WO2020169455A1

WO2020169455A1 - Additive manufacture of a workpiece

Info

Publication number: WO2020169455A1
Application number: PCT/EP2020/053823
Authority: WO
Inventors: Thomas Engel; Matthias Goldammer; Andreas Graichen; Clemens Otte; Axel Reitinger
Original assignee: Siemens Aktiengesellschaft
Priority date: 2019-02-19
Filing date: 2020-02-14
Publication date: 2020-08-27

Abstract

In order to achieve a higher degree of precision during the additive manufacture of a workpiece, a method is disclosed, in which a fusing beam (3) is directed onto a tracing spot (4) on a tracing surface (1). A marker structure is projected onto the tracing surface (1), at least one portion of the tracing surface (1), onto which at least part of the marker structure is projected, is imaged on an optical detector (5), and at least one detector signal is generated using the image. A position of the tracing spot (4) is determined on the basis of the at least one detector signal and the manufacture of the workpiece continues, taking into consideration said determined position of the tracing spot (4).

Description

description

Additive manufacturing of a workpiece

The invention relates to a method for additive manufacturing of a workpiece, wherein a melt jet is directed to a writing point on a writing surface. The invention also relates to a device for additive manufacturing of a workpiece with a scanning unit, an optical detector, a control unit and an imaging unit and a computer program.

In additive manufacturing, material is applied in one writing point and applied in layers to the workpiece or a workpiece carrier. The melt jet, which is required for heating or melting, is guided over the workpiece. This can be done mechanically, for example, by moving a write head relative to the workpiece, or by means of a scanning process, the melt beam being deflected with a scanning unit and, for example, a focusing unit being focused on the component surface.

Since the melt beam passes through one or more lenses at different locations with different beam inclinations, geometric mapping errors can occur which cannot be compensated for by a lens design, or only inadequately.

Known methods for partially correcting such misrepresentation or geometry errors are based on static Ka libration methods that determine at a given point in time, in particular special before the start of production, distortions and other imaging errors and perform the management of the melt beam according to the calibration.

However, such known approaches have the disadvantage that an arrangement of components of the device for additive Production and its properties or parameters may no longer be changed after calibration, as otherwise the correction will no longer work or no longer work completely. In typical systems for additive manufacturing, however, various assemblies, for example the scanning unit or the focusing unit, air in a work area, mechanical distances and relative positioning of the writing surface or the writing location to the scanning unit and the focusing unit are not constant over time during production and possibly not for different temperatures. For example, the ambient temperature can have an influence on a system geometry, but the melt jet itself also heats parts and components of the system, so that properties of the system can also change via thermal expansion effects, which leads to further geometric errors that are within the scope of known Correction methods are not taken into account.

The document EP 3 421 225 A1 describes the calibration of a device for fully parallelized additive manufacturing. For this purpose, three marker devices are provided, each of which project a light reference marking onto a component or a construction field. A laser device has a detection device which detects the light reference markings and a control unit calibrates the laser device as a function of the light reference markings detected.

Document WO 2017/187147 A1 describes an apparatus for additive manufacturing by selective laser melting. In one embodiment, a reference pattern is projected onto a work surface. Two different optical modules, one of which is calibrated and one uncalibrated, are moved to nominally the same position and the associated images, each of which is the reference pattern, are compared with one another in order to correct the uncalibrated optical module. Document EP 3 208 077 A1 relates to a method for 3D printing. A sensing energy beam can form a pattern on a surface. Different receivers or transmitters can view a target position from different spatial positions.

Against this background, it is an object of the present invention to provide an improved concept for additive manufacturing with a melt jet that enables greater accuracy in the manufacture of the workpiece.

This object is achieved according to the invention by a method and a device as well as a computer program according to the independent claims. Advantageous embodiment forms and developments are the subject of the dependent claims.

The improved concept is based on the idea of a

Projecting the marker structure onto the writing surface, recording at least part of the marker structure with a detector and determining a current position of the writing point based on the recording.

According to a first independent aspect of the improved concept, a method for additive manufacturing of a workpiece is specified, wherein a melt beam is directed to a writing point on a writing surface. A marker structure is projected onto the writing surface, at least a portion of the writing surface on which at least part of the marker structure was projected is imaged on an optical detector and at least one detector signal is generated based on the image. Based on the at least one detector signal, a position, in particular an actual or current position, of the writing point is determined and the production of the workpiece is continued taking into account the determined position of the writing point. The marker structure is projected onto the writing surface, for example, by means of a projector, in particular optically using light, in particular Laserstrah sources.

