Abstract:
A method for monitoring quality of a weld ( 56 ) being formed between first and second pieces of material ( 10  and  12 ) includes the steps of obtaining a thermal image of the weld ( 56 ) being formed by collecting infrared radiation passing through the second piece of material ( 12 ); and analyzing the obtained thermal image for characteristics indicative of a properly formed weld ( 56 ).

Description:
TECHNICAL FIELD  
       [0001]     The present invention relates to a method for monitoring the quality of a weld being formed between two pieces of material. More particularly, the present invention relates to a method for monitoring the quality of a transmissive laser weld being formed between two pieces of plastic material.  
       BACKGROUND OF THE INVENTION  
       [0002]     Transmissive laser welding is used for bonding together first and second pieces of plastic material. The weld in transmissive laser welding is formed using a beam of electromagnetic energy that is emitted from a laser. One of the two pieces of plastic material, for example, the first piece of plastic material, is transmissive to the beam of electromagnetic energy, i.e., the beam passes through the first piece of plastic material. The other of the two pieces of plastic material, for example, the second piece of plastic material, absorbs the electromagnetic energy of the beam.  
         [0003]     During the transmissive laser welding process, the beam of electromagnetic energy emitted from the laser passes through the first piece of plastic material and is absorbed by the second piece of plastic material. The absorbed electromagnetic energy heats a portion of the second piece of plastic material and the heated portion of the second piece of plastic material begins to melt. Also, heat of the heated portion of the second piece of plastic material is conducted to an adjacent portion of the first piece of plastic material. The adjacent portion of the first piece of plastic material also begins to melt. The melted portions of the first and second pieces of plastic material collectively form a weld pool in which the melted portions of the first and second pieces of plastic material mix together. When the electromagnetic energy is removed, the weld pool begins to cool and hardens into a weld that bonds together the first and second pieces of plastic material.  
         [0004]     Many problems can occur during the formation of a transmissive laser weld. For example, when a portion of the weld pool is not heated to a proper temperature, i.e., the temperature of the portion is too low, the width of the weld pool at that portion may be too narrow and a weak point or even a void in the weld may result. On the other hand, when a portion of the weld pool is overheated, i.e., the temperature of the portion is too high, gas voids may form in the weld pool. As the weld pool cools, the gas voids may result in weak points or voids in the weld.  
         [0005]     Thus, it is desirable to monitor the formation of the weld during the transmissive laser welding process to ensure that a quality weld is being formed between the first and second pieces of plastic material. Currently, the formation of a transmissive laser weld is monitored using an infrared pyrometer. The infrared pyrometer monitors the temperature of the weld pool at a single point that is located immediately behind the beam of electromagnetic energy. The infrared pyrometer is turned off when the laser is turned off.  
         [0006]     One drawback to the use of the infrared pyrometer is that the infrared pyrometer is limited to monitoring the temperature of a single point of the weld pool at a time. Thus, the infrared pyrometer is not capable of sensing temperature changes of the weld pool that occur over time, for example, during the period of time that another portion of the weld pool is being heated.  
       SUMMARY OF THE INVENTION  
       [0007]     The present invention relates to a method for monitoring quality of a weld being formed between first and second pieces of material. The method comprises the steps of: obtaining a thermal image of the weld being formed by collecting infrared radiation passing through the second piece of material; and analyzing the obtained thermal image for characteristics indicative of a properly formed weld.  
