Calibration method and printing system configured to produce three-dimensional workpiece

Abstract

A calibration method for a printing system configured to produce a three-dimensional workpiece is provided. A first image of a first portion of the calibration plate is obtained, a position of at least one portion of the calibration mark of the calibration plate is detected in the first image, an irradiation system of the printing system irradiates a spot on the first portion of the calibration plate with a laser beam, a second image of the first portion is obtained, a position of a spot formed by the laser beam is detected in the second image, and the position of the spot is detected. And calibrating the printing system based on the detected position of the at least one portion in the first image and the detected position of the spot in the second image. The invention further provides a corresponding printing system.

Description

Technical Field

[0001] The invention relates to a calibration method for a printing system which is configured to produce three-dimensional workpieces. Furthermore, the invention relates to a printing system of this type. Background Art

[0002] Powder bed fusion is a layer-by-layer additive process by which powdered raw materials, in particular metal and / or ceramic raw materials, can be processed into three-dimensional workpieces of complex shape. To this end, layers of raw powder are applied to a carrier and are irradiated with laser light in a position-selective manner according to the desired geometry of the workpiece to be produced. The laser radiation penetrating the powder layer results in heating of the raw powder particles and thus in melting or sintering of the raw powder particles. Further layers of raw powder are then applied successively to the already laser-treated layer on the carrier until the workpiece has the desired shape and size. Powder bed fusion can be used to produce or repair prototypes, tools, replacement parts, high-value parts or medical prostheses (e.g. dental or orthopedic prostheses) based on CAD data.

[0003] A printing system for producing a three-dimensional workpiece by powder bed fusion, as described, for example, in WO 2019 / 141381 A1, comprises a carrier (also referred to as a building platform) and an irradiation unit (also referred to as an irradiation system), the carrier being configured to receive a plurality of layers of raw material, the irradiation unit being configured to selectively irradiate laser radiation onto the raw material on the carrier to produce the workpiece. The irradiation unit may be provided with a spatial light modulator configured to split the laser beam into at least two sub-beams. Thus, a plurality of sub-beams, generally referred to as laser beams, may be used to selectively irradiate the raw material on the building platform.

[0004] The planned locations on the stock material to be irradiated by one or more laser beams may be defined or derived from workpiece data describing the workpiece to be manufactured (e.g., computer assisted design (CAD)). Due to manufacturing tolerances, temperature variations, and other reasons, the irradiation system may direct the laser beam to a location on the stock material that deviates from the planned location. In other words, there may be a misalignment between the irradiation location on the stock material and the corresponding planned location. This misalignment may result in a printed workpiece having lower rigidity, workpiece dimensions exceeding an acceptable predetermined manufacturing tolerance, and other disadvantages. In order to reduce or eliminate the misalignment between the irradiation location on the stock material and the planned location, the printing system may be calibrated. Summary of the invention

[0005] It is an object of the present invention to provide a calibration method for a printing system configured to produce three-dimensional workpieces and a printing system of this type, which enables an effective and precise calibration of the printing system.

[0006] A layer of raw material powder can be applied to the surface of the building platform by a powder applying device, which moves across the building platform to distribute the raw material powder. The building platform can be a rigidly fixed carrier. However, preferably, the building platform is designed to be displaceable (change height) in the vertical direction, so that when the workpiece is built layer by layer with raw material powder, the building platform can move downward in the vertical direction as the building height of the workpiece increases. It can be said that different heights of the building platform correspond to different vertical positions of the building platform. In addition, the building platform can be provided with a cooling device and / or a heating device, which is configured to cool and / or heat the building platform.

[0007] The building platform and the powder application device can be accommodated in a processing chamber, which can be sealed relative to the ambient atmosphere. An inert gas atmosphere can be established in the processing chamber by introducing a gas flow into the processing chamber via a gas inlet. After the gas flow is guided through the processing chamber and passes over the raw material powder layer applied to the carrier, the gas flow is discharged from the processing chamber via a gas outlet. The raw material powder applied to the building platform in the processing chamber is preferably a metal powder, in particular a metal alloy powder, but can also be a ceramic powder or a powder containing different materials. The powder can have any suitable particle size or particle size distribution. However, it is preferred to process powders with a particle size of less than 100 μm.

[0008] The illumination system may include a laser beam source configured to emit at least one laser beam. In particular, the laser beam source of the illumination system may emit a laser with a wavelength of 450 nm (e.g., linearly polarized) (i.e., a "blue" laser), or a laser with a wavelength of 532 nm (i.e., a "green" laser), or a laser with a wavelength in the range of 1000 nm to 1090 nm or 1530 nm to 1610 nm (i.e., an "infrared" laser). If one or more beam splitter cubes are used to split the laser beam into two or more partial beams, only one partial beam may be used as an illumination beam while blocking the other partial beams. Alternatively, one or more partial beams may be directed to different illumination systems in one or more printing systems.

[0009] The irradiation system may irradiate the building area with a single laser beam. However, it is also conceivable that the irradiation system irradiates two or more laser beams onto the building area. The plurality of laser beams irradiated by the irradiation system onto the building area may be emitted by suitable subunits of the laser beam source. The building area may correspond to an area of ​​the printing system where workpieces to be produced are located, in particular an area located on and above the building platform in a vertical direction. The irradiation system may be controlled by a processor, in particular to irradiate (e.g. planned) positions and / or points (e.g. on a layer of raw powder) in the building area.

[0010] The irradiation system may also include at least one optical scanning system for separating, guiding and / or processing at least one laser beam emitted by the laser beam source. The optical scanning system may include one or more optical elements (such as an objective lens and a scanner unit), preferably, the scanner unit includes a diffractive optical element and a deflection mirror. At least one optical scanning system may be configured to guide the laser beam to the building area. The irradiation system may include a plurality of optical scanning systems, each of which is configured to guide different laser beams in the laser beam to the building area. In one example, the building area is divided into a plurality of (e.g., non-overlapping) sections, wherein each of the optical scanning systems is configured to guide different laser beams in the laser beam to a set of different sections. This set of different sections may differ from each other in shape, size and / or position, and may overlap each other.

[0011] The printing system may include an adjustment system having a plurality of configurations, each configuration producing a different size and / or shape of a spot irradiated with one or more laser beams. The adjustment system may be configured to change the laser beam diameter, the shape of the cross section of the laser beam and / or the focus of the laser beam. The adjustment system may include one or more optical elements, such as lenses, apertures and / or reflectors. The adjustment system may be part of the irradiation system, in particular part of the optical scanning system. Alternatively, the adjustment system may be separated from the optical scanning system. Different adjustment systems may be provided for different laser beams.

[0012] A calibration plate can be provided, which is configured to be arranged in the build area. The calibration plate can, for example, be arranged on a build platform, on a processing room floor, on top of a powder application device, or the calibration plate can be implemented to be movable into a space in a processing room located in the build area or above the build area. The calibration plate can carry at least one calibration mark (for example, on the surface of the calibration plate). The calibration plate can carry a plurality of calibration marks, for example, arranged in a symmetrical pattern. One or more calibration marks can have a circular profile. Each calibration mark can have a higher light reflectivity than a portion of the calibration plate adjacent to or surrounding the corresponding calibration mark. The calibration plate can be an anodized aluminum plate. Each calibration mark can be formed by the following portion of the aluminum plate: in the portion, the surface layer formed by anodization is removed (for example, by milling, scraping or laser evaporation).

