Abstract:
A short-cycle arc-welding system ( 10 ) and a method for welding elements ( 32 ), such as metal bolts ( 32 ) to components ( 34 ) such as sheet metal ( 34 ) includes a robot ( 12 ), having at least one arm ( 16 ) displaceable on at least two co-ordinate axes (x, y, z), a welding head base ( 20 ) fixed to robot arm ( 16 ), a welding head ( 22 ) displaceably mounted on welding head base ( 20 ), a retaining device ( 30 ) retaining an element, a lifting device ( 36 ) for setting and re-positioning retaining device ( 30 ) in relation to welding head ( 22 ), and a measuring system ( 44,46 ) determining the relative position between component ( 34 ) and element ( 32 ). Measuring system ( 44,46 ) has a control device ( 46 ), controlling lifting device ( 36 ) such that element ( 32 ) displaces towards component ( 34 ) until contacting component ( 34 ) to determine the relative position.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application is a continuation of PCT/EP02/08568, filed Aug. 1, 2002, which claims priority to German application 101 38 947.7, filed Aug. 2, 2001. The disclosures of the above applications are incorporated herein by reference. 

   BACKGROUND AND SUMMARY OF THE INVENTION 
   The present invention relates to a short-time arc welding system for welding elements, such as, for example metal studs, onto components, such as, for example metal sheets, with:
         a robot, having at least one arm movable in at least two coordinate axes,   a welding head base fixed to the robot arm,   a welding head, mounted movably on the welding head base, on which a holding device for holding an element and a lifting device for advancing and returning the holding device relative to the welding head are provided, and   a measuring system for determining the relative position between a component and an element to be welded onto the component.       

   The invention further relates to a method for short-time arc welding, in particular for stud welding, of elements, such as, for example metal studs, onto components, such as, for example metal sheets. A short-time arc welding system of this kind and an associated method are generally known. 
   With short-time arc welding an element is welded onto a component. An arc is therein formed between element and component, which melts the end faces. Then the element and the component are moved towards one another, so the molten masses are mixed. The arc is short-circuited and the combined molten mass solidified. 
   It is usual to draw the arc. The element is therein first placed onto the component. Then a pilot current is switched on and the element is lifted with respect to the component up to a desired height, wherein an arc is drawn. Only after this is the welding current connected. 
   In order to achieve consistently good welding results, it is important to know, inter alia, the relative position between element and component, in particular to lift the element to the right height, before the welding current is connected. For this purpose the relative position is usually measured before each welding process, in particular in the form of determining a zero position. 
   This is of particular significance with robot-based systems. Though modern robots are generally capable of positioning comparatively accurately, it is not possible to achieve great precision with very high dynamics, in particular owing to the large masses moved. 
   Stud welding systems are used in particular in the automotive industry. There they serve, above all, to weld elements such as studs with and without thread, eyes, nuts, etc., onto the vehicle body panel. These elements then serve as holding anchors to fix, for example interior fittings. 
   In the automotive industry manufacturing speed is what really matters. Within a few minutes hundreds of elements have to be welded on at different positions automatically by means of robots. The robots therefore have to be moved with high dynamics. 
   It is therefore known to attach to the arm of a robot a welding head base, which bears a cradle. The cradle can be moved with high dynamics with great precision, usually by means of a pneumatic or hydraulic system. On the cradle is mounted the actual welding head, which again has a lifting device for moving the element. 
   From DE 41 208 11 A1 a stud welding device is known, which uses a screw thread reluctance motor (SGRM) as linear motor for axially adjusting a stud holder. For this purpose the moved member of the SGRM forms an adjusting member, the movement of which is transmitted to the stud holder. The non-moved member forms the holder of the stud welding device. The SGRM allows the axial movement forwards and backwards of the stud holder to be controlled in a defined manner with respect to the setting position and speed. 