         [0008]     According to another aspect, the present invention relates to a method for monitoring quality of a weld being formed between first and second pieces of material. The method comprises the steps of: determining a range of wavelengths of infrared radiation that will pass through the second piece of material; positioning an infrared detector that is configured to detect infrared radiation within the determined range of wavelengths on a side of the second piece of material opposite the first piece of material; obtaining a thermal image of the weld being formed between the first and second pieces of material by collecting infrared radiation within the determined range of wavelengths; and analyzing the obtained thermal image for characteristics indicative of a properly formed weld. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]     The foregoing and other features and advantages of the present invention will become apparent to those skilled in the art to which the present invention relates upon reading the following description with reference to the accompanying drawings, in which:  
         [0010]      FIG. 1  schematically illustrates first and second pieces of plastic material that are being welded together and an apparatus for performing the method of the present invention;  
         [0011]      FIG. 2  is a top view illustrating the first and second pieces of plastic material during formation of the weld; and  
         [0012]      FIGS. 3A and 3B  collectively form a flow diagram that illustrates an exemplary control process that may be performed in accordance with the method of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0013]      FIG. 1  schematically illustrates first and second pieces of plastic material  10  and  12 , respectively. The first piece of plastic material  10  is generally planar and includes upper and lower surfaces  14  and  16 , respectively. Similarly, the second piece of plastic material  12  is generally planar and includes upper and lower surfaces  18  and  20 , respectively.  FIG. 1  illustrates the second piece of plastic material  12  overlaying the first piece of plastic material  10 . Two clamping devices  22  are illustrated in  FIG. 1  holding the first and second pieces of plastic material together. When the first and second pieces of plastic material  10  and  12  are held together, the upper surface  14  of the first piece of plastic material  10  abuts the lower surface  20  of the second piece of plastic material.  
         [0014]     The first and second pieces of plastic material  10  and  12  have different material properties. Particularly, the second piece of plastic material  12  is transmissive to a range of infrared wavelengths, while the first piece of plastic material  10  absorbs the same range of infrared wavelengths. The second piece of plastic material  12  may be transparent to the range of infrared wavelengths or may be translucent to the range of infrared wavelengths. This range of infrared wavelengths is determined prior to beginning the transmissive laser welding process.  
         [0015]      FIG. 1  also schematically illustrates an apparatus  30  for performing the method of the present invention. The apparatus  30  includes a welding portion  32  and a monitoring portion  34 . The welding portion  32  includes devices for forming a transmissive laser weld to bond together the first and second pieces of plastic material  10  and  12 . These devices include a laser  40 , mirror  42 , a mirror adjustment device  44 , and a weld controller  46 .  
         [0016]     The laser  40  of the welding portion  32  of the apparatus  30  is actuatable to provide a beam  50  of electromagnetic energy having a wavelength that is within the range of wavelengths determined to be transmissive to the second piece of plastic material  12  and determined to be absorptive to the first piece of plastic material  10 . As illustrated in  FIG. 1 , the laser  40  is fixed in a located above the first and second pieces of plastic material  10  and  12  and directs the beam  50  of electromagnetic energy in a direction generally parallel to the upper surfaces  14  and  18  of the first and second pieces of plastic material  10  and  12 .  
         [0017]     The mirror  42  of the welding portion  32  of the apparatus  30  includes a reflective surface  52  for reflecting the beam  50  of electromagnetic energy from the laser  40  toward the first and second pieces of plastic material  10  and  12 . The mirror adjustment device  44  is coupled to the mirror  42  and controls the positioning of the mirror  42 .  FIG. 1  shows an actuator arm  54  of the mirror adjustment device  44  being fixedly attached to the mirror  42 . The mirror adjustment device  44  is actuatable for changing the position of the mirror  42  and thus, changing the angle of the reflective surface  52  relative to the laser  40 . Changes in position of the mirror  42  result in a movement of the beam  50  of electromagnetic energy relative to the first and second pieces of plastic material  10  and  12 .  
         [0018]     The weld controller  46  of the welding portion  32  of the apparatus  30  is preferably a microcomputer. Alternatively, the weld controller  46  may be formed from discrete circuitry, an application-specific-integrated-circuit (“ASIC”), or any other type of control circuitry. The weld controller  46  is operatively coupled to the laser  40  and to the mirror adjustment device  44 . The weld controller  46  controls actuation of the laser  40  and the mirror adjustment device  44  to form a weld of a predetermined shape between the first and second pieces of plastic material  10  and  12 .  
