Technique for determining a pose of a calibration element arranged in an additive manufacturing apparatus

Abstract

Disclosed is a method for determining a pose of a calibration element, arranged in an irradiation area of an additive manufacturing apparatus configured to produce three-dimensional workpieces by selective layer-wise irradiation of a raw material. An irradiation beam is scanned along predefined scan paths across the calibration element, each scan path having two intersection points with scattering features of the calibration element. For each scan path, a time-dependent intensity of irradiation scattered by the calibration element is detected. The intensities are analyzed to identify intensity peaks. The pose of the calibration element is determined at which the identified intensity peaks match the intersection points between the scan paths and the scattering features. A system is also disclosed herein.

Description

[0001] Nikon SLM Solutions AG - 1 - 30A-168 961

[0002] Technique for determining a pose of a calibration element arranged in an additive manufacturing apparatus

[0003] The present disclosure generally relates to a method for determining a pose of a calibration element arranged in an irradiation area of an additive manufacturing apparatus, and to a system.

[0004] As understood herein, an additive manufacturing apparatus is configured to produce three-dimensional workpieces by selective layer-wise irradiation of a raw material. The raw material may be a raw material powder such as a metal powder, a metal alloy powder or a polymer powder. The irradiation of the raw material may be performed with at least one irradiation beam, for example a laser beam. In such an apparatus, a first layer of raw material can be deposited on a height-adjustable build platform, and the irradiation beam can then be scanned across portions of the raw material to be solidified that lie within an irradiation area reachable by the irradiation beam. The energy that is input by the irradiation beam leads to a heating of the raw material at the irradiation positions, which, in turn, leads to melting and / or sintering of the raw material within the scanned portions. One may say that a corresponding additive manufacturing apparatus is configured to perform selective laser melting and / or selective laser sintering. The material then cools down, thereby forming a hardened volume of a to-be-produced workpiece. After such a selective solidification of portions of a raw material layer, the build plate is lowered by a given distance (e.g., 100pm), another raw material layer is deposited atop the previously irradiated layer, and the another layer is then also irradiated where needed. In this manner, a three-dimensional workpiece can be produced layer by layer.

[0005] Known additive manufacturing apparatuses comprise one or more optical scanning units, each configured to scan an irradiation beam across an irradiation area of the apparatus. By controlling these optical scanning units, irradiation beams can be guided to intended positions on the raw material layers. Due to manufacturing tolerances, deformation of components of the apparatus over time, thermal expansion of components of the apparatus, and other factors, an irradiated position on the raw material layer, i.e., an impingement point of an irradiation beam, may not correspond to an intended position. For example, there may be a mismatch between a coordinate system of an optical scanning unit of the apparatus and a coordinate system of the powder layer or process chamber containing said powder layer. A calibration element may be used to calibrate the apparatus in order to compensate said mismatch. In this Nikon SLM Solutions AG - 2 - 30A-168 961 case, a pose of the calibration element could be determined, for example relative to the coordinate system of the optical scanning unit.

[0006] In view of the above, there remains a need for a technique for determining a pose of a calibration element arranged in an irradiation area of an additive manufacturing apparatus. The technique should preferably be quick and accurate.

[0007] According to a first aspect of the present disclosure, a method for determining a pose of a calibration element, arranged in an irradiation area of an additive manufacturing apparatus configured to produce three-dimensional workpieces by selective layer-wise irradiation of a raw material, is provided. The method comprises controlling the apparatus such that an irradiation beam is scanned along a plurality of predefined scan paths across the calibration element, wherein each scan path has at least two intersection points with one or more scattering features of the calibration element. The method comprises, for each of the plurality of predefined scan paths: {i} detecting, for each of a plurality of different points in time during scanning of the scan path, an intensity of irradiation scattered by the calibration element, and {ii} analyzing the detected intensities to identify intensity peaks associated with the at least two intersection points of the scan path. The method comprises determining a pose of the calibration element at which the identified intensity peaks match the intersection points between the scan paths and the one or more scattering features of the calibration element.

[0008] The pose of the calibration element may comprise at least one of a position and an orientation. The pose of the calibration element may be determined in at least or exactly two dimensions, for example in two dimensions covering all (e.g., x- and y-) scanning directions of the irradiation beam. The pose of the calibration element may be determined in a coordinate system of an optical scanning unit of the apparatus that guides the irradiation beam along the plurality of predefined scan paths.

[0009] The calibration element may be arranged such that its entire upper surface lies within the irradiation area. The irradiation area may correspond to an area that can be irradiated by the apparatus (e.g., reached by the irradiation beam(s)). Said area may lie in or lie parallel to the x-y-plane of the coordinate system of the optical scanning unit. The irradiation area may also be referred to as a build area or a manufacturing area. The irradiation area may correspond to or enclose an outline of a build plate of the apparatus (e.g., when viewed along the z-direction). A surface of the calibration Nikon SLM Solutions AG - 3 - 30A-168 961 element may lie essentially normal to a z-direction that is orthogonal to both the x- direction and the y-direction of the coordinate system of the optical scanning unit.

[0010] Controlling the apparatus such that an irradiation beam is scanned along the plurality of predefined scan paths across the calibration element may comprise controlling the (e.g., at least one) optical scanning unit of the apparatus to guide the irradiation beam along the plurality of predefined scan paths. Controlling the apparatus accordingly may comprise controlling at least one beam source such as a laser beam source to emit the irradiation beam.

[0011] In one example, the irradiation beam that is scanned along the plurality of predefined scan paths is also configured to selectively solidify the base material when it is scanned across (e.g., selected portions of) the base material. Alternatively, the irradiation beam that is scanned along the plurality of light paths may differ from an irradiation beam configured to selectively solidify the base material, for example in laser power, wavelength and / or beam shape. The irradiation beam that is scanned along the plurality of light paths may in this case be referred to as a guide beam, which for example has a wavelength perceptible by a human eye.

[0012] As each scan path has at least two intersection points with one or more scattering features of the calibration element, one may say that each scan path crosses the scattering features multiple times. When scanning the irradiation beam (e.g., the guide beam) across the calibration element along a predefined scan path, the irradiation beam may cross the one or more scattering features at a plurality of (i.e., at least two) different time points.

[0013] The calibration element may be formed as a calibration plate and the one or more scattering features may be arranged on a same surface of said calibration plate. Each scattering feature may be configured to scatter light that falls thereon. Each of the one or more scattering features may be bounded by portions of the calibration element that exhibit lower light scattering. Each scattering feature may have a degree of light scattering that falls within a predefined range and / or that is higher (e.g., by a predefined minimum amount) compared with a degree of light scattering of portions of the calibration element that surround the respective scattering feature. A scattering feature may comprise or consist of an area on the calibration element having a rougher surface than portions of the calibration element that surround the respective scattering feature. The rougher surface may be formed by selectively etching the calibration element and / or by depositing particles onto the calibration element. The scattering Nikon SLM Solutions AG - 4 - 30A-168 961 features may be arranged in a pattern. The scattering features of the calibration element may be arranged in groups, each group forming a pattern. Examples of such patterns include a spiral, concentrically arranged circles and a plurality of intersecting lines (e.g., forming a T-shape, an X-shape, a H-shape or a (e.g., rectangular) grid).

[0014] In one example, the calibration element is formed as a component that is (e.g., substantially) transparent for light having the wavelength or range of wavelengths of the irradiation beam (e.g., the guide beam) that is scanned along the plurality of predefined scan paths. The calibration element may comprise a glass substrate or a ceramic substrate. For example, the calibration element may be formed of glass or a ceramic material. Compared with calibration elements made from metal, such a calibration element has a lower thermal coefficient and cannot be significantly bent without breaking. Thus, a risk of degradation of accuracy of the method disclosed herein due to an (e.g., unnoticed) deformation of the calibration element can be reduced further.

[0015] The step of detecting, for each of a plurality of different points in time during scanning of the scan path, an intensity of irradiation scattered by the calibration element, may be referred to as a time-dependent intensity measurement. The plurality of different points in time may be referred to as measurement time points or detection time points. The plurality of different points in time may be equidistant. The number of detection time points per time interval may be referred to as sampling rate. The sampling rate may be above 100 / s, above 1000 / s or even above 10.000 / s. The detected intensity of irradiation may be specific for the scanned irradiation beam. The intensity of irradiation of scattered light may be detected separately for each irradiation beam.

[0016] Analyzing the detected irradiation intensities may comprise applying data filtering thereto, for example to filter out outliers and / or detected intensities that fulfil one or more filtering criteria (e.g., lie above a predefined maximum intensity value or below a predefined minimum intensity value). Analyzing the detected irradiation intensities may comprise selecting detected intensities that fall into a timespan that presumably matches to the time the irradiation beam crossed the one or more scattering features, for example using a masking or window selection approach.

[0017] Analyzing the detected irradiation intensities may alternatively or subsequently comprise generating a plot or function representing the (e.g., selected and / or not filtered-out) detected irradiation intensities for the different detection time points, and identifying the peaks of said plot or function as the intensity peaks. Each of the Nikon SLM Solutions AG - 5 - 30A-168 961 intensity peaks that is identified based on the irradiation intensities detected for a given scan path is associated with one of the intersection points between said given scan path and the one or more scattering features. Detected irradiation intensities may be identified as part of such an intensity peak if they fall above a predefined threshold value and / or if they are higher (e.g., by at least predefined amount) compared to a reference such as the lowest detected intensity, an average of all intensities or an average of all intensities falling below a predefined threshold. Another possible condition for detecting a peak may be that the peak shall be associated with a predefined time width and / or scan distance.

