Abstract:
The invention relates to a short-time arc welding system ( 10 ) and method for welding elements ( 32 ) on pars ( 34   a ). The system is equipped with: a robot ( 12 ) having at least one arm ( 16 ) that can move in at least one coordinate axis (x,y,z); a welding head ( 22 ), which is mounted on the arm ( 16 ) and on which a holding device ( 30 ) for holding an element ( 32 ) and a lifting device ( 36 ) for advancing and returning the holding device ( 30 ) with regard to the welding head ( 22 ) are provided, and; a measuring system ( 31   a,    44, 46 ) for determining the relative position between a part ( 34 ) and an element ( 32 ), which is to be welded onto the part ( 34 ) and is held by the holding device ( 30 ). The measuring system has a stand ( 31   a ), which is mounted on the welding head ( 22 ) and which is designed to be, during operation, in contact with the part ( 34   a ) in order to determine the relative position between the element ( 32 ) and the part ( 34 ). The inventive system is also equipped with a storage device ( 46 ), inside of which a number of welding positions can be stored. The measuring system additionally comprises positioning means ( 50 ) for positioning the stand ( 31   a ) with regard to an element ( 32 ) held inside the welding head ( 22 ) in a multitude of at least two different operating positions (a-c), and the measuring system ( 46 ) is designed for positioning the stand ( 31   a ) each time into an appropriate operating position for each stored welding position.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application is a continuation of PCT/EP03/01468, filed Feb. 14, 2003 and designating the U.S., which was not published under PCT Article 21(2) in English, and claims priority to German application DE 102 07 726.6, filed Feb. 20, 2002, whose disclosures are incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION  
       [0002]     The present invention relates to a short-time arc welding system for welding elements such as metal studs onto components such as metal sheets, having: 
        a robot which has at least one arm which can move on at least one co-ordinate axis,     a welding head which is mounted on the arm and on which a holding apparatus for holding an element, and a linear-movement apparatus for moving the holding apparatus forwards and backwards relative to the welding head are provided,     a measurement system for determining the relative position between a component and an element which is to be welded onto the component and is held by the holding apparatus, with the measurement system having a foot which is mounted on the welding head and is designed, during operation, to make contact with the component in order to determine the relative position between the element and the component, and     a memory device in which a number of welding positions can be stored.        
 
         [0007]     The present invention also relates to a corresponding method for short-time arc welding. A short-time arc welding system such as this and an associated method are generally known.  
       BACKGROUND OF THE INVENTION  
       [0008]     During short-time arc welding, an element is welded onto a component. In this case, an arc is formed between the element and the component, which melts the end surfaces. The element and the component are then moved towards one another, so that the melts mix. The arc is short-circuited, and the entire melt solidifies.  
         [0009]     Normally, the arc is drawn (welding with drawn arc). In this case, the element is first of all placed on the component. A pilot current is then switched on, and the element is raised to a desired height above the component, with an arc being drawn. The welding current is not switched on until this point.  
         [0010]     In order to achieve constantly good welding results, it is important, inter alia, to know the relative position between the element and the component, in particular to raise the element to the correct height before the welding current is switched on. To this end, the relative position is generally measured before each welding process, in particular in the form of a null position being determined.  
         [0011]     This is particularly important for robot-based systems. Modern robots are admittedly generally able to position themselves comparatively precisely. However, particularly owing to the large moving masses, high precision cannot be achieved at the same time as a very fast dynamic response. The robots can normally move in three co-ordinates. In the simplest case, a robot is a linear guide (slide) which is driven in an automated manner and on which a welding head is mounted.  
         [0012]     Stud welding systems are used in particular in the motor vehicle industry where they are used, in particular, to weld elements such as bolts or studs with or without a thread, eyelet, nuts etc. to the sheet-metal bodywork. These elements are then used as holding anchors in order, for example, to fix internal linings.  
         [0013]     The production speed is a major factor in the motor vehicle industry. Hundreds of the elements must be welded automatically by means of robots at different positions within a few minutes. In consequence, the robots have to have a short-time dynamic response.  