         [0019]     For example, with reference to  FIG. 2 , the weld controller  46  may control actuation of the laser  40  and the mirror adjustment device  44  to form a generally square-shaped weld, indicated at  56 , near a periphery  58  of the second piece of plastic material  12 . Weld  56  may be used, for example, for welding a cover piece formed from the second piece of plastic material  12  over an opening in a housing formed from the first piece of plastic material  10 . In such an example, the opening in the housing would be located radially inside the weld  56  and the weld  56  would close the opening. The opening is not illustrated in  FIGS. 1 and 2 .  
         [0020]     During the transmissive laser welding process, the weld controller  46  actuates the laser  40  to provide the beam  50  of electromagnetic energy. The weld controller  46  also controls the mirror adjustment device  44  for moving the mirror  42  so as to direction the beam  50  of electromagnetic energy over the desired weld path, e.g., the square-shaped path of weld  56  in  FIG. 2 . Since the beam  50  of electromagnetic energy has a wavelength in the range that is transmissive to the second piece of plastic material  12 , the beam  50  of electromagnetic energy, after being reflected by the reflective surface  52  of the mirror  42 , passes through the second piece of plastic material  12 . Since the first piece of plastic material  10  is absorptive to the wavelength of the beam  50  of electromagnetic energy, the beam  50  of electromagnetic energy heats the first piece of plastic material  10  in a location adjacent the upper surface  14 . The heat resulting from absorption of the beam  50  of electromagnetic energy begins to melt a portion the first piece of plastic material  10  in a location adjacent the upper surface  14 . The heat is also conducted to the lower surface  20  of the second piece of plastic material  12  and begins to melt an adjacent portion of the second piece of plastic material  12 .  
         [0021]     Melted portions of the first and second pieces of plastic material  10  and  12  mix together to forms a weld pool  60  in a location between the first and second pieces of plastic material  10  and  12 . As is discussed below, cooling of the weld pool  60  forms the weld  56  that bonds together the first and second pieces of plastic material  10  and  12 . Thus, with reference to  FIG. 2 , the weld pool  60  has the identical, generally square-shape as the weld  56 .  
         [0022]     When forming weld pool  60 , the weld controller  46  may control the mirror adjustment surface  44  so that the mirror  42  directs the beam  50  of electromagnetic energy over the path of weld pool  60  multiple times. For example, with reference to  FIG. 2 , the beam  50  of electromagnetic energy may start in the upper, right corner of the weld pool  60 , as viewed in  FIG. 2 , and may be moved around the generally square-shaped path in a clockwise direction multiple times. By moving the beam  50  of electromagnetic energy around the path of weld pool  60  multiple times, a more uniform heating of the weld pool  60  occurs and the entire weld  56  may be formed simultaneously. By forming the entire weld  56  simultaneously, the occurrence of stress points within the weld  56  is reduced as compared to when the weld  56  is formed a portion at a time.  
         [0023]     The monitoring portion  34  of the apparatus  30  includes an infrared camera  70 , an optical filter  72 , an image controller  74 , and may optionally include an alarm device  76 . The infrared camera  70  is positioned above the second piece of plastic material  12 , as viewed in  FIG. 1 , on an side of the second piece of plastic material  12  opposite the first piece of plastic material  10 . Preferably, the infrared camera  70  is centrally located above an area, indicated generally by  80 , in which the weld  56  is to be formed to bond together the first and second pieces of material  10  and  12 .  
         [0024]     The infrared camera  70  has a field of view  84 , shown in  FIG. 1  as the area between dashed lines  82 . The field of view  84  includes the area  80  in which the weld  56  is to be formed. The infrared camera  70  is configured for obtaining infrared images of the field of view  84  during the process of forming the weld  56 . Thus, the infrared camera  70  obtains infrared images of the weld pool  60 , in its entirety, during the process of forming the weld  56 .  
