the construction plane 23 to construct the object 50. The movement or "guidance" of the laser spot 27 on the construction plane 23 is accomplished by adjusting the mirror system MS along with the optical and / or mechanical elements within the optical system 25. In an exemplary embodiment, the optical system 25 has the dimensions (ie, diameter or width W) of the laser spot 27._LS, See Figure 11A) to adjust the resolution of the laser scan and the resolution of the construction process.

In an exemplary embodiment, the guidance of the laser spot 27 on the plane 23 is controlled by the computer control system 30. In an exemplary embodiment, the computer control system 30 controls such guidance based on the CAD data generated by the CAD data generator 32 in a computer-aided design (CAD design) system or the like. The CAD data generator 32 is then operatively connected to the computerized CAD data conversion system 34, which is operably connected to (or contained within) the computer control system 30. The CAD data conversion system 34 converts the CAD data from the CAD data generator 32 into a suitable layer data format for stereolithography so that the controller can guide the laser spot 27 in a manner that forms the object 50. Convert.

System 10 has an elevating system 40 that is operatively connected to computer control system 30 (also called platform surface). The elevating system 40 includes a mobile construction platform 42 having a top surface 43. The construction platform 42 is operably connected to an elevator drive 44, such as a drive screw or piston base, controlled by the platform drive 46. The elevator drive unit 44 selectively moves the construction platform 42 up and down (that is, along the Z direction) by the platform drive unit 46 under the control of the computer control system 30.

The system 10 further comprises a laser leveling system 48 that is operatively connected to the computer control system 30. The laser leveling system 48 is configured to generate a laser beam 49 that reflects away from any of the surfaces below it to measure the plane level with respect to the horizontal reference plane.

The construction plane 23 of the UV curable liquid 22 is maintained at a constant level within the vessel 20 and is strong enough to cure the liquid into a solid material with a laser spot 27 or other suitable form of reactive stimulus. Is moved on the construction plane in a programmed manner. The UV curable liquid 22 cures to form a solid material, and the elevator platform 42 (originally directly below the construction plane 23) moves downward from the construction plane in a manner programmed by the action of the elevator drive 44. Will be done. In this way, the first solid material formed is obtained underneath the construction plane 23 and the new liquid 22 is on the construction plane in the presence of assistance such as repainting equipment or without assistance. be introduced. Part of this new liquid is then converted to solids by the chemical line 26 from the laser spot 27, which attaches and connects the new material to the material beneath it. As the device is activated, the device produces a three-dimensional object 50 by stepwise stacking of stacked layers (stacks) 52. This process continues until the entire 3D object 50 is built on the platform surface 43. The object 50 is then removed from the container 20 and the device is prepared to produce another object. Then another one of the same object is manufactured, or some new object is created by modifying the CAD data supplied to the computer control system 30.

<u style="single">Optical system</u> Figure 2 shows the exemplary mirror system MS and the relationship between the angular coordinates (θx, θy) of the mirror system and the Cartesian coordinates (x, y, z) associated with the platform plane 42, with mirrors as the main elements. It is an enlarged perspective view of a part of the optical system 25. The mirror system MS is a first mirror MX and a second mirror MY that are mechanically rotated around their respective axes X, Y by their respective mirror drives (eg, mirror motor or galvanometer) MDX, MDY. including. The mirror motors MDX and MDY are operably connected to and controlled by the computer controller 30. The mirror MX controls the X coordinate on the platform surface 43, and the mirror MY controls the Y coordinate on the platform surface. The laser beam 26 generated by the laser LS is guided to the point P = P (x, y, z) by the operation of the mirrors MX and MY under the control of the mirror motors MDX and MDY, and the calibration plate surface 132. The origin is determined on the center of the Y-dimensional mirror MY and its rotation axis. The calibration plate surface 132 is separated from the center of the Y-dimensional mirror MY by a distance Z. The angle θy corresponds to the angle of the laser beam 26 from the vertical in the Y dimension, and the angle θx corresponds to the angle of the laser beam 26 from the vertical in the X dimension. Laser scanning where the distance (interval) between the center of the Y-dimensional mirror MY (its rotation axis) and the center of the X-dimensional mirror MY (its rotation axis) is S, and the system scans a planar surface. According to well-known relationships in the field, the correction value of the angle of the laser beam 26 from the vertical direction to irradiate the point P can be determined as follows: θy = TAN^-1(y / Z) θx = TAN^-1(x / (Z²+ Y²)^1/2+ S))) The focal radius FR (not shown) of this system can also be determined by the conventional method as follows: FR = [((Z²+ Y²) ^1/2+ S)²+ X²]^1/2From these relationships, the geometric errors caused by the planar surfaces of the mirrors MX and MY and their spacing S are corrected given the distance Z between the platform surface 43 and the Y-dimensional mirror MY. can do.

