Abstract:
A system for joining articles includes a joining unit; a positioning unit coupled to the joining unit, the positioning unit positioning the joining unit; a vision system obtaining an image of an item to be joined on a workpiece; a controller for processing the image and controlling a position of the joining unit relative to the item to be joined in response to the image; and a temperature probe for monitoring a temperature; wherein the controller adjusts the position of the joining unit in response to the temperature.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of U.S. Provisional patent application serial number 61/670,176, filed Jul. 11, 2012, the entire content of which is incorporated herein by reference. 
     
    
     BACKGROUND 
       [0002]    The subject matter disclosed herein generally relates to joining workpieces, and in particular to a method and system for joining aluminum workpieces. 
         [0003]    Brazing is one technique to join workpieces. Brazing is used to join metal workpieces by heating a joint of the workpieces (e.g., via a torch) and applying a filler to the joint once the workpieces have reached a suitable temperature, or by fusing both parts together. The filler melts into the joint, and when cooled creates a mechanical attachment between the workpieces. 
         [0004]    Controlling heat during brazing can be challenging when working with certain materials. There exist automatic brazing systems, but complicated shapes or hard to reach joints are particularly challenging for automated brazing systems. Overheating and melting of the part is easy to do especially when the window between the base metal alloy melt point and the melting point for the brazing materials is close. This challenge makes the use of manual brazing for complicated or hard to access joints the preferred method, at the expense of cycle time and cost. Manual joining quality is highly dependent on operator skills. Existing automated joining systems are in use for simple geometries, but are not as effective or reliable for hard to reach joints or parts with high variability (e.g., manually assembled parts). 
       BRIEF DESCRIPTION OF THE INVENTION 
       [0005]    One embodiment is a system for joining articles including a joining unit; a positioning unit coupled to the joining unit, the positioning unit positioning the joining unit; a vision system obtaining an image of an item to be joined on a workpiece; a controller for processing the image and controlling a position of the joining unit relative to the item to be joined in response to the image; and a temperature probe for monitoring a temperature; wherein the controller adjusts the position of the joining unit in response to the temperature. 
         [0006]    Another embodiment is a method for joining articles, the method including obtaining an image of an item to be joined on a workpiece; processing the image; positioning a joining unit relative to the item to be joined on the workpiece in response to processing the image; monitoring a temperature; and adjusting the position of the joining unit in response to the temperature. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]      FIG. 1  depicts a joining system in an exemplary embodiment; 
           [0008]      FIG. 2  depicts a gripper in an exemplary embodiment; 
           [0009]      FIG. 3  depicts an item to be joined in an exemplary embodiment; 
           [0010]      FIG. 4  depicts positioning of a joining unit relative to a workpiece in an exemplary embodiment; 
           [0011]      FIG. 5  is a block diagram of a joining system in an exemplary embodiment; and 
           [0012]      FIG. 6  is a flowchart of a process for joining workpieces in an exemplary embodiment. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0013]      FIG. 1  depicts a joining system  10  (e.g., a brazing system) in an exemplary embodiment. In the examples described herein, the joining is achieved by brazing. It is understood that other joining techniques may be used, and embodiments are not limited to brazing. 
         [0014]    A positioning unit  12  moves a joining unit  14  (e.g. a brazing unit) to apply heat to a workpiece  16  to join/braze the items on the workpiece  16 . The positioning unit  12  positions the brazing unit  14  in three dimensions to position the brazing unit  14  relative to the workpiece  16 . The positioning unit  12  may employ axial motion using three, single-axis, modular linear actuators  18 . The actuators  18  use internal rail guides and a drive such as a screw, belt or gear. It is understood that the positioning unit  12  may employ other constructions (e.g., a multijointed arm) in order to position brazing unit  14  in three-dimensional space. 
         [0015]    Brazing unit  14  applies heat to an item of the workpiece  16  in response to commands from a controller  200  ( FIG. 5 ). Brazing unit  14  may be implemented using a torch or an induction brazer. Embodiments described herein refer to a torch, but the processes described herein are applicable to induction brazers and other types of brazers. Brazing unit  14  may use known fuel types (e.g., propane, oxy-acetylene, propylene, natural gas, MAPP, hydrogen, LP, acetylene). The fuel type may be any fuel-air/oxygen combination. The fuel-air mix may be adjusted by controller  200  as described in further detail herein. 