         [0014]     It is thus known for a welding head base supporting a slide to be fitted to the arm of a robot. The slide can be moved highly dynamically with high precision, normally by means of a pneumatic or hydraulic system. The actual welding head is mounted on the slide and itself has a linear movement apparatus for moving the element.  
         [0015]     In order to determine the relative position between the element and the component, it is known for a so-called supporting foot to be attached to the welding head (for example from “Neue TUCKER-Technologie. Bolzenschweiβen mit System”, Emhart Tucker, September 1999). The supporting foot is aligned approximately parallel to the holding apparatus of the welding head. In an initial position, the element which is held in the holding apparatus projects somewhat beyond the supporting foot.  
         [0016]     In order to determine the relative position, the welding head is moved towards the component. In the process, the element first of all makes contact with the component. The welding head is fed further forwards until the supporting foot makes contact with the component. In the process, the holding apparatus is generally offset relative to the welding head, against an elastic biasing force. The interlocking contact between the supporting foot and the welding head, together with a suitable measurement system in consequence allow the relative position between the element and the component to be determined.  
         [0017]     As an alternative to this, so-called measurement systems without supporting feet are also known for determining the relative position between the element and the component. For example, U.S. Pat. No. 5,252,802 discloses a bolt welding apparatus with a housing which is in the form of a handheld pistol. A position motor first of all moves the housing to a position in which a bolt is arranged in the vicinity of a component. A linear motor is provided in the housing, in order to move a linear-movement shaft, which carries the stud, axially. A movement measurement system is provided in order to control the linear motor. To determine the relative position between the stud and the workpiece, the linear motor is driven in order to move the stud towards the workpiece at a predetermined speed. As soon as the stud touches the workpiece, an electrical contact is closed. Furthermore, it is known from WO 96/11767 for the stud holder to be elastically biased in the direction of the workpiece, and to be moved axially against the bias force by means of a linear motor.  
         [0018]     WO 96/05015 also discloses a stud welding apparatus without a supporting foot. However, stud welding without a supporting foot has the disadvantage that, particularly if the components are thin metal sheets, it is not always possible to exactly maintain an exact position relationship between this component and the element or bolt. This is due to the fact that thin sheet metal is frequently bent somewhat when the bolt makes contact with it. As soon as the bolt is lifted off the metal sheet again during the process of raising it to draw an arc, the metal sheet moves back, and the exact position relationship is lost.  
         [0019]     Essentially, this problem does not exist in stud welding with a supporting foot. This is because the supporting foot makes contact with the component or metal sheet throughout the entire welding process. In consequence, this mechanical coupling always makes it possible to maintain an exact position relationship.  
         [0020]     However, stud welding with a supporting foot has the disadvantage that the supporting foot, which is necessarily arranged close to the stud, increases the amount of space required for stud welding. Furthermore, spot welds must generally be arranged where the component is planar. In the region of steps or depressions, it is frequently necessary to rotate the arm of the robot so that the supporting foot does not impede the movement of the welding head. This means that the supply lines to the welding head are subject to high stress levels resulting from torsion, compression etc.  
       SUMMARY OF THE INVENTION  
       [0021]     Against the above background, one object of the present invention is to specify an improved short-time arc welding system and method for short-time arc welding. In the case of the short-time arc welding system mentioned at the outset, this object is achieved in that the measurement system furthermore has positioning means in order to position the foot in a large number of at least two different operating positions relative to an element which is held in the welding head, and in that the measurement system is designed to position the foot in a respectively suitable operating position for each stored welding position.  
         [0022]     In the case of the short-time arc welding method mentioned at the outset, the object is achieved by using a short-time arc welding system for the method, which has a control device in which a respective suitable operating position for a foot is stored for a number of welding processes which can be carried out automatically, which foot is designed to make contact with the component in order to determine the relative position between the element and the component, in which the method, furthermore, has the following steps: 
        a) actuation of a robot with an arm such that a welding head which is fixed on the arm is moved to a welding position for a chosen welding process,     b) positioning of the foot in an operating position which is suitable for the chosen welding process,     c) movement of the element towards the component until the foot makes contact with the component, and     d) carrying out the chosen welding process.        
