Resistance projection welding system and method for welding a fastener element to a workpiece

Abstract

A system and method is disclosed for monitoring and controlling a resistance projection welder for welding a projection weld nut to a workpiece. A displacement sensor measures the displacement of a welding electrode of a weld gun in real time during the welding procedure. A computer is coupled to the displacement sensor and determines whether the projection weld nut is correctly loaded between the workpiece and the welding electrode. The computer instructs a programmable logic controller (PLC) to trigger a weld controller to apply power to the welding electrode if the projection weld nut is correctly loaded onto the workpiece. The computer instructs the PLC to inhibit power if it determines the projection weld nut to be inverted, missing, or misaligned in order to allow for the removal and replacement of the projection weld nut. The computer further instructs the PLC to control and terminate the power to the welding electrode in real time in order to form a projection weld between the projection weld nut and the workpiece that is substantially similar to a predetermined projection weld. The computer processes the displacement data and provides a fault signal when the projection weld is unacceptable. A marker marks the unacceptable workpieces in response to the fault signal.

Description

This DRAWINGS

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 illustrates a block diagram of a resistance projection welding system 10. Welding system 10 includes a computer 12, a programmable logic controller (PLC) 13, a weld controller 14, an interface 15, a silicon controlled rectifier (SCR) 16, a transformer 17, a weld gun 18, and a displacement sensor 20. As is well known in the art, welding system 10 includes many readily available components. For example, PLC 13 may be obtained from a manufacturer such as Allen Bradley. Weld controller 14 is manufactured by Weltronics, Medar, or Square D. Farrah makes SCR 16. Roman produces transformer 17 and weld gun 18 can be obtained from Milco, Savair, Grossel, OHMA or Centerline.

Weld gun 18 has a first electrode 32 and a second electrode 34. First electrode 32 engages a projection weld nut 36. Second electrode 34 engages a workpiece 38. Projection weld nut 36 is to be welded onto workpiece 38. As is known in the art, weld gun 18 may be a single electrode system.

According to the preferred procedure, computer 12 transmits control signals to PLC 13 through interface 15. Weld controller 14 and weld gun 18 are operatively responsive to PLC 13. Weld controller 14 is provided with SCR 16 which couples weld controller 14 to weld gun 18 via transformer 17. Weld controller 14, SCR 16, and transformer 17 act together to supply electric current to electrodes 32 and 34 in weld gun 18. Displacement sensor 20 monitors the relative position between the weld electrodes in a two electrode weld gun or the position of the electrode relative to the workpiece in the single electrode system. Displacement sensor 20 is a transducer such as a linear variable displacement transducer (LVDT) described in U.S. Pat. Nos. 5,288,968, 5,220,145 and 4,542,277, which are incorporated herein for the purpose of describing the sensor and related control logic. Displacement sensor 20 provides the electrode position data to computer 12 through a device such as an analog-to-digital converter. A vibrational nut feeder 31 responsive to PLC 13 supplies projection weld nuts to weld gun 18. Of course, as illustrated in FIG. 1, a plurality of weld guns may be operated and coordinated by welding system 10. The workpieces move from each weld gun along a production line (now shown). Each weld gun is coupled to an SCR and a displacement sensor. Each weld gun is individually coupled to computer 12, PLC 13, and weld controller 14 as described above.

Briefly describing a general overview of the operation of welding system 10, computer 12 executes a supervisory routine which triggers PLC 13 to perform its functions if projection weld nut 36 is correctly loaded onto workpiece 38. Functions of PLC 13 include opening and closing weld gun 18, automatically positioning weld gun 18 in various positions, turning weld controller 14 on and off, supplying cooling water to weld gun 18, controlling the clamping pressure on electrodes 32 and 34 of weld gun 18, controlling nut feeder 31 for supplying projection weld nut 36 to weld gun 18, and controlling automation for moving workpieces along the production line. In an alternative embodiment, computer 12 can be programmed to directly perform the functions of PLC 13. In another embodiment, computer 12 may be controlled through a computer communication network.

