Abstract:
A resistance welding controller for supplying a substantially constant level of AC current to a resistance welder is provided. In one embodiment, the resistance welding controller automatically switches modes to provide compatibility with both AC and DC resistance welders.

Description:
This application claims the benefit of provisional application Ser. No. 60/113,705 filed Dec. 24, 1998. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to a resistance welding controller which supplies a given level of alternating (“AC”) current to a resistance welder. 
     A resistance welder welds a workpiece sandwiched between two electrodes as current flows. The current flow heats the workpiece and forms a molten metal weld “nugget.” After the welding current stops flowing, the weld nugget solidifies to form the weld. 
     There are two types of resistance welders: AC and direct (“DC”) current. A DC resistance welder and an associated resistance welding controller provide the advantage that the current supplied to resistance welder (and, in turn, to the electrodes) can be controlled within stringent limits. However, there are two major disadvantages: the equipment required is expensive and the electrodes wear out quickly because current flows in one direction only during welding. In contrast, an AC resistance welder and an associated resistance welding controller provide the advantages that the equipment required is inexpensive and the electrodes wear out very slowly. However, a disadvantage is that current supplied to the AC resistance welder  80  (and, in turn, to the electrodes) can be controlled only within fairly loose limits. 
     One way to control both types of resistance welders is to install a separate resistance welding controller for each. However, having two separate controllers is costly. 
     In view of the foregoing, it would be desirable to provide a resistance welding controller which can control both AC and DC resistance welders while using electricity efficiently and improving welding performance. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a resistance welding controller which can control both AC and DC resistance welders while using electricity efficiently and improving welding performance. 
     The disadvantages and limitations of previous resistance welder controllers are overcome by the present invention which provides a resistance welding controller for supplying a substantially constant level of AC current to a resistance welder. 
     In one embodiment, the resistance welding controller automatically switches modes to provide compatibility with both AC and DC resistance welders. Operators are not required to visually check the type of the resistance welder and switch the mode, making the welder easier to use. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other objects and advantages of the invention will be apparent upon consideration of the following detailed description, taken in conjunction with the accompanying drawings, in which reference characters refer to like parts throughout and in which: 
     FIG. 1 illustrates the principles of resistance welding. 
     FIG. 2 shows a comparison of the welding performance between the present invention and the conventional technology. 
     FIG. 3 is a circuit showing a DC resistance welding controller. 
     FIG. 4 is a circuit showing an AC resistance welding controller. 
     FIG. 5 is a circuit showing a resistance welding controller capable of switching between AC and DC modes according to the present invention. 
     FIG. 6 is a diagram showing the current in the primary and secondary coils when an AC resistance welding controller is connected to a resistance welder. 
     FIG. 7 is a diagram showing the current in the primary and secondary coils when a DC resistance welding controller is connected to a resistance welder. 
     FIG. 8A shows an AC resistance welder primary current waveform according to the prior art. 
     FIG. 8B shows an AC resistance welder primary current waveform according to the present invention. 
     FIG. 9 shows the operation of a DC resistance welding controller according to the present invention. 
     FIG. 10 shows the operation of an AC resistance welding controller according to the present invention. 
     FIG. 11A shows a portion of a DC resistance welder. 
     FIG. 11B shows DC resistance welder primary and secondary current waveforms according to the present invention. 
     FIG. 12A shows a portion of an AC resistance welder. 
     FIG. 12B shows AC resistance welder primary and secondary current waveforms obtained by the pulse width modulation (PWM) method according to the prior art. 
     FIG. 12C shows AC resistance welder primary and secondary current waveforms shaped in trapezoid according to the present invention. 
     FIG. 12D shows how a variable frequency control method is performed. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The principle of resistance welding is described with respect to FIG. 1. A “nugget”  210  is the melted portion of workpieces W, W. In principle, the best welding performance is provided by the largest nugget  210  (in mm diameter) produced with uniform repeatability. As shown in FIG. 1, a large current I of preferably about 10,000 A is applied via electrodes  201 ,  202  to workpieces W, W for preferably about 0.2-0.3 seconds. Heat is generated between the two workpieces and portions of them are melted, thereby forming nugget  210 . 