   In the position of rest the moved member rests against a stop connected to the holder under the tension of a spring. Only by triggering the SGRM is the moved member displaced out of the position of rest towards the workpiece which is to be provided with a welding stud. The stud holder therein takes up an intermediate position according to a predetermined path before the welding stud impacts on the workpiece, whereby by making contact the welding arc is struck in the known manner. 
   U.S. Pat. No. 5,252,802 discloses a further stud welding device with a housing, constructed as a hand pistol. A positioning motor first brings the housing into a position in which a stud is arranged near a component. In the housing a linear motor is provided, to move axially a lifting shank, which carries the stud. A path-measuring system is provided for controlling the linear motor. To determine the relative position between stud and workpiece the linear motor is triggered to move the stud at a certain speed towards the workpiece. As soon as the stud touches the workpiece an electrical contact closes. 
   It is further known from WO 96/11767 to bias the stud holder elastically towards the workpiece and by means of a linear motor to move it axially against the bias. 
   For detecting the relative position between stud and workpiece it is also known (for example from “Neue TUCKER Technologie. Bolzenschweiβen mit System”, Emhart Tucker, September 1999) to determine a zero position by means of a support foot. 
   Finally WO96/05015 discloses a stud welding device without support foot in which an entire welding head can be adjusted by means of an adjustment drive. A holding device holding a stud is provided on the welding head. An adjustment device serves to displace the holding device axially with respect to the welding head. The adjustment device can be a servo-pneumatic or a servo-hydraulic operating cylinder. The relative position between holding device and welding head is detected by means of a path measuring system. 
   For determining a zero position between stud and workpiece the welding head is moved towards the workpiece until it reaches an end position. In the course of this movement the stud impacts on the workpiece. As the stud from this time can no longer follow the movement of the welding head, from then on the holding device is displaced with respect to the welding head against the pressing movement. This displacement is measured by the path-measuring system and the end position of the welding head thus exactly detected. 
   Against the above background an object of the present invention is to cite an improved short-time arc welding system and method for short-time arc welding with which high positioning speeds are possible with great precision. 
   This object is achieved in a first aspect of the invention by a short-time arc welding system, as mentioned initially, wherein the measuring system has a control device which triggers the lifting device in such a way that the element is moved towards the component until it contacts the component in order thus to determine the relative position. 
   The object is further achieved in the first aspect of the invention by a method for short-time arc welding, in particular for stud welding, of elements, such as, for example metal studs, onto components, such as, for example metal sheets, with the steps:
         a) triggering a robot with an arm in such a way that a welding head base with a welding head, fixed to the arm, goes into a basic welding position,   b) triggering a cradle bearing the welding head on the welding head base or moving a component in relation to the welding head base in such a way that the welding head goes into a head welding position,   c) triggering a lifting device of the welding head in such a way that a holding device with an element held thereon is moved towards the component, and   d) detecting the position of the lifting device in which the element contacts the component.       

   According to a second aspect of the invention the object is achieved by a short-time arc welding system for welding elements, such as, for example metal studs, onto components, such as, for example metal sheets, with a welding head, on which a holding device for holding an element and a lifting device for advancing and returning the holding device relative to the welding head are provided, and a measuring system for determining the relative position between a component and an element to be welded onto the component, wherein the measuring system has a control device which triggers the lifting device in such a way that the element is moved towards the component until it contacts the component, in order thus to determine the relative position, and wherein the welding head has elastic means to bias the holding device elastically into a positioning direction. 
   The above object is fully achieved by the first aspect of the invention. By means of the combination of a robot and a welding head, fixed movably on the robot arm via a welding head base, high positioning speeds can be achieved with great precision. By the measure of designing the measuring system for determining the relative position between component and element in such a way that in particular no support foot is necessary, the speed can be further increased. This further has the advantage that the component is not contacted by any auxiliary means. In this respect damage of the component in the surrounding region of the welding is ruled out. 