[0018] As mentioned above, the method comprises determining a pose of the calibration element at which the identified intensity peaks match the intersection points between the scan paths and the one or more scattering features of the calibration element. The determination of the pose of the calibration element may be based on geometric properties of the calibration element, including, for example, a size and / or shape and / or position and / or orientation of one or more (e.g., each) of the scattering elements of the calibration element. The size and / or shape and / or position and / or orientation of each scattering element may be defined relative to a first common reference (e.g., a coordinate system of the calibration element). The determination of the pose of the calibration element may be based on scan path properties, including, for example, a length and / or shape and / or position and / or orientation of one or more (e.g., each) of the plurality of predefined scan paths. The length and / or shape and / or position and / or orientation of each scan path may be defined relative to a second common reference (e.g., a coordinate system of the optical scanning unit of the apparatus via which the irradiation beam is guided along the predefined scan paths). The coordinate system of the optical scanning unit may be associated with an intended and / or expected impingement point of the irradiation beam on a powder layer or an object (e.g., the calibration element) arranged in the irradiation area.

[0019] Determining the pose of the calibration element may comprise determining positions of the intersection points relative to the first common reference and the second common reference. Determining the pose of the calibration element may comprise determining positions of the intersection points in the coordinate system of the calibration element and determining positions of the intersection points in the coordinate system of the optical scanning unit.

[0020] Determining the pose of the calibration element may comprise determining a pose of the first common reference relative to the second common reference (e.g., by Nikon SLM Solutions AG - 6 - 30A-168 961 matching the positions of the intersection points determined relative to the first and the second common reference). Determining the pose of the calibration element may comprise determining a pose of the coordinate system of the calibration element relative to the coordinate system of the optical scanning unit (e.g., by matching the positions of the intersection points determined in the coordinate system of the calibration element and in the coordinate system of the optical scanning unit). Each of these relative poses may be expressed as a transformation function and / or a transformation matrix between the two poses.

[0021] The method may comprise, for each of the plurality of predefined scan paths, determining at least one parameter selected from {a} a time difference between the identified intensity peaks and {b} a scan distance between the identified intensity peaks. The positions of the intersection points and / or the pose of the calibration element may be determined based on the at least one parameter and, optionally, based on the geometric properties of the calibration element and / or the scan path properties.

[0022] The time differences comprised in the at least one parameter may be normalized relative to a common reference scanning velocity (e.g., before the time differences are used for determining the positions of the intersection points and / or the pose of the calibration element based thereon). The common reference scanning velocity may be predefined. The common reference scanning velocity may correspond to a minimum, an average or a maximum (e.g., intended, observed and / or predefined) speed of travel of the impingement point of the scanned irradiation beam across the calibration element.

[0023] Alternatively, or in addition, the scan distances comprised in the at least one parameter may be determined based on an actual scanning velocity. The actual scanning velocity may correspond to an actual (e.g., observed and / or predefined) speed of travel of the impingement point of the scanned irradiation beam across the calibration element.

[0024] The method may further comprise selecting at least one of the scan paths for which the at least one parameter is an extremum (e.g., has a highest or lowest value) compared with the parameters (e.g., of the same parameter type) of the other scan paths of the plurality of predefined scan paths. For example, the method may comprise selecting one of the scan paths for which the (e.g., normalized) time difference is the smallest among the (e.g., normalized) time differences of all scan paths. As another example, the method may comprise selecting one of the scan paths for which the (e.g., Nikon SLM Solutions AG - 7 - 30A-168 961 normalized) time difference is the largest among the (e.g., normalized) time differences of all scan paths. As another example, the method may comprise selecting one of the scan paths for which the scan distance is the smallest among the determined scan distances of all scan paths. As another example, the method may comprise selecting one of the scan paths for which the scan distance is the largest among the determined scan distances of all scan paths.

[0025] The pose of the calibration element may be determined based on a location of the selected at least one scan path. This location may correspond to at least one of a position and an orientation of the selected at least one scan path. This location may be defined relative to the second common reference and / or in the coordinate system of the optical scanning unit. This location may be indicative of an x- and / or y-position in the coordinate system of the optical scanning unit.

[0026] Alternatively, the method may comprise estimating at least one location of a (e.g., virtual) scan path for which the at least one parameter would be an extremum (e.g., has a highest or lowest value) compared with the parameters (e.g., of the same parameter type) of the scan paths of the plurality of predefined scan paths. This (e.g., virtual) scan path may have a similar shape and / or size and / or length and / or orientation as the scan paths of the plurality of predefined scan paths. This (e.g., virtual) scan path may differ from the plurality of predefined scan paths only in its location (e.g., in x- and / or y- coordinates). This (e.g., virtual) scan path may lie (e.g., parallel to and) between neighboring scan paths of the plurality of predefined scan paths. For example, the method may comprise estimating a location of a (e.g., virtual) scan path for which the (e.g., normalized) time difference would be the smallest among the (e.g., normalized) time differences of all scan paths. As another example, the method may comprise estimating a location of a (e.g., virtual) scan path for which the (e.g., normalized) time difference would be the largest among the (e.g., normalized) time differences of all scan paths. As another example, the method may comprise estimating a location of a (e.g., virtual) scan path for which the scan distance would be the smallest among the determined scan path lengths of all scan paths. As another example, the method may comprise estimating a location of a (e.g., virtual) scan path for which the scan distance would be the largest among the determined scan distances of all scan paths.

[0027] The pose of the calibration element may be determined based on the estimated at least one location. This estimated location may correspond to at least one of an estimated position and an estimated orientation of the scan path. This estimated Nikon SLM Solutions AG - 8 - 30A-168 961 location may be defined relative to the second common reference and / or in the coordinate system of the optical scanning unit.

[0028] Determining the pose of the calibration element may comprise identifying a portion of the calibration element that is associated with the selected at least one scan path or with the scan path the location of which was estimated. This identified portion may have a size and / or length and / or shape that corresponds to a size and / or length and / or shape of the selected at least one scan path or of the scan path the location of which was estimated. Determining the pose of the calibration element may comprise determining a pose of the identified portion of the calibration element relative to the first common reference and / or in the coordinate system of the calibration element. Determining the pose of the calibration element may comprise matching the pose of the identified portion of the calibration element to the location of the at least one selected scan path or to the estimated location of the scan path.

[0029] The one or more scattering features may comprise two scattering features that each extend longitudinally along an axis and intersect one another. The one or more scattering features may be arranged on a same surface of the calibration element. The axes may intersect one another on the (e.g., same surface of the) calibration element. The at least two scattering features may form a V-shape, a T-shape or an X- shape. The two scattering features may intersect one another at a common intersection point. This common intersection point may define the origin of the coordinate system of the calibration element. The direction of the x- and y-axis of the coordinate system of the calibration element may be defined by the one or more scattering features, in particular by the axes thereof. For example, the x-axis and the y-axis may each extend at an angle of 45° relative to the axes of the scattering features.

[0030] Each of the plurality of predefined scan paths may be defined in the coordinate system of the optical scanning unit. Each of the plurality of predefined scan paths may extend in the x-y-plane of the coordinate system of the optical scanning unit.

[0031] The plurality of scan paths may comprise or consist of a first plurality of parallel linear scan paths and a second plurality of parallel linear scan paths. The second plurality of parallel linear scan paths may be oblique to the first plurality of parallel linear scan paths. An angle between the scan paths of the first and second plurality of parallel linear scan paths may for example correspond to 45°, 60° or 90°. The first plurality of scan paths may each extend parallel to an x-axis of the coordinate system of the optical Nikon SLM Solutions AG - 9 - 30A-168 961 scanning unit. The second plurality of scan paths may each extend parallel to a y-axis of the of the optical scanning unit. One of the first plurality of scan paths may lie on the x-axis of the coordinate system of the optical scanning unit. One of the second plurality of scan paths may lie on the y-axis of the coordinate system of the optical scanning unit.

[0032] The plurality of scan paths may comprise or consist of a plurality of concentrically arranged circular scan paths. The radius may differ between these circular scan paths (e.g., by a predefined amount). The circular scan paths may have a common centerpoint. This centerpoint may correspond to the origin of the coordinate system of the optical scanning unit.

[0033] The plurality of scan paths may comprise or consist of a set of scan paths that (e.g., together) form a spiral. Each of the set of scan paths may form a section of the spiral. A centerpoint of the spiral, an innermost end of the spiral or an outermost end of the spiral may correspond to the origin of the coordinate system of the optical scanning unit.

[0034] In one particular variant, the one or more scattering features comprise at least one reference feature. The reference feature is configured such that an intensity peak generated upon a laser beam scanning across the at least one reference feature can be identified among a plurality of detected intensity peaks. This / these intensity peak(s), also referrable to as reference peak(s), may be used as a reference for aligning the detected intensity peaks and / or compensating for different scanning speeds (e.g., differing from one scan path to another). In one example, the at least one reference feature comprises two or more (e.g., parallel) lines. At least one of these two or more lines may be oriented oblique, for example orthogonal, to the plurality of scan paths.

