Abstract:
The invention relates to a short-cycle arc welding system comprising a robot having at least one arm which can be displaced in at least one coordinate axis (x, y, z), a welding head which is positioned on the arm and on which a holding device or holding an element and a lifting device are provided, said lifting device moving the holding device back and forth in relation to the welding head and a measuring system for determining the relative position between a component and an element which is held by the holding device and is to be welded to the component. Said measuring system comprises a foot which is mounted on the welding head and which is embodied in such a way that, during operation, it is in contact with the component in order to determine the relative position between the element and the component.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application is a continuation of PCT application PCT/EP02/09760, filed on Sep. 2, 2002, which claims priority to German application no. DE  101   44   256 . 4 , filed on Sep. 3, 2001, both of which are incorporated by reference herein. 
     
    
     
       BACKGROUND AND SUMMARY OF THE INVENTION  
         [0002]    The present invention relates to a short-cycle arc welding system for welding elements, such as metal studs, to components, such as metal sheets, having:  
           [0003]    a robot that has at least one arm that can move in at least one coordinate axis  
           [0004]    a welding head that is mounted on the arm and on which is provided a holder for holding an element and a linear motion device for advancing and retracting the holder relative to the welding head, and  
           [0005]    a measurement system for determining the relative position of a component to an element to be welded to the component and held by the holder, wherein the measurement system has a foot mounted on the welding head that is designed to contact the component during operation in order to determine the relative position of the element to the component.  
           [0006]    The present invention also relates to a corresponding method for short-cycle arc welding. Such a short-cycle arc welding system and an associated method are generally known. In short-cycle arc welding, an element is welded onto a component. In this process, an arc that melts the end faces is formed between the element and the component. The element and component are then moved toward one another so that the molten materials combine. The arc is short-circuited and all the molten material solidifies. It is customary for the arc to be drawn. In this process, the element is first placed in contact with the component. A pilot current is then switched on and the element is raised to a desired height above the component, drawing an arc. Only then is the welding current switched on.  
           [0007]    To achieve consistently good welding results, it is important to know the relative positions of the element and component, and in particular to raise the element to the correct height before the welding current is switched on. To this end, a measurement of the relative position is generally performed before each welding operation, particularly in the form of a determination of the zero point. This is especially important in robot-based systems. It is true that modern robots generally are able to position relatively precisely. However, due to the large moving masses, high precision cannot be achieved with very high speeds. The robots customarily have a range of motion in three coordinates. In the simplest case, a robot is an automatically operated linear guide (carriage) on which is mounted a welding head.  
           [0008]    Stud welding systems are used in the automotive industry in particular, where they are used primarily to weld elements such as threaded and unthreaded studs, eyes, nuts, etc. to the car body sheet metal. These elements are then used as anchors for attaching such items as interior paneling and trim. Speed of production is a critical factor in the automotive industry. Hundreds of elements must be automatically welded to different positions by robots within a space of just a few minutes. Consequently, the robots must be moved at high speeds. Accordingly, it is known to affix to the arm of a robot a welding head base that supports a carriage. The carriage is capable of moving at high speeds with high precision, typically by means of a pneumatic or hydraulic system. Mounted on the carriage is the actual welding head, which in turn has a linear motion device for moving the element.  
           [0009]    It is known to affix a so-called support foot to the welding head in order to determine the relative position of the element to the component (for example, from the publication “Neue TUCKER-Technologie. Bolzenschweiβen mit System,” Emhart Tucker, September 1999). The support foot is aligned approximately parallel to the welding head holder. In an initial position, the element held in the holder projects somewhat beyond the support foot. In order to determine the relative position, the welding head is made to approach the component. During this process, the element contacts the component first. The welding head is advanced further until the support foot contacts the component. In this process, the holder is displaced relative to the welding head in opposition to an elastic preloading force. As a result, the relative position of the element to the component can be determined by means of a suitable measurement system and the rigid connection between the support foot and the welding head.  