In the context of the present disclosure, the term light can be understood in such a way that it includes electromagnetic waves in the visible, infrared and / or ultraviolet range. Accordingly, the term optical can be understood such that it relates to light according to this understanding.

The marker structure can in particular contain one or more marks or structures.

The writing surface can be, for example, a surface or part of the surface of a powder bed from which the workpiece is manufactured. The writing surface can include part of the powder bed surface and part of a workpiece surface.

The position of the writing point is in particular a position in the powder bed and / or on the workpiece, that is to say on the writing surface. The position can in particular be two-dimensional or three-dimensional coordinates, for example Cartesian coordinates and / or angle coordinates. The position can in particular also be determined by a current or actual deflection, in particular angular deflection or angular position

Describe melt jet.

According to various embodiments, the melt jet by means of a scanning unit, which for example the

Can deflect melt jet controlled, for example by means of deflecting mirrors, directed to the writing point of the writing surface.

In various embodiments, the melt beam is detected by means of the scanning unit and / or by means of a focusing purity, which is arranged, for example, between the scanning unit and the writing surface, and includes, for example, an F-theta lens, directed to the writing point.

According to various embodiments, the melt beam is directed onto the writing point by means of an optical device which, for example, can contain the scanning unit and / or the focusing unit.

According to a method for additive manufacturing according to the improved concept, an actual position of the writing point on the writing surface can advantageously be recorded and determined directly and compared with a target position. Deviations from the actual position and the target position can, for example, be due to imaging errors caused by the optical device, in particular the focusing unit, for example the F-theta lens. According to the improved concept, such deviations can be compensated for by determining the actual position of the writing point and continuing production taking into account the determined position.

Such a compensation can advantageously take place in real time, that is to say online, or approximately in real time during production. In particular, the compensation is dynamic in that it is based on the actual current mapping errors or deviations between the actual and target value of the position and not on a calibration carried out before the start of production, which essentially has to rely on the Do not differentiate conditions during production from conditions during calibration.

Accordingly, additive manufacturing according to the improved concept can achieve better error compensation and thus increased accuracy and product quality in the manufacture of the workpiece. The marker structure contains a first and a second group of lines, in particular straight lines. Each line in the first group intersects at least one of the lines in the second group. The position of the writing point is determined by means of a spatial coding which is based on at least one geometric property of the lines of the two groups.

Preferably, both the first and second groups each include at least two lines.

The geometric property can be a property of the lines themselves, for example a line width of the respective lines or a property of several lines in relation to one another, for example a sequence of distances between adjacent lines or angles between intersecting lines.

As a result, more complex patterns can be used to advantage, which allow location coding and thus greater accuracy in determining the writing point and greater reliability in determining the writing point.

According to at least one embodiment, the position of the writing point is compared with a target position and the production of the workpiece is continued as a function of a result of the comparison.

According to at least one embodiment, the position of the writing point relative to a reference feature is

Mark structure determined.

The reference feature can, for example, contain a point, a line, in particular a straight line, or a line segment marked out in a defined manner. For example, the reference feature can contain an intersection of two lines of the marker structure. The position of the writing point determined according to the method can in particular be an estimated, a calculated or a current, in particular actual, position of the writing point.

The position of the writing point can in particular also be determined or given by an estimated, calculated or actual angular design of the melt jet, in particular if a distance between the scanning unit and the reference feature is known.

The determination of the position of the writing point is advantageously based on a direct determination of a deflection of the writing beam or a direct determination of the position of the writing point on the writing surface itself and in particular not on an indirect conclusion based on a setting or angle setting or target setting of the scanning unit or a target deflection of the write beam, which can be prone to errors due to the geometric imaging errors described above.

According to at least one embodiment, the position of the writing point is determined relative to at least two reference features of the marker structure.

The at least two reference features can contain, for example, two intersection points of lines, in particular of at least three lines, of the marker structure.

From a known position of the intersection points of the lines it can be determined, for example, which location or position of the writing point with respect to one or more of the

Occupies intersections of the lines. The position of the writing point can, for example, be determined approximately as the coordinate of the closest intersection.

Alternatively or additionally, for example, via a la terale, that is, in a plane of the writing surface or a Main plane of the powder bed, shifting the lines the position of the writing point can be determined via interpolation.

In this way, it is also possible to estimate positions or coordinates between the positions or coordinates of the respective intersection points.

According to at least one embodiment, the marker structure comprises three or more lines and the reference features contain two or more points of intersection of the three or more lines.

