Calibration method and printing system configured to manufacture a three-dimensional workpiece

Abstract

Provided is a calibration method for a printing system configured to manufacture a three-dimensional workpiece. An image of a first portion of a calibration plate is obtained, at least a partial position of a calibration mark on the calibration plate is detected in the first image, a laser beam is irradiated by an irradiation system of the printing system at a point on the first portion of the calibration plate, a second image of the first portion is obtained, the position of a light spot formed by the laser beam is detected in the second image, and the printing system is 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. Additionally, a corresponding printing system is provided.

Description

Technical Field

[0001] The present invention relates to a method for calibrating a printing system configured to manufacture a three-dimensional workpiece. Further, the present invention relates to a printing system of this kind.

Background Art

[0002] Powder bed fusion is an additive manufacturing process capable of processing powdery raw materials, particularly metal raw materials and / or ceramic raw materials, into three-dimensional workpieces with complex shapes. To achieve this purpose, a powder layer of the raw material is applied onto a carrier, and selectively receives laser radiation according to the desired shape of the workpiece to be manufactured. The laser radiation penetrating the powder layer heats the powder particles of the raw material, and as a result, they are melted or sintered. Thereafter, powder layers of the raw material are continuously applied onto the already laser-treated layer on the carrier until the workpiece reaches the desired shape and size. Powder bed fusion may be employed for the manufacture or repair based on CAD data of prototypes, tools, replacement parts, high-value parts, or medical prostheses such as dental or orthopedic prostheses.

[0003] For example, as described in Patent Document 1 (International Publication No. 2019 / 141381), a printing system for manufacturing a three-dimensional workpiece by powder bed fusion includes a carrier (also called a build platform) configured to receive multiple layers of raw material, and an irradiation unit (also called an irradiation system) configured to selectively irradiate the raw material on the carrier with laser radiation to manufacture the workpiece. The irradiation unit may be provided with a spatial light modulator configured to split a laser beam into at least two sub-beams. Therefore, generally, a plurality of sub-beams, generally called laser beams, can be used to selectively irradiate the raw material on the build platform.

[0004] The planned position on the raw material to be irradiated with the laser beam may be defined by workpiece data that describes the workpiece to be manufactured (e.g., computer-aided design, CAD) or derived from the workpiece data. It cannot be denied that the irradiation system may direct the laser beam to a position on the raw material that deviates from the planned position due to causes such as manufacturing tolerances and temperature changes. In other words, there may be a deviation between the irradiation position on the raw material and the corresponding planned position. Such a deviation may lead to disadvantages such as a decrease in the rigidity of the printed workpiece and the dimensions of the workpiece exceeding the allowable pre-defined manufacturing tolerances. The printing system may be calibrated to reduce or eliminate the deviation between the irradiation position and the planned position on the raw material.

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Summary of the Invention

[0006] An object of the present invention is to provide a printing system configured to manufacture a three-dimensional workpiece and a calibration method for this type of printing system. This calibration method enables efficient and accurate calibration of the printing system.

[0007] The raw material powder layer may be applied onto the surface of the construction platform by a powder application device that moves across the construction platform to distribute the raw material powder. The construction platform may be a rigidly fixed carrier. However, preferably, the construction platform is designed to be movable in the vertical direction (e.g., the height can be changed), so that as the construction height of the workpiece constructed layer by layer from the raw material powder increases, the construction platform can be moved downward in the vertical direction. It can be said that the various heights of the construction platform correspond to its various vertical positions. Furthermore, the construction platform may be provided with a cooling device and / or a heating device configured to cool and / or heat the construction platform.

[0008] The construction platform and the powder application device may be housed in a processing chamber that can be sealed against the surrounding atmosphere. By introducing a gas flow into the processing chamber through a gas inlet, an inert gas atmosphere may be established in the processing chamber. The gas flow may pass through the processing chamber and be guided across the raw material powder layer applied on the carrier, and then be discharged from the processing chamber through a gas outlet. The raw material powder applied on the construction platform in the processing chamber is preferably a metal powder, particularly a metal alloy powder, but may also be a ceramic powder or a powder containing various materials. The powder may 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.

[0009] The irradiation system may comprise a laser beam source configured to emit at least one laser beam. In particular, the laser beam source of the irradiation system may emit a laser beam with a wavelength of 450 nm (for example, linearly polarized), i.e., a "blue" laser beam, or a laser beam with a wavelength of 532 nm, i.e., a "green" laser beam, or a laser beam in the range of 1000 nm to 1090 nm or 1530 nm to 1610 nm, i.e., an "infrared" laser beam. When using one or more beam splitter cubes to split the laser beam into two or more partial beams, only one partial beam may be used as the irradiation beam and the other partial beams may be blocked. Alternatively, one or more partial beams may be directed to different irradiation systems within one or more printing systems.

[0010] The irradiation system may irradiate the build area using a single laser beam. However, alternatively, it is conceivable that the irradiation system irradiates the build area with two or more laser beams. The plurality of laser beams irradiated onto the build area by the irradiation system may be emitted by a suitable sub-unit of the laser beam source. The build area may correspond to the area in which the workpiece within the printing system is to be manufactured, in particular, the area on and above the build platform in the vertical direction. The irradiation system may be controlled by a processor to irradiate, in particular, positions (for example, planned positions) within the build area and / or points (for example, on the powder bed of the raw material).

[0011] The irradiation system may also include at least one optical scanning system for splitting, guiding, and / or processing at least one laser beam emitted from a laser beam source. The optical scanning system may include one or more optical elements such as an objective lens and a scanner unit. The scanner unit preferably includes a diffractive optical element and / or a deflection mirror. The at least one optical scanning system may be configured to guide the laser beam to a construction area. The irradiation system may include a plurality of optical scanning systems each configured to guide a different laser beam to the construction area. In one example, the construction area is divided into a plurality of (e.g., non-overlapping) sections. Here, each of the optical scanning systems is configured to guide a different laser beam to a different set of sections. The different sets of sections may differ from each other in shape, size, and / or position, and may overlap each other.

[0012] The printing system may include an adjustment system having a plurality of configurations. Each configuration provides a different size and / or shape of the point irradiated by the laser beam. The adjustment system may be configured to change the diameter of the laser beam, 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 mirrors. The adjustment system may be part of the irradiation system, particularly part of the optical scanning system. Alternatively, the adjustment system may be separate from the optical scanning system. Different adjustment systems may be provided for different laser beams.

[0013] A calibration plate configured to be disposed in a construction area may be provided. The calibration plate may be disposed, for example, on a construction platform, on the floor of a processing chamber, on a powder coating apparatus, or may be realized as movable within a space in the processing chamber inside or above the construction area. The calibration plate may carry at least one calibration mark (e.g., on the surface of the calibration plate). The calibration plate may carry a plurality of calibration marks, and may be arranged, for example, in a symmetric pattern. One or more calibration marks may have a circular contour. Each calibration mark may have a high light reflectivity compared to a part of the calibration plate that touches or surrounds the respective calibration mark. The calibration plate may be an anodized aluminum plate. Each calibration mark may be formed by a part of an aluminum plate from which a surface layer formed by anodization has been removed (e.g., by milling, scratching, or laser vapor deposition).

