Automatic assembling system of galvanized steel sheet by spot welding

Abstract

A method and apparatus for continuously spot-welding galvanized steel sheets overlapped to be bonded by way of a spot welding machine having a pair of electrodes. The galvanized steel sheets clamp a resistance increasing material including a spacer to ensure a gap between the sheets. Bonding surfaces of the galvanized steel sheets partly contact each other when the pair of electrodes pressurize the galvanized steel sheets such that a part of the gap is retained around the spacer between the bonding surfaces. The retained gap has a size in which zinc melted or vapored when a weld current flows between the electrodes can escape through a weld section of the galvanized steel sheets to outside. The method comprises the step of recording an inter-electrode resistance for each spot when the continuous spot welding is executed by the pair of electrodes. The method estimates an electrode lifetime defined by one of the number of spots and a duration of the spot welding until a sufficient nugget will not be formed by way of the electrodes according to the record of the inter-electrode resistance. The method automatically changes weld conditions to enable the continuous spot welding when the electrode lifetime reaches a predetermined electrode lifetime.

What is claimed is:  
     
       1. A spot welding method for assembling at least two initial members formed from a galvanized  steel sheet into a structural member by spot welding bonding surfaces of the initial members through a galvanized layers on  layer formed on at least one of the bonding surfaces by way of a spot welding machine having a pair of weld electrodes, wherein said method comprises the steps of: 
       placing a resistance increasing material at a predetermined position on the bonding surface of one of the initial members;  
       overlapping the other of the initial members on the one of the initial members while clamping the resistance increasing material between the initial members;  
       positioning a center axis passing through the pair of weld electrodes over substantially the center of the resistance increasing material clamped between the bonding surfaces to apply a predetermined pressure by the weld electrodes to the resistance increasing material and the initial members;  
       flowing a weld current having a predetermined value between the weld electrodes in a predetermined time;  
       detecting electric characteristics with respect to the weld electrodes in the predetermined time;  
       calculating an inter-electrode resistance based on the detected electric characteristics and calculating characteristics of resistance change based on the inter-electrode resistance;  
       determining success or failure in forming a nugget between the bonding surfaces by comparing the characteristics of resistance change with a predetermined standard;  
       automatically changing weld conditions upon the determination of failure in the determining step and primarily compensating for the failure;  
       comparing another predetermined standard with characteristics of resistance change additionally calculated after the compensating step and secondarily determining success or failure in forming the nugget;  
       recording the determination of failure in forming the nugget in the secondarily determining step;  
       continuously recording at least one of the electric characteristics, the inter-electrode resistance and the characteristics of resistance change during continuous spot welding by using the identical weld electrodes;  
       estimating the number of spots or the duration of spot welding until the successful nugget will not be formed according to the record in the continuously recording step;  
       automatically controlling to change subsequent weld conditions when the estimated member or duration reaches a predetermined standard for estimating;  
       secondarily comprising the forming of nugget by activating an additional back-up system when it is determined that the nugget is not formed according to the record in the continuously recording step or due to an unexpected accident occurred in the series of the steps; and  
       conveying the initial members between the steps, the steps being adapted to constitute a production line totally controlled by a host computer.  
     
     
       2. A method according to  claim 1 , wherein said resistance increasing material includes a spacer which ensures a gap between the bonding surfaces of the galvanized  steel sheets in the overlapping step. 
     
     
       3. A method according to  claim 2 , wherein said resistance increasing material retains a part of the gap around the spacer between the bonding surfaces to permit the bonding surfaces to contact each other in the pressurizing step. 
     
     
       4. A method according to  claim 2 , wherein said resistance increasing material retains a part of the gap around the spacer between the bonding surfaces to permit the bonding surfaces to contact each other in the pressurizing step, the retained gap having a size in which zinc, melted or vapored in the current flowing step, can escape from a weld section of the galvanized  steel sheets. 
     
     
       5. A method according to  claim 2 , wherein said resistance increasing material includes the mixture of bad electric conductive particles and adhesive material, said bad electric conductive particles functioning as the spacer and said adhesive material foaming or increasing the adhesion when heated or aged. 
     
     
       6. A method according to  claim 2 , wherein said resistance increasing material includes a perforated tape having an adhesive coated on its opposing faces. 
     
     
       7. A method according to  claim 1 , wherein said step of changing the weld conditions for the primarily compensating includes prolonging the duration of the weld current. 
     