           [0010]    Alternatively, so-called footless measurement systems are also known for determining the relative position of the element to component. Thus, U.S. Pat. No. 5,252,802 discloses a stud welding apparatus with a housing designed as a gun. A positioning motor first brings the housing into a position in which a stud is located in the vicinity of a component. Provided in the housing is a linear motor for the purpose of axially moving a shaft that carries the stud. A position measurement system is provided to control the linear motor. To determine the relative position of the stud and the workpiece, the linear motor is actuated so as to move the stud toward the workpiece at a specific speed. As soon as the stud touches the workpiece, an electrical contact closes. In addition, it is known from WO 96/11767 to elastically preload the stud holder in the direction of the workpiece and to move it axially against the preloading by means of a linear motor. Finally, WO 96/05015 discloses a stud welding apparatus with no support foot, wherein a welding head can be moved as a whole by means of a positioning drive. Provided on the welding head is a holder that holds a stud. A positioning device serves to move the holder axially relative to the welding head. The positioning device can be a servo-pneumatic or servo-hydraulic working cylinder. The relative position of the holder to the welding head is determined by means of a position measurement system.  
           [0011]    In order to determine a zero position between the stud and the workpiece, the welding head is moved to an end position in the direction of the workpiece. In the course of this movement, the stud contacts the workpiece. Because from this point on the stud can no longer follow the motion of the welding head, the holder is then displaced relative to the welding head in opposition to the pressing motion. This displacement is measured by the position measurement system with the result that the final position of the welding head is determined precisely.  
           [0012]    With the above in mind, an object of the present invention is to specify an improved short-cycle arc welding system and method for short-cycle arc welding. This object is achieved in the aforementioned short-cycle arc welding system in that the measurement system is additionally designed to determine the relative position of the element to the component without any contact between the foot and the component, and in that the measurement system has means to move the foot from an operating position to a rest position in which the foot is inactive.  
           [0013]    In the aforementioned short-cycle arc welding method, the object is achieved in that the method uses a short-cycle arc welding system that has a control unit in which is stored, for a plurality of automatic welding processes, information on whether determination of the relative position of the element to the component should take place in a given welding process with or without a foot that is designed to contact the component, and further wherein the method has the following steps:  
           [0014]    a) operating a robot with an arm such that a welding head attached to the arm reaches a welding position for a selected welding process,  
           [0015]    b) establishing whether the selected welding process requires the relative position of the element to the component to be determined with or without the foot,  
           [0016]    c) determining the relative position of the element to the component as a function of the conclusion reached in step b), and  
           [0017]    d) carrying out the selected welding process.  
           [0018]    With the invention, it is possible to determine the relative position of the element to the component with or without a foot or support foot. Generally, the relative position is determined without a support foot in weld locations where the component is relatively stable. This has the advantage that the welding stud can be welded especially close to contours since more space is available that is not occupied by a support foot. The higher stiffness in corners and at folds etc. also makes it possible to do without a support foot.  
           [0019]    In contrast, when a welding process is to be carried out on a relatively unstable component, for example unsupported thin sheet metal, it is preferred to determine the relative position of the element to the component with a support foot. In such a case, the foot provides a type of “bracing” of the component relative to the welding head. As a result, the component cannot deflect. In such a case, the relative instability of the component could result in incorrect measurements if the relative position is determined without using the support foot. The object is attained in full in this way.  
           [0020]    It is particularly advantageous for the measurement system to be designed to determine the relative position of the element to the component without contact between the foot and the component in that the point when the element contacts the component is measured when the element approaches the component. In general, this embodiment permits especially rapid determination of the relative position. In this context, it is particularly advantageous for the contact between the element and the component to be measured electrically. This measurement can, for example, be accomplished in that the rise of the motor current in an electric motor that is used to move the element toward the component is measured. Alternatively, a voltage could also be applied between the element and the component. The collapse of this voltage to zero then indicates that the element has contacted the component electrically and consequently also mechanically.  