FIGS. 2(a-e) illustrate how computer 12 determines whether projection weld nut 36 is correctly loaded onto workpiece 38. First, FIG. 2a illustrates projection weld nut 36 separated from workpiece 38. Projection weld nut 36 has an upper surface 40 and a lower surface 42. Lower surface 42 has projections 44 that are adapted to bear against workpiece 38. Projection weld nut 36 also has a threaded bore 46 for allowing a bolt or fastener to extend through projection weld nut 36 and an aperture 48 in workpiece 38.

FIG. 2b illustrates a side view of projection weld nut 36 properly welded on workpiece 38. Projections 44 of lower surface 42 have penetrated into workpiece 38 by the intense heat created because of the high density electric current.

FIG. 2c illustrates a side view of projection weld nut 36 welded onto workpiece 38 in an inverted position. Projection weld nut 36 is inverted when upper surface 40 is positioned onto workpiece 38. As discussed earlier, upper surface 40 provides a low resistance path because its entire surface makes electrical contact with workpiece 38. The low electric current density inadequately heats upper surface 40 forming a weak projection weld with workpiece 38.

FIG. 2d is an illustration of a cross-sectional view of electrodes 32 and 34 having a distance d_MAX between them indicative of projection weld nut 36 being correctly positioned on workpiece 38 prior to welding. First electrode 32 has a contact face 50 for engaging projection weld nut 36. Contact face 50 has an annular groove 52 that is the same size as projections 44. Second electrode 34 has a contact face 54 that engages workpiece 38. Electrodes 32 and 34 are shown in a closed welding position forming an electrical welding contact between them through projection weld nut 36 and workpiece 38. The distance between electrodes 32 and 34 is a function of projections 44 supporting projection weld nut 36 above workpiece 38.

FIG. 2e is a cross-sectional view of electrodes 32 and 34 having a distance d_MIN between them indicative of projection weld nut 36 being in an inverted position on workpiece 38 prior to welding. The inverted position allows projections 44 to fit into annular groove 52. Because projections 44 are enclosed by annular groove 52, the distance d_MIN is smaller than the distance d_MAX by the length of projection 44.

As can be appreciated from FIGS. 2(a-e), it can be determined which surface of projection weld nut 36 engages workpiece 38 as a function of the distance between electrodes 32 and 34. Specifically, d_MAX indicates projection weld nut 36 to be correctly loaded. The distance d_MIN indicates that projection weld nut 36 is incorrectly loaded. When the distance between electrodes 32 and 34 is negligible, electrodes 32 and 34 may be directly engaged with each other causing a certain electrical short if electrical power is applied. Also, a distance in a range near d_MIN indicates that projection weld nut 36 has already been welded to workpiece 38. Thus, d_MAX, d_MIN or other distances can be used as an indicator to decide whether to apply or inhibit welding power. Furthermore, if the distance between electrodes 32 and 34 remains constant during the application of power, this could also be used to indicate that projection weld nut 36 is inverted because due to lack of penetration.

With reference to FIGS. 1 and 3, a flow chart for a weld routine executed by welding system 10 is shown. Initially, PLC 13 instructs a production line to advance and load a new workpiece into weld gun 18 as shown in advance block 60. PLC 13 then triggers nut feeder 31 to supply projection weld nut 36 to first electrode 32 in block 62. Upon approval by computer 12, PLC 13 commands electrodes 32 and 34 to shift to the closed welding position in block 64. The step of closing the electrodes encompasses the step of advancing electrodes 32 and 34 of weld gun 18 upon projection weld nut 36 and workpiece 38.

In spacing condition block 66, computer 12 determines the initial distance between first electrode 32 relative to second electrode 34 after receiving the distance data from displacement sensor 20. If the distance between electrodes 32 and 34 is d_MAX, then projection weld nut 36 is correctly loaded onto workpiece 38 and computer 12 will instruct PLC 13 to trigger weld controller 14 for applying weld power as shown in initiate power block 68. SCR 16 applies the power to electrodes 32 and 34 through transformer 17.