   In a particularly preferred embodiment the robot arm can be moved in three coordinate axes. Robots of this kind are of particular advantage in the use of welding metal elements on to body panels of motor vehicles. For, in this field of use there is a requirement for welding the studs onto the body panel at any number of places and consequently in the most varied positions. 
   It is further of advantage if the welding head base has a cradle on which the welding head is mounted. A cradle, or generally speaking a device for carrying out a movement along an axis, is of advantage in so far as highly dynamic movements can be achieved therewith with comparatively great precision. It is therein particularly preferred if the cradle is pneumatically driven. It is herein of advantage that pneumatic power is usually available in the generic short-time arc welding systems as a matter of course. 
   In a further preferred embodiment the welding head has elastic means to bias the holding device elastically into a positioning direction. By means of this measure the lifting device for advancing and moving back the holding device can in most operating states be held without power. Consequently there is low power consumption. 
   This feature at the same time forms one of the core features according to the second aspect of the invention. The second aspect of the invention is directed at the design of a welding head, irrespective of whether the welding head is arranged on a robot and/or a welding head base. Decisive in the second aspect of the invention is the combination of a measuring system without a support foot with elastic means for elastic biasing of the holding device into a positioning direction. 
   With the second aspect of the invention it is consequently advantageous that the relative position between component and element can be measured without the component being touched by auxiliary means. The lifting device used for determining the relative position can further be held in most operating states without power, as the holding device is elastically biased in a positioning direction into a suitable position of rest. 
   According to a particularly preferred embodiment the elastic means bias the holding device into a return direction. As the lifting device for determining the relative position between component and element is triggered in the advancing direction, the holding device is always located in its elastically biased position of rest in the right starting position, so altogether particularly low energy consumption is achieved. Further, under certain preconditions higher dynamics can be achieved in comparison with other configurations. 
   In an alternative embodiment the elastic means bias the holding device in the advancing direction. In this embodiment higher dynamics can be achieved in the actual welding process in the advancing direction. 
   In this embodiment, when using the method according to the invention, it is so that drawing in the holding device during the approach of the welding head into the head welding position takes place against the bias force of the elastic means. 
   In contrast to this, in the embodiment in which the elastic means bias the holding device in the return direction it is so that the movement of the holding device in method step (c) takes place against the bias force of the elastic means. 
   Altogether it is preferably so in the method according to the invention that the holding device releases the element after the welding process and the lifting device is switched off, so the holding device is biased by the elastic means into a position of rest. 
   In the short-time arc welding system according to the invention the measuring system preferably has a path sensor which detects the path of the holding device relative to the welding head. 
   It goes without saying that the above mentioned features and those still to be described below can be used not only in the combination cited in each case, but also in other combinations or standing alone, without leaving the scope of the present invention. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Embodiments of the invention are illustrated in the drawings and are explained in greater detail in the following description. 
       FIG. 1  shows a schematic view of a first embodiment of a short-time arc welding system according to the invention; 
       FIG. 2  shows a schematic path-time graph, illustrating the relative position between a component and an element to be welded onto the component (or its holding device) over the time, wherein the position of the component is defined by the time axis; wherein the path-time diagram emerges during operation of the first embodiment of the short-time arc welding system according to the invention according to  FIG. 1 ; 
       FIG. 3  is a path-time graph, which emerges during operation of an alternative embodiment of a short-time arc welding system, as illustrated schematically in  FIG. 4 ; and 
       FIG. 4  is a schematic illustration of a welding head of an alternative embodiment of a short-time arc welding system according to the invention. 
   

   DETAILED DESCRIPTION 
   In  FIG. 1  a first embodiment of a short-time arc welding system according to the invention is generally designated by  10 . The short-time arc welding system  10 , which is abbreviated below as stud welding system  10 , comprises a robot  12 . The robot  12  has a rotary head  14 , by means of which a singly or multiply articulated arm  16  can be rotated. In all the robot  12  is designed to move the end of the arm freely in three coordinate axes x, y, z. 
