Abstract:
A stud welding gun includes a closed conductivity path indicating a stud properly positioned for welding. Another aspect includes a conductivity detection circuit closed when a stud is present. A further aspect provides a method to operate a stud welding gun including closing a conductivity path to permit a welding operation. A still further aspect provides a method to detect a stud presence by passing a conductivity current from a conductivity detection circuit through a conductivity path closed by a stud.

Description:
BACKGROUND 
   The invention relates generally to stud welding and more particularly to a stud gun and method for using a stud gun having a stud confirmed in location for welding prior to initiating a welding sequence. 
   Stud welding guns are used to weld a variety of sizes of studs onto various work pieces for further attachment of additional items to the work pieces. Stud welding guns are widely used in the automotive industry to attach studs for further attachment of trim pieces on automobiles. Furthermore, stud welding guns can be used in both a manual and an automated system. Studs for use in a stud welding gun include a shaft or body often having a smooth or fastener threaded surface, and are formed of electrically conductive material. A welding current is passed through the stud which creates an arc used to fuse the stud to an electrically conductive surface (that is (i.e.), the work piece). 
   Common stud welding guns operate by feeding an individual stud to a collet or similar chuck device which temporarily holds the stud. The stud is then positioned approximate a work piece and a small electric current passed through the stud creates an arc between the stud and the work piece. Once the arc forms, a full welding current is applied between the stud and the work piece to generate a fusion area between the two. The stud is then rammed into the fusion area to complete the welding process. A disadvantage may result from a stud being misaligned or missing when the collet or chuck is positioned to weld. When a stud is not in position for welding, a welding arc generated between the collet and the work piece results in the collet potentially being welded to the work piece. 
   In either of the above situations, i.e., where the stud is missing and the process must be repeated to provide a stud in the appropriate location, and where the collet is inadvertently welded to the work piece, additional time and costs are incurred due to the delay in providing a stud or the rework required to remove the attached collet from the work piece. Stud welding gun systems are known which provide a conductivity check using a circuit path including the work piece such that the presence of a stud in position for welding is required before the arc current is generated to start the welding process. The potential for inadvertently welding the collet to the work piece is still present with these systems because the conductivity circuit is completed through the work piece, therefore requiring the stud welding gun and collet to be brought into close alignment with the work piece. 
   It is therefore desirable to provide a stud welding gun and stud welding gun system which reduces the potential for starting a welding process when a stud is not present and reduces the potential for welding the collet to the work piece. 
   SUMMARY OF THE INVENTION 
   In accordance with a preferred embodiment of the present invention, a stud welding gun includes a gun body having a collet mechanically and electrically connected to a receiver. A ram disposed in the receiver section is contacted by a biased conductive element. A conductive stud is driven into physical contact with the collet by the ram which closes a conductivity path through the conductive element, ram, metallic stud, collet, and receiver. A closed conductivity path indicates a stud properly positioned for welding. The conductivity path is formed in the stud welding gun, independent of a work piece. 
   A conductivity detection circuit in communication with the stud welding gun is connected external to the stud gun. The conductivity detection circuit applies a small voltage potential across the conductivity path. The conductivity path is closed and a small conductivity current flows when a stud is present and open when a stud is absent. A stud welding gun having a conductivity detection circuit of the present invention prevents the application of a welding current if a stud is not present, and therefore the conventional potential to weld the collet to the work piece. A stud welding gun of the present invention also improves cycle time (i.e., the time to feed a new stud before attempting to weld). 
   The conductive element is mechanically connected to the stud welding gun and includes a biasing device to bias the conductive element into contact with the ram. By insulating the ram from the stud welding gun, the biased conductive element permits conductivity current flow through the ram, without shorting through the gun body. The addition of the conductive element of the present invention provides a low cost, efficient way to modify a known stud welding gun to connect a closed loop conductivity path to the stud welding gun. 
   A method of operating a welding gun with a stud and a method of detecting a stud presence in a stud welding gun are also provided. 
