Abstract:
The present invention provides an electrode assembly for a spot welder which has an acoustic sensor built therein. In a preferred embodiment of the present invention, a spot welder has a first and second electrode assembly according to the present invention. During welding, the acoustic sensor from the first electrode assembly selectively generates a burst of acoustic energy which passes through a weld subject and is received by the second electrode assembly. The acoustic sensor in the second transducer then emits an output signal, representative of the geometry of the weld nugget, to a computer.

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     1. Technical Field 
     The present invention relates generally to a transducer built into an electrode and, more particularly, to a transducer built into an electrode for real time resistance spot welding monitoring and feedback. 
     2. Discussion 
     Welding is a common process for attaching one metal member to another. This process generally involves heating an interface between the items which are to be welded, thereby melting the interface into one joint or weld nugget. Because this process has its application in many different types of manufacturing, such as automobile manufacturing, inspection ensuring that the weld nugget meets certain quality standards is a must. Specifically, it is desirable to inspect the area, size and configuration of the weld nugget and to determine if any defects exist therein. Uninspected welds may result in weld failure after the welded item is sold or distributed to a final user. 
     Ideally, a weld is inspected either during or shortly after the welding process so that added inspection does not increase weld time, and to allow weld problems to be identified when they occur. Furthermore, non-destructive testing is preferred so that welded parts which pass inspection may still be sold or distributed to the end user. 
     Visual inspection systems have been employed in the weld environment for this purpose. Specifically, an individual, such as a quality control person, may gage the size of the weld nugget or destructively test a welded item to determine its internal characteristics. However, these methods have several drawbacks. First, because of the bright light and harsh conditions generated by welding, visual inspection of a weld cannot be performed during the welding process. Instead, the welded item must be inspected off line, adding more time and cost to manufacturing. Second, to properly inspect the weld for defects, the internal structure of the weld nugget must be observed. This, in many instances, requires the welded item to be destructively tested, rendering the welded item useless. Besides the increased cost associated with scrapping an item for the purpose of inspection, it is practically impossible to destructively test all items. As such, destructive testing results in a lower number of samples tested and increased cost to manufacturing. The present invention was developed in light of these drawbacks. 
     SUMMARY OF THE INVENTION 
     The present invention addresses the aforementioned drawbacks, among others, by providing an electrode assembly for a spot welder which has an ultrasonic probe built therein. In a preferred embodiment of the present invention, a spot welder has a first and second electrode assembly containing first and second ultrasonic probes respectively. During welding, the ultrasonic probe from the first electrode assembly generates a burst of acoustic energy. One portion of this acoustic energy passes through a weld subject and resonates the first ultrasonic probe and another portion is reflected back by the weld subject and is received by the second ultrasonic probe. The ultrasonic probe in the first or second electrode assembly then emits an output signal, representative of the geometry of the weld nugget, to a computer. 
     Additional advantages and features of the present invention will become apparent from the subsequent description and the appended claims taken in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the drawings which illustrate the best mode presently contemplated for carrying out the present invention: 
     FIG. 1 is a cross-sectional view of an electrode assembly according to the present invention; 
     FIG. 2 is a cross-sectional view of an electrode assembly according to the present invention; 
     FIG. 3 is a cross-sectional view of a lower adaptor of an electrode assembly according to the present invention; 
     FIG. 4 is a cross-sectional view of a lower adaptor of an electrode assembly according to the present invention; 
     FIG. 5 is a cross-sectional view of a lower ultrasonic probe holder of an electrode assembly according to the present invention; 
     FIG. 6 is a cross-sectional view of a lower ultrasonic probe holder of an electrode assembly according to the present invention; 
     FIG. 7 is a schematic view of electrode assemblies being used in conjunction with a spot welder according to the present invention; 
     FIG. 8 is a schematic view of ultrasonic probes of electrode assemblies being used according to the present invention; 
     FIG. 9 is an exploded view of an ultrasonic probe according to the present invention. 
