Patent Publication Number: US-11648730-B2

Title: System and method for calibrating an acoustic monitoring system of an additive manufacturing machine

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     The present specification is a continuation of U.S. patent application Ser. No. 16/202,539 filed Nov. 28, 2018, entitled “System and Method for Calibrating an Acoustic Monitoring System of an Additive Manufacturing Machine” which is incorporated by reference herein. 
    
    
     FIELD 
     The present disclosure generally relates to additive manufacturing machines, or more particularly, to a calibration system for an additive manufacturing machine. 
     BACKGROUND 
     Additive manufacturing (AM) processes generally involve the buildup of one or more materials to make a net or near net shape (NNS) object, in contrast to subtractive manufacturing methods. Though “additive manufacturing” is an industry standard term (ISO/ASTM52900), AM encompasses various manufacturing and prototyping techniques known under a variety of names, including freeform fabrication, 3D printing, rapid prototyping/tooling, etc. AM techniques are capable of fabricating complex components from a wide variety of materials. Generally, a freestanding object can be fabricated from a computer aided design (CAD) model. 
     A particular type of AM process uses an energy source such as an irradiation emission directing device that directs an energy beam, for example, an electron beam or a laser beam, to sinter or melt a powder material, creating a solid three-dimensional object in which particles of the powder material are bonded together. AM processes may use different material systems or additive powders, such as engineering plastics, thermoplastic elastomers, metals, and/or ceramics. Laser sintering or melting is a notable AM process for rapid fabrication of functional prototypes and tools. Applications include direct manufacturing of complex workpieces, patterns for investment casting, metal molds for injection molding and die casting, and molds and cores for sand casting. Fabrication of prototype objects to enhance communication and testing of concepts during the design cycle are other common usages of AM processes. 
     Selective laser sintering, direct laser sintering, selective laser melting, and direct laser melting are common industry terms used to refer to producing three-dimensional (3D) objects by using a laser beam to sinter or melt a fine powder. More accurately, sintering entails fusing (agglomerating) particles of a powder at a temperature below the melting point of the powder material, whereas melting entails fully melting particles of a powder to form a solid homogeneous mass. The physical processes associated with laser sintering or laser melting include heat transfer to a powder material and then either sintering or melting the powder material. Although the laser sintering and melting processes can be applied to a broad range of powder materials, the scientific and technical aspects of the production route, for example, sintering or melting rate and the effects of processing parameters on the microstructural evolution during the layer manufacturing process have not been well understood. This method of fabrication is accompanied by multiple modes of heat, mass, and momentum transfer, and chemical reactions that make the process very complex. 
     During direct metal laser sintering (DMLS) or direct metal laser melting (DMLM), an apparatus builds objects in a layer-by-layer manner by sintering or melting a powder material using an energy beam. The powder to be melted by the energy beam is spread evenly over a powder bed on a build platform, and the energy beam sinters or melts a cross sectional layer of the object being built under control of an irradiation emission directing device. The build platform is lowered and another layer of powder is spread over the powder bed and object being built, followed by successive melting/sintering of the powder. The process is repeated until the part is completely built up from the melted/sintered powder material. 
     After fabrication of the part is complete, various post-processing procedures may be applied to the part. Post processing procedures include removal of excess powder by, for example, blowing or vacuuming. Other post processing procedures include a stress relief process. Additionally, thermal, mechanical, and chemical post processing procedures can be used to finish the part. 
     In order to monitor the additive manufacturing process, certain conventional additive manufacturing machines include acoustic monitoring systems. These monitoring systems typically include one or more microphones or acoustic wave sensors for detecting acoustic waves or vibrations generated or otherwise emitted during the process. The measured acoustic waves or sensor values can be used to evaluate the quality of the build, during or after completion of the build process. The quality evaluation may be used to adjust the build process, stop the build process, troubleshoot build process anomalies, issue a warning to the machine operator, and/or identify suspect or poor quality parts resulting from the build. However, such acoustic monitoring systems are frequently not calibrated, making it impossible to determine if variations in measured acoustic signals are a product of variations in the additive build process or melt pool or are the product of variations in the acoustic monitoring system itself when comparing data from multiple machines. In practice, additive manufacturing has been primarily used for prototyping and small lot production with acoustic monitoring used as a research and development tool rather than part quality assessment. The recent emergence of mass production applications where build-to-build and machine-to-machine repeatability is far more critical has highlighted the value of acoustic monitoring as a quality tool. 