   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 
     The present invention will become more fully understood from the detailed description and the accompanying drawings, wherein: 
       FIG. 1  is a diagrammatic view identifying a component system for performing stud welding using a modified stud welding gun according to the preferred embodiment of the present invention; 
       FIG. 2  is a diagrammatic view simplified from  FIG. 1 , further showing an exemplary conductivity detection circuit connected to the stud welding gun; 
       FIG. 3  is a cross section view taken through the stud welding gun of  FIG. 2 ; 
       FIG. 4  is an elevational view of the preferred embodiment stud gun identifying a conductivity current flow path completed via the conductivity detection circuit and the conductive element; 
       FIGS. 5-9  are a series of diagrammatic views showing the preferred embodiment stud welding gun throughout a welding procedure; 
       FIG. 6  is a diagrammatic view similar to  FIG. 5  showing the initial step of retracting the ram to initiate a stud welding sequence; 
       FIG. 7  is a diagrammatic view similar to  FIG. 6  showing a new stud in position prior to pneumatic pressure being applied to the ram to press the stud into contact with the collet; 
       FIG. 8  is a diagrammatic view similar to  FIG. 7  showing the ram contacting the stud to drive the stud into contact with the collet; 
       FIG. 9  is a diagrammatic view similar to  FIG. 8  showing the new stud in contact with the collet, closing the conductivity path and initiating a next welding sequence; 
       FIG. 10  is a flow chart identifying the steps to operate the stud welding gun of the preferred embodiment; 
       FIG. 11  is a flow chart identifying the steps to detect a stud according to the preferred embodiment; 
       FIG. 12  is a flow chart identifying the sequence of logic and welding signals produced during a welding procedure for the preferred embodiment stud welding gun; and 
       FIG. 13  is a diagrammatic view of the conductivity signal path for the preferred embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The following description of the preferred embodiments is merely exemplary in nature and is in no way intended to limit the invention, its application, or uses. 
     FIG. 1  shows a stud welding gun of the preferred embodiment of the present invention. A stud detection system  10  includes a stud gun  12  holding a stud  14  in position for welding to a work piece  16 . It is envisioned that work piece  16  is a steel, sheet metal automobile panel. Each stud  14  is provided via a stud feeder supply  18  and a pneumatic tube  20  to stud gun  12 . A weld tool process control unit  22  is connected via a welding current supply line  24  to stud gun  12 . Weld tool process control unit  22  is electrically connected on a feed side to a process control unit  26  via control lines  28 . The process control unit  26  also includes a welding current supply  27 . The process control unit  26  is connected to a conductive element  30  of stud gun  12  and a body of the stud gun  12  via conductivity signal lines  32  and  33 , respectively. Alternately, conductivity signal lines  32  and  33  can be included with welding current supply line  24 . Process control unit  26  also includes a microprocessor, a memory unit, an input device (e.g., a keyboard) and a display screen, as known in the art. 
   Referring now to  FIG. 2 , stud gun  12  includes a receiver  34 , a collet  36  having a plurality of longitudinal slits  37 , and a union  38  which connectably joins collet  36  to receiver  34 . Longitudinal slits  37  provided in collet  36  enable the distal end of collet  36  to radially expand and contract about the changing diameter of a stud  14  in the direction of radial arrows “A”. A positive grip is therefore maintained on stud  14  by the biasing force provided by a plurality of deflectable portions  39  of collet  36  between longitudinal slits  37 . Conductive element  30  is releasably connected to receiver  34  such that conductive element  30  can be removed and replaced. In the preferred embodiment, the outer diameter of conductive element  30  has male threads (not shown) which engage associated female threads (not shown) of receiver  34 . An electrical ground connection  40  is also provided on receiver  34  whose purpose will be further described in reference to FIG.  4 . 
   Stud gun  12  is connected to a conductivity detection circuit  42  via a conductive element line  44  connected to conductive element  30  and a ground path line  46  connected to ground connection  40 . Conductivity detection circuit  42  is connected to weld tool process control unit  22  via at least one signal line  48 . Weld tool process control unit  22  is connected to welding current supply  27  of process control unit  26  via a conductivity signal line  50 . These lines are further described in reference to FIG.  12 . 