     
       8. A method according to  claim 1 , wherein said estimating step includes estimating the number of spots or the duration of spot welding until the successful nugget will not be formed by comparing the characteristics of resistance change with the predetermined standard, and wherein said step of changing the weld conditions in the controlling step includes automatically grinding the weld electrodes. 
     
     
       9. A method according to  claim 1 , wherein said estimating step includes estimating the number of spots or the duration of spot welding until the successful nugget will not be formed by comparing the frequency of irregular current flow between the weld electrodes, and wherein said step of changing the weld conditions in the controlling step includes increasing the pressure applied to the galvanized  steel sheets. 
     
     
       10. A method according to  claim 1 , wherein said step of changing the weld conditions in the controlling step includes prolonging the duration of the weld current. 
     
     
       11. A method according to  claim 1 , wherein said step of changing the weld conditions in the controlling step includes increasing the predetermined value of the weld current. 
     
     
       12. A spot welding method for assembling at least two initial members formed from a galvanized  steel sheet into a structural member by spot welding bonding surfaces of the initial members through a galvanized layers on  layer formed on at least one of the bonding surfaces by way of a spot welding machine having a pair of weld electrodes, wherein said method comprises the steps of: 
       placing a resistance increasing material at a predetermined position on the bonding surface of one of the initial members;  
       overlapping the other of the initial members on the one of the initial members while clamping the resistance increasing material between the initial members;  
       positioning a center axis passing through the pair of weld electrodes over substantially the center of the resistance increasing material clamped between the bonding surfaces to apply a predetermined pressure by the weld electrodes to the resistance increasing material and the initial members;  
       flowing a weld current having a predetermined value between the weld electrodes in a predetermined time;  
       detecting electric characteristics with respect to the weld electrodes in the predetermined time;  
       calculating an inter-electrode resistance based on the detected electric characteristics and calculating characteristics of resistance change based on the inter-electrode resistance;  
       determining success or failing in forming a nugget between the bonding surfaces by comparing the characteristics of resistance change with a predetermined standard;  
       automatically changing weld conditions upon the determination of failure in the determining step and compensating for the failure;  
       comparing the predetermined standard with characteristics of resistance change additionally calculated after the compensating step and secondarily determining success or failure in forming the nugget; and  
       recording the determination of failure in forming the nugget in the secondarily determining step.  
     
     
       13. A method according to  claim 12 , wherein said changing the weld conditions includes prolonging the duration for flowing the weld current. 
     
     
       14. A method according to  claim 12 , wherein said resistance increasing material includes a spacer which ensures a gap between the bonding surfaces of the galvanized  steel sheets in the overlapping step. 
     
     
       15. A method according to  claim 14 , wherein said resistance increasing material retains a part of the gap around the spacer between the bonding surfaces to permit the bonding surfaces to contact each other in the pressurizing step. 
     
     
       16. A method according to  claim 14 , wherein said resistance increasing material retains a part of the gap around the spacer between the bonding surfaces to permit the bonding surfaces to contact each other in the pressurizing step, the retained gap having a size in which zinc, melted or vapored in the current flowing step, can escape from a weld section of the galvanized  steel sheets. 
     
     
       17. A method according to  claim 14 , wherein said resistance increasing material includes the mixture of bad electric conductive particles and adhesive material, said bad electric conductive particles functioning as the spacer and said adhesive material foaming or increasing the adhesion when heated or aged. 
     
     
       18. A method according to  claim 14 , wherein said resistance increasing material includes a perforated tape having an adhesive coated on its opposing faces. 
     
     
       19. A method according to  claim 12  further comprising secondarily compensating the forming of nugget by activating an additional back-up system when it is determined that the nugget is not formed according to the record in the continuously recording step or due to an unexpected accident occurred in the series of the steps. 
     
     
       20. A spot welding method for assembling at least two initial members formed from a galvanized  steel sheet into a structural member by spot welding bonding surfaces of the initial members through a galvanized layers on  layer formed on at least one of the bonding surfaces by way of a spot welding machine having a pair of weld electrodes, wherein said method comprises the steps of: 
       placing a resistance increasing material at a predetermined position on the bonding surface of one of the initial members;  
       overlapping the other of the initial members on the one of the initial members while clamping the resistance increasing material between the initial members;  
       positioning a center axis passing through the pair of weld electrodes over substantially the center of the resistance increasing material clamped between the bonding surfaces to apply a predetermined pressure by the weld electrodes to the resistance increasing material and the initial members;  
       flowing a weld current having a predetermined value between the weld electrodes in a predetermined time;  
       detecting electric characteristics with respect to the weld electrodes in the predetermined view;  
       calculating an inter-electrode resistance based on the detected electric characteristics and calculating characteristics of resistance change based on the inter-electrode resistance;  
       determining success or failure in forming a nugget between the bonding surfaces by comparing the characteristics of resistance change with a predetermined standard;  
       automatically changing weld conditions upon the determination of failure in the determining step and primarily compensating the forming of the nugget;  
       continuously recording at least one of the electric characteristics, the inter-electrode resistance and the characteristics of resistance change during continuously spot welding;  
       estimating the number of spots or the duration of spot welding until the successful nugget will not be formed according to the record in the continuously recording step; and  
       automatically controlling to change subsequent weld conditions when the estimated number or duration reaches a predetermined standard.  
     