     Accordingly, an additive manufacturing machine with improved calibration features would be useful. More particularly, a system and method for calibrating an acoustic monitoring system of an additive manufacturing machine would be particularly beneficial. 
     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 practice of the invention. 
     According to one embodiment of the present subject matter, a method of calibrating an acoustic monitoring system of an additive manufacturing machine is provided. The method includes generating acoustic waves from one or more calibrated acoustic sources from one or more fixed positions in the additive manufacturing machine and measuring an acoustic signal of the generated acoustic waves using the acoustic monitoring system. The method further includes comparing the measured acoustic signal to a measurement standard for use in calibrating the acoustic monitoring system. 
     According to another exemplary embodiment, a calibration system for an acoustic monitoring system of an additive manufacturing machine is provided. The calibration system includes a mounting mechanism positionable at desired locations within the additive manufacturing machine and one or more calibrated acoustic sources mounted to the mounting mechanism, the one or more calibrated acoustic sources defining a measurement standard when operating. 
     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 embodiments of the invention and, together with the description, serve to explain certain principles of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A full and enabling disclosure of the present invention, 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. 
         FIG.  1    shows a schematic view of an additive manufacturing machine according to an exemplary embodiment of the present subject matter. 
         FIG.  2    shows a close-up schematic view of a build platform of the exemplary additive manufacturing machine of  FIG.  1    according to an exemplary embodiment of the present subject matter. 
         FIG.  3    shows a calibration assembly of the exemplary additive manufacturing machine of  FIG.  1    according to an exemplary embodiment of the present subject matter. 
         FIG.  4    is a method of calibrating an acoustic monitoring system of an additive manufacturing machine in accordance with one embodiment of the present disclosure. 
     
    
    
     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 invention. 
     DETAILED DESCRIPTION 
     Reference now will be made in detail to embodiments of the invention, one or more examples of which are illustrated in the drawings. Each example is provided by way of explanation of the invention, not limitation of the invention. In fact, it will be apparent to those skilled in the art that various modifications and variations can be made in the present invention without departing from the scope or spirit of the invention. 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 invention covers such modifications and variations as come within the scope of the appended claims and their equivalents. 
     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. In addition, as used herein, terms of approximation, such as “approximately,” “substantially,” or “about,” refer to being within a ten percent margin of error. 
     A system and method for calibrating an acoustic monitoring system of an additive manufacturing machine includes installing a calibration system on the machine and performing a calibration process. Specifically, the calibration system includes a calibration platform removably mountable to a build platform of the additive manufacturing machine and having calibrated acoustic sources mounted thereon for defining a measurement standard. The generated acoustic waves are measured by the acoustic monitoring system and compared to the known measurement standard to determine whether system adjustments would improve process tolerances or uniformity. 
     Referring to  FIG.  1   , a laser powder bed fusion system, such as a DMLS or DMLM system  100  will be described according to an exemplary embodiment. As illustrated, system  100  includes a fixed enclosure or build area  102  which provides a contaminant-free and controlled environment for performing an additive manufacturing process. In this regard, for example, enclosure  102  serves to isolate and protect the other components of the system  100 . In addition, enclosure  102  may be provided with a flow of an appropriate shielding gas, such as nitrogen, argon, or another suitable gas or gas mixture. In this regard, enclosure  102  may define a gas inlet port  104  and a gas outlet port  106  for receiving a flow of gas to create a static pressurized volume or a dynamic flow of gas. 