The lines are in particular straight lines or straight lines.

The lines are in particular partly parallel lines, that is to say at least two of the three or more lines are parallel to one another.

According to various embodiments, at least two of the three or more lines are arranged in a fan shape, that is to say at least two of the at least three lines have a common origin, in particular defined by a common light source, and are emitted at different angles.

According to at least one embodiment, the lines of the first group are generated with light of a first wavelength and the lines of the second group are generated with light of a second wavelength different from the first.

The wavelength of the light of the first group corresponds to a characteristic laser wavelength or a wavelength distribution of the light. The same applies accordingly to the wavelength of the light of the second group.

By using different wavelengths, color coding can be used to carry out the location coding. In this way, different groups are advantageously distinguishable visually and can be used for location coding.

In particular, the lines of the different groups of lines are generated with different projectors, in particular line projectors.

According to at least one embodiment, the lines of the first group are generated alternately with the lines of the second group and in particular are switched on and off alternately with the lines of the second group.

In particular, the lines of the first and second group are time-multiplexed.

Correspondingly, time coding can advantageously be undertaken, which enables a differentiation between the various lines of different groups and can thus be used for location coding.

According to at least one embodiment, an area is imaged on the detector which contains the entire writing surface and in which at least one reference structure is additionally arranged, in particular a reference structure of the

Marker structure. The area is adapted using the reference structure or a position and / or orientation of the detector is calibrated using the reference structure.

The calibration can include, for example, a calibration or a definition of a field of view of the detector or an adaptation of the field of view of the detector.

To adapt the area or to calibrate the position or alignment of the detector, for example, a position, alignment or orientation of the reference structure can be determined on the detector image. According to at least one embodiment, a projection beam is coupled into a beam path of the melt beam in order to project the marker structure onto the writing surface. During production, the melt beam and the projection beam are moved synchronously.

In particular, the entire marker structure is coupled into the beam path of the measuring beam, in particular by means of several projection beams. In this context, the projection beam is not to be understood as an isolated light beam, but can for example contain a large number of light beams.

A change in the position of the writing point thus directly affects a position and / or shape of the

Marker structure. Accordingly, the position

and / or the shape of the marker structure serve as a direct measure of the position of the writing point.

According to at least one embodiment, a geometric parameter of the marker structure is determined and the position of the writing point is determined based on the geometric parameter.

This is particularly advantageous in embodiments of the method in which the projection beam is coupled into the beam path of the melt beam and the projection beam is moved synchronously with the melt beam.

The geometric parameter can be, for example, a shape or contour or a distortion of the marker structure or of a part of the marker structure. Due to the coupling of the projection beam into the beam path of the melting beam, a marker structure can be used, for example, which is then imaged concentrically with the melting beam on the writing surface. For example, the marker structure can be selected in such a way that when the melt jet impinges perpendicularly on the writing surface, the marker structure represents a circle in the center of which the position of the writing point is located. If the melt beam is deflected by the scanning unit, the circular shape of the marker structure is thus distorted.

From the strength of the distortion, a center of the

Marker structure and thus the position of the writing point can be determined.

According to at least one embodiment, the melt jet and at least one further melt jet are used for additive manufacturing of the workpiece, which in particular can be deflected independently of one another by the scanning unit or by respective scanning units and directed onto the writing surface. Based on the at least one detector signal, a common coordinate system can then be defined for all melt jets.

According to a further independent aspect of the improved concept, a device for additive manufacturing of a workpiece is specified. The device has a scanning unit which is set up to direct a melt beam onto a writing point on a writing plane. The device also has an optical detector, a control unit and an imaging unit. The device also has a projection unit, in particular with at least one projector, which is set up and arranged to project a marker structure onto the writing surface. The imaging unit is set up to image at least a partial area of the writing surface onto which at least a part of the marker structure was projected on the optical detector. The detector is set up to generate at least one detector signal as a function of the image and the control unit is set up to control the steering unit as a function of the at least one detector signal. According to at least one embodiment, the projection unit contains at least one line projector, in particular a laser line projector, which is set up and arranged in such a way that it can project one or more lines, in particular laser lines, onto the writing surface that are parallel or substantially parallel to one The main plane of the writing surface.

The main plane can, for example, be a plane that is perpendicular to the melt beam when the melt beam is not deflected by the scanning unit. In particular, the main plane of the writing surface can be an initial

Corresponding to the writing surface or writing plane at the start of production or before the start of production.