<u style="single">Calibration criteria</u> There are many criteria that must be met by calibration equipment for solid imaging equipment. For example, in order to weaken the "roofing" effect of the solidified layer of the formed part, the laser beam needs to have an angle range close to 90 degrees with respect to the construction plane. This is achieved by having a large distance from the scanning mirror to the construction plane. This large working distance hinders the calibration process. The reason is any geometric of the scanning system, such as imperfect mounting of mirrors, non-uniformity and lack of flatness of chamber windows, warpage of mirror axes, non-parallel mounting of galvo motors (ie mirror motors). This is because various defects are also amplified.

Moreover, the theoretical mapping of the angular coordinates associated with a pair of scanning mirrors to the construction plane presents inherent difficulties known to those skilled in the art of laser scanning techniques. The difficulty is that the mapping of the coordinate system is non-linear because the laser beam originates from two points, a vector with an unknown start and a vector with an unknown end, rather than the center point of space. is there. With this configuration, an arc is drawn on the construction plane instead of a straight line, and a mapping error in the shape of pincushion distortion is generated.

Another criterion is that during data acquisition, there must be no movement of the solid imaging device in any direction or rotation about any axis other than by a scanning mirror. Any movement can be factored into the calculation, but its accuracy is limited by how well the movement is detected. Any rotation distorts the error map to some extent, and such rotations are difficult to measure with the required accuracy.

Moreover, the exact angle and position of the laser beam incident on each mirror, the distance between both mirrors, and the distance from the second mirror to the construction plane are unknown. The combination of image non-linearity with many unknown parameters and various possible geometric defects requires large amounts of data to achieve the desired accuracy. Another criterion is that the calibration should be relatively quick, for example within an hour, so expansion or contraction of the system due to temperature or humidity during the course of the calibration measurement is ignored. Is Rukoto. A relevant criterion is that the relatively large amount of computational information must be processed on the fly without the need to reinstall any calibration equipment and to allow the next calibration measurement to be repeated. It must be.

<u style="single">Calibration device</u> As mentioned above, the laser spot 27 is guided to the desired object coordinates with high accuracy and accuracy so that the intended object is faithfully duplicated prior to the operation of the system 10 for constructing the object 50. , The system needs to be calibrated.

FIG. 3 is a schematic representation of the stereolithography system 10 of FIG. 1, which further comprises a calibration device according to an exemplary embodiment of the invention that allows the calibration method of the invention to be carried out. Including. The calibration device has a calibration plate 110 having an upper surface 112 and a lower surface 113, and the calibration plate is arranged such that the lower surface is located on the platform surface 43. Details of the exemplary calibration plate 110 will be described in more detail below.

The calibration device has a photodetector 130 that is located above the top surface 112 of the calibration plate (ie, in the + Z direction) so as to deviate from the path of the light beam 26. In an exemplary embodiment, the photodetector 130 has a Si-PIN photodiode having a diameter of, for example, 5.8 mm and a wide wavelength detection range from 190 nm to 1100 nm. Other types of photodetectors capable of detecting light of UV wavelength (or other required wavelengths), such as GaP and GaAsP photodetectors, can also be used. The photodetector 130 produces an electrical detection signal SD in response to the detected light, as described in more detail below. In an exemplary embodiment, the chemical wavelength λ₀An optical filter 131 with a passband Δλ centered on substantially drives the detection process to the chemical wavelength λ.₀Used to limit to.