     Enclosure  102  may generally contain some or all components of AM system  100 . According to an exemplary embodiment, AM system  100  generally includes a table  110 , a powder supply  112 , a scraper or recoater mechanism  114 , an overflow container or reservoir  116 , and a build platform  118  positioned within enclosure  102 . In addition, an energy source  120  generates an energy beam  122  and a beam steering apparatus  124  directs energy beam  122  to facilitate the AM process as described in more detail below. Each of these components will be described in more detail below. 
     According to the illustrated embodiment, table  110  is a rigid structure defining a planar build surface  130 . In addition, planar build surface  130  defines a build opening  132  through which build chamber  134  may be accessed. More specifically, according to the illustrated embodiment, build chamber  134  is defined at least in part by vertical walls  136  and build platform  118 . In addition, build surface  130  defines a supply opening  140  through which additive powder  142  may be supplied from powder supply  112  and a reservoir opening  144  through which excess additive powder  142  may pass into overflow reservoir  116 . Collected additive powders may optionally be treated to sieve out loose, agglomerated particles before re-use. 
     Powder supply  112  generally includes an additive powder supply container  150  which generally contains a volume of additive powder  142  sufficient for some or all of the additive manufacturing process for a specific part or parts. In addition, powder supply  112  includes a supply platform  152 , which is a plate-like structure that is movable along the vertical direction within powder supply container  150 . More specifically, a supply actuator  154  vertically supports supply platform  152  and selectively moves it up and down during the additive manufacturing process. 
     AM system  100  further includes recoater mechanism  114 , which is a rigid, laterally-elongated structure that lies proximate build surface  130 . For example, recoater mechanism  114  may be a hard scraper, a soft squeegee, or a roller. Recoater mechanism  114  is operably coupled to a recoater actuator  160  which is operable to selectively move recoater mechanism  114  along build surface  130 . In addition, a platform actuator  164  is operably coupled to build platform  118  and is generally operable for moving build platform  118  along the vertical direction during the build process. Although actuators  154 ,  160 , and  164  are illustrated as being hydraulic actuators, it should be appreciated that any other type and configuration of actuators may be used according to alternative embodiments, such as pneumatic actuators, hydraulic actuators, ball screw linear electric actuators, or any other suitable vertical support means. Other configurations are possible and within the scope of the present subject matter. 
     Energy source  120  may include any known device operable to generate a beam of suitable power and other operating characteristics to melt and fuse the metallic powder during the build process. For example, energy source  120  may be a laser or any other suitable irradiation emission directing device or irradiation device. Other directed-energy sources such as electron beam guns are suitable alternatives to a laser. 
     According to an exemplary embodiment, beam steering apparatus  124  includes one or more mirrors, prisms, lenses, and/or electromagnets operably coupled with suitable actuators and arranged to direct and focus energy beam  122 . In this regard, for example, beam steering apparatus  124  may be a galvanometer scanner that moves or scans the focal point of the laser beam  122  emitted by energy source  120  across the build surface  130  during the laser melting and sintering processes. In this regard, energy beam  122  can be focused to a desired spot size and steered to a desired position in plane coincident with build surface  130 . The galvanometer scanner in powder bed fusion technologies is typically of a fixed position but the movable mirrors/lenses contained therein allow various properties of the laser beam to be controlled and adjusted. It should be appreciated that other types of energy sources  120  may be used which may use and alternative beam steering apparatus  124 . For example, if the energy source  120  is an electronic control unit for directing an electron beam, beam steering apparatus  124  may be, e.g. a deflecting coil. 
     Prior to an additive manufacturing process, recoater actuator  160  may be lowered to provide a supply of powder  142  of a desired composition (for example, metallic, ceramic, and/or organic powder) into supply container  150 . In addition, platform actuator  164  may move build platform  118  to an initial high position, e.g., such that it substantially flush or coplanar with build surface  130 . Build platform  118  is then lowered below build surface  130  by a selected layer increment. The layer increment affects the speed of the additive manufacturing process and the resolution of a components or parts  170  being manufactured. As an example, the layer increment may be about 10 to 100 micrometers (0.0004 to 0.004 in.). 