     A comparison of the welding performance between the present invention and the conventional technology is shown in FIG.  2 . As shown in FIG. 2, the present invention, represented by the inverter type AC controller, provides larger nuggets with uniform repeatability. As described herein, the present invention provides excellent welding performance. 
     FIG. 3 shows a DC resistance welder  60 . Transformer  70  and a pair of diodes  76  convert AC current supplied by resistance welding controller  110  to DC current such that DC current flows between two electrodes  61 ,  62 . DC resistance welder  60  uses resistance welding controller  110  of the following specification as shown in FIG.  3 . DC resistance welding controller  110  comprises power supply  112 , which is made up of three phase power supply  114  and diodes  116 . Inverter  120  and capacitor  150  are connected in parallel with power supply  112 . AC current from inverter  120  is supplied to resistance welder  60 . DC controller  140  controls the ON/OFF operation of each of the transistors constituting inverter  120 , thereby supplying a given level of AC current to DC resistance welder  60 . 
     FIG. 4 shows an AC resistance welder  80 . Transformer  90  transforms AC current supplied by resistance welding controller  210  such that AC current flows between two electrodes  81 ,  82  without AC/DC conversion. The second type (hereafter referred to as AC) resistance welder  80  uses AC resistance welding controller  210  of the following specification as shown in FIG.  4 . Thyristors  220  are connected such that AC current is supplied to AC resistance welder  80  from power supply  214 . AC controller  240  controls each of the thyristors  220 , thereby supplying a given level of AC current to AC controller  240 . 
     The frequency that DC resistance welding controller  110  supplies to DC resistance welder  60  is as much as 10 times higher than AC resistance welding controller  210  supplies to AC resistance welder  80 . 
     As shown in FIG. 5, resistance welding controller  10  may be coupled to either DC resistance welder  60  or AC resistance welder  80  for supplying a given level of AC current. Resistance welding controller  10  comprises: power supply  12 , inverter  20 , AC/DC controller  40 , a pair of resistance welder connection terminals  48 , and capacitor  50 . 
     Power supply  12  comprises: three phase power supply  14  and diodes  16 . 
     Inverter  20  and capacitor  50  are arranged in parallel with power supply  12 . Inverter  20  comprises four transistors  21 ,  22 ,  23 ,  24 . 
     Groups consisting of first and second transistors  21 ,  22  and third and fourth transistors  23 ,  24  are connected in parallel with power supply  12 . First transistor  21  and second transistor  22  are connected in series with power supply  12 . Third transistor  23  and fourth transistor  24  are also connected in series with power supply  12 . Current flows through transistors  21 ,  22 ,  23 ,  24  in the same direction. The bases of each of transistors  21 ,  22 ,  23 ,  24  are connected to AC/DC controller  40 . 
     Diodes  31 ,  32 ,  33 ,  34  are connected in parallel with transistors  21 ,  22 ,  23 ,  24 , respectively. Current flows through transistors  21 ,  22 ,  23 ,  24  inversely with respect to diodes  31 ,  32 ,  33 ,  34 , respectively. 
     A pair of resistance welder connecting terminals  48  are connected between first transistor  21  and second transistor  22  and between third transistor  23  and fourth transistor  24 . 
     Current detection circuit  42  is coupled to the lead connecting power supply  12  and inverter  20 . Current detection circuit  42  is also coupled to AC/DC tester  44 . AC/DC tester  44  is coupled to AC/DC controller  40 . AC/DC controller  40  is coupled to switching element  46 . 
     DC resistance welder  60  comprises a pair of electrodes (first electrode  61  and second electrode  62 ) and controller connection terminals  64 . Transformer  70  is connected to controller connection terminals  64 . Both sides of the secondary coil  72  of transformer  70  are connected to first electrode  61  via diode  76  located in the direction of the first electrode  61 . The center of secondary coil  72  is connected to second electrode  62 . 
     AC resistance welder  80  comprises a pair of electrodes (first electrode  81  and second electrode  82 ) and controller connection terminals  84 . Controller connection terminals  84  are connected to transformer  90 . One side of secondary coil  92  of transformer  90  is connected to the first electrode  81 . The other side of secondary coil  92  is connected to second electrode  82 . 
     Resistance welding controller  10  supplies a given level of AC current to each of the resistance welders  60  and  80  when controller connection terminals  64 ,  84  are connected to resistance welder connection terminals  48  of resistance welding controller  10 . 
     Manual Operation 
     The manual operation of the resistance welding controller shown in FIG. 5 is described herein. 
     For DC resistance welding, switching element  46  turns on the DC mode and resistance welding controller  10  supplies AC current of preferably about 400-1200 Hz to DC resistance welder  60 . 
