Patent Publication Number: US-2021187861-A1

Title: Build plane measurement systems and related methods

Description:
PRIORITY INFORMATION 
     The present application claims priority to U.S. Provisional Patent Application Ser. No. 62/950,636 filed on Dec. 19, 2019, which is incorporated by reference herein for all purposes. 
    
    
     FIELD 
     The present disclosure generally pertains to systems and assemblies for determining a position of a build plane in an additive manufacturing machine, including systems and assemblies for measuring a distance between an energy beam generating device and a build plane. 
     BACKGROUND 
     An additive manufacturing machine may include one or more energy beam generating devices uses to irradiate sequential layers of powder material defining a powder bed distributed across a build plane. The position of an energy beam generating device relative to the build plane affects properties of the energy beam incident upon the powder material defining the powder bed. An energy beam generating device may be configured with a reference position that defines the position of the energy beam generating device relative to the build plane. However, deviations from the reference position may cause a corresponding undesirable deviations in one or more properties of the energy beam incident upon the powder bed. Such variability may affect the quality of components produced using the additive manufacturing machine. Accordingly, there exists a need for improved additive manufacturing machines and related systems and methods that address actual or potential deviations from a reference position for an energy beam generating device relative to a build plane and/or a powder bed. 
     BRIEF DESCRIPTION 
     Aspects and advantages will be set forth in part in the following description, or may be obvious from the description, or may be learned through practicing the presently disclosed subject matter. 
     In one aspect, the present disclosure embraces additive manufacturing machines. An exemplary additive manufacturing machine may include an energy beam system situated in a fixed position relative to a reference plane coinciding with an expected location of a build plane, an energy beam system with an irradiation device configured to generate an energy beam and to direct the energy beam upon the build plane, and a position measurement system configured to determine a position of the build plane. 
     In another aspect, the present disclosure embraces position measurement assemblies for use with an additive manufacturing machine. An exemplary position measurement assembly may include one or more position sensors, and one or more mounting brackets configured to attach the one or more position sensors to an energy beam system of an additive manufacturing machine. The position measurement assembly may be configured to determine a position of a build plane with the energy beam system situated in a fixed position relative to a reference plane coinciding with an expected location of the build plane. 
     In yet another aspect, the present disclosure embraces methods of determining a position of a build plane of an additive manufacturing machine. An exemplary method may include projecting an energy beam incident upon a build plane, detecting one or more properties of the energy beam upon having been reflected from the build plane, and using the one or more properties of the detected energy beam to determine the position of the build plane. 
     In yet another aspect, the present disclosure embraces methods of aligning a reference plane to a build plane of an additive manufacturing machine, and/or methods of calibrating a position of a reference plane of an additive manufacturing machine. An exemplary method may include determining a position of a build plane of an additive manufacturing machine, comparing the position of the build plane to a corresponding position of a reference plane, and adjusting a position of the reference plane based at least in part on the comparison of the position of the build plane to the corresponding position of the reference plane. 
     In yet another aspect, the present disclosure embraces methods of operating an additive manufacturing machine and/or methods of additively manufacturing an object. In some embodiments, an exemplary method may include determining a position of a build plane of an additive manufacturing machine, and transmitting a control command to one or more controllable components of an additive manufacturing machine based at least in part on the determined position of the build plane. Additionally, or in the alternative, an exemplary method may include additively manufacturing at least a portion of an object, interrupting one or more operations of the additive manufacturing machine, calibrating a reference plane based at least in part on a position of a build plane determined using a position measurement system, and resuming the one or more operations of the additive manufacturing machine. 
     In yet another aspect, the present disclosure embraces additive manufacturing systems. An exemplary system may include an additive manufacturing machine, one or more build modules configured to be installed into and removed from the additive manufacturing machine, one or more powder modules configured to be installed into and removed from the additive manufacturing machine; and a control system. In some embodiments, an exemplary additive manufacturing machine may include an energy beam system situated in a fixed position relative to a reference plane coinciding with an expected location of a build plane, an energy beam system comprising an irradiation device configured to generate an energy beam and to direct the energy beam upon the build plane, and a position measurement system configured to determine a position of the build plane. The control system may be configured to cause the position measurement system to determine the position of the build plane. 
     In yet another aspect, the present disclosure embraces computer-readable medium comprising computer-executable instructions, which when executed by a processor associated with an additive manufacturing system, causes the additive manufacturing system to perform operations in accordance with the present disclosure. Exemplary operations may include determining a position of a build plane of an additive manufacturing machine, aligning a reference plane to a build plane of an additive manufacturing machine, calibrating a position of a reference plane of an additive manufacturing machine, operating an additive manufacturing machine, and/or additively manufacturing an object. 
     These and other features, aspects and advantages will become better understood with reference to the following description and appended claims. The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate exemplary embodiments and, together with the description, serve to explain certain principles of the presently disclosed subject matter. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A full and enabling disclosure, including the best mode thereof, directed to one of ordinary skill in the art, is set forth in the specification, which makes reference to the appended Figures, in which: 
         FIG. 1  schematically depicts an exemplary additive manufacturing system; 
         FIG. 2  schematically depicts a perspective view of an exemplary additive manufacturing machine; 
         FIG. 3  schematically depicts an exemplary irradiation unit; 
         FIGS. 4A and 4B  schematically depict exemplary position measurement systems; 
         FIGS. 5A and 5B  schematically depict an exemplary position measurement systems determining a position of a build plane; 
         FIG. 6  schematically depicts an exemplary comparison of a build plane to a reference plane; 
         FIGS. 7A-7D  schematically depict exemplary features of a build plane that may be associated with a divergence from a reference plane; 
         FIG. 8  schematically depicts an exemplary depth map of a surface of a build plane; 
         FIGS. 9A-9D  show flowcharts depicting exemplary methods in accordance with the present disclosure; and 
         FIG. 10  schematically depicts an exemplary control system. 