"Essentially parallel" can be understood here, for example, to mean that the laser line encloses an angle with the main plane that is small enough for the laser line to run over an entire working area of the writing surface, i.e. over the entire writing surface or the entire powder bed and is recognizable on this.

According to at least one embodiment, the device has a beam splitter which is net angeord on an input side of the scanning unit in a beam path of the melt beam. The projection device is set up and arranged in such a way that it can generate a projection beam which can be coupled into the beam path of the fusible source via the beam splitter.

According to at least one embodiment, the device has a camera system, at least one camera of the camera system, which contains the detector, being arranged between the scanning unit and the writing surface.

In particular, the camera is between the focusing unit or the F-theta lens and the writing surface or the powder bed arranged. The camera can also be arranged at the same height as the focusing unit or the F-theta lens.

In particular, the camera is arranged outside a Arbeitsraumgehäu ses of the device, within which the workpiece and the writing plane are located, in particular between the scanning unit and a cover window of the Arbeitsraumge housing.

As a result, the entire work space can advantageously be observed by the camera or the at least one camera. The arrangement outside the workspace housing protects against dirt, for example.

According to at least one embodiment, at least one further camera of the camera system is arranged and set up in such a way that it uses light decoupled from a beam path of the melt beam for imaging the partial area of the

Can detect writing surface on the detector.

The at least one further camera includes the detector, for example.

Embodiments are also possible which contain at least one camera and at least one further camera. In this case, the device has several detectors, in particular each of the at least one cameras includes a corresponding detector and each of the other cameras includes a corresponding further detector.

The detectors and further detectors can then jointly or individually take on one or more tasks that are described above with regard to the detector, in particular the optical detector. In particular, the detectors of the cameras and the further detectors of the further cameras can generate the detector signals.

According to a further independent aspect of the improved concept, a computer program is specified with commands which, when the computer program is executed by a computer system, cause a device according to the improved concept to carry out a method according to the improved concept.

The computer system is in particular a computer system of the device. In particular, the computer system contains the control unit, the evaluation unit and / or another processor unit of the device.

According to a further independent aspect of the improved concept, a computer-readable storage medium is specified on which a computer program according to the improved concept is stored.

Further embodiments of the device according to the improved concept follow directly from the various embodiments of the method according to the improved concept and vice versa. In particular, a device according to the improved concept is set up to carry out a method according to the improved concept.

The invention is explained in more detail below with reference to specific embodiment examples and associated schematic drawings. Identical or functionally identical elements are provided with the same reference symbols in the figures. The description of identical or functionally identical elements is possibly not necessarily repeated in different figures.

In the figures show: 1 shows a schematic representation of an exemplary

Embodiment of a device according to the improved concept;

2 shows a schematic representation of a marker structure according to an exemplary embodiment of a method according to the improved concept;

3 shows a schematic representation of another

Marker structure according to a further exemplary embodiment of a method according to the improved concept;

4 shows a schematic representation of a projection unit of an exemplary embodiment of a device according to the improved concept;

5 shows a schematic illustration of a further projection unit according to a further exemplary embodiment of a device according to the improved concept; and

6 shows a schematic representation of a further projection unit according to a further exemplary embodiment of a device according to the improved concept.

In FIG. 1, a device according to an exemplary embodiment is shown according to the improved concept.

The device has a scanning unit or scanning unit 2, which can systematically deflect a melt beam 3, for example a laser beam or electron beam, in accordance with a work order for the production of a workpiece and direct or direct it to a writing point 4 in a powder bed. The device also has a control unit 6, which is coupled, for example, to the scanning unit 2 in order to control it. The melt beam 3 is generated by a heating source 9, in particular a laser source or electron beam source, and coupled into the scanning unit 2 by means of optional beam-shaping optics 12 on an input side of the scanning unit 2. To image the melt jet 3 on the writing point 4, the device can have a focusing unit 10, for example with an F-theta lens.

The device also has a projection unit 11, which is set up to project a marker structure onto the powder bed or a writing surface 1. The writing surface 1 can be a surface, in particular a current surface given at a certain point in time of production, of the powder bed and / or correspond to a workpiece to be manufactured.