[0035] The method may be performed multiple times, each time for a different plurality of predefined scan paths. The pluralities of scan paths may differ in their location (e.g., in the coordinate system of the optical scanning unit(s) and / or relative to the calibration element). In this way, a plurality of poses of the calibration element can be determined. The plurality of poses may be indicative of and / or be used to determine a plurality of coordinate transformations between the coordinate system of the calibration element and the coordinate system of the optical scanning unit. This plurality of transformations may be expressed as a function or a matrix and can be referred to as a scanfield correction. One may thus say that the plurality of poses can Nikon SLM Solutions AG - 10 - 30A-168 961 be used to determine a scan field distortion and / or a scan field correction. In this way, parameters such as gain, offset, rotation and local irregularities can be compensated.

[0036] The method may comprise calibrating the apparatus based on the determined pose(s) of the calibration element. The method may comprise determining a calibration function or calibration matrix based on the determined pose of the calibration element. The determined pose of the calibration element may be indicative of the calibration function (e.g., coordinate transformation between the coordinate system of the optical scanning unit and the coordinate system of the calibration element) or calibration matrix (e.g., transformation matrix).

[0037] In one example, the additive manufacturing apparatus comprises an (e.g., the) optical scanning unit configured to scan the irradiation beam along the plurality of predefined scan paths across the calibration element. The method may further comprise calibrating the (e.g., at least or exactly one) optical scanning unit based on the determined pose of the calibration element. Calibrating the optical scanning unit may comprise determining a calibration parameter, function or matrix that, when applied to a subsequent position control of the optical scanning unit, results in the position of the impingement point of the irradiation beam falling onto the desired position (e.g., as defined in the coordinate system of the calibration element). Calibrating the optical scanning unit may comprise compensating for a difference between the pose of the first common reference and the second common reference and / or compensating for a difference between the pose of the coordinate system of the calibration element and the pose of the coordinate system of the optical scanning unit and / or applying the (e.g., inverse of the) transformation function and / or the (e.g., inverse of the) transformation matrix to subsequent scan paths.

[0038] In one example, the additive manufacturing apparatus comprises a plurality of optical scanning units, each configured to scan an irradiation beam along a or the plurality of predefined scan paths across the calibration element. One or more or all method steps may be performed for each of the plurality of optical scanning units, for example such that the pose of the calibration element is separately determined for each optical scanning unit. Two or more of the optical scanning units may be controlled to simultaneously scan the respective irradiation beam along the respective plurality of scan paths. In this case, the intensity of scattered light may be detected separately for each of the irradiation beams scanned by the two or more optical scanning units. Nikon SLM Solutions AG - 11 - 30A-168 961

[0039] In one particular variant, a plurality of irradiation beams may be scanned simultaneously by the plurality of scanning units. The plurality of irradiation beams may be scanned such that they intersect calibration features of the calibration element at different points in time. The plurality of irradiation beams may be simultaneously scanned (i) across separate regions of the calibration element and / or (ii) along different scan paths, for example along different ones of the same plurality of scan paths scanned by each optical scanning unit.

[0040] The method may further comprise calibrating the plurality of optical scanning units based on the poses of the calibration element that have been separately determined for each optical scanning unit. This may comprise calibrating the plurality of optical scanning units relative to one another and / or relative to a common reference (e.g., relative to the pose of the calibration element, relative to the first reference and / or relative to a pose of the coordinate system of the calibration element).

[0041] The method may comprise determining a three-dimensional pose of the calibration element based on the determined two-dimensional pose of the calibration element, based on the scan distance(s) and based on an (e.g., angular) scanning speed of the optical scanning unit. Once the two-dimensional pose of the calibration element has been determined, the alignment of the scan paths relative to the scattering features is known. This means that the scan distance between intersection points of the laser light beam and the scattering features can be determined based on the known geometric properties of the calibration element and / or the scattering features. The time difference between the two time points at which the laser light beam following a scan path intersects the scattering feature(s) can be derived from the detected light intensities. As the (e.g., angular) scanning speed of the optical scanning unit during scanning of the laser light beam along said scan path is also known, a distance be said scan path to the optical scanning unit can be determined. In combination with the previously determined two-dimensional pose of the calibration element, this may yield the three-dimensional pose of the calibration element.

[0042] The method may comprise determining a three-dimensional pose and / or position of the calibration element based on the (e.g., three-dimensional) poses and / or positions of the calibration element that have been separately determined for each optical scanning unit and, optionally, based on known relative poses between the optical scanning units. The three-dimensional position may thus be determined based on triangulation. Nikon SLM Solutions AG - 12 - 30A-168 961

[0043] The additive manufacturing apparatus may comprise a process chamber. One or more or all method steps may be performed for a plurality of different relative poses between the calibration element and the process chamber.

[0044] For example, the calibration element is fixedly arranged relative to a build plate carrier or a build plate of the additive manufacturing apparatus. The build plate carrier or build plate may be height-adjustable relative to the process chamber. The plurality of different relative poses between the calibration element and the process chamber may be defined by different height positions of the build plate carrier or build plate relative to the process chamber. The different height positions of the build plate carrier or build plate may be referred to as different z-positions.

[0045] In one particular variant, the calibration element is fixedly arranged relative to a component of the additive manufacturing apparatus such as a (e.g., the) build plate carrier or a (e.g., the) build plate. The determined pose of the calibration element may then be used to determine a pose of said component.

[0046] The scanning of the irradiation beam along the plurality of scan paths may take place during manufacturing of a three-dimensional workpiece. The method may comprise controlling the optical scanning unit such that the irradiation beam is scanned across a first portion of a raw material powder layer (e.g., to solidify the same), and is then (e.g., continuously) scanned along the plurality of predefined scan paths across the calibration element. Alternatively, or in addition, the method may comprise controlling the optical scanning unit such that the irradiation beam is scanned along the plurality of predefined scan paths across the calibration element and is then scanned across a second portion of a raw material powder layer (e.g., to solidify the same).

[0047] For example, the calibration element is fixedly arranged relative to a powder deposition unit of the additive manufacturing apparatus. The powder deposition unit may be configured to move across at least a part of the irradiation area when depositing (e.g., a to-be-irradiated layer of) raw material powder. The calibration element may be arranged on an upper surface of the powder deposition unit that faces the optical scanning unit. The scanning of the irradiation beam across the calibration element may be performed while the powder deposition unit is positioned within the irradiation area, while the powder deposition unit is depositing raw material powder and / or or while the powder deposition unit is moving (e.g., through the irradiation area). The scanning of the irradiation beam along the plurality of scan paths may thus take place while a to-be-irradiated raw material powder layer is being deposited. Nikon SLM Solutions AG - 13 - 30A-168 961

[0048] The method may further comprise determining a (e.g., two-dimensional) pose of a surface of a raw material powder layer (e.g., in the coordinate system of at least one of the optical scanning units). In case a three-dimensional pose of the surface is determined, this pose may be indicative of a surface level or z-height of the raw material powder layer. The pose of the surface of the raw material powder layer may be determined based on the (e.g., three-dimensional) pose and / or position of the calibration element and, optionally, based on a known relative distance between the calibration element and the surface of the raw material powder layer. The known relative distance may be defined by an attachment position of the calibration element relative to (e.g., a powder-spreading and / or surface-forming element of) the powder deposition unit.

[0049] Multiple poses of the surface of the raw material powder layer may be determined based on a plurality of poses and / or positions of the calibration element fixedly arranged relative to the powder deposition unit. In this case, each pose and / or position of the calibration element may be associated with a different position of the powder deposition unit along a movement path of the powder deposition unit associated with a deposition of a raw material powder layer. A plurality of poses of the calibration element fixedly arranged relative to the powder deposition unit may be determined, and, for each of said poses, a pose of the surface of the deposited raw material powder layer (e.g., a surface layer level of the deposited raw material powder layer) may be determined. Said poses of the surface may be used to determine an estimation of the shape of the surface of the deposited powder layer.

[0050] The method may comprise calibrating the powder deposition unit based on the determined pose(s) of the calibration element and / or the determined pose(s) of the surface of the raw material powder layer and / or the determined shape of the surface of the deposited powder layer. This may comprise actively changing a pose or configuration of the powder deposition unit to compensate an offset from an intended (e.g., predefined) optimal pose or configuration, and / or applying an offset compensation factor to a subsequent control of the powder deposition unit. Alternatively, or in addition, the method may comprise calibrating at least one of the optical scanning units of the apparatus based on at least one determined pose of the calibration element fixedly arranged relative to the powder deposition unit and / or based on the determined pose(s) of the surface of the raw material powder layer. This may comprise applying an offset compensation factor and / or a correction to a subsequent control of the at least one optical unit. Nikon SLM Solutions AG - 14 - 30A-168 961

[0051] The method may further comprise controlling (e.g., at least one optical scanning unit of) the apparatus such that an irradiation beam is scanned along at least one scan path across the calibration element, wherein the at least one scan path crosses one or more scattering features of the calibration element that are part of a (e.g., two- dimensional) code such as a barcode or a Quick Response, QR, code. The method may comprise detecting, for each of the at least one scan path, and for each of a plurality of different points in time during scanning of said scan path, an intensity of irradiation scattered by the calibration element. The method may comprise analyzing the detected intensities to determine a content of the code. In short, when scanning the irradiation beam across the code that is formed by the scattering features, the content of the code can be determined based on the detected intensities of scattered light.

