Abstract:
A method for optimizing a schedule for impulse welding including generating a weld lobe using a preexisting welding schedule, the schedule prescribing a current, a weld time, and a pressure; identifying an operating window on the weld lobe; analyzing the operating window, the analysis including determining a maximum time range of the operating window, and determining a maximum current range of the operating window; until (i) the determined maximum time range of the operating window is greater than a predetermined percentage of the prescribed weld time of the schedule and (ii) the determined maximum current range is greater than the prescribed current of the schedule, creating additional weld lobes by varying the pressure and repeating the identifying an operating window and the analyzing the operating window steps; and selecting, within the operating window satisfying conditions (i) and (ii), a second schedule including a second weld time, a second current, and a second pressure.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates generally to resistance welding. More particularly, the present invention is directed to generating spot weld schedules to be implemented by weld controllers. 
         [0003]    2. Description of the Related Art 
         [0004]    Automobile body panels are spot-welded using a welding apparatus including a robot and a spot welding servo gun coupled to a wrist portion of the robot. The spot welding servo gun has a pair of electrode tips for pressing a work piece therebetween. The electrode tips are electrically connected to a welding transformer. The welding transformer is electrically connected to a welding power source via a controller having a timer function. At least one tip of the spot welding servo gun is movably installed in a servo motor which performs position control or pressure control of the electrode tip. 
         [0005]    Many spot welds experience expulsion. That is, molten metal is ejected from the weld decreasing the cleanliness of the mass produced automobile body and compromising the weld integrity. There are five primary causes of expulsion: (i) gun alignment, (ii) condition of the tip of the electrodes, (iii) weld schedule, (iv) misfit parts, and (v) adhesives between the parts. 
       SUMMARY OF THE INVENTION 
       [0006]    It is an object of the present invention to reduce the level and frequency of expulsion during the spot welding process. To that end, the inventors studied the interaction of the five causes of expulsion and identified the most important—weld schedules were giving off too much heat and misfit parts. 
         [0007]    To reduce the frequency of expulsions, the present invention optimizes existing welding schedules by analyzing an operating window of a weld lobe and uses impulse welding to address, among other things, the misfit parts issue. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]    A more complete appreciation of the invention and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein: 
           [0009]      FIG. 1  is a flowchart illustrating a process for implementing a method according to the present invention; 
           [0010]      FIG. 2  illustrates a setup for creating a weld lobe according to the present invention; 
           [0011]      FIG. 3  illustrates a weld lobe created using the setup illustrated in  FIG. 2 ; 
           [0012]      FIG. 4  illustrates a method for identifying on the weld lobe of  FIG. 3  a weld time constraint; 
           [0013]      FIG. 5  illustrates an operating window not meeting predetermined conditions according to an embodiment of the present invention; 
           [0014]      FIG. 6  illustrates an operating window meeting predetermined conditions according to an embodiment of the present invention; 
           [0015]      FIG. 7  illustrates identifying a preferred location within the operating window; 
           [0016]      FIG. 8  illustrates identifying a base schedule within the preferred location of the operating window; 
           [0017]      FIG. 9  illustrates a method for testing impulse welding according to an embodiment of the present invention; 
           [0018]      FIG. 10  illustrates a 3D weld lobe analysis; and 
           [0019]      FIG. 11  illustrates a method for identifying an operating window during a part fit study. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0020]    The inventors conducted a Taguchi experiment measuring the effect and interaction of weld time, uses of impulses, part fit, pressure, pulsation, and current. The five factor experiment required 2 5  permutations. Based on the Taguchi experiment, the inventors concluded that the interaction of current and time were dominant over the other factors. The interaction between pulse and time to address misfit parts, while not as significant as the interaction between current and time, was also found to be significant. Thus, a parameter identified by the inventors for optimization was the number of pulses in the weld time (while operating within the weld lobe). The inventors concluded from the Taguchi experiment that: (i) impulse welding should be implemented due to its effectiveness on expulsion; (ii) the top two traceable factors that affect nugget size and expulsion are current and time. Based on these conclusions, among other things, the inventors developed a methodology for spatter-less welding including optimizing a welding schedule and using impulse welding. 
