Abstract:
A method for controlling weld energy used in a welding process. The method includes establishing a weld energy profile having a total weld energy used over the period of the weld cycle. During the welding process, the expulsion rate is monitored and used to modify the weld energy. Based on the expulsion rate occurring during the welding process, the total weld energy can be shifted between various phases or cycles. In addition, the overall amount of or total weld energy can be increased and decreased.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     Not applicable.  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates generally to resistance welding and more specifically to a method for controlling and adaptively changing weld energy based on expulsion occurrence.  
         [0004]     2. Description of Related Art  
         [0005]     Resistance welding is used to weld pieces of metal together. Typically resistance welding uses two large electrodes that are placed on opposite sides of the surface to be welded. A large electric current passes through the electrodes and correspondingly heats up the metal between the electrodes. The result is a small area that is quickly heated to the melting point, forming a small spot of welded metal.  
         [0006]     Achieving a good weld starts with a good process design that controls the welding variables such as weld energy and time. For example, if not enough weld energy is applied, a cold weld may result caused by insufficient heat at the welding interface whereby fusion occurs only between the point contacts of the two pieces of metal. Conversely, if too much weld energy is applied for too long, expulsion may occur. Expulsion is the forceful ejection of molten metal from the welds and often results in material burn through.  
         [0007]     In the interest of obtaining good welds, is desirable to monitor the welding process and adaptively change the weld energy as necessary to maintain weld quality.  
       SUMMARY OF THE INVENTION  
       [0008]     Accordingly, the present invention is a method for controlling weld energy used in a welding process. The method includes the steps of establishing a weld energy profile having a total weld energy used over the period of the weld cycle to perform the welding process or operation. During the welding process the expulsion rate occurrence is monitored and recorded. Based on the expulsion rate, the weld energy is modified after which the modified weld energy is used to perform subsequent welding processes.  
         [0009]     In one embodiment, the total weld energy is partitioned into a first phase and a second phase and the expulsion rate for each phase is determined. Based on the expulsion rate occurring in each phase, the total weld energy is shifted to increase or decrease the amount of weld energy in the particular phases.  
         [0010]     In an additional embodiment, the total weld energy is either increased or decreased based on the overall expulsion rate occurrence. Accordingly, if the overall expulsion rate is high, the total weld energy is reduced and if the overall expulsion rate is low the total weld energy is increased.  
         [0011]     Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]      FIG. 1  is a schematic view of a weld energy profile.  
         [0013]      FIG. 2  is a block diagram illustrating an overall method of one embodiment of the present invention.  
         [0014]      FIG. 3  illustrates fuzzy logic membership functions for input and output variables useful in adapting the weld energy profile.  
         [0015]      FIG. 4  is a schematic view of a weld energy profile wherein the total factored energy is shifted between first and second parts or phases based on weld expulsion rates.  
         [0016]      FIG. 5  is a schematic view of a weld energy profile wherein the total factored energy is increased or decreased based on weld expulsion rates. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0017]      FIG. 1  schematically illustrates the dynamic electrical properties occurring during spot welding. Welds with an acceptable button size have a weld energy profile  10  based on cycles or time and weld energy. The weld energy profile  10  represents the total factored energy F t . In accordance with the present invention, the total factored energy F t  is partitioned or divided at partition point  12  into two parts having partial factored energies F t   (1)  and F t   (2)  of a duration or time period N* and (N−N*) not necessarily equal. The partition point can be defined for instance by the melting point or based on other considerations needed to produce a satisfactory weld. The total factored energy F t  is a product of the sum of the factored energy F t   (1)  and F t   (2) , where F t =F t   (1) +F t   (2) .  
         [0018]     Referring to  FIG. 2 a  block diagram of a method  14  for controlling the total factored energy F t  used in the welding process is illustrated. In operation, the method  14  begins with block  16  wherein a weld energy profile  10 , as illustrated in  FIG. 1 , is. established. As set forth previously, the weld energy profile  10  has a total factored energy F t . The weld energy profile  10  can be created to empirical data or it can be provided based on data for the particular material.  
         [0019]     Block  18  illustrates determining the partition point  12 . Once the time periods of the F t   (1)  and F t   (2) , that is the partition point  12  is established, the time for completion of a single weld is divided into two intervals, including half cycles, (1, . . . , N*) and (N*+1, . . . , N). Denoting α as the current partitioning of the total factored energy, that is, viewing each part F t   (1)  and F t   (2)  as a percentage or ratio of the total factored energy F t  then:
 
α= F   t   (1) /( F   t   (1)   +F   t   (2) ).
 