[0013] The printing system may include an imaging system. The imaging system may be arranged and configured to capture an image of at least a portion of the building area, in particular to capture an image of the following section of the building area: the calibration plate is arranged or can be arranged in the section. The imaging system may include an image sensor, such as a camera. The imaging system may be configured to capture images using a predetermined electromagnetic spectrum. In other words, the imaging system may be sensitive only to light having a wavelength falling within the predetermined electromagnetic spectrum. For example, the imaging system may include one or more filters, and the one or more filters are arranged so that all light originating from the building area (for example, from the surface of the calibration plate arranged in the building area) and falling on the image sensor falls within the predetermined electromagnetic spectrum. The predetermined electromagnetic spectrum may be different from the wavelength of one or more laser beams. That is, the imaging system may be configured to be insensitive to light having one or more wavelengths of one or more laser beams. One or more wavelengths of one or more laser beams may be outside the predetermined electromagnetic spectrum. The imaging system may include one or more optical components, such as a reflector, a lens, and an aperture, and the one or more optical components are arranged and configured to guide light from the building area to the image sensor. The imaging system may share at least one of the optical components with the illumination system. For example, the image sensor may be arranged and configured to capture images via an optical scanning system of the illumination system.

[0014] The printing system may include an illumination unit. The illumination unit may include one or more light emitting elements, such as one or more light emitting diodes. The illumination unit may be arranged and configured to illuminate at least a portion of the calibration plate when the calibration plate is arranged in the build area. The illumination unit may be configured to illuminate at least the build area. The illumination unit may be configured to emit light having a predetermined wavelength spectrum (e.g., for illuminating at least a portion of the calibration plate). The predetermined wavelength spectrum may correspond to, fall within, or overlap with a predetermined electromagnetic spectrum. In other words, the imaging system may be sensitive to light emitted from the illumination unit (e.g., and reflected by a calibration mark of the calibration plate). The calibration mark may exhibit a higher reflectivity of light having a predetermined wavelength spectrum than a portion of the calibration plate adjacent to or surrounding the corresponding calibration mark.

[0015] According to the present disclosure, a calibration method for a (e.g., described) printing system is provided, the printing system being configured to produce a three-dimensional workpiece. The method is performed by a (e.g., described) processor and includes step (a): obtaining a first image of a first portion of a (e.g., described) calibration plate arranged in a (e.g., described) building area of ​​the printing system captured by an (e.g., described) imaging system of the printing system, the first portion including at least one portion of at least one calibration mark carried by the calibration plate. The method may include the step of triggering the imaging system to capture the first image. Alternatively, the first image may be captured in advance and then retrieved from a database.

[0016] The method also includes step (b): detecting the position of at least one portion in (e.g., only) the first image. The position may be detected in a frame coordinate system FCS of the imaging system. The FCS may be associated with the first image. The position may be detected based on the first image using one or more of feature extraction, feature recognition, template matching, and machine vision (e.g., based on one or more predetermined (e.g., geometric and / or optical) properties of at least one calibration mark). At least one calibration mark (e.g., at least one portion of at least one calibration mark) may have a contour or shape such that the orientation of the at least one calibration mark cannot be detected and / or determined based on the first image (e.g., unambiguously). Since only the position of the at least one portion needs to be detected in the first image, the method may still produce a reliable calibration in the case of circular calibration marks.

[0017] The method further comprises: step (c): controlling an illumination system (e.g., the illumination system) of the printing system to illuminate a point on a first portion of the calibration plate arranged in the building area with a laser beam; step (d): obtaining a second image of the first portion of the calibration plate arranged in the building area captured by the imaging system, the second image comprising a light point formed by the laser beam illuminating the point on the first portion of the calibration plate arranged in the building area; and step (e): detecting the position of the light point in (e.g., only) the second image. The method may comprise the step of triggering the imaging system to capture the second image. The position of the light point may be detected in an FCS, which may be associated with the second image. The position of the light point may be detected based on the second image using one or more of feature extraction, feature recognition, template matching, and machine vision (e.g., based on one or more predetermined (e.g., geometric and / or optical) properties of at least one light point). The light point may have a contour or shape such that the orientation of the light point cannot be detected and / or determined based on the second image (e.g., unambiguously). Since the position of the light point only needs to be detected in the second image, the method may still produce a reliable calibration in the case of a circular light point.

[0018] The method includes a step (f): calibrating the printing system based on the detected position of at least one portion in the first image and the detected position of the light spot in the second image. The printing system can be calibrated based on a comparison of the detected position of at least one portion in the first image and the detected position of the light spot in the second image. The first image and the second image can cover the same field of view and / or the same area. Alternatively or in addition, the alignment of the first portion in the first image can be the same as the alignment of the first portion in the second image. During the capture of the first image and the capture of the second image, the calibration plate can have the same orientation in the build area. The printing system can be calibrated based on a comparison of the detected position of at least one portion in the FCS and the detected position of the light spot in the FCS. For example, the illumination system or (e.g., the) optical scanning system included in the illumination system (e.g., and used to direct a laser beam to a point on the calibration plate) can be calibrated to calibrate the printing system. In order to calibrate the printing system, the coordinate systems of the optical scanning system, the imaging system and / or other components of the printing system can be adjusted (e.g., relative to each other). Calibrating the printing system can include determining one or more transformations between these coordinate systems.

[0019] Thus, the calibration method uses two different images, wherein a first image is used to detect the position of at least a portion of a calibration mark carried by a calibration plate, and a second image is used to detect the position of a light spot formed by a laser beam irradiating a point on the calibration plate. The images can be captured using imaging parameters suitable for optimizing the corresponding detection (e.g., illumination settings, focus settings, exposure time settings and / or contrast settings), and preferably the images are captured at different times. For example, the (e.g., described) lighting unit can be configured to illuminate at least a first portion of the calibration plate when the calibration plate is arranged in the build area. The first image can be captured during the illumination of the first portion, and the second image can be captured when the lighting unit is turned off. At least one calibration pattern (e.g., at least a portion of at least one calibration pattern) may be invisible and / or undetectable in the second image.

[0020] The method may include obtaining an alignment image set including one or more images of at least a portion of a calibration plate disposed in a build area captured by an imaging system. An orientation of the calibration plate may be determined based on the alignment image set (e.g., relative to the FCS). The printing system may be calibrated (e.g., further) based on the determined orientation of the calibration plate.

[0021] The method may include detecting a geometric element on the calibration plate in at least one of the images of the alignment image set, and determining an orientation of the detected geometric element in the at least one image. The orientation of the calibration plate may then be determined based on the determined orientation of the detected geometric element. The orientation of a single geometric element detected in a single image may be sufficient to determine the orientation of the calibration plate.

[0022] The geometric element can be a non-rotationally symmetrical element. The geometric element can have a non-circular profile or shape. The geometric element can have a limited number of axes of symmetry. The geometric element can include at least two lines or be composed of at least two lines. Two or more lines may not be parallel and / or intersect each other. The example of such a geometric element includes an L-shaped pair of lines, a cross-shaped or plus-shaped pair of lines, four lines forming a rectangle or a square, and an arrangement of a circle in an L-shaped or square shape. Alternatively, the geometric element can include an asymmetric two-dimensional pattern or be composed of an asymmetric two-dimensional pattern, such as a (for example, computer-readable) two-dimensional code, such as a quick response (Quick Response, QR) code. The geometric element can be arranged on a calibration board, adjacent to a calibration mark or so that the geometric element surrounds the calibration mark. In one example, the geometric element is connected to (for example, converted to) a calibration mark. The geometric element can show a higher reflectivity of light with a predetermined wavelength spectrum than the adjacent or surrounding portion of the corresponding calibration mark of the calibration board.