           [0021]    In an alternative embodiment, the contact between the element and the component is measured in that, while the welding head approaches the component, the element is displaced relative to the welding head after contact with the component and the relative position of the element to the welding head is measured. This embodiment corresponds to the method disclosed in the aforementioned WO 96/05015. It is advantageous overall for a control unit to be provided in which is stored, for a plurality of automatic welding processes, information on whether determination of the relative position of the element to the component should take place with or without a foot in each welding process. In this way, determination of the relative position with or without the foot can be performed on a case-by-case basis when a plurality of welding processes are performed one after another by a robot. Prior to each welding process, the foot is moved, either to its operating position or to its rest position. This can even take place during movement of the welding head from one weld point to a next weld point by the robot, for example.  
           [0022]    In another preferred embodiment, the welding head has elastic means for preloading the holder in an actuating direction. As a result, the linear motion device for advancing and retracting the holder can be left de-energized in most operating states. The result is low energy consumption. In accordance with a preferred embodiment, the elastic means preload the holder in the retracting direction. When the linear motion device is operated in the advancing direction in order to determine the relative position of the component to the element without a foot, the holder in its elastically preloaded rest position is always in the correct initial position. Thus, especially low energy consumption is achieved on the whole. Moreover, under certain conditions, higher speeds can be achieved than in other embodiments.  
           [0023]    In an alternative embodiment, the elastic means preload the holder in the advancing direction. In this embodiment, a higher speed can be achieved in the advancing direction in the actual welding process. In the event of preloading in the retracting direction, the holder must be extended against the preloading during determination of the relative position with the support foot, either before or after the support foot has mechanically contacted the component. With preloading in the advancing direction, active movement of the holder by means of the linear motion device is not absolutely necessary when determining the relative position using the support foot.  
           [0024]    In another preferred embodiment, the measurement system has a position sensor that measures the position of the holder relative to the welding head. Of course, the features mentioned above and those explained below can be used not only in the combinations specified, but also in other combinations or by themselves without exceeding the scope of the present invention. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0025]    Example embodiments of the invention are shown in the drawings and are explained in detail in the description below. Shown are:  
         [0026]    [0026]FIG. 1 a schematic view of a first embodiment of a short-cycle arc welding system in accordance with the invention; and  
         [0027]    [0027]FIG. 2 a schematic representation of a welding head of an alternative embodiment of a short-cycle arc welding system in accordance with the invention. 
     
    
     DETAILED DESCRIPTION  
       [0028]    In FIG. 1, a first embodiment of a short-cycle arc welding system in accordance with the invention is labeled  10  overall. The short-cycle arc welding system  10 , which hereinafter is referred to in abbreviated form as stud welding system  10 , includes a robot  12 . The robot  12  has a turret  14 , by which means a single-jointed or multiple-jointed arm  16  can be rotated. The robot  12  as a whole 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  supports a carriage  21  that can be moved back and forth along an axis  26  relative to the welding head base  20 . A welding head  22  is mounted on the carriage  21 . A pneumatic arrangement  24  serves to move the welding head  22  back and forth with respect to the welding head base  20  by means of the carriage  21 . The stroke through which the pneumatic arrangement  24  can move the welding head  22  with respect to the welding head base  20  is labeled  28 . The welding head  22  has a holder  30  that is designed to releasably hold a metal element such as a stud  32 . To this end, the holder  30  has appropriate clamping means that are not shown in detail in FIG. 1. FIG. 1 also shows a metal component, such as a metal sheet  34 , that is oriented essentially perpendicular to the axis  26 .  
         [0029]    In addition, a support foot  31  is provided on the welding head base  20 . The support foot  31  can be moved back and forth in an axial direction between an operating position shown in solid lines in FIG. 1 and a rest position labeled  31 ′. A displacement mechanism that is schematically indicated in FIG. 1 as  31 A is provided for this purpose. As an alternative to a movable mounting of the support foot  31  on the welding head base  20 , the support foot  31  could also be mounted directly on the welding head  22  in an appropriately movable fashion.  