[0014] The printing system may include an imaging system. The imaging system may be arranged and configured to capture an image of at least a part of the construction area, in particular, of the section of the construction area where the calibration plate is or can be arranged. The imaging system may include an image sensor such as a camera. The imaging system may be configured to capture images using a predefined electromagnetic spectrum. In other words, the imaging system may be highly sensitive only to light having wavelengths within the predefined electromagnetic spectrum. For example, the imaging system may include one or more wavelength filters arranged such that any light generated in the construction area (e.g., on the surface of the calibration plate arranged in the construction area) and incident on the image sensor falls within the predefined electromagnetic spectrum. The predefined electromagnetic spectrum may be different from the wavelength of the laser beam. That is, the imaging system may be configured to be less sensitive to light having the wavelength of the laser beam. The wavelength of the laser beam may be outside the predefined electromagnetic spectrum. The imaging system may include one or more optical components such as mirrors, lenses, apertures, etc., and the one or more optical components are arranged and configured to direct light from the construction area towards the image sensor. The imaging system may share at least one of the optical components with the irradiation system. For example, the image sensor may be arranged and configured to capture images via the optical scanning system of the irradiation system.

[0015] The printing system may include an illumination unit. The illumination unit may include one or more light-emitting elements, for example, one or more light-emitting diodes. The illumination unit may be arranged and configured to illuminate at least a part of the calibration plate when the calibration plate is disposed 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 predefined wavelength spectrum (e.g., for illuminating at least a part of the calibration plate). The predefined wavelength spectrum may coincide with, be included in, or overlap with the predefined electromagnetic spectrum. In other words, the imaging system may be sensitive to the light emitted from the illumination unit (and further, for example, the light reflected by the calibration marks of the calibration plate). The calibration marks may have a higher reflectivity of light in the predefined wavelength spectrum than the portions adjacent to each calibration mark of the calibration plate or the portions surrounding the calibration marks.

[0016] According to the present disclosure, there is provided a calibration method for a printing system configured to manufacture a three-dimensional workpiece (e.g., the above-described one). The method is implemented by a processor (e.g., the above-described one) and includes step (a) of obtaining a first image of a first portion of a calibration plate (e.g., the above-described one) disposed in a build area of the printing system (e.g., the above-described one), the first image being captured by an imaging system of the printing system (e.g., the above-described one). The first portion includes at least one portion of at least one calibration mark carried by the calibration plate. The method may include the step of activating the imaging system to capture the first image. Alternatively, the first image may be captured in advance and then read from a database.

[0017] This method further includes step (b) of detecting the position (e.g., only the position) of at least one part in the first image. This position may be detected in the frame coordinate system (FCS) of the imaging system. The FCS may be associated with the first image. This position may be detected based on the first image using one or more of feature extraction, feature recognition, template matching, and machine vision, for example, based on one or more pre-defined (e.g., geometric and / or optical) characteristics of at least one calibration mark. At least one calibration mark (e.g., at least one part thereof) may have a contour or shape such that its orientation cannot be detected (e.g., unambiguously) and / or determined based on the first image. Since only the position of at least one part needs to be detected in the first image, even in the case of a circular calibration mark, it is possible to achieve reliable calibration with this method.

[0018] This method further includes: step (c) of controlling an irradiation system (such as the above-mentioned one) of a printing system to irradiate a point on a first portion of a calibration plate disposed within a build area with a laser beam; step (d) of obtaining a second image of the first portion of the calibration plate disposed within the build area, the second image being captured by an imaging system, the second image including a light spot formed by the laser beam irradiating the point on the first portion of the calibration plate disposed within the build area; and step (e) of detecting the position (such as only the position) of the light spot within the second image. This method may further include the step of activating the imaging system to capture the second image. The position of the light spot may be detected within an FCS associated with the second image. The position of the light spot may be detected based on the second image using one or more of feature extraction, feature recognition, template matching, and machine vision, for example, based on one or more predefined (such as geometric and / or optical) characteristics of at least one light spot. The light spot may have a contour or shape such that its orientation cannot be (such as clearly) detected and / or determined based on the second image. Since only the position of the light spot needs to be detected in the second image, even in the case of a circular light spot, this method may provide a highly reliable calibration.

[0019] This method includes step (f) of calibrating a printing system based on the detected positions of at least one part of the first image and the detected positions of the light points of the second image. The printing system may be calibrated based on a comparison between the detected position of at least one part of the first image and the detected position of the light points of the second image. The first image and the second image may be directed at the same field of view and / or the same area. Alternatively, or in addition to this, the alignment of the first part of the first image may be the same as the alignment of the first part of the second image. The calibration plate may be in the same orientation within the construction area during the capture of the first image and during the capture of the second image. The printing system may be calibrated based on a comparison between the detected position of at least one part of the FCS and the detected position of the light points within the FCS. The irradiation system or the optical scanning system included in the irradiation system (for example, further used to direct a laser beam at a point on the calibration plate) (for example, the above) may be calibrated to calibrate the printing system. To calibrate the printing system, the coordinate systems of the optical scanning system, the imaging system and / or other components of the printing system may be adjusted (for example, relative to each other). The step of calibrating the printing system may include the step of determining one or more conversions between such coordinate systems.

[0020] The calibration method uses two different images as described above. Here, the first image is used to detect the position of at least a part of the calibration mark carried by the calibration plate, and the second image is used to detect the position of the light spot formed by the laser beam irradiating the point on the calibration plate. The images may be captured using image parameters (e.g., illumination settings, focus settings, exposure time settings, and / or contrast settings) configured to optimize each detection, and preferably, are captured at different times. For example, the illumination unit (e.g., as described above) may be configured to illuminate at least a first portion of the calibration plate when the calibration plate is disposed in the construction area. The first image may be captured while illuminating the first portion, and the second image may be captured while the illumination unit is off. At least one calibration pattern (e.g., at least one part of the calibration pattern) may not be visible and / or may not be detectable in the second image.

[0021] This method may include obtaining an alignment image set including one or more images captured by the imaging system, which are one or more images of at least one part of the calibration plate disposed in the construction area. The orientation of the calibration plate may be determined based on the alignment image set (e.g., with respect to the FCS). The printing system may be calibrated (e.g., further) based on the determined orientation of the calibration plate.

[0022] This 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 the orientation of the detected geometric element of at least one image. Next, the orientation of the calibration plate may 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.

[0023] The geometric element may be a non-rotationally symmetric element. The geometric element may have a non-circular contour or shape. The geometric element may have a finite number of symmetry axes. The geometric element may include at least two lines or be composed of at least two lines. The two or more lines may be non-parallel to each other and / or may intersect each other. Examples of such geometric elements include a pair of L-shaped lines, a cross-shaped or plus-shaped pair of lines, four lines forming a rectangle or a square, and an L-shaped or square arrangement of circles. Alternatively, the geometric element may include an asymmetric two-dimensional pattern such as a two-dimensional code (e.g., computer-readable), such as a Quick Response (QR) code, or may be composed of an asymmetric two-dimensional pattern. The geometric element may be disposed on a calibration plate adjacent to the calibration mark or the calibration plate may be disposed so as to surround the calibration mark. In one example, the geometric element is joined to (e.g., transitions to) the calibration mark. The geometric element may have a higher reflectivity of light in a predefined wavelength spectrum than the portion surrounding the same configuration mark or in contact with each calibration mark of the calibration plate.