     
    
    
     Repeat use of reference characters in the present specification and drawings is intended to represent the same or analogous features or elements of the present disclosure. 
     DETAILED DESCRIPTION 
     Reference now will be made in detail to exemplary embodiments of the presently disclosed subject matter, one or more examples of which are illustrated in the drawings. Each example is provided by way of explanation and should not be interpreted as limiting the present disclosure. In fact, it will be apparent to those skilled in the art that various modifications and variations can be made in the present disclosure without departing from the scope of the present disclosure. For instance, features illustrated or described as part of one embodiment can be used with another embodiment to yield a still further embodiment. Thus, it is intended that the present disclosure covers such modifications and variations as come within the scope of the appended claims and their equivalents. 
     It is understood that terms such as “top”, “bottom”, “outward”, “inward”, and the like are words of convenience and are not to be construed as limiting terms. As used herein, the terms “first”, “second”, and “third” may be used interchangeably to distinguish one component from another and are not intended to signify location or importance of the individual components. The terms “a” and “an” do not denote a limitation of quantity, but rather denote the presence of at least one of the referenced item. 
     Here and throughout the specification and claims, range limitations are combined and interchanged, and such ranges are identified and include all the sub-ranges contained therein unless context or language indicates otherwise. For example, all ranges disclosed herein are inclusive of the endpoints, and the endpoints are independently combinable with each other. 
     Approximating language, as used herein throughout the specification and claims, is applied to modify any quantitative representation that could permissibly vary without resulting in a change in the basic function to which it is related. Accordingly, a value modified by a term or terms, such as “about”, “approximately”, and “substantially”, are not to be limited to the precise value specified. In at least some instances, the approximating language may correspond to the precision of an instrument for measuring the value, or the precision of the methods or machines for constructing or manufacturing the components and/or systems. 
     The present disclosure provides additive manufacturing machines and related systems and methods, as well as systems and assemblies for determining a position of a build plane in an additive manufacturing machine. The position of the build plane may be compared to a corresponding position of a reference plane, for example, to determine a divergence between the build plane and the reference plane. The position of the reference plane may be adjusted based at least in part on the comparison of the position of the build plane to the corresponding position of the reference plane. 
     In some embodiments, an adjustment to a position of a reference plane may be performed, for example, in connection with a calibration procedure, such as to align the build plane with the reference plane. Such a calibration procedure may be performed at any time, including before, during, and/or after operating the additive manufacturing machine. In some embodiments, a calibration procedure may be performed when additive manufacturing operation has been interrupted, such as prior to resuming operations. For example, with large format additive manufacturing machines, in some instances additive manufacturing operations may be interrupted to make operational changes, such as replenishing a supply of powder material and/or exchanging build modules, powder modules, and/or overflow modules, and so forth. The calibration may mitigate a divergence of a build plane from a reference plane, thereby improving precision and accuracy of the additive manufacturing and the resulting quality of objects manufactured using the additive manufacturing machine. 
     As used herein, the term “build plane” refers to a plane defined by a surface upon which an energy beam impinges during an additive manufacturing process. Generally, the surface of a powder bed defines the build plane; however, prior to distributing powder material across a build module, a build plate that supports the powder bed generally defines the build plane. When performing an additive repair process on a previously fabricated object, the build plane refers to a plane defined by a surface of the previously fabricated object upon which the energy beam impinges during the additive repair process. 
     As used herein, the term “reference plane” refers to a plane that defines an expected position of a build plane. A reference plane may be defined, for example, according to a calibration, which may be performed periodically and updated from time to time. The reference plane may be utilized by an additive manufacturing machine to direct an energy beam onto a build plane to melt or fuse a layer of powered material corresponding to the build plane. 
     In some embodiments, a “scan field calibration” may be performed using a position of the build plane determined in accordance with the present disclosure and/or using a position of a reference plane that has been adjusted based at least in part on a determined position of a build plane. The scan field calibration may be performed to ensure that the energy beam system provides an energy beam that is properly focused across the working surface of the build plane. Scan field calibrations may be performed at any time, including before, during, and/or after sequential layers of powder material have been distributed across the build module and/or before, during, and/or after the sequential layers of powder have been selectively irradiated by an energy beam to form the respective layer of the object. Scan field accuracy may be improved with respect to the operation of a particular energy beam generating device, and/or with respect to relative operations of cooperative energy beam devices such as those used to perform overlapping contour paths (e.g., multi-beam stitching). 
     In some embodiments, the presently disclosed subject matter may be employed to mitigate erroneous calibrations, unintended movements of various componentry of the additive manufacturing machine such as movements of the build platform. Additionally, recoater wear or damage can be mitigated that may otherwise impact the position of the build plane relative to the reference plane. 
     In some embodiments, the presently disclosed subject matter may be employed to determine a movement of an object being additively manufactured, to determine a probability of a recoater contacting an object or a portion thereof, and/or to determine a location of an object in connection with an additive repair process and/or a machine restart. 
     As described herein, exemplary embodiments of the present subject matter involve the use of additive manufacturing machines or methods. As used herein, the terms “additively manufactured” or “additive manufacturing techniques or processes” refer generally to manufacturing processes wherein successive layers of material(s) are provided on each other to “build-up,” layer-by-layer, a three-dimensional component. The successive layers generally fuse together to form a monolithic component which may have a variety of integral sub-components. 