 Alternatively, the partial factored energies F t   (1)  and F t   (2)  can be expressed as:
 
 F   t   (1)   =αF   t   ; F   t   (2) =(1−α)F t  
 
 Thus, α defines the current partitioning of the total factored energy. 
 
         [0020]     In addition, in order to account for any uncertainties during the setup phase and eventual process/material changes the present invention also provides for an increase and decrease of the total factored energy F t  whereby:
 
 F   t   =F   t (1+γ F   t ).
 
 Parameter γ is by default equal to 0 but can be dynamically adjusted as necessary to increase or decrease the total factored energy F t . 
 
         [0021]     Parameters α and γ introduce flexibility to manage the total factored energy. Block  20  illustrates that the expulsion rate occurring during the welding process is monitored and recorded. Block  22  then performs a fuzzy logic analysis on the expulsion rates and converts the expulsion occurrence data into output control values. Block  24  uses the output control values along with the expulsion rates to generate weld energy adjustments. Whereby block  26  applies the adjustments to the weld energy profile  10  to adaptively change the total factored energy based on weld expulsion rates. Accordingly, the present invention links the rate of expulsion to weld energy management; specifically, parameters α and γ. As set forth more fully herein, the invention provides necessary adjustments of those parameters based on the actual conditions including dynamic shifting of the total factored energy F t  based on expulsion occurrence in the first and second parts or phases F t   (1)  and F t   (2)  of the welding process and fine tuning of the total factored energy F t  based on the overall expulsion rate.  
         [0022]     Accordingly, the original distribution of the total factored energy F t  of the weld profile is automatically compensated by gradually shifting the energy in a direction that minimizes the expulsion rate. Monitoring the expulsion rate for a given process window or number of welds (p) establishes the rates R 1 , R 2  in the first and second parts or intervals as:
 
 R   1   =n   1   /p; R   2   =n   2   /p. 
 
 Where n 1  is the total number of expulsions during half cycles (1, . . . , N*) and n 2  is the total number of expulsions during half cycles (N*+1, . . . , N) for the last p welds. 
 
         [0023]     A fuzzy logic controller executes fuzzy logic inference rules from a fuzzy rule base. Input and output variables are defined as members of fuzzy sets with degrees of membership in the respective fuzzy sets determined by specific membership functions. The rule base defines the fuzzy inference system and is based on knowledge of weld energy parameters and expulsion occurrences and rates during the weld process. The input data, or expulsion rates defines the membership functions used in the fuzzy rules. The reasoning mechanism executes the fuzzy inference rules converting the expulsion occurrence data into output control values using the database membership functions.  
         [0024]     In general, systems using fuzzy logic inference rules are well-known. A preferred embodiment of the fuzzy logic controller disclosed herein is based on the fuzzy reasoning system using weld expulsion occurrence rates, including overall expulsion rate occurring during the total target factored energy profile F t  of the welding process and the expulsion rate occurring during the first and second parts or phases F t   (1)  and F t   (2)  of the total target factored energy F t . By adjusting the target factored energy, the fuzzy logic controller attempts to maintain weld energy based on weld expulsion occurrence.  
         [0025]      FIG. 3  illustrates an embodiment of the fuzzy logic controller using a fuzzy membership function. As illustrated in  FIG. 3 , the membership functions include overlapping membership ranges for the expulsion rates R 1 , R 2  in the first and second parts or phases F t   (1)  and F t   (2)  of the total factored energy F t . As set forth below, the overlapping membership functions for the rate of expulsion of variables R 1 , R 2  result in a corresponding subset membership determined from the fuzzy rule base.  
         [0026]     Turning first to the adaptation of the distribution of the total factored energy F t  according to parameter α, the mechanism for adjusting the total factored energy F t  based on the expulsion rates is accomplished through the following rules: 
        IF R 1  is High AND R 2  is Low THEN dα=N a       IF R 1  is Low AND R 2  is High THEN dα=P a       IF R 1  is High AND R 2  is High THEN dα=Z     IF R 1  is Low AND R 2  is Low THEN dα=Z 
 
 wherein N a , P a , Z are constants defining positive, negative, or zero percent change of the ratio parameter α and energy gain γ. 
       