 
         [0027]     The invention makes it possible to carry out stud welding with a supporting foot on components with any desired contour. In the present context, the expression of an operating position for a foot in each case means a position of the foot in which the foot is used to determine the relative position between the element and the component. In consequence the foot is designed to make contact with the component in every operating position, in contrast to being in a rest position.  
         [0028]     This makes it possible to make use of the advantages of stud welding with a supporting foot for all welding processes. The relative movement capability between the supporting foot and the stud makes it possible to carry out welding processes at any desired points without the supporting foot being “in the way”.  
         [0029]     It is particularly advantageous for the positioning means to be designed such that the foot can be positioned in a large number of positions relative to the element being held, approximately parallel to a welding axis along which the elements are welded to the components. This makes it possible to weld on welding elements even in regions in which there are depressions in the components. The relative “height” of the supporting foot can then be matched to the depth of this depression.  
         [0030]     According to a further preferred embodiment, positioning means are designed to position the foot in at least two different positions relative to the element being held, about an axis which is parallel to a welding axis along which the elements are welded to the components. These features are regarded as an invention of their own, in conjunction with the precharacterizing clause of the main claim.  
         [0031]     As a result of the measure of designing the foot such that it can move around a parallel axis, in particular around the welding axis itself, it is possible to weld on elements even in the region of steps or edges of the components, without the robot arm having to carry out extreme movements. In consequence, the supply lines to the welding head at the end of the robot arm are protected against excessive stresses such as compression, twisting etc. Furthermore, since the extent of the robot movements can be reduced, it is possible to achieve a shorter cycle time between individual welding processes.  
         [0032]     Furthermore, it is preferable for the positioning means to have a foot movement sensor which detects the position of the foot. This makes it possible to control the positioning of the foot relative to the element held in the welding head. This makes it possible to achieve a high positioning accuracy even in the case of frequent repetitions. Since the foot length can vary over the course of time owing to wear, erosion or material deposits etc., a closed control system for foot positioning also allows regular calibration. This makes it possible to compensate for changes to the foot length.  
         [0033]     In a further preferred embodiment, the welding head has elastic means in order to elastically bias the holding apparatus in the feed direction. This measure allows the linear-movement apparatus for moving the holding apparatus forwards and backwards, to be held in most operating states without consuming any energy. This reduces the energy consumption.  
         [0034]     In a further preferred embodiment, the measurement system has a movement sensor which detects the movement of the holding apparatus relative to the welding head. Furthermore, it is preferable for the positioning means to be designed to position the foot relative to the welding head. Since an element which is held by the holding apparatus generally has a known position relationship with the welding head, the relative movement between the foot and welding head can be used to determine an exact position relationship between the foot and the element. Alternatively, relative positioning between the foot and holding apparatus is feasible.  
         [0035]     Furthermore, it is advantageous for the positioning means to have a motor and, in the rest state, for the motor to apply a resistance torque which is preferably greater than approximately 150 N with respect to axial motor movements. It is particularly preferable for the resistance torque to be more than 200 N. When it is approaching the component, the foot is moved with a not inconsiderable force until the foot makes contact with the component. A motor with the intended resistance torque with respect to motor movements from the rest state allows the selected position to be maintained even if the foot strikes the component relatively hard. In consequence, the position which is set in advance by the motor is maintained.  
         [0036]     According to a further preferred embodiment, the positioning means have clamping means in order to fix the foot in the axial direction with respect to the element being held, in the rest state. In this embodiment, it is not absolutely essential for a motor for the positioning means to have a high resistance torque with respect to axial movements. In addition to, or instead of, this, the clamping means ensure that the foot remains in the position set by the motor even if the foot strikes the component with a high force, or at a high speed.  
         [0037]     In the method according to the invention for short-time arc welding, it is furthermore advantageous for steps a) and b) to be carried out such that they overlap. This allows the supporting foot to be moved in a time-saving manner to the respectively suitable foot position during the movement of the robot to a new welding position.  