     FIG. 10 is a schematic view of ultrasonic probes of electrode assemblies being used according to the present invention; and 
     FIG. 11 is a graphical representation of operating characteristics according to the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to FIG. 1, an electrode assembly  10  according to the present invention is shown attached to an electrode holder  12 . As illustrated, electrode assembly  10  generally has four parts, ultrasonic probe  20 ; structural support system  21 ; shell  13 ; and cooling circuit  23 , each playing a vital roll in its operation. At its core, electrode assembly  10  has ultrasonic probe  20  which is responsible for generating acoustic energy. Supporting ultrasonic probe  20  within electrode assembly  10  is structural support system  21 . This structural support system  21  maintains ultrasonic probe in position while allowing coolant to flow around ultrasonic probe  20  and through electrode assembly  10 . The outer periphery of electrode assembly  10  consists of shell  13  which conducts electrical current for spot welding and provides protection to the internal components of electrode assembly  10 . Between shell  13  and structural support assembly  21  lies a cooling circuit  23  for cooling shell  13  and ultrasonic probe  20 . In the following discussion, each of these elements will be discussed in greater detail. 
     Referring now to FIG. 2, electrode assembly  10  is shown in greater detail. Here, shell  13  has an has an electrode cap  14 , lower adaptor  16  and an upper adaptor  18 . To facilitate the flow of coolant from electrode holder  12  to electrode assembly  10 , electrode holder  12  is provided with an internal sleeve  22 . Sleeve  22  is radially spaced from outer sleeve  24 , creating a gap  26  therebetween. This gap  26  allows coolant to flow from electrode assembly  10  and into electrode holder  12 . Similar to gap  26 , the internal diameter of internal sleeve  22  forms a passage  28  which channels coolant into electrode assembly  10 . As such, passage  28  and gap  26  provide the entrance and exit passages for internal cooling circuit  21  within electrode assembly  10 . 
     Internal sleeve  22  and lower adaptor  16  serve as the structural base for structural support system  21  designed to support ultrasonic probe  20 . Besides these elements, structural support system  21  generally comprises upper ultrasonic probe holder  32  and lower ultrasonic probe holder  34 . Internal sleeve  22 , at its lower periphery as shown, attaches to upper ultrasonic probe holder  32 . Upper ultrasonic probe holder  32 , in turn, attaches to lower ultrasonic probe holder  34 . Lower ultrasonic probe holder  34  is then attached to and supported by lower adaptor  16  by sleeve  38 . Because only internal sleeve  22  and lower adaptor  16  connect structural support system  21  to shell  13 , a gap  36  is formed between structural support system  21  and shell  13 . This gap  36  provides a major portion of internal cooling circuit  21  which passage  28  and gap  26  is designed to service. 
     In FIG. 3, a cross-sectional view of lower adaptor  16  is shown. Channels  40  fluidly connect an upper area  42  with passage  44 . In FIG. 4, a cross-sectional view at Section A of FIG. 3 illustrates channels  40  in greater detail. From the illustration, it may be seen that channels  40  generally comprise a plurality of small cylindrical-like tubes. These tubes allow coolant flow through lower adaptor  16  while maintaining its structural integrity. 
     In FIG. 5, a cross-sectional view of lower ultrasonic probe holder  34  is shown. As illustrated, passages  46  connect upper chamber  50  with lower chamber  48 . In FIG. 6 a cross-sectional view at Section B of FIG. 5, similar to lower adaptor  16 , shows that passages  46  generally comprise a plurality of cylindrical channels which allow coolant flow and maintain the structural integrity of ultrasonic probe holder  34 . 
     As discussed previously, structural support system  21  provides support for ultrasonic probe  20  and passages for cooling circuit  21 . Ultrasonic probe  20  is supported in position by support plate  52 , lower ultrasonic probe holder  34 , upper ultrasonic probe holder  32 , and upper plate  56  as shown. Ultrasonic probe  20 , itself, generally comprises piezoelectric crystal  58  sandwiched between conductive plates  60 . Conductive plates  60  serve to provide the required current and voltage across piezoelectric crystal  60  to create vibration, thereby inducing a burst of acoustic energy  67 . As such, conductive plates  60  are electrically connected to plug  62  by electrical leads  64  to provide the required voltage and current. A power source  66 , controlled by computer  68 , is connected to plug  62  to provide the required power thereto. During welding, shell  13  provides a conductive path for welding current to be transmitted from electrode holder  12  to a weld subject. As such, electrical current is conducted from electrode holder  12  through upper adaptor  18  and lower adaptor  16 , terminating at electrode cap  14 . The electrode cap  14 , itself, is the element which is in contact with items which are to be welded. 