The device also has a detector 5 and an imaging unit 2, the imaging unit being set up to image at least part of the writing surface onto which at least part of the marker structure was projected onto the optical detector 5. The imaging unit can be designed, for example, as a camera that contains the detector 5. The camera can, for example, be aimed directly at the writing surface 1 or the powder bed. Alternatively or additionally, the imaging unit, as shown in FIG. 1, contain a beam splitter 7, which is arranged in a beam path of the melt beam 3, in particular on the input side to the scanning unit 2, that is between the scanning unit 2 and the heating source 9. Of the

Beam splitter 7 is used to decouple imaging light, which starts from the writing surface 1 and hits the beam splitter 7 via the scanning unit 2, from the beam path of the melt beam 3, so that the imaging light, for example, via an optional lens 8, for example the camera, onto the Detector 5 can be imaged. The imaging unit, in particular the camera or the detector 5, can therefore observe and recognize the marker structure projected onto the writing surface 1. Based on an image which is mapped onto the detector 5, the detector 5 can generate one or more detector signals which, for example, correspond to an observation of the marker structure and / or the writing point 4. The control unit 6 can control the scanning unit 2, for example, as a function of the detector signals.

FIG. 2 shows an exemplary version of a marker structure as can be used in FIG. In the example in FIG. 2, the marker structure contains, for example, a plurality of n parallel and straight lines, which for example run parallel to a y-axis of the writing surface 1 and are arranged at respective distances d1, d2... Dn from one another. The distances d1, d2, dn can be identical, but can also be different. In various embodiments, all distances between all lines parallel to the y-axis dl, d2 ... dn are different from one another.

The marker structure also has N parallel and straight lines, which are arranged parallel to an x-axis of the writing surface 1 and are in particular perpendicular to the n lines parallel to the y-axis. Distances between the N lines parallel to the x-axis are marked with D1, D2 ... DN. These distances D1, D2, DN can also be identical or different, in particular all different.

The lines of the marker structure shown in FIG. 2 can be, for example, laser lines that were projected onto the writing plane by means of the projector 11.

The respective line widths of the different lines can be identical or different from one another. For example, all parallel lines can have identical line widths which, for example, differ from those of lines perpendicular thereto. In embodiments, the liabili Change the width of the lines for parallel lines along their arrangement along the x-axis or along the y-axis.

By using different distances dl, d2, dn or Dl, D2, DN and / or by using different line widths, location coding can take place. By observing in particular the intersection points between the lines parallel to the y and x axes and depending on the position of the writing point relative to these intersection points, it is possible, in particular, using the location coding to determine where the writing point 4 is positioned with respect to one or more of the intersection points. A position of the writing point 4 can be determined accordingly. The position of the writing point 4 can be understood, for example, as a position of one of the intersection points of the lines to which the

Is closest to writing point 4. But alternative assignments are also possible.

If, for example, an alignment of the detector 5 does not match a line direction of the lines, a coordinate value between intersection points of the lines can also be determined with high accuracy via a lateral displacement of the lines in the marker structure. It is advantageous if at least two lines are in the field of view of the detector 5. Alternatively or additionally, a line of the marker structure together with the information on the current writing point 4 or a setting of the scanning unit 2 can be offset, so that an oblique line over the intersection on two marked axes or pixel lines in the detector 5 with the displacement of the Points of intersection of the lines a current position of the field of view of the detector 5 can be checked and possibly also coded.

Because of possible laminar gas flows in connection with a discharge of vapors from the writing area directly above the writing surface, disturbances of the line projection and in particular lateral deflections can occur. This late- Real deflections can for example also be monitored with a sensor for at least one line. Both pixel-based detectors such as cameras or line arrays or also integrating detectors such as position sensing devices for one or two measuring directions can be used for detection.

As a camera, for example, as shown in FIG. 1, a camera or a detector 5 can be used which capture an area around the writing point 4 coaxially with the melt jet 3 via the writing system, i.e. via the scanning unit 2 and the focus unit 10 can. For this purpose, the observation beam path is decoupled from the melt beam 3, for example via the beam splitter 7, preferably between the heating source 9 for the melt beam 3 and the scanning unit 2.

The projection unit 11 in FIG. 1 is only an example of one or more projectors, for example line projectors. In the example in FIG. 2, for example, a separate line projector can be used for each line, as is sketched in FIG. 4, for example.

Alternatively or in addition, multi-line generators can be used for several of the lines, as explained with respect to FIG. 5 and FIG.

FIG. 3 shows an alternative variant of a marker structure as it can also be used in a device according to FIG. For example, instead of parallel lines, several lines diverge in a fan shape

Beams or line bundles projected onto the writing surface. In the example shown in FIG. 3, starting from three points A, B, C at respective edges of the writing surface, bundles of lines diverging in a fan shape are sent out. The positions of the source points of the compartments A, B, C are only chosen as examples and can be distributed, for example, in such a way that one as possible homogeneous spatial resolution is generated on the writing surface. In particular, the corresponding line generators do not have to be arranged in corners or other prominent positions on the writing surface.