   A pneumatic pressure source  52  is connected to stud gun  12  via a pressure tube  54  and is connected to the stud feeder supply  18  via a stud supply pressure tube  56 . The pneumatic pressure source  52  provides a source of positive pressure for driving the stud  14  into the receiver  34  via the pneumatic tube  20 , and for driving the stud  14  into the welding position shown (as will be described in reference to FIG.  3 ). The pressure tube  54  connects to a piston head  58  of the stud gun  12  at a piston head cap  59 . A stud  14  shown in proper position for welding deflects the deflectable portions  39  of the collet  36  in the radial deflection direction “A” as shown. 
   As best seen in reference to  FIG. 3 , piston head  58  receives air from pneumatic pressure source  52  (shown in  FIG. 2 ) in a chamber  60 , operable to advance piston  62  in a linear direction “B”. The pneumatic fluid acts on a piston  62  having a distally extending ram  64  attached thereto. The ram  64  in turn contacts stud  14  to drive stud  14  into the position shown for welding in the linear direction “B”. Ram  64  is slidably disposed within receiver  34  in a bearing sleeve  66 . Ram  64  is electrically isolated from receiver  34  by insulation  68  circumferentially surrounding ram  64 . Bearing sleeve  66  is also preferably provided of a non-conductive material such as a polyamide to further electrically insulate ram  64  from receiver  34 . 
   An aperture  69  within insulation  68  is provided for exposing a contact surface of ram  64 . A conductive element housing  70  is removably disposed in receiver  34  in alignment with aperture  69  in insulation  68 . The conductive element housing  70  is a non-conductive material. Studs are received in receiver  34  in a stud loading direction “C” via a delivery tube  71  when ram  64  retracts in a piston upstroke direction “E” (as further described in reference to FIG.  6 ). This overall process is fully described with reference to  FIGS. 5-9 . Collet  36  is necessarily a conductive material (e.g., copper) which is releasably fastened to the conductive material of receiver  34  via union  38 . The union  38 , receiver  34 , and ram  64  are electrically conductive materials such as copper or steel, which when mechanically linked, form a portion of a conductivity path for stud gun  12 , as further discussed below. The conductivity path is closed when a stud  14  is present, and open when a stud  14  is absent. 
   As detailed in  FIG. 4 , a conductivity current flow path for the preferred embodiment is shown by arrows D through a closed conductivity path. A ball  72  is slidably disposed within conductive element housing  70 . A compression spring  74  is connectably attached to ball  72  at a first end and to an electrical contact  75  at a second end. Ball  72 , conductive element housing  70 , compression spring  74 , and electrical contact  75  form conductive element  30 . Compression spring  74  is preferably a coil spring (as shown) but can also be any type of spring or biasing means capable of providing an electrical contact path between ball  72  and electrical contact  75 . Compression spring  74  biases ball  72  into contact with ram  64 . 
   Ram  64  advances in the linear direction “B” until stud  14  contacts and is held by contact surfaces  76  at the distal end of collet  36 . With stud  14  in the position shown, a conductivity path closes between conductivity detection circuit  42  via conductive element line  44 , electrical contact  75 , compression spring  74 , ball  72 , ram  64 , stud  14 , contact surfaces  76 , collet  36 , union  38 , receiver  34 , receiver ground connection  40 , and ground path line  46 , respectively. The conductivity current flow path is exemplified by direction arrows D. When the conductivity circuit path closes as shown, conductivity detection circuit  42  directs a signal (described in detail with reference to FIGS.  11  and  12 ) to weld tool process control unit  22  (shown in  FIG. 2 ) indicating that a stud  14  is in position for welding. The weld tool process control unit  22  then signals welding current supply  27  (within process control unit  26 ) to initiate a welding sequence. When no stud  14  is present, the conductivity circuit path is open. A length of ram  64  is controlled to prevent ram  64  contacting the contact surfaces  76  for any position of ram  64 . 