         [0016]    Workpiece  16 , in the example shown in  FIG. 1 , is a round tube plate fin heat exchanger having a plurality of aluminum hairpin tubes  20  mounted to fins  22 . The brazing system  10  brazes an aluminum return bend  24  ( FIG. 3 ) to the tubes  20 . Return bend  24  is positioned in a cup  26  formed in one end of tube  20 . 
         [0017]    One or more vision systems  30  (e.g., cameras) are mounted to the positioning unit  12  to obtain two-dimensional images of the workpiece  16  at a brazing location. Vision systems  30  are positioned, for example, on orthogonal axes to provide a side view and a top view of the workpiece  16 . The images from vision systems  30  are processed to detect the location of an item to be brazed on the workpiece and position brazing unit  14  accordingly. Although two vision systems  30  are shown in  FIG. 1 , a single vision system  30  may be used (e.g., the side view) for location of the workpiece. 
         [0018]    Positioning unit  12  includes a gripper  40  for mounting brazing unit  14 , shown in  FIG. 2 . Gripper  40  includes a multi-dimensional linkage  42  that allows for positioning the gripper  40 , and thus brazing unit  14 , in one or more dimensions. The linkage  42  includes joints  44  that may be adjusted to define a location and orientation in three dimensional space. In one embodiment, joints  44  include set screws that are used to provide rotation about the Y axis and Z axis, as shown in  FIG. 2 . Joints  44  may also include electrically driven actuators that are adjusted by a controller  200  to position the brazing unit  14  remotely. 
         [0019]    Also shown in  FIG. 2  is a temperature probe  46 . Temperature probe  46  detects a temperature of an item to be brazed on workpiece  16  in order to control the brazing operation. Temperature probe  46  may be implemented using a variety of techniques. In an exemplary embodiment, temperature probe  46  is a contact probe that measures temperature of the workpiece  16  at a location a fixed distance from the projected centerline of the brazing unit  14 . In another embodiment, temperature probe  46  is a non-contact probe (e.g., IR or pyromenter) that monitors temperature at a fixed point close to the tube  20  being brazed. This may include using a non-reflective coating on tube  20 , to minimize emissivity and reflectivity issues with aluminum. In another embodiment, temperature probe  46  is a photosensor that monitors and detects change in flame color at the proper brazing temperature, if brazing unit  14  includes a torch. Using the change in flame color to detect proper brazing temperature for aluminum is described in further detail in published PCT application, PCT/US11/057192, the contents of which are incorporated herein by reference. 
         [0020]    To properly braze the workpiece  16 , the brazing unit  14  is positioned by the positioning unit  12 . To determine the location of an item to be brazed on workpiece  16 , an image from the vision system  30  is processed by controller  200 .  FIG. 3  depicts and exemplary image of a side view of tube  20  with return tube  24  positioned therein. Brazing material  50  may be packed in the joint between the return tube  24 . Alternatively, brazing material may be provided in the form of a brazing rod  52 . If a brazing rod  52  is used, the positioning unit  12  includes a feed unit for delivering the brazing rod to the joint between tube  20  and return tube  24 . 
         [0021]    Controller  200  ( FIG. 5 ) analyzes the image of the tube  20  to detect the location of the tube  20 . Other components of the system may perform image processing functions described herein. For example, vision system  30  may perform pre-processing of an image to provide feature recognition. As such, operations described herein as performed by controller  200  may have portions of those operations performed by other components. 
         [0022]    Several techniques may be used to determine the location of tube  20 . Feature extraction may be performed on the image to extract features such as the edges of the tube  20  and edges of the cup  26 . Once the edges of the tube  20  and cup  26  are located, measurements of the tube width at the tube base, cup width at the cup base and a distance between the tube  20  and cup  26  are acquired from the image as shown in  FIG. 3 . The measurements from the image are compared to reference values to determine the location of the tube  20  relative to the vision system  30 . For example, if the measurement of tube width is greater than an expected reference value, the vision system  30  and tube  20  are too close, and the positioning unit  12  can move away from the tube  20  in the x direction. 