   A welding head base  20  is attached to the end of the robot arm  16 . The welding head base  20  carries a cradle  21 , which can be moved forwards and backwards relative to the welding head base  20  in the direction of an axis  26 . A welding head  22  is mounted on the cradle  21 . 
   A pneumatic arrangement  24  serves to move the welding head  22  forwards and backwards in relation to the welding head base  20  by means of the cradle  21 . At  28  the lift of the stroke is shown by which the pneumatic arrangement  24  can move the welding head  22  in relation to the welding head base  20 . 
   The welding head  22  has a holding device  30 , which is designed to hold detachably a metal element such as a stud  32 . For this purpose the holding device has suitable fixing means, which are not illustrated in greater detail in  FIG. 1 .  FIG. 1  shows a metal component, such as a sheet of metal  34 , which is substantially aligned vertically to the axis  26 . The welding head  22  further has a lifting device  36 , which is formed by a linear motor, in particular an electric linear motor. 
   The lifting device  36  serves to displace the holding device  30  in relation to the welding head  22  in an axial direction, aligned parallel to the axis  26 . The lifting device  36  has a lift of stroke  38  which can 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 with this the lift of stroke  28  of the pneumatic arrangement  24  can be in the range between 2 cm and 10 cm, in particular in the range between 4 cm and 6 cm. 
   Further, the holding device  30  is biased in relation to the welding head  22  in the direction away from the component  34 , in other words in the return direction, by means of a pressure spring  40 . The pressure spring  40  on the one hand acts on the welding head  22  or the movable part of the cradle  21  and on the other hand on the holding device  30 . 
   The welding head  22  further has a path sensor  44 , which in  FIG. 1  is indicated only schematically. The path sensor  44  serves to detect the relative position between holding device  30  and welding head  22 . For this purpose the path sensor  44  can have a code reader on the holding device  30 , which reads a linear coding on the welding head  22 . 
   A control device  46  is further provided. The control device  46  is connected to the robot  12  and to the pneumatic arrangement  24 , the lifting device  36  and the path sensor  44 . The control device  46  serves to control the movable elements of the stud welding system  10  in coordination with one another or to regulate their movement, speed and/or acceleration on the basis of the signals of the path sensor  44 . The control device  46  further serves, as will be described below, to determine the relative position between the element  32  and the component  34  before a welding process. 
   For this purpose the control device  46  is electrically connected via leads, not illustrated in greater detail, to the stud  32 . Further indicated in  FIG. 1  is that the control device  46  is capable of applying a measuring voltage V to the stud  32 . The component  34  can, for example, be earthed, so via a suitable current measuring device it is possible to detect as soon as the element  32  electrically contacts the component  34 . 
   The operation of the stud welding system of  FIG. 1  is explained below using the path-time diagram of  FIG. 2 . Before a time T 1  the robot  12  is triggered in order to bring the welding head base  20  by means of the rotary head  14  and the robot arm  16  into a basic welding position, which is reached at T 1 . The basic welding position of the welding head base  20  is illustrated in  FIG. 1 . In this position the welding head base  20  is located at a certain distance above the component  34 , wherein the axis  26  stands vertically on the desired welding position of the component  34 . 
   From T 1  onwards the pneumatic arrangement  24  is actuated, so the welding head  22  is moved out towards the component  34 , and in fact by the full lift of stroke  28 , until it reaches the head welding position. The end position of the element  32  is shown in  FIG. 1  as  32 ′, in  FIG. 2  as T 2 . The element  32 ′ is therein located at a distance  48  from the component  34  which is smaller than the maximum lift of stroke  38  of the lifting device  36 . 
   From T 2  onwards the lifting device  36  is actuated, so the element  32  is moved towards the component  34  until it contacts the component  34  at time T 3 . This movement preferably takes place at constant speed. The path covered is therein detected by means of the path sensor  44 . At time T 3  the element  32  contacts the component  34 , so a current circuit starting from the measuring voltage V is closed. This is detected by the control device  46  and the lifting device  36  is stopped. 