     AC/DC controller  40  causes resistance welding controller  10  to cycle through the following four states at the above frequencies: first and fourth transistors  21 ,  24  are ON; second and third transistors  22 ,  23  are OFF (hereafter referred to as the first state); all transistors  21 ,  22 ,  23 ,  24  are OFF (hereafter referred to as the second state): second and third transistors  22 ,  23  are ON; first and fourth transistors  21 ,  24  are OFF (hereafter referred to as the third state); and all transistors  21 ,  22 ,  23 ,  24  are OFF (hereafter referred to as the fourth state). 
     In the first state, current flows in the following order: power supply  12 , first transistor  21 , primary coil  71 , forth transistor  24 , and power supply  12 . 
     In primary coil  71 , current flows from the upper level to lower level, as shown in FIG.  5 . This is referred to as the a-direction. 
     In the third state, current flows in the following order: power supply  12 , third transistor  23 , primary coil  71 , second transistor  22 , and power supply  12 . 
     In primary coil  71 , current flows to the upper level from lower level in the drawing. This is referred to as the b-direction. 
     A given level of high frequency AC current is supplied to DC resistance welder  60  (secondary coil  72  side) in the above manner. 
     For DC resistance welding, workpieces W, W (sandwiched between electrodes  61  and  62 ) are welded together by the above AC current which flows on the secondary coil  72  side in DC resistance welder  60  in the loop comprising: one end of secondary coil  72 , first electrode  61 , second electrode  62 , the center portion of secondary coil  72 , the other end of secondary coil  72 , and first electrode  61 . 
     For AC resistance welding, switching element  46  turns on the AC mode and resistance welding controller  10  supplies AC current of preferably about 50 or 60 Hz. AC/DC controller  40  alternately provides two states: the first state (first and fourth transistors  21 ,  24  are ON) and the third state (second and third transistors  22 ,  23  are ON) at the above frequencies in resistance welding controller  10 , supplying AC current of preferably about 50-60 Hz to AC resistance welder  80 . For AC resistance welder  80 , both transistors  21 ,  24  (or  22 ,  23 ) are duty-controlled to continuously switch on and off. As a result, the AC current assumes a generally rectangular wave form. Workpieces W, W (sandwiched between first electrode  81  and second electrode  82  of AC resistance welder  80 ) are welded together by the above AC current supplied in the above manner. 
     Each of the current flows that can occur in primary coil  91  is referred to as the a- or b-direction and the corresponding current flows that can occur in secondary coil  92  are referred to as the a′- or b′-direction. 
     Automatic Operation 
     AC/DC controller  40  alternately provides the first, second, third, and fourth states at a given frequency. Current detection circuit  42  detects different levels of current for DC and AC resistance welders  60  and  80 . 
     The operation of AC resistance welder  80  is shown in FIG.  6 . In the first state (first and fourth transistors  21 ,  24  are ON), the current level gradually increases from zero as current flows through current detection circuit  42  in the following order: power supply  12 , first transistor  21 , primary coil  91  (a-direction), fourth transistor  24 , and power supply  12 . Current taken by the coil in the current detection circuit  24  causes the current level to increase gradually. The current level of current flowing in the a′-direction also increases gradually from zero for the same reason. 
     Note that the current flow from left to right through current detection circuit  42  (i.e., the lead portion at which current detection circuit  42  measures current gain) is referred to as the A direction. The reverse current flow is referred to as the B direction. In this case, current flows in the A-direction. 
     The sequence goes to the second state (all transistors  21 ,  22 ,  23 ,  24  are OFF). Current flowing through primary coil  91  of transformer  90  is interrupted. However, current remaining in the secondary coil  92  continues flowing to the secondary coil  92  side of transformer  90  in the a′-direction, inducing current flowing through primary coil  91  in the a-direction without interruption. 
     On the primary coil  91  side, current flows in the following order as capacitor  50  is charged: primary coil  91 , third diode  33 , capacitor  50 , second diode  32 , and primary coil  91 . 
     Current flows past current detection circuit  42  in the B-direction. The current level gradually decreases as capacitor  50  is charged. Current stops flowing to both the first and second coil sides when capacitor  50  is charged completely. 
     For AC resistance welder  80 , in the second state, current remains in a loop including not only the secondary coil  92  side of transformer  90  but also the primary coil  91  side (including resistance welding controller  10 ) as described. The current stops flowing when capacitor  50  is charged completely. 
     The sequence goes to the third state (second and third transistors  22  and  23  are ON). The current level gradually increases from zero in the same manner as in the first state, as current flows through current detection circuit  42  in the A-direction (from power supply  12  to third transistor  23  to primary coil  91  (b-direction) to second transistor  22  to power supply  12 ). The current level of current flowing in the b′-direction also increases gradually from zero. 