If computer 12 determines the distance between electrodes 32 and 34 not to be within an acceptable range of d_MAX, for example, by reading the distance to be d_MIN, then computer 12 instructs PLC 13 to command weld controller 14 not to apply power to electrodes 32 and 34. This sequence is shown in inhibit power block 70. In addition to projection weld nut 36 being inverted, the distance between electrodes 32 and 34 may be outside of the acceptable range of d_MAX because projection weld nut 36 has not been placed on either of electrodes 32 and 34 or because projection weld nut 36 has already been welded to workpiece 38.

After initiating welding power in block 68, computer 12 monitors displacement sensor 20 in monitor displacement block 72 to determine the relative positions of electrodes 32 and 34 in real time during the application of welding current. The positions of electrodes 32 and 34 will change depending upon the penetration of projection weld nut 36 into workpiece 38. Vary power block 74 allows computer 12 to control the weld quality in real time by varying the power applied to electrodes 32 and 34. Computer 12 continuously varies the welding time in order for the displacement of electrodes 32 and 34 to substantially simulate a predetermined displacement. Since projection weld nut strength is a function of electrode displacement, the predetermined displacement can be chosen to correspond to the strongest possible weld. The repeated scanning of displacement sensor 20 over the duration of the weld allows computer 12 to obtain a time sequence of measured positions of electrodes 32 and 34. Computer 12 uses the time sequence displacement data to determine in diagnose block 76 whether the weld has been completed or whether it is defective.

Computer 12 determines the projection weld to be completed when the displacement of electrodes 32 and 34 is substantially similar to the predetermined displacement. Computer 12 then commands PLC 13 to instruct weld controller 14 to inhibit welding power.

If the weld has not been completed, meaning that the displacement of electrodes 32 and 34 has not yet reached the predetermined displacement but is currently within an acceptable tolerance, then the routine branches back to monitor displacement block 70. The cycle of monitoring electrode displacement while applying power continually repeats until the weld is completed or is defective.

With reference still to FIGS. 1 and 3, computer 12 determines the weld to be defective in block 76 when the electrode displacement is not within an acceptable tolerance range of the predetermined displacement. As the case for a completed weld, computer 12 commands PLC 13 to instruct weld controller 14 to inhibit power. Diagnose block 76 then proceeds to analyze reject block 78. Analyze reject block 78 determines why the weld is defective. Possible reasons for defective welds include a plurality of fault conditions such as low projection penetration due to lack of generated heat because of a failed transformer, high penetration due to welding power fluctuations, expulsion, or other characteristics indicative of an inadequate weld. Analyze reject block 78 detects the fault condition by comparing the measured time sequence displacement data with the predetermined displacement.

After determining the fault condition, reject block 78 directs the process to marking block 80 which marks the workpiece having the defective weld. Marking block 80 could mark the defective workpiece with data indicating time, date, count number of the workpiece along the production line or other identifying indicia. Marking block 80 could use paint, bar coding, laser, or stamping to mark the workpieces.

After the projection weld has been completed or rejected, computer 12 instructs PLC 13 to shift electrodes 32 and 34 to the opened position as indicated by open block 82. Also, if computer 12 commanded PLC 13 to inhibit power from weld controller 14 to electrodes 32 and 34 in block 70, then power block 70 also proceeds to open block 82.

Conditional open block 84 determines if electrodes 32 and 34 are shifted to the opened position. If any one of electrodes 32 and 34 are not in the opened position, meaning that at least one electrode is fused to a surface of projection weld nut 36 or workpiece 38 which are now welded together, then a stuck gun alarm is activated in stuck alarm block 86. Further execution of the routine is halted until the stuck gun condition is corrected, as indicated by conditional stuck block 88.

If electrodes 32 and 34 are in the opened position, then computer 12 determines whether power was applied to electrodes 32 and 34 during the sequence of spacing condition block 66 as shown in conditional block 90. If computer 12 inhibited power because it determined the initial distance between electrodes 32 and 34 to be improper due to an inverted projection nut, then the inverted nut is removed from first electrode 32 in block 92. Block 92 then directs the process to block 62 where nut feeder 31 places a new projection weld nut 36 on first electrode 32, thus repeating the welding process on the same workpiece. Of course, the old inverted projection nut could be reoriented instead of being replaced with a new projection nut.