<u style="single">First exemplary calibration plate</u> FIG. 4 is a plan view of the calibration plate 110 according to the first exemplary embodiment, and FIG. 5 is a cross-sectional view of the calibration plate of FIG. 4 taken along lines 5-5. The calibration plate 110 includes a rigid plate-like substrate 130 having a flat top surface 132 and a bottom surface 134. The substrate 130 has a width Wp, a length Lp, and a thickness Tp. An exemplary material for substrate 130 is aluminum. In an exemplary embodiment, the substrate 130 has an overall uniform thickness Tp ranging from about 0.5 inches (about 12.5 mm) to about 2 inches (about 50 mm), preferably about 0.75 inches (about 19 mm). It is an aluminum plate. In an exemplary embodiment, the top surface 132 of the aluminum substrate is further formed to have a surface roughness FL 0.005 inch (about 0.127 mm) at each 20 inch (about 50 mm) span. In an exemplary embodiment, a Blanchard grinding process (also called "rotary surface grinding") is used to achieve the desired surface roughness FL. In an exemplary embodiment, the substrate 130 has a width Wp of 1 foot (about 30 cm) Wp 3 feet (about 90 cm) and a length Lp of 1 foot (about 30 cm) Lp 4 feet. It is the range specified by (about 120 cm). Other board dimensions (including thickness) can also be used, using dimensions that are limited only by the requirements of the particular system 10, such as the need to minimize board deflection. In an exemplary embodiment, the dimensions of the substrate 130 define the dimensions of the calibration plate 110.

In an exemplary embodiment, the calibration plate 110 includes a leveling tab 111 having a surface 113 extending outward and positioned relative to the calibration plate surface 112. The leveling tab 111 is positioned such that its surface 113 is directly below the laser alignment system 48 and provides a reference plane for precisely leveling the calibration plate 110 within the system 10.

The top surface 132 of the calibration plate is formed so as to be substantially non-scattering (ie, substantially non-scattering) and preferably substantially absorbs the chemical line 26. In an exemplary embodiment, for this purpose, the calibration plate top surface 132 comprises a light absorbing layer 140 formed on it. In an exemplary embodiment, the light absorbing layer 140 is formed by anodizing, preferably by photoanodizing. The light absorbing layer can also be formed by other techniques and / or other materials such as dyes, paints, plastics, ceramics and the like.

The light absorption layer 140 is capable of absorbing a significant amount of chemical lines 26 to reduce unwanted scattering when the light spots 27 are scanned between reference marks as described below on the calibration plate 110. Is formed in. In this sense, the top surface 132 of the calibration plate serves as a "dark" or "non-scattering" background.

The calibration plate 110 further includes a periodic array 150 of reference marks 156 formed, for example, on the surface or inside of the light absorbing layer 140, in contact with the top surface 132 of the calibration plate. The plurality of reference marks 156 are formed so that the chemical line 26 can be scattered. In an exemplary embodiment, the reference mark has silver halide formed within the light absorption layer 140 during the photoanodizing process described above. FIG. 4 includes an inset showing a magnified view of an exemplary light absorbing layer 140 having a circular reference mark 156. In an exemplary embodiment, the reference mark 156 has approximately the same dimensions as the laser spot 27.

In an exemplary embodiment, the reference mark 156 is formed within the photosensitive anodized aluminum substrate surface 132 using computer numerically controlled (CNC) milling to create the reference mark. In a further embodiment, alternative techniques and / or materials are used to provide multiple reference marks on the substrate surface. The reference mark is associated with the surface of the substrate by being positioned in contact with or within the surface of the substrate, or by being connected to or in close proximity to the surface of the substrate.

In an exemplary embodiment, the reference mark 156 is the center-to-center distance D._FOnly separated from each other and also width W_FHave. The reference mark 156 is preferably the center-to-center spacing D._FIs not greater than 1 inch (about 25 mm), more preferably not greater than 0.5 inch (about 12.5 mm), and even more preferably about 0.25 inch (about 6.25 mm). Reference mark 156 is preferably width W_FIs not greater than 0.005 inch (about 0.125 mm), more preferably not greater than 0.004 inch (about 0.1 mm), and even 0.002 inch (about 0.05 mm) W_FThe range specified by 0.004 inch (about 0.1 mm) is more preferred. The placement accuracy of the reference mark 156 is preferably 0.001 inch (about 0.025 mm) or less.

An exemplary calibration plate 110 includes, for example, 1,000 to 20,000 reference marks 156. FIG. 6 shows, for example, the distance D.₁Shown is an exemplary 1 foot x 1 foot calibration plate containing 48 x 48 = 2,304 reference marks 156, separated by = 0.25 inches. In the calibration plate of FIG. 6, the contrast is reversed in order to facilitate the display, that is, the background surface 140 is displayed in white and the reference mark is displayed in black.

The 2-foot x 3-foot version of the calibration plate 110 in Figure 6 contains approximately 13,824 standard marks 156. However, the dimensions of the calibration plate and the spacing between the reference marks D_FDepending on, a number of reference marks 156 outside the above range can also be used.