[0024] This method may include the step of detecting a plurality of reference elements of the calibration plate based on the alignment image set. The plurality of reference elements may be detected in one image of the alignment image set, or different reference elements of the plurality of reference elements may be detected in different images of the alignment image set. Based on the alignment image set, the position of each detected reference element may be determined. For example, the positions of all reference elements within one image of the alignment image set may be determined, or the positions of each different reference element within different images may be detected. This method may include the step of determining the orientation of the calibration plate based on the determined positions of the detected reference elements. That is, the orientation of the calibration plate may be determined using the positions of the plurality of reference elements detected in a single image or multiple images of the alignment image set. It is not necessary to detect the orientation of individual reference elements. Thereby, reference elements for which the individual orientation cannot be (e.g., clearly) detected in the images of the alignment image set can be used. For this reason, the reference elements according to the present disclosure may have a circular shape or contour. An example of a reference element includes a calibration mark. That is, the reference element may correspond to a calibration mark carried by the calibration plate. In one example, the diameters of the reference element and the calibration mark may be different. The reference element may have a higher reflectance of light in a predetermined wavelength spectrum than a portion that touches or surrounds the respective calibration mark of the calibration plate.

[0025] The alignment image set may include or be composed of one or more of the following images: (i) a first image, (ii) a third image of a second portion of the calibration plate disposed within the construction area (the second portion being different from the first portion), (iii) a plurality of images of different portions of the calibration plate disposed within the construction area.

[0026] Calibration may be performed and / or repeated for (i) different laser beams of the irradiation system, (ii) different optical scanning systems of the irradiation system, each configured to direct a different laser beam to the construction area, (iii) different sizes of points on the calibration plate irradiated with the laser beam, (iv) different shapes of points on the calibration plate irradiated with the laser beam, (v) different configurations of the adjustment system (e.g., the above) of the irradiation system, each configuration resulting in different sizes and / or shapes of the points irradiated with the laser beam, (vi) different first portions, and / or (vii) different heights of the construction platform disposed within the construction area and carrying the calibration plate.

[0027] When the irradiation system includes a plurality of optical scanning systems each configured to direct a different laser beam to the construction area, at least one (e.g., all) of the plurality of optical scanning systems may be calibrated when calibrating the printing system. Two variations of this procedure are described herein. 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.

[0028] As a first variation, steps (c) to (e) may be performed for at least one additional laser beam, and the printing system may be calibrated based on the detected positions of at least one portion in the first image and the detected positions of the respective light spots in the second image. In other words, the position of at least one portion may be detected using a single first image, while the respective light spots of different laser beams may be detected using a plurality of second images.

[0029] As a second variant, the method may include a step (c´) of controlling the irradiation system to irradiate at least one additional laser beam at different points on a first portion of a calibration plate disposed in the construction area. In step (d), a second image of the first portion of the calibration plate disposed in the construction area is obtained, the second image being captured by the imaging system. The second image in this variant includes light spots formed by a laser beam irradiating points on the first portion of the calibration plate disposed in the construction area, and further includes at least one additional light spot formed by at least one additional laser beam irradiating different points on the first portion of the calibration plate disposed in the construction area. The method may further include a step (e´) of detecting the position of at least one additional light spot in the second image. Next, the printing system may be calibrated based on the detected position of at least one portion of the first image and the detected position of the light spots in the second image. In other words, it is possible to detect the position of at least one portion using a single first image and to detect a plurality of light spots of different laser beams using a single second image. The light spots may be distinguished based on their respective (e.g., geometric and / or optical) characteristics. Such characteristics may include one or more of light intensity, light color, light spectrum, light wavelength, shape, contour, beam profile (e.g., Gaussian, top hat or donut). The characteristics of the light spots may be associated with each laser beam, for example, using a known relationship between the characteristics and the laser beam. This may make it possible to individually calibrate the optical scanning systems of different laser beams using a single second image including a plurality of light spots generated by different laser beams irradiating different points on the calibration plate.

[0030] The irradiation system comprises an adjustment system having a plurality of configurations, and if different sizes and / or shapes of the points where the laser beam is irradiated on the calibration element arranged in the construction area are obtained by each configuration, this method may include the steps of performing steps (c) to (e) for each of the plurality of configurations of the adjustment system. Next, the printing system may be calibrated based on the detected positions of at least one part in the first image and the detected positions of the respective light points in the second image.

[0031] This method is a step of performing steps (a) to (e) for each of a plurality of different first parts, and the printing system may include a step of being calibrated based on the detected positions of at least one part in the first image and the detected positions of the respective light points in the second image.

[0032] As described above, the printing system may include a construction platform arranged in the construction area and configured to carry a calibration plate. This method is a step of performing steps (a) to (e) for each of a plurality of heights of the construction platform when carrying the calibration plate, and the printing system may include a step of being calibrated based on the detected positions of at least one part in the first image and the detected positions of the respective light points in the second image.

[0033] This method may include a step of obtaining correction data indicating the geometric parameters of the calibration plate measured using an external measurement system, and the printing system is calibrated based on the correction data.

[0034] This method may include the step of determining a transformation between (i) a coordinate system of an optical scanning system (e.g., the above-described) of an irradiation system, wherein the optical scanning system is configured to direct a laser beam to a build area, and (ii) a coordinate system of an imaging system (e.g., coordinate system FCS), and 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.

[0035] This method may include the step of controlling the imaging system to capture a first image while at least a first portion of a calibration plate disposed in the build area is illuminated by an illumination unit. Alternatively, or in addition thereto, the imaging system may be controlled to capture a second image while at least a first portion of a calibration plate disposed 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 while at least a first portion of a calibration plate disposed in the build area is illuminated by the illumination unit.

[0036] The illumination unit may be controlled (e.g., by a processor or a user) to activate illumination when the first image is captured. The illumination unit may be controlled (e.g., by a processor or a user) to activate illumination when images other than the second image are captured. The illumination unit may be controlled (e.g., by a processor or a user) to deactivate illumination when the second image is captured.

[0037] The imaging unit may be configured to acquire a first image only when the illumination unit illuminates at least a first portion of the calibration plate disposed within the construction area. The imaging unit may be configured to acquire all images except a second image only when the illumination unit illuminates at least a first portion of the calibration plate disposed within the construction area. The imaging unit may be configured to acquire a second image only when the illumination unit does not illuminate the calibration plate disposed in the construction area. For this purpose, the imaging unit may be coupled to the illumination unit or may include a light sensor configured to detect light emitted by the illumination unit.

[0038] The imaging system may be arranged to capture at least a first image and a second image via an optical scanning system (e.g., the above-described one) included in the irradiation system, and the optical scanning system is configured to direct a laser beam into the construction area. In this case, the imaging system may include an on-axis camera.