If computer 12 initiated the welding power in block 66, then conditional block 92 proceeds to advance block 60. PLC 13 advances and loads a new workpiece into weld gun 18. The welding process then repeats by placing a new projection weld nut on first electrode 32. Of course, if the previous workpiece was defective, it could be removed from the production line before advancement of all of the workpieces.

Now turning to FIG. 4, a block diagram of welding system 10 with a detailed view of an illustrative weld gun 18 having a first electrode 32 and a second electrode 34 with a retrofitted displacement sensor 20 is shown. First electrode 32 is connected to electrode shaft 120. Electrode shaft 120 is secured by means of an electrode holder 122 which is connected to a terminal 124 for supplying a voltage to first electrode 32 and second electrode 34. Terminal 124 is electrically coupled in a closed circuit with SCR 16. First electrode 32 is positionable by means of fluid-powered cylinder 126. This cylinder consists of a cylinder rod 128 rigidly attached to an electrode positioning bracket 130 and cylinder housing 132.

Displacement sensor 20 comprises electrode positioning bracket 130 and a linear variable displacement transducer (LVDT) 133 used for determining the displacement of first electrode 32 relative to second electrode 34 or to workpiece 38 depending upon whether a two electrode or a single electrode weld gun is utilized. Electrode positioning bracket 130 is secured to electrode holder 122. Electrode positioning bracket 130 is electrically isolated from electrode holder 122 by an insulator 134. This insulation is important, because of the high currents produced in the welding process and the resulting high intensity magnetic fields which will be present. Electrode positioning bracket 130 is stabilized by means of a shaft assembly 136 which consists of a shaft 138, a bushing 137 which holds shaft 138 and is affixed to electrode positioning bracket 130, and shaft clamps 142 which affixes shaft 138 to an arm 144. This shaft 138 eliminates rotation of electrode positioning bracket 130 in the plane radial to cylinder 128. Although shaft assembly 136 is beneficial in providing rotational stability of electrode positioning bracket 130, its inclusion is not required in alternative embodiments of the present invention.

The position of electrode positioning bracket 130 and thus the position of first electrode 32 is determined by LVDT 133. Alternatively, a laser based translational sensor could also be employed. LVDT 133 consists of a transducer body 148 having a cylindrical core receptacle, a transformer core 150 and a core rod 152. Transducer body 148 is attached to arm 144 by means of an isolating clamp 154. Isolating clamp 154 is constructed of phenolic material so as to insulate transducer body 148 from arm 144. This insulation is important, since LVDT 133 is susceptible to induced noise. The high currents and magnetic fields previously mentioned require transducer 133 to be insulated in this manner. For the same reason, transducer body 148 is preferably sheathed in stainless steel which is substantially not magnetic. Further, core rod 152 is preferably constructed of stainless steel.

During the welding process, computer 12 monitors the amount of penetration by projections 44 into workpiece 38 using displacements of electrodes 32 and 34 via displacement sensor 20. FIG. 5a presents the displacement voltage characteristic, as given by LVDT 133 for a representative good projection weld. As time increases, projections 44 penetrate into workpiece 38 as shown by negative displacement slope 160. Weld voltage is cut-off at 162 once sufficient weld penetration is obtained. The weld voltage to weld gun 18 is cut-off after a fixed amount of penetration so as to avoid expulsion, a condition whereby penetration is taken to such a point that the weld material is forced out of a weld junction. Electrodes 32 and 34 are held in place for a short interval of time before being retracted 172 to ensure proper solidification of the weld.

An expansion weld curve is shown in FIG. 5b. This condition occurs when projection weld nut 36 is inverted, presently welded, or projections 44 are not in contact with workpiece 38. There is no penetration by projections 44 into workpiece 38. Electrodes 32 and 34 are then moved to the opened position after the application of power at 168 as shown by positive displacement slope 170 and an appropriate fault signal is provided by computer 12.

