Patent Publication Number: US-11040500-B2

Title: Thermographic inspection for tape layup machines

Description:
FIELD 
     The disclosure relates to the field of fabrication, and in particular, to tape layup machines that create laminates comprising multiple layers of tape. 
     BACKGROUND 
     Multi-layer laminates of constituent material (e.g., Carbon Fiber Reinforced Polymer (CFRP)) may be formed into any of a variety of shapes for curing into a composite part. To facilitate the fabrication of composite parts, a tape layup machine, such as an Automated Fiber Placement (AFP) machine or Automated Tape Layup (ATL) machine, may be utilized. For example, a tape layup machine may lay up one or more layers of tows of constituent material that form a laminate which is then hardened (e.g., cured or consolidated) to form a composite part. 
     The operations of a tape layup machine may be directed by a Numerical Control (NC) program that dictates movements of the tape layup machine. A tape layup machine may dispense multiple tows at once onto a laminate in a single course (e.g., a single “run” across a laminate), and a tape layup machine may initiate or terminate individual lanes of tape within a course at different locations, in response to instructions from the NC program. 
     The final laminate generated by a tape layup machine may vary from what is intended in an NC program, owing to factors that are not always controllable. For example, lanes of tape may be placed some distance apart from their intended locations due to the machine being in need of calibration, foreign debris may fall onto the laminate, and fabrication inconsistencies such as twists or folds within a lane of tape may occur owing to inconsistencies in the lamination process. These conditions are difficult to visually detect during layup, because lanes of tape are made of the same material and hence are the same color (e.g., black). Furthermore, human inspection of a laminate prior to curing may result in additional foreign debris (e.g., lint, etc.) landing upon the laminate. Furthermore, current inspection techniques do not allow real time course by course inspection of the lay down process. The above-recited problems also apply to laminates made from tapes that are not fiber reinforced, and laminates that are not capable of hardening into composite parts. It is desirable to detect all conditions described above, and especially desirable to detect conditions cause portions of layup to be out of tolerance. 
     It remains possible to perform inspection of a composite part via ultrasonic techniques after hardening a laminate. However, if out of tolerance conditions within the composite part indicate a level of quality below a desired level, the entire composite part may need to be reworked or discarded. For large composite parts such as aircraft wings, a single reworked or discarded composite part results in a substantial waste of resources, time, and labor. 
     Therefore, it would be desirable to have a method and apparatus that take into account at least some of the issues discussed above, as well as other possible issues. 
     SUMMARY 
     Embodiments described herein include thermographic inspection systems that are mounted to the head of a tape layup machine. These inspection systems utilize infrared cameras to acquire thermal images of lanes of tape applied by the head. Different portions of the laminate will exhibit different temperatures, depending on whether they are the underlying laminate, foreign debris, or lanes of tape applied atop the underlying laminate. For example, a heater at the head may generate a detectable temperature difference between the underlying laminate and the lanes of tape, by heating either the underlying laminate or the lanes of tape. In a further example, a heater may heat both a laminate and a foreign object on the laminate. However, because the laminate and the foreign object have fundamentally different thermal properties, the foreign object will respond to the application of heat differently than the underlying laminate, resulting in a detectable difference in temperature. These differences are detected by reviewing thermal images acquired during the layup process. The location and nature of features that impact the quality of the laminate may therefore be reliably detected and reported, by analyzing thermal images from infrared cameras mounted to a head of the tape layup machine. One embodiment is a method for performing inspection of a tape layup. The method comprises laying up tape onto a surface of a laminate, applying heat to tack the tape to the surface, and generating thermographic images of the tape as applied to the surface. 
     A further embodiment is a method for determining applied tape boundaries. The method includes laying up lanes of tape onto a surface of a laminate, applying heat to tack the lanes of tape to the surface of the laminate, generating thermographic images of the lanes of tape as applied to the laminate, analyzing contrast within the thermographic images to identify the lanes of tape, and reporting locations of ends of the lanes of tape, based on boundaries depicted in the thermographic images. 
     A further embodiment is a non-transitory computer readable medium embodying programmed instructions which, when executed by a processor, are operable for performing a method for performing tape layup inspection. The method includes laying up lanes of tape onto a surface of a laminate, applying heat to tack the lanes of tape to the surface of the laminate, generating thermographic images of the lanes of tape as applied to the laminate, analyzing contrast within the thermographic images to identify the lanes of tape, and reporting locations of ends of the lanes of tape, based on boundaries depicted in the thermographic images. 
     A still further embodiment is a tape layup end detection system. The system includes a head of a tape layup machine. The head includes tape dispensers that lay up lanes of tape onto a surface of a laminate, a heater that applies heat to tack the lanes of tape to the surface, and an infrared camera disposed downstream of the tape dispensers that generates thermographic images of the lanes of tape as applied to the laminate. The system also includes a controller that analyzes contrast within the thermographic images to identify the lanes of tape, and reports locations of ends of the lanes of tape, based on boundaries depicted in the thermographic images. 
     A still further embodiment is a method of controlling a tape laying process. The method comprises laying up tape on surface, while laying up the tape, inspecting the surface on which it is laid up as well as the laid-up tape using IR imaging, reviewing the IR imaging for out of tolerance conditions, and stopping the tape laying if an out of tolerance condition is detected. 
     A still further embodiment is a method of detecting out of tolerance inconsistencies during a tape laying process. The method comprises heating a surface on which a tape will be applied, acquiring an IR image of the surface, and determining that an out of tolerance inconsistency is depicted in the IR image. 
     A still further embodiment is a method of inspecting a composite surface. The method includes creating temperature differentials on a surface that has been heated, detecting the temperature differentials on the surface, and determining that an out of tolerance inconsistency is present based upon the temperature differentials. 
     A still further embodiment is a method of creating a composite structure. The method includes inspecting a surface on which a laminate is to be laid, with IR imaging, reviewing the IR imaging for out of tolerance conditions, and stopping tape layup prior to reaching an out of tolerance condition. 
     A still further embodiment is a method that includes laying up lanes of tape at a laminate, operating an IR camera to thermally image the lanes of tape, reviewing thermal images to identify ends of the lanes of tape, and determining whether an end of a lane of tape is out of tolerance, and reporting the out of tolerance lane of tape for dispositioning. 