         [0038]     It is self-evident that the features mentioned above and those which are still to be explained in the following text can be used not only in the respectively stated combination, but also in other combinations or on their own, without departing from the scope of the present invention. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0039]     Exemplary embodiments of the invention are explained in more detail in the following description and are illustrated in the drawing, in which:  
         [0040]      FIG. 1  shows a schematic view of a first embodiment of a short-time arc welding system according to the invention;  
         [0041]      FIG. 2  shows a schematic view of the welding head of the short-time arc welding system, shown in  FIG. 1 , from below;  
         [0042]      FIG. 3  shows a schematic illustration of a welding process in a depression of a component; and  
         [0043]      FIG. 4  shows a schematic illustration of a welding process on a shoulder of a component. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0044]     In  FIG. 1 , a first embodiment of a short-time arc welding system according to the invention is annotated  10  in general. The short-time arc welding system  10 , which is referred to for short in the following text as the stud welding system  10 , contains a robot  12 . The robot  12  has a rotating head  14 , by means of which a single-joint or multiple-joint arm  16  can be rotated. Overall, the robot  12  is designed to move the end of the arm freely on three co-ordinate axes x, y, z.  
         [0045]     A welding head base  20  is fitted to the end of the robot arm  16 . The welding head base  20  supports a slide  21 , which can be moved in a reciprocating manner parallel to an axis  26  relative to the welding head base  20 . A welding head  22  is mounted on the slide  21 . A pneumatic arrangement  24  is used to move the welding head  22  in a reciprocating manner with respect to the welding head base  20  by means of the slide  21 . The welding head  22  has a holding apparatus  30 , which is designed to hold a metal element such as a bolt  32 , detachably. For this purpose, the holding apparatus  30  has suitable clamping means, which are not shown in any greater detail in  FIG. 1 .  FIG. 1  furthermore shows a metal component, such as a metal sheet  34 , which is essentially aligned at right angles to the axis  26 . A supporting foot  31  is furthermore provided on the welding head base  20 .  
         [0046]     The welding head  22  furthermore has a linear-movement apparatus  36 . The linear-movement apparatus  36  is used to cause the holding apparatus  30  to move in an axial direction, which is aligned parallel to the axis  26 , with respect to the welding head  22 . The linear-movement apparatus  30  has a linear movement stroke  37  which may be, for example, in the range between 8 mm and 20 mm, in particular in the range between 10 mm and 15 mm. In comparison to this, the linear movement stroke of the pneumatic arrangement  24  may be in the range between 2 cm and 10 cm, in particular in the range between 4 cm and 6 cm.  
         [0047]     As a rule, the linear-movement apparatus  36  is formed by a linear motor. In the illustrated embodiment, the linear-movement apparatus  36  is, however, formed by a spring and magnet system. The welding head  22  has a permanent magnet  38 . A coil  39 , which can be energized in a controlled manner, is formed on a sleeve-shaped section of the holding apparatus  30 . When energized, the holding apparatus  30  is drawn into the welding head  22 , against the influence of a compression spring  40 . The compression spring  40  acts, on the one hand, on the welding head  22  or on the moving part of the slide  21 , and, on the other hand, on the holding apparatus  30 .  
         [0048]     The welding head  22  also has a movement sensor  44 , which is indicated only schematically in  FIG. 1 . The movement sensor  44  is used to detect the relative position between the holding apparatus  30  and the welding head  22 . For this purpose, the movement sensor  44  may have a code reader on the holding apparatus  30 , which reads a linear code on the welding head  22  (or vice versa).  
         [0049]     A control device  46  is also provided. The control device  46  is connected to the robot  12  and to the pneumatic arrangement  24 , to the linear-movement apparatus  36 , and to the movement sensor  44 . The control device  46  is used to control the moving elements of the bolt welding system  10 , in a manner coordinated with respect to one another, and to close-loop control their movement, speed and/or acceleration on the basis of the signals from the movement sensor  44 .  