     Additive powder is then deposited over the build platform  118  before being fused by energy source  120 . Specifically, supply actuator  154  may raise supply platform  152  to push powder through supply opening  140 , exposing it above build surface  130 . Recoater mechanism  114  may then be moved across build surface  130  by recoater actuator  160  to spread the raised additive powder  142  horizontally over build platform  118  (e.g., at the selected layer increment or thickness). Any excess additive powder  142  drops through the reservoir opening  144  into the overflow reservoir  116  as recoater mechanism  114  passes from left to right (as shown in  FIG.  1   ). Subsequently, recoater mechanism  114  may be moved back to a starting position. Notably, the step of recoater mechanism  114  spreading powder over build surface  130  inherently generates an acoustic wave or vibration due to friction between the recoater mechanism  114 , powder layer  142 , and component  170  being built. This acoustic wave travels through the build area or system enclosure  102 , through the structure of the table  110 , through the gas within the enclosure  102 , and even outside enclosure  102 . 
     Therefore, as explained herein and illustrated in  FIG.  1   , recoater mechanism  114 , recoater actuator  160 , supply platform  152 , and supply actuator  154  may generally operate to successively deposit layers of additive powder  142  or other additive material to facilitate the print process. As such, these components may collectively be referred to herein as powder depositing apparatus, system, or assembly. The leveled additive powder  142  may be referred to as a “build layer”  172  (see  FIG.  2   ) and the exposed upper surface thereof may be referred to as build surface  130 . When build platform  118  is lowered into build chamber  134  during a build process, build chamber  134  and build platform  118  collectively surround and support a mass of additive powder  142  along with any components  170  being built. This mass of powder is generally referred to as a “powder bed,” and this specific category of additive manufacturing process may be referred to as a “powder bed process.” 
     During the additive manufacturing process, the directed energy source  120  is used to melt a two-dimensional cross-section or layer of the component  170  being built. More specifically, energy beam  122  is emitted from energy source  120  and beam steering apparatus  124  is used to steer the focal point  174  of energy beam  122  over the exposed powder surface in an appropriate pattern. A small portion of exposed layer of the additive powder  142  surrounding focal point  174 , referred to herein as a “weld pool” or “melt pool” or “heat effected zone”  176  (best seen in  FIG.  2   ) is heated by energy beam  122  to a temperature allowing it to sinter or melt, flow, and consolidate. As an example, melt pool  176  may be on the order of 100 micrometers (0.004 in.) wide. This step may be referred to as fusing additive powder  142 . 
     Build platform  118  is moved vertically downward by the layer increment, and another layer of additive powder  142  is applied in a similar thickness. The directed energy source  120  again emits energy beam  122  and beam steering apparatus  124  is used to steer the focal point  174  of energy beam  122  over the exposed powder surface in an appropriate pattern. The exposed layer of additive powder  142  is heated by energy beam  122  to a temperature allowing it to sinter or melt, flow, and consolidate both within the top layer and with the lower, previously-solidified layer. This cycle of moving build platform  118 , applying additive powder  142 , and then directed energy beam  122  to melt additive powder  142  is repeated until the entire component  170  is complete. 
     As explained briefly above, as energy source  120  and beam steering apparatus  124  direct energy beam  122 , e.g., a laser beam or electron beam, onto the powder bed or build surface  130 , the additive powders  142  are heated and begin to melt into melt pool  176  where they may fused to form the final component  170 . Notably, during the step of sintering or melting additive powder, the heated material inherently generates an acoustic wave or vibration radiating out from the melt pool  176  or from focal point  174 . This acoustic wave travels through the build area or system enclosure  102 , through the structure of the table  110 , through the gas within the enclosure  102 , and even outside enclosure  102 . 
