CALIBRATION OF A CAMERA PROVIDED FOR MONITORING AN ADDITIVE MANUFACTURING PROCESS

Abstract

A method for the calibration of a camera for monitoring additive manufacturing of an object in which material is applied in a plurality of layers is provided. The method includes: a) providing the camera and providing means for additive manufacturing of the object, b) capturing an image of the object being manufactured or already manufactured by the camera, c) comparing the image captured with a model of the object, d) determining a calibration function on the basis of the comparison from step c), which is intended to transform the image captured into a corrected image, wherein the corrected image of the object substantially corresponds to the model of the object, and e) calibrating the camera by the calibration function. Also provided is a computer program comprising commands which, when executed by a computer, cause the computer to execute the steps of the method as well as a related apparatus.

Claims

1. A method for calibrating a camera, wherein the camera is provided for monitoring additive manufacturing of an object, which involves applying material in a plurality of layers, and wherein the method comprises: a) providing the camera and providing means for carrying out the additive manufacturing of the object, b) capturing an image of the object being produced or the already completed object by the camera, c) comparing the captured image with a pattern of the object, d) determining a calibration function on the basis of the comparison from step c), said calibration function being provided for transforming the captured image into a corrected image, wherein the corrected image of the object substantially corresponds to the pattern of the object, and e) calibrating the camera by the calibration function.

2. The method as claimed in claim 1, wherein the pattern corresponds to a sectional contour of a 3D design model of the object.

3. The method as claimed in claim 2, wherein the sectional contour is provided as a layer file.

4. The method as claimed in claim 1, wherein each respective layer applied in the additive manufacturing is assigned a respective individual pattern, in particular an individual sectional contour, and wherein the captured image is compared with the respective pattern which corresponds to the respective layer applied.

5. The method as claimed in claim 1, wherein after the image has been captured, the object in the captured image is segmented and the segmented image is subsequently compared with the pattern.

6. The method as claimed in claim 1, wherein the comparison between the captured image and the pattern includes a comparison of distances between selected reference points.

7. The method as claimed in claim 1, wherein an outline of the object is used for the comparison between the captured image and the pattern.

8. The method as claimed in claim 7, wherein a Kullback-Leibler divergence of two frequency distributions representing in each case a distance between the respective pixels describing the outline of the object and a reference point is used as a measure of a similarity of the captured image and the pattern.

9. The method as claimed in claim 1, wherein the calibration function is determined by the following steps: d1) initializing the calibration function with initialization parameters, d2) transforming the captured image into the corrected image by the calibration function, d3) determining a deviation between the corrected image and the pattern, d4) changing the parameters of the calibration function in order to reduce the deviation, d5) repeating steps d2) to d4) until the deviation is less than a predetermined threshold value.

10. The method as claimed in claim 1, wherein in the additive manufacturing a material to be processed is applied in a thin layer in powder form on a build plate, after layer application, the pulverulent material is locally remelted by laser radiation, the remelted layer forms a solid material layer after it has solidified, and this cycle is repeated until the object to be manufactured has attained its planned shape and size.

11. The method as claimed in claim 10, wherein the image is captured in accordance with step b) after the remelting by the laser radiation and before the application of the pulverulent material for a next material layer.

12. The method as claimed in claim 1, wherein the calibration is carried out automatically at predefined points in time and, in the case of changes in the calibration function, a user is informed.

13. The method as claimed in claim 1, wherein the calibration function is stored in a blockchain.

14. A computer program, comprising instructions which, when the program is executed by a computer, cause the computer to perform the method as claimed in claim 1.

15. An apparatus comprising means for carrying out additive manufacturing of an object, which involves applying material in a plurality of layers, a camera provided for monitoring the additive manufacturing of the object, and a calibration unit for calibrating the camera, wherein the calibration unit is configured to cause capture of an image of the object being produced or already completed by the camera, to compare the captured image with a pattern of the object, to determine a calibration function on the basis of the comparison, wherein the calibration function is provided for transforming the captured image into a corrected image, wherein the corrected image of the object substantially corresponds to the pattern of the object, and to calibrate the camera by the calibration function.

Description

BRIEF DESCRIPTION

[0073] Some of the embodiments will be described in detail, with references to the following Figures, wherein like designations denote like members, wherein:

[0074] FIG. 1 illustrates an apparatus comprising a 3D printing device, a camera and a calibration unit, according to an embodiment;

[0075] FIG. 2 illustrates an image of an object being produced, said image having been captured by a camera, according to an embodiment;

[0076] FIG. 3 illustrates a pattern associated with the image from FIG. 2;

[0077] FIG. 4 illustrates the outline of an individual component from the pattern from FIG. 3; and

[0078] FIG. 5 illustrates a histogram of the distances between the outline and a reference point of the individual component represented in FIG. 4.