[0052] The code may contain (e.g., encode) information defining at least one geometric property of the calibration element and / or a type of the calibration element and / or a configuration of the one or more scattering features to be crossed by scan paths of the plurality of predefined scan paths. The code may contain (e.g., encode) the geometric properties of the calibration element described herein above and / or information indicative of a location of the common intersection point on the calibration element and / or information indicative of locations of a plurality of common intersection points of the scattering features of the calibration element (e.g., relative to a common reference such as an origin of the coordinate system of the calibration element). The information encoded in the code may be used in the subsequent steps of the method.

[0053] For example, the calibration element is configured to extend throughout the entire irradiation area of the additive manufacturing apparatus. For example, the surface of the calibration element that faces the one or more optical scanning units may span an area of at least 10cm x 10cm, at least 30cm x 30cm or at least 60cm x 60cm. The calibration element may be configured such that each optical scanning unit of the apparatus can guide an irradiation beam thereon (e.g., at the same time and / or without having to reposition the calibration element).

[0054] The intensity of scattered irradiation may be detected by at least one optical detector. The at least one optical detector may comprise or consist of a light-detecting diode or transistor. The at least one optical detector may be configured as a light sensor, in particular a light sensor having no spatial resolution. In one example, the at least one optical detector comprises or consists of a detector that is arranged within the process chamber of the apparatus. In one example, the at least one optical detector comprises Nikon SLM Solutions AG - 15 - 30A-168 961 or consists of a detector that is arranged in a build cylinder mounted on the process chamber. In one example, the at least one optical detector comprises or consists of a detector that is arranged such that it captures at least a portion of the irradiation area via an optical scanning unit of the apparatus. In this case, the detector may be referred to as an "on-axis sensor". In one example, the at least one optical detector comprises or consists of a detector that is arranged such that it captures at least a portion of the irradiation area without using an optical path of an optical scanning unit of the apparatus. In this case, the detector may be referred to as an "off-axis sensor". In one example, the at least one optical detector comprises or consists of a detector that includes an optical fiber communicatively coupled to a photodiode. In one example, the at least one optical detector comprises or consists of a detector that is arranged on an opposite side of the calibration element compared to the scanned irradiation beam. The at least one optical detector may be part of the apparatus or part of a system comprising the apparatus.

[0055] The calibration element may comprise one or more waveguides. Such a waveguide may be embedded within the calibration element or arranged on a surface thereof. The calibration element may comprise, for each of (e.g., the) at least one optical detector, one or more waveguides configured to guide irradiation scattered by a scattering feature of the calibration element towards the respective optical detector. Each waveguide may be associated with a predefined portion of the calibration element. For example, each waveguide may be associated with one or more scattering features arranged within said predefined portion of the calibration element. Each waveguide may be configured to (e.g., exclusively) guide light that is scattered on (e.g., scattering feature(s) arranged within) the associated predefined portion towards (e.g., a distinct) one of the at least one optical detector. Multiple optical scanning units of the apparatus may simultaneously scan irradiation beams across different ones of these portions of the calibration element. In this case, each of the different pluralities of scan paths scanned by the different optical scanning units may be associated with a different one of these portions of the calibration element. Different detectors, each associated with one of these portions, may be used to detect the light intensities of scattered light separately for each of the irradiation beams.

[0056] The method of the first aspect may be performed by at least one processor. The present disclosure provides for a computer program storing instructions which, when executed by at least one processor, causes the at least one processor to perform the method of the first aspect. The computer program may be carried by at least one Nikon SLM Solutions AG - 16 - 30A-168 961 carrier such as a data stream or a (e.g., non-transitory) computer-readable storage medium.

[0057] According to a second aspect, a system is provided. The system comprises an additive manufacturing apparatus configured to produce three-dimensional workpieces by selective layer-wise irradiation of a raw material. The system further comprises at least one calibration element arranged in an irradiation area of the apparatus and having one or more scattering features. The system comprises at least one optical detector configured to detect an intensity of irradiation scattered by the calibration element at different points in time. The system further comprises (e.g., the) at least one processor that is configured to perform the method of the first aspect.

[0058] For sake of brevity, only some of the features discussed with reference to the first aspect are repeated herein below. However, it is to be noted that one or more or all (e.g., optional) features described herein above for the method according to the first aspect may be provided in the system according to the second aspect and vice versa.

[0059] The one or more scattering features of the calibration element may comprise two scattering features that each extend longitudinally along an axis, the axes intersecting one another on the calibration element.

[0060] In one particular variant, the one or more scattering features may comprise two or more parallel lines oriented oblique, for example orthogonal, to the plurality of scan paths.

[0061] The additive manufacturing apparatus may comprise at least one optical scanning unit configured to scan the irradiation beam along the plurality of predefined scan paths across the calibration element.

[0062] The additive manufacturing apparatus may comprise a plurality of optical scanning units, each configured to scan an irradiation beam along a plurality of predefined scan paths across the calibration element.

[0063] The additive manufacturing apparatus may comprise a process chamber and the calibration element may be configured to be arranged in different relative poses relative to the process chamber. Nikon SLM Solutions AG - 17 - 30A-168 961

[0064] The additive manufacturing apparatus may comprise a build plate carrier or a build plate that is height-adjustable relative to the process chamber. The calibration element may be fixedly arranged relative to the build plate carrier or the build plate.

[0065] In one particular variant, the calibration element may be fixedly arranged relative to a component of the additive manufacturing apparatus such as a (e.g., the) build plate carrier or a (e.g., the) build plate.

[0066] The additive manufacturing apparatus may comprise a powder deposition unit configured to move across the irradiation area when depositing raw material powder and the calibration element is fixedly arranged relative to the powder deposition unit.

[0067] One or more scattering features of the calibration element may be part of a code, the code optionally containing information defining at least one geometric property of the calibration element and / or a type of the calibration element and / or a configuration of the one or more scattering features to be crossed by scan paths of the plurality of predefined scan paths.

[0068] The calibration element may be configured to extend throughout the entire irradiation area of the additive manufacturing apparatus.

[0069] The calibration element may comprise, for each of at least one optical detector, one or more waveguides configured to guide irradiation scattered by a scattering feature of the calibration element towards the respective optical detector.

[0070] The at least one optical detector may comprise a detector that is arranged within a process chamber of the apparatus and / or a detector that is arranged in a build cylinder mounted on the process chamber and / or a detector that is arranged such that it captures at least a portion of the irradiation area via an optical scanning unit of the apparatus (e.g., the optical scanning unit that is simultaneously scanning the irradiation beam across the calibration element, or another optical scanning unit of the apparatus) and / or a detector that includes an optical fiber communicatively coupled to a photodiode and / or a detector that is arranged on an opposite side of the calibration element compared to the scanned irradiation beam.

[0071] The calibration element may comprise, for each of at least one optical detector, one or more waveguides configured to guide irradiation scattered on a scattering feature towards the respective optical detector. Nikon SLM Solutions AG - 18 - 30A-168 961

[0072] Exemplary embodiments will now be explained with reference to the figures, wherein similar reference signs denote the same functional or structural features and wherein:

[0073] Fig. 1 shows a schematic illustration of a system during manufacturing of a three-dimensional workpiece;

[0074] Fig. 2 shows a schematic illustration of the system with a calibration element;

[0075] Fig. 3a shows exemplary scattering features and scan paths crossing said scattering features;

[0076] Fig. 3b-3d shows exemplary detected intensities or scattered irradiation for the scan paths and scattering features of Fig. 3a;

[0077] Fig. 4a shows exemplary scattering features and scan paths crossing said scattering features;

[0078] Fig. 4b shows a diagram with time differences between intensity peaks identified based on detected intensities for the respective scan paths of Fig. 4a;

[0079] Fig. 4c shows a linear fit performed based on the diagram of Fig. 4b;

[0080] Fig. 5 shows exemplary scattering features and a spiral of scan paths crossing said scattering features;

[0081] Fig. 6 shows exemplary scattering features and a set of concentrically arranged circular scan paths crossing said scattering features;

[0082] Fig. 7 shows a flowchart of an exemplary method;

[0083] Fig. 8a shows the scattering features and scan paths of Fig. 4a as well as an indication of exemplary groups of scan paths;

[0084] Fig. 8b shows exemplary scanning results using a group-based scanning strategy; Nikon SLM Solutions AG - 19 - 30A-168 961

[0085] Fig. 9 shows exemplary scattering features and scan paths crossing said scattering features; and

[0086] Fig. lOa-lOi show different examples of scattering features.