     An exemplary additive manufacturing machine may be configured to utilize any desired additive manufacturing technique. In an exemplary embodiment, the additive manufacturing machine  102  may perform a powder bed fusion (PBF) process, such as a direct metal laser melting (DMLM) process, an electron beam melting (EBM) process, an electron beam sintering (EBS) process, a selective laser melting (SLM) process, a directed metal laser sintering (DMLS) process, or a selective laser sintering (SLS) process. In an exemplary PBF process, the additive manufacturing machine builds components in a layer-by-layer manner by melting or fusing sequential layers of a powder material to one another. 
     Additive manufacturing technology may generally be described as enabling fabrication of complex objects by building objects point-by-point, layer-by-layer, typically in a vertical direction; however, other methods of fabrication are contemplated and within the scope of the present disclosure. For example, although the discussion herein refers to the addition of material to form successive layers, the presently disclosed subject matter may be practiced with any additive manufacturing technique or other manufacturing technology, including layer-additive processes, layer-subtractive processes, or hybrid processes. 
     The additive manufacturing processes described herein may be used for forming components using any suitable material. For example, the material may be metal, concrete, ceramic, polymer, epoxy, photopolymer resin, plastic, or any other suitable material that may be in solid, liquid, powder, sheet material, wire, or any other suitable form. 
     Each successive layer may be, for example, between about 10 μm and 200 μm, although the thickness may be selected based on any number of parameters and may be any suitable size according to alternative embodiments. In exemplary embodiments, the position of a reference plane and/or other operational parameters of an additive manufacturing machine may be adjusted based at least in part on a position of a build plane having been determined in accordance with the present disclosure. 
     Such adjustments to the position of the reference plane may made in any one or more directions of a cartesian coordinate system. In an exemplary embodiment a position of a reference plane may be adjusted vertically (e.g., along a z-axis); however, lateral (x-axis) and horizontal (y-axis) adjustments are also contemplated. Additionally, or in the alternative, adjustments to one or more operating parameters of the additive manufacturing machine may effectively equate to an adjustment to the position of the reference plane. With reference to the position of a reference plane-point, such adjustments may actually or effectively move the reference plane-point a distance of from about 1 micrometers (μm) and about 2 millimeters (mm), such as from about 1 μm and 1 mm, such as from about 1 μm and 10 μm, such as from about 2 μm and 10 μm, such as from about 5 μm and 50 μm, such as from about 10 μm and 1 mm, such as from about 10 μm and 500 μm, or such as from about 25 μm and 250 μm. 
     A reference plane-point may be adjusted by relatively small increments or relatively large increments. For example, a reference plane-point may be adjusted by at least 1 μm, such as at least 2 μm, such as at least 5 μm, such as at least 10 μm, such as at least 25 μm, such as at least 50 μm, such as at least 100 μm, such as at least 250 μm, such as at least 500 μm, or such as at least 1 mm. Additionally, or in the alternative, a reference plane-point may be adjusted by less than 1 mm, such as less than 500 μm, such as less than 250 μm, such as less than 100 μm, such as less than 50 μm, such as less than 25 μm, such as less than 10 μm, such as less than 5 μm, or such as less than 2 μm. 
     In some embodiments, an adjustment to the reference plane may provide improved accuracy and precision of additive manufacturing operations so as to allow for objects to be additively manufactured with dimensional tolerances of +/− 50 μm or less, such as +/− 30 μm or less, such as +/− 25 μm or less, such as +/− 10 μm or less, or such as +/− 5 μm or less. 
     Exemplary embodiments of the present disclosure will now be described in further detail.  FIG. 1  schematically depicts an exemplary additive manufacturing system  100 . The additive manufacturing system  100  may include one or more additive manufacturing machines  102 . The one or more additive manufacturing machines  102  may include a control system  104 . The control system may include componentry integrated as part of the additive manufacturing machine  102  and/or componentry that is provided separately from the additive manufacturing machine  102 . Various componentry of the control system  104  may be communicatively coupled to various componentry of the additive manufacturing machine  102 . 
     The control system  104  may be communicatively coupled with a management system  106  and/or a user interface  108 . The management system  106  may be configured to interact with the control system  104  in connection with enterprise-level operations pertaining to the additive manufacturing system  100 . Such enterprise level operations may include transmitting data from the management system  106  to the control system  104  and/or transmitting data from the control system  104  to the management system  106 . The user interface  108  may include one or more user input/output devices to allow a user to interact with the additive manufacturing system. 
     As shown, an additive manufacturing machine  102  may include build module  110  that includes a build chamber  112  within which an object  114  may be additively manufactured in a layer-by-layer manner. In some embodiments, an additive manufacturing machine  102  may include a powder module  116  and/or an overflow module  118 . The build module  110 , the powder module  116 , and/or the overflow module  118  may be provided in the form of modular containers configured to be installed into and removed from the additive manufacturing machine such as in an assembly-line process. Additionally, or in the alternative, the build module  110 , the powder module  116 , and/or the overflow module  118  may define a fixed portion of the additive manufacturing machine  102 . 
     The powder module  116  contains a supply of powder material  120  housed within a supply chamber  122 . The powder module  116  includes a powder piston  124  that elevates a powder floor  126  during operation of the additive manufacturing machine  102 . As the powder floor  126  elevates, a portion of the powder material  120  is forced out of the powder module  116 . A recoater  128  such as a blade or roller sequentially distributes thin layers of powder material  120  across the build module  110 . A build platform  130  supports the sequential layers of powder material  120  distributed across the build module  110 . 