         [0025]     The infrared camera  70  may be a complimentary metal-oxide semiconductor (“CMOS”) camera, a charge-coupled device (“CCD”) camera, or any other type of infrared camera that is capable of collecting infrared radiation having a wavelength that is transmissive to the second piece of plastic material and is also capable of obtaining infrared images of the entire field of view  84 . The infrared camera  70  is designed for imaging light in the range of infrared wavelengths that is transmissive to the second piece of plastic material  12  and is absorbed by the first piece of plastic material  10 . Preferably, the infrared camera  70  obtains an infrared image of the viewable field  84  at a wavelength that is different than the wavelength of the beam  50  of electromagnetic energy that is emitted from the laser  40 . As a result, the beam  50  of electromagnetic energy is not seen in the infrared images obtained by the infrared camera  70 .  
         [0026]     The optical filter  72  is associated with the infrared camera  70  and is located in the field of view  84  between the infrared camera  70  and the first and second pieces of plastic material  10  and  12 . The optical filter  72  enables a range of wavelengths of light to pass through the filter and blocks wavelengths of light that are outside the range. Preferably, the range of infrared light that may pass through the optical filter  72  includes the wavelength of light obtained by the infrared camera  70  and does not include the wavelength of the beam  50  of electrical energy. For example, the beam  50  of electromagnetic energy may have a wavelength of 1062 nanometers, the infrared camera  70  may obtain images at 900 nanometers, and the optical filter  72  may block wavelengths of light outside of an 820 to 1000 nanometer range.  
         [0027]     The image controller  74  of the monitoring portion  34  of the apparatus  30  is preferably a microcomputer. Alternatively, the image controller  74  may be formed from discrete circuitry, an application-specific-integrated-circuit (“ASIC”), or any other type of control circuitry. The image controller  74  is operatively coupled to the infrared camera  70  and images obtained by the infrared camera  70  are provided to the image controller  74 . The image controller  74  is also operatively coupled to the weld controller  46  and communicates with the weld controller  46 . As an alternative to the apparatus  30  including separate and distinct weld and image controllers  46  and  74 , a single controller may form both the weld controller  46  and the image controller  74 .  
         [0028]     The image controller  74  includes an internal timer. Alternatively, a separate timer (not shown) may be operatively connected to the image controller  74 . The image controller  74  controls the starting, stopping, and resetting of the timer. After being started, the timer provides the image controller  74  with signals that indicate the amount of time that has elapsed since the start of the timer.  
         [0029]     The image controller  74  controls the infrared camera  70 . The image controller  74  turns on the infrared camera  70  when the welding process begins and turns the infrared camera  70  off after the image controller  74  determines that the weld  56  has been formed. While on, the infrared camera  70  continuously obtains images and provides the images to the image controller  74 . The images provided from the infrared camera  70  to the image controller  74  may be color images or may be grayscale images. Preferably, the infrared camera  70  provides the image controller  74  with color images.  
         [0030]     The image controller  74  performs a pattern recognition algorithm on the received images. The image controller  74  analyzes various characteristics of the received images to determine whether the weld  56  is being formed properly. The various characteristics are discussed in detail below. A properly formed weld is a weld in which each analyzed characteristic conforms to associated criteria. For example, when the analyzed characteristic is the temperature of the weld pool  60 , the associated criteria include threshold temperature ranges that vary over time. The threshold temperature ranges increase over time as the beam  50  of electromagnetic energy heats the first and second pieces of plastic material  10  and  12  to form the weld pool  60 , then the threshold temperature ranges decreases over time after the beam  50  of electromagnetic energy is removed and the weld pool  60  begins to cool and harden into the weld  56 .  
         [0031]     In addition to analyzing the various characteristics of the received images, the image controller  74  also provides feedback signals to the weld controller  46 . The feedback signals indicate to the weld controller  46  that an analyzed characteristic is out of conformance with its associated criteria, a location in the weld pool  60  of the nonconforming characteristic, and a description of the nonconforming characteristic, e.g., low temperature.  