         [0030]    The welding head  22  also has a linear motion device  36  that is comprised of a linear motor, more particularly a linear electric motor. The linear motion device  36  serves to move the holder  30  relative to the welding head  22  in an axial direction that is aligned parallel to the axis  26 . The linear motion device  30  has a stroke  38  that can, for example, be in the range from 8 mm to 20 mm, more particularly in the range from 10 mm to 15 mm. In comparison, the stroke  28  of the pneumatic arrangement  24  can be in the range from 2 cm to 10 cm, more particularly in the range from 4 cm to 6 cm. Moreover, the holder  30  is preloaded in the direction away from the component  34  relative to the welding head  22 , which is to say in the retracting direction, by means of a compression spring  40 . The compression spring  40  engages the welding head  22  or the movable part of the carriage  21  on one and, and the holder  30  on the other end.  
         [0031]    The welding head  22  also has a position sensor  44  that is merely indicated schematically in FIG. 1. The position sensor  44  serves to measure the relative position of the holder  30  to the welding head  22 . To this end, the position sensor  44  can have a code reader on the holder  30  that reads a linear coding on the welding head  22 . In addition, a control unit  46  is provided. The control unit  46  is connected to the robot  12 , and also to the pneumatic arrangement  24 , the linear motion device  36  and the position sensor  44 . The control unit  46  serves to operate the movable elements of the stud welding system  10  in a coordinated manner, and to control their movement, speed and/or acceleration on the basis of the signals from the position sensor  44 . Moreover, as is described below, the control unit  46  serves to determine the relative position of the element  32  to the component  34  prior to a welding process. The welding system  10  is designed to determine the relative position of the element  32  to the component  34  alternately using the support foot  31  in its operating position and not using the support foot  31  (which is then in the rest position  31 ′).  
         [0032]    For the case of determining the relative position with the support foot  31 , the welding head  22  generally is brought closer to the component  34  by means of the carriage  21  until the end of the support foot  31  contacts the component  34 . Since the holder  30  in this embodiment is preloaded in retraction, the element  32  is located in the position shown at  32 ′ after the component  34  is contacted by the support foot  31 . Subsequently, the linear motion device  36  is actuated until the element  32  contacts the component  34 . As a result of the fixed positional relationship between the element  34 , the support foot  31 , the welding head base  20 , the welding head  22  and the holder  30 , the relative position of the element  32  to the element  34  can be established unambiguously. Alternatively, it is also possible to operate the linear motion device  36  prior to moving the support foot  31  toward the component  34  in such a way that the element  32  projects beyond the support foot  31  in the axial direction. In this case, after initial contact with the component  34  by the element  32 , the approach continues, displacing the linear motion device  36 , until the support foot  31  contacts the component  34 . In the alternate method of determining the relative position of the element  32  and the component  34 , the support foot  31  is moved to the rest position  31 ′ and is not used. In this method, the relative position is determined as follows, for example. First, the control unit  46  is electrically connected to the stud  32  by means of lines that are not shown. In addition, it is indicated in FIG. 1 that the control unit  46  is capable of applying a measurement voltage V to the stud  32 . The component  34  can be grounded, for example, such a suitable current measurement device can be used to determine when the element  32  electrically contacts the component  34 . First, the robot  12  is operated to bring the welding head base  20  into a base welding position, which is shown in FIG. 1, by means of the turret  14  and the robot arm  16 . In this position, the welding head base  20  is located a certain distance above the component  34 , wherein the axis  26  is perpendicular to the desired welding position of the component  34 .  
         [0033]    Subsequently, the pneumatic arrangement  24  is actuated so that the welding head  22  is extended toward the component  34 , specifically by the full stroke  28 , into a head welding position. The end position of the element  32  is shown in FIG. 1 at  32 ′. In this context, the element  32 ′ is located a distance  48  from the component  34  that is shorter than the maximum stroke  38  of the linear motion device  36 . Then the linear motion device  36  is actuated such that the element  32  is moved toward the component  34  until it contacts the component  34 . This movement preferably takes place at a constant speed. During the process, the distance traveled is measured by the position sensor  44 . As soon as the element  32  contacts the component  34 , an electric circuit originating at the measurement voltage V is closed. This is detected by the control unit  46 , and the linear motion device  36  is stopped. In addition, the contact position of element  32  and component  34  that is present at this point in time is used as the “zero position” for the further welding process. Consequently, the exact relative position of element  32  to component  34  is known throughout the entire welding process that follows by means of the position sensor  44 . As a result, the welding process can take place independently of any existing tolerances in positioning by the robot  12  or by the pneumatic arrangement  24  with the desired positional relationship between the element  32  and the component  34 .  