[0039] One or more position markers may be provided adjacent to the calibration plate disposed in the construction area. The one or more position markers may be provided in an area that is not covered with powder material while a new powder layer is being formed by the powder application device. The calibration plate may carry one or more additional position markers. The method may include the step of capturing, by the imaging system, a fourth image of at least one of the position markers provided adjacent to the calibration plate and at least one of the position markers carried by the calibration plate. Based on the captured fourth image, the position and / or orientation of each position marker may be determined. The positions and / or orientations may be compared to determine the offset of the calibration plate from a predefined calibration pose of the calibration plate with respect to the construction area. Next, the position and / or orientation of the calibration plate may be adjusted (e.g., by mechanical and / or manual adjustment) to minimize or correct the offset. Next, the method may proceed to steps (a) to (f).

[0040] After performing the calibration, this method may be repeated for verification and / or a three-dimensional workpiece may be manufactured by the printing system.

[0041] According to the present disclosure, there is provided a printing system for manufacturing a three-dimensional workpiece, comprising: an irradiation system configured to selectively irradiate one or more laser beams onto a build area; an imaging system configured to capture an image of at least a part of a calibration plate when the calibration plate is disposed within the build area; and a processor. The processor of the printing system is configured to perform: (a) obtaining a first image of a first portion of a calibration plate disposed in the build area, the first image being captured by the imaging system, the first portion including at least a part of at least one calibration mark carried by the calibration plate; (b) detecting a position of at least a part of the first image; (c) controlling the irradiation system to irradiate a point on the first portion of the calibration plate disposed in the build area with one of the one or more laser beams; (d) obtaining a second image of the first portion of the calibration plate disposed in the build area, the second image being captured by the imaging system, the second image including a light spot formed by one of the one or more laser beams irradiating a point on the first portion of the calibration plate disposed in the build area; (e) detecting a position of the light spot of the second image; and (f) calibrating the printing system based on the detected position of at least a part of the first image and the detected position of the light spot of the second image.

[0042] A printing system, in particular, the processor may be configured to implement the method described above herein. The printing system may include (i) one or more optical scanning systems (e.g., as described above), each configured to direct a different one of the laser beams to a build area, (ii) an adjustment system (e.g., as described above) having a plurality of configurations, each configuration resulting in a different size and / or shape of the point irradiated by the laser beam, (iii) a build platform (e.g., as described above) disposed in the build area and configured to carry a calibration plate, (iv) an illumination unit (e.g., as described above) configured to illuminate at least a first portion of the calibration plate when the calibration plate is disposed in the build area, (v) an on-axis camera (e.g., as described above), (vi) a calibration plate, and may include one or more of the foregoing.

[0043] The processor may be configured to calibrate the printing system based on a comparison (e.g., as described above) of the detected position of at least one portion of the first image and the detected position of the light spot of the second image.

[0044] 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 target the same field of view and / or the alignment of the first portion of the first image is the same as the alignment of the first portion of the second image.

[0045] The processor may be configured to calibrate the irradiation system to calibrate the printing system.

[0046] The processor may be configured to obtain an alignment image set including one or more images of at least a part of a calibration plate arranged within a construction area, the one or more images being captured by an imaging system (e.g., the above). The processor may be configured to determine the 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.

[0047] The processor may be configured to detect geometric elements (e.g., the above) on the calibration plate in at least one of the images of the alignment image set, determine the orientation of the detected geometric elements in the at least one image, and determine the orientation of the calibration plate based on the determined orientation of the detected geometric elements.

[0048] The processor may be configured to detect a plurality of reference elements (e.g., the above) of the calibration plate based on the alignment image set, determine the position of each detected reference element based on the alignment image set, and determine the orientation of the calibration plate based on the determined positions of the detected reference elements. Additionally, the orientation of the calibration plate may be determined step by step through the step of evaluating each image of the alignment image set immediately after incorporating, inspecting, and correcting the calculated orientation for each additional image.

[0049] The alignment image set may include, or be composed of, one or more of the following images: (i) a first image; (ii) a third image of a second part (e.g., the above) of a calibration plate arranged in the construction area, the second part being different from the first part; (iii) a plurality of images of various parts of a calibration plate arranged in the construction area (e.g., the above).

[0050] The processor may be configured to perform calibration for (i) various laser beams, (ii) various optical scanning systems of the printing system, each configured to direct a different single laser beam to the build area, (iii) various sizes of points on the calibration plate irradiated with the laser beam, (iv) various shapes of points on the calibration plate irradiated with the laser beam, (v) various configurations of the adjustment system (e.g., as described above) of the printing system, the adjustment system having a plurality of configurations, each configuration resulting in a different size and / or shape of the point irradiated with the laser beam, (vi) various first portions, and / or (vii) various heights of the build platform disposed within the build area and configured to carry the calibration plate.

[0051] The irradiation system may comprise a plurality of optical scanning systems (e.g., as described above), each optical scanning system being configured to direct a different single laser beam to the build area. The processor may be configured to calibrate at least one of the plurality of optical scanning systems when calibrating the printing system.

[0052] The processor may be configured to perform steps (c) to (e) for at least one additional laser beam of the laser beams, in particular for each different single laser beam, and to calibrate the printing system based on the detected positions of at least one portion of the first image and the detected positions of the respective light points of the second image.

[0053] The processor may be configured to control the irradiation system to irradiate at least one additional laser beam of the (c´) laser beam, in particular, one different laser beam each, at different points (e.g., the above) on the first part of the calibration plate arranged in the construction area. The processor may be configured to obtain, in step (d), a second image of the first part of the calibration plate arranged in the construction area, the second image being captured by the imaging system. The second image includes a light spot formed by one of the one or more laser beams irradiating points on the first part of the calibration plate arranged in the construction area, and the second image further includes at least one additional light spot (e.g., the above) formed by at least one additional laser beam irradiating different points on the first part of the calibration plate arranged in the construction area. The processor may be configured to detect the position of at least one additional light spot in the (e´) second image and calibrate the printing system based on the detected position of at least one part in the first image and the detected position of the light spot in the second image.

[0054] The irradiation system may comprise an adjustment system having a plurality of configurations (e.g., the above), each configuration resulting in different sizes and / or shapes of the points where the laser beam is irradiated on the calibration element arranged in the construction area. The processor may perform steps (c) to (e) for each of the plurality of configurations of the adjustment system and be configured to calibrate the printing system based on the detected position of at least one part in the first image and the detected position of each light spot in the second image.

[0055] The processor may perform steps (a) to (e) for each of the plurality of different first parts (e.g., the above) and be configured to calibrate the printing system based on the detected position of at least one part of each first image and the detected position of each light spot of the second image.

[0056] The printing system may include a construction platform (e.g., the above-described one) arranged in a construction area and configured to carry a calibration plate. In this case, when carrying the calibration plate, the processor may perform steps (a) to (e) for each of a plurality of heights (e.g., the above-described ones) of the construction platform, and calibrate the printing system based on the detected positions of at least one part of each of the first images and the detected positions of the light points of each of the second images.

[0057] The processor may be configured to obtain correction data indicating (e.g., the above-described) geometric parameters of the calibration plate measured using (e.g., the above-described) an external measurement system, and calibrate the printing system based on the correction data.