         [0032]     As stated above, one characteristic of the obtained images that the image controller  74  analyzes is the temperature of the weld pool  60 . When the infrared camera  70  provides color images to the image controller  74 , the image controller  74  analyzes the color of each pixel of the received image to determine the temperature associated with the pixel. The image controller  74  then compares the determined temperatures of the pixels associated with the weld pool  60  to a threshold temperature range to determine whether the determined temperatures are within the threshold temperature range. Since the obtained image includes the weld pool  60 , in its entirety, the image controller  74  monitors the temperature of the entire weld pool  60 . This enables the image controller  74  to determine whether any portion of the weld pool  60  is at a temperature outside of the threshold temperature range, whether too low or too high. When the image controller  74  determines that a portion of the weld pool  60  is outside of the threshold temperature range, the image controller  74  provides a feedback signal to the weld controller  46 .  
         [0033]     The weld controller  46  is responsive to the feedback signal from the image controller  74  for adjusting the welding process. For example, when the feedback signal from the image controller  74  indicates that the temperature of the nonconforming portion of the weld pool  60  is too low, the weld controller  46  may increase the time that the beam  50  of electromagnetic energy is concentrated on the nonconforming portion of the weld pool  60 . Similarly, when the feedback signal from the image controller  74  indicates that the temperature of the nonconforming portion of the weld pool  60  is too high, the weld controller  46  may reduce the time that the beam  50  of electromagnetic energy is concentrated on the nonconforming portion of the weld pool  60 .  
         [0034]     Another characteristic of the received images that the image controller  74  analyzes is the width of the weld pool  60 . The image controller  74  determines the width of the weld pool  60  and compares the determined width to a threshold width range to determine whether the width of the weld pool  60  is within the threshold width range. Since each received image includes the entire weld pool  60 , the image controller  74  analyzes the width of the weld pool  60  along the entire path of the weld pool. Again, the image controller  74  provides a feedback signal to the weld controller  74  when a determination is made that the width of a portion of the weld pool  60  is outside of the threshold width range. The weld controller  46  is responsive to the feedback signal to adjust the welding process to bring the width of the portion back into the threshold width range. Since the width of the weld pool  60  varies over time as portions of the first and second pieces of plastic material  10  and  12  melt, the threshold width range may vary over time. Alternatively, the image controller  74  may only monitor the width of the weld pool  60  after a determination has been made that the weld pool  60  has reached a predetermined temperature. When monitoring the width of the weld pool  60  only after the determination that the weld pool  60  has reached the predetermined temperature, only one threshold width range is necessary.  
         [0035]     The image controller  74  also analyzes the received images for any voids that may be located in the weld pool  60 . In response to finding a void, the image controller  74  provides a feedback signal to the weld controller  46  so that the weld controller may alter the welding process to fill the void.  
         [0036]     In addition to providing feedback signals to the weld controller  46 , the image controller  74  also provides alarm signals to the alarm device  76  of the monitoring portion  34  of the apparatus  30 . The alarm device  76  is operable for providing an indication to an operator of the apparatus  30  that the image controller  74  determined the existence of a nonconforming characteristic. The alarm device  76  may provide any one or more of visual, audio, and tactile signals to the operator.  
         [0037]      FIGS. 3A and 3B  collectively form a flow diagram that illustrates an exemplary control process  300  that may be performed by the image controller  74  in accordance with the method of the present invention. The process  300  begins at step  302  in response the apparatus  30  being powered on. From step  302 , the process  300  proceeds to step  304  in which a determination is made as to whether the welding process is beginning. For example, the image controller  74  may receive a signal from the weld controller  46  indicating that the welding process is beginning. When the determination at step  304  is negative, step  304  is repeated. When the determination at step  304  is affirmative, the process  300  proceeds to step  306 .  
         [0038]     At step  306 , the image controller  74  actuates the infrared camera  70  to begin obtaining images. At step  308 , the image controller  74  starts its internal timer and at step  310 , the image controller  74  monitors the infrared camera  70  for an image. After the image controller  74  receives an image from the infrared camera  70 , the process  300  proceeds to step  312  in which the image controller  74  monitors the timer to determine the time at which the infrared camera  70  obtained the image.  