[0087] Fig. 1 shows a schematic illustration of a system 2 in accordance with the present disclosure. The system 2 comprises an additive manufacturing apparatus 4 configured to produce three-dimensional workpieces by selective layer-wise irradiation of raw material. The apparatus 4 comprises a plurality of optical scanning units 6, each configured to guide an irradiation beam 7 onto a layer of raw material 8. In the illustrated example, the optical scanning units 6 each comprise a laser beam source 9 configured to emit a laser beam as the irradiation beam 7. The optical scanning units

[0088] 6 also comprise a scan mirror arrangement that is configured to control a path of the laser beam through the process chamber 10 of the apparatus 4 onto the layer of raw material 8. It is to be understood that exactly one optical scanning unit 6 may be provided or that a plurality of scanning units 6 may be provided. Also, depending on the use case, the irradiation beam 7 may be configured as a high-power beam to consolidate raw material, or it may be configured as a low-power beam to simply act as a visual guide for a user. In the latter case, the irradiation beam 7 may be referred to as a guide beam and may have a human-visible wavelength. Separate beam sources 9 may be provided for each of these two laser beam types. The low-power and the high-power beams are preferably guided into the irradiation area of the build apparatus 4 via one and the same optical scanning unit 6, although other variants are also possible.

[0089] The system 2 furthermore comprises at least one optical detector 12. In Fig. 1, a plurality of such optical detectors 12 are shown for illustrative purposes. It is to be understood that only one or more of the illustrated optical detectors 12 may be provided. Each of the optical detectors 12 is configured to detect an intensity of scattered light, as discussed in detail hereinbelow. As can be seen, some of the optical sensors 12 are arranged such that they share at least a part of a path of the laser beam 7 ("on-axis" arrangement). It is also possible for an optical sensor 12 to capture a field of view via a first one of the optical scanning units 6 while the irradiation beam

[0090] 7 is scanned within the irradiation area via a second one of the optical scanning units 6. In this case, the first and second optical scanning units 6 may be controlled such that their views of the irradiation area are similar. Other optical sensors 12 may be arranged within the process chamber 10 and / or attached to a wall thereof. An optical sensor 12 may be coupled to the apparatus 4 via an optical fiber 13. Each optical Nikon SLM Solutions AG - 20 - 30A-168 961 sensor 12 may comprise a photodiode or a phototransistor configured to detect an intensity of light. It is not required for the respective optical sensor 12 to have a spatial resolution. In other words, the optical sensors 12 do not need to be configured as image sensors or cameras.

[0091] In the illustrated example, the apparatus 4 further comprises a powder deposition unit 14 arranged within the process chamber 10 that is configured to deposit a layer of raw material 8. The raw material 8 may be deposited in the form of a powder, for example a metal powder or a metal alloy powder, onto a build plate 17 arranged in a build cylinder 15. To this end, the powder deposition unit 14 may travel across an opening 19 in a bottom of the process chamber 10 that opens into an interior of the build cylinder 15. In this manner, the raw material powder layer 8 can be deposited within the build cylinder 15.

[0092] When scanning the laser beam 7 across a portion 16 of this raw material powder layer 8, said scanned portion 16 can be solidified. This process can be referred to as selective laser sintering or selective laser melting. After such a selective solidification, a build plate carrier 18 of the apparatus 4 that carries the build plate 17 can be lowered (i.e., moved away from the process chamber 10) by a predefined amount that defines the thickness of a subsequent powder layer. In order to move the build plate carrier 18 relative to the process chamber 10, a movement unit 20 (e.g. a spindle moved by a motor) can be used.

[0093] After having moved the build plate carrier 18 away from the process chamber 10 by the predefined amount, the powder deposition unit 14 can be moved once more across the opening 19 in the bottom of the process chamber 10 to deposit a further layer of raw material powder 8 onto the previously irradiated layer. By repeating this process layer by layer, a three-dimensional workpiece 22 can be produced.

[0094] The system 2 further comprises at least one processor 24 communicatively coupled to at least one memory 25. The at least one memory 25 stores instructions which, when executed by the at least one processor 24, cause the at least one processor 24 to perform the method(s) as disclosed herein. The at least one processor 24 may be configured to control the other components of the system 2, in particular the optical scanning units 6, the powder deposition unit 14 and the movement unit 20. The optical detectors 12 may be communicatively coupled to the at least one processor 24 to feed intensity measurements to said processor(s). Nikon SLM Solutions AG - 21 - 30A-168 961

[0095] Fig. 2 shows a schematic illustration of the system 2 with a plurality of calibration elements 26. The system 2 may comprise one or more calibration elements 26. Such a calibration element 26 may be arranged on the powder deposition unit 14 or on a bottom or side wall of the process chamber 10. In the illustrated example, one calibration element 26 is arranged parallel to the build plate 17 by a holding assemblyJ fixed to the build plate 17. In this case, instead of the build plate 17, a reference plate may be used that carries attachment features for the holding assembly 27. This calibration element 26 spans across the entire opening 19 in the bottom of the process chamber 10 of the apparatus 4 and, thus, covers the entire build area of the apparatus 4. An optical detector 12 may be arranged between this calibration element 26 and the build plate or reference plate 17.

[0096] It is also possible for an optical detector 12 to be arranged adjacent to said calibration element 26, as shown in Fig. 2. In this case, the calibration element 26 may comprise one or more waveguides 29 that guide scattered light towards this particular optical detector 12. Each of the waveguides 29 may be associated with a distinct portion of the calibration element such that it only guides scattered light originating from an irradiation beam 7 falling onto said distinct portion towards an associated optical detector 12. In this manner, multiple light beams 7 may be scanned across the calibration element 26 at the same time via multiple optical scanning units 6, while the waveguides 29 and the associated detectors 12 still enable a separate detection of the light intensities of scattered light for each of the irradiation beams 7.

[0097] Each calibration element 26 comprises one or more scattering features 28. Each calibration element 26 may be made from glass or another (e.g., ceramic) material that is transparent for light having the wavelength or range of wavelengths of the laser beam 7 (e.g., configured to melt the raw material or configured as a guide beam). The one or more scattering features 28 are configured to scatter the laser light beam 7 when being hit thereby. To this end, the one or more scattering features 28 may exhibit a higher surface roughness compared with surrounding portions of the calibration element 26. The one or more scattering features 28 may be formed by selectively sanding or selectively etching portions of the calibration element 26 and / or by depositing small particles onto selected portions of the calibration element 26. When the laser light beam 7 is scanned across a calibration element 26, it can enter the substantially transparent calibration element 26. When the laser light beam 7 falls onto one of the scattering features 28, the laser light beam is scattered into various directions. In this case, the intensity of light received by the optical detectors 12 will increase. In case the optical detector 12 is arranged below the calibration element 26, Nikon SLM Solutions AG - 22 - 30A-168 961 the intensity of received light will decrease. In either case, the intensity of light detected by the respective optical detector 12 will change abruptly and significantly (e.g., by a factor of 2, 3, 4, 5, 10 or more) upon the laser light beam 7 crossing one of the scattering features 28. The scattering features 28 may be arranged in multiple groups, each group forming a pattern (e.g., an X-shape) and being arranged at a different x-y-position of the calibration element 26. For example, multiple groups of two linear scattering features 28, each group forming an X-shape, may be arranged across the calibration element 26. In this case, the overall combination of scattering elements 28 may form a rectangular grid pattern.

[0098] Fig. 7 shows an exemplary method in accordance with the present disclosure with optional steps being indicated in dashed boxes. The system 2, in particular the at least one processor 24 thereof, may be configured to perform said method. This method will now be explained in detail with further reference to Fig. 3a to Fig. 6.

[0099] In step 602, the apparatus 2 is controlled such that an irradiation beam 7 is scanned across a plurality of scattering features of a calibration element 26 that form a two- dimensional code such as a barcode or a QR code. In particular, an optical scanning unit 6 of the apparatus 4 can be controlled such that the irradiation beam 7 is scanned along a first plurality of predefined scan paths across the calibration element 26, wherein at least one of the plurality of predefined scan paths crosses the scattering features that form the two-dimensional code. At the same time, light intensities of scattered light are detected with one or more of the optical detectors 12 and fed to the processor 24.

[0100] In step 604, a content of the scanned code is determined based on the detected light intensities. This content may be indicative of a type, a serial number or a physical property of the calibration element 26. In particular, the encoded information may be indicative of a type, size, shape, position and / orientation of the scattering features 28 of the calibration element 26 that are to be used for determining a pose of the calibration element 26.

[0101] In step 606, the apparatus 4 is controlled such that an irradiation beam 7 is scanned across the same or different scattering features 28 of the calibration element 26, which scattering features 28 are to be used for determining a pose of the calibration element 26. To this end, the at least one processor 24 may control an optical scanning unit 6 of the apparatus 4 such that the laser beam 7 is scanned along a second plurality of predefined scan paths 30 across the calibration element 26. Nikon SLM Solutions AG - 23 - 30A-168 961

[0102] Different combinations of scan paths 30 that are used in step 606 and scattering features 28 of the calibration element 26 are shown in Fig. 3a, Fig. 4a, Fig. 5 and Fig. 6. In the example of Fig. 3a and Fig. 4a, the scattering features 28 consist of two orthogonal lines forming an X-shape and the plurality of scan paths 30 comprises a plurality of equidistant parallel scan paths 30 of equal length. In the example of Fig. 5, the same scattering features 28 are used, but in this case the plurality of scan paths 30 comprises a set of scan paths 30 that each form a different revolution of a common spiral. In the example of Fig. 6, the plurality of scan paths 30 comprises a plurality of concentrically aligned circular scan paths 30 forming circles with different radii. As can be seen, in each of these examples, two or more of the scan paths 30 each have at least two intersection points with the scattering features 28.