[0058] The processor may be configured to determine a transformation between (i) a coordinate system (e.g., the above-described one) of an optical scanning system of an irradiation system, where the optical scanning system is configured to direct one of one or more laser beams into the construction area, and (ii) a coordinate system (e.g., the above-described one) of an imaging system. The processor may be configured to calibrate the printing system based on the determined transformation.

[0059] The printing system may further include an illumination unit (e.g., the above-described one) configured to illuminate at least a first part of the calibration plate when the calibration plate is arranged in the construction area. The processor may be configured to control the imaging system to capture a first image while at least the first part of the calibration plate arranged in the construction area is illuminated by the illumination unit, and / or control the imaging system to capture a second image while at least the first part of the calibration plate arranged in the construction area is not illuminated by the illumination unit, and / or control the imaging system to capture all images except the second image while at least the first part of the calibration plate arranged in the construction area is illuminated by the illumination unit.

[0060] The imaging system may be arranged to capture at least a first image and a second image via an optical scanning system included in the irradiation system, and the optical scanning system is configured to direct at least one of one or more laser beams to the build area.

[0061] The printing system may further include a calibration plate, and the calibration plate may optionally be disposed within the build area.

[0062] When executed by a processor, a computer program product may be provided that stores instructions for causing the processor to perform 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

[0063] Preferred embodiments of the present invention will be described in further detail with reference to the accompanying schematic drawings.

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5a

Figure 5b

Figure 5c

Figure 6a

Figure 6b

Figure 6c

Figure 6d

Figure 7a

Figure 7b

Figure 7c

Figure 7d

Figure 7e

Figure 7f

Figure 8a

Figure 8b

Figure 9a

Figure 9b

Figure 10

Figure 11

DETAILED DESCRIPTION OF THE INVENTION

[0064] FIG. 1 shows a printing system 100 for manufacturing a three-dimensional workpiece by an additive manufacturing process. The 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 housed within a processing chamber 106 that can be sealed against the ambient atmosphere. Since the build platform 102 is vertically movable within the build cylinder 108, the build platform 102 can be moved downward as the layer height of the workpiece 110 stacked layer by layer from the raw material powder on the build platform 102 increases. The build platform 102 may include a heater and / or a cooler.

[0065] The apparatus 100 further includes an irradiation system 10 for selectively irradiating a raw material powder layer 11 coated on a carrier 102 with laser radiation. In the embodiment of the apparatus 100 shown in FIG. 1, the irradiation system 10 includes two laser beam sources 12a, 12b each configured to emit a laser beam 14a, 14b. Optical scanning systems 16a, 16b for guiding and processing the laser beams 14a, 14b emitted by the laser beam sources 12a, 12b are 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 as a result, the irradiation system 10 emits only a single laser beam. Adjustment systems 13a, 13b may be formed by some optical components of the respective optical scanning systems 16a, 16b. The adjustment system 13a for the laser beam 14a may include, for example, a lens 15a of the optical scanning system 16a. The same also applies to the adjustment system 13b for the laser beam 14b and the lenses 15b, 15c of the optical scanning system 16b. Each adjustment system 13a, 13b may be configured to adjust the width, focus and / or shape of the respective laser beams 14a, 14b before the respective laser beams enter the construction area. A processor 18, also called a control device, is provided to control the operation of components of the apparatus 100 including the irradiation system 10, for example, a powder coating device 104.

[0066] By supplying a shielding gas into the processing chamber 106 through the process gas inlet 112, a controlled gas atmosphere, preferably an inert gas atmosphere, is established in the processing chamber 106. The gas passes through the processing chamber 106, crosses the raw material powder layer 11 coated on the carrier 102, and is then discharged from the processing chamber 106 through the process gas outlet 114. The process gas is recycled from the process gas outlet 114 to the process gas inlet 112 and may then be cooled or heated.

[0067] During operation of the apparatus 100 for manufacturing a three-dimensional workpiece, a layer of raw material powder is applied onto the carrier 102 by the powder application device 104. To apply the raw material powder layer 11, the powder application device 104 moves across the carrier 102 under the control of the control unit 18. Next, again under the control of the control unit 18, the layer of raw material powder is selectively irradiated with laser radiation by the irradiation device 10 according to the shape of the corresponding layer of the workpiece 110 to be manufactured. The steps of applying a layer of raw material powder onto the carrier 102 and selectively irradiating the layer of raw material powder with laser radiation according to the shape of the corresponding layer of the workpiece 110 to be manufactured are repeated until the workpiece 110 reaches the desired shape and size.

[0068] The printing system 100 may be calibrated before and / or after manufacturing the three-dimensional workpiece. For this purpose, a calibration plate 116 may be placed on the construction platform 102. The construction platform 102 may be positioned such that the upper surface of the calibration plate 116 is located in a plane corresponding to the surface of the powder layer during manufacturing of the workpiece. The calibration plate 116 is shaped and dimensioned so as to be positioned within the construction area formed by the inner wall surface of the cylinder 108. The printing system 100 includes an imaging system 118 configured to capture an image of at least a first portion of the calibration plate 116 when the calibration plate 116 is disposed within the construction area.

[0069] In the example of FIG. 1, the imaging system 118 includes a camera 120 and a set of scanning mirrors 122 for shifting the field of view of the camera 120 on the calibration plate 116 in response to a request. In contrast, in the example of FIG. 2, the imaging system 118 shares optical components with the optical scanning system 16b. In particular, a beam splitter 124 is disposed in the optical path of the laser beam 14b. The beam splitter 124 directs the light emitted from the calibration plate 116 toward the camera 120. In this configuration, the laser deflection mirror of the optical scanning system 16b may be used to shift the field of view of the camera 120. It should be noted that the field of view of the camera 120 may be larger than the laser beam irradiating the construction area, as shown in FIG. 2. The camera 120 in the configuration of FIG. 1 is called an off-axis camera, while the camera 120 in the configuration of FIG. 2 is called an on-axis camera.

[0070] The printing system 100 includes an illumination unit 126 including a set of light emitting elements 128a, 128b configured to emit light of a predefined spectrum such that the calibration plate 116 is illuminated. The predefined 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 image acquisition of the calibration plate 116 by the imaging unit 118. The imaging system 118 may be non-responsive to incident light of a wavelength corresponding to the wavelengths of the 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 laser light. Such a configuration may protect the camera 120 from high-intensity reflected laser light.

[0071] Figure 3 shows a flowchart of the method according to the present disclosure. The method is implemented by the processor 18. In step (a), a first image captured by the imaging system 118 of the printing system 100 is acquired. The first image includes a first portion of the calibration plate 116 disposed in the build area of the printing system 100. The first portion of the calibration plate 116 includes at least one portion of at least one calibration mark carried by the calibration plate 116. During acquisition of the first image, the illumination unit 126 may illuminate the calibration plate 116.

[0072] In step (b), the position of at least one portion in the first image is detected using, for example, feature recognition techniques. By illuminating the calibration plate 116 during acquisition of the first image, an image with a high optical contrast between the calibration mark and the remaining portion of the calibration plate 116 is obtained. This may improve the accuracy of the detected position of at least one portion of the calibration mark.