     As described below, the acoustic waves and vibrations generated by the recoat process or by the weld pool may be detected by microphones, vibration sensors, and other devices in order to generate a measured acoustic profile. It should be appreciated that the measured acoustic profile may be recorded at any suitable predetermined interval of layer increments and using any suitable devices. Generally speaking, the measured acoustic profile may be used to improve process monitoring and control. An exemplary system for monitoring the additive manufacturing process is described below according to exemplary embodiments. 
     Referring still to  FIG.  1   , an acoustic monitoring system  200  which may be used with system  100  for monitoring melt pool  176  and the manufacturing process in general will be described according to an exemplary embodiment of the present subject matter. Acoustic monitoring system  200  includes one or more acoustic sensors or vibration sensors for measuring the acoustic waves or vibrations from the recoat process or melt pool  176 . More specifically, according to the illustrated embodiment, acoustic monitoring system  200  includes one platform sensor  202 , one enclosure sensor  204 , and one external sensor  206 . Although three sensors  202 - 206  are described herein, it should be appreciated that only a single sensor is needed according to exemplary embodiments. In this regard, three sensors  202 - 206  are described in order to illustrate various exemplary positions of such acoustic or vibration sensors, but such positioning is not intended to be limiting. Thus, it should be appreciated that acoustic monitoring system  200  may include any other suitable type, number, and configuration of sensors for detecting acoustic waves, vibrations, and other properties of melt pool  176  or the additive manufacturing process in general. 
     It should be appreciated that sensors  202 - 206  may be any sensor suitable for measuring acoustic waves or vibrations generated by melt pool  176 . In addition, sensors  202 - 206  may be mounted at any suitable location within build enclosure  102  (e.g., platform sensor  202  or enclosure sensor  204 ) or outside of enclosure  102  (e.g., external sensor  206 ). According to exemplary embodiments, sensors  202 - 206  may include at least one of a microphone, a contact microphone, an acoustic wave sensor, an accelerometer, a vibration sensor, or a piezoelectric acoustic sensor. Other suitable acoustic sensors, positions, and configurations are possible and within the scope of the present subject matter. 
     Acoustic sensors  202 - 206  may be positioned within enclosure  102  or outside enclosure  102 , and may be mounted to a rigid structure, suspended, or mounted in any other suitable manner. For example,  FIG.  1    illustrates platform sensor  202  as being mounted to the underside of the build platform  118 , enclosure sensor  204  as being suspended within enclosure  102 , and external sensor  206  as being mounted to a wall of enclosure  102 . In general, the process of measuring acoustic waves or vibrations using sensors  202 - 206  may be referred to herein as “acoustic monitoring” or the like. As used herein, the term “acoustic” refers generally to the transmission of energy through a medium such as a gas or a solid, without regard to the frequency range of such energy. 
     Acoustic monitoring system  200  further includes a controller  220  which is operably coupled with sensors  202 - 206  for receiving signals corresponding to the detected acoustic waves. Controller  220  may be a dedicated controller for acoustic monitoring system  200  or may be a system controller for operating AM system  100 . Controller  220  may include one or more memory devices and one or more microprocessors, such as general or special purpose microprocessors operable to execute programming instructions or micro-control code associated with an additive manufacturing process or process monitoring. The memory may represent random access memory such as DRAM, or read only memory such as ROM or FLASH. In one embodiment, the processor executes programming instructions stored in memory. The memory may be a separate component from the processor or may be included onboard within the processor. Alternatively, controller  220  may be constructed without using a microprocessor, e.g., using a combination of discrete analog and/or digital logic circuitry (such as switches, amplifiers, integrators, comparators, flip-flops, AND gates, and the like) to perform control functionality instead of relying upon software. 
     As explained briefly above, conventional acoustic monitoring systems are not calibrated. Moreover, when multiple additive manufacturing machines are set up for manufacturing a series of identical parts, the acoustic monitoring systems on each of these machines may have slightly different response characteristics. Therefore, aspects of the present subject matter are directed to systems and methods of calibrating acoustic monitoring systems for additive manufacturing machines. 