     Other illustrative embodiments (e.g., methods and computer-readable media relating to the foregoing embodiments) may be described below. The features, functions, and advantages that have been discussed can be achieved independently in various embodiments or may be combined in yet other embodiments further details of which can be seen with reference to the following description and drawings. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
       Some embodiments of the present disclosure are now described, by way of example only, and with reference to the accompanying drawings. The same reference number represents the same element or the same type of element on all drawings. 
         FIG. 1  is a block diagram of a tape layup inspection system in an illustrative embodiment. 
         FIG. 2  is a flowchart illustrating a method for detecting and classifying features found within a layup for a laminate, based on thermographic images in an illustrative embodiment. 
         FIG. 3  is a diagram illustrating a tape layup machine in an illustrative embodiment. 
         FIG. 4  is a top view of courses laid-up by a tape layup machine in an illustrative embodiment. 
         FIGS. 5A-5B, 6, and 7  are side views of heads of a tape layup machine in an illustrative embodiment. 
         FIG. 8  is a flowchart illustrating a method for determining locations of ends of lanes of tape based on a thermographic image in an illustrative embodiment. 
         FIG. 9  is a thermographic image of a portion of a course that includes ends of lanes of tape in an illustrative embodiment. 
         FIG. 10  is a flowchart illustrating a method for detecting layup inconsistencies in an illustrative embodiment. 
         FIG. 11  is a thermographic image of a portion of a course that includes a layup inconsistency in an illustrative embodiment. 
         FIG. 12  is a flowchart illustrating a method for determining locations of debris based on a thermographic image in an illustrative embodiment. 
         FIG. 13  is a thermographic image of a portion of a course that includes debris in an illustrative embodiment. 
         FIG. 14  is a flowchart illustrating a method of correlating image coordinates with physical locations in an illustrative embodiment. 
         FIGS. 15-18  illustrate further methods pertaining to thermographic inspection in illustrative embodiments. 
         FIGS. 19A-19B  illustrate further methods pertaining to thermographic inspection in illustrative embodiments. 
         FIG. 20  is a flow diagram of aircraft production and service methodology in an illustrative embodiment. 
         FIG. 21  is a block diagram of an aircraft in an illustrative embodiment. 
     
    
    
     DESCRIPTION 
     The figures and the following description provide specific illustrative embodiments of the disclosure. It will thus be appreciated that those skilled in the art will be able to devise various arrangements that, although not explicitly described or shown herein, embody the principles of the disclosure and are included within the scope of the disclosure. Furthermore, any examples described herein are intended to aid in understanding the principles of the disclosure, and are to be construed as being without limitation to such specifically recited examples and conditions. As a result, the disclosure is not limited to the specific embodiments or examples described below, but by the claims and their equivalents. 
     As used herein, “tape” may comprise fiber reinforced tapes or slit tape tows. In this disclosure the terms tape and tow are used interchangeably to indicate strips of material of varying widths. Tapes may be utilized to fabricate a variety of laminates, including laminates that will be cured into composite parts. Composite parts, such as Carbon Fiber Reinforced Polymer (CFRP) parts, are initially laid-up in a multi-layer laminate. Individual fibers within each layer of the laminate are aligned parallel with each other within the plane of the laminate, but different layers may exhibit different fiber orientations in order to increase the strength of the resulting composite along different dimensions. The laminate may include a viscous resin that solidifies in order to harden the preform into a composite part (e.g., for use in an aircraft). Carbon fiber that has been impregnated with an uncured thermoset resin or a thermoplastic resin is referred to as “prepreg.” Other types of carbon fiber include “dry fiber” which has not been impregnated with thermoset resin but may include a tackifier or binder. Dry fiber may be infused with resin prior to curing. For thermoset resins, the hardening is a one-way process referred to as curing. For thermoplastic resins, a hardened (or consolidated) resin may reach a viscous form if it is re-heated. 
       FIG. 1  is a block diagram of a tape layup inspection system  100  in an illustrative embodiment. Tape layup inspection system  100  comprises any system, component, or device operable to lay up tape to form a laminate, and to inspect the laminate for quality control purposes. Tape layup inspection system  100  has been enhanced to utilize thermal imaging devices mounted to a head of a tape layup machine in order to identify features that may be pertinent to quality control. 
     In this embodiment, tape layup inspection system  100  includes inspection server  110  and tape layup machine  130 . Tape layup machine  130  operates head  140  in accordance with NC program  135  in order to lay up lanes  160  of tape  154  that form one or more layers  152  of laminate  150 . For example, controller  132  of tape layup machine  130  may direct the operations of motors  138  based on instructions stored in memory  134 , in order to move head  140  to various locations at laminate  150 . Controller  132  may further direct tape reserve  136  to provide additional tape to tape dispensers  143  of head  140 . Controller  132  may be implemented, for example, as custom circuitry, as a hardware processor executing programmed instructions, or some combination thereof. 
     Head  140  includes tape dispensers  143 , which apply lanes  160  of tape  154  to surface  156  of laminate  150 . Heater  141  and/or heater  144  apply heat that facilitates tacking of lanes of tape  154  to laminate  150 . For example, these heaters may heat laminate  150  (or lanes  160  of tape  154 ) to a temperature at which a thermoplastic or thermoset resin within the lanes of tape  154  either tackifies or becomes molten. Heaters  141  and  144  may comprise lasers, infrared heat lamps, etc. 
     In embodiments where heaters  141  and  144  heat either laminate  150  or the lanes of tape, a substantial temperature difference (e.g., one to fifty degrees Fahrenheit (F) for thermoset tapes, five hundred to eight hundred degrees Fahrenheit for thermoplastic tapes) exists between the lanes  160  of tape  154  and the laminate  150 . This means that thermographic images (having a sensitivity of, for example, a fifth of one degree Fahrenheit) will exhibit a high degree of contrast between the lanes  160  of freshly laid tape and the laminate  150 . 
     In embodiments where heaters  141  and  144  are operated to heat the lanes  160  of tape  154  and also the laminate  150 , foreign objects (which are made from different types of material) will contrast strongly against the underlying laminate material, because they will reach a different temperature and have a different thermal emissivity than the laminate material in response to being exposed to the same amount of heat. 