[0023] The method may include detecting a plurality of reference elements of a calibration plate based on an alignment image group. The plurality of reference elements may be detected in one image of the alignment image group, or different reference elements of the plurality of reference elements may be detected in different images of the alignment image group. The position of each detected reference element may be determined based on the alignment image group. For example, the positions of all reference elements in one image of the alignment image group may be determined, or the positions of corresponding different reference elements in different images may be detected. The method may include determining the orientation of the calibration plate based on the determined positions of the detected reference elements. That is, the positions of the plurality of reference elements detected in a single image or in a plurality of images of the alignment image group may be used to determine the orientation of the calibration plate. It may not be necessary to detect the orientation of each reference element. This enables the use of reference elements whose individual orientations cannot be detected (e.g., unambiguously) in one or more images of the alignment image group. Therefore, a reference element according to the present disclosure may have a circular shape or contour. An example of a reference element is a calibration mark. That is, the reference element may correspond to a calibration mark carried by the calibration plate. In one example, the reference element and the calibration mark may have different diameters. The reference element may exhibit a higher reflectivity of light having a predetermined wavelength spectrum than a portion of the calibration plate adjacent to or surrounding the corresponding calibration mark.

[0024] The alignment image set may include one or more of the following images or consist of one or more of the following images: (i) a first image, (ii) a third image of a second portion of the calibration plate arranged in the building area, the second portion being different from the first portion, (iii) multiple images of different portions of the calibration plate arranged in the building area.

[0025] The calibration can be performed and / or repeated for: (i) different laser beams of the illumination system, (ii) different optical scanning systems of the illumination system, each optical scanning system being configured to direct a different laser beam to the building area, (iii) different sizes of the spots on the calibration plate illuminated by the laser beam, (iv) different shapes of the spots on the calibration plate illuminated by the laser beam, (v) different configurations of the (e.g., described) adjustment system of the illumination system, each configuration producing a different size and / or shape of the spots illuminated by the laser beam, (vi) different first portions, and / or (vii) different heights of a building platform arranged in the building area and carrying the calibration plate.

[0026] In the case where the illumination system includes multiple optical scanning systems, each optical scanning system is configured to direct a different laser beam to the building area, when the printing system is calibrated, at least one (e.g., all) of the multiple optical scanning systems can be calibrated. Two variations of this process will now be described. It should be noted that the following variations are not limited to the calibration of at least one of the optical scanning systems, and are equally applicable to the calibration of the entire printing system or other components of the printing system.

[0027] As a first variant, steps (c) to (e) may be performed for at least one further laser beam, and the printing system may be calibrated based on the detected position of the at least one portion in the first image and the detected position of the corresponding light spot in the second image. In other words, a single first image may be used to detect the position of the at least one portion, while a plurality of second images may be used to detect the corresponding light spots of different laser beams.

[0028] As a second variant, the method may include a step (c'): controlling the illumination system to illuminate different points on the first portion of the calibration plate arranged in the building area with at least one additional laser beam. In step (d), a second image of the first portion of the calibration plate arranged in the building area captured by the imaging system is obtained. The second image in this variant includes light spots formed by the laser beam illuminating the points on the first portion of the calibration plate arranged in the building area, and also includes at least one additional light spot formed by at least one additional laser beam illuminating different points on the first portion of the calibration plate arranged in the building area. The method may also include a step (e'): detecting the position of at least one additional light spot in the second image. Then, the printing system may be calibrated based on the detected position of at least one portion in the first image and the detected position of the light spot in the second image. In other words, a single first image may be used to detect the position of at least one portion, and a single second image may be used to detect multiple light spots of different laser beams. The light spots may be distinguished based on (e.g., geometrical and / or optical) properties of the light spots. The characteristics may include one or more of the following: light intensity, light color, spectrum, light wavelength, shape, profile, beam profile (e.g., Gaussian beam profile, Top Hat beam profile, or Donut beam profile). The characteristics of the light spot may be associated with the corresponding laser beam, for example, using a known relationship between the characteristics and the laser beam. This enables the optical scanning system of different laser beams to be calibrated separately using a single second image that includes multiple light spots generated by different laser beams illuminating different points on a calibration plate.

[0029] In the case where the illumination system comprises an adjustment system having a plurality of configurations, each configuration producing a different size and / or shape of a spot illuminated with the laser beam on a calibration element arranged in the build area, the method may comprise: performing steps (c) to (e) for each of the plurality of configurations of the adjustment system. The printing system may then be calibrated based on the detected position of the at least one portion in the first image and the detected position of the corresponding spot in the second image.

[0030] The method may comprise performing steps (a) to (e) for each of a plurality of different first portions, wherein the printing system is calibrated based on the detected position of the corresponding at least one portion in the first image and the detected position of the corresponding light spot in the second image.

[0031] As described above, the printing system may include a building platform arranged in the building area and configured to carry a calibration plate. The method may include performing steps (a) to (e) for each of a plurality of heights of the building platform when carrying the calibration plate, wherein the printing system is calibrated based on the detected position of the corresponding at least one portion in the first image and the detected position of the corresponding light point in the second image.

[0032] The method may include obtaining correction data indicative of geometrical parameters of the calibration plate measured with an external measurement system, wherein the printing system is calibrated based on the correction data.

[0033] The method may include determining a transformation between (i) a coordinate system of an (e.g., described) optical scanning system of an illumination system and (ii) a coordinate system of an imaging system (e.g., coordinate system FCS), wherein the printing system is calibrated based on the determined transformation. The coordinate system of the optical scanning system may be referred to as a scanning coordinate system (SCS).

[0034] The method may include controlling the imaging system to capture a first image when a first portion of the calibration plate arranged in the build area is at least illuminated by the illumination unit. Alternatively or additionally, the imaging system may be controlled to capture a second image when at least the first portion of the calibration plate arranged in the build area is not illuminated by the illumination unit. The imaging system may be controlled to capture all images except the second image when at least the first portion of the calibration plate arranged in the build area is illuminated by the illumination unit.

[0035] The lighting unit may be controlled (e.g., by a processor or by a user) to activate the lighting when capturing the first image. The lighting unit may be controlled (e.g., by a processor or by a user) to activate the lighting when capturing one or more other images in addition to the second image. The lighting unit may be controlled (e.g., by a processor or by a user) to deactivate the lighting when capturing the one or more second images.

[0036] The imaging unit may be configured to capture the first image only when the lighting unit illuminates at least a first portion of the calibration plate arranged in the build area. The imaging unit may be configured to capture all images except the second image only when the lighting unit illuminates at least a first portion of the calibration plate arranged in the build area. The imaging unit may be configured to capture the second image only when the lighting unit does not illuminate the calibration plate arranged in the build area. To this end, the imaging unit may be coupled to the lighting unit or may include a light sensor configured to detect light emitted by the lighting unit.

[0037] The imaging system may be arranged such that the imaging system captures at least the first image and the second image via an (e.g., described) optical scanning system included in the illumination system, the optical scanning system being configured to direct the laser beam to the building area. In this case, the imaging system may include a coaxial camera.

[0038] One or more position marks may be set adjacent to a calibration plate arranged in the build area. One or more position marks may be set in an area that is not covered by powder material during the formation of a new powder layer by a powder application device. One or more additional position marks may be carried by a calibration plate. The method may include the following steps: a fourth image of at least one of the position marks set adjacent to the calibration plate and at least one of the position marks carried by the calibration plate is captured by an imaging system. The position and / or orientation of each position mark can be determined based on the captured fourth image. One or more positions and / or one or more orientations can be compared to determine the offset of the calibration plate and the calibration plate relative to a predetermined calibration posture of the build area. Then, the position and / or orientation of the calibration plate can be adjusted (e.g., mechanically and / or manually) to minimize or compensate for the offset. Then, the method can perform steps (a) to (f).

[0039] After calibration has been performed, the method may be repeated for verification and / or a three-dimensional workpiece may be produced by a printing system.