     Referring now specifically to  FIGS.  3  and  4   , a calibration system  230  which may be used to calibrate acoustic monitoring system  200  of additive manufacturing system  100  will be described according to an exemplary embodiment of the present subject matter. Although exemplary embodiments of calibration system  230  are provided herein and specifically described as being used for calibrating acoustic monitoring system  200 , it should be appreciated that according to alternative embodiments, calibration system  230  may include modifications and variations, e.g., to provide more tailored calibration for a specific machine set up. The exemplary embodiments described herein and are not intended to limit the scope of the present subject matter. 
     As illustrated, calibration system  230  includes a calibration platform  232  which is removably mountable to build platform  118  of additive manufacturing system  100 . In this regard, calibration platform  232  may be a rigid, planar mounting structure that may be mounted in any suitable additive manufacturing machine such that the position of calibration platform  232  is in a known position relative to the acoustic monitoring system  200 . For example, calibration platform  232  may have the same dimensions as build platform  118  (e.g., in a horizontal plane defined by the build surface  130 ). In this manner, by positioning the calibration platform  232  through build opening  132 , vertical walls  136  may ensure that calibration platform  232  is repeatedly positioned in a fixed, known location and orientation. 
     According to the exemplary embodiment, calibration platform  232  defines one or more alignment features  234  that are configured for engaging complementary features  236  defined on build platform  118  or on a build platform location of the AM machine  100 . More specifically, for example, alignment features  234  may be bolts that extend down from calibration platform  232  toward build platform  118  and complementary features  236  may be holes defined in build platform  118  for receiving the bolts. Notably, each additive manufacturing machine may include an identical build platform  118  having identical complementary features  236  for receipt of alignment features  234 . In this manner, build platform  118  may be moved from machine to machine by having a fixed relative position that may be known by controller  220  to achieve a precise and improved calibration process. 
     In addition, calibration system  230  includes one or more calibrated acoustic sources  240  that are mounted to calibration platform  232  at a fixed, known position. Calibrated acoustic sources  240  are generally configured for defining a measurement standard (such as an acoustic calibration standard) when energized or otherwise operating. In this regard, for example, calibrated acoustic sources  240  may generate acoustic waves or vibrational energy having a known signal amplitude, signal intensity, signal frequency, signal frequency spectrum, or any other suitable measurable quantity or quality of acoustic energy. It should be appreciated that as used herein, “measurement standard” may be used interchangeably to refer to the actual acoustic waves or energy generated by calibrated acoustic sources  240  or the signal generated by acoustic monitoring system  200  when measuring those acoustic waves. 
     Vibrations due to recoat process are typically in the audible range, 20 Hz to 20 kHz with the measured amplitude of the vibration depending largely on the placement of the sensors  202 - 206  of acoustic monitoring system  200 . Recoater contact with the component  170  or increased friction due to distortion of the component  170  or any other effect that reduces the effective powder layer thickness during the recoat process will result in an increase in amplitude and frequency of the vibration. Acoustic noise generated by the melt pool formation is typically in the audible range as well, 20 Hz to 20 kHz, while further vibrations may be generated in the ultrasonic range, 20 kHz to 50 Mhz, by residual stresses resulting in crack, porosity, or other mechanical defect formation. Specific measured frequencies and amplitudes will also depend on the nature of the sensor used and attenuation of the signal as it travels from source to sensor. 
     In general, calibrated acoustic sources  240  may be any source of acoustic waves or vibrational energy, such as a speaker, a sound generating device, a vibration mechanism, a piezoelectric generator, or an acoustic wave generator, or any other device which generates sound waves having a known and measurable quantity or quality of acoustic emissions, e.g., as described above. The terms “acoustic source,” “vibration source,” and the like may be used interchangeably herein. 