     The additive manufacturing machine includes an energy beam system  132  configured to generate an energy beam  134  such as a laser or an electron beam, and to direct the energy beam  134  onto the surface of a powder bed  136  defined by the sequential layers of powder material  120 , thereby selectively melting or fusing the sequential layers of powder material  120  to form the object  114 . Typically with a DMLM, EBM, or SLM system, the powder material  120  is fully melted, with respective layers being melted or re-melted with respective passes of the energy beam  134 . Conversely, with DMLS or SLS systems, layers of powder material  120  are sintered, fusing particles of powder material  120  to one another generally without reaching the melting point of the powder material  120 . The energy beam system  132  may include componentry integrated as part of the additive manufacturing machine  102  and/or componentry that is provided separately from the additive manufacturing machine  102 . 
     The energy beam system  132  includes an irradiation device  138  configured to generate an energy beam and to direct the energy beam upon a build plane. To irradiate a layer of powder having been distributed across the build module  110 , the irradiation device directs the path of the energy beam  134  across the surface of the powder bed  136  so as to melt or fuse only the portions of the powder material  120  that are to become part of the object  114 . The first layer or series of layers of powder material  120  are typically melted or fused to the build platform  130 , and then sequential layers of powder material  120  are melted or fused to one another to additively manufacture the object  114 . 
     The energy beam system  132  is situated in a fixed position relative to a reference plane  141  coinciding with the expected location of the build plane  142 . The operation of the energy beam system  132  may be calibrated to fixed refence position  143  that may be used to determine a position of the energy beam system  132  relative to various locations of the reference plane  141 . As sequential layers of powder material  120  are melted or fused to one another, a build piston  140  gradually lowers the build platform  130  so as to make room for the recoater  128  to distribute sequential layers of powder material  120 . The as the build piston gradually lowers  140  and sequential layers of powdered material  120  are applied across the build module  110 , the next sequential layer of power material  120  defines the surface of the powder bed  136  coinciding with the build plane  142 . Sequential layers of powder material  120  may be selectively melted or fused to the object  114  until a completed object  114  has been additively manufactured. 
     In some embodiments, an additive manufacturing machine may utilize an overflow module  118  to capture excess powder material in an overflow chamber  144 . The overflow module  118  may include an overflow piston  146  that gradually lowers to make room within the overflow chamber  144  for additional excess powder material  120 . 
     It will be appreciated that in some embodiments an additive manufacturing machine may not utilize a powder module  116  and/or an overflow module  118 , and that other systems may be provided for handling powder material, including different powder supply systems and/or excess powder recapture systems. However, the subject matter of the present disclosure may be practiced with any suitable additive manufacturing machine without departing from the scope hereof. 
     Still referring to  FIG. 1 , in accordance with the present disclosure, an additive manufacturing machine  102  may include a position measurement system  150 . The position measurement system  150  is configured to project a measurement beam  152  upon the build plane  142  and to detect one or more properties of the measurement beam  152  upon having been reflected from the build plane  142 . The one or more properties of the reflected measurement beam  152  may be used to determine a distance between the energy beam system  132  and the build plane  142 . The position measurement system  150  may be an imaging system configured to project an electromagnetic beam such as an infrared beam from a laser diode. The position measurement system  150  may determine the spatial position of one or more points of the build plane  142  relative to one or more reference positions  143  for the energy beam system  132  and/or the position measurement system  150 . The spatial position of the one or more points of the build plane  142  may be determined on the basis of any non-contact distance measurement technology including, for example, laser triangulation, interferometry, confocal displacement, structured light, modulated light, and/or time of flight, as well as combinations of these. Additionally, or in the alternative, any other suitable position sensing technology may be used, including ultrasound, magnetic displacement, and/or capacitive displacement technology. 
     The position measurement system  150  may project a measurement beam  152  in the form of a point, and/or the position measurement system  150  may project a linear or two-dimensional field of view  154 . With a measurement beam  152  projected in the form of a point, the position measurement system  150  may determine a position of a static location of the build plane  142 , and/or the measurement beam  152  may be scanned across at least a portion of the build plane  142  so as to determine a position with respect to a plurality of locations across the build plane  142 . With the measurement beam  152  projected across a linear and/or three dimensional field of view, the position measurement system  150  may determine a position of a plurality of location across the build plane  142 . The field of view  154  may remain static and/or the field of view may be scanned across at least a portion of the build plane  142 . For example, a measurement beam  152  projected as a linear field of view  154  may be scanned across the build plane  142  to define a two-dimensional field of view  154 . 
     The position measurement system  150  may include componentry integrated as part of the additive manufacturing machine  102  and/or componentry that is provided separately from the additive manufacturing machine  102 . For example, the position measurement system  150  may include componentry integrated as part of the energy beam system  132 . Additionally, or in the alternative, the position measurement system  150  may include separate componentry, such as in the form of an assembly, that can be installed as part of the energy beam system  132  and/or as part of the additive manufacturing machine  102 . 
     By way of example,  FIG. 2  shows an exemplary position measurement system  150  that includes a position measurement assembly  200  attached to an energy beam system  132 . The position measurement assembly  200  may include one or more position sensors  202 . As shown, two position sensors  202  are provided. However, it will be appreciated that a position measurement assembly  200  may include any number of position sensors  202 . The energy beam system  132  may be configured to generate one or more energy beams  134 . For example, as shown, the energy beam system  132  may generate two energy beams  134  from one, two, or more energy beam sources (see, e.g.,  FIG. 7 ). In some embodiments, the number of position sensors  202  may correspond to the number of energy beams  134  generated by the energy beam system  132  and/or the number of energy beam generating sources. 