         [0023]    Further, pattern matching may be used to compare the acquired image to reference images. Shifts in the Y or Z direction can be detected by overlaps between the acquired image and reference image. The positioning unit  12  can then move to obtain the proper position between the positioning unit  12  and the workpiece  16 . An image of a top view of the tube  20  may also be processed to detect features of the tube and then adjust the location of the positioning unit  12  based on measurements from the top view image and/or pattern recognition of the top view image. 
         [0024]    By performing image analysis on the images from the vision system  30 , the initial position of positioning unit  12 , and thus brazing unit  14 , can be established.  FIG. 4  shows an exemplary initial position for brazing unit  14  relative to tube  20 . The brazing unit  14  is positioned at an x distance from the tube  20  and at a z distance from the top of cup  26 . Brazing unit  14  may be centered with tube  20  along the y axis or offset along the y axis as desired. Further, joints  44  in linkage  42  may be set to provide rotation of the brazing unit  14  about the y axis or the z axis, as desired. 
         [0025]      FIG. 5  is a block diagram of a brazing system in an exemplary embodiment. The system includes a controller  200  that controls operation of the positioning unit  12  and brazing unit  14 . As noted above, other types of joining units may be used, and embodiments are not limited to brazing. Controller  200  may be implemented using a general-purpose microprocessor executing instructions stored in a storage medium to perform the operations described herein. Actuators  202  are part of positioning unit  12 , and includes actuators  18  to move the brazing unit  14  in the x, y, and z directions as described above. Actuators  202  may also include joints  44  of linkage  42  so that rotation of the brazing unit  14  about the y and z axes may be remotely controlled. 
         [0026]    Vision system  30  corresponds to one or more elements  30  in  FIG. 1 . Controller  200  obtains images from the vision systems(s)  30  and performs image analysis to locate the position of the tube  20  as described above. As noted above, vision system  30  may perform some of the image analysis. Brazing unit  14  is controlled by controller  200  as well. Controller  200  can alter the output heat of the brazing unit  14  by adjusting a fuel/air mix to a torch or electrical power to an induction brazer. Database  204  stores information about workpieces that is used by controller  200  to automate the brazing process. This information includes characteristics of the workpiece (e.g., distance between tubes  20 ), expected temperature ranges, time to complete brazing, reference patterns for image pattern matching, etc. These workpiece characteristics are used to control the brazing operation and permit the system to be used on a variety of different types of workpieces without any system reconfiguration. A user interface  206  allows a user to access the controller  200 . User interface  206  may include typically computer peripherals such as a keyboard, mouse, display, etc. 
         [0027]      FIG. 6  is a flowchart of a process for brazing workpieces in an exemplary embodiment. The example of  FIG. 6  is directed to brazing, but other joining techniques may be used. The process begins at  300  where the workpiece to be brazed is identified. This may be performed by reading an identifier on the workpiece (e.g., a barcode) or by entering a workpiece identifier through user interface  206 . 
         [0028]    At  302 , brazing parameters for the workpiece are obtained from database  204  based on the workpiece identifier. The brazing parameters include a number of items such as the initial position of brazing unit  14  relative to a first tube  20  in the x, y and z directions, rotation of the brazing unit  14  about the y and z axes, a desired brazing temperature, initial heat output for brazing unit  14 , tube-to-tube spacing on the workpiece  16 , reference measurements to be compared to measurements from the vision system, and reference patterns for matching patterns from camera images. The heat output of brazing unit  14  may be controlled by the air-fuel mixture to a torch or electrical power to an induction brazer. This allows the heat output of brazing unit  14  to be adjusted to lower levels for thinner parts, for example, to reduce likelihood of overheating the workpiece. 
         [0029]    At  304  the brazing unit  14  is initialized by controller  200  in response to the retrieved brazing parameters. The brazing unit  14  may be positioned by controller  200  sending commands to actuators  202  and sending control signals to the brazing unit  14  to set a heat output. 