     For DC resistance welder  60 , the controller of the present invention operates as shown in FIG.  7 . In the first state (first and fourth transistors  21 ,  24  are ON), the current level gradually increases from zero as current flows into current detection circuit  42  in the A-direction (from power supply  12  to first transistor  21  to primary coil  91  (a-direction) to fourth transistor  24  to power supply  12 ). The current level of current flowing through secondary coil  72  in the a′-direction also increases gradually from zero. 
     When the sequence goes to the second state (all transistors  21 ,  22 ,  23 ,  24  are OFF), current flowing through primary coil  71  of transformer  70  is interrupted. As is the case for AC resistance welder  80 , current continues to flow on the secondary coil  72  side of transformer  70  as it does in the first state. Nevertheless, in this case, on the secondary coil  72  side, current flows in a loop including: one end of secondary coil  72 , first electrode  61 , second electrode  62 , the center of second coil  72 , the other end of secondary coil  72 , and first electrode  61 , as described above. 
     Thus, in secondary coil  72 , current flows from the center to both ends. Current in primary coil  71  induced by current flowing through secondary coil  72  is canceled in the a- and b-directions and little current flows on the primary coil  71  side. Therefore, on the primary coil  71  side, current remaining within the coil of current detection circuit  24  flows independently from current flowing on the secondary coil  72  side in the following order as capacitor  50  is charged: primary coil  71 , third diode  33 , capacitor  50 , second diode  32 , and primary coil  71 . 
     Current gain is so small that current stops flowing shortly. On the secondary coil  72  side, current continues to flow independently from the current in primary coil  71 . Unlike what happens for AC resistance welder  80 , resistivity in the loop remains small and current does not stop flowing when capacitor  50  is charged completely. For this reason, on the secondary coil  72  side, current continues to flow in the second state, as shown in FIG.  6 . Resistivity somewhat decreases current level. 
     The sequence goes to the third state (first and fourth transistors  21  and  24  are ON) with the above condition. The secondary coil  72  side is still conducting electricity, therefore, there is no chance for current flowing through primary coil  71  to induce current in secondary coil  72 . As a result, amperage does not start from zero. It starts from a given value and increases gradually. 
     During the transition from the second to third state described above, the current level of current flowing in the primary coil  71  side (current flowing through current detection circuit  42 ) starts from zero for AC resistance welder  80 . The current level starts from a given value, not zero, for DC resistance welder  60 . For this reason, AC/DC tester  44  is able to determine the type of resistance welder,  60  or  80 . Based on the result, AC/DC controller  40  turns on the DC or AC mode for the type of resistance welder  60  or  80 . The specific sequence of how each of the resistance welders are controlled is the same as that of manual switching. 
     As described above, resistance welding controller  10  of the present invention can control both DC and AC resistance welders  60  and  80 , providing an easy to use controller which does not require separate controllers for different resistance welders,  60  and  80 . Also, the controller can automatically switch the mode based on the type of resistance welder,  60  or  80 , eliminating the need for manual switching for different types of resistance welders,  60  and  80 . 
     The primary current as provided to welding controllers for AC resistance welders according to the prior art and the present invention is shown in FIGS. 8A and 8B, respectively. 
     According to the present invention, FIG. 12C shows the trapezoid wave form control method in which T is constant. FIG. 12D shows the variable frequency control method in which the following condition is fulfilled: 
     
       
           fa= 1 /Ta   
       
     
     
       
           fb= 1 /Tb   
       
     
     when 
     fa&gt;fb 
     Tz&lt;Tb and 
     current (i) is controlled by varying f(frequency). 
     FIG. 12B shows the pulse width modulation (PWM) in which the following condition is fulfilled: 
     
       
           f= 1 /T   
       
     
     In other words, frequency f is constant. Current is controlled by varying t. 
     For an AC resistance welder as shown in FIGS. 10,  12 A,  12 C, and  12 D according to the present invention, the secondary current flow is substantially constant, thus electricity is efficiently used and welding performance is improved. This is in contrast to the prior art AC resistance welder primary current waveform shown in FIG.  12 B. 
     For a DC welder, as shown in FIGS. 9,  11 A, and  11 B, the secondary current flows when the primary current does not flow, theoretically making better use of electricity than an interrupted power supply. 
     It will be understood that the foregoing is only illustrative of the principles of this invention, and that various modifications can be made by those skilled in the art without departing from the scope and spirit of the invention.