[0040] According to the present disclosure, a printing system for producing a three-dimensional workpiece is provided, the printing system comprising an irradiation system, an imaging system and a processor, the irradiation system being configured to selectively irradiate a building area with one or more laser beams, the imaging system being configured to capture an image of at least a portion of a calibration plate when the calibration plate is arranged in the building area. The processor of the printing system is configured to: (a) obtain a first image of a first portion of the calibration plate arranged in the building area captured by the imaging system, the first portion comprising at least one portion of at least one calibration mark carried by the calibration plate, (b) detect the position of at least one portion in the first image, (c) control the irradiation system to irradiate a point on the first portion of the calibration plate arranged in the building area with one of the one or more laser beams, (d) obtain a second image of the first portion of the calibration plate arranged in the building area captured by the imaging system, the second image comprising a light spot formed by one of the one or more laser beams irradiating a point on the first portion of the calibration plate arranged in the building area, (e) detect the position of the light spot in the second image, and (f) calibrate the printing system based on the detected position of at least one portion in the first image and the detected position of the light spot in the second image.

[0041] The printing system (in particular the processor) may be configured to perform or implement the method as described above. The printing system may include one or more of the following: (i) (e.g., described) one or more optical scanning systems, each of which is configured to direct a different one of the laser beams to the building area, (ii) (e.g., described) an adjustment system having a plurality of configurations, each configuration producing a different size and / or shape of a spot irradiated with the laser beam, (iii) (e.g., described) a building platform, which is arranged in the building area and is configured to carry a calibration plate, (iv) (e.g., described) an illumination unit, which is configured to illuminate at least a first portion of the calibration plate when the calibration plate is arranged in the building area, (v) (e.g., described) a coaxial camera, (vi) a calibration plate.

[0042] The processor may be configured to calibrate the printing system based on (eg, the) comparison of the detected position of the at least one portion in the first image and the detected position of the light spot in the second image.

[0043] The processor may be configured to control the imaging system to capture the first image and the second image such that the first image and the second image cover the same field of view and / or such that an alignment of the first portion in the first image is the same as an alignment of the first portion in the second image.

[0044] The processor may be configured to calibrate the illumination system to calibrate the printing system.

[0045] The processor may be configured to obtain (e.g., the) alignment image set including one or more images captured by the imaging system of at least a portion of the calibration plate disposed in the build area. The processor may be configured to determine an orientation of the calibration plate based on the alignment image set, and calibrate the printing system based on the determined orientation of the calibration plate.

[0046] The processor can be configured to detect (e.g., the) geometric element on the calibration plate in at least one image in the images of the alignment image group, determine the orientation of the detected geometric element in at least one image, and determine the orientation of the calibration plate based on the determined orientation of the detected geometric element.

[0047] The processor may be configured to detect (e.g., the) plurality of reference elements of the calibration plate based on the alignment image set, determine a position of each detected reference element based on the alignment image set, and determine an orientation of the calibration plate based on the determined position of the detected reference elements. The determination of the orientation of the calibration plate may also be determined stepwise by evaluating each image of the alignment image set directly after obtaining, checking, and correcting the calculated orientation with each additional image.

[0048] The alignment image group may include one or more of the following images or consist of one or more of the following images: (i) a first image; (ii) a third image (e.g., described) of a second portion of the calibration plate arranged in the building area, the second portion being different from the first portion; (iii) multiple images (e.g., described) of different portions of the calibration plate arranged in the building area.

[0049] The processor can be configured to perform calibration for the following situations: (i) different laser beams, (ii) different optical scanning systems of the printing system, each optical scanning system being configured to direct a different one of the laser beams to the building area, (iii) different sizes of the points on the calibration plate illuminated by the laser beam, (iv) different shapes of the points on the calibration plate illuminated by the laser beam, (v) different configurations of an adjustment system (e.g., described) of the printing system, the adjustment system having multiple configurations, each configuration producing a different size and / or shape of the points illuminated by the laser beam, (vi) different first parts, and / or (vii) different heights of a building platform arranged in the building area and configured to carry the calibration plate.

[0050] The illumination system may include (e.g., described) a plurality of optical scanning systems, each optical scanning system configured to direct a different one of the laser beams to the building area. The processor may be configured to calibrate at least one of the plurality of optical scanning systems when calibrating the printing system.

[0051] The processor can be configured to perform steps (c) to (e) for at least one additional one of the laser beams (in particular, each different one of the laser beams) and to calibrate the printing system based on the detected position of at least one portion in the first image and the detected position of the corresponding light spot in the second image.

[0052] The processor may be configured to (c') control the illumination system to illuminate (e.g., the) different points on the first portion of the calibration plate arranged in the building area with (e.g., the) at least one additional laser beam of the laser beams (in particular, each different laser beam of the laser beams). The processor may be configured to obtain in step (d) a second image of the first portion of the calibration plate arranged in the building area captured by the imaging system, the second image comprising light points formed by one of the one or more laser beams illuminating the points on the first portion of the calibration plate arranged in the building area, the second image also comprising (e.g., the) at least one additional light point formed by at least one additional laser beam illuminating different points on the first portion of the calibration plate arranged in the building area. The processor may be configured to (e') detect the position of the at least one additional light point in the second image, and calibrate the printing system based on the detected position of the at least one portion in the first image and the detected position of the light point in the second image.

[0053] The illumination system may include an adjustment system (e.g., described) having a plurality of configurations, each configuration producing a different size and / or shape of a spot illuminated with the laser beam on a calibration element arranged in the build area. The processor may be configured to perform steps (c) to (e) for each of the plurality of configurations of the adjustment system and calibrate the printing system based on the detected position of at least one portion in the first image and the detected position of a corresponding spot in the second image.

[0054] The processor can be configured to perform steps (a) to (e) for each of (e.g., the) multiple different first parts and calibrate the printing system based on the detected position of the corresponding at least one part in the first image and the detected position of the corresponding light spot in the second image.

[0055] The printing system may include a (e.g., described) building platform arranged in the building area and configured to carry a calibration plate. In this case, the processor may be configured to perform steps (a) to (e) for each of the (e.g., described) plurality of heights of the building platform when the building platform carries the calibration plate, and calibrate the printing system based on the detected position of the corresponding at least one portion in the first image and the detected position of the corresponding light spot in the second image.

[0056] The processor may be configured to obtain correction data indicative of (eg, the) geometrical parameters of the calibration plate measured with (eg, the) external measurement system, and to calibrate the printing system based on the correction data.

[0057] The processor can be configured to determine a transformation between (i) a (e.g., described) coordinate system of an optical scanning system of the illumination system and (ii) a (e.g., described) coordinate system of the imaging system, the optical scanning system being configured to direct one of the one or more laser beams to the building area. The processor can be configured to calibrate the printing system based on the determined transformation.

[0058] The printing system may further include (e.g., described) an illumination unit configured to illuminate at least a first portion of the calibration plate when the calibration plate is arranged in the build area. The processor may be configured to control the imaging system to capture a first image when at least the first portion of the calibration plate arranged in the build area is illuminated by the illumination unit, and / or may be configured to control the imaging system to capture one or more second images when at least the first portion of the calibration plate arranged in the build area is not illuminated by the illumination unit, and / or may be configured to control the imaging system to capture all images except the one or more second images when at least the first portion of the calibration plate arranged in the build area is illuminated by the illumination unit.

[0059] The imaging system may be arranged such that it captures at least the first image and the second image via an optical scanning system included in the illumination system, the optical scanning system being configured to direct at least one of the one or more laser beams to the building area.

[0060] The printing system may further comprise a calibration plate, which may optionally be arranged in the build area.