         [0030]    At  306 , the workpiece  16  is moved into an initial position. The workpiece  16  is transported by a carriage or rail system and moved to an initial position in which the first tube  20  is positioned proximate to brazing unit  14 . 
         [0031]    At  308 , the brazing unit is positioned relative to the first tube  20 . As noted above, this is performed by controller  200  receiving images and/or features from the vision system  30  and processing the images and/or features to detect a location of the tube  20 . The brazing unit  14  is positioned by controller  200  sending commands to actuators  202  until the desired location of the brazing unit  14  relative to tube  20  is reached. Using vision system(s)  30  to adjust the location of the brazing unit  14  relative to workpiece  16  allows the system to find the tube location independently of variability in the workpiece  16 . 
         [0032]    At  310 , the brazing process in begun by controller  200  commanding the brazing unit  14  to apply heat to the tube  20 . At  312 , controller  200  monitors the temperature of workpiece  16  to determine if the temperature at the tube  20  meets the desired brazing temperature obtained from the brazing parameters. The desired brazing temperature may be indicated as a single temperature or as a range of temperature values. 
         [0033]    At  314 , controller  200  determines if the temperature at tube  20  has reached the desired brazing temperature. This may be done in a variety of ways, depending on the type of temperature sensing performed. If a contact temperature probe or a non-contact temperature probe is used, then controller  200  receives the temperature signal from the probe and determines whether the measured temperature meets the desired brazing temperature. If the temperature probe  46  is a photosensor, then controller  200  determines if the color of the flame applied by brazing unit  14  has changed to indicate that the proper brazing temperature has been reached. This technique is described in published PCT application, PCT/US11/057192. 
         [0034]    If at  314 , the desired brazing temperature is not met, then flow proceeds to  316  where the heat applied at tube  20  may be adjusted by controller  200 . This may entail moving the brazing unit  14  closer to tube  20  (if heat is too low) or farther from tube  20  (if heat is too high) along the x axis. This may also entail moving the brazing unit  14  along the z axis or the y axis to alter temperature at tube  20 . Adjusting the heat at tube  20  may also involve increasing heat output of the brazing unit  14  by adjusting a fuel-air mix (for a torch) or adjusting electrical power applied (for an induction brazer). After the adjustments at  316 , the process feeds back to  314  to further evaluate the brazing temperature. 
         [0035]    Once the desired brazing temperature has been reached at  314 , flow proceeds to  318  where controller  200  determines if the final tube  20  of the workpiece  16  has been brazed. If so, flow proceeds to  320  where the process ends. If the final tube  20  has not been brazed at  318 , flow proceeds to  322  where the brazing unit  14  is positioned at the next tube  20 . This may be performed by controller  200  moving the brazing unit  14  along the y axis an amount determined by the tube-to-tube spacing on the workpiece  16 . Alternatively, the carriage handling workpiece  16  is moved along the y axis an amount determined by the tube-to-tube spacing on the workpiece  16 . Flow then proceeds to  308  where the vision system  30  provide images and/or features to controller  200  to adjust the position of the brazing unit  14  relative to the next tube  20 . Again, by using image analysis to position the brazing unit  14 , variability in workpiece  16  does not interfere with locating the brazing unit  14  at the proper location. 
         [0036]    The continuous positioning and heat monitoring allows the brazing system  10  to react to changing conditions. Brazing system  10  provides consistent quality with repeatable heat input control, brazer alignment and brazer displacement. The system  10  automatically aligns brazing unit  14  with the joints to be brazed, and adjusts to manufacturing tolerances stack-ups. 
         [0037]    While the invention has been described in detail in connection with only a limited number of embodiments, it should be readily understood that the invention is not limited to such disclosed embodiments. Rather, the invention can be modified to incorporate any number of variations, alterations, substitutions or equivalent arrangements not heretofore described, but which are commensurate with the spirit and scope of the invention. Additionally, while various embodiments of the invention have been described, it is to be understood that aspects of the invention may include only some of the described embodiments. Accordingly, the invention is not to be seen as limited by the foregoing description, but is only limited by the scope of the appended claims.