   Further, the contact position existing at this time between element  32  and component  34  is brought into play as the “zero position” for the further welding process. Via the path sensor  44  the respective exact relative position between element  32  and component  34  is consequently known over the entire following welding process. The welding process can consequently take place irrespective of any existing tolerances in the positioning by the robot  12  or by the pneumatic arrangement  24  with the desired position relationship between element  32  and component  34 . In other words before each welding process there is a “calibration” of the relative position between element  32  and component  34 . 
   From T 3  to T 5  a stud welding process takes place in a manner known per se. Therein, after cutting off the measuring voltage V, a pilot current is applied to the element  32 . Then the element  32  is lifted with respect to the component  34 , so an arc is drawn. After a certain height has been reached, the actual welding current is connected, by which the power of the arc is increased in such a way that the end face of the element  32  and the associated place on the component  34  are melted together. 
   This lifting process takes place only by means of the lifting device  36 . The pneumatic arrangement  24  is not actuated herein. Subsequently the lifting device  36  advances the element  32  onto the component again. As soon as the electrical contact has been reached again, at T 4 , the arc is short-circuited and the welding current is switched off. 
   Then in general another advancing process takes place to slightly below the surface of the component  34 , so good mixing of the mutual molten masses takes place. The combined molten mass solidifies and the actual welding process is complete at time T 5 . At this time the holding device releases the element  32 . The lifting device  36  is further switched off. The holding device  30  is consequently moved back by the spring  40  into the drawn-in position of rest. Further, after this or simultaneously therewith the pneumatic arrangement  24  is triggered by the control device  46  in such a way that the welding head  22  goes back into the drawn-in starting position. At time T 6  the stud welding system  10  is again located in the position shown in  FIG. 1 . Then a new element  32  is supplied to the holding device  30  and the robot  12  is triggered in such a way that the welding head base  20  goes into a suitable basic welding position for the welding process of the new element  32 . 
     FIG. 2  shows that the course of the path-time graph from T 1  to T 2  is linear. It is herein assumed that the robot first transfers the welding head base  20  into the basic welding position and then the pneumatic arrangement  24  is actuated. In many cases the movement of the pneumatic arrangement  24  can also be superimposed on the movement of the robot arm  16 . In this case a non-linear curve would occur between T 1  and T 2 . For example a non-linear course is shown between T 5  and T 6 . This occurs in that after the holding device  30  has been detached the movements at least of the lifting device  36  and the pneumatic arrangement, optionally also superimpose those of the robot arm  16 . 
     FIG. 3  shows a similar path-time graph, which differs from the path-time diagram of  FIG. 2  in two aspects only. On the one hand the course of the graph between T 1  and T 2  is shown as a non-linear curve. On the other hand a different non-linear course occurs between T 5  and T 6  from in the curve of  FIG. 2 . These differences occur during operation of an alternative embodiment of a stud welding system, the welding head of which is designated in  FIG. 4  in general by  50 . 
   Welding head  50  can be mounted instead of welding head  22  on a welding head base  20  by means of a cradle  21 . The structure and operation of a stud welding system with welding head  50  can also otherwise be identical to the structure of stud welding system  10  in  FIG. 1 . Consequently for this second embodiment reference is made to the description of stud welding system  10  and only the differences are explained below. 
   Welding head  50  has a holding device  52  for an element  32  and a lifting device  54  respectively. The lifting device  54  serves to move the holding device  52  in relation to the welding head  50  in the direction of an axis  26 , in order to advance the element  32  onto the component  34  or to return it therefrom. The lifting device  54  has a permanent magnet  56  comprising a circular bore. 