For a marker structure as in FIG. 3, for example, three multiline generators can be used, as are shown in FIG. 5 or FIG. In such a case, a multiline generator can be arranged at each of the points A, B, C, from each of which a fan-shaped divergent beam emanates.

Such an arrangement of line generators and the resulting marker structure intersect lines of different starting points A, B, C with defined angles. Instead of the location coding, as explained with regard to FIG. 2 with regard to different distances between the lines and / or different line widths of the lines, location coding can be carried out with a marker structure as in FIG. 3 via angle coding at the intersection points. Of course, the angle coding can also be combined with a spatial coding using a spacing sequence or line widths or both of the lines.

In various embodiments, different ones of the line generators at locations A, B, C can emit laser lines with different wavelengths, so that lines emanating from A have a different wavelength than those emanating from B and / or C, for example.

In particular, laser sources with different colors can be used. This makes the lines of different sources visually distinguishable. It is thus possible to provide color coding for the source locations A, B, C.

Alternatively or additionally, the line generators at points A, B, C can generate the respective lines offset in time with respect to one another, that is to say according to time multiplexing become. In such embodiments, a time coding of the points on the writing surface is then possible.

In further embodiments, projection light from the projection unit 11 can also be coupled into the beam path of the melt beam 3, for example via a further beam splitter, so that the melt beam 3 is coupled or deflected synchronously with the projection beam. This enables marker structures which are always arranged around a writing point 4 of the melt jet 3 or around the melt jet 3.

In addition to the melt jet 3, a pattern, which can consist of points, line pieces, circles, ellipses and / or other geometric elements, can be projected onto the writing surface 1. As described, the pattern to be projected can be superimposed on the melt beam 3 by coupling into the beam path of the melt beam 3.

Advantageously, the melt jet 3 and the pattern or marker structure have different wavelengths. This leads to very low-loss transmission, for example by means of dichroic beam splitters. The camera or the detector 5 can also be protected from the much higher intensity of the melt beam 3, for example by attenuating filters, at least to the extent that the melt beam 3 and the marker structure can be detected and measured together. Wavelengths for the marker structure can be selected, for example, in such a way that the effects of vapors emanating from the writing surface on absorption or beam deflection or possible thermal effects are largely minimized.

For this purpose it can be advantageous to select the wavelength for the

To choose marker structure in an area in which on the one hand the detector 5, or the camera, which can be designed as an overview camera, is sensitive, as well as the dispersion of the atmosphere, especially in the blue sky rich is as low as possible, especially when it is very warm. A single camera or an arrangement of several cameras, which can then for example capture different and in particular overlapping areas of the writing surface 1, can serve as an overview camera.

The procedure for the overview camera is shown using the example of a camera, but it can also be directly functionally and analogously transferred to several cameras. The marker structure can then be imaged on the writing surface 1 around the melt stream 3 or, in the case of several melt streams, around each of the melt streams. In the illustration, the marker structure is mapped in an oblique direction of incidence of the respective melt beam 3 according to the inclination of the beam 3 and due to the distortion of the focus unit, for example the F-theta lens. If, for example, a circle is assumed as the marker structure, in the center of which the melt jet 3 is arranged, the circle generally becomes an ellipse. However, since the marker structure can be significantly larger than an extension of the melt beam 3 or writing point 4, it can be detected by the detector 5 with more pixels and thus a corresponding distorted image of the marker structure, also known as an oblique image, is a fit Center position of the marker structure and thus of the

Melt jet 3 can be determined with high precision.

To increase the precision of the fit and to avoid interference from the writing process, it can be advantageous to evaluate only that part of the image that lies on one side of the melt jet 3 assigned to the camera. If several cameras are used to record the marker structure, the marker structure is then recorded from different perspectives and thus different sub-areas of the marker structure are also evaluated. Measurement results from the several cameras can then be converted into a common measurement result, for example. Weighting factors can also be taken into account, for example a quality ("Quality of Sensing") of the respective measurement result of a camera are taken into account.

Furthermore, it can be advantageous if a detection area of the camera or cameras is somewhat larger than an entire production area of the device, that is to say than the entire production area

Writing surface. For example, a reference structure, in particular in the sense of a reference frame, possibly expanded to include reference marks that can be one-, two- or three-dimensional, can be provided to provide a common and stable reference for the field of view of the respective camera or the plurality to create cameras. A displacement and / or tilting of the respective camera can then also be detected via the reference structure and, if necessary, also compensated by software.