     To cool the electrode assembly  10  and protect the ultrasonic probe  20 , cooling circuit  23  is provided. In cooling circuit  21 , coolant is transmitted from electrode holder  12  through internal portion  28  and into upper chamber  50 . Coolant moves from upper chamber  50 , through passages  46  of lower ultrasonic probe holder  34  and into lower chamber  48 , thereby ensuring that ultrasonic probe  20  remains cool. Coolant then passes from lower chamber  48 , through passage  44  of lower adaptor  16 , to area  70  within electrode cap  14 , thereby cooling electrode cap  14 . Coolant next moves from area  70  into channels  40  of lower adaptor  16 , through gap  36 , thereby cooling lower adaptor  16  and upper adaptor  18 , and exits through gap  26 . 
     Referring now to FIG. 7, the operation of the present invention will now be described. In FIG. 7, upper electrode assembly  110  and lower electrode assembly  210 , having the same components as electrode assembly  10 , are shown attached to upper electrode holder  112  and lower electrode holder  212 , respectively. Upper electrode holder  112  and lower electrode holder  212  are mechanically and electrically engaged with spot welder  41  as is known. 
     During operation, weld subject  80 , here consisting of two or more overlapping plates  84  and  86 , are clamped between lower electrode assembly  210  and upper electrode assembly  110 . Electrical current is then transmitted from upper electrode assembly  110  to lower electrode assembly  210 , through weld subject  80 , creating weld nugget  86 . 
     When weld subject  80  is initially clamped and before weld current flow and formation of weld nugget  86 , computer  68  instructs upper electrode assembly  110  to generate bursts of acoustic energy  67  which pass through and are reflected by upper plate  82 , weld nugget  86 , and lower plate  84 . These bursts continue until after weld nugget  86  has been formed and cooled. The portion of each Burst of acoustic energy  67  which passes through these elements, intersects and resonates piezoelectric crystal  58  of lower electrode assembly  210 . The portion which is reflected by these elements, resonates piezoelectric crystal  58  of upper electrode assembly  110 . This resonation induces a current in conductive plates  60 , sending electrical signals to computer  68 . 
     Referring now to FIG. 10, a schematic illustrating weld subject  80 , upper electrode assembly  110  and lower electrode assembly  210  is provided. Rays  250 ,  252 ,  254 , and  258  are reflected portions of initial burst of acoustic energy  67 . Different portions of burst of acoustic energy  67  reflect off different portions of weld subject  80 . Specifically, ray  250  represents acoustic energy reflected upward from the upper surface of upper weld plate  82 , ray  252  represents acoustic energy reflected from upper portion of weld nugget  86 , rays  254  represent acoustic energy reflected from the interface between upper plate  82  and lower plate  84 , and ray  258  represents acoustic energy reflected from the interface between weld nugget  86  and lower plate  84 . Similarly, ray  256  represents acoustic energy which passes through weld subject  80  and ultimately intersects and resonates piezoelectric crystal  58  of lower electrode assembly  210 . 
     The time of flight (TOF), time from transmission of burst of acoustic energy  67  until reception, is indicative of certain characteristics of weld subject  80 , weld nugget  86  and even upper electrode  110 . 