[0061] A computer program product storing instructions may be provided which, when executed by a processor, causes the processor to implement the methods described herein. The computer program may be stored on one or more computer readable media or may be carried by a data stream. BRIEF DESCRIPTION OF THE DRAWINGS

[0062] Preferred embodiments of the present invention will be described in more detail with reference to the accompanying schematic drawings, in which:

[0063] Figure 1 An apparatus for producing a three-dimensional workpiece by irradiating a layer of raw powder with laser radiation is shown;

[0064] Figure 2 Another apparatus for producing a three-dimensional workpiece by irradiating a layer of raw powder with laser radiation is shown;

[0065] Figure 3 A flow chart showing a calibration method is shown;

[0066] Figure 4 A calibration plate is shown;

[0067] Figure 5a to Figure 5c An exemplary first image of a calibration plate is shown;

[0068] Figures 6a to 6d showing an exemplary second image of a calibration plate;

[0069] Figures 7a to 7f Details of the calibration plate are shown;

[0070] Figure 8a and Figure 8b A technique for determining the orientation of a calibration plate is shown;

[0071] Figure 9a and Figure 9b Techniques for determining registration between two coordinate systems are shown;

[0072] Fig.10 A detailed flow chart showing a first variation of the calibration method; and

[0073] Fig.11 A detailed flow chart of a second variant of the calibration method is shown. DETAILED DESCRIPTION

[0074] Figure 1 A printing system 100 for producing a three-dimensional workpiece by a layer-by-layer additive process is shown. System 100 includes a build platform 102 and a powder application device 104 for applying raw material powder to the build platform 102. The build platform or carrier 102 and the powder application device 104 are contained in a processing chamber 106, which can be sealed relative to the ambient atmosphere. The build platform 102 can be displaced in a vertical direction into a build cylinder 108, so that when a workpiece 110 is built layer by layer with raw material powder on the build platform 102, the build platform 102 can move downward as the build height of the workpiece increases. The build platform 102 can include a heater and / or a cooler.

[0075] The apparatus 100 further comprises an irradiation system 10 for selectively irradiating laser radiation onto a layer 11 of raw material powder applied to a carrier 102. Figure 1In the embodiment of the device 100 shown, the irradiation system 10 comprises two laser beam sources 12a, 12b, each of which is configured to emit a laser beam 14a, 14b. An optical scanning system 16a, 16b for guiding and processing the laser beams 14a, 14b emitted by the laser beam sources 12a, 12b is associated with each of the laser beam sources 12a, 12b. However, it is also conceivable that the irradiation system 10 is equipped with only one laser beam source and one optical scanning system and therefore emits only a single laser beam. The adjustment systems 13a, 13b can be formed by some optical components of the corresponding optical scanning systems 16a, 16b. The adjustment system 13a of the laser beam 14a can, for example, include a lens 15a of the optical scanning system 16a. The same applies to the adjustment system 13b of the laser beam 14b and one or more lenses 15b, 15c of the optical scanning system 16b. Each adjustment system 13a, 13b can be configured to adjust the width, focus and / or shape of the respective laser beam 14a, 14b before the respective laser beam 14a, 14b penetrates the building area. The processor 18 (also referred to as the control device) is arranged to control the operation of the irradiation system 10 and other components (e.g., the powder application device 104) of the apparatus 100.

[0076] A controlled gas atmosphere (preferably, an inert gas atmosphere) is established within the process chamber 106 by supplying a protective gas to the process chamber 106 via the process gas inlet 112. After the gas is directed through the process chamber 106 and passes through the raw material powder layer 11 applied to the carrier 102, the gas is exhausted from the process chamber 106 via the process gas outlet 114. The process gas may recirculate from the process gas outlet 114 to the process gas inlet 112 and thereby may be cooled or heated.

[0077] During operation of the apparatus 100 for producing a three-dimensional workpiece, a raw powder layer is applied to the carrier 102 by means of the powder application device 104. To apply the raw powder layer 11, the powder application device 104 is moved over the carrier 102 under the control of the control unit 18. Then, the raw powder layer is selectively irradiated with laser radiation by means of the irradiation device 10, again under the control of the control unit 18, in accordance with the geometry of the corresponding layer of the workpiece 110 to be produced. The steps of applying the raw powder layer to the carrier 102 and selectively irradiating the raw powder layer with laser radiation in accordance with the geometry of the corresponding layer of the workpiece 110 to be produced are repeated until the workpiece 110 has reached the desired shape and size.

[0078] Before and / or after the production of a three-dimensional workpiece, printing system 100 may be calibrated. To this end, calibration plate 116 may be arranged on building platform 102. Building platform 102 may be positioned such that an upper surface of calibration plate 116 is located in a plane corresponding to a surface of a powder layer during production of a workpiece. The shape and size of calibration plate 116 are designed such that the calibration plate may be positioned in a building area defined by an inner wall surface of cylinder 108. Printing system 100 includes an imaging system 118 configured to capture an image of at least a first portion of calibration plate 116 when calibration plate 116 is arranged in the building area.

[0079] exist Figure 1 In the example of FIG. 1 , the imaging system 118 includes a camera 120 and a set of scanning mirrors 122 for moving the field of view of the camera 120 across the calibration plate 116 as needed. Figure 2 In the example of FIG. 1 , imaging system 118 shares optical components with optical scanning system 16 b. In particular, beam splitter 124 is arranged in the optical path of laser beam 14 b, which directs light originating from calibration plate 116 to camera 120. In this configuration, the laser deflection mirror of optical scanning system 16 b can be used to move the field of view of camera 120. It should be noted that the field of view of camera 120 can be larger than the laser beam illuminating the build area, such as Figure 2 shown. Figure 1 The camera 120 in the configuration may be referred to as an off-axis camera, and Figure 2 The camera 120 in the configuration may be referred to as a coaxial camera.

[0080] The printing system 100 also includes an illumination unit 126 including a set of light emitting elements 128a, 128b configured to emit light having a predetermined spectrum so that the calibration plate 116 is illuminated. The predetermined spectrum may be selected according to the sensitivity of the imaging system 118, and vice versa. In other words, the illumination unit 126 may be configured to provide illumination for capturing an image of the calibration plate 116 by the imaging unit 118. The imaging system 118 may be insensitive to incident light having a wavelength corresponding to the wavelength of one or more laser beam sources 12a, 12b. For example, the imaging system 118 may be configured to capture images using light visible to the human eye, and the laser light sources 14a, 14b may be configured to emit infrared lasers. This configuration may protect the camera 120 from reflected laser light having a high intensity.

[0081] Figure 3A flow chart of a method according to the present disclosure is shown. The method is performed by processor 18. In step (a), a first image captured by imaging system 118 of printing system 100 is obtained. The first image includes a first portion of calibration plate 116 arranged in a build area of ​​printing system 100. The first portion of calibration plate 116 includes at least one portion of at least one calibration mark carried by calibration plate 116. During acquisition of the first image, illumination unit 126 may illuminate calibration plate 116.

[0082] In step (b), the position of at least one portion in the first image is detected, for example using feature recognition techniques. Illuminating the calibration plate 116 during acquisition of the first image can produce an image in which there is a high optical contrast between the calibration mark and the rest of the calibration plate 116. This can improve the accuracy of the detected position of the at least one portion of the calibration mark.

[0083] In step (c), the illumination system 10 of the printing system 100 is controlled to illuminate the points on the first part of the calibration plate 116 arranged in the building area with one of the laser beams. At the same time, the illumination unit 126 can be deactivated so that the calibration plate 116 is illuminated only with light from the laser source 12a or 12b.

[0084] In step (d), a second image of the first portion of calibration plate 116 arranged in the build area, captured by imaging system 118, is obtained, the second image comprising light spots formed by one or more laser beams illuminating points on the first portion of calibration plate 116 arranged in the build area. Since the illumination unit is deactivated during acquisition of the second image, the light spots may be highly visible in the second image.

