METHOD AND APPARATUS

Abstract

We describe a calibration method for calibrating one or more optical elements of an additive layer manufacturing apparatus useable for producing a three-dimensional workpiece, the method comprising: projecting, using the one or more optical elements, an optical pattern onto a material in order to prepare, from said material, solidified material layers using an additive layer manufacturing technique to form a test sample; determining a geometry of the test sample; comparing the determined geometry with a nominal geometry to generate calibration data; and calibrating the one or more optical elements using said calibration data.

Claims

1-21. (canceled)

22. A calibration method for calibrating one or more optical elements of an additive layer manufacturing apparatus useable for producing a three-dimensional workpiece, the method comprising: projecting, using the one or more optical elements, an optical pattern onto a material in order to prepare, from said material, solidified material layers using an additive layer manufacturing technique to form a test sample; determining a geometry of the test sample; comparing the determined geometry with a nominal geometry to generate calibration data; and calibrating the one or more optical elements using said calibration data.

23. A calibration method as claimed in claim 22, wherein one or more of the material layers of the test sample are prepared only when a first temperature variation rate of a first temperature of a said optical element and/or a second temperature variation rate of a second temperature of an enclosure in which the one or more material layers are prepared and/or a third temperature variation rate of a third temperature of a substrate on which the test sample is formed is below a threshold rate.

24. A calibration method as claimed in claim 22, wherein the geometry and the nominal geometry comprise coordinates and nominal coordinates, respectively, of a location of the test sample.

25. A calibration method as claimed in claim 22, wherein said determining comprises determining geometrical data relating to a said material layer of the test sample prepared last using the additive layer manufacturing technique’

26. A calibration method as claimed in claim 22, wherein said preparing using the additive layer manufacturing technique comprises supplying different layers of said material on a same height level for preparing the test sample using the additive layer manufacturing technique.

27. A calibration method as claimed in claim 26, wherein said supplying of the different layers of said material on the same height level comprises lowering the test sample prior to supplying consecutive ones of the different layers.

28. A calibration method as claimed in claim 22, wherein said forming of the test sample is performed over a period longer than a threshold period.

29. A calibration method as claimed in claim 22, wherein the nominal geometry is based on data used for defining the optical pattern.

30. A calibration method as claimed in claim 22, wherein the substrate is lowerable during the forming of the test sample.

31. A calibration method as claimed in claim 22, wherein a substrate material of the substrate on which the test sample is formed is identical with the material used to form the test sample.

32. A calibration method as claimed in claim 23, wherein the third temperature of the substrate is maintained at a target temperature and/or within a target temperature range during one or both of (i) the forming of the test sample and (ii) the determining of the geometry of the test sample.

33. A calibration method as claimed in claim 22, wherein a build height of the test sample is above a threshold height which is dependent on an amount of heat transferable, via the test sample, from an area of the material onto which the optical pattern is projected to an opposite side of the material.

34. A calibration method as claimed in claim 22, further comprising taking an optical image of the test sample, and wherein said determining of the geometry of the test sample comprises analyzing the optical image.

35. A calibration method as claimed in claim 34, further comprising: placing a real and/or virtual grid over the optical image and/or placing one or more reference marks on a floor of an enclosure, in which the test sample is formed, and identifying, in the optical image, a location of the test sample relative to the grid and/or the one or more reference marks to determine the geometry of the test sample; and/or wherein the optical image is taken upon the forming of the test sample having been completed.

36. A calibration method as claimed in claim 22, wherein the determining of the geometry of the test sample is performed using a coordinate-measuring machine.

37. An apparatus for producing a three-dimensional workpiece, the apparatus comprising: a substrate adapted to receive material useable for producing the three-dimensional workpiece; one or more optical elements adapted to project an optical pattern onto the material in order to prepare, from said material, solidified material layers using an additive layer manufacturing technique to form a test sample; and a calibration unit adapted to: determine a geometry of the test sample; compare the determined geometry with a nominal geometry to generate calibration data; and calibrate the one or more optical elements using said calibration data.