     Head  140  also includes a compaction roller  146 , which applies pressure to the lanes  160  of tape  154  (e.g., after the lanes  160  have been tackified), pressing them onto laminate  150  and physically integrating them into laminate  150 . Infrared (IR) cameras  142  and  145  image the laminate  150  as well as the lanes  160  of tape  154  that are applied to laminate  150 . Position sensors  139  detect the location of head  140  as thermographic images are acquired by IR cameras  142  and  145 . This enables pixels within the thermographic images to be correlated with real-world locations at the laminate  150 . Position sensors  139  may, for example, comprise laser or visual tracking systems, rotation and/or extension sensors mounted to actuators within tape layup machine  130 , etc. 
     Thermographic images  118  produced by tape layup machine  130  during layup are processed by inspection server  110 . Inspection server  110  includes controller  112 , which analyzes thermographic images  118  stored in memory  114 , and identifies and classifies features within the thermographic images  118  based on detection functions  122 . Detection functions  122  may also include instructions for implementing one or more of the methods described herein. Controller  112  further determines, based on position data  120  acquired from position sensors  139 , locations of the features on the laminate  150 . This information may be passed on to a technician either as a report or an annotated image of the laminate for review. Controller  112  may be implemented, for example, as custom circuitry, as a hardware processor executing programmed instructions, or some combination thereof. 
     Illustrative details of the operation of tape layup inspection system  100  will be discussed with regard to  FIG. 2 . Specifically,  FIG. 2  illustrates a method for detecting and classifying features found within a layup for a laminate, based on thermographic images. Assume, for this embodiment, that tape layup machine  130  has been programmed to follow instructions in NC program  135  for laying up a laminate (e.g., a laminate that will be hardened into a composite part). Further, assume that tape layup machine  130  is loaded with rows of tape (e.g., prepreg thermoset or thermoplastic CFRP) and is ready to initiate fabrication of the laminate. To this end, the tape layup machine  130  operates head  140  to lay up a base layer of the laminate  150  by dispensing one or more courses comprising lanes of tape. 
       FIG. 2  is a flowchart illustrating a method for operating a tape layup inspection system in an illustrative embodiment. The steps of method  200  are described with reference to tape layup inspection system  100  of  FIG. 1 , but those skilled in the art will appreciate that method  200  may be performed in other systems. The steps of the flowcharts described herein are not all inclusive and may include other steps not shown. The steps described herein may also be performed in an alternative order. 
     In step  202 , head  140  of tape layup machine  130  lays up tape  154  onto surface  156  of laminate  150 . This may comprise following instructions in NC program  135  to cut and/or dispense multiple lanes of tape in a course. This may further comprise operating compaction roller  146  to physically integrate the newly dispensed lanes of tape with the laminate  150 . 
     In step  204 , heater  141  or heater  144  apply heat to tack the lanes of tape to the surface of the laminate  150 . Step  204  may occur concurrently with, before, or even after step  202 . Thus, in many embodiments, laminate  150  or lanes  160  are heated prior to contacting each other. In embodiments where the tape  154  comprises a prepreg thermoset resin tape, heater  141  may be activated to alter the temperature of the surface of laminate  150  with respect to ambient temperature before the lanes of tape are applied to the surface. In embodiments where the tape  154  comprises prepreg thermoplastic tape, heater  144  may comprise one or more lasers that heat the tape  154  resulting in a temperature differential from approximately four hundred to eight hundred degrees Fahrenheit between the surface of the laminate before the tape  154  is applied. In either case, the heaters generate a substantial difference in temperature, between the lanes of the tape  154  leaving the tape dispenser  143 , and the surface of laminate  150 . 
     In step  206 , IR camera  145  generates thermographic images  118  of the lanes  160  of the tape  154  as applied to the laminate  150 . Each thermographic image  118  may depict a portion of all lanes within a course, and thermographic images  118  may be stitched together to depict the layup resulting from an entire course. Because lanes may extend for tens of feet, multiple thermographic images  118  may need to be analyzed in order to detect the specific start locations and stop locations of individual lanes within a course. Thus, the thermographic images  118  may be acquired periodically (e.g., once every few seconds, once every ten feet of movement of head  140 , etc.), to ensure that there are no gaps in coverage between images during layup. 
     In step  208 , controller  112  analyzes contrast within the thermographic images  118  to identify a feature at laminate  150  that is thermally distinct from its surroundings. Each pixel within a thermographic image  118  is assigned a value corresponding with a temperature, and thermally distinct features may be detected by identifying contiguous sets of pixels that are within a range of temperatures (e.g., fifty degrees Fahrenheit, ten degrees Fahrenheit, etc.) that are surrounded by pixels outside of the range (e.g., more than one degree Fahrenheit different than the contiguous set of pixels). Each feature may have an associated temperature or range of temperatures, a known shape, and a known size in terms of width or number of pixels. In further embodiments, a thermographic image may be altered by applying an edge detection algorithm (such as a Laplacian or other filter) before the image is analyzed. 
     In step  210 , controller  112  classifies the feature based on at least one of a size of the feature, a shape of the feature, or a difference in temperature between the feature and its surroundings. For example, lanes  160  of tape  154  are expected to exhibit known ranges of temperature differences from an underlying laminate. These ranges are discussed above. If a region is within the expected range of temperature difference with respect to another region, it may be classified based on whether is hotter or colder than that other region. 
     In step  212  controller  112  determines features that are out of tolerance (e.g., too large, as identified by a filtering process performed on the feature&#39;s properties). If features are out of tolerance, then controller  112  reports the out of tolerance features for review. These features/conditions may be reported graphically on a representation of laminate  150 , or in a textual report. During this step, controller  112  may further filter the features based on their size and type, in order to automatically indicate and highlight out of tolerance features without a need for human intervention. 
     In a further example, a feature that exhibits rounded borders (e.g., a puddle of liquid) may be classified differently than a feature having sharp, linear edges (e.g., an edge of a lane of tape). Size also plays a role in these determinations, as small features may be indicative of debris at the laminate, while large features may be indicative of entire courses or lanes of tape. 