[0085] In step (e), the position of the light spot in the second image is detected. When the second image captured during the off period of the lighting unit 126 is used, the accuracy of the detected position can be improved.

[0086] In step (f), the printing system 100 (particularly an optical scanning system 16a or 16b that directs a laser beam to a point on the first portion of the calibration plate 116) is calibrated based on the detected position of at least one portion in the first image and the detected position of the light point in the second image. Using separate images can improve the respective detected positions and thus produce a more accurate calibration. Calibrating the optical scanning system 16a, 16b can include, for example, adjusting the coordinate system of the respective optical scanning system relative to a second coordinate system (such as the coordinate system of the imaging system 118 or the central coordinate system of the printing system 100). Print data (e.g., irradiation instructions) used by the printing system 100 to form a three-dimensional object can be defined relative to the central coordinate system. By calibrating one or more optical scanning systems 16a, 16b, any subsequently irradiated positions correspond to the expected positions to be irradiated (e.g., based on the print data).

[0087] Figure 4 An exemplary calibration plate 116 is shown. It can be seen that the calibration plate 116 has a black upper surface, which is formed, for example, by anodizing an aluminum plate. A plurality of calibration marks 130 are carried by the calibration plate 116. Compared to the black upper surface, the calibration marks 130 have a higher light reflectivity at least in a predetermined spectrum and are therefore displayed as white dots. Each calibration mark 130 corresponds to a circular area on the upper surface of the calibration plate 116, which is coated with a reflective color or which does not include an anodized layer. The calibration marks 130 can be formed by selective laser evaporation or computer-controlled milling of portions of the anodized layer. The calibration marks 130 are arranged in a symmetrical matrix having rows and columns. Other arrangements of the calibration marks 130 are possible.

[0088] Since the calibration mark 130 has a higher optical reflectivity than the surrounding portion of the calibration plate 116 (e.g., has an optical reflectivity that is 2, 5, 10, or 100 times higher than the surrounding portion of the calibration plate 116), at least a portion of at least one calibration mark 130 can be easily detected in the first image, especially if the illumination unit 126 illuminates the calibration plate 116 during the acquisition of the first image. Circular calibration marks are more easily detected in the first image than more complex geometric shapes. Even if only a portion of the circular calibration mark 130 is visible in the first image, the center point of the circular calibration mark 130 can be detected or determined based on the first image.

[0089] Figure 5a to Figure 5cAn exemplary first image 133 of the calibration plate 116 is shown. In FIG5 , a section 132 of the calibration plate 116 including the calibration mark 130 is shown. A rectangular outline 134 represents the area covered by the first image 133. That is, all contents within the outline 134 are visible in the first image 133. The first image 133 has a frame coordinate system FCS, in which Figure 5a to Figure 5c , and is exemplarily shown in the lower left corner of the first image 133. Figure 5b As shown, an outline 136 of the circular calibration mark 130 may be detected in the first image 133 .

[0090] Thus, the center point 138 of the circular contour 136 can be determined as the location of the calibration mark 130 in the FCS, as shown in FIG. Figure 5c As shown. Figure 5c In the example, the plate coordinate system PCS is represented by the axes "X" and "Y". The orientation of the PCS relative to the FCS can be referred to as follows Figures 7a to 8b Determined as further described.

[0091] Figures 6a to 6d An exemplary second image 140 of the calibration plate 116 is shown. The second image 140 includes a portion of the calibration plate 116 that is sized and oriented similar to the calibration plate 116. Figure 5a to Figure 5c The first image 133 is of the same size and orientation as the first segment 132. It can be said that the first image 133 and the second image 140 are views of exactly the same portion of the calibration plate 116. Unlike the first image 133, the second image 140 includes a light spot 142 corresponding to a point on the calibration plate 116, which was illuminated by the laser beam during the acquisition of the second image 140. Figures 6a to 6d The calibration mark 130 in the example of FIG. 1 can be seen in the second image 140 , but this is only for the purpose of illustration and is not required. That is, the position of the calibration mark 130 may not be detected from the second image 140 .

[0092] The position of the light spot 142 can be determined in the FCS (see Figure 6b ).

[0093] An offset between a position of the calibration mark 130 detected in the first image 133 (e.g., a center point 138 of the calibration mark 130 in the FCS) and a position of the light spot 142 detected in the second image 140 (e.g., in the FCS) may be determined, such as Figure 6c and Figure 6d This offset can be determined in the FCS. Figure 6cIn the example of , the offset is shown in the PCS. The offset in the PCS can be determined based on the offset in the FCS and the transformation between the FCS and the PCS. Figure 6d In the example of , the offset is shown in the SCS. The offset in the SCS can be determined based on the offset in the FCS and the transformation between the FCS and the SCS. Figure 9a and Figure 9b Exemplary techniques for determining the transformation between the FCS and the SCS are discussed. The transformation between the FCS and the SCS can be determined based on images of light spots obtained by illuminating different locations and / or based on one or more images of (e.g., asymmetric or non-circular) light patterns generated by the illumination system.

[0094] The one or more offsets may then be used to calibrate a scanning system 16a or 16b used to illuminate a point on the calibration plate 116 during acquisition of the second image 140. In other words, the detected position 138 may be compared to the detected position of the light point 142 to calibrate the optical scanning system 16a, 16b of the printing system 100, and in particular the illumination system 10, used to illuminate the light point on the calibration plate 116 when capturing the second image 140. The calibration may include correcting any future position of illumination with a laser beam via the calibrated scanning system 16a, 16b by the one or more offsets.

[0095] The orientation of the calibration plate 116 can be determined based on the alignment image set including one or more images acquired by the camera 120. Then, calibration can also be performed based on the orientation of the calibration plate 116, in particular based on the transformation between the FCS and the PCS. Two main variants for determining the orientation of the calibration plate 116 relative to the FCS will be described below. The first variant refers to Figures 7a to 7f , the second variant refers to Figure 8a and Figure 8b It should be noted that these two variations can be combined to obtain a more accurate orientation of the calibration plate 116 .

[0096] Figures 7a to 7f Details of the calibration plate 116 are shown. In particular, each of these figures shows a different example of one or more geometric elements 144 that may be provided on the calibration plate 116. The geometric elements 144 may be formed in a similar manner as the one or more calibration marks 130, for example by selective laser ablation.

[0097] exist Figure 7aIn the example of , three QR codes as geometric elements 144 are carried by the calibration plate 116, each QR code being arranged adjacent to the calibration mark 130. By detecting one or all of these QR codes in the first image or another image of the calibration plate 116 included in the alignment image set, the orientation of the detected QR code in the FCS can be determined. Each QR code can have a predetermined orientation (e.g., rotation) relative to the calibration plate 116 (e.g., PCS). This enables the orientation of the calibration plate 116 to be determined, and in particular the rotation between the PCS and the FCS to be determined. This rotation can then be used during the calibration process (see Figure 5c , Figure 6c and Figure 6d ).

[0098] exist Figure 7b In the example of , the geometric element 144 includes radial lines extending outward from the calibration pattern 130. In particular, four lines circumferentially spaced 90° from each other are provided as the geometric element 144. This enables detection of misalignments between the PCS and the FCS of less than 45°. More (e.g., asymmetric) lines may be added to enable detection of misalignments between the PCS and the FCS of greater than 45° without creating ambiguity.

[0099] exist Figure 7c In the example of , the geometric element 144 is a cross or plus sign formed by two orthogonal intersecting lines. Also in this case, some ambiguity about the orientation of the calibration plate 116 may still exist.