DETAILED DESCRIPTION

[0079] FIG. 1 shows an apparatus comprising a 3D printing device 10, a camera 20 and a calibration unit 30. An apparatus for selective laser melting is shown by way of example as 3D printing device 10. The 3D printing device 10 has a material supply container 13 for filling with material 12. Furthermore, the printing device 10 has a printing region 18, in which the object to be manufactured is produced. The material 12 is present in powder form and contains for example a metal or a metallic compound. The material supply container 13 has side walls and an adjustable base 14. The base 14 is height-adjustable, such that the volume of the material supply container 13 is variable. The height of the base 14 of the material supply container 13 is able to be set or regulated by a controller and corresponding actuators.

[0080] The printing region 18 likewise has a height-adjustable base, the so-called build plate 11. The build plate 11, too, is able to be set or regulated by a controller and corresponding actuators. The object 15 being produced is situated on the build plate 11. At the beginning of the manufacturing process, the height of the build plate 11 is maximal. It moves downward in the direction of the arrow bearing the reference sign 111 during the construction of the material layers 151 of the object. The direction 141 of movement of the base 14 of the material supply container 13, said direction being identified by the arrow bearing the reference sign 141, is opposite to the direction 111 of movement of the build plate 11.

[0081] A roller 16 distributes material 12 from the material supply container 13 uniformly into the printing region 18. Customary layer thicknesses in selective laser melting are in the range of 15 μm to 500 μm. After the pulverulent material 12 has been distributed this process is also referred to as “recoating” in the technical jargon -, in a predefined region the material 12 is irradiated with a laser beam 172, the so-called exposure. The laser beam 172 is emitted by a laser 17 and is directed onto a desired point by a deflection mirror 171 mounted in a rotatable fashion. The pulverulent material 12 is locally completely remelted by laser radiation and forms a solid material layer 151 after solidification. The build plate 11 is subsequently lowered by the magnitude of a layer thickness and material 12 is applied once again. This cycle is repeated until all material layers have been remelted.

[0082] A camera 20 is positioned such that it can capture an image of the build plate 11 covered with the material 12 and of the object 15 being produced. However, it is generally unavoidable that the image captured by the camera 20 has distortions or similar artefacts. Consequently, a calibration of the camera 20 that corrects these optical effects is necessary.

[0083] In the conventional art, cameras are calibrated by calibration plates, for example. Instead of the use of a calibration plate, embodiments of the present invention propose the comparison of an image of the object being produced, or the already completed object, with a pattern. FIG. 2 shows an image 21 of an object, said image having been captured by a camera. Here the object consists of eight identical ring-shaped individual components. The fact of whether the object is still being produced or else it is already possible to see the finished object after laser irradiation (exposure) of the last layer is irrelevant to the elucidation of the inventive concept. In any case FIG. 3 shows the corresponding pattern 40, in this case from a layer file, which is assigned to the material layer represented in the image in FIG. 2.

[0084] It is already possible to discern with the naked eye that the image of the object that can be seen in FIG. 2 is distorted in comparison with the pattern illustrated in FIG. 3. By a calibration function, this distorted image is intended to be corrected so that it substantially corresponds to the pattern.

[0085] For this purpose, e.g., the frequency distributions (also called histograms) of the pixels representing the outline of the object of the captured image and of the pattern can be compared. Specifically, in this case the distances of the pixels of the outline in relation to a reference point are represented in the histograms and compared.

[0086] The object shown by way of example in FIGS. 2 and 3 has eight individual components 41 of identical type, which are separated from one another. It is appropriate to compare the frequency distributions for each of the individual components 41 separately.

[0087] FIG. 4 shows the outline 42 of an individual component 41 of the object mentioned. The resolution of the camera has already been taken into account here, which is why the outline shown in FIG. 5 has a certain unsharpness. The center point, that is to say the center, of the individual component is chosen as a reference point 43.

[0088] FIG. 5 shows the frequency distribution of the distances of the pixels of the outline 42 shown in FIG. 4. The distance from the reference point 53 (in millimeters) is plotted on the x-axis, and the relative frequency is plotted on the y-axis.

[0089] This frequency distribution is then to be compared with the frequency distribution of an individual component such as can be seen in the captured image 21 in FIG. 2. It will become apparent that the frequency distributions deviate from one another. By optimizing the parameters of the calibration function, an attempt should then be made to attain a (relative) minimum of the Kullback-Leibler divergence.

[0090] A calibration of the camera monitoring the additive manufacturing is thus possible, without, as in the conventional art, having recourse to a calibration plate or apparatus-specific reference markers.

[0091] Although the present invention has been disclosed in the form of preferred embodiments and variations thereon, it will be understood that numerous additional modifications and variations could be made thereto without departing from the scope of the invention.

[0092] For the sake of clarity, it is to be understood that the use of “a” or “an” throughout this application does not exclude a plurality, and “comprising” does not exclude other steps or elements.