     Method  200  provides a substantial advantage over prior inspection techniques, because it utilizes differences in temperature, not color, to identify and classify layup features. For example, there is likely to be a pre-existing temperature difference between layup components (e.g., because one of them is heated to facilitate tacking), while there is likely to be almost no color difference between the laminate and the lanes of tape in visible light spectra. Therefore, method  200  enables the signal to noise ratio of layup inspection techniques to increase by orders of magnitude with respect to prior techniques. Furthermore, because thermal imaging technology is tightly coupled with the head  140  of the tape layup machine  130 , there is no need for manual imaging of the laminate, or other human interactions with the laminate  150 . This reduces the chance of technicians dropping foreign debris onto the laminate, stepping on the laminate  150 , or otherwise unintentionally altering the laminate  150  during human inspection. This also enables layup inspection to be performed much faster, and to occur contemporaneously with tape laydown of each course for each layer of the laminate, especially compared with stopping tape lay down to facilitate human access/inspection. 
       FIG. 3  is a diagram illustrating a tape layup machine  300  that is mounted to a support  370  in an illustrative embodiment. Tape layup machine  300  comprises any system or device capable of laying up lanes  352  of tape that form a laminate  350  (e.g., for curing into a composite part). Tape layup machine  300  includes head  380 , which dispenses lanes  352  of tape (e.g., CFRP) during layup. Lanes  352  are laid-up to form laminate  350 , which comprises one or more layers of material that will be cured into a single monolithic composite part. In this embodiment, laminate  350  comprises a section for an aircraft, and is held in place by rotational holder  360 . 
     As tape layup machine  300  operates to lay up the lanes  352  of tape onto laminate  350 , tape layup machine  300  may move directly towards/away from laminate  350  along axis X  366 , vertically upwards/downwards along axis Y  364 , and/or laterally along axis Z  362 . As used herein, when tape layup machine  300  lays up multiple lanes  352  concurrently during a single “sweep” of head  380 , those lanes  352  are collectively referred to as a single “course.” A set of non-overlapping courses that are applied consecutively may be referred to as a layer. As layers are added to laminate  350 , the strength of the resulting composite part is increased. 
     Laying up material for a laminate  350  that is large (e.g., a section of fuselage) is a time-consuming and complex process. In order to ensure that lanes  352  are laid-up quickly and efficiently, the operations of tape layup machine  300  are controlled by an NC program. In one embodiment, the NC program provides instructions on a course-by-course basis for aligning/repositioning the tape layup machine  300 , to control layup processes all the way down to the application of individual tows/tapes, moving the head  380 , and laying up lanes  352  onto laminate  350 . In this manner, by performing the instructions in the NC program, tape layup machine  300  fabricates a laminate (e.g., a laminate for curing into a composite part). 
       FIG. 4  is a top view of courses  420  laid-up by a tape layup machine in an illustrative embodiment, and corresponds with view arrows  4  of  FIG. 3 .  FIG. 4  illustrates a laminate  400 , which itself comprises lanes of tape that have been tacked together (not shown for clarity). As head  380  moves across surface  430  of laminate  400 , it deposits a course  420  comprising one or more lanes  352  of tape onto the laminate  400 . In this embodiment, each course comprises eight lanes of tows. For fiber reinforced laminates, each lane in a course will exhibit the same fiber direction, although different courses for different layers of the laminate may exhibit different fiber directions. 
     An NC program directing the head  380  may indicate locations at which to place each lane  352  within a course  420 . However, the actual ends of the lanes  352  as placed onto the laminate  400  may vary. Thus, a distance D may exist between the actual end location of a lane  353 , and the intended end location of the lane  353 . Additionally, debris  440  may fall onto the laminate during or after layup, and one or more layup inconsistencies  450  may also occur during the layup process. Debris  440  may comprise pills or pulls of fiber at the tape (“fuzz balls”), liquids (e.g., oil or water), particles (e.g., metal shavings, granules of plastic material, etc.), and a backing for the tape. Layup inconsistency  450  may comprise a twisted tape, a folded tape, a bridging of tape, a pucker, a wrinkle, an untacked tow or portion thereof, a missing tow, a double tow, a split or damaged tow, missing material, or other conditions. 
     Because thermal imaging may be utilized to quantify aspects of various features such as the locations of ends of lanes, the locations of foreign debris, and the locations of layup inconsistencies, the time and labor spent reworking the laminate  400  is reduced. That is, because out of tolerance features of the laminate  400  are immediately detected during layup, only a section of a course will need to be dispositioned. Furthermore, because the laminate  400  remains green and uncured during the inspection process, the rebuilding process is a simple matter of directly removing and re-applying lanes of tape to the laminate. This is not possible after the laminate has been cured into a composite part. 
       FIGS. 5-7  are side views of various configurations of a head  380  of a tape layup machine, and corresponds with view arrows  5  of  FIG. 4 . Specifically,  FIGS. 5A-5B  illustrate heads configured to inspect thermoset lanes of tape applied to laminates,  FIG. 6  illustrates a head configured to inspect thermoplastic lanes of tape applied to laminates, and  FIG. 7  illustrates a head configured to inspect a laminate for debris. 
       FIG. 5A  is a side view of a head  380  for inspecting thermoset lanes of tape applied to laminates in an illustrative embodiment. As shown in  FIG. 5A , head  380  proceeds in direction  500 , and includes a compaction roller  520  for compacting lanes  352  of tape onto surface  430  of laminate  400 . Heater  530  applies heat (A) to the surface  430 , in order to increase the temperature of surface  430  to a temperature where it softens and becomes tacky (i.e., to increase tack for the layer that was previously laid-up and is about to be covered up by a new layer). Although heater  530  is shown as being a distance in front of compaction roller  520 , in further embodiments the heater  530  is placed immediately in front of compaction roller  520 . When lanes  352  are applied to surface  430 , a temperature differential exists between the lanes and the underlying (e.g., unheated) laminate. This makes the lanes  352  distinguishable from the underlying laminate when reviewing thermographic images from IR camera  510 , which trails (i.e., is located downstream of) compaction roller  520 .  FIG. 5B  illustrates a further view wherein a heater  532  is disposed upstream of compaction roller  520 , and heats tape for one or more lanes  352  prior to the tape reaching the compaction roller  520 . 