[0073] In step (c), the irradiation system 10 of the printing system 100 is controlled to irradiate one of the laser beams at a point on the first portion of the calibration plate 116 disposed in the build area. At the same time, the illumination unit 126 is stopped from irradiating the calibration plate 116 with only the light from the laser source 12a or 12b.

[0074] In step (d), a second image of the first portion of the calibration plate 116 disposed in the build area, which is the second image captured by the imaging system 118, is acquired. The second image includes light spots formed by one or more laser beams that irradiate a point on the first portion of the calibration plate 116 disposed in the build area. The light spots may be very clearly visible in the second image because the illumination unit is stopped during acquisition of the second image.

[0075] In step (e), the position of the light spots in the second image is detected. The accuracy of the detected position may be improved using the second image captured during the stop period of the illumination unit 126.

[0076] In step (f), the optical scanning system 16a or 16b that directs the laser beam to a point on the first part of the calibration plate 116, particularly in the printing system 100, is calibrated based on the detected position of at least one part in the first image and the detected position of the light spot in the second image. When using separate images, the detected positions are each improved, and a more accurate calibration is obtained. The step of calibrating the optical scanning systems 16a, 16b may include, for example, the step of adjusting the coordinate system of each optical scanning system with reference 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. The printing data (e.g., irradiation instructions) used by the printing system 100 to form a three-dimensional object may be defined with reference to the central coordinate system. By calibrating the optical scanning systems 16a, 16b, any position to be irradiated subsequently corresponds to the position scheduled to be irradiated (e.g., based on the printing data).

[0077] FIG. 4 shows an exemplary calibration plate 116. As can be seen from the figure, the calibration plate 116 has, for example, a black upper surface formed by anodizing an aluminum plate. The calibration plate 116 carries a plurality of calibration marks 130. The calibration marks 130 are shown as white dots because they have a high light reflectivity compared to the black upper surface at least in a predefined spectrum. Each calibration mark 130 corresponds to a circular area on the upper surface of the calibration plate 116, and this area is either covered with a reflective color or an area that does not include an anodized layer. The calibration marks 130 may be formed by selective laser evaporation or computer-controlled milling of a part of the anodized layer. The calibration marks 130 are provided in a symmetric matrix having rows and columns. Other arrangements of the calibration marks 130 are also possible.

[0078] Since the calibration mark 130 has a higher light reflectivity (e.g., more than 2 times, 5 times, 10 times, or 100 times) than the surrounding portion of the calibration plate 116, in particular, when the illumination unit 126 illuminates the calibration plate 116 during the acquisition of the first image, at least one portion of at least one calibration mark 130 may be easily detected in the first image. Circular calibration marks are easier to detect in the first image compared to complex geometric shapes. Even if only a part of the circular calibration mark 130 is visible in the first image, it is possible to detect or determine the center point of the circular calibration mark 130 based on the first image.

[0079] Figures 5a to 5c show exemplary first images 133 of the calibration plate 116. In FIG. 5, a section 132 of the calibration plate 116 including the calibration mark 130 is shown. The rectangular contour 134 represents the area covered by the first image 133. That is, everything within the outline 134 is made visible in the first image 133. The first image 133 has a frame coordinate system FCS exemplified in FIGS. 5a to 5c at the lower left corner of the first image 133. As shown in FIG. 5b, the contour 136 of the circular calibration mark 130 may be detected in the first image 133.

[0080] Next, as shown in FIG. 5c, the center point 138 of the circular contour 136 is determined as the position of the calibration mark 130 within the FCS. In FIG. 5c, the plate coordinate system PCS is shown with axes "X" and "Y". The orientation of the PCS with respect to the FCS is determined as will be further described below with reference to FIGS. 7a to 8b.

[0081] Figures 6a - 6d show an exemplary second image 140 of the calibration plate 116. The second image 140 includes the same section 132 of the calibration plate 116 that is the same size and in the same orientation as the first image 133 of FIGS. 5a - 5c. The first image 133 and the second image 140 may be said to be displays of exactly the same part of the calibration plate 116. Different from the first image 133, the second image 140 includes light spots 142 corresponding to points on the calibration plate 116 where a laser beam was irradiated during acquisition of the second image 140. The calibration marks 130 in the example of FIGS. 6a - 6d can be seen in the second image 140, but this is for illustrative purposes only and is not necessarily the case. That is, the position of the calibration marks 130 may not be detectable from the second image 140.

[0082] The position of the light spots 142 may be determined by FCS (see FIG. 6b).

[0083] An offset between the position of the calibration marks 130 detected in the first image 133 (e.g., the center point 138 of the calibration marks 130 within the FCS) and the position of the light spots 142 (e.g., within the FCS) detected in the second image 140 may be determined as shown in FIGS. 6c and 6d. This offset may be determined by the FCS. In the example of FIG. 6c, the offset is shown by the PCS. The PCS offset may be determined based on the FCS offset and the conversion between the FCS and the PCS. In the example of FIG. 6d, the offset is shown by the SCS. The SCS offset may be determined based on the FCS offset and the conversion between the FCS and the SCS. For an exemplary technique for determining the conversion between the FCS and the SCS, see FIGS. 9a and 9b and the description below. The conversion between the FCS and the SCS may be determined based on images of light spots obtained by irradiating different positions and / or based on one or more images of (e.g., asymmetric or non - circular) light patterns generated by the irradiation system.

[0084] Next, the offset may be used to calibrate the scanning system 16a or 16b that was used to irradiate points on the calibration plate 116 during acquisition of the second image 140. In other words, the detected position 138 may be compared with the position of the light spot 142 to calibrate the printing system 100, particularly the optical scanning systems 16a, 16b of the irradiation system 10. These optical scanning systems 16a, 16b were used to irradiate the light spots on the calibration plate 116 when capturing the second image 140. Calibration may include the step of correcting any future positions at which the laser beam is irradiated via the calibrated scanning systems 16a, 16b by the offset.

[0085] The orientation of the calibration plate 116 may be determined based on an alignment image set including one or more images acquired by the camera 120. Next, the configuration may otherwise be implemented based on the orientation of the calibration plate 116, particularly the transformation between the FCS and the PCS. Two main variants for determining the orientation of the calibration plate 116 with respect to the FCS will be described below with reference to FIGS. 7a to 7f for the first variant and FIGS. 8a and 8b for the second variant. It should be noted that combining these two variants may result in a more accurate orientation of the calibration plate 116.

[0086] FIGS. 7a to 7f show details of the calibration plate 116. In particular, such figures each show various examples of one or more geometric elements 144 that may be provided on the calibration plate 116. The geometric elements 144 may be formed, for example, by selective laser ablation in substantially the same manner as the calibration markings 130.

[0087] In the example of FIG. 7a, three QR codes are carried on the calibration plate 116 as geometric elements 144, each arranged adjacent to a calibration mark 130. By detecting one or all of such 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 within the FCS can be determined. Each QR code may have a predetermined orientation (e.g., rotation) with respect to the calibration plate 116 (e.g., PCS). Thereby, the orientation of the calibration plate 116, particularly the rotation between the PCS and the FCS, can be determined. This rotation may be used during the calibration process (see FIGS. 5c, 6c, and 6d).

