in fig1 , 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 . 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 fig1 . fig1 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 . 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 . 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 . the welding head 22 also has a movement sensor 44 , which is indicated only schematically in fig1 . 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 ). 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 . 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 fig1 . 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 fig1 , 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 . 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 . 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 fig1 , 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 . as is shown in fig2 , 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 . furthermore , a clamping device 58 is indicated schematically in fig1 . 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 . 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 . fig1 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 fig1 is used for detecting the axial position of the supporting foot 31 with respect to the welding head 22 . fig2 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 . 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 . 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 . with regard to the contour of the component 34 , fig1 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 . fig3 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 fig1 . 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 . fig4 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 fig4 . 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 . 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 fig1 . 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 . 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 . 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 . 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 . 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 . 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 . 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 . 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 . 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 . 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 . 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 . 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 fig3 may be a rest position for a welding process on a planar metal sheet 34 a , as is shown in fig1 .