         [0021]    Referring now to the drawings, wherein like reference numerals designate identical or corresponding parts throughout the several views. In developing a methodology for generating spatter-less welds which would be effective where the parts being welded may have a poor fit, the inventors created a weld lobe using a preexisting weld schedule on a perfect part fit and a weld lobe using the preexisting schedule on a poor part fit. The part fits were simulated using coupons. As can be seen in  FIG. 11 , weld lobes for the perfect part fit and the poor part fit were generated using a fixed pressure of 2940 N and by varying the current and weld time. Expulsions were observed visually during the welding process. In order to identify undersized welds and good welds, the coupons were sent to a lab for a peel test. 
         [0022]    The overlapping window illustrated in  FIG. 11  where good welds were created on both the perfect part fit and the poor fit had a small tolerance for changes in current. Because in most real world applications the fit of the two parts being welded together can vary frequently (e.g., on a day to day basis) from poor to perfect, it would be difficult to operate consistently in the overlapping window identified in  FIG. 11 . Consequently, in order to create a better fit between the parts during the welding process and in accordance with an embodiment of the present invention, multiple impulse welding is used in lieu of conventional single pulse welding. 
         [0023]    With conventional single pulse welding, heat greatly dissipates in the metal. In contrast, with impulse welding, heat is focused on the weld reducing metal temperature and expulsion. Heat is a function of the current squared, the weld time, and the pressure. 
         [0000]      Q α (I 2 t)/P 
         [0000]    As reflected in  FIG. 10 , the inventors discovered that current has a larger effect on expulsion than weld time because the heat generated is a function of the current squared. Further, the effect of pressure diminishes as it is increased. Thus, higher pressure welds are recommended. Welds with low current have no expulsion even if welded for a long period of time. However, because in most real world applications the weld time cannot be increased to preserve manufacturing efficiencies, the time parameter cannot be optimized. 
         [0024]    A preferred methodology for creating an optimized weld schedule is next explained in conjunction with  FIG. 1 . Although specific values for current, pressure, and time are identified, the scope of the invention is not intended to be limited to such an implementation. 
         [0025]    In step  101 , a weld lobe for an existing weld schedule is generated. Welding schedules typically include a prescribed current, time, and pressure for the weld. According to an embodiment of the present invention, step  101  includes the following sub-steps. First, taking two metal coupons  201  of the appropriate type and thickness associated with the existing weld schedule and clamping them down to back blocks  203  and  205  as shown in  FIG. 2 . The coupons may have to be turned slightly to ensure that one end of the coupons  201  touches a back block and the other end of the coupons  201  does not. The clamps should not touch the fixture where it is not insulated or the process will ground out. 
         [0026]    To generate the data to create the weld lobe, for a first weld, apply the suggested pressure of the existing schedule, apply half the suggested weld time found in the existing schedule, apply 2 kA plus the prescribed current found in the existing schedule, and record expulsion whether an expulsion occurs (Yes/No). Increments of current smaller or larger than 2 kA can be added to the prescribed current dependent on the number of data points wanted. 
         [0027]    For the next few welds, hold the current and the pressure constant, vary the prescribed weld time in predetermined increments (e.g., prescribed total weld time/6), and increase the weld time by the calculated interval for each weld until either expulsion occurs or a time cap is reached. According to an embodiment of the invention, the time cap can be established at for example the prescribed weld time plus (6 to 10) predetermined increments. Record expulsion data for each weld. In a preferred embodiment, decrease the current (originally the prescribed current plus 2 kA) by 1 kA, reduce the weld time back to half of the prescribed weld time and repeat the process described in this paragraph until the current being applied is the prescribed current minus 2 kA. 