38. An apparatus as claimed in claim 37, wherein the apparatus is configured to prepare one or more of the material layers of the test sample only when a first temperature variation rate of a first temperature of a said optical element and/or a second temperature variation rate of a second temperature of an enclosure in which the one or more material layers are prepared and/or a third temperature variation rate of a third temperature of the substrate is below a threshold rate.

39. An apparatus as claimed in claim 37, further comprising a heating unit comprising a heating frame, wherein the heating unit is thermally coupled to the substrate to heat the substrate to and/or maintain the substrate at a target temperature and/or a target temperature range.

40. An apparatus as claimed in claim 37, further comprising: one or more temperature sensors adapted to measure the temperature variation rate of the temperature of one or more of (i) the one or more optical elements, (ii) the substrate, and (iii) the enclosure of the apparatus in which the substrate is arranged; a controller for controlling the one or more optical elements, wherein the one or more temperature sensors are coupled to the controller; and wherein the controller is adapted to control the one or more optical elements to project the optical pattern onto the material in order to prepare the one or more material layers of the test sample only when the first temperature variation rate and/or the second temperature variation rate and/or the third temperature variation rate is below the threshold rate.

41. An apparatus as claimed in claim 37, wherein the nominal geometry is based on data used for defining the optical pattern.

Description

[0055] These and other aspects of the invention will now be further described, by way of example only, with reference to the accompanying figures, wherein like reference numerals refer to like parts, and in which:

[0056] FIGS. 1 (a) to (e) show an exposure method according to the prior art;

[0057] FIG. 2 shows temperature of components of an apparatus for producing a three-dimensional workpiece versus time;

[0058] FIG. 3 shows an image of a test sample according to some example implementations as described herein;

[0059] FIG. 4 shows an image for an image field correction according to some example implementations as described herein;

[0060] FIG. 5 shows an image of a test sample and a virtual grid according to some example implementations as described herein;

[0061] FIG. 6 shows an image of a test sample and reference marks according to some example implementations as described herein;

[0062] FIG. 7 shows a schematic block diagram of an apparatus according to some example implementations as described herein; and

[0063] FIG. 8 shows a schematic flow diagram of a method according to some example implementations as described herein.

[0064] Example methods and apparatus as described herein may be used for image field correction in particular of a powder bed.

[0065] In order to overcome the problems of the prior art as stated above, in some examples, a (dot) pattern is not exposed onto a foil or plate, but is exposed into the powder. This may eliminate all the disadvantages of the calibration method used in the prior art in which a foil or plate is used.

[0066] In order to implement examples of the calibration method as described herein, a coater may, in some examples, proceed always at the same level. With a suitable tool/measuring device, the coater may always be set at the same position and/or height. This may allow for preparing the test sample in the same plane as, for example, in the laser melting or laser sintering process performed subsequently in order to prepare a three-dimensional workpiece using calibrated optics.

[0067] It may be advantageous if preparing the test sample takes a longer time (for example longer than a predefined threshold), as the optics (for example scanners) may need a warm-up phase and it may take, in some example, several hours to reach a constant temperature. A delta T (i.e. temperature change) of the optics (for example scanner) may lead to a positional shift of the laser beam in the x-y direction.

[0068] FIG. 2 shows temperature (in degree Celsius) of components of an apparatus for producing a three-dimensional workpiece versus time (in arbitrary units).

[0069] In this example, a reference pattern (in this example a cylinder) was generated from each of a first optics/optical element and a second optics/optical element.

[0070] Measurements taken and shown in FIG. 2 relate to the temperature versus time for a flow sensor (206) of the apparatus, a process chamber (208) of the apparatus, a first galvanometer scanner (210) of the apparatus, a second galvanometer scanner (212) of the apparatus, a first (front) part of a substrate platform (214) of the apparatus, and a second (back) part of the substrate platform (216) of the apparatus.