Both when using marker structures overlaying the melt jet 3 and when using lines or patterns projected directly onto the writing surface, it can be helpful to manufacture reference elements in addition to the actual workpiece, especially in the edge or corner areas of the writing surface 1 in order to have a reference directly in a plane of the writing surface 1 for a current height of the structure or the workpiece, which reference can be continuously measured, for example.

FIG. 4 shows an exemplary line projector 11a, which can be part of the projection unit 11 from FIG. 1, for example. The line projector 11a is designed, for example, as a laser line projector, which is preceded by a collimator lens 13a, for example, in order to shape the laser beam in accordance with a desired line shape.

The line projector 11a can, for example, contain a semiconductor laser diode for generating the laser beam for the marker structure. In the upper part of FIG 4 is a plan view of the Laserli never or the writing surface 1 shown in the lower part of FIG 4 is a corresponding side view. Semiconductor lasers can, for example, have different divergences in different directions of propagation. This can be seen, for example, in the upper part of FIG. In a first direction b, the divergence of the laser beam is less than in a second direction perpendicular to the first direction, direction a.

A respective cross-sectional view of the laser line at two different distances XI, X2 from the laser source, that is to say from the generator 11a, is also shown schematically in the upper part of FIG. At point XI, an expansion in the b direction is bl and an expansion of the line in a direction al. At point X2, which is further away from the line projector 11a than XI, there is the extension in the b direction b2, which is, for example, similar or almost identical to the extension bl. Due to the greater divergence in the a direction, however, an extension of the line in the a direction a2 at point x2 is increased compared to a1.

Due to the different divergences, it can be advantageous, for example, to collimate only in one direction, for example by means of a rotationally symmetrical lens, and not to collimate the other direction. The axis with the remaining higher divergence can then be used for the height of the line.

The laser line can preferably be incident with a line direction perpendicular to the writing surface 1 or to the powder bed with a beam axis that runs almost or almost parallel over the writing surface 1. For example, the beam axis is tilted slightly in the direction of the writing surface in order to ensure that the line is easily recognizable at every location on the writing surface or writing surface. This is shown in the lower part of FIG. Since the writing surface 1 can, for example, have a linear extent in the range of a few 10 mm to 100 mm, it can be advantageous to keep the point of impact on the writing surface 1 in the line generation that varies over the line height. For example, a Powell lens can have a different focal length over its height. Diffractive optical elements, DOE, can also be appropriately adapted and used, for example, in that over a direction of the DOE in which the line extends, structures also become larger with increasing focus distance. Alternatively, purely collimated beams can also be used if the collimation is good enough, at least in the direction of the line width, that the projected line is mapped sufficiently sharp.

In particular, for the generation of marker structures as in FIG. 3, several line or fan-type projectors can also be used. This is shown schematically in FIGS. 5 and 6. FIG. 5 shows a further line projector 11b with an optional upstream collimator lens 13b. The line projector 11b is located behind a DOE or an optical grating 14. Correspondingly, the grating or DOE 14 leads to a diffraction of the laser beams and, accordingly, to a fanning out of the laser lines.

FIG. 6 shows a further variant of a projection unit or a line projector 11c, which is preceded by an optional collimator lens 13c and a DOE 15. Furthermore, a beam splitter 16 is provided. Beams fanned out by the DOE are divided further, for example by the beam splitter 16, so that the number of resulting lines is further increased.

The illustrations in FIG. 5 and FIG. 6 serve only to explain that a larger number of exiting beams, in particular with a larger number of beam directions, is generated from a number of incident beams. The beam splitter 16 can, for example, be dielectric, birefringent, polarizing and / or diffractive.

It is also possible to superimpose several wavelengths or laser sources in order to generate more different beam directions with one DOE.

According to a method and a device according to the improved concept, imaging errors in the beam deflection by the scanning unit, in particular by the focusing unit or the F-theta lens, can be detected and compensated effectively and in real time during the production of the workpiece. In contrast to static calibration methods, deviations and changes in the system and in temperature as well as other environmental conditions that occur during production can be taken into account.

According to the improved concept, a method for geometric referencing for additive manufacturing systems can be specified which is able to perform geometric referencing of the writing point during the writing process, ie online, and to guide the writing point exactly to a desired location.