If projections 44 do not sufficiently penetrate workpiece 38 after a preset amount of time, as presented by the displacement voltage characteristic in FIG. 5c due to some factor such as a failed transformer or the presence of oil or other contaminants on the surface of projection weld nut 36 or workpiece 38, the weld voltage is cut-off at 172 and computer 12 indicates a fault. If the displacement of electrode 32 and 34 increases dramatically as presented by the displacement voltage characteristic in FIG. 5d at 174, due to some factors such as over heating projection weld nut 36 at the end of the cycle time, the weld voltage is cut-off and a fault is indicated.

A further condition that can be detected using an embodiment of the present invention is the condition of electrode cladding. This condition is of concern when resistance welding workpieces made of aluminum. The affinity of aluminum to a copper electrode results in an impregnation of molten aluminum to the electrode during welding. When a significant amount of aluminum impregnates into the electrode, the possibility exists of blowing a hole in the workpieces during the next weld. Typically, the exhibited fit-up behavior leading up to such a condition is one where steady wear on the electrode contact face is present for a number of welds, followed by a significant apparent growth on the electrode contact face after a single weld. In addition to space gauging, the present invention can examine the differences in the spacing of electrodes 32 and 34 when they initially form the closed welding position over successive welds to detect the condition of electrode cladding. After detecting such a condition, the electrode can be either replaced or redressed before any further welds are performed by the electrode.

In order to appreciate how the welding system and method of the present invention is not limited to a specific type of projection weld nut, an alternative projection weld nut and electrode assembly is shown in cross sectional view in FIG. 6a. First electrode 200 has a contact face 202 with a shaft 204. Contact face 202 further includes a groove 206 extending around shaft 204. A projection weld nut 208 has an upper surface 210 and a lower surface 212. Projections 214 extend outward from lower surface 212. Projection weld nut 208 has a bore 216 engaging shaft 204 of first electrode 200. A ring 218 on upper surface 210 engages groove 206. Bore 216 is aligned with aperture 48 to allow a bolt or a fastener to extend through aperture 48. A second electrode 224 has a contact face 226 with a hollowed-out portion 228.

Projection weld nut 208 and workpiece 38 are engaged electrodes 200 and 224. Portion 228 is shaped in such a way as to avoid contact with shaft 204 of first electrode 200 in order to prevent electrical shorts.

FIG. 6b shows an exploded cross sectional view of the alternative projection weld nut and electrode assembly shown in FIG. 6a taken along the line X-X'.

As can be appreciated, the distance between electrodes 200 and 224 will vary depending upon which surface of projection weld nut 208 engages workpiece 38. These spacing differences can be used by computer 12 to determine whether or not to apply power to electrodes 200 and 224.

FIG. 7a shows a side elevation view of yet another embodiment of a projection weld nut. A projection weld nut 230 is shown separated from workpiece 38. Projection weld nut 230 has projections 232 extending from a lower surface 233. Projection weld nut 230 has a bore 234. A pilot ring 236 extends from lower surface 233 and runs substantially around bore 234.

FIG. 7b is a side view of projection weld nut 230 properly welded onto workpiece 38. Pilot ring 236 fits snugly into aperture 48. Pilot ring 236 is used to properly align projection weld nut 230 with workpiece 38.

FIG. 7c is a cross-sectional view of a first electrode 240 and a second electrode 242 engaging projection weld nut 230 and workpiece 38 having a distance d'_MIN between them prior to welding. The distance d'_MIN indicates that projection weld nut 230 is correctly positioned on workpiece 38.

FIG. 7d is a cross-sectional view of electrodes 240 and 242 having a distance d'_MAX between them indicative of projection weld nut 230 being in an inverted position on workpiece 38. Pilot ring 236 engages first electrode 240 causing the separation between electrodes 240 and 242 to be equal to the distance d'_MAX. Note that d'_MAX is greater than d'_MIN by a distance equal to the length of pilot ring 236 extending from projection weld nut 230.

FIG. 7e is a cross-sectional view of projection weld nut 230 being loaded laterally off position with respect to aperture 48 of workpiece 38. Pilot ring 236 is not properly aligned with aperture 48 causing the distance between electrodes 240 and 242 to be different then d'_MIN. As taught in the preferred embodiment, distance d'_MAX, d'_MIN or other distances can be used by computer 12 as an indicator of whether projection weld nut 230 is properly aligned and in correct contact with workpiece 38, whether projection nut 230 is missing from workpiece 38 or whether projection nut 230 has already been welded to workpiece 38.