[0071] It can be seen that in the last exposed layer of the test sample, the temperatures of the galvanometer scanners and of the process chamber have settled to a constant temperature.

[0072] The platform heating was turned off, such that the increase in temperature of the substrate platform originated from the energy stemming from the laser emission.

[0073] Another point which may need to be kept in mind is the temperature of the substrate plate on which the test sample is built. The material of the substrate plate corresponds, in some examples, to the powder that is melted or sintered. A connection between the generated test sample and the substrate plate may hereby be achieved.

[0074] Different materials may have different coefficients of linear expansion a, as shown in the following table for some examples.

TABLE-US-00001 TABLE 1 coefficients of linear expansion α for different materials. α.sub.0 . . . 100° C. α.sub.0 . . . 500° C. Aluminum 23.8 × 10.sup.−6 K.sup.−1 27.4 × 10.sup.−6 K.sup.−1 Steel C 60 11.1 × 10.sup.−6 K.sup.−1 13.9 × 10.sup.−6 K.sup.−1 Stainless steel 16.4 × 10.sup.−6 K.sup.−1 18.2 × 10.sup.−6 K.sup.−1 Invar(RTM) steel  0.9 × 10.sup.−6 K.sup.−1  1.2 × 10.sup.−6 K.sup.−1

[0075] If the substrate plate is heated to, for example, 100° C. (preheating temperature for steel) during the calibration process/preparation of the test sample, it expands according to the coefficient α. The heating may deliberately be provided through a heater, but additionally or alternatively the laser(s) also introduce heat into the substrate plate. If the test sample is evaluated after completion at, for example, room temperature under, for example, a coordinate-measuring machine, the substrate plate shrinks together with the generated test sample. Even for a substrate plate (side length of, for example, 600 mm) made of Invar®, the shrinkage would be 600 mm×0.9×10.sup.−6 K.sup.−1×(100 K−20 K)=0.0432 mm.

[0076] To eliminate this problem, a high (higher than a predefined threshold) test sample may be advantageous. Heat induced by the beam (for example laser beam) may be partly released into the surrounding powder and only a part may reach the substrate plate. At the same time, the substrate plate may be kept at a certain temperature with a heating unit and corresponding control.

[0077] When evaluating the test sample, the substrate plate may be brought to the same temperature (ideally the same temperature as in the test sample preparation process). This can be done using, for example, a specially designed frame for the coordinate-measuring machine with integrated heating. In addition, the substrate plate may be centered both in the apparatus (for example selective laser melting machine) and in the frame. Alternatively or additionally, a software-based simulation/correction of the shrunken substrate plate (for example when assuming a central positioning of the substrate plate in the apparatus) may be used when evaluating the test sample.

[0078] FIG. 3 shows an image of a test sample according to some example implementations as described herein.

[0079] The test sample comprises, in this example, a pattern of individual elements 302 with a build height above a predefined threshold.

[0080] FIG. 4 shows an image for an image field correction according to some example implementations as described herein. The image field correction is applied on the substrate plate over multiple layers 402.

[0081] In this example, using a coordinate-measuring machine, the last exposed layer of the reference pattern is then measured and a correction file for each optic is created from it, in some examples based on a difference between nominal and actual coordinates. This correction is, in this example, performed based on the apparatus having been under process conditions during the test sample preparation and calibration process.

[0082] In some examples, an additional or alternative evaluation may be carried out in the apparatus by a camera. The camera may take a picture or, when using several optics, several cameras may each take a picture. These pictures may then be stitched together.

[0083] After the (high, i.e. above a threshold height) test sample preparation, one or more images are taken, whereby the two cameras may, in some examples, be misaligned with each other. The images may be taken from varying angles and/or positions of a camera and/or a camera with an x-y guide may be used in order to obtain multiple images. A virtual and/or real grid may be placed over the one or more images.