Various shapes such as circle (Fig. 7A), hexagon (Fig. 7B) and square (Fig. 7C) can be used for the reference mark 156. Other shapes such as crosses, double boxes, other polygonal shapes, or curved shapes are also available. In general, the reference mark 156 may have any shape that can be scanned by the laser spot 70 to obtain the center (x, y) position of the mark using the detected scattered light and an appropriate algorithm. ..

<u style="single">Second exemplary calibration plate</u> The calibration plate 110 described above uses a single thick substrate 130 and it can be relatively expensive to replace that substrate. For example, a 2-foot x 3-foot aluminum substrate 130 with a thickness of 0.75 inches (18.75 mm) can be polished to a high degree of flatness on its surface 132 for about $ 2,000. The cost is required. If surface 132 is scratched or damaged, the entire calibration plate needs to be replaced.

FIG. 8 is a schematic side view similar to that of FIG. 5, but substantially rigid first support substrate 136 or substrate 130 as a support plate and substantially supported by the substrate 130 on the top surface 132. A calibration plate 110 of a second exemplary embodiment, including a flexible thin second substrate 136 or a "target plate", is shown. The substrate 136 has a light absorbing layer 140 and an upper surface 138 on which a plurality of reference marks 156 are formed. In this embodiment, the substrate surface 132 does not require anodizing. The substrate 136 is preferably aluminum and is relatively thin, for example, with a thickness ranging from about 0.0015 inches (about 0.0375 mm) to about 0.004 inches (about 0.1 mm), preferably about 0.002 inches (about 0.05 mm). Has a thickness of. The thickness of the substrate 136 is selected according to the roughness FL of the surface 132 of the substrate 130 below. In an exemplary embodiment, the substrate 136 is attached to the surface 132 of the substrate 130 using alcohol and the surface tension thus obtained.

The advantage of the two-board form of the calibration plate 110 is that if the surface 138 on which the array 150 of reference marks 156 is mounted is damaged, only the relatively thin board 136 needs to be replaced at a cost of about $ 200.

<u style="single">Calibration method</u> A method according to a preferred embodiment of the present invention will be described below with reference to the flowchart 200 of FIG. The exemplary calibration method can be used before shipment at the manufacturing site and / or after installation. If any mechanical movement, laser removal, or substantial laser drift occurs, the calibration process should be repeated.

In step 201, the calibration plate 110 is inserted into the construction chamber 21 of the system 10 so that the surface 112 of the calibration plate is substantially coplanar with the construction plane 23. As mentioned above, in an exemplary embodiment, the calibration plate 110 includes a leveling tab 111 used to accurately level the calibration plate within the system 10. The leveling tab 111, or other mechanism of the calibration plate 110, is used to align or orient the calibration plate with respect to the solid imaging system.

At step 202, the light beam 26 is guided towards reference mark 156 and the reference mark inspection profile is performed. This includes a two-dimensional (2D) raster scan of a particular reference mark 156 in the X and Y directions.

Figure 10A-10D shows how the light beam 26 raster scans the reference mark in the X and Y directions during the profiling process to determine the "optimal location" or "center position" 156C of the reference mark. It is an enlarged plan view of an exemplary reference mark which shows whether it is done. The arrow 170 indicated by the broken line indicates the scanning direction of the light spot 27.

The data from the 2D raster scan of the selected reference mark 156 is then used to infer the center position of the reference mark 156C, as well as the appropriate contrast and black level for scanning the entire calibration plate 110. In an exemplary embodiment, the center position 156C is determined using two different algorithms, such as the centroid algorithm and the Gaussian approximation algorithm. Both algorithms must match so that the center position 156C is determined within a very small margin of error (eg, less than 0.001 inch), or a raster scan of that reference mark. Is repeated. Other algorithms or efforts to determine the center position 156C may be used alone or in combination. Laser power is automatically adjusted in a closed loop by the computer control system 30 for each raster scan or "profiling" to maximize contrast. In an exemplary embodiment, some reference mark 156 is "profiling" multiple times for redundant measurements.

11A-11C show the light beams 26 before, during, and after passing over the reference mark when scanning the calibration plate 110, an expanded cross section of the exemplary reference mark 156 and the light absorption layer 140. It is a top view. In FIG. 11B, the light spot 27 substantially overlaps the reference mark 156 to form scattered light 26S, and the scattered light is detected by the detector 130 (FIG. 3). Between the reference marks 156, the light beam 26 is largely absorbed by the light absorbing layer 140 as the light spots 27 pass between the reference marks 156, so there is in principle no light scattered towards the detector 130.