       FIG. 6  is a side view of head  380  for inspecting thermoplastic lanes of tape applied to a laminate in an illustrative embodiment. As shown in  FIG. 6 , head  380  proceeds in direction  600 , and includes a compaction roller  620  for compacting lanes  352  of tape onto surface  430  of laminate  400 . A heater  632  applies targeted heat (A) to tape within lanes  352  in order to increase the temperature up to or in excess of the thermoplastic material melt temperature. This heat is applied at or just before tape for the lanes  352  is compacted onto laminated  400 . This enables detection of added or lost lanes of tape, as well as debris. When tape for lanes  352  is applied to surface  430 , a temperature differential exists between the lanes and the underlying laminate. This makes the lanes distinguishable from the underlying laminate when reviewing thermographic images from the IR camera  610 , which trails compaction roller  620 . 
       FIG. 7  is a side view of a head  380  for inspecting a laminate in order to detect debris in an illustrative embodiment. As shown in  FIG. 7 , head  380  proceeds in direction  700 , and includes a compaction roller  720  for compacting lanes  352  of tape onto surface  430  of laminate  400 . Head  380  also includes heater  730  and IR camera  710 , as well as heater  732  and IR camera  712 . Heater  730  applies heat (A) to the surface  430 , in order to increase the temperature of surface  430 . Debris  750  (e.g., Foreign Object Debris (FOD), a pill of fiber, etc.) located at surface  430  is therefore heated by heater  730 . Because the underlying thermal properties of the debris are likely to vary from that of surface  430 , or because the shape of the debris may alter its ability to retain heat as compared to surface  430  and/or because the shape of the debris may be recognizable via image analysis, the debris  750  may be detected in thermographic images acquired by IR camera  710 . 
     Head  380  additionally includes heater  732  and IR camera  712 . Heater  732  increases a temperature of lanes  352 . This helps IR camera  712  to better distinguish between debris  760  (e.g., new debris falling off of head  380 , such as oil) and the lanes  352 . Thus, when debris exists on surface  430  before tape for one or more lanes  352  are laid-up, the debris can be detected by IR camera  710 , while debris that lands on lanes  352  after the lanes  352  are laid-up can be detected by IR camera  712 . 
     The arrangement depicted in  FIG. 7  has an additional advantage in that the IR camera  732  may also indirectly detect debris  750 . Debris  750  often conducts heat differently between lanes  352  and laminate surface  430  than does direct contact between lanes  352  and laminate surface  430 . If debris  750  is more insulating (i.e., conducts heat worse) than direct contact, or debris  750  induces an air gap (which also insulates) between lanes  352  and laminate surface  430 , then the area of one or more lanes  352  above the debris  750  will appear hotter than surrounding lanes, and this hot spot may be used as detection of debris  750  buried under lanes  352 . If debris  750  is more conductive (i.e., conducts heat better) than direct contact between lanes  352  and laminate surface  430 , then the area of lanes  352  which are above the debris  750  will appear colder than the surrounding lanes  352 , and this cold spot may be used as detection of debris  750  buried under lanes  352 . 
       FIGS. 8-13  illustrate various specific techniques for identifying ends of lanes of tape, layup inconsistencies, and debris respectively. Specifically,  FIGS. 8-9  describe identifying the ends of lanes of tape,  FIGS. 10-11  describe identifying layup inconsistencies, and  FIGS. 12-13  describe identifying debris. 
       FIG. 8  is a flowchart illustrating a method  800  for operating a tape layup inspection system to detect ends of lanes of tape in an illustrative embodiment. The steps of method  800  are described with respect to tape layup inspection system  100  of  FIG. 1 , and may be performed via the head  380  depicted in  FIG. 5  or  FIG. 6 , or even the head  380  depicted in  FIG. 7 . Method  800  may initiate with steps  202 - 206  of method  200  described above, to lay up and image lanes of tape at a laminate. 
     In step  802 , controller  112  analyzes contrast within thermographic images  118  to identify lanes  160  of tape  154 . Controller  112  may identify values (e.g. intensity levels, or brightness levels) at each of multiple pixels within the thermographic images  118 . Pixel values within the thermographic images correspond with temperatures. Hence, controller  112  may identify regions that have different temperatures, based on differences between values of neighboring pixels. 
     This process may include identifying contiguous regions of pixels that have a temperature differential of more than a predetermined threshold amount with respect to neighboring contiguous regions of pixels, or grouping all contiguous pixels that are within a threshold range of temperatures (e.g., five degrees Fahrenheit, fifty degrees Fahrenheit) together with each other into a region. For example, if the lanes  160  of tape  154  are known to have a temperature differential between one and fifty degrees Fahrenheit with respect to the laminate  150 , controller  112  may identify contiguous regions of pixels that have a corresponding temperature differential to surrounding regions as being lanes of tape. 
     In step  804 , controller  112  determines a direction of the lanes  160  of tape  154 . The direction of a lane of tape is the direction in which head  140  moves while laying up the lane. The direction may be predefined based on a known orientation of the camera with respect to the head  140 , considered in combination with position data  120  and/or directions specified by the NC program  135 . Controller  112  may even use position data  120  to confirm that the head  140  moves in a direction indicated by NC program  135 . Alternatively, the direction or may be dynamically determined based on the longest axis found for lanes depicted within a thermographic image. 
     In step  806 , for each lane of tape, controller  112  identifies a boundary at which temperature changes by more than a threshold amount when proceeding in the direction determined in step  804 . That is, within the bounds of each lane of tape, controller  112  reviews the values of adjacent/neighboring pixels, while moving pixel-by-pixel in the direction until a boundary is detected. The threshold amount used for boundary detection may vary between thermoset and thermoplastic materials, as discussed above. In some embodiments, step  806  may comprise running an edge detection algorithm (e.g., applying a Laplacian or other filter) to the thermographic image  118 , and identifying regions where a sharp transition between temperatures occurs. 
     In step  808 , controller  112  determines a location of a corresponding boundary for each of the lanes  160  of tape  154 . This may comprise transforming coordinates at the thermographic image  118  into locations at the laminate  150 , for example, based upon a known position and/or orientation of an IR camera at the time that the IR camera generated the thermal image, and a known offset between the IR camera and coordinates of pixels. 