         [0039]     From step  312 , the process  300  proceeds to step  314 . At step  314 , the image controller  74  analyzes the received image to determine the temperature of each pixel associated with the weld pool  60 . Thus, in performing step  314 , the image controller  74  analyzes the entire path of the weld pool  60 . As stated above, when the image received by the image controller  74  is a color image, the color of each pixel of the received image indicates the temperature associated with that pixel. When the image received by the image controller  74  is a grayscale image, the intensity or brightness of the pixel indicates the temperature associated with that pixel. After the temperature of each pixel associated with the weld pool  60  has been determined, the process  300  proceeds to step  316  in which the determined temperatures are compared to a threshold temperature range for the determined image time. Since the temperature of the weld pool  60  gradually increases as the time that the beam  50  of electromagnetic energy is applied increases, the threshold temperature range varies as a function of time, as was discussed above.  
         [0040]     At step  318 , a determination is made as to whether any portion of the weld pool  60  has a temperature that is outside of the threshold temperature range for the determined image time. When the determination at step  318  is affirmative and a portion of the weld pool  60  is outside of the threshold temperature range, the process  300  proceeds to step  320  in which a feedback signal is provided to the weld controller  46  and an alarm signal is provided to the alarm device  76 . As discussed above, the weld controller  46  may be responsive to the feedback signal for adjusting the weld process to correct the nonconforming portion of the weld pool  60 . When the determination at step  318  is negative, the process  300  proceeds to step  322 .  
         [0041]     At step  322 , a determination is made as to whether any voids are present in the weld pool  60 . To determine whether any voids are present, the image controller  74  compares the pixels of the received image that represent the weld pool  60  to surrounding pixels of the weld pool to determine if any unusual differences in temperature exist. Although each pixel may be within the threshold temperature range for the determined image time, a large temperature difference between adjacent pixels may indicate the occurrence of a void, for example, a gas bubble, in the weld pool  60 . When the determination at step  322  is affirmative, the process  300  proceeds to step  318  in which a feedback signal is provided to the weld controller  46  and an alarm signal is provided to the alarm device  76 . When the determination at step  322  is negative, the process  300  proceeds to step  324  ( FIG. 3B ).  
         [0042]     At step  324 , a determination is made as to whether the temperature of the weld pool  60  has previously exceeded a predetermined temperature. The predetermined temperature is a temperature at which the width of the weld pool  60  should be fully formed, i.e., a temperature at which the width of the weld pool  60  should be within its desired range. When the determination at step  324  is negative, the process  300  proceeds to step  326 . At step  326 , a determination is made as to whether the temperature of the weld pool  60  is currently above the predetermined temperature. When the determination at step  326  is negative, the process  300  returns to step  310 . When the determination at step  326  is affirmative, the process  300  proceeds to step  328 .  
         [0043]     At step  328 , the image controller  74  determines the width of the weld pool  60 , in its entirety, i.e., along the entire path of the weld pool  60 . At step  330 , the determined width of the weld pool  60  is compared to a threshold width range. At step  332 , a determination is made as to whether the determined width of the weld pool  60  at any location along the path of the weld pool is outside of the threshold width range. When the determination at step  332  is affirmative and a portion of the weld pool  60  is determined to have a width that is outside of the threshold width range, the process  300  proceeds to step  320  in which a feedback signal is provided to the weld controller  46  and an alarm signal is provided to the alarm device  76 . When the determination at step  332  is negative, the process  300  returns to step  310 .  
         [0044]     Returning to step  324 , when a determination at step  324  is affirmative and the weld pool  60  has previously exceeded the predetermined temperature, the process  300  proceeds to step  334 . At step  334 , a determination is made as to whether the temperature of the weld pool  60  indicates that the weld  56  has been formed. More specifically, at step  334 , a determination is made as to whether the weld pool  60  temperature is below a temperature at which the weld pool  60  hardens to form the weld  56 . When the determination at step  334  is negative, the process  300  returns to step  310 . When the determination at step  334  is affirmative, the process  300  proceeds to step  336  and the process  300  ends.  
         [0045]     From the above description of the invention, those skilled in the art will perceive improvements, changes and modifications. Such improvements, changes and modifications within the skill of the art are intended to be covered by the appended claims.