Referring again to Flowchart 200 of FIG. 9, in step 203, in the center row and column of the calibration plate 110, that is, in the center row (x, 0) in the X direction, and then in the center column (0, y) in the Y direction. The X-axis and Y-axis are identified by performing the first (or early) beam scan of one row and one column. In general, the first beam scan can be performed along any two orthogonal rows / columns (orthogonal columns) of the reference mark 156. However, in an exemplary embodiment of the invention, it is preferred to select a central row and a central column to determine the origin of the Cartesian coordinate system in the center of the calibration plate 110.

In the above procedure, it is preferable to use a relatively large laser spot size for the first or initial scan of a relatively large area. In an exemplary embodiment, the width W of the laser spot 27_LSIs the width W of the reference mark 156 to be searched_FApproximately 4 times the width of the reference mark W_FIf is 0.03 inch (about 0.75 mm), the laser spot size W_LSIs about 0.12 inches (about 3 mm). This relatively wide first scan is performed for each reference mark in one row and one column, preferably in the middle row and middle column. The relatively wide laser spot 27 ensures that the reference mark 156 is searched by the first scan. Once all the positions of the reference marks 156 in the center row and center column have been determined, the theoretical positions of the remaining reference marks 156 on the calibration plate 110 are calculated.

This initial scan provides important information for solving almost all theoretical model parameters, including rotation, displacement, mirror distance, distance from the second mirror to the plane, incident and exit angles at the origin. Be done. These theoretical model parameters are determined for all reference marks based on the data collected for the central row and central column, using known regression analysis techniques using the equations described above. Regression analysis iterations are continued until the RMS error is less than 0.005 inches. This theoretical model allows a rapid scan of the remaining reference mark 156 in the second scan, as described below. The remaining reference marks are relatively large (eg, W), as described as being done for the middle row and middle column for the first scan, for example._LSIs narrower than scanning the reference mark with a laser spot 27 (of approximately 0.12 inches), eg W_LS= Scanned in a second scan using a 0.040 inch laser spot 27.

By moving the mirrors MX and MY in small increments dθx, dθy and guiding the laser beam in the corresponding small increments dY and dX in Cartesian coordinates, the relationship between the angular movement of the mirror and the distance traveled is established. , The theoretical origin of the initial coordinate system can be set.

If successful in step 203, the rotation, magnification and offset values are calculated, which either form the first coordinate system or map the mirror angular coordinates (θx, θy) to the XY coordinates of the calibration plate. It is used to create a "theoretical model". This theoretical model is the "theoretical" position (x) of the other (ie, unscanned) reference mark 156._T, y_T) Is used to predict. Before calibration, system 10 is incomplete as a whole, so the theoretical (center) position (x) of the reference mark._T, y_T) And the measured actual (center) position (x)_A, Y_A) And there will be a big difference.

In step 204, preferably all reference marks 156 of at least an important portion of the calibration plate 110 are measured by a second (or measurement) light beam scan and the actual center position (X) of the scanned reference marks 156._A,y_A) Is determined. In this second scan, the theoretical position (x) to search for reference mark 156_T, y_T) Is used. In an exemplary embodiment, this entire scanning process takes about 20 minutes for about 10,000 reference marks. This compares the actual center position to be compared with the theoretical center position and the error between them: (δx, δy) = (x_AX_T, y_AY_T) Allows you to calculate. This in turn allows the identification of local and overall errors in the theoretical model introduced by the flaws in System 10. Therefore, in step 204, the actual center position (x) measured in the second scan._A, Y_A) Is given to the geometric table. In one embodiment of the invention, the geometric table is 0.25 inch (about 6.25 mm) increments of Cartesian coordinate x along the X axis and 0.25 inch of Cartesian coordinate y along the Y axis. Includes the scanning mirror angular coordinates θy for each inch (about 6.25 mm) increment. In step 205, the geometric table established in step 204 is used, for all coordinates (x, y) of the calibration plate, for all scanning mirrors (eg, using a polynomial of degree 5). Interpolate the angular coordinates (θx, θy). This interpolation is generated from an equation that defines the entire scan area, and thus "tiling errors" such as local deviations generated using only the closest reference marks and using a simple averaging algorithm. Is not affected. These "interpolated coordinates" are the calibrated coordinates (x) that are then used by the computer control system 30 to guide the laser beam 26 in generating the object 50._C, Y_C). In an exemplary embodiment of the invention, polynomial interpolation of order 4 and degree 2 is performed to smooth the collected data. To obtain accurate (ie, calibrated) θχ and θy from this smoothed data, traditional bilinear interpolation is performed on the four closest surrounding data points in the geometric table. It is done in.