In addition to the preferred and alternative embodiments of the present invention, one skilled in the art will see application of this technology beyond the realm of projection weld nut configurations herein described. It should be noted that the present invention may be used in a wide variety of different constructions encompassing many alternatives, modifications, and variations which are apparent to those skilled in the art. Accordingly, the present invention is intended to embrace all such alternatives, modifications, and variations as found within the spirit and broad scope of the appended claims.

Claims

1. A welding system for welding a weld nut to a workpiece, wherein the weld nut has a first face provided with at least one projection and an opposed second face, the system comprising:

a power source for supplying power;

a weld gun having a welding electrode coupled to the power source;

a displacement sensor coupled to the weld gun for generating a signal responsive to an orientation of the welding electrode relative to the workpiece; and

a programmable logic controller (PLC) coupled to the power source and the displacement sensor, the PLC processes the displacement sensor signal to determine which face of the weld nut engages the workpiece, wherein the PLC transmits a control signal to the power source to prevent the initiation of power if the second face of the weld nut engages the workpiece.

2. The system of claim 1 further comprising:

a vibrational weld nut feeder coupled to the PLC and the weld gun for supplying a steady stream of weld nuts to the welding electrode upon command of the PLC.

3. The system of claim 1 wherein:

the welding electrode has an annular groove which fits over the at least one projection of the weld nut when the welding electrode engages the first face causing the distance between the welding electrode and the workpiece to be smaller than the distance between the welding electrode and the workpiece when the welding electrode engages the second face.

4. The system of claim 1 wherein:

the displacement sensor comprises a linear variable displacement transducer.

5. The system of claim 1 wherein:

the weld gun further comprises a ground electrode.

6. A welding system for welding a weld nut to a workpiece, wherein the weld nut has a first face provided with a pilot ring and a second face, the system comprising:

a power source for supplying power;

a weld gun having a welding electrode coupled to the power source;

a displacement sensor coupled to the weld gun for generating a signal responsive to an orientation of the welding electrode relative to the workpiece; and

a programmable logic controller (PLC) coupled to the power source and the displacement sensor to process the displacement sensor signal for determining which face of the weld nut engages the welding electrode, wherein the PLC transmits a control signal to the power source to prevent the initiation of power if the first face of the weld nut engages the welding electrode.

7. The system of claim 6 wherein:

the PLC transmits a control signal to the power source to initiate power if the second face of the weld nut engages the welding electrode and the pilot ring abuts and is properly aligned with a receiving portion of the workpiece.

8. A welding system for welding a desired face of a weld nut to a workpiece, wherein the weld nut has first and second dissimilar faces, the system comprising:

a power source for supplying power;

a weld gun having a welding electrode coupled to the power source;

a displacement sensor coupled to the weld gun for generating a signal responsive to an orientation of the welding electrode relative to the workpiece; and

a PLC coupled to the power source and the displacement sensor to process the displacement sensor signal for determining which face of the weld nut engages the workpiece, wherein the PLC transmits a control signal to the power source to prevent the initiation of power if the welding electrode engages the first face of the weld nut.

9. The system of claim 8 wherein:

the PLC transmits a control signal to the power source to initiate power if the welding electrode engages the second face of the weld nut.

10. The system of claim 8 wherein:

the PLC transmits control signals to the power source to vary the power in real time based upon the displacement sensor signal such that the orientation substantially simulates a predetermined orientation sequence.

11. For use with a weld gun having a welding electrode, a method for welding a weld nut to a workpiece, wherein the weld nut has a first face provided with at least one projection and an opposed second face, the method comprising:

generating a signal responsive to an orientation of the welding electrode relative to the workpiece;

processing the signal to determine which face of the weld nut engages the workpiece; and

preventing the initiation of power if the second face of the projection weld nut engages the workpiece.