         [0050]     The relative position between the element  32  and the component  34  is in general determined as follows. In an initial position, the coil  39  is not energized. The holding apparatus  30  protrudes from the welding head  22  to the maximum extent. The component  32 , in this situation, projects somewhat beyond the axial end of the supporting foot  31 . The welding head  22  is then moved towards the component  34  by means of the actuating drive  24 , until the element  32  makes contact with the component  34 . This movement is continued until the axial end of the supporting foot  31  makes contact with the component  34 . The movement of the actuating drive  24  is interrupted at this point. This position is shown in  FIG. 1 .  
         [0051]     The welding process is then carried out as normal, by supplying a pilot current through the element  32  and through the component  34 , with the element  32  then being raised off the component  34 . The supporting foot  31  prevents the “component  34  from bouncing back”. As soon as the element  32  has been raised through a suitable distance from the component  34 , the actual welding current is switched on. The mutually opposite surfaces melt. The bolt  32  is then lowered onto the component  34 , generally to below the position illustrated in  FIG. 1 , so that it enters the melt of the component  34 . On reaching the component  34 , the welding current is switched off. The entire melt solidifies, and the welding process is complete.  
         [0052]     It is self-evident that, if a linear motor is used as the linear-movement apparatus  36 , it is also possible for the welding head  22  to approach the component  34 , with the holding apparatus  30  being located in a drawn-back position, that is to say with the element  32  not projecting beyond the axial end of the supporting foot  31 . In this situation, the welding head  22  approaches until the supporting foot  31  makes contact with the component  34 . The linear motor of the linear-movement apparatus  36  is then operated, in order to lower the element  32  as far as onto the component  34 . Both methods ensure that there is an exact position relationship between the element  32  and the supporting foot  31  or component  34  throughout the entire welding process.  
         [0053]     According to the invention, the supporting foot  31  is designed such that it can be moved by means of an actuating drive  50 . The actuating drive  50  is provided in a drive housing  52 , which is mounted on an outer face of the welding head  22  such that it can move. As is shown in  FIG. 1 , the actuating drive  50  is able to move the supporting foot  31 , which is mounted rigidly on the drive housing  52 , up and down in a direction  54  approximately parallel to the welding axis  26 . In this case, it is preferable for the actuating drive  50  to have a stepping motor, which can move the supporting foot  31  to a large number of different axial positions relative to the welding head  22 , in increments which are generally less than 1 mm.  
         [0054]     As is shown in  FIG. 2 , which illustrates the welding head  22  from below, the drive housing  52  can be moved together with the supporting foot  31  that is mounted on it, around the pivoting axis  26 , as is indicated schematically at  56 . The range of movement is generally between 90 and 270° and is 180° in the illustrated exemplary embodiment. The capability of the supporting foot  31  to move relative to the welding head  22 , and hence also relative to the element  32  allows welding processes to be carried out on any desired contours of the component  34 .  
         [0055]     Furthermore, a clamping device  58  is indicated schematically in  FIG. 1 . The clamping device  58  is used to clamp the supporting foot  31  firmly in a respective operating position on the welding head  22 . The clamping effect in this case acts primary in the axial direction. This prevents the supporting foot  31  from being moved out of the operating position relative to the welding head  22  when the welding head  22  is moved towards the metal sheet  34  and the supporting foot  31  touches the metal sheet  34 .  
         [0056]     If the actuating drive  50  applies a sufficiently large resistance torque with respect to axial movements of the supporting foot  31  relative to the welding head  22  in the rest state, there is no need for the clamping device  58 . The holding force from the motor (its resistance torque) should preferably be greater than approximately 150 N and particularly preferably greater than 200 N.  
         [0057]      FIG. 1  also shows a foot movement sensor  60 . The foot movement sensor  60  is designed to detect the relative position between the supporting foot  31  and the welding head  22 . In the simplest case, the foot movement sensor  60  detects a marking  62  on a part of the supporting foot  31  or of the drive housing  52 , which can move relative to the welding head  22 . The marking which is indicated at  62  in  FIG. 1  is used for detecting the axial position of the supporting foot  31  with respect to the welding head  22 .  FIG. 2  also shows that a crown  64 , which rotates with the drive housing  52 , is provided on the welding head  22 . A further marking  66  is provided on the crown  64 , and can be detected by the movement sensor  60 . In consequence, the foot movement sensor  60  can also detect the angular position of the supporting foot  31  with respect to the welding head  22 .  