[0084] FIG. 5 shows an image 500 of a test sample and a virtual grid as mentioned above. In this example, the test sample comprises a pattern of multiple elements (for example pins) 502, and a virtual grid 504 is placed over this pattern.

[0085] Additionally or alternatively to using a virtual grid, reference marks for the cameras in the enclosure/on the process chamber floor may be used. Additionally or alternatively, an optical pattern (using, for example, one or more LEDs and/or one or more lasers) may be projected onto the test sample. The optical pattern may, for example, comprise one or more of a transparent (for example glass) plate comprising a reference pattern, a transparent foil comprising a reference pattern, one or more diffractive optical elements (for example one or more DOE laser modules), and one or more pattern projectors (for example one or more LED pattern projectors).

[0086] The transparent (for example glass) plate and/or foil may be placed on the material layer (e.g. powder bed) after completion of calibration. The plate and/or foil may be positioned using pins (for example dowel pins).

[0087] When using one or more diffractive optical elements (for example one or more DOE laser modules) and/or one or more pattern projectors (for example one or more LED pattern projectors), these may be arranged within and/or outside of the process chamber and may project the grid onto the material layer (e.g. powder bed).

[0088] Afterwards, when the transparent (for example glass) plate comprising a reference pattern and/or the transparent foil comprising a reference pattern and/or the one or more diffractive optical elements (for example one or more DOE laser modules) and/or the one or more pattern projectors (for example one or more LED pattern projectors) are positioned, the one or more images may be taken.

[0089] FIG. 6 shows an image 600 of a test sample and such reference marks 602 arranged on the process chamber floor 604 (in this example in the form of reference marks which are actually projected onto the chamber floor) on which the substrate 606 is arranged.

[0090] The location of the elements of the test sample may then be identified relative to the virtual grid and/or the one more reference marks in order to determine the geometry of the test sample.

[0091] This evaluation method prevents the substrate plate from shrinking as the temperature may not drop.

[0092] Examples of the evaluation and correction method using one or more optical images may be combined with examples of the calibration method as outlined above using a coordinate-measuring machine.

[0093] FIG. 7 shows a block diagram of an apparatus 700 according to some example implementations as described herein.

[0094] In this example, the apparatus comprises a substrate 702, one or more optical elements 704 (such as, but not limited to one or more beam sources, such as laser sources, and/or one or more beam steering elements and/or beam splitting elements), an optical element controller 706 for controlling the one or more optical elements 704, a calibration unit 708 (comprising, in some examples, a coordinate-measuring machine) used to calibrate, via the optical element controller 706, the one or more optical elements 704, a heating unit 710 adapted to heat the substrate 702 (and/or keep the substrate 702 at a target temperature or target temperature range), one or more temperature sensors 712 adapted to measure a temperature of one or more of the substrate 702, the one or more optical elements 704 and a process chamber 716 of the apparatus, and a coater 714 for coating material layers on the substrate 702.

[0095] The optical element controller 706 and the calibration unit 708 may be integral to a single unit.

[0096] The temperature sensors 712 are, in some examples, coupled to the optical element controller 706 and a controller for the heating unit 710.

[0097] FIG. 8 shows a schematic flow diagram of a method 800 according to some example implementations as described herein.

[0098] The method comprises the following steps. At step S802, using one or more optical elements, an optical pattern is projected onto a material in order to prepare, from said material, solidified material layers using an additive layer manufacturing technique to form a test sample. At step S804, a geometry of the test sample is determined. At step S806, the determined geometry is compared with a nominal geometry to generate calibration data, wherein the nominal geometry is based on data used for defining the optical pattern. At step S808, the one or more optical elements are calibrated using said calibration data.

[0099] No doubt many other effective alternatives will occur to the skilled person. It will be understood that the invention is not limited to the described embodiments and example implementations and encompasses modifications apparent to those skilled in the art and lying within the scope of the claims appended hereto.