     
       21. A method according to  claim 20 , wherein said step of changing the weld conditions for the primarily compensating includes prolonging the duration of the weld current. 
     
     
       22. A method according to  claim 20 , wherein said resistance increasing material includes a spacer which ensures a gap between the bonding surfaces of the galvanized  steel sheets in the overlapping step. 
     
     
       23. A method according to  claim 22 , wherein said resistance increasing material retains a part of the gap around the spacer between the bonding surfaces to permit the bonding surfaces to contact each other in the pressurizing step. 
     
     
       24. A method according to  claim 22 , wherein said resistance increasing material retains a part of the gap around the spacer between the bonding surfaces to permit the bonding surfaces to contact each other in the pressurizing step, the retained gap having a size in which zinc, melted or vapored in the current flowing step, can escape from a weld section of the galvanized  steel sheets. 
     
     
       25. A method according to  claim 22 , wherein said resistance increasing material includes the mixture of poorly electrically conductive particles and adhesive material, said poorly electrically conductive particles functioning as the spacer and said adhesive material foaming or increasing the adhesion when heated or aged. 
     
     
       26. A method according to  claim 22 , wherein said resistance increasing material includes a perforated tape having an adhesive coated on its opposing faces. 
     
     
       27. A method according to  claim 20  further comprising secondarily compensating the forming of nugget by activating an additional back-up system when it is determined that the nugget is not formed according to the record in the continuously recording step or due to an unexpected accident occurred in the series of the steps. 
     
     
       28. A method according to  claim 20 , wherein said estimating step includes estimating the number of spots or the duration of spot welding until the successful nugget will not be formed by comparing the characteristics of resistance change with the predetermined standard and wherein said step of changing the weld conditions in the controlling step includes automatically grinding the weld electrodes. 
     
     
       29. A method according to  claim 20 , wherein said estimating step includes estimating the number of spots or the duration of spot welding until the successful nugget will not be formed by comparing the frequency of irregular current flow between the weld electrodes, and wherein said step of changing the weld conditions in the controlling step includes increasing the pressure applied to the galvanized  steel sheets. 
     
     
       30. A method according to  claim 20 , wherein said step of changing the weld conditions in the controlling step includes prolonging the duration of the weld current. 
     
     
       31. A method according to  claim 20 , wherein said step of changing the weld conditions in the controlling step includes increasing the predetermined value of the weld current. 
     
     
       32. A spot welding apparatus for assembling at least two initial members formed from a galvanized  steel sheet into a structural member by spot welding bonding surfaces of the initial members through a galvanized layers on  layer formed on at least one of the bonding surfaces by way of a spot welding machine having a pair of weld electrodes, said apparatus comprising: 
       means for fixing one of the initial members;  
       means for placing a resistance increasing material at a predetermined position on the bonding surface of the one of the initial members;  
       means for overlapping the other of the initial members on the one of the initial members while clamping the resistance increasing material between the initial members;  
       means for positioning a center axis passing through the pair of weld electrodes over substantially centrally with respect to the resistance increasing material clamped between the bonding surfaces to apply a predetermined pressure by the weld electrodes to the resistance increasing material and the initial members;  
       means for flowing a weld current having a predetermined value between the weld electrodes in a predetermined time;  
       means for detecting electric characteristics with respect to the weld electrodes in the predetermined time;  
       means for calculating an inter-electrode resistance based on the detected electric characteristics and for calculating characteristics of resistance change based on the inter-electrode resistance;  
       means for determining success or failure in forming a nugget between the bonding surfaces by comparing the characteristics of resistance change with a predetermined standard;  
       means for automatically changing weld conditions upon the determination of failure and for primarily compensating the forming of the nugget;  
       means for comparing the predetermined standard with characteristics of resistance change additionally calculated after compensating the forming of the nugget and for secondarily determining success or failure in forming the nugget; and  
       means for recording the determination of failure in forming the nugget.  
     