 
         [0031]     As illustrated in  FIG. 4 , the first two rules deal with shifting the total factored energy F t  towards the first or the second part of the weld process based on expulsion rate. For a high expulsion rate R 1  in the first part or phase F t   (1)  it is desirable to gradually shift the energy towards the following or second part or phase F t   (2) , illustrated by the dotted weld energy profile  10   a.  For a high expulsion rate R 2  in the second part or phase F t   (2)  it is desirable to gradually shift the energy towards the initial or first part or phase F t   (1) , illustrated by the dashed weld energy profile  10   b.  Shifting the weld energy profile  10  is accomplished by modifying the rate of change of parameter α controlling the ratio of the total factored energies. No major changes occur with the gain parameter γ.  
         [0032]     The third and forth rule are activated when the overall expulsion rate is either high or low. In these cases, the focus is on increasing or decreasing of the overall target energy rather than redistributing or shifting the energy.  
         [0033]     Turning now to the adaptation of the overall target factored energy F t  according to parameter γ. A high overall expulsion rate, i.e. expulsions consistently occurring in the first F t   (1)  and in the second F t   (2)  parts or phases of the total factored energy F t  of welding process, requires an overall reduction of the target factored energy. Decreasing the energy gain parameter γ, which controls the overall level of the target energy, results in a reduction of the target factored energy F t . Alternatively, a consistent lack of expulsions should be considered as an indicator that the overall target factored energy F t  is below the minimum for an acceptable weld. Thus, increasing the energy gain parameter γ, results in an increase of the target factored energy F t .  
         [0034]     Accordingly, increasing or decreasing the total factored energy F t  according to parameter γ, the mechanism for adjusting the total factored energy F t  based on the expulsion rates, is accomplished through the following rules: 
        IF R 1  is High AND R 2  is High THEN dγ=N g       IF R 1  is Low AND R 2  is Low THEN dγ=P g       IF R 1  is High AND R 2  is Low THEN dγ=Z     IF R 1  is Low AND R 2  is High THEN dγ=Z 
 
 wherein N g , P g , Z are constants defining positive, negative, or zero percent change of the energy gain γ. 
       
 
         [0039]     As illustrated in  FIG. 5 , the first two rules are activated when both expulsion rates; i.e., the rates in both the first F t   (1)  and in the second F t   (2)  parts or phases of the total factored energy F t  are low or high. In the case of a high overall expulsion rate it is desirable to gradually shift or decrease the overall level of the total factored energy F t , illustrated by the dashed weld energy profile  10   c.  The second rule reflects the situation when the overall expulsion rate is low. Since this may result in a cold weld, it is desirable to gradually shift or increase the overall level of the total factored energy F t , illustrated by the dotted line  10   d.  If the increase is too great, then it would be immediately limited by the first rule. This allows the target factored energy F t  to stay close to the edge of creating expulsions while minimizing the chance for a cold weld. The third and forth rule deactivate the changes of parameter γ when an energy shift is required.  
         [0040]     Accordingly, adaptation of parameters α and γ can be combined into a single set of rules as follows: 
        IF R 1  is High AND R 2  is Low THEN dα=N a  and dγ=Z     IF R 1  is Low AND R 2  is High THEN dα=P a  and dγ=Z     IF R 1  is High AND R 2  is High THEN dα=Z and dγ=N g       IF R 1  is Low AND R 2  is Low THEN dα=Z and dγ=P g  
 
 Applying the fuzzy reasoning algorithm we obtain an analytical expression for the change of parameters α and γ in dependence on the rates R 1  and R 2 :
 
 dα=R   1 (1− R   2 ) N   a   +R   2 (1− R   1 ) P   a   +R   1   R   2   Z+ (1− R   1 )(1− R   2 ) Z 
 
 dγ=R   1   R   2   N   g +(1− R   1 )(1− R   2 ) P   g   +R   1 (1− R   2 ) Z+R   2 (1− R   1 ) Z 
 
 Solving for the corresponding dα and dγ and applying the change to the original rate of change of parameter α and energy gain parameter γ yields:
 
α=α+ dα 
 
γ=γ+ dγ 
 
 Applying the new rate of change of parameter α to the total factored energy F t  changes the partial target factored energies F t   (1)  and F t   (2)  as follows:
 
 F   t   (1)   =αF   t   ;F   t   (2) =(1−α) F   t 
 
 Applying the new energy gain parameter γ to the total factored energy F t  changes the total factored energy as follows:
 
 F   t   =F   t (1+γ F   t ).
       
 
         [0045]     As demonstrated above, the present invention optimizes the weld energy, that is the energy per unit volume of weld, such that the total factored energy supplied to the weld reaches a point at or near the energy at which expulsion occurs.  
         [0046]     The description of the invention is merely exemplary in nature and, thus, variations that do not depart from the gist of the invention are intended to be within the scope of the invention. Such variations are not to be regarded as a departure from the spirit and scope of the invention.