[0100] exist Figure 7d In the example of , geometric element 144 is a square surrounding one of calibration marks 130. Also in this case, some ambiguity may still exist regarding the orientation of calibration plate 116. The ambiguity may be acceptable (e.g., in the case where calibration plate 116 cannot be arranged in the build area with a misalignment greater than 45°), or may be minimized by adding additional geometric elements 144 such as lines, points, etc.

[0101] exist Figure 7e In the example of , the geometric element 144 is an L-shape formed by two orthogonal lines with joined ends. In this case, the geometric element 144 has only one axis of symmetry. Therefore, similar to the asymmetric geometric element 144, the orientation of the calibration plate 116 can be accurately determined without ambiguity.

[0102] exist Figure 7fIn the example of , three circles as geometric elements 144 are carried by the calibration plate 116, each circle being arranged adjacent to the calibration mark 130. These circles together form a pattern having only one axis of symmetry. Therefore, similar to the asymmetric geometric elements 144, the orientation of the calibration plate 116 can be accurately determined without ambiguity.

[0103] Figure 8a to Figure 8b A technique for determining the orientation of the calibration plate 116 using a plurality of reference elements 146 carried by the calibration plate 116 is shown. In this case, the orientation of the calibration plate 116 is determined based on a plurality of positions (rather than a plurality of orientations) of the plurality of reference elements 146. In the example shown, five of the calibration elements 130 are used as reference elements 146: one reference element 146-1 located at the center of the calibration plate 116, two reference elements 146-2, 146-3 located at the ends of the vertical center axis of the calibration plate 116, and two reference elements 146-4, 146-5 located at the ends of the horizontal center axis of the plate 116. Figure 8a As shown, three images may be captured as a set of alignment images, each image including a different one of the reference elements 146, wherein at least one of the images preferably includes a center reference element 146-1. The position of the reference element 146 may be determined in the three captured images. Then, the orientation of the calibration plate 116 relative to the imaging system 118 (particularly the rotational offset between the FCS and the PCS) may be determined based on the detected position of the reference element 146, as shown in FIG. Figure 8b It is schematically shown in FIG.

[0104] Calibration can also be performed based on a transformation between coordinate systems FCS and SCS. Figure 9a and Figure 9b Techniques for determining this transformation (also called "registration") between coordinate systems FCS and SCS are shown.

[0105] In accordance with Figure 9a In the technology, a coaxial camera 120 is used, for example Figure 2 As shown. During the illumination of the point on the calibration plate 116 by the laser beam, an image (e.g., the second image 140) is captured. In the FCS, the light point 142-1 is detected in the captured image. Next, the field of view of the camera is moved in a predetermined direction by a predetermined amount. Figure 9a In the example of , the field of view of the camera is moved to the position [0, 1] according to the FCS. Then, another image is captured while the laser beam illuminates the same point on the calibration plate 116. Due to the moved field of view of the camera 120, the position of the detected light spot will be different. Figure 9aIn the example shown, the detected position of the light spot 142-2 in the FCS is offset by Δx in the x-axis direction and Δy in the y-axis direction of the FCS from the detected position of the light spot 142-1 in the FCS. The offset angle α between the FCS and the SCS can be determined based on the detected positions of the light spots 142-1 and 142-2. x Thus, the technique enables determination of such a transformation (also referred to as registration) between coordinate systems FCS and SCS. The FCS may be defined relative to each image acquired by the imaging system 118. It should be noted that instead of detecting the light spots 142-1, 142-2, the positions of the same calibration marks, geometrical elements or reference elements depicted by both images may be detected in the FCS.

[0106] In accordance with Figure 9b In the technique, an off-axis camera 120 is used, for example Figure 1 As shown. During the illumination of the first point on the calibration plate 116 by the laser beam, an image (e.g., the second image 140) is captured. In the FCS, the light point 142-1 is detected in the captured image. Next, the second point on the calibration plate 116 is illuminated, and the second point is offset from the first point by a predetermined amount in a predetermined direction. Figure 9b In the example of , the second point is offset from the first point by an exemplary amount 1 in the x-axis direction of the SCS. Thus, the first point can be described as having coordinates [0, 0] in the SCS, while the second point has coordinates [1, 0] in the SCS. An image is captured, and the position of the light spot 142-2 formed by the laser beam irradiating the second point is detected in the FCS. Next, a third point on the calibration plate 116 is irradiated, and the third point is offset from the first point by another predetermined amount in another predetermined direction. Figure 9b In the example of , the third point is offset from the first point in the y-axis direction of the SCS by an exemplary amount of 1. Therefore, the first point can be described as having coordinates [0, 0] in the SCS, while the third point has coordinates [0, 1] in the SCS. An image is captured, and the position of a light spot 142-3 formed by a laser beam irradiating the third point is detected in the FCS. The detected positions of the light spots 142-1, 142-2, and 142-3 can be used to determine the orientation of the SCS relative to the FCS. That is, the alignment between the SCS and the FCS can be determined based on the detected positions of the light spots 142-1, 142-2, and 142-3.

[0107] Fig.10A detailed flow chart of a first variation of the method is shown. The calibration plate is referred to as a Scan Field Correction Plate (SFCP). The calibration mark 130 is referred to as a SFCP circle. The optical scanning system of the laser beam is referred to as a "scanner". The reference point may be a groove, notch or other feature of the calibration plate 116. Optional steps are marked with question marks, i.e., the MCF may be used in step 3.3, in step 4, or not at all.

[0108] The MCF describes the geometric parameters of the calibration plate 116 as measured by the external measurement system. For example, the MCF may describe the deviation between theoretical geometric properties of the calibration plate 116 (e.g., the positions of the calibration marks 130, geometric elements 144, and other features of the calibration plate 116) and the actual geometric properties of the calibration plate 116 as measured by the external measurement system. Therefore, the MCF 116 may be considered a "gold standard" for the positions of the corresponding marks, elements, and features of the calibration plate 116 (e.g., relative to a PCS defined by one or more reference points or position marks carried by the calibration plate 116).

[0109] The magnification offset data from each scanner corresponds to calibration parameters used to calibrate the respective scanner for different configurations of the adjustment system of the respective scanner. Fig.10 In the example of , the magnification offset data is predetermined, such as provided by the manufacturer of the optical scanning system or provided during maintenance. A "calibration file (Cal file)" may correspond to a calibration file that defines calibration parameters (e.g., spatial offsets applied during illumination) for each optical scanning system of the printing system 100.

[0110] Fig.11 A detailed flow chart of a second variant of the method is shown. Fig.10 Unlike, in this case, the magnification offset data are not predetermined. Therefore, steps 3.4.2.1 and 3.4.2.2 are repeated for all magnification steps M, each corresponding to a different configuration of the adjustment system of the respective optical scanning system or "scanner". Apart from this difference, Fig.11 The method corresponds to Fig.10 Both methods result in the printing system 100 being calibrated via the Cal file.

[0111] like Fig.10 and Fig.11As shown, the calibration method described herein may be performed for a variety of configurations of printing system 100. For example, the calibration may be performed for different laser beams 14a, 14b of illumination system 10, and / or the calibration may be performed for different optical scanning systems 16a, 16b of illumination system 10, each optical scanning system being configured to direct a different laser beam 14a, 14b to the building area, and / or the calibration may be performed for different sizes of spots on calibration plate 116 illuminated by laser beams 14a, 14b, and / or the calibration may be performed for different shapes of spots on calibration plate 116 illuminated by laser beams 14a, 14b, and / or the calibration may be performed for different configurations of adjustment systems 13a, 13b of illumination system 10, each configuration of adjustment systems 13a, 13b producing a different size and / or shape of spots illuminated by laser beams 14a, 14b, and / or the calibration may be performed for different first portions, and / or the calibration may be performed for different vertical positions of building platform 102 arranged in the building area and carrying calibration plate 116.