         [0058]     It is self-evident that the illustrated embodiments of foot movement sensors are only by way of example. The critical factor is that the foot movement sensor  60  can detect the relative position between the supporting foot  31  and the welding head  22 . The information about this relative position is supplied to the control device  46 . The control device  46  has a closed control loop, in order to set the respectively desired operating position of the supporting foot  31  with respect to the welding head  22 . It is self-evident that this control loop can be calibrated in order to compensate for changes in the effective length of the supporting foot  31 , for example resulting from wear, erosion, material deposits, or the like.  
         [0059]     It is furthermore self-evident that the movement measurement system for detecting the relative position between the supporting foot  31  and the welding head  22 , and the movement measurement system  44  for detecting the relative position between the bolt  32  and the welding head  22  can be matched to one another. In consequence, the relative position of all the critical moving elements of the welding head base  22  is known at all times.  
         [0060]     With regard to the contour of the component  34 ,  FIG. 1  shows a standard situation, which is identified by the index “a”. The component  34  is virtually planar in the region of the welding point. In consequence, the supporting foot  31  is located in a position in which known welding systems have mounted the supporting foot rigidly.  
         [0061]      FIG. 3  shows, in a schematic form, a welding process for a bolt-like element  32  in a groove in a component  34   b . The groove is so narrow that the supporting foot  31  makes contact in the edge region of the groove, as is shown at  31   b . The position  31   b  is likewise shown by dashed lines in  FIG. 1 .  
         [0062]     A welding process with the supporting foot would not have been possible with known welding systems. However, in the present case, the supporting foot  31  is moved to a suitable position, namely the position  31   b  as determined by a teach-in process, before the welding head  22  approaches the component  34   b . It then makes its approach in the same way as that described above.  
         [0063]      FIG. 4  illustrates a situation in which a welding bolt is intended to be placed in the vicinity of a shoulder on a component  34   c . In this embodiment, the shoulder is located at the point at which the supporting foot  31  would conventionally be located if the welding head were to approach the shoulder in a straight line. In consequence, in the prior art, the robot  12  would have to rotate the welding head  22  through at least 90° before making its approach. This would result in the supply lines to the welding head  22  being twisted and possibly compressed. In the present case, the welding head  22  can be guided on a straight path directly to the welding point. Even before its approach or during the movement of the welding head base  22  by means of the robot  12 , the supporting foot  31  is pivoted through 180° by means of the actuating drive  50 , to the position  31   c  shown in  FIG. 4 .  
         [0064]     In an alternative process for determining the relative position between the element  32  and the component  34  without a supporting foot, the supporting foot  31  is moved to a rest position, and is not used. In this method, which is not illustrated in any more detail in the figures, the relative position is determined, for example, as follows: 
        First of all, the control device  46  is electrically connected to the stud  32  via cables which are not illustrated in any more detail. Furthermore, the control device  46  is able to apply a measurement voltage to the stud  32 . For example, the component  34  may be grounded, so that it is possible to use a suitable current measurement apparatus to detect the point of time when the element  32  makes electrical contact with the component  34 .        
 
         [0066]     The robot  12  is first of all actuated in order to move the welding head base  20  by means of the rotary head  14  and the robot arm  16  to a basic welding position, which is shown in  FIG. 1 . In this position, the welding head base  20  is located at a specific distance from the component  34 , with the axis  26  being at right angles to the desired welding position on the component  34 .  
         [0067]     The pneumatic arrangement  24  is then operated, so that the welding head  32  is moved in the direction towards the component  34 , to be precise through its full stroke, as far as a head welding position. In the final position, the element  32  is located at a distance from the component  34  which is shorter than the maximum stroke  37  of the linear-movement apparatus  36 .  