     The position measurement assembly  200  may include various componentry for installing or attaching the one or more position sensors  202  in a fixed position. For example, the one or more position sensors  202  and other componentry may be attached to the energy beam system  132  in a suitable manner. As shown, the position measurement assembly  200  may include one or more mounting brackets  204  configured to attach the one or more position sensors  202  to the energy beam system  132 . In some embodiments, an energy beam system  132  may include an energy beam generating assembly  206  mounted to an energy beam rail  208 . For example, an energy beam housing  210  may be mounted to the energy beam rail  208 , and the one or more energy beam sources may be contained within the energy beam housing  210 . By mounting the one or more position sensors  202  to the energy beam rail  208 , such as using the one or more mounting brackets  204 , the one or more position sensors  202  are given a fixed position relative to the one or more energy beam sources  210 , which facilitates measurement accuracy. For example, any movement or vibration of the additive manufacturing machine generally would be either absorbed by the energy beam rail  208  or would affect the of the energy beam housing and position sensors relatively equivalently. However, the one or more position sensors  202  may be mounted in any location that allows the respective one or more measurement beams  152  to be projected upon the build plane  142 . Vibration damping materials and componentry may be utilized in the event such movement or vibration are of concern. 
     The energy beam system  132  and/or the position measurement system  150  may be contained within an instrument cladding  212  prevents against a stray energy beam from exiting the additive manufacturing machine while also providing some protection to the energy beam system  132  and/or the position measurement system  150  from loose powder  120  that may be disrupted from the powder bed  136 . The instrument cladding  212  includes a bottom surface  214  with one or more ports  216  through which the one or more energy beams  134  and the one or more measurement beams  152  are projected onto the powder bed  136  and/or the build plane  142 . The instrument housing may be surrounded by an outer housing  218  defining a build chamber  220  within which the additive manufacturing process takes place. 
     Now turning to  FIG. 3 , an exemplary irradiation device  138  will be described. As shown, an irradiation device  138  may include a beam generating unit  300 , a scanner  302 , and an optical assembly  304 . The beam generating unit  300  is configured to generate an energy beam  152 , such as a laser beam or an electron beam. The scanner  302  is configured to control the orientation of the energy beam  152  so as to direct the path of the energy beam  152  across the powder bed  136 . The optical assembly  304  includes one or more lenses configured to focus the energy beam  152 . 
     In some embodiments, an irradiation device  138  may include a beam splitter  306  configured to direct at least a portion of the energy beam  134  to an evaluation unit  308  configured to evaluate one or more properties of the energy beam  134 . The evaluation unit  308  may be configured to monitor one or more properties of the energy beam  134 , such as energy density. 
     Operation of the irradiation device may be performed according to control commands from an irradiation control module  310 . The control commands may be transmitted to one or more controllable components associated with the irradiation device. 
     Now turning to  FIGS. 4A and 4B , exemplary position measurement systems  150  will be described. A position measurement system  150  may include any non-contact measurement device. A position measurement system  150  may include a laser triangulation device, an interferometer, a confocal light sensor, a structured light device, a modulated light device, as well as combinations of these. Alternatively, a position measurement system  150  may utilize an ultrasonic distance sensor, a magnetic displacement sensor, and/or a capacitive displacement sensor. 
     As shown in  FIGS. 4A and 4B , an exemplary position measurement system  150  includes a light source  400  such as a laser diode, and an image sensor  402 , such as a charge-coupled device. The light source  400  may emit a measurement beam  152  that impinges on the surface of the powder bed  136  in a field of view  154  of the image sensor  402 . The field or view  154  may include all or a portion of the build plane  142 . The measurement beam  152  may follow an optical path that passes through one or more projection optical elements  404  prior to impinging upon the powder bed  136 . The one or more projection optical elements  404  may include one or more lenses, filters, diffusers, apertures or other optical element, as well as combinations of these. The projection optical element  404  may be configured to provide a collimated and/or focused measurement beam. The optical element may additionally, or alternatively filter the measurement beam  152  to provide a particular wavelength range. 
     In some embodiments, the projection optical element  404  may be configured to project the measurement beam  152  onto the surface of the powder bed  136  in the form of a structured light pattern and/or a modulated light pattern. An exemplary pattern may include an array of dots or any other suitable pattern. The structured and/or modulated light pattern may be utilized to determine a three-dimensional topography of the powder bed  136 . 
     After impinging upon the build plane  142  (e.g., the powder bed  136 ), the measurement beam reflected from the build plane  142  follows an optical path to the image sensor  402 . The optical path from the build plane  142  to the image sensor  402  may include one or more measurement optical elements  406 . The one or more measurement optical elements  406  may include one or more lenses, filters, diffusers, apertures or other optical element, as well as combinations of these. For example, as shown, the one or more measurement optical elements  406  may include an imaging lens assembly  408  and/or a filter element  410 . The imaging lens assembly may include one or more lenses or other optical elements configured to focus the measurement beam incident upon the image sensor  402 . The filter element  410  may include one or more filters, diffusers, and/or apertures configured to conform the measurement beam  152  incident upon the image sensor  402  to a certain wavelength or range of wavelengths. As shown in  FIG. 4B , in some embodiments, the position measurement system  150  may include an interferometer  412  or a series of optical elements configured to perform interferometry. By way of example, the interferometer may be configured as a Michelson interferometer, a Mach-Zehnder interferometer, an Fabry-Pérot Interferometer, a Sagnac Interferometer, a common-path Interferometer, and so forth. 
     Operation of the position measurement system  150  may be performed according to control commands from a measurement control module  414 . The control commands may be transmitted to one or more controllable components associated with the position measurement system  150 . 