     
       33. An apparatus according to  claim 32 , wherein said resistance increasing material includes a spacer which ensures a gap between the bonding surfaces of the galvanized  steel sheets when the galvanized  steel sheets are overlapped. 
     
     
       34. An apparatus according to  claim 33 , wherein said resistance increasing material retains a part of the gap around the spacer between the bonding surfaces to permit the bonding surfaces to contact each other when the galvanized  steel sheets are pressurized. 
     
     
       35. An apparatus according to  claim 33 , wherein said resistance increasing material retains a part of the gap around the spacer between the bonding surfaces to permit the bonding surfaces to contact each other when the galvanized  steel sheets are pressurized, the retained gap having a size in which zinc, melted or vapored, can escape from a weld section of the galvanized  steel sheets when the weld current flows the galvanized  steel sheets. 
     
     
       36. An apparatus according to  claim 33 , wherein said resistance increasing material includes the mixture of poorly electrically conductive particles and adhesive material, said poorly electrically conductive particles functioning as the spacer and said adhesive material foaming or increasing the adhesion when heated or aged. 
     
     
       37. An apparatus according to  claim 33 , wherein said resistance increasing material includes a perforated tape having an adhesive coated on its opposing faces. 
     
     
       38. An apparatus according to  claim 32  further comprising: 
       means for continuously recording at least one of the electric characteristics, the inter-electrode resistance and the characteristics of resistance change during continuously spot welding;  
       means for estimating the number of spots or the duration of spot welding while using the identical weld electrodes until the successful nugget will not be formed according to the record; and  
       means for controlling to automatically change subsequent weld conditions when the estimated number or duration reaches a predetermined standard.  
     
     
       39. An apparatus according to  claim 38 , wherein said estimating means estimates the number of spots or the duration of spot welding until the successful nugget will not be formed by comparing the characteristics of resistance change with the predetermined standard, and wherein said controlling means changes the weld conditions by automatically grinding the weld electrodes. 
     
     
       40. An apparatus according to  claim 38 , wherein said estimating means estimates the number of spots or the duration of spot welding until the successful nugget will not be formed by comparing the frequency of irregular current flow between the weld electrodes, and wherein said controlling means changes the weld conditions by increasing the pressure applied to the galvanized  steel sheets. 
     
     
       41. An apparatus according to  claim 38 , wherein said controlling means changes the weld conditions by increasing the predetermined value of the weld current. 
     
     
       42. An apparatus according to  claim 38 , wherein said controlling means changes the weld conditions by prolonging the duration of the weld current. 
     
     
       43. An apparatus according to  claim 32  further comprising means for secondarily compensating the forming of nugget by activating an additional back-up system when it is determined that the nugget is not formed according to the record by the recording means or due to an unexpected accident. 
     
     
       44. An apparatus according to  claim 32  further comprising means for conveying the initial members between each means, each means being adapted to constitute a production line totally controlled by a host computer. 
     
     
       45. An apparatus according to  claim 32 , wherein said changing means prolongs the duration of the weld current. 
     
     
       46. A method for continuously spot-welding galvanized  steel sheets overlapped to be bonded by way of a spot welding machine having a pair of electrodes, at least one of said steel sheets being galvanized, said galvanized  steel sheets clamping a resistance increasing material including a spacer to ensure a gap therebetween, bonding surfaces of said galvanized  steel sheets partly contacting each other when the pair of electrodes pressurize the galvanized  steel sheets such that a part of the gap is retained around the spacer between the bonding surfaces, the retained gap having a size in which zinc melted or vapored when a weld current flows between the electrodes can escape through a weld section of the galvanized  steel sheets, said method comprising the steps of: 
       recording an inter-electrode resistance for each spot when the continuous spot welding is executed by the pair of electrodes;  
       estimating an electrode lifetime defined by one of the number of spots and a duration of the spot welding until a sufficient nugget will not be formed by way of the electrodes according to the record of the inter-electrode resistance; and  
       automatically changing weld conditions to enable the continuous spot welding when the electrode lifetime reaches a predetermined electrode lifetime.  
     
     
       47. A method according to  claim 46 , wherein said electrode lifetime is defined by the number of spots when a frequency of irregular current flow occurred in a predetermined interval of the series of spots reaches a predetermined standard value, said irregular current flow being defined in a case when the inter-electrode resistance in a first cycle of an alternative current supplied to each spot exceeds a predetermined value. 
     