     According to the illustrated embodiment, calibration system  230  includes a single calibrated acoustic source  240  mounted at a center of calibration platform  232 . In this regard, calibration system  230  may include a mounting structure  242  which may include one or more struts, slats, or other supports to hold calibrated acoustic source  240  in its fixed position. Although a single calibrated acoustic source  240  is illustrated in the exemplary embodiments, it should be appreciated that according to alternative embodiments any suitable number, type, and positioning of calibrated acoustic sources  240  may be used to enable calibration of multiple sensors associated respectively with multiple laser systems and/or to account of normal spatial variation in the sensor response due to acoustic system design. For example, according to another embodiment, calibration system  230  includes four calibrated acoustic sources  240  spaced equidistant from each other on calibration platform  232  (e.g. proximate each of the four corners of calibration platform  232 ). 
     In addition, calibrated acoustic sources  240  may be positioned at a fixed location within AM system  100  using any other suitable device or mechanism, which may be referred to herein as an acoustic mounting mechanism. For example, according to another exemplary embodiment, one or more calibrated acoustic sources  240  may be mounted to a retractable arm (not shown) mounted on a side of build chamber  134 . In this manner, prior to a manufacturing process, the retractable arm could extend to position the acoustic source  240  in the desired position, the calibration process could be performed, and the arm could be retracted before starting the build. 
     According to still other embodiments, the location of calibrated acoustic sources  240  may be adjusted by at least one of computer controls (e.g., via controller  220 ), using an alignment motor, based in response to a position determined by an alignment sensor, etc. Alternatively, one or more calibrated acoustic sources  240  may be positioned elsewhere within enclosure  102  or even outside of enclosure  120 , so long as the output of such acoustic sources  240  is measurable by acoustic monitoring system  200 , e.g., via sensors  202 - 206 . Other suitable mounting structures and solutions are possible and within the scope of the present subject matter. 
     Notably, according to an exemplary embodiment, calibration system  230  may be movable among multiple additive manufacturing machines to provide consistent calibration values across all machines. As used herein, “calibration” may be used generally refer to the process of comparing a measured value from a sensor to a known value or calibration signal standard. In this regard, for example, calibration system  230  may generate a known “measurement standard” that may be measured by one or more acoustic sensors, such as sensors  202 - 206  of acoustic monitoring system  200 . The sensor parameters may be adjusted until the output of the sensor is indicative of measured acoustic waves or vibrations that are substantially equivalent to the measurement standard. Alternatively, controller  220  may be configured to compensate for differences between the measured acoustic waves and the measurement standard. In this manner, a relationship between a known value (the measurement standard) and an unknown value (the measured acoustic waves) may be used to adjust the sensor output and reduce measurement uncertainty. 
     By adjusting all acoustic monitoring systems  200  in a group of additive manufacturing machines based on their response to the acoustic waves generated by the calibration system  230  (i.e., the measurement standard), subsequent measurements obtained during the additive manufacturing of parts may be reliable indicators of the process and melt pool characteristics. Controller  220  may be configured for receiving, manipulating, and using this information to make necessary process corrections to achieve more precision by a single machine and more uniformity among a group of machines which have been similarly calibrated. 
     It should be appreciated that AM system  100 , acoustic monitoring system  200 , and calibration system  230  are illustrated and described herein only for explaining aspects of the present subject matter. However, the scope of the present subject matter is not limited to such exemplary embodiments, but is instead contemplated as including embodiments including variations and modifications. For example, although the actuators are illustrated herein as being a hydraulic actuators, other suitable types and configurations of actuators may be used according to alternative embodiments. In addition, other suitable forms and/or types of powder supply  112  may be used, such as a powder container that moves along build surface  130  while depositing additive powder at a predetermined flow rate. Furthermore, any suitable configuration of beam steering apparatus  124  may be used, e.g., based on the type of energy beam  122  generated. Other configurations are possible and within the scope of the present subject matter. 
     Now that the construction and configuration of AM system  100 , acoustic monitoring system  200 , and calibration system  230  have been described according to exemplary embodiments of the present subject matter, an exemplary method  300  for calibrating process monitoring system of an additive manufacturing system will be described according to an exemplary embodiment of the present subject matter. Method  300  can be used to calibrate acoustic monitoring system  200  of AM system  100 , or any other suitable additive manufacturing machine. In this regard, for example, controller  220  may be configured for implementing some or all steps of method  300 . Further, it should be appreciated that the exemplary method  300  is discussed herein only to describe exemplary aspects of the present subject matter, and is not intended to be limiting. 