         [0028]    After all of the welds have been created, perform peel tests on the welds to check weld quality. The quality of the welds should be recorded as pass or fail. A plot the current versus time data should be generated, using a triangle, for example, to represent expulsions, circles, for example, to represent good welds without expulsion, and an “x” or a square, for example, to represent bad welds. See  FIG. 3 . 
         [0029]    In step  103 , draw the prescribed weld time on the generated weld lobe. See  FIG. 4 . 
         [0030]    In step  105 , an operating window is identified. As discussed above, the inventors performed a part fit study to investigate the effect of poor part fit on spot welds, and discovered that part fit had a significant impact on the operating window of a spot weld. 
         [0031]    According to an embodiment of the present invention, the operating window only includes the region of good welds to the left of the weld time constraint. Measure the current and time range for the operating window. According to a preferred embodiment, the maximum current range should be greater than 2 kA and the maximum time range should be greater than 20 percent of the suggested weld time. If these constraints are met, then go to step  109 . Otherwise, go to step  107 .  FIG. 5  illustrates an operating window which does not meet the maximum current range and maximum time range constraints 
         [0032]    In step  107 , if the pressure is being varied for a first time, then increase the pressure by 200 N or another predetermined increment, recreate the weld lobe, and repeat steps  103  and  105 . If the pressure is not being varied for the first time, then judge whether the window improved (i.e., increased in size) or worsened (i.e., decreased in size) based on the maximum current range and maximum time range constraints disclosed above and vary pressure in the up or down in intervals of 200 N. Recreate the weld lobe, and repeat steps  103  and  105  until an operating window is generated meeting the maximum current range and maximum time range constraints. When this is accomplished, proceed to step  109 . 
         [0033]    If, after several pressure changes, there is indication that the operating window will not meet the maximum current range and maximum time range constraints, then select the pressure examined with the biggest operating window, and continue to step  111 .  FIG. 6  illustrates an operating window which does meet the maximum current range and maximum time range constraints. 
         [0034]    The inventors discovered that in a preferred but non-limiting embodiment that an ideal location for an optimized weld schedule is a region on the weld lobe that meets the following conditions: 
         [0035]    a. 0.5 kA away from any expulsion; 
         [0036]    b. 1.5 kA away from any bad weld; 
         [0037]    c. within the prescribed weld time constraint of the original schedule; and 
         [0038]    d. 20 percent of the prescribed weld time away from bad welds. (Example: the suggested weld time is 20 cycles, then the ideal location should be at least 4 cycles away from bad welds). 
       See FIG. 7. 
       [0039]    If an operating window satisfying the maximum current range and maximum time range constraints is identified in step  107  and an ideal window is identified within the window, then in step  111  select a schedule that is at the very right side of the ideal location, because it is the furthest from bad welds. The selected schedule is the optimum schedule. See  FIG. 8 . If an operating window satisfying the maximum current range and maximum time range constraints is not identified in step  107 , then in step  111  select a schedule from the very right side of the largest operating window not satisfying the constraints. The selected schedule is the optimum schedule. 
         [0040]    In step  113 , impulse welding is tested in order to determine a number of cool cycles to include with the pulses during the optimized weld schedule. According to an embodiment of the present invention, in order to test the impulse welding, a part gap is simulated as shown in  FIG. 9 . 
         [0041]    When implementing the impulse welding, the total weld time should not increase for the reasons discussed above. Therefore, if cool time is added, then actual weld time must be reduced. See the following formula for detail: 
         [0000]      Total Cool Time=(Pulse#−1)*Cool Time 
         [0000]      Total Weld Time=Actual Weld Time+Total Cool Time 
         [0000]    Note, if cool time is 1 cycle, and there are 3 pulses, then the total cool time would be 2 cycles. 
         [0042]    To select the appropriate number of pulses and amount of cool times, the following algorithm can be used:
       a. Incrementally add cool time, starting with total cool time of 1 cycle. Try all possible # of pulses before increasing total cool time, however, according to a preferred embodiment of the present invention, do not go above 4 pulses, and stop when any of the following conditions is reached:
           i. expulsion is eliminated, or   ii. actual weld time drops below the weld lobe. If this occurs and there is still expulsion, reduce current by 0.1 kA and repeat step 2. Note, the current should not decrease below the ideal location boundary.   
               