[0088] In the example of FIG. 7b, the geometric element 144 is composed of radial lines extending outward from the calibration pattern 130. In particular, four lines spaced 90° apart circumferentially from each other are provided as the geometric element 144. Thereby, it may be possible to detect a misalignment of less than 45° between the PCS and the FCS. Adding more (e.g., asymmetric) lines may make it possible to detect misalignments exceeding 45° between the PCS and the FCS unambiguously.

[0089] In the example of FIG. 7c, the geometric element 144 is a cross or plus shape formed by two orthogonal intersecting lines. Also, in this case, there may remain some ambiguity regarding the orientation of the calibration plate 116.

[0090] In the example of FIG. 7d, the geometric element 144 is a square surrounding one of the calibration marks 130. Also, in this case, there may remain some ambiguity regarding the orientation of the calibration plate 116. This ambiguity may be acceptable (e.g., when the calibration plate 116 cannot be placed within the construction area with a misalignment exceeding 45°), or it may be minimized by adding additional geometric elements 144 such as lines, points, etc.

[0091] In the example of FIG. 7e, the geometric element 144 is in an L-shape formed by two orthogonal lines joined at their 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.

[0092] In the example of FIG. 7f, there are three circles as the geometric element 144 on the calibration plate 116, each arranged adjacent to the calibration mark 130. Overall, the circles form a pattern with 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.

[0093] FIGS. 8a to 8b show a technique for determining the orientation of the calibration plate 116 using a plurality of reference elements 146 carried on the calibration plate 116. In this case, the orientation of the calibration plate 116 is determined based on a plurality of positions, rather than the plurality of orientations of the plurality of reference elements 146. In the illustrated example, five of the calibration elements 130 function as reference elements 146, one reference element 146-1 is at the center of the calibration plate 116, two reference elements 146-2, 146-3 are at both ends of the vertical central axis of the calibration plate 116, and two reference elements 146-4, 146-5 are at both ends of the horizontal central axis of the plate 116. As shown in FIG. 8a, three images are captured as an alignment image set, each containing a different one of the reference elements 146. Here, it is preferable that at least one of the images includes the central reference element 146-1. The positions of the reference elements 146 can be determined from the three captured images. Next, as schematically shown in FIG. 8b, the orientation of the calibration plate 116 with respect to the image system 118, particularly the rotational offset between the FCS and the PCS, may be determined based on the detected positions of the reference elements 146.

[0094] Calibration may be further performed based on the transformation between the coordinate systems FCS and SCS. FIGS. 9a and 9b show a technique for determining such a transformation, also referred to as "alignment" between the coordinate systems FCS and SCS.

[0095] In the technique of FIG. 9a, for example, as shown in FIG. 2, an on-axis camera 120 is used. While irradiating a point on the calibration plate 116 with a laser beam, an image is captured (e.g., a second image 140). In the FCS, a light spot 142-1 is detected in the captured image. Next, the field of view of the camera is shifted by a predetermined amount in a predetermined direction. In the example of FIG. 9a, it is shifted to the position [0,1] by the FCS. Next, while the laser beam is irradiating the same point on the calibration plate 116, another image is captured. Since the field of view of the camera 120 is shifted, the detected position of the light spot will be different here. In the example shown in FIG. 9a, the detected position of the light spot 142-2 in the FCS is offset by an amount Δx in the x-axis direction and by an amount 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 αx between the FCS and the SCS may be determined based on the detected positions of the light spots 142-1, 142-2. Thus, with this technique, the conversion (also called alignment) between the coordinate systems FCS and SCS can be determined. The FCS may be defined for 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 mark, geometric element or reference element shown by two images may be detected in the FCS.

[0096] In the technique of FIG. 9b, for example, as shown in FIG. 1, an off-axis camera 120 may be used. While irradiating a first point on the calibration plate 116 with a laser beam, an image is captured (e.g., a second image 140). In the FCS, a light spot 142-1 is detected in the captured image. Next, a second point on the calibration plate 116 is irradiated. The second point is offset from the first point by a predetermined amount in a predetermined direction. In the example of FIG. 9b, the second point is offset from the first point by an exemplary amount of 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 by the FCS. Next, a third point on the calibration plate 116 is irradiated. The third point is offset from the first point by another predetermined amount in another predetermined direction. In the example of FIG. 9b, the third point is offset from the first point by an exemplary amount of 1 in the Y-axis direction of the SCS. Thus, 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 the light spot 142-3 formed by the laser beam irradiating the third point is detected by the FCS. The detected positions of the light spots 142-1, 142-2, and 142-3 may be used to determine the orientation of the SCS with respect to the FCS. That is, the alignment between the SCS and the FCS may be determined based on the detected positions of the light spots 142-1, 142-2, and 142-3.

[0097] FIG. 10 shows a detailed flowchart of a first modification of this method. The calibration plate is called a scanning range correction plate (SFCP). The calibration mark 130 is called an SFCP circle. The optical scanning system of the laser beam is called a "scanner". The reference point may be a groove, a depression, or other feature of the calibration plate 116. An optional step is marked with a question mark. That is, the MCF is either used in step 3.3, used in step 4, or not used at all.

[0098] The MCF describes the geometric parameters of the calibration plate 116 measured using an external measurement system. For example, the MCF may describe the deviation between the theoretical geometric characteristics of the calibration plate 116 (e.g., the positions of features such as the calibration marks 130 and geometric elements 144 of the calibration plate 116) and the actual geometric characteristics of the calibration plate 116 measured using the external measurement system. For this reason, the MCF 116 may be considered as an "absolute reference" for the positions of the respective marks, elements, and features of the calibration plate 116 (e.g., with respect to the PCS defined by one or more reference points or position markers carried on the calibration plate 116).

[0099] The magnification offset data from each scanner corresponds to the calibration parameters used to calibrate each scanner according to the different configurations of the adjustment systems of the respective scanners. In the example of FIG. 10, the magnification offset data is determined in advance and is provided, for example, by the manufacturer of the optical scanning system or during use. The "Cal file" may correspond to a calibration file that defines the calibration parameters (e.g., the spatial offset to be applied during irradiation) of each optical scanning system of the printing system 100.

[0100] FIG. 11 shows a detailed flowchart of a second variant of the method. Different from FIG. 10, in this case, the magnification offset data is not determined in advance. For this reason, steps 3.4.2.1 and 3.4.2.2 are repeated for all magnification steps M, and each magnification step corresponds to a different configuration of the adjustment system of the respective optical scanning system or "scanner". Except for that difference, the method of FIG. 11 corresponds to the method of FIG. 10. In either method, the calibration of the printing system 100 is performed via the Cal file.

[0101] As is apparent from FIGS. 10 and 11, the calibration method described herein may be implemented for multiple components of the printing system 100. For example, calibration may be performed for different laser beams 14a, 14b of the irradiation system 10 and / or for different optical scanning systems 16a, 16b of the irradiation system 10, each configured to direct a different laser beam 14a, 14b to the build area, and / or for different sizes of points on the calibration plate 116 irradiated by the laser beams 14a, 14b, and / or for different shapes of points on the calibration plate 116 irradiated by the laser beams 14a, 14b, and / or for different configurations of the adjustment systems 13a, 13b of the irradiation system 10, each configuration resulting in different sizes and / or shapes of the points irradiated by the laser beams 14a, 14b, and / or for different first portions and / or different vertical positions of the build platform 102 disposed within the build area and carrying the calibration plate 116.