     Referring now to  FIG.  4   , method  300  includes, at step  310 , positioning one or more calibrated acoustic sources in or on a calibration platform. In this regard, continuing the example from above, calibrated acoustic source  240  may be mounted on calibration platform  232  at a known location. Although the figures illustrate a single calibrated acoustic source  240  positioned in the center of calibration platform  232 , it should be appreciated that any suitable number, type, spacing, and configuration of acoustic sources can be used according to alternative embodiments. Step  220  includes mounting the calibration platform in or on a build platform location of an additive manufacturing machine. For example, calibration platform  232  may be mounted to build platform  118  and may be aligned and oriented by vertical walls  136  and/or by the seating of alignment features  234  within complementary features  236  defined by build platform  118 . In this manner, the position of calibrated acoustic source  240  is in a known location and orientation within a three-dimensional space within enclosure  102 . According to alternative embodiments, calibrated acoustic source  240  could be positioned at any other suitable location within or outside of enclosure  102 . 
     Step  330  includes generating acoustic waves from the one or more calibrated acoustic sources from one or more fixed positions in the additive manufacturing machine. In this regard, for example, calibrated acoustic source  240  may be energized or otherwise operated to generate noise, acoustic waves, or vibrational energy that may be measured by acoustic monitoring system  200 , e.g., using sensors  202 - 206 . Notably, as explained above, when calibrated acoustic source  240  is operated, the acoustic waves generated define a measurement standard having a known signal amplitude, signal intensity, frequency, frequency response, etc. 
     Step  340  includes measuring and acoustic signal of the generated acoustic waves using an acoustic monitoring system. In this regard, acoustic monitoring system  200  uses one or more of sensors  202 - 206  to measure the acoustic waves or vibrational energy generated by calibrated acoustic source  240 . Steps  350  and  360  are generally directed toward comparing the measured acoustic signal to a known standard to determine the accuracy/calibration of the acoustic sensor. 
     Specifically, step  350  includes obtaining a measurement standard generated by the one or more calibrated acoustic sources. This measurement standard may be provided by a manufacturer of the acoustic source, may be determined by syncing up the output of the acoustic source with a known and reliable acoustic source, or may be determined in any other suitable manner. Step  360  includes comparing the measured acoustic signal to the measurement standard. Notably, if sensors  202 - 206  are properly calibrated, the acoustic signal which is measured should be substantially equivalent to the measurement standard of calibrated acoustic source  240 . By contrast, if there is a difference between the acoustic signal and the measurement standard, a calibration procedure may be desirable to ensure improved process monitoring. 
     Step  370  includes adjusting the acoustic monitoring system such that the measured acoustic signal is substantially identical to the measurement standard. As described above, such an adjustment may be made by adjusting a physical gain of one or more of sensors  202 - 206  or making an electronic compensation or adjustment of the measured signals (e.g., using controller  220 ). Alternatively, it may be determined that a sensor must be replaced entirely or that a maintenance procedure should be performed. 
       FIG.  4    depicts an exemplary control method having steps performed in a particular order for purposes of illustration and discussion. Those of ordinary skill in the art, using the disclosures provided herein, will understand that the steps of any of the methods discussed herein can be adapted, rearranged, expanded, omitted, or modified in various ways without deviating from the scope of the present disclosure. Moreover, although aspects of the methods are explained using AM system  100 , acoustic monitoring system  200 , and calibration system  230  as an example, it should be appreciated that these methods may be applied to the calibration and operation of any suitable monitoring system of any suitable additive manufacturing machine. 
     This written description uses exemplary embodiments to disclose the invention, including the best mode, and also to enable any person skilled in the art to practice the invention, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the invention 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.