     Now turning to  FIGS. 5A and 5B , exemplary position measurements will be described. As shown in  FIGS. 5A and 5B , an exemplary position measurement system  150  may determine a position of a build plane  142  relative to a reference plane  141 . A position of the build plane  142  may be determined at any time or series of times prior to, during, or after operation of an additive manufacturing machine  100 . In some embodiments, a position of the build plane  142  may be determined prior to commencing operation of the additive manufacturing machine  100 , such as prior to commencing a build process to build an object  114 . As shown in  FIGS. 5A and 5B , the position of the build plane  142  may be determined prior to distributing powder material  122  across the build module  110 . In that instance, the build plane  142  may correspond to the top surface of the build plate  130 . Additionally, or in the alternative, the position of the build plane  142  may be determined after to distributing powder material  122  across the build module  110 . In that instance, the build plane  142  may correspond to the surface of the powder bed  136 . 
     In some embodiments, the position of the build plane  142  may be determined before, during, and/or after sequential layers of powder material  122  have been distributed across the build module  110  and/or before, during, and/or after the sequential layers of powder have been selectively irradiated by an energy beam to form the respective layer of the object  114 . These measurements may be obtained intermittently and/or after each sequential layer, for example, to confirm proper alignment of the build plane  142  with the reference plane  141  at respective points in time during operation of the additive manufacturing machine  100 . 
     The position of the build plane  142  may be determined using a distance measurement d m , representing a distance between a position on the build plane  142  and a reference position  143  for the energy beam system  132  and/or the position measurement system  150 . The distance measurement d m  may be compared to a reference value d r , representing a distance between a corresponding position on a reference plane  141  and the reference position  143 . An error value “e” may be determined by comparing the distance measurement d m  to the reference value d r . For example, the error value e may be determined according to the relationship: e=d m −d r . An error value of zero may indicate congruence between the measured position on the build plane  142  and the corresponding position on the reference plan  141 . An error value of greater than zero or less than zero may indicate divergence between the measured position on the build plane  142  the corresponding position on the reference plane  141 . 
     For example, as shown in  FIG. 5A , comparison of a distance measurement d m  to a reference value d r  yields an error value e indicating that the measured position on the build plane  142  is situated below the corresponding position on the reference plane  141 . An adjustment may be applied with respect to all or a portion of the reference plane  141  to compensate for the error value e. In some embodiments, the reference plane  141  may be adjusted based at least in part on the error value e., for example, by shifting vertically the reference plane  141  in an amount corresponding to the error value e. The adjustment to the reference plane  141  may align the reference plane  141  with the build plane  142 . 
     In some embodiments, the build plane  142  may be presumed to be sufficiently flat or otherwise known that a distance measurement d m  for a single build plane-point  142  may provide sufficient information for determining an adjustment to the reference plane  141 . However, the present disclosure is not limited to obtaining a distance measurement d m  for only single build plane-point  142 . Rather, in some embodiments a distance measurement d m  may be obtained for a plurality of points on a build plane  142 . 
     For example, as shown in  FIG. 5B , a plurality of distance measurements may be used to determine a degree of levelness and/or a degree of parallelism between a reference plane  141  and a build plane  142 . As shown in  FIG. 5B , a comparison of a first distance measurement d m1  to a first reference value d r1  yields a first error value e 1 , and a comparison of a second distance measurement d m2  to a second reference value d r2  yields a second error value e 2 . A comparison of the first error value e 1  and the second error value e 2  may indicate a divergence in the levelness and/or the parallelism of the build plane  142  relative to the reference plane  141 . For example, as shown in  FIG. 5B , the build platform  130  is tilted, positioning a portion of the build platform  130  above the reference plane  141 . A slope may be determined by comparing the first error value to the second error value. An adjustment may be applied with respect to all or a portion of the reference plane  141  to compensate for the first error value e 1  and the second error value e 2 , for example, by tilting and/or shifting vertically the reference plane  141  in an amount corresponding to the error value e. The adjustment to the reference plane  141  may align the reference plane  141  with the build plane  142 . 
     Now referring to  FIG. 6 , in some embodiments, a build plane  142  may be determined using a plurality of distance measurements. The plurality of distance measurements may be used to compare an orientation of the build plane  142  to an orientation of the reference plane  141 . The orientation of the build plane  142  may include levelness, parallelism, vertical shift, horizontal shift, twist, and so forth. In some embodiments, the build plane  142  may be determined using distance measurements d m  corresponding to three different points on the build plane  142 . The build plane  142  may be compared to the reference plane  141  by comparing the respective distance measurements d m  to corresponding points on the reference plane  141 . For example, as shown in  FIG. 6 , a first build plane-point p b1  may be compared to a first reference plane-point p r1  to obtain a first error value e 1 , a second build plane-point p b2  may be compared to a second reference plane-point pre to obtain a second error value e 2 , and a third build plane-point p b3  may be compared to a third reference plane-point p r3  to obtain a third error value e 3 . A comparison of the build plane  142  to the reference plane  141  may indicate a divergence of the build plane  142  relative to the reference plane  141 . For example, as shown in  FIG. 6 , a divergent between the build plane  142  and the reference plane  141  may include elements of levelness, parallelism, shift, twist. 
     The position measurement system  150  may be utilized to compare a build plane  142  to a reference plane  141  for a variety of different purposes. By way of example,  FIGS. 5A and 5B , and  FIG. 6  show examples of determining orientation of the build plane  142  with respect to levelness, parallelism, vertical shift, horizontal shift, or twist, as well as combinations of these. Further examples will now be described with reference to  FIGS. 7A-7D . 