[0103] It can also be seen that the scan paths 30 are defined in a coordinate system 32 of the optical scanning unit 6 used to scan the laser light beam 7 along these scan paths 30. In difference thereto, a position and orientation of the scattering features 28 is defined in a coordinate system 34 of the calibration element 26, which may have its origin at a common intersection point between the two linear scattering elements 28, at the center of the X-shape. One task that can be addressed with the technique disclosed herein is how to determine a relative alignment between these two coordinate systems 32, 34. To this end, it is proposed to determine a pose of the calibration element 26, preferably in the coordinate system 32.

[0104] In step 608, an intensity of scattered irradiation is detected by one or more of the optical detectors 12. This is done for each of a plurality of different points in time during scanning of the scan paths 30, and for each scan path 30. That is, the detection takes place at the same time as the laser light beam 7 is scanned across the calibration element 26 along the plurality of scan paths 30. The intensity measurements can then be transferred from the optical detectors 12 to the at least one processor 24.

[0105] Three exemplary intensity measurements as obtained in step 608 are shown in Fig. 3b, Fig. 3c and Fig. 3d for separate scan paths 30. In the illustrated example, the scanning speed along each of the scan paths 30 was the same. It can be seen that the detected intensities of scattered light show two peaks for each scan path 30. Each of these peaks is associated with an intersection point between the respective scan path 30 and the scattering features 28 of the calibration element 26. In other words, when the laser light beam 7 is scanned along a scan past 30 and crosses one of the scattering Nikon SLM Solutions AG - 24 - 30A-168 961 features 28, the intensity of scattered light that is detected by the one or more optical detectors 12 changes significantly, in the illustrated example by a factor of at least 4.

[0106] In step 610, the detected intensities are analyzed to identify, for each of the scan paths 30, (e.g., minimum or maximum) intensity peaks associated with the at least two intersection points between the scan path 30 and the scattering features 28. This step may comprise filtering out outliers and / or masking portions of the detection signal (s) for peak detection that are expected to contain peaks. Other measurement data processing actions such as smoothing, averaging, resolution reduction or the like may also be performed for detecting the intensity peaks.

[0107] In step 612, a time difference between the intensity peaks is determined for each scan path 30. In case a scanning speed between the associated intersection points is known, a scan distance between these intersection points can also be determined based on the time difference.

[0108] If the scanning speed differs over time, either within a single scan path 30 or between scan paths 30, the time difference may be normalized relative to a common scanning speed and / or the scan distance may be determined based on the actual scanning speed in step 614.

[0109] In the example of Fig. 3b, the time difference between the two detected peaks corresponds to approximately 6.2 ms, in Fig. 3c, the time difference amounts to approximately 2.3 ms, and in Fig. 3d the time difference is around 1 ms. As each time difference stems from a scan path 30 that has a known location in at least the y- coordinates of the coordinate system 32, each time difference is associated with such a location.

[0110] In a first variant (step 616), the location of the path 30 that exhibits the lowest time difference is selected. In the example of Figs. 3a-3d, the location of the path 30 resulting in the lowest time difference of 1 ms has a location in the y-coordinates of the coordinate system 32 of, for example, +5mm. This location may thus be selected.

[0111] In a second variant (step 618), a location of a virtual scan path that was not scanned but for which the time difference would be minimal compared with the determined time differences of the scan paths 30 is estimated. In the example of Fig. 4a, the scan paths 30 are all of equal length, equidistant and parallel to the x-axis of the coordinate system 34. The virtual scan path may be shaped similarly to the scan paths 30, in the Nikon SLM Solutions AG - 25 - 30A-168 961 shown example as a linear scan path extending parallel to the x-axis of the coordinate system 32. Exemplary time differences resulting from the scanned scan paths 30 and the scattering features 28 are shown in Fig. 4b for the different y-positions of the different scan paths 30 in the coordinate system 32. A linear fit may be applied as illustrated in Fig. 4c to estimate a y-position of a similarly shaped virtual scan path in the coordinate system 32 for which the time difference would be zero. In the illustrated example, this y-position lies at approximately y = +2.1 mm in the coordinate system 32.

[0112] In step 620, a pose of the calibration element at which the identified intensity peaks match the intersection points between the scan paths 30 and the one or more scattering features 28 of the calibration element 26 is determined.

[0113] In case of the first variant, it may be assumed that the common intersection point of the scattering features 28, at which the time difference would be zero, lies at the location selected in step 616, in the illustrated example of Figs. 3a-3d at a position of y = +5mm in the coordinate system 32. That is, the position of the origin of the coordinate system 34 may be assumed to lie at y = +5mm of the coordinate system 32.

[0114] In case of the second variant, it may be assumed that the common intersection point of the scattering features 28, at which the time difference would be zero, lies at the location estimated in step 618, in the illustrated example of Fig. 4a-4c at a position of y = +2.1 in the coordinate system 32. That is, the position of the origin of the coordinate system 34 may be assumed to lie at y = +2.1mm of the coordinate system 32.

[0115] A position of the origin of the coordinate system 34 in x-coordinates of the coordinate system 32 may be determined in a similar manner based on intensity measurements detected for scan paths 30 that extend parallel to the y-axis of the coordinate system 32 and that intersect the scattering features 28. It is also possible to use spiral-shaped or circular scan paths (see, e.g., Fig. 5 or Fig. 6) instead of linear scan paths to derive the two-dimensional pose of the coordinate system 34 in the coordinate system 32.

[0116] The pose of the calibration element 26 may even be determined in three dimensions, for example based on two-dimensional poses of the calibration element 26 determined for each of a plurality of different optical scanning units 6. The three-dimensional position of the calibration element 26 may be determined based on the scan distance, Nikon SLM Solutions AG - 26 - 30A-168 961 the time difference associated with this scan distance, and a known (e.g., angular) scanning speed of the optical scanning unit 6.

[0117] In case the steps 606-620 are performed for different pluralities of predefined scan paths, each being associated with a different location on the calibration plate, multiple poses of the calibration element 26 are obtained, namely one for each of the pluralities of predefined scan paths. Based on these multiple poses, multiple coordinate transformations between the coordinate systems 32, 34 can be determined. In some cases, these coordinate transformations may differ from one another due to a scan field distortion. Such a scan field distortion may be compensated based on the plurality of coordinate transformations and, optionally, based on additional coordinate transformations (e.g., associated with different coordinates of the coordinate systems 32, 34) obtained by interpolation, averaging or the like of the so-determined coordinate transformations.

[0118] In step 622, a pose of a surface of a raw material powder layer is determined based on the determined pose of the calibration element 26. To this end, the calibration element 26 is fixedly arranged in a predefined pose relative to the powder deposition unit 14. Once the pose of the calibration element 26 has been determined, the pose of the powder layer surface can be determined based on said predefined pose. It is also possible to determine multiple poses of the raw material powder layer, for example one pose for each of a plurality of positions of the powder deposition unit 14 during movement thereof when depositing a raw material powder layer.

[0119] The method may be performed for different calibration elements 26, for example for a first calibration element 26 arranged on the build plate 18 and for a second calibration element 26 arranged on the powder deposition unit 14. This may enable determining a relative pose between the powder deposition unit and the build plate 18. The pose(s) of the surface of the raw material powder layer may then be determined based on this relative pose. The sequence in which the calibration element's poses are determined can be adapted as needed. By determining the poses of multiple calibration elements 26, factors such as thermal expansion of the process chamber 10 can be determined (e.g., during manufacturing of a three-dimensional workpiece) and used to calibration the apparatus 4 in order to maintain build quality during the subsequent manufacturing steps.

[0120] One or more of the steps 602 - 624 may be performed during manufacturing of a three-dimensional workpiece 22. For instance, the optical scanning unit(s) 6 may be Nikon SLM Solutions AG - 27 - 30A-168 961 controlled to scan the laser light beam across one or more portions 16 of the raw material powder layer 8, and then, immediately before, during or immediately after moving the powder deposition unit 14 across the opening 19, scan the laser light beam across the calibration element 26 arranged on the powder deposition unit 14. It is also possible to do this whilst changing or switching the irradiation beam 7 from a consolidating laser beam to a guide beam which is then scanned across the calibration element 26.

[0121] In step 624, the optical scanning unit(s) 6 for which the pose(s) of the calibration element 26 was / were determined is / are calibrated. This may comprise compensating an offset between the two coordinate systems 32, 34 for each of the scanning units 6. The scanning units 6 may thereby be calibrated relative to one another via the calibration element 26. It is also possible to calibrate one or more of the optical scanning units 6 based on the determined pose(s) of the surface of the raw material powder layer. For example, a planned x-y-position in a coordinate system of said optical scanning unit(s) may be adjusted based on the pose(s) of the surface of the powder layer and / or based thickness(es) thereof derived from the pose(s) of the surface of the powder layer. This may ensure that a subsequent irradiation of the powder layer can be performed with the laser beam 8 impinging on the correct x-y- position of the powder layer (e.g., in a coordinate system of the powder layer).