     In step  810 , controller  112  reports locations of the ends of the lanes  160  of tape  154 , based on the boundaries detected in the thermographic images. For example, controller  112  may report the locations determined in step  808 , in either a textual report or an overlay provided atop an image of the laminate  150 . If the locations of the ends of the lanes  160  are more than a threshold amount (e.g., one inch, ten inches, etc.) from their intended start locations and stop locations, controller  112  may indicate this condition as part of the report. In step  812 , controller  112  determines whether the ends of the lanes are within tolerance. If any of the ends of the lanes are out of tolerance, step  814  comprises dispositioning these ends. Dispositioning may include any type of determination of the course of action to deal with a discovered out of tolerance condition for an end. 
       FIG. 9  is a thermographic image  900  of a portion of a course in an illustrative embodiment. Within the thermographic image, pixels that represent objects having different temperatures will have different values (e.g., levels of brightness). Thus, a pixel for a cool object may appear darker than a pixel for a warm object. Controller  112  may analyze thermographic image  900  by applying an edge-detection algorithm, or otherwise searching for transitions in temperature along the Y direction  930  that are greater than a threshold amount (e.g., greater than one degree Fahrenheit). Controller  112  may then determine a location and thickness of each lane depicted within the thermographic image  900 . Fibers may be oriented in any suitable direction within each of the lanes of tape. For example, a region  922  depicting a top lane has a width W 1 , a region  924  depicting a middle lane has a width W 2 , and a region  926  depicting bottom lane has a width W 3 . These regions are surrounded by region  920 , which represents the underlying laminate (as determined by differences in temperature). Within the Y coordinates occupied by each lane, controller  112  may traverse pixels in the X direction  940  to identify boundaries  910  between pixels that are greater than the threshold. For each boundary  910  identified in this manner, controller  112  may identify the X and Y coordinate of the boundary  910  within the image as the end of a lane. For example, the top lane ends at X 1 , the middle lane ends at X 2 , and the bottom lane ends at X 3 . In embodiments where the boundaries comprise regions of pixels, controller  112  may calculate a centroid of the region, and use the coordinate of the centroid. The coordinates may be transformed into locations at the laminate  150  based on position data  120 , and the locations may be compared against desired locations indicated in an NC program. If the locations are more than a threshold distance (e.g., one foot, one inch, etc.) from the desired locations, then a technician may elect to pause layup processes for the laminate, in order to disposition (e.g., rework) any out-of-tolerance conditions. In addition, a statistical report may be provided to the technician that compares desired lane locations to the identified lane locations for each layer. 
       FIG. 10  is a flowchart illustrating a method  1000  for detecting layup inconsistencies in an illustrative embodiment. The steps of method  1000  are described with respect to tape layup inspection system  100  of  FIG. 1 , and may be performed via the head  380  depicted in  FIG. 5  or  FIG. 6 , or even the head  380  depicted in  FIG. 7 . Method  1000  may initiate with steps  202 - 206  of method  200  described above, to lay up and image lanes of tape at a laminate. 
     Step  1002  includes analyzing contrast within thermographic images  118  to identify lanes  160  of tape  154 . This may be performed based on an expected amount of temperature difference between the laminate  150  and the lanes  160 , and may be performed in a similar manner to the steps of method  800  provided above. 
     In step  1004 , for each lane of tape, controller  112  reviews an interior of the lane for differences in temperature. These differences in temperature may be low enough that the interior of the lane is not considered a different region, but may be high enough to indicate that an inconsistency may exist. Step  1004  therefore facilitates detection of layup inconsistencies found within a lane of tape. 
     In step  1006 , for each lane of tape, controller  112  reviews a boundary of the lane for inconsistencies in shape. For example, lanes may be expected to have boundaries that are roughly rectangular in shape, and are composed of long straight lines. If a boundary exhibits a high curvature or irregularity, this may indicate the presence of a layup inconsistency. Step  1006  therefore facilitates detection of layup inconsistencies found at the edge of one or more lanes of tape. 
     In step  1008 , controller  112  determines the existence of a layup inconsistency, for example based upon the reviews of step  1004  and step  1006 . In step  1010 , controller  112  categorizes the layup inconsistency based on at least one of a size of the layup inconsistency, a shape of the layup inconsistency, or a difference in temperature at the layup inconsistency. For example, detection functions  122  may indicate that a inconsistency exists if the width of a tow changes to less than a predetermined amount, if a gap between tows increases beyond or decreases below a threshold value, if a boundary of a tow is jagged, etc. Different ones of detection functions  122  may be triggered (and hence different categories of inconsistency may be assigned by controller  112 ) based on various combinations of shape, size, and temperature. In step  1012 , controller  112  identifies out-of-tolerance layup inconsistencies, and in step  1014 , controller  112  reports out of tolerance layup inconsistencies for review (e.g., in order to enable a technician to engage in dispositioning of the out of tolerance conditions. 
       FIG. 11  is a thermographic image  1100  of a portion of a course that includes layup inconsistencies in an illustrative embodiment. Within the thermographic image, pixels that represent objects having different temperatures will have different values (e.g., levels of brightness). Thus, a pixel for a cool object may appear darker than a pixel for a warm object. The lanes have a length along the X direction  1160 , and width along the Y direction  1150 . In this embodiment, lanes  1122  and  1124  do not include inconsistencies, while lane  1126  includes layup inconsistencies  1130  in the form of a pucker, and layup inconsistency  1140  in the form of a wrinkle. Layup inconsistencies  1130  may be determined based on lane  1126  dropping below an expected width or having a varying width, or may be detected by determining that a curvature of the edge of lane  1126  changes or is within a predefined range. Layup inconsistencies may also be detected by temperature differences (beyond a threshold) between lane  1126  and an underlying layer  1120  of the laminate. Meanwhile, layup inconsistency  1140  may be detected based on its long, narrow shape, and having a known temperature difference with respect to the rest of the lane  1126 . 