[0102] The method may be repeated after calibration has been performed. Here, 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. Depending on the result of the comparison with the offset threshold, another calibration may be performed as described herein. Upon verification that the difference between the detected positions is found to be lower than the predetermined offset threshold, preferably after removing the calibration plate 116 from the build platform 102, a three-dimensional workpiece may be manufactured. The three-dimensional workpiece so manufactured may have advantageous physical properties such as lower manufacturing tolerances and higher rigidity compared to workpieces manufactured using the printing system 100 before calibration was performed.

Claims

Claim 1 A calibration method for a printing system (100), wherein the printing system (100) is configured to manufacture a three-dimensional workpiece, and the method is implemented by a processor (18), (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), the first image (133) being captured by an imaging system (118) of the printing system (100), the first portion including at least one 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 within the first image (133); (c) controlling an irradiation system (10) of the printing system (100) to irradiate a point on the first portion of the calibration plate (116) disposed in the build area with a laser beam (14a, 14b); (d) obtaining a second image (140) of the first portion of the calibration plate (116) disposed in the build area, the second image (140) being captured by the imaging system (118), the second image including a light spot (142) formed by the laser beam (14a, 14b) irradiating the point on the first portion of the calibration plate (116) disposed in the build area; (e) detecting a position of the light spot (142) within the second image (140); (f) calibrating the printing system (100) based on the detected position (138) of the at least one portion within the first image (133) and the detected position of the light spot (142) within the second image (140). Claim 2 The method according to claim 1, wherein the printing system (100) is calibrated based on a comparison between the detected position (138) of the at least one portion within the first image (133) and the detected position of the light spot (142) within the second image (140). Claim 3 The method according to claim 1 or 2, wherein the first image (133) and the second image (140) target the same field of view and / or the alignment of the first part in the first image (133) is the same as the alignment of the first part in the second image (140).

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

5. Obtaining an alignment image set including one or more images of at least one part of the calibration plate (116) arranged in the construction area, the one or more images being captured by the imaging system (118); Determining the orientation of the calibration plate (116) based on the alignment image set, the method according to any one of claims 1 to 4, wherein the printing system (100) is calibrated based on the determined orientation of the calibration plate (116).

6. Detecting a geometric element (144) on the calibration plate in at least one of the images of the alignment image set; Determining the orientation of the detected geometric element (144) in the at least one image; Determining the orientation of the calibration plate (116) based on the determined orientation of the detected geometric element (144), the method according to claim 5.

7. Detecting a plurality of reference elements (146) of the calibration plate (116) based on the alignment image set; Determining the position of each detected reference element (146) based on the alignment image set; Determining the orientation of the calibration plate (116) based on the determined positions of the detected reference elements (146), the method according to claim 5 or 6.

8. The calibration is (i) for different laser beams (14a, 14b) of the irradiation system (10), (ii) for different optical scanning systems (16a, 16b) of the irradiation system (10), each configured to direct a different laser beam (14a, 14b) to the construction area (iii) for different sizes of the points on the calibration plate (116) irradiated with the laser beams (14a, 14b), (iv) for different shapes of the points on the calibration plate (116) irradiated with the laser beams (14a, 14b), (v) for different configurations of the adjustment systems (13a, 13b) of the irradiation system (10), each configuration resulting in different sizes and / or shapes of the points irradiated with the laser beams (14a, 14b), (vi) for different first portions, and / or (vii) for different heights of the construction platform (102) disposed within the construction area and carrying the calibration plate (116), the method according to any one of claims 1 to 7, wherein the method is implemented.

9. The method according to any one of claims 1 to 8, comprising the step of obtaining correction data indicating geometric parameters of the calibration plate (116) measured using an external measurement system, wherein the printing system (100) is calibrated based on the correction data.

10. (i) a coordinate system (SCS) of the optical scanning systems (16a, 16b) of the irradiation system (10), wherein the optical scanning systems (16a, 16b) are configured to direct the laser beams (14a, 14b) into the construction area, and (ii) determining a transformation between the coordinate system (SCS) and the coordinate system (FCS) of the imaging system (118), the method according to any one of claims 1 to 9, wherein the printing system (100) is calibrated based on the determined transformation.

11. 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 disposed in the construction area, the method according to any one of claims 1 to 10, wherein the method comprises controlling the imaging system (118) to capture the first image (133) while at least the first portion of the calibration plate (116) disposed in the construction area is illuminated by the illumination unit (126), and / or Controlling the imaging system (118) to capture the second image (140) while at least the first portion of the calibration plate (116) disposed in the construction area is not illuminated by the illumination unit (126), and / or Controlling the imaging system (118) to capture all images except the second image (140) while at least the first portion of the calibration plate (116) disposed in the construction area is illuminated by the illumination unit (126), the method comprising. **Claim 12** The imaging system (118) is arranged to capture at least the first image (133) and the second image (140) via the optical scanning systems (16a, 16b) included in the irradiation system (10), and the optical scanning systems (16a, 16b) are configured to direct the laser beams (14a, 14b) to the construction area. The method according to any one of claims 1 to 11. **Claim 13** A printing system (100) for manufacturing a three-dimensional workpiece, An irradiation system (10) configured to selectively irradiate one or more laser beams (14a, 14b) in a construction area; An imaging system (118) configured to capture an image of at least a part of the calibration plate (116) when the calibration plate (116) is disposed in the construction area; A processor (18), (a) Obtaining a first image (133) of a first portion of the calibration plate (116) disposed in the construction area, the first image (133) being captured by the imaging system (118), the first portion including at least one part of at least one calibration mark (130) carried by the calibration plate (116); (b) Detecting the position (138) of the at least one part in the first image (133); (c) Controlling the irradiation system (10) to irradiate a point on the first portion of the calibration plate (116) disposed in the construction area with one of the one or more laser beams (14a, 14b); Step of obtaining a second image (140) of the first portion of the calibration plate (116) disposed in the construction area, the second image (140) being captured by the imaging system (118), wherein the second image (140) includes a light spot (142) formed by one of the one or more laser beams (14a, 14b) that irradiate the point on the first portion of the calibration plate (116) disposed in the construction area, Step of detecting the position of the light spot (142) within the second image, A processor (18) configured to perform the steps of calibrating the printing system (100) based on the detected position (138) of the at least one portion within the first image (133) and the detected position of the light spot (142) within the second image (140). A printing system (100) comprising:

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. One or more optical scanning systems (16a, 16b), each configured to direct a different one of the laser beams (14a, 14b) to the construction area, An adjustment system (13a, 13b) having a plurality of configurations, each configuration providing a different size and / or shape of the point irradiated by the laser beam (14a, 14b), A construction platform (102) disposed within the construction area and configured to carry the calibration plate (116), An illumination unit (126) configured to illuminate at least the first portion of the calibration plate (116) when the calibration plate is disposed in the construction area, An on-axis camera (120), The printing system (100) according to claim 13 or 14, further comprising at least one of the calibration plate (116).

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