   Referring now to  FIGS. 5-9 , the sequence for loading a new stud, the delivery of the stud to the collet and the welding of the stud will be described.  FIG. 5  shows stud welding gun  12  following a completed welding cycle (i.e., a previous stud is not shown) and ram  64  is fully extended in the ram linear direction “B”. Piston  62  is in a fully extended position within piston head  58  and chamber  60  is fully pressurized. Ball  72  is biased into contact with ram  64 , however, the absence of a stud results in an open conductivity path in conductivity detection circuit  42 . 
     FIG. 6  shows the initiation of a stud loading step. Pneumatic pressure from pneumatic pressure source  52  (shown in  FIG. 2 ) is applied to a piston bottom face  78  to drive piston  62  in the piston upstroke direction “E” as shown. Pressure in chamber  60  is released to permit piston  62  to travel in piston upstroke direction “E”. Ram  64 , which is connectably disposed to piston  62 , retracts in piston upstroke direction “E”. Stud  14  is pneumatically delivered through delivery tube  71  of receiver  34  prior to reaching a stud chamber  80 . 
     FIG. 7  shows stud  14  positioned within stud chamber  80  prior to ram  64  being driven into contact with stud  14 . Piston  62  is fully retracted prior to pneumatic pressure entering chamber  60 . 
   In the step shown in  FIG. 8 , pneumatic fluid (e.g., air) from pneumatic pressure source  52  (shown in  FIG. 2 ) is directed into chamber  60  above piston  62  and is bled from the piston bottom face  78  side of piston  62 . This process is known and is therefore not further detailed herein. Piston  62 , connected to ram  64 , drives ram  64  into contact with stud  14  within stud chamber  80 . Although ball  72  is biased into contact with ram  64 , stud  14  has not yet reached contact surfaces  76  therefore the conductivity path is still open at this time. 
   Referring to  FIG. 9 , ball  72  remains biased into contact with ram  64 . Stud  14 , driven by ram  64 , physically deflects or contacts contact surfaces  76  of collet  36 . A conductivity path as shown in  FIG. 4  thereby closes and an electrical potential across the conductivity path induces a small current flow. The insulation  68  precludes the conductivity current from shorting between ram  64  and receiver  34 . As best shown in  FIG. 4 , conductivity current flows through conductive element line  44 , ground path line  46 , and conductivity detection circuit  42  to complete the conductivity circuit. The direction of conductivity current flow is exemplary to demonstrate the components in the path. 
   As best described with reference to  FIG. 10 , the steps to operate a stud welding gun having a stud detection conductivity path of the present invention are provided. In an initial step  100 , a first conductive portion of the gun is mechanically and electrically connected to a second conductive portion of the gun. Next, at step  102 , a third conductive portion of the gun is advanced to extend the stud toward the second conductive portion of the gun. As described by step  104 , the third conductive portion is electrically isolated from both the first and second conductive portions of the gun. Further at step  106 , a conductive element is biased into contact with the third conductive portion of the gun. Following at step  108 , an electrical current is conducted through a current path including the conductive element, the third conductive portion, the second conductive portion and the first conductive portion of the gun when the stud contacts the second conductive portion of the gun. For step  110 , a welding current supply initiating signal is generated when the electrical current flows through the current path. During final step  112 , the stud is welded when the electrical current supply initiating signal is generated. 
   As shown in  FIG. 11 , a method of detecting a stud presence in a stud welding gun is provided. At the initial step  114 , components of the stud welding gun are mechanically and electrically joined to partially form a conductivity path. Next, in a step  116 , a conductivity detection circuit is connected in series with the partially formed conductivity path. In a following step  118 , the stud is disposed in the welding gun to close the conductivity path. In a final step  120 , a conductivity current is induced to flow from the conductivity detection circuit through the conductivity path and the stud. 