       FIG. 12  is a flowchart illustrating a method  1200  for determining locations of debris based on a thermographic image in an illustrative embodiment, and may be performed via the head  380  depicted in  FIG. 7 . The steps of method  1200  are described with respect to tape layup inspection system  100  of  FIG. 1 , but may be performed in other systems as desired. Method  1200  includes heating a surface  156  of laminate  150  via heater  141  in step  1202 , and generating thermographic images  118  of the surface  156  via IR camera  142  in step  1204 . Method  1200  also includes laying up lanes  160  of tape  154  onto the surface  156  of laminate  150  via tape dispensers  143  in step  1206 . In step  1208 , method  1200  includes heating the lanes  160  of tape  154  via heater  144 , and step  1210  includes generating thermographic images of the lanes  160  of tape  154  as applied to the laminate  150 . Ideally, the lanes  160  are heated to the same temperature as the laminate  150 . This enables debris (e.g., FOD) to be more easily distinguished from the tape  154  that lanes  160  and laminate  150  are made from. 
     Having acquired the thermographic images  118  depicting both the laminate  150  and the lanes  160  applied to the laminate  150 , foreign object debris can be spotted accurately and efficiently by identifying differences in temperature. In step  1212 , controller  112  analyzes contrast within the thermographic images to identify different regions having different temperatures, which may be performed in a similar manner to the techniques described above. However, because different categories of debris may be associated with substantially different thermal properties, regions may be distinguished based on a variety of different temperature thresholds, each corresponding to a different type of debris. For example, pills of fiber at the tape may be expected to be a first range of temperatures higher than the underlying laminate, to be small in size and to have irregular borders, while liquids may be expected to be a second range of temperatures cooler than the underlying laminate, have a wide range of sizes, and have smooth borders. Thus, the amount of temperature difference used as criteria to define separate regions in method  1200  (e.g., less than five degrees, less than two degrees, etc.) may be much smaller than the amount of temperature difference described with respect to other methods. 
     Because thermographic images are acquired both before and after laying up lanes of tape, analyzing contrast within the thermographic images may comprise reviewing the thermographic images of the surface of the laminate to identify debris covered by at least one layer of tape, and also reviewing the thermographic images of the lanes of tape as applied to the laminate to identify debris at a surface of the lanes of tape. The process may even be stopped prior to laying up a course over an out of tolerance piece of debris detected by IR camera  710 . This allows disposition (e.g. removal of the piece of debris) prior to applying the course over the debris. If the debris is not out of tolerance, applying a course over it might be a desired action. To facilitate detection of debris at boundaries between lanes or courses, images may have a wide enough field of view to capture likely locations at which the debris will be located. 
     In step  1214 , controller  112  categorizes a type of debris within a region based upon at least one of a size of the region, a shape of the region, or a difference in temperature between the region and other regions. For example, pills of fiber may be expected to have irregular shapes, to have a specific amount of temperature differential from the underlying laminate, and to be small (e.g., having a maximum number of pixels corresponding with an area of less than a centimeter across). Particles such as metal shavings may be expected to be particularly small or a different temperature than their surroundings, and liquids may be expected to be have a different range of temperature differentials with their surroundings, and also to have rounded borders. Furthermore, in some embodiments metal shavings of any size are considered out of tolerance, while pills below a certain size might be considered within tolerance. Detection functions  122  may indicate conditions for categorizing each of a variety of regions at a laminate into categories of debris. After debris has been categorized and identified by controller  112 , controller  112  may generate a report indicating the nature, location, and/or severity of the debris that was detected. In step  1216 , debris that is out of tolerance is identified (e.g., based on its size and classification) by controller  112 , and in step  1218 , out of tolerance debris is reported to a technician for dispositioning (e.g., removal). 
       FIG. 13  is a thermographic image  1300  of a portion of a course that includes debris in an illustrative embodiment. Within the thermographic image, pixels that represent objects having different temperatures will have different values (e.g., levels of brightness). Thus, a pixel for a cool object may appear darker than a pixel for a warm object. In this embodiment, the course includes lane  1322 , lane  1324 , and lane  1326 . Lane  1322  includes debris in the form of a metal pellet  1330  that happens to be hotter than its surroundings, and lane  1326  includes debris in the form of a liquid puddle  1340  that happens to be cooler than its surroundings. In this case, the metal pellet  1330  straddles a boundary of lane  1322  and an underlying laminate  1320 . The courses proceed in the X direction  1360 , and have a width along the Y direction  1350 . 
       FIG. 14  is a flowchart illustrating a method  1400  of correlating image coordinates with physical locations in an illustrative embodiment. In step  1402  of method  1400 , controller  1412  determines a position and/or orientation of an IR camera  145  at the time when a thermographic image was generated by the IR camera  145 . This may come in the form of position data  120  reported by position sensors  139  of  FIG. 1 . In step  1404 , controller  112  determines a coordinate (e.g., an X and Y position) of a feature depicted within the thermographic image. This may comprise identifying a region having a different temperature than neighboring regions, and calculating a centroid of the region. 
     Step  1406  includes determining a location at the laminate based on the position of the IR camera  145  and the coordinate of the feature. For example, position data may indicate a position and orientation of the IR camera  145  when the thermographic image was taken. Because the camera is fixed with respect to a head of a tape layup machine, each coordinate within all images may correspond with a known physical offset from the IR camera  145 . Hence, by applying the offset, the actual location of a feature at the laminate can be reliably determined. 
       FIG. 15  illustrates a method  1500  of controlling a tape laying process in an illustrative embodiment. The method includes laying up tape on surface (step  1502 ), and while laying up the tape, inspecting the surface on which it is laid up as well as the laid up tape using IR imaging (step  1504 ). The method further comprises reviewing the IR imaging for out of tolerance conditions, and stopping the tape laying if an out of tolerance condition is detected (step  1508 ). 
       FIG. 16  illustrates a method  1600  of detecting out of tolerance inconsistencies during a tape laying process in an illustrative embodiment. The method includes heating a surface on which a tape will be applied (step  1602 ), acquiring an IR image of the surface (step  1604 ), and determining that an out of tolerance inconsistency is depicted in the IR image (step  1606 ). 
       FIG. 17  illustrates a method  1700  of inspecting a composite surface in an illustrative embodiment. Method  1700  includes creating temperature differentials on a surface that has been heated (step  1702 ), detecting the temperature differentials on the surface (step  1704 ), and determining that an out of tolerance inconsistency is present based upon the temperature differentials (step  1706 ). 