[0112] The method may be repeated after performing a calibration, wherein, in step (f), the difference between the detected positions may be compared to a predetermined offset threshold to verify whether the previously performed calibration was sufficient. Based on the result of the comparison with the offset threshold, another calibration may be performed as described herein. Once the verification indicates that the difference between the detected positions is below the predetermined offset threshold, a three-dimensional workpiece may be produced, preferably after removing calibration plate 116 from build platform 102. The three-dimensional workpiece so produced may have lower manufacturing tolerances, higher rigidity, and other advantageous physical properties than workpieces produced with printing system 100 before performing the calibration.

Claims

1. A calibration method for a printing system (100), the printing system (100) being configured to produce a three-dimensional workpiece, the method being executed by a processor (18) and comprising: (a) obtaining a first image (133) of a first portion of a calibration plate (116) disposed in a build area of ​​the printing system (100), captured by an imaging system (118) of the printing system (100), the first portion comprising at least a portion of at least one calibration mark (130) carried by the calibration plate (116); (b) detecting a position (138) of the at least one portion in the first image (133); (c) controlling an illumination system (10) of the printing system (100) to illuminate points on the first portion of the calibration plate (116) arranged in the building area with a laser beam (14a, 14b); (d) obtaining a second image (140) of the first portion of the calibration plate (116) arranged in the building area, captured by the imaging system (118), the second image comprising a light spot (142) formed by the laser beam (14a, 14b) illuminating the point on the first portion of the calibration plate (116) arranged in the building area; (e) detecting the position of the light spot (142) in the second image (140); as well as (f) calibrating the printing system (100) based on the detected position (138) of the at least one portion in the first image (133) and the detected position of the light spot (142) in the second image (140).

2. The method according to claim 1, wherein: The printing system (100) is calibrated based on a comparison of the detected position (138) of the at least one portion in the first image (133) and the detected position of the light point (142) in the second image (140).

3. The method according to claim 1 or 2, wherein: The first image (133) and the second image (140) cover the same field of view and / or an alignment of the first portion in the first image (133) is the same as an alignment of the first portion in the second image (140).

4. The method according to any one of claims 1 to 3, wherein: The illumination system (10) is calibrated to calibrate the printing system (100).

5. The method according to any one of claims 1 to 4, comprising: obtaining an alignment image set comprising one or more images captured by the imaging system (118) of at least a portion of the calibration plate (116) disposed in the build area; as well as determining an orientation of the calibration plate (116) based on the set of alignment images, Wherein, the printing system (100) is calibrated based on the determined orientation of the calibration plate (116).

6. The method according to claim 5, comprising: detecting a geometric element on the calibration plate in at least one of the images in the alignment image set (144); determining an orientation of the detected geometric element (144) in the at least one image; as well as The orientation of the calibration plate (116) is determined based on the determined orientation of the detected geometrical element (144).

7. The method according to claim 5 or 6, comprising: detecting a plurality of reference elements (146) of the calibration plate (116) based on the set of alignment images; determining a position of each detected reference element (146) based on the set of alignment images; as well as The orientation of the calibration plate (116) is determined based on the determined positions of the detected reference elements (146).

8. The method according to any one of claims 1 to 7, wherein: The calibration is performed for the following situations: (i) different laser beams (14a, 14b) of the irradiation system (10), (ii) different optical scanning systems (16a, 16b) of the illumination system (10), each optical scanning system being configured to direct a different laser beam (14a, 14b) to the building area, (iii) different sizes of the spots on the calibration plate (116) illuminated by the laser beams (14a, 14b), (iv) different shapes of the points on the calibration plate (116) illuminated by the laser beams (14a, 14b), (v) different configurations of the regulating system (13a, 13b) of the irradiation system (10), each configuration producing a different size and / or shape of the spot irradiated with the laser beam (14a, 14b), (vi) a different first part, and / or (vii) different heights of a building platform (102) arranged in the building area and carrying the calibration plate (116).

9. The method according to any one of claims 1 to 8, comprising: obtaining correction data indicative of geometrical parameters of the calibration plate (116) measured with an external measurement system, Therein, the printing system (100) is calibrated based on the correction data.

10. The method according to any one of claims 1 to 9, comprising: Determine the transformation between the following coordinate systems: (i) a coordinate system (SCS) of an optical scanning system (16a, 16b) of the illumination system (10), the optical scanning system (16a, 16b) being configured to direct the laser beam (14a, 14b) to the building area, and (ii) a coordinate system (FCS) of the imaging system (118), Therein, the printing system (100) is calibrated based on the determined transformation.

11. The method according to any one of claims 1 to 10, wherein: The printing system (100) further comprises an illumination unit (126) configured to illuminate at least the first portion of the calibration plate (116) when the calibration plate (116) is arranged in the building area, the method comprising: controlling the imaging system (118) to capture the first image (133) when the first portion of the calibration plate (116) arranged in the building area is illuminated by at least the illumination unit (126); and / or controlling the imaging system (118) to capture the second image (140) when the first portion of the calibration plate (116) arranged in the building area is at least not illuminated by the illumination unit (126); and / or The imaging system (118) is controlled to capture all images except the second image (140) when the first portion of the calibration plate (116) disposed in the building area is illuminated by at least the illumination unit (126).

12. The method according to any one of claims 1 to 11, wherein: The imaging system (118) is arranged such that the imaging system captures at least the first image (133) and the second image (140) via an optical scanning system (16a, 16b) included in the illumination system (10), the optical scanning system (16a, 16b) being configured to direct the laser beam (14a, 14b) to the building area.

13. A printing system (100) for producing a three-dimensional workpiece, comprising: an irradiation system (10) configured to selectively irradiate a build area with one or more laser beams (14a, 14b); an imaging system (118) configured to capture an image of at least a portion of the calibration plate (116) when the calibration plate (116) is disposed in the build area; and A processor (18), the processor being configured to: (a) obtaining a first image (133) captured by the imaging system (118) of a first portion of the calibration plate (116) arranged in the build area, the first portion comprising at least a portion of at least one calibration mark (130) carried by the calibration plate (116); (b) detecting a position (138) of the at least one portion in the first image (133); (c) controlling the illumination system (10) to illuminate a point on the first portion of the calibration plate (116) arranged in the building area with one of the one or more laser beams (14a, 14b); (d) obtaining a second image (140) of the first portion of the calibration plate (116) arranged in the build area, captured by the imaging system (118), the second image (140) comprising a light spot (142) formed by one of the one or more laser beams (14a, 14b) illuminating the point on the first portion of the calibration plate (116) arranged in the build area; (e) detecting the position of the light spot (142) in the second image; and (f) calibrating the printing system (100) based on the detected position (138) of the at least one portion in the first image (133) and the detected position of the light spot (142) in the second image (140).

14. The printing system (100) according to claim 13, wherein: The processor (18) is configured to perform the method according to any one of claims 1 to 12.

15. The printing system (100) according to claim 13 or 14, further comprising at least one of the following components: (i) one or more optical scanning systems (16a, 16b), each optical scanning system being configured to direct a different one of the laser beams (14a, 14b) to the building area, (ii) a conditioning system (13a, 13b) having a plurality of configurations, each configuration producing a different size and / or shape of the spot irradiated with the laser beam (14a, 14b), (iii) a building platform (1029) arranged in the building area and configured to carry the calibration plate (116), (iv) an illumination unit (126) configured to illuminate at least the first portion of the calibration plate (116) when the calibration plate is arranged in the building area, (v) a coaxial camera (120), (vi) The calibration plate (116).

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