If the device for additive manufacturing uses more than one melt jet, then these are provided, for example, by different scanning units. Each of the scanning units is corrected in the manner described in accordance with the improved concept. In the case of devices with two or more melting jets or scanning units, according to the improved concept, geometrical referencing can be specified in such a way that the writing surfaces or writing areas of the different melting jets or scanning units have a common coordinate system or separate coordinate systems which, however, are free of offset or seams Rotation errors or scaling errors can connect to one another. The improved concept allows the workpiece to be manufactured with reference to a reference coordinate system, for example a machine or workpiece coordinate system. Disruptive influences on the dimensional precision of the manufacturing process can be compensated. A real-time correction of detected dimensional deviations can also be carried out. To evaluate the detector signals and to compensate for the

Errors caused by appropriate activation of the scanning unit, the control unit can contain, for example, a geometrical correction unit to compensate for detected geometrical deviations for a current production order. For example, an interface to the production machine can be provided for correcting the machine movements for the production order or for transferring new coordinate values for the production order.

Claims

1. A method for additive manufacturing of a workpiece, wherein a melt jet (3) is directed onto a writing point (4) on a writing surface (1);

in which

- A marker structure is projected onto the writing surface (1);

- at least a partial area of the writing surface (1), onto which at least a part of the marker structure was projected, is imaged onto an optical detector (5);

- at least one detector signal is generated based on the image;

- A position of the writing point (4) is determined based on the at least one detector signal;

- The production of the workpiece is continued, taking into account the position of the writing point (4);

the marker structure contains a first and a second group of lines;

- each line of the first group intersects at least one of the lines of the second group; and

- The position of the writing point (4) is determined by means of a Ortsko dation based on at least one geometric property of the lines of the two groups.

2. The method according to claim 1, wherein the position of the writing point (4) is compared with a target position and the production of the workpiece is continued depending on a result of the comparison.

3. The method according to any one of claims 1 or 2, wherein the lines of the first group with light of a first wavelength he testifies and the lines of the second group with light ei ner second, different from the first wavelength, wavelengths are generated.

4. The method according to any one of claims 1 to 3, wherein the lines of the first group are generated alternately with the lines of the second group.

5. The method according to any one of claims 1 to 4, wherein

- A region is imaged on the detector (5) which contains the entire writing surface (1) and in which at least one reference structure is additionally arranged; and

- the area is adapted using the reference structure or a position and / or alignment of the detector (5) is calibrated using the reference structure.

6. The method according to any one of claims 1 to 5, wherein

- To project the marker structure onto the writing surface (5), a projection beam is coupled into a beam path of the melt beam (3); and

- During the production of the melt beam (3) and the projection beam are moved synchronously.

7. The method according to claim 6, wherein a geometric parameter of the marker structure is determined and the position of the writing point (4) is determined based on the geometric parameter.

8. Device for additive manufacturing of a workpiece, the device having a scanning unit (2), which is directed to a melt beam (3) on a writing point (4) on a writing surface (1), an optical detector (5) , a control unit (6) and an imaging unit, wherein

- The device has a projection unit (11) which is set up and arranged to project a marker structure onto the writing surface (1);

the marker structure contains a first and a second group of lines;

- each line of the first group intersects at least one of the lines of the second group; - the imaging unit is set up to image at least one partial area of the writing surface (1) onto which at least part of the marker structure was projected onto the optical detector (5);

- The detector (5) is set up to generate at least one detector signal as a function of the image;

- The control unit (6) is set up to control the deflection unit (2) as a function of the at least one detector signal, with a position of the writing point (4) being determined based on the at least one detector signal and the position of the writing point (4) is determined by means of a spatial coding which is based on at least one geometric property of the lines of the two groups.

9. Apparatus according to claim 8,

- Having a beam splitter (7) on an input side of the scanning unit (2) in a beam path of the

Melt jet (3) is arranged;

- The projection device (11) being set up and arranged to generate a projection beam which can be coupled into the beam path of the melt beam (3) via the beam splitter (7).

10. Device according to one of claims 8 or 9, having a camera system, wherein at least one camera of the camera system, which includes the detector (5), is arranged between the scanning unit (2) and the writing surface (1).

11. Device according to one of claims 8 to 10, wherein we at least one further camera of the camera system is arranged and set up to detect light decoupled from a beam path of the melt beam (3) for imaging the portion of the writing surface (1) on the detector (5) .

12. Computer program with instructions which, when the computer program is executed by a computer system, cause a device according to one of claims 8 to 11 to carry out a process according to one of claims 1 to 7.