     As shown  FIGS. 7A-7D , various features of a build plane  142  (e.g., a powder bed  136  and/or a build platform  130 ) may be associated with a divergence from a reference plane  141 . As shown in  FIG. 7A , a build plane  142  may be situated at an elevation that differs from an elevation of a reference plane  141 . This may occur, for example, when the volume of powder material  120  distributed across the build module  110  differs from an expected volume (greater than or less than expected), resulting in a powder bed defining a build plane  142  being positioned above or below the reference plane  141 . Additionally, or in the alternative, movement of the build platform may result in a difference between the build plane  142  the reference plane  141 . Such movement may include incremental movements that are greater or less than expected, or unintended movements, such as due to slippage, machine vibrations, and so forth. 
     As another example, a powder bed may settle over time, such as during interruptions in machine operations, resulting in a build plane  142  being that differs from a reference plane  141 . For example, with larger additive manufacturing machines and/or larger builds, machine operations may be interrupted temporarily to replenish a powder supply and/or exchange powder modules, which may introduce a potential divergence between the build plane  142  and the reference plane  141 , associated with powder settling, vibration, or unintended movements of the additive manufacturing machine componentry. 
     As shown in  FIGS. 7B-7D , in some embodiments, a position measurement system  150  may be utilized to determine a local difference between a build plane  142  and a reference plane  141 , such as due to uneven powder distribution ( FIG. 7B ), thermal expansion of an object  114  ( FIG. 7C ), and/or partial separation of an object  114  ( FIG. 7D ). A position measurement system  150  may apply adjustments compensate for these and other local differences between a build plane  142  and a reference plane  141 . 
     As shown in  FIG. 8 , in some embodiments, a depth map  800  may be generated for a surface of a build plane  142  using position measurements from a position measurement system  150 . A depth map  800  may be generated using triangulation techniques and/or structured or modulated light techniques. A depth may include an array of discrete points on a build plane  142 , corresponding, for example, to respective pixels of an image sensor  402 . The depth map may reveal differences between a build plane  142  and a reference plane  141  that may exist for any reason, including those described herein with reference to  FIGS. 5A and 5B ,  FIG. 6 , and  FIGS. 7A-7D . 
     Regardless of the measurement methodology used to determine a position of a build plane  142 , and regardless of how many discrete points on a build plane  142  are determined, an additive manufacturing system  100  and/or additive manufacturing machine  102  may utilize information about the build plane  142  determined using the position measurement system  150  for a variety of different purposes. In some embodiments, a reference plane  141  may be adjusted based at least in part on information about a build plane  142  determined by the position measurement system  150 . Additionally, or in the alternative, one or more operations of the additive manufacturing system  100  and/or additive manufacturing machine may be adjusted based at least in part on information about a build plane  142  by the position measurement system  150 . The information about the build plane  142  used for making adjustments may include discrete position measurements and/or data determined using measurements, such as data obtained over time, as well as data subjected to various modeling and/or statistical analysis algorithms. 
     Now turning to  FIGS. 9A-9D , an exemplary methods will be described. An exemplary method  900  may include determining a position of a build plane  142 . As shown in  FIG. 9A , the exemplary method  900  may include, at block  902 , projecting an energy beam incident upon a build plane  142 ; at block  904 , detecting one or more properties of the energy beam upon having been reflected from the build plane  142 ; and at block  906 , using the one or more properties of the detected energy beam to determine the position of the build plane  142 . In an exemplary embodiment, a position of the build plane  142  may be determined at least in part using laser triangulation. In another exemplary embodiment, a position of the build plane  142  may be determined at least in part using structured light and/or modulated light. 
     An exemplary method  910  may include aligning a reference plane  141  to a build plane  142 , and/or calibrating a position of a reference plane  141 . As shown in  FIG. 9B , an exemplary method  910  may include, at block  912 , determining a position of a build plane  142 ; at block  914 , comparing the position of the build plane  142  to a corresponding position of a reference plane  141 ; and at block  916 , adjusting a position of the reference plane  141  based at least in part on the comparison of the position of the build plane  142  to the corresponding position of the reference plane  141 . 
     An exemplary method  920  may include operating an additive manufacturing machine and/or additively manufacturing an object. As shown in  FIG. 9C , an exemplary method  920  may include, at block  922 , determining a position of a build plane  142 ; and at block  924 , transmitting a control command to one or more controllable components of an additive manufacturing machine based at least in part on the determined position of the build plane  142 . 
     In some embodiments, an exemplary method  930  may include, at block  932 , additively manufacturing at least a portion of an object; at block  934 , interrupting one or more operations of the additive manufacturing machine; at block  936 , calibrating a reference plane  141  based at least in part on a position of a build plane  142  determined using a position measurement system  150 ; and at block  938 , resuming the one or more operations of the additive manufacturing machine. In some embodiments, the one or more operations of the additive manufacturing machine may be interrupted to replenish a powder supply and/or exchange powder modules. 
     Now turning to  FIG. 10 , and exemplary control system  104  will be described. An exemplary control system  104  includes a controller  1000  communicatively coupled with an additive manufacturing machine  102 . For example, the controller may be communicatively coupled with a measurement system  150  and/or an energy beam system  132 . In some embodiments, the controller  1000  may be communicatively coupled with a calibration system  1002  configured to perform calibration procedures. Additionally, or in the alternative, the controller  1000  may be communicatively coupled with a build module system  1004  configured to perform build module operations, including functionalities of a build module  110 , powder module  116 , and/or overflow module  118 . Such build module operations may additionally or alternatively include exchanging powder modules and/or overflow modules. The controller  1000  may also be communicatively coupled with a user interface  108  and/or a management system  106 . 