 
         [0046]    The following table can be used as a guide for the number of pulses and total cool time combinations. This table should be followed in an ascending manner. 
         [0000]    
       
         
               
               
               
               
               
             
           
               
                   
                   
               
               
                   
                 Total Cool 
                   
                   
                   
               
               
                   
                 Time 
                 2 Pulses 
                 3 Pulses 
                 4 Pulses 
               
               
                   
                   
               
             
             
               
                   
                 1 cycle 
                 1 cool 
                   
                   
               
               
                   
                 2 cycle 
                 2 cool 
                 1 cool 
               
               
                   
                 3 cycle 
                 3 cool 
                   
                 1 cool 
               
               
                   
                 4 cycle 
                 4 cool 
                 2 cool 
               
               
                   
                 5 cycle 
                 5 cool 
               
               
                   
                 6 cycle 
                 6 cool 
                 3 cool 
                 2 cool 
               
               
                   
                   
               
             
          
         
       
     
         [0047]    To determine the appropriate individual pulse durations, the following formulas can be used: 
         [0000]    
       
         
           
             
               
                 First 
                  
                 
                   
                       
                   
                    
                   
                       
                   
                 
                  
                 Pulse 
                  
                 
                     
                 
                  
                 Duration 
               
               = 
               
                 
                   
                     
                       ( 
                       
                         Actual 
                          
                         
                             
                         
                          
                         Weld 
                          
                         
                             
                         
                          
                         Time 
                       
                       ) 
                     
                     / 
                     
                       ( 
                       
                         # 
                          
                         
                             
                         
                          
                         of 
                          
                         
                             
                         
                          
                         Pulses 
                       
                       ) 
                     
                   
                    
                 
                 + 
                 Remainder 
               
             
              
             
                 
             
           
         
       
       
         
           
             
                 
             
              
             
               The 
                
               
                   
               
                
               result 
                
               
                   
               
                
               of 
                
               
                   
               
                
               this 
                
               
                   
               
                
               division 
                
               
                   
               
                
               is 
                
               
                   
               
                
               rounded 
                
               
                   
               
                
               down 
                
               
                   
               
                
               to 
                
               
                   
               
                
               the 
                
               
                   
               
                
               nearest 
                
               
                   
               
                
               integer 
             
           
         
       
       
         
           
             
                 
             
              
             
               
                 Second 
                  
                 
                     
                 
                  
                 Pulse 
                  
                 
                     
                 
                  
                 Duration 
               
               = 
               
                 
                   
                     ( 
                     
                       Actual 
                        
                       
                           
                       
                        
                       Weld 
                        
                       
                           
                       
                        
                       Time 
                     
                     ) 
                   
                   / 
                   
                     ( 
                     
                       # 
                        
                       
                           
                       
                        
                       of 
                        
                       
                           
                       
                        
                       Pulses 
                     
                     ) 
                   
                 
                  
               
             
           
         
       
     
         [0048]    Due to most weld controller limitations, pulses after the second pulse repeat the duration of the second pulse. For example:
       Total weld time=23 cycles   Total cool time=4 cycles   Number of pulses=3       
 
         [0000]      First pulse duration=(23−4)/3+remainder=19/3+remainder=6+1=7 cycles 
         [0000]      Second and Third pulse duration=(23−4)/3=6 cycles 
         [0052]    An example of data obtained by the inventors applying the method of the present invention is provided below: 
       EXAMPLE 
       [0053]    Base Schedule: 9.6 kA, 23 cycles, 3340N (From step  111 ) 
         [0054]    Incrementally add cool time: 
         [0000]    
       
         
               
             
           
               
                   
               
             
             
               
                 
                   
                             
                     
                         
                         
                     
                   
                 
               
               
                   
               
             
          
         
       
     
       Final Spatter-Less Schedule. 
       [0000]    
       
         Current: 9.6 kA 
         Number of Pulses: 3 pulses 
         Cool times: 2 cool cycles in between pulses, total cool time of 4 cycles 
         Weld time: total weld time 23 cycles, individual pulse times→7 cycles, 6 cycles, 6 cycles 
         Pressure: 3340N 
       
     
         [0060]    Although less practical than a 2D lobe, according to an embodiment of the present invention, a 3D weld lobe (current, pressure, and time) based on the heat equation Q α (I 2 t)/P can be generated. See e.g.,  FIG. 10 . This would allow for simultaneously finding the optimal spatter-less weld schedule with the largest operating window, (i.e., the window having the most distance between expulsion and separation). However, to create a 3D weld lobe takes a considerable amount of time. Due to this time constraint, as described above a preexisting schedule can be used to create a 2D weld lobe, varying current and time. Using the visual results of this lobe, a different pressure can be chosen for study until an adequate optimal spatter-less weld schedule is found. 
         [0061]    Obviously, numerous modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described herein.