   As best described in  FIG. 12 , the signals generated in the process control unit  26  (as shown in  FIG. 2 ) during a conductivity path check are identified. At step  130 , a welding subroutine is initiated. Next, during the step  132 , a “start weld” signal is required to be present before a welding sequence can be initiated, and is examined if present. Following at a step  134 , if a “start weld” signal is sensed, a “stud present signal” must exist and is examined if present. If a stud is not present, (i.e., a high voltage is sensed across the conductivity path in the stud welding gun, at a step  136 , a “no stud present” at “head #” fault signal is generated. Following thereafter, during a step  138 , the process control unit  26  monitors for a change in condition, indicated by a “no fault, head #” status signal, and if a stud is thereafter sensed, the subroutine returns to step  132 . In a parallel step  140 , if the voltage across the conductivity path is low (as determined during the step  134 ), which indicates a stud present for welding, a weld cycle is initiated. At a step  142 , the process control unit  26  examines the status of a “weld complete, head #” signal and if complete, returns to step  132  to initiate a further stud welding cycle. Finally, at step  144 , the process ends when all welding is complete and the system is down-powered. 
   Referring finally to  FIG. 13 , an exemplary circuit diagram for the preferred embodiment conductivity current path is shown. A conductivity current path  150  includes stud  14  which closes the conductivity path through stud gun  12  (best shown in  FIG. 4 ) across conductive element  30  and ground connection  40 . When stud  14  closes the conductivity path, a circuit path is completed between conductive element line  44  and ground path line  46  across the conductivity detection circuit  42 . A 24 volt direct current (DC) voltage potential provided by a voltage source  152  in weld tool process control unit  22  causes a small current flow through conductivity detection circuit  42 . The voltage potential across signal line  48  and a ground path line  154  at process control unit  26  is “low”, dropping to approximately zero. Signal line  48  therefore produces a “low” voltage signal, indicating a “stud present” condition. 
   Conversely, if stud  14  is not present, the conductivity path across conductivity detection circuit  42  remains open. The voltage potential across signal line  48  and ground path line  154  at weld tool process control unit  22  is “high” (i.e., approximately 24 volts DC) owing to a resistor  156  connected between the positive terminal of voltage source  152  and signal line  48 . Signal line  48  therefore produces a “high” voltage signal, indicating a “no stud present” condition. The “no stud present” condition or signal is also traceable to a particular stud welding gun  12  or head number (i.e., head #) if more than one stud gun  12  is being monitored concurrently by process control unit  26 . Each “head #” is preassigned. It is noted that resistor  156  is identified as a 3.3 K-Ohm, ¼ Watt resistor. This resistor size is exemplary of a variety of resistor sizes possible for the stud detection system  10  of the present invention, and can vary with voltage potential across voltage source  152  and resistances in the various signal and ground lines, and equipment used. 
   The preferred embodiment for the stud welding gun of the present invention is exemplary in nature. In the preferred embodiment, the conductive element  30  includes a ball biased into contact with the ram by a compression spring. In alternate embodiments, the ball can be replaced by any suitable shape including a cone shape or a cylinder sized to slidably dispose within the contact the ram. The compression spring can alternately include any suitable type of biasing device including a leaf type spring, or an electrically conductive compressible material. The collet is herein described as a copper material due to the reduced expense of copper when the collet requires replacement due to wear. The collet, the receiver, the union nut, the conductive element housing, and the ram can alternately be provided of any suitable electrically conductive material including a copper alloy material, a carbon steel or a stainless steel. The conductive element housing can alternately be welded, press fit, or otherwise mechanically connected to the receiver. 
   The conductive element and conductivity detection circuit of the present invention can be modified to suit alternate stud gun designs without departing from the scope and gist of the present invention, although all of the present advantages may not be achieved. For example, stud welding guns having the collet or chuck electrically isolated from the stud gun body can be modified to have the conductive element connected to the collet or chuck. Studs can also be provided by a mechanical delivery device in place of the pneumatically driven device shown in FIG.  1 . While a preferred and alternate embodiments have been disclosed, it will be appreciated that other configurations may be employed within the spirit and scope of the present invention. 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.