       FIG. 18  illustrates a method  1800  of creating a composite structure in an illustrative embodiment. The method includes inspecting a surface on which a laminate is to be laid (step  1802 ), with IR imaging. The method also includes reviewing the IR imaging for out of tolerance conditions (step  1804 ) and stopping tape layup prior to reaching an out of tolerance condition (step  1806 ). 
       FIGS. 19A-19B  illustrate methods  1900  and  1950  of inspecting tape end layup in an illustrative embodiment. Method  1900  includes laying up lanes of tape at a laminate (step  1902 ), operating an IR camera to thermally image the lanes of tape (step  1904 ), reviewing thermal images to identify ends of the lanes of tape (step  1906 ), determining whether an end of a lane of tape is out of tolerance (step  1908 ), and reporting the out of tolerance lane of tape for dispositioning (step  1910 ). 
       FIG. 19B  illustrates method  1950  for inspecting tape end layup. Method  1950  if focused upon heating the tape prior to placing the tape, in order to improve tack. Method  1950  may be used to detect out of tolerance inconsistencies during a tape laying process in an illustrative embodiment. The method includes heating tape prior to application of the tape onto a surface (step  1952 ), acquiring an IR image of the surface (step  1954 ), and determining that an out of tolerance inconsistency is depicted in the IR image (step  1956 ). 
     EXAMPLES 
     Referring more particularly to the drawings, embodiments of the disclosure may be described in the context of aircraft manufacturing and service in method  2000  as shown in  FIG. 20  and an aircraft  2002  as shown in  FIG. 21 . During pre-production, method  2000  may include specification and design  2004  of the aircraft  2002  and material procurement  2006 . During production, component and subassembly manufacturing  2008  and system integration  2010  of the aircraft  2002  takes place. Thereafter, the aircraft  2002  may go through certification and delivery  2012  in order to be placed in service  2014 . While in service by a customer, the aircraft  2002  is scheduled for routine work in maintenance and service  2016  (which may also include modification, reconfiguration, refurbishment, and so on). Apparatus and methods embodied herein may be employed during any one or more suitable stages of the production and service described in method  2000  (e.g., specification and design  2004 , material procurement  2006 , component and subassembly manufacturing  2008 , system integration  2010 , certification and delivery  2012 , service  2014 , maintenance and service  2016 ) and/or any suitable component of aircraft  2002  (e.g., airframe  2018 , systems  2020 , interior  2022 , propulsion system  2024 , electrical system  2026 , hydraulic system  2028 , environmental  2030 ). 
     Each of the processes of method  2000  may be performed or carried out by a system integrator, a third party, and/or an operator (e.g., a customer). For the purposes of this description, a system integrator may include without limitation any number of aircraft manufacturers and major-system subcontractors; a third party may include without limitation any number of vendors, subcontractors, and suppliers; and an operator may be an airline, leasing company, military entity, service organization, and so on. 
     As shown in  FIG. 21 , the aircraft  2002  produced by method  2000  may include an airframe  2018  with a plurality of systems  2020  and an interior  2022 . Examples of systems  2020  include one or more of a propulsion system  2024 , an electrical system  2026 , a hydraulic system  2028 , and an environmental system  2030 . Any number of other systems may be included. Although an aerospace example is shown, the principles of the invention may be applied to other industries, such as the automotive industry. 
     As already mentioned above, apparatus and methods embodied herein may be employed during any one or more of the stages of the production and service described in method  2000 . For example, components or subassemblies corresponding to component and subassembly manufacturing  2008  may be fabricated or manufactured in a manner similar to components or subassemblies produced while the aircraft  2002  is in service. Also, one or more apparatus embodiments, method embodiments, or a combination thereof may be utilized during the subassembly manufacturing  2008  and system integration  2010 , for example, by substantially expediting assembly of or reducing the cost of an aircraft  2002 . Similarly, one or more of apparatus embodiments, method embodiments, or a combination thereof may be utilized while the aircraft  2002  is in service, for example and without limitation during the maintenance and service  2016 . For example, the techniques and systems described herein may be used for material procurement  2006 , component and subassembly manufacturing  2008 , system integration  2010 , service  2014 , and/or maintenance and service  2016 , and/or may be used for airframe  2018  and/or interior  2022 . These techniques and systems may even be utilized for systems  2020 , including, for example, propulsion system  2024 , electrical system  2026 , hydraulic  2028 , and/or environmental system  2030 . 
     In one embodiment, a part comprises a portion of airframe  2018 , and is manufactured during component and subassembly manufacturing  2008 . The part may then be assembled into an aircraft in system integration  2010 , and then be utilized in service  2014  until wear renders the part unusable. Then, in maintenance and service  2016 , the part may be discarded and replaced with a newly manufactured part. Inventive components and methods may be utilized throughout component and subassembly manufacturing  2008  in order to fabricate laminates that are hardened into new parts. 
     Any of the various control elements (e.g., electrical or electronic components) shown in the figures or described herein may be implemented as hardware, a processor implementing software, a processor implementing firmware, or some combination of these. For example, an element may be implemented as dedicated hardware. Dedicated hardware elements may be referred to as “processors”, “controllers”, or some similar terminology. When provided by a processor, the functions may be provided by a single dedicated processor, by a single shared processor, or by a plurality of individual processors, some of which may be shared. Moreover, explicit use of the term “processor” or “controller” should not be construed to refer exclusively to hardware capable of executing software, and may implicitly include, without limitation, digital signal processor (DSP) hardware, a network processor, application specific integrated circuit (ASIC) or other circuitry, field programmable gate array (FPGA), read only memory (ROM) for storing software, random access memory (RAM), non-volatile storage, logic, or some other physical hardware component or module. 
     Also, a control element may be implemented as instructions executable by a processor or a computer to perform the functions of the element. Some examples of instructions are software, program code, and firmware. The instructions are operational when executed by the processor to direct the processor to perform the functions of the element. The instructions may be stored on storage devices that are readable by the processor. Some examples of the storage devices are digital or solid-state memories, magnetic storage media such as a magnetic disks and magnetic tapes, hard drives, or optically readable digital data storage media. 
     Although specific embodiments are described herein, the scope of the disclosure is not limited to those specific embodiments. The scope of the disclosure is defined by the following claims and any equivalents thereof