   The holding device  52  has a blind hole  60  matched to the circular bore  58 , so an end of the holding device  52  remote from the component  34  forms a sleeve portion  62 , which is inserted into the circular bore  58 . On the outer periphery of the sleeve portion  62  a coil  64  is constructed. The coil  64  is connected to a power part  66 , which in turn is triggered by a control unit  68 , for example by pulse width modulation. Further provided is a path sensor  70 , which measures the path of the holding device  52  in relation to the welding head  50 . 
   A pressure spring  72  is arranged between the permanent magnet  56  and a flange  74  of the holding device  52  projecting on the side of the component. The pressure spring  72  biases the holding device  52  into a position of rest and in contrast to the embodiment in  FIG. 1  the position of rest is in the advancing direction, so the holding device  52  in the position of rest is drawn out to its maximum with respect to the welding head  50 . 
   By exciting the coil  64  the holding device  52  can be drawn into the welding head  50  with respect to this position of rest against the bias of the pressure spring  72 . A code reader  78 , rigidly connected to the welding head  50 , therein scans a linear coding  76  on the holding device  52 . The code reader  78  consequently emits a path actual signal  80  to the control device  68 . The control device  68  compares the actual signal  80  with a desired signal  82  and emits a positioning signal  84  to the power part  66 . It goes without saying that there is consequently a suitable regulator in the control device  68 . Operation of the stud welding system with the alternative welding head  50  is basically identical to operation of the stud welding system  10  of  FIG. 1 , with the following exceptions: As shown in  FIG. 3 , in the period between T 1  and T 2  there occurs a non-linear course of the path of the element  32  over the time. This is because the movement of the pneumatic arrangement  24  is superimposed by a movement of the lifting device  54  into the opposite direction. For during the approach of the welding head  50  to the component  34  the holding device  52  with the element  32  held thereon is first drawn in against the advancing direction and against the force of the spring  72 . 
   In time T 2  the holding device  52  is consequently in the fully drawn in position, at its position of maximum distance from the component  34 . Further operation between T 2  and T 5  is identical to the operation of the embodiment of  FIGS. 1 and 2 . 
   From time T 5  onwards, after the holding device  52  has released the element  32 , the movements of the pneumatic arrangement  24  and the holding device  52  takes place by means of the spring  72 , which biases the holding device  52  are superimposed into the drawn out position of rest. This movement can be further superimposed by a movement of the robot arm  16 . 
   In both embodiments the element  32  can approach the component  34  highly dynamically and with great precision. Then the combination of control device  46  or  68 , path sensor  44  or  70  and lifting device  36  or  54  is used to determine the relative position between element  32  and component  34 . Consequently successive welding processes can be carried out with consistently high quality, irrespective of the precision of the positioning of the welding head  22  or  50 . 
     FIG. 1  illustrates another alternative embodiment by a dotted line, in which the component  34 A can be adjusted by means of a schematically indicated adjusting device by a stroke  28 A. This configuration represents an alternative to the arrangement of a cradle  21  on the welding head base  20 . If consequently an adjusting device of this kind for the component  34 A is present, it is possible to fix the welding head rigidly to the welding head base  20 . 
   Further, in many cases it can be sufficient to bring the end of the robot arm directly into a position in which the element  32  is in the approximate position designated by  32 ′ in  FIG. 1 . This applies in particular if the holding device  30  or  52  is biased into a position of rest with respect to the welding head  22  or  50  by means of elastic means, such as the spring  40  or  72 . 
   The above-mentioned range data for the stroke of the pneumatic arrangement  28  (or  28 A) and the lifting device  36  or  54  are designed to achieve particularly high dynamics with the greatest possible precision of positioning. 
   In particular is use in the automotive industry to weld holding anchors such as studs with and without thread, eyes, T-studs, etc., on vehicle bodywork panels, high dynamics of this kind with great precision are of particular importance as part of an automated production line. It is of further advantage in the embodiments illustrated that the component  34  is not touched by other elements of the welding head before, during and after the welding process, but only by the element  32  itself.