     
       48. A method according to  claim 46 , wherein said electrode lifetime is defined by the number of spots when a frequency of a compensation for finalizing an insufficient nugget formed in a predetermined interval of the series of spots reaches a predetermined standard value, said insufficient nugget being defined by a nugget formed when a difference between the maximum and the minimum of the inter-electrode resistances in the cycles of current supplied to each spot becomes lower than a predetermined standard value, and said compensation being performed for the insufficient nugget by extending a time of current supply. 
     
     
       49. A method according to  claim 46 , wherein said electrode lifetime is defined by the number of spots when a frequency of a compensation for finalizing an insufficient nugget formed in a predetermined interval of the series of spots reaches a predetermined standard value, said insufficient nugget being defined by a nugget formed when a pattern previously selected from a plurality of patterns each representing a change of the inter-electrode resistance during supplying the current to each spot, and said compensation being performed for the insufficient nugget by extending a time of current supply. 
     
     
       50. A method according to  claim 46 , wherein said electrode lifetime is defined by the number of spots when a frequency of a compensation for finalizing an insufficient nugget formed in a predetermined interval of the series of spots reaches a predetermined standard value, said insufficient nugget being defined by a nugget formed when a pattern previously selected from a plurality of patterns each representing a change of the inter-electrode resistance during supplying the current to each spot and when a difference between the maximum and the minimum of the inter-electrode resistances in the cycles of current supplied to each spot becomes lower than a predetermined standard value, and said compensation being performed for the insufficient nugget by extending a time of current supply. 
     
     
       51. A method according to  claim 46 , wherein said resistance increasing material includes the mixture of an adhesive material and powders, said powders having a function as a spacer. 
     
     
       52. A spot welding method for spot welding bonding at least two galvanized  steel sheets by way of a pair of weld electrodes, at least one of said steel sheets being galvanized, said method being characterized by the steps of: 
       placing a resistance increasing material having a spacer between said galvanized  steel sheets to ensure a gap between said galvanized  steel sheets;  
       clamping said galvanized  steel sheets by said weld electrodes;  
       flowing a welding current having a predetermined value between said weld electrodes in a predetermined time;  
       detecting electric characteristics with respect to said weld electrodes during flowing of the welding current; and  
       comparing said electric characteristics with a predetermined standard to determine success or failure in forming a nugget between said galvanized  steel sheets based on the flowing of the predetermined value welding current between said weld electrodes in the predetermined time.  
     
     
       53. A method according to  claim 52 , wherein said resistance increasing material retains a part of the gap around the spacer between the galvanized  steel sheets in the clamping step, the retained gap having a size in which zinc, melted or vapored in the current flowing step, can escape from a weld section of the galvanized  steel sheets. 
     
     
       54. A method according to  claim 52 , wherein the increase in the resistance between said electrodes caused by said resistance increasing material occurs substantially during said current flowing step. 
     
     
       55. A method according to  claim 52 , wherein said electric characteristics include characteristics of resistance change between said weld electrodes. 
     
     
       56. A method according to  claim 52 , wherein said electric characteristics include a difference between the maximum and the minimum of the resistances between said weld electrodes, said difference representing a reduced amount of said resistances between said weld electrodes. 
     
     
       57. A method according to  claim 52 , wherein said electric characteristics include a pattern representative of the change of the resistances between said weld electrodes in the current flowing step. 
     
     
       58. A method according to  claim 52 , wherein said electric characteristics comprise a difference between the maximum and the minimum of the resistances between said weld electrodes and a pattern representative of the change of the resistances between said electrodes in the current flowing step, said difference representing a reduced amount of said resistances between said weld electrodes. 
     
     
       59. A method according to  claim 52 , wherein said resistance increasing material includes the mixture of an adhesive material and powders, said powders having a function as a spacer. 
     
     
       60. A method according to  claim 52 , wherein said resistance increasing material includes the mixture of an adhesive material and powders, said powders having a function as a spacer, said adhesive material foaming or the adhesive force being increased when heated or aged. 
     
     
       61. A method according to  claim 52 , wherein said resistance increasing material includes a perforated tape having an adhesive coated on its opposing faces. 
     
     
       62. A method according to  claim 52 , wherein said galvanized  steel sheets partly contact each other while said resistance increasing material retains a part of the gap around the spacer between the galvanized  steel sheets in the clamping step, the retained gap having a size in which zinc, melted or vapored in the current flowing step, can escape from a weld section of the galvanized  steel sheets. 
     
     
       63. A method according to  claim 52  further comprising: automatically taking corrective action upon the determination of a failure.