     The controller  1000  may include one or more computing devices  1006 , which may be located locally or remotely relative to the additive manufacturing machine  102 . The one or more computing devices  1006  may include one or more processors  1008  and one or more memory devices  1010 . The one or more processors  1008  may include any suitable processing device, such as a microprocessor, microcontroller, integrated circuit, logic device, and/or other suitable processing device. The one or more memory devices  1010  may include one or more computer-readable media, including but not limited to non-transitory computer-readable media, RAM, ROM, hard drives, flash drives, and/or other memory devices. 
     The one or more memory devices  1010  may store information accessible by the one or more processors  1008 , including machine-executable instructions  1012  that can be executed by the one or more processors  1008 . The instructions  1012  may include any set of instructions which when executed by the one or more processors  1008  cause the one or more processors  1008  to perform operations. In some embodiments, the instructions  1012  may be configured to cause the one or more processors  1008  to perform operations for which the controller  1000  and/or the one or more computing devices  1006  are configured. 
     The memory devices  1010  may store data  1014  accessible by the one or more processors  1008 . The data  1014  can include current or real-time data, past data, or a combination thereof. The data  1014  may be stored in a data library  1016 . As examples, the data  1014  may include data associated with or generated by additive manufacturing system  100 , including data  1014  associated with or generated by a controller  1000 , the additive manufacturing machine  102 , the user interface  108 , the management system  106 , and/or a computing device  1006 . The data  1014  may also include other data sets, parameters, outputs, information, associated with an additive manufacturing system  100 , such as those associated with the additive manufacturing machine  102 , the user interface  108 , and/or the management system  106 . 
     The one or more computing devices  1006  may also include a communication interface  1018 , which may be used for communications with a communication network  1020  via wired or wireless communication lines  1022 . The communication interface  1018  may include any suitable components for interfacing with one or more network(s), including for example, transmitters, receivers, ports, controllers, antennas, and/or other suitable components. The communication interface  1018  may allow the computing device  1006  to communicate with the additive manufacturing machine  102 . The communication network  1020  may include, for example, a local area network (LAN), a wide area network (WAN), SATCOM network, VHF network, a HF network, a Wi-Fi network, a WiMAX network, a gatelink network, and/or any other suitable communication network  1020  for transmitting messages to and/or from the controller  1000  across the communication lines  1022 . The communication lines  1022  of communication network  1020  may include a data bus or a combination of wired and/or wireless communication links. 
     The communication interface  1018  may additionally or alternatively allow the computing device  1006  to communicate with a user interface  108  and/or a management system  106 . The management system  106  may include a server  1024  and/or a data warehouse  1026 . As an example, at least a portion of the data  1014  may be stored in the data warehouse  1026 , and the server  1024  may be configured to transmit data  1014  from the data warehouse  1026  to the computing device  1006 , and/or to receive data  1014  from the computing device  1006  and to store the received data  1014  in the data warehouse  1026  for further purposes. The server  1024  and/or the data warehouse  1026  may be implemented as part of a control system  104 . 
     The controller  1000  may include one or more control models  1028 , which may utilize the data  1014 , including the data library  1016 , and/or other data sets, parameters, outputs, information, associated with the additive manufacturing system  100 , such as those associated with the additive manufacturing machine  102 , the user interface  108 , and/or the management system  106 . The one or more control models  1028  may additionally or alternatively utilize data from the data warehouse  1026 , which may be transmitted to the controller  1000  from the server  1024 . 
     Referring again to  FIG. 1 , in some embodiments, a control system  104  may be configured to perform a calibration of the reference plane  141 . The calibration may be utilized by the additive manufacturing system to generate control commands to the additive manufacturing machine. Exemplary control commands may include directing the position and/or energy density of an energy beam, including imparting a desired spot size incident upon the build plane  142  and/or imparting a desired energy input to a respective portion of the build plane  142 . Exemplary control commands may additionally or alternatively include control commands configured to operate motors used to operate the powder module, build module, and/or overflow module. The control commands may be used to control a motor that operates the piston that raises or lowers the platform of the respective module. For example, a build module  110  may include a build module motor  156 , and the control system  104  may transmit a control command to the build module motor  156  based at least in part on a position of the build plane  142  determined by the position measurement system  150 . Additionally, or in the alternative, a powder module  116  may include a powder module motor  158 , and the control system  104  may transmit a control command to the powder module motor  158  based at least in part on a position of the build plane  142  determined by the position measurement system  150 . The control commands to the build module motor  156  and/or the powder module motor  158  may be provided to align the build plane  142  with the reference plane  141 . In some embodiments, an overflow module  116  may include an overflow module motor  160 , and the control system  104  may transmit a control command to the overflow module motor  158  based at least in part on a position of the build plane  142  determined by the position measurement system  150 , for example, to accommodate a volume of overflow powder material  120  being recaptured in the overflow chamber  144 . 
     Referring again to  FIG. 3 , exemplary control commands may be configured to impart a desired energy input to a respective portion of the build plane  142 , based at least in part on a position of the build plane  142  determined by the position measurement system  150 . For example, a control command may be transmitted to a beam generating device  300 , such as to impart a desired energy input to a respective portion of the build plane  142 . Additionally, or in the alternative, a control command may be transmitted to an optical assembly  304 , such as to impart a desired spot size incident upon the build plane  142  based at least in part on a position of the build plane  142  determined by the position measurement system  150 . Still further additionally, or in the alternative, a control command may be transmitted to a scanner  302 , such as to control movements of an energy beam (e.g., speed and/or contour path) upon based at least in part on a position of the build plane  142  determined by the position measurement system  150 . In some embodiments, an evaluation unit  300  may provide feedback pertaining to one or more properties of the energy beam. 
     This written description uses exemplary embodiments to describe the presently disclosed subject matter, including the best mode, and also to enable any person skilled in the art to practice such subject matter, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the presently disclosed subject matter is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they include structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal languages of the claims.