[0122] Alternatively, or in addition, the powder deposition unit 14 may be calibrated in step 624. This may comprise actively changing a pose of the powder deposition unit 14 to compensate an offset of the determined pose thereof from an intended, predefined optimal pose, and / or applying an offset compensation factor to a subsequent control of the powder deposition unit 14.

[0123] It is to be understood that the sequence of steps 602-624 may be changed if needed. It is also possible for several steps to be combined into a common step or for steps to be split up into various sub-steps performed at different points in time. The examples described with reference to the Figures are not limiting. The steps and features described herein with reference to the Figures may be combined with and / or replaced with the steps and features described herein with reference to the first and / or second aspect and vice versa. Further modifications of the technique disclosed herein may be apparent to those skilled in the art.

[0124] Further developments of the technique disclosed above are now explained with reference to Figs. 8a-10i. Nikon SLM Solutions AG - 28 - 30A-168 961

[0125] Fig. 8a shows the scattering features 28 and scan paths of Fig. 4a as well as an indication of exemplary groups G1-G4 of the scan paths 30. In case the scan paths 30 are scanned one after another, starting with the topmost scan path in Fig. 8a (highest y-value), and detecting the peaks based on each scan path, the cross-pattern of the scattering features 28 is obtained. However, at most times during the scanning, no scattering feature 28 is crossed with the scanning light beam. The procedure thus leaves room for a simultaneous scanning of multiple scan paths, as long as the respective peaks can be associated to the respective scan paths afterwards. For example, the scan paths 30 of the first group G1 could be scanned at the same time as the scan paths 30 of the second group G2, and the scan paths 30 of the third group G3 could be scanned at the same time as the scan paths 30 of the fourth group G4. This simultaneous scanning may be performed with a plurality of beams 7, directed by several optical scanning units 6.

[0126] Fig. 8b shows exemplary scanning results using such a group-based scanning strategy. In this case, Al illustrates the results of scanning groups G1+G2 simultaneously, A2 illustrates the results of scanning groups G2+G1 simultaneously, A3 illustrates the results of scanning groups G3+G4 simultaneously, and A4 illustrates the results of scanning groups G4+ G3 simultaneously. The position and orientation of the scattering features 28 can be determined for each scanning unit 6 separately by using the scanning results forming the cross for the scanning unit that scanned the scan paths 30 in y-order and by using the scanning results forming the outer V-shapes for the scanning unit that scanned the scan paths 30 of G2, then Gl, then G4 and then G3.

[0127] This approach can also be used in case the scan paths 30 to be scanned via the scanning units 6 differ from one unit 6 to the other. In this case, for example, a first scanning unit 6 may scan the scan paths 30 of Fig. 4a that are parallel to the x-axis, while a second scanning unit 6 may scan other scan paths 30 that are parallel to the y-axis. Also in this case, a group-based scan strategy may be employed rather than scanning one neighboring scan path after another.

[0128] Fig. 9 shows exemplary scattering features 28 and scan paths 30 crossing said scattering features 28. Also indicated are unwanted optical artifacts 40 of the calibration element which could include, for example, dirt, dust or surface scratches. In addition to the X-shaped scattering features 28, the calibration element in this example further comprises scattering features 28a that are arranged beside said X- shaped scattering features 28. The scattering features 28a are crossed by each scan Nikon SLM Solutions AG - 29 - 30A-168 961 path 30 and will also result in respective peaks in the detection signal. These peaks can also be referred to as reference peaks and can be used as a reference for starting acquisition of the measurement signal indicating the intensities and / or for aligning the measurement signals indicating the intensities associated with the respective scan paths with one another. This could be particularly useful in case the start points of the scan paths 30 differ from one another or in case another trigger indicating scan path start is to be avoided. In the illustrated example, the features 28a are formed as a set of three parallel lines parallel to the y-direction. Each scan path 30 crosses these three lines. This also allows to adjust the measurement results by compensating for scanning speeds that vary between scan paths 30. Other shapes and arrangements of the features 28a are also possible, as long as the associated peaks can be reliably detected from the detected intensities. Peaks generated by the artifacts 40 can be filtered out from the measurement signals based on their detection position, associated peak height and / or associated peak width. It is also possible to detect presence, type and / or location of such artifacts based on the measurement signals. A warning or cleaning instructions may then be output for a user.

[0129] Fig. lOa-lOi show different examples of scattering features 28 that could be used on a calibration element. In Fig. 10a, the features 28 comprise circles and two orthogonal lines 28a. In Fig. 10b, the features 28 comprise squares and two orthogonal lines 28a. In Fig. 10c, the features 28 are lines forming rectangular triangles. In Fig. lOd, the features 28 are lines forming squares. In Fig. lOe, the features 28 comprise rectangular triangles inside a respective rectangle formed by scattering lines 28a. In Fig. lOf, the features 28 form a QR code inside a rectangle formed by scattering lines 28a. In Fig. 10g, the features 28 form a barcode. In Fig. lOh, the features 28 form a QR code. In Fig. lOi, the features 28 form a barcode and text or numbers 32. Other variants are also possible. The codes, text and numbers could encode information regarding the calibration plate and / or the scattering features 28.

[0130] It is possible for the scattering features 28 to comprise one or more of (a) printed features, (b) engraved features, (c) scratched features, (d) laser marked features.

[0131] In one example, all scan paths of the plurality of scan paths are scanned in a similar scanning direction. In another example, directions in which the plurality of scan paths are scanned differ between at least two of the scan paths. For example, a first set of scan paths may be scanned in a first direction (e.g., positive x-direction), whereas a second set of scan paths may be scanned in a second direction (e.g., negative x- direction). It is also possible to switch the scanning direction from one scan path to Nikon SLM Solutions AG - 30 - 30A-168 961 another. In one example, the scan paths are alternatively scanned in a first and in an opposite second direction (e.g., in case of so-called hatch scanning). This may reduce the time required for the overall scanning.

[0132] As disclosed above, the calibration element may be attached to the powder deposition unit 14. It is also possible to attach the calibration element to another component, for example the build plate carrier 18 or the build plate 17, and to use the determined pose of the calibration element to derive a pose of said another component (e.g., the carrier 18 or the build plate 17). In one particular variant, a component of the apparatus 4 may be used as the calibration element. For example, the plurality of scan paths 30 may be scanned across surface features of said component (e.g., edges thereof) having a known geometry. The resulting intensities may then be used to determine the position and orientation of said component, as disclosed above with reference to the calibration element. To improve peak detectability, scattering features 28 such as grooves, treated surface portions or reflective markers may be provided thereon. A dualistic approach in which both the calibration element and the component as such are scanned to obtain a position and orientation of the component is also possible. In either case, a position and orientation of a component of the apparatus 4 can also be tracked over time by performing the scanning, peak detection and localization as disclosed herein at different points in time (e.g., before and after a manufacturing process). Such tracking may involve short-term tracking (e.g., during a manufacturing process) or long-term tracking (e.g., over the course of a plurality of manufacturing processes, for example over several weeks). In case of a change in position and / or orientation that exceeds a predefined threshold, a warning may be output and / or a manufacturing process may be stopped or inhibited.

Claims

Nikon SLM Solutions AG - 31 - 30A-168 961Claims1. A method for determining a pose of a calibration element (26), arranged in an irradiation area of an additive manufacturing apparatus (4) configured to produce three-dimensional workpieces (22) by selective layer-wise irradiation of a raw material (8), the method comprising: controlling (606) the apparatus (4) such that an irradiation beam (7) is scanned along a plurality of predefined scan paths (30) across the calibration element (26), wherein each scan path (30) has at least two intersection points with one or more scattering features (28) of the calibration element (26); for each of the plurality of predefined scan paths (30):{i} detecting (608), for each of a plurality of different points in time during scanning of the scan path (30), an intensity of irradiation scattered by the calibration element (26), and{ii} analyzing (610) the detected intensities to identify intensity peaks associated with the at least two intersection points of the scan path (30); and determining (620) a pose of the calibration element (26) at which the identified intensity peaks match the intersection points between the scan paths (30) and the one or more scattering features (28) of the calibration element (26).

2. The method of claim 1, further comprising, for each of the plurality of predefined scan paths (30), determining (612) at least one parameter selected from {a} a time difference between the identified intensity peaks and {b} a scan distance between the identified intensity peaks, wherein the pose of the calibration element (26) is determined based on the at least one parameter.

3. The method of claim 2, wherein the time differences comprised in the at least one parameter are normalized (614) relative to a common reference scanning velocity and / or wherein the scan distances comprised in the at least one parameter are determined based on an actual scanning velocity.

4. The method of claim 2 or 3, further comprising: selecting (616) at least one of the scan paths (30) for which the at least one parameter is an extremum compared with the parameters of the other scan paths (30)Nikon SLM Solutions AG - 32 - 30A-168 961 of the plurality of predefined scan paths, wherein the pose of the calibration element (26) is determined based on a location of the selected at least one scan path (30); or estimating (618) at least one location of a scan path for which the at least one parameter would be an extremum compared with the parameters of the scan paths (30) of the plurality of predefined scan paths (30), wherein the pose of the calibration element (26) is determined based on the estimated at least one location.