         [0068]     The linear-movement apparatus  36  is then operated such that the element  32  is moved towards the component  34 , until it makes contact with the component  34 . This movement is preferably carried out at a constant speed. In this case, the distance traveled is detected by means of the movement sensor  44 . As soon as the element  32  makes contact with the component  34 , a circuit created by the measurement voltage is closed. This is detected by the control device  46 , and the linear-movement apparatus  36  is stopped.  
         [0069]     Furthermore, the contact position between the element  32  and the component  34  at this time is used as the “null position” for the rest of the welding process. In consequence, the exact relative position between the element  32  and the component  34  is in each case known throughout the entire subsequent welding process via the movement sensor  44 , provided the component  34  does not “bounce back” when the element  32  is lifted off. The welding process can in consequence be carried out independently of any tolerances that there may be in the positioning by the robot  12  or by the pneumatic arrangement  24 , with the desired position relationship between the element  32  and the component  34 .  
         [0070]     The actual stud welding process is then carried out in a manner known per se. In this case—once the measurement voltage has been switched off—a pilot current is applied to the element  32 . The element  32  is then raised off the component  34 , so that an arc is drawn. Once a specific height is reached, the actual welding current is switched on, thus increasing the energy of the arc, so that the end face of the element  32  and the associated point on the component  34  are melted. The linear-movement apparatus  36  then moves the element  32  back towards the component. As soon as electrical contact is made once again, the arc is short-circuited, and the welding current is switched off.  
         [0071]     In general, the feed movement process continues to a point somewhat below the surface of the component  34 , so that the melts on both sides are mixed well together. The entire melt solidifies, and the actual welding process is complete. The holding apparatus releases the element  32 . The linear-movement apparatus  36  is then switched off. The linear-movement apparatus  30  is then moved back to the retracted rest position by the spring  40 . Furthermore, after this or in parallel with it, the pneumatic arrangement  24  is actuated by the control device  46  so that the welding head  22  is moved back to the retracted initial position.  
         [0072]     In all the embodiments, the element  32  can approach the component  34  in a highly dynamic manner and with high precision. The combination of the control device  46 , the movement sensor and the linear-movement apparatus  36  is then preferably used to determine the relative position between the element  32  and the component  34 , preferably with, or possibly also without, a supporting foot  31 .  
         [0073]     In some cases, it may be sufficient to move the end of the robot arm directly, that is to say without a slide  24  or a moveable component, to a position in which the element  32  is located in the position at the end of the approach process. This applies in particular when the holding apparatus  30  is pre-stressed or biased elastically to a rest position with respect to the welding head  22  by elastic means, such as the spring  40 . In one alternative embodiment, the robot  12  and the slide  21  may also be replaced by a single linear guide, which is driven in an automated manner.  
         [0074]     From what has been stated above, it is self-evident that, in a simplified embodiment, the supporting foot  31  is merely moved backwards and forwards between two axial positions. Both axial positions are operating positions, which means that the supporting foot  31  is used to make contact with the component (metal sheet)  34  in both operating positions, in order to determine the relative position between the element (bolt)  32  and the component (metal sheet)  34 . A freely variable axial position is, of course, more ideal.  
         [0075]     In an alternative simplified embodiment, the supporting foot  31  is simply pivoted backwards and forwards in a corresponding manner between two angular positions. Both angular positions are operating positions, in the above sense.  
         [0076]     Although an embodiment in which the supporting foot is pivoted around the pivoting axis is described above, embodiments are also feasible in which it is pivoted about an axis parallel to this. Since the supporting foot is frequently designed with a bend at its free end, it is also possible to pivot the supporting foot about its own axis.  
         [0077]     In addition, it is also possible to set up not only the operating positions but also a single rest position, in which the supporting foot  31  is not in operation. This is because, in certain circumstances, it may be worthwhile determining the relative position between the element and the component without using a supporting foot. A rest position is worthwhile for special cases such as this. It is self-evident that the rest position may be different for different welding positions. By way of example, the position of the supporting foot in  FIG. 3  may be a rest position for a welding process on a planar metal sheet  34   a , as is shown in  FIG. 1 .