12. A welding system for welding a fastener element to a workpiece, wherein the fastener element has a first face provided with at least one projection and an opposed second face, the fastener element further having a central fastening portion for receiving a cooperating threaded member, the system comprising:

a power source for supplying weld power;

a weld gun having a welding electrode coupled to the power source, the welding electrode having a contact face configured to engage a face of the fastener element;

a displacement sensor coupled to the weld gun for generating a signal responsive to an orientation of the welding electrode relative to the workpiece; and

a processor coupled to the power source and the displacement sensor, the processor processes the displacement sensor signal to determine if the spacing between the welding electrode and the workpiece falls within an acceptable spacing region when a face of the fastener element engages the workpiece prior to welding, wherein the processor transmits a control signal to the power source to prevent the initiation of weld power if the spacing falls outside of the acceptable spacing region.

13. The system of claim 12 wherein:

the weld gun further comprises a ground electrode configured to engage the workpiece.

14. The system of claim 12 wherein:

spacing between the welding electrode and the workpiece falling within the acceptable spacing region prior to welding is indicative of the first face of the fastener element engaging the workpiece.

15. The system of claim 12 wherein:

the processor transmits a control signal to the power source to enable the initiation of weld power if the spacing falls within the acceptable spacing region.

16. For use with a weld gun having a welding electrode, a method for welding a fastener element to a workpiece, wherein the fastener element has a first face provided with at least one projection and an opposed second face, the fastener element further having a central fastening portion for receiving a cooperating threaded member, the method comprising:

generating a signal responsive to an orientation of the welding electrode relative to the workpiece when the welding electrode engages a face of the fastener element;

processing the signal to determine if the spacing between the welding electrode and the workpiece falls within an acceptable spacing region when a face of the fastener element engages the workpiece prior to welding; and

preventing the initiation of weld power if the spacing falls outside of the acceptable spacing region.

17. The method of claim 16 further comprising:

transmitting a control signal to the power source to enable the initiation of weld power if the spacing falls within the acceptable spacing region.

18. A welding system for welding a projection fastener to a workpiece, wherein the projection fastener has a first face provided with a projection and an opposed second face, the system comprising:

a pair of welding electrodes one of which cooperating with the projection fastener to be welded;

a displacement sensor operable with at least one of the welding electrodes for generating a signal responsive to an orientation of the welding electrode cooperating with the projection fastener and the workpiece; and

a processor operable with the displacement sensor for processing the displacement sensor signal to determine which face of the projection fastener engages the workpiece;

wherein the processor generates a control signal output indicative of an orientation of the projection fastener which terminates welding power to the welding electrode if the second face of the projection fastener engages the workpiece so that the projection fastener is not improperly welded.

19. The system of claim 18 wherein:

the control signal output causes welding power to be supplied to the welding electrode only if the first face of the projection fastener engages the workpiece.

20. The system of claim 18 wherein:

the control signal output causes welding power to the welding electrode to be inhibited if the second face of the projection fastener engages the workpiece.

21. The system of claim 18 wherein:

the projection fastener is a weld nut.

22. The system of claim 18 wherein:

the processor includes a programmable logic controller (PLC).

23. The system of claim 18 wherein:

the processor includes a computer.

24. A method for welding a fastener element to a workpiece using a pair of welding electrodes coupled to a weld power source, wherein the fastener element has a first face provided with a projection to be welded to the workpiece and an opposed second face which cooperates with one of the weld electrodes, the fastener element further having a central fastening portion for receiving a cooperating fastener member, the method comprising:

generating a signal indicative of the spacing between the welding electrode cooperating with the fastener and the workpiece when the fastener engages both the workpiece and an electrode; and

processing the signal to determine if the first face of the fastener provided with a projection is properly oriented relative to the workpiece; and

terminating the application welding power by the weld power source to the welding electrodes if the first face of the projection fastener is not properly oriented to the workpiece so that the projection fastener is not welded if the fastener is misoriented.

25. The method of claim 24 wherein terminating the application welding power comprises providing a fault signal inhibiting the application welding power to the welding electrodes by the weld power source if the first face of the projection fastener is not properly oriented to the workpiece.

26. The method of claim 24 wherein terminating the application welding power comprises providing a weld signal causing the application welding power to the welding electrodes by the weld power source only if the first face of the projection fastener is properly oriented to the workpiece.