     Referring now to FIG. 11, a time amplitude graph is shown which plots each ray  250 ,  252 ,  254  or  258  in time. Each spike represents the signal strength, generated by resonation of piezoelectric crystal  58  in upper electrode assembly  110 . The time between each of these spikes is used to determine certain characteristics about weld subject  80  and upper electrode assembly  110 . For instance, the TOF of ray  250  (TOF  260  in FIG.  11 ), which represents time between transmission and reception of acoustic energy which results in ray  250 , can be used to determine the wear of upper electrode assembly  110 . Likewise, ray  252  together with ray  250  can be used to generate TOF  262  used to determine the residual thickness of upper plate  82 , and together with ray  258  can be used to generate TOF  264  and determine the thickness of weld nugget  86 . Rays  254 , which are reflected only outside weld nugget  86 , can be used to determine the cross section of weld nugget  86 . 
     Ray  256  is used by computer  68  generate a time history of the welding process. This time history follows a somewhat predictable pattern from which characteristics of the weld nugget  86  may be configured. Initially, when plates  84  and  86  are clamped between lower electrode assembly  210  and upper electrode assembly  110 , before the flow of electrical current begins and before maximum clamping pressure, the strength of ray  256  is small and the electrical signal generated from lower electrode assembly  210 , in response to ray  256 , is near zero. As the applied force from the clamping action of upper electrode assembly  110  and lower electrode assembly  210  increases, the strength of ray  256  increases to a peak, then remains constant until the welding current is generated. During heating, the strength of ray  256  increases causing the signal produced by piezoelectric crystal  58  to likewise increase. After current flow and during cooling of weld nugget  86 , the signal strength fluctuates according to temperature and phase transition of the cooling metal. 
     This signal strength and fluctuation during the welding process can be used to form a kind of acoustic signature of the process and determine certain characteristics of the weld nugget  86 . In particular, primary informative parameters of the signal (magnitude and phase) tend to follow the metal heating and melting stages. Experimental ultrasonic patterns, as a function of time, as well as real time welding current values, tend to correlate with the diameter of the weld nugget  86 . By using a representative set of the signatures and comparing them with destructive tests (peel tests), quantitative calibration characteristics can be established. Those calibration characteristics can be explicit ones, or the final guess about the weld could be established using neural networking algorithms. Either way, this information can be used to determine valuable information about the weld subject  80 . 
     Referring now to FIGS. 8 and 9, ultrasonic probe  20  of lower electrode assembly  210  is replaced with ultrasonic array  120 . Ultrasonic array  120  differs from ultrasonic probe  20  in that ultrasonic array  120  has a plurality of sensing elements as opposed to only one. In FIG. 8, ultrasonic array  120  is shown having a plurality of independent ultrasonic probes  120 A,  120 B, and  120 C, each generating an output signal independent from the remainder. To form these elements, each independent piezoelectric crystal  58 A,  58 B, and  58 C is sandwiched by independent conductive plates  60 A,  60 B, and  60 C. Each set of conductive plates  60 A,  60 B, and  60 C communicates with computer  68 , allowing each independent piezoelectric crystal  58 A,  58 B, and  58 C to provide a different output signal to computer  68 . 
     In operation, each burst of acoustic energy  67  intersects various and different portions of ultrasonic array  120 . As such, bursts of acoustic energy  67  which pass through weld nugget  86  may intersect one portion of ultrasonic array  120  while other bursts of acoustic energy  67  intersect other portions of ultrasonic array  120 . For example, as shown in FIG. 7, bursts of acoustic energy  67  which pass through weld nugget  86  intersect independent piezoelectric crystal  58 A while bursts of acoustic energy  67  which bypass weld nugget  86  intersect independent piezoelectric crystals  58 B and  58 C. As such, the outputs generated by independent piezoelectric crystal  58 A will be different than the outputs from independent piezoelectric crystals  58 B and  58 C. The result is that computer  68  is able to analyze the received information and provide a more accurate result of the size and geometry of weld nugget  86 . It is noted that ultrasonic probe  20  of upper electrode assembly  110  may be also constructed similar to ultrasonic array  120 , thereby providing a plurality of independent and separate bursts of acoustic energy  67 . 
     While the above detailed description describes the preferred embodiment of the present invention, it should be understood that the present invention is susceptible to modification, variation, and alteration without deviating from the scope and fair meaning of the subadjoined claims.