5. The method of any one of claims 1 to 4, wherein the one or more scattering features (28) comprise two scattering features (28) that each extend longitudinally along an axis, the axes intersecting one another on the calibration element (26), and wherein, optionally: {i} the plurality of scan paths (30) comprises a first plurality of parallel linear scan paths and a second plurality of parallel linear scan paths that are oblique to the first plurality of parallel linear scan paths and / or {ii} the plurality of scan paths (30) comprises a plurality of concentrically arranged circular scan paths and / or {iii} the plurality of scan paths (30) comprise a set of scan paths that form a spiral.

6. The method of any one of claims 1 to 5, wherein the one or more scattering features comprise at least one reference feature configured such that an intensity peak generated upon a laser beam scanning across the at least one reference feature can be identified among a plurality of detected intensity peaks; wherein, optionally, the at least one reference feature comprises two or more parallel lines that are oriented oblique to the plurality of scan paths.

7. The method of claim 6, wherein the intensity peak(s) associated with the at least one reference feature is / are used as a reference for aligning detected intensity peaks and / or compensating for different scanning speeds.

8. The method of any one of claims 1 to 7, wherein the additive manufacturing apparatus (4) comprises at least one optical scanning unit (6) configured to scan the irradiation beam (7) along the plurality of predefined scan paths (30) across the calibration element (26), the method further comprising calibrating (624) the at least one optical scanning unit (6) based on the determined pose of the calibration element (26).

9. The method of any one of claims 1 to 8, wherein the additive manufacturing apparatus (4) comprises a plurality of optical scanning units (6), each configured to scan an irradiation beam (7) along a or the plurality of predefined scan paths (30) across the calibration element (26), wherein all method steps are performed for eachNikon SLM Solutions AG - 33 - 30A-168 961 of the plurality of optical scanning units (6) such that the pose of the calibration element (26) is separately determined for each optical scanning unit (6).

10. The method of claim 9, wherein a plurality of irradiation beams (7) are scanned simultaneously by the plurality of scanning units (6).

11. The method of claim 10, wherein the plurality of irradiation beams (7) are scanned such that they intersect calibration features (28) of the calibration element (26) at different points in time.

12. The method of claim 10 or 11, wherein the plurality of irradiation beams (7) are simultaneously scanned across separate regions of the calibration element (26) and / or along different scan paths, for example along different ones of the same plurality of scan paths scanned by each optical scanning unit (6).

13. The method of any one of claims 9 to 12, further comprising: calibrating (624) the plurality of optical scanning units (6) based on the poses of the calibration element (26) that have been separately determined for each optical scanning unit (6); and / or determining (620) a three-dimensional position of the calibration element (26) based on the poses of the calibration element (26) that have been separately determined for each optical scanning unit (6) and based on known relative poses between the optical scanning units (6).

14. The method of any one of claims 1 to 13, wherein the additive manufacturing apparatus (4) comprises a process chamber (10), wherein all method steps are performed for a plurality of different relative poses between the calibration element (26) and the process chamber (10).

15. The method of claims 13 and 14, wherein the calibration element (26) is fixedly arranged relative to a build plate carrier (18) or a build plate (17) of the additive manufacturing apparatus (4), which build plate carrier (18) or build plate (17) is height- adjustable relative to the process chamber (10), wherein the plurality of different relative poses between the calibration element (26) and the process chamber (10) are defined by different height positions of the build plate carrier (18) or build plate (17) relative to the process chamber (10).Nikon SLM Solutions AG - 34 - 30A-168 96116. The method of any one of claims 1 to 15, wherein the calibration element (26) is fixedly arranged relative to a component of the additive manufacturing apparatus (4) such as a build plate carrier (18) or a build plate (17), wherein the pose of the calibration element is used to determine a pose of said component.

17. The method of any one of claims 1 to 16, wherein the calibration element (26) is fixedly arranged relative to a powder deposition unit (14) of the additive manufacturing apparatus (4), which powder deposition unit (14) is configured to move across the irradiation area when depositing raw material powder (8).

18. The method of claim 17, further comprising: determining (622) a pose of a surface of a raw material powder layer (8) based on the determined pose of the calibration element (26) and based on a known relative pose between the calibration element (26) and the surface of the raw material powder layer (8).

19. The method of any one of claims 1 to 18, further comprising: controlling (602) the apparatus (4) such that an irradiation beam (7) is scanned along at least one scan path across the calibration element (26), wherein the at least one scan path crosses one or more scattering features of the calibration element (26) that are part of a code; detecting, for each of the at least one scan path, and for each of a plurality of different points in time during scanning of said scan path, an intensity of irradiation scattered by the calibration element (26); and analyzing the detected intensities to determine a content of the code.

20. The method of claim 19, wherein the code contains information defining at least one geometric property of the calibration element (26) and / or a type of the calibration element (26) and / or a configuration of the one or more scattering features (28) to be crossed by scan paths (30) of the plurality of predefined scan paths (30).

21. The method of any one of claims 1 to 20, wherein the calibration element (26) is configured to extend throughout the entire irradiation area of the additive manufacturing apparatus (4) and / or wherein the calibration element (26) comprises, for each of at least one optical detector (12), one or more waveguides (29) configured to guide irradiation scattered by a scattering feature (28) of the calibration element (26) towards the respective optical detector (12).Nikon SLM Solutions AG - 35 - 30A-168 96122. The method of any one of claims 1 to 21, wherein the intensity of scattered irradiation is detected by at least one optical detector (12), wherein the at least one optical detector (12) comprises a detector (12) that is arranged within a process chamber (10) of the apparatus (4) and / or a detector (12) that is arranged in a build cylinder (15) mounted on the process chamber (10) and / or a detector (12) that is arranged such that it captures at least a portion of the irradiation area via an optical scanning unit (6) of the apparatus (4) and / or a detector (12) that includes an optical fiber (13) communicatively coupled to a photodiode and / or a detector (12) that is arranged on an opposite side of the calibration element (26) compared to the scanned irradiation beam (7).

23. A system (2) comprising: an additive manufacturing apparatus (4) configured to produce three- dimensional workpieces (22) by selective layer-wise irradiation of a raw material (8), the apparatus (4) comprising a process chamber (10); at least one calibration element (26) arranged in an irradiation area of the apparatus (4) and having one or more scattering features (28); at least one optical detector (12) configured to detect an intensity of irradiation scattered by the calibration element (28) at different points in time; and at least one processor (24) that is configured to perform the method of any one of claims 1 to 22.

24. The system of claim 23, wherein the one or more scattering features (28) of the calibration element (26) comprise two scattering features (28) that each extend longitudinally along an axis, the axes intersecting one another on the calibration element (26); and / or the one or more scattering features comprise two or more parallel lines oriented oblique, for example orthogonal, to the plurality of scan paths (30); and / or the additive manufacturing apparatus (4) comprises at least one optical scanning unit (6) configured to scan the irradiation beam (7) along the plurality of predefined scan paths (30) across the calibration element (26); and / or the additive manufacturing apparatus (4) comprises a plurality of optical scanning units (6), each configured to scan an irradiation beam (7) along a plurality of predefined scan paths (30) across the calibration element (26); and / or the additive manufacturing apparatus (4) comprises a process chamber (10) and the calibration element (26) is configured to be arranged in different relative poses relative to the process chamber (10); and / orNikon SLM Solutions AG - 36 - 30A-168 961 the additive manufacturing apparatus (4) comprises a build plate carrier (18) or a build plate (17) that is height-adjustable relative to the process chamber (10) and the calibration element (26) is fixedly arranged relative to the build plate carrier (18) or the build plate (17); and / or the calibration element (26) is fixedly arranged relative to a component of the additive manufacturing apparatus (4) such as a build plate carrier (18) or a build plate (17); and / or the additive manufacturing apparatus (4) comprises a powder deposition unit (14) configured to move across the irradiation area when depositing raw material powder and the calibration element (26) is fixedly arranged relative to the powder deposition unit (14); and / or one or more scattering features of the calibration element (26) are part of a code, the code optionally containing information defining at least one geometric property of the calibration element (26) and / or a type of the calibration element (26) and / or a configuration of the one or more scattering features (28) to be crossed by scan paths (30) of the plurality of predefined scan paths (30); and / or the calibration element (26) is configured to extend throughout the entire irradiation area of the additive manufacturing apparatus (4); and / or the calibration element (26) comprises, for each of at least one optical detector (12), one or more waveguides (29) configured to guide irradiation scattered by a scattering feature (28) of the calibration element (26) towards the respective optical detector (12); and / or the at least one optical detector (12) comprises a detector (12) that is arranged within a process chamber (10) of the apparatus (4) and / or a detector (12) that is arranged in a build cylinder (15) mounted on the process chamber (10) and / or a detector (12) that is arranged such that it captures at least a portion of the irradiation area via an optical scanning unit (6) of the apparatus (4) and / or a detector (12) that includes an optical fiber (13) communicatively coupled to a photodiode and / or a detector (12) that is arranged on an opposite side of the calibration element (26) compared to the scanned irradiation beam (7).

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