<u style="single">Calculation of calibration plate error</u> The surface 112 of the calibration plate can introduce errors into the calibration process. Any flatness or rotation error (eg, overall and local flatness change or rotation) causes a positional error in the scanned data. The theoretical flatness of the suspended surface is known to follow parabolic properties and can be modeled. However, unless measured, it is not known what kind of deviation will occur, and it may change depending on temperature and humidity.

Thus, in an exemplary embodiment, the calibration method comprises an optional step 206, in which the calibration plate 110 is rotated (eg, 45 ° or 90 °) and the central X-row and central Y-row th. One beam scan is repeated. If the calibration plate 112 is not perfectly horizontal, or if there is a flatness defect, or if the periodic array 150 of the reference marks 156 has a rotation error, these discrepancies will be rotated as well. There will be. When one X-column and one Y-column of reference mark 156 are scanned to determine flatness and / or rotation error or offset, these errors are calculated in the ("theoretical") coordinate system. it can. In a similar technique, the surface 112 of the calibration plate is lowered or raised to measure and compensate for any flatness error.

<u style="single">Focus map</u> Since the solid imaging system 10 often includes a relatively large construction plane 23, the mirror-based optical system 25 is the focus of the laser beam as the laser beam 26 moves over the construction plane. Must be adjusted dynamically. Although the focal length mechanism has been known for some time, any moving part has its own offset and rotational error, thus moving the focused beam spot to a position different from the intended position. Therefore, in step 207, the interpolated coordinate information from step 205 is used to generate a focus map that provides the laser beam 26 with the appropriate focus for the given (x, y) coordinates.

<u style="single">Calibration verification</u> In optional step 208, a visual verification process is performed. The scanning mirror uses a geometric table to create a vector that illuminates the selected reference mark 156, which appears to shine when illuminated due to the scattered light 26S described above. The selected reference mark 156 is illuminated within a selected pattern that allows the user of the system to perform a visual verification of the calibration.

Another optional step 209 requires or guarantees verification beyond visual calibration verification, such as black MYLAR® film (not shown), other polyester films, or other materials. A film is supplied and etched using a laser beam 26 guided using calibrated coordinates so that it becomes the selected calibration pattern (ie, the reference mark illumination pattern). The calibration pattern formed on the photosensitive material is then inspected using conventional measurement methods that confirm the calibration of System 10.

<u style="single">Double scanning system</u> After addressing all of the errors described above, it is possible to add another scanning system adjacent to that system in order to increase the dimensions of the construction plane. FIG. 12 is a schematic representation of a pair of stereolithography systems 300, including two construction chambers operatively arranged side by side, with two calibration plates 110 used to perform calibration in the manner described above. is there. This dual stereolithography system 330 is a separate optical system with a mirror as the main element to move the light beams generated by a single beam generator (not shown) or a separate beam generator. It can have a computer control system 30 that controls 25. In certain embodiments, each set of data points can be used to calibrate for each construction chamber. However, in other embodiments, each set of data points may be used to form a single calibration data set that combines the calibration data for a single (combination) construction chamber. Good. Many modifications and other embodiments of the invention described herein that have the advantages of the teachings provided in the specification and accompanying drawings will be appreciated by those skilled in the art to which the present invention belongs. You will notice. Accordingly, it is intended that the present invention is not limited to the particular embodiments described or modifications thereof, and that the modifications and other embodiments are included in the appended claims. Will be understood. The present invention is intended to include any amendment or modification as long as the amendment or modification does not deviate from the scope of the appended claims. Although specific terms have been adopted herein, they are used only in a general or descriptive sense and are not intended to be limiting.

10: Solid Imaging System 20: Container 21: Construction chamber 23: Construction plane 25: Optical system 26: Light beam 26S: Scattered light 27: Laser spot 30: Computer control system 32: CAD data generator 34: CAD data conversion system 46: Platform drive 110: Calibration plate