         [0034]    The actual stud welding process is accomplished in a manner that is known per se. In this context, a pilot current is applied to the element  32  after disconnection of the measurement voltage V. Then the element  32  is raised relative to the component  34  so that an arc is drawn. After a certain height has been reached, the actual welding current is switched on, which raises the energy of the arc such that the end face of the element  32  and the associated location on the component  34  melt. Afterward, the linear motion device  36  again advances the element  32  toward the component. As soon as electrical contact has been made again, the arc is short-circuited and the welding current is switched off. In general, the advancing step takes place somewhat below the surface of component  34  so that good mixing of the molten materials on both sides takes place. Then all the molten material solidifies and the actual welding process is concluded. The holder releases the element  32 . Then the linear motion device  36  is switched off. The holder  30  is consequently returned to the retracted rest position by the spring  40 . Furthermore, after this or at the same time, the pneumatic arrangement  24  is operated by the control unit  46  in such a way that the welding head  22  again reaches the retracted initial position.  
         [0035]    In FIG. 2, an alternative embodiment of a welding head is labeled  50  overall. The welding head  50  can be mounted instead of the welding head  22  on a welding head base  20  by means of a carriage  21  or without a carriage. In other respects as well, the structure and operation of a stud welding system with the welding head  50  can be identical to the structure of the stud welding system  10  from FIG. 1. Consequently, for this second embodiment, reference is made to the description of the stud welding system  10 , and only the differences are explained below. The welding head  50  has a holder  52  for one element  32  at a time and also has a linear motion device  54 . The linear motion device  54  serves to move the holder  52  toward an axis  26  relative to the welding head  50  in order to advance the element  32  toward or retract it from the component  34 . The linear motion device  54  has a permanent magnet  56  which contains a circular hole  58 .  
         [0036]    The holder  52  has a blind hole  60  that is matched to the circular hole  58  such that an end of the holder  52  facing away from the component  34  forms a sleeve section  62  that is introduced into the circular hole  58 . A coil  64  is formed on the outer circumference of the sleeve section  62 . The coil  64  is connected to a power supply  66  which in turn is operated by a control unit  68 , for example via pulse width modulation. Moreover, a position sensor  70  is provided which measures the position of the holder  52  relative to the welding head  50 . A compression spring  72  is arranged between the permanent magnet  56  and a flange  74  of the holder  52  which projects on the component side. The compression spring  72  preloads the holder  52  in a rest position, and in contrast to the embodiment in FIG. 1 the rest position lies in the advancing direction so that the holder  52  is maximally extended relative to the welding head  50  in the rest position. By excitation of the coil  64 , the holder  52  can be retracted into the welding head  50  relative to this rest position against the preloading of the compression spring  72 . In this process, a code reader  78  that is rigidly attached to the welding head  50  passes over a linear coding  76  on the holder  52 . The code reader  78  thus provides an actual position signal  80  to the control unit  68 . The control unit  68  compares the actual signal  80  with a target signal  82  and outputs a positioning signal  84  to the power supply  66 . Naturally, an appropriate regulator is therefore present in the control unit  68 .  
         [0037]    Also shown in FIG. 2 is that a support foot  86  is mounted on the welding head  50 . In similar fashion to the support foot  31  in FIG. 1, the support foot  86  can be moved back and forth in the axial direction between an operating position shown in FIG. 2 and a rest position  86 ′ by a displacement mechanism  86 A. In this embodiment as well, the relative position of the element  32  to the component  34  can be determined either with the support foot  86  or without the support foot  86 . When the relative position is determined with the support foot  86 , said foot is in the operating position. Before the welding head  50  approaches the element  34 , the linear motion device  54  is in the initial position, in which the holder  52  is maximally extended relative to the welding head  50 . In this position, the held element  32  extends a little ahead of the support foot  86  such that the component  34  is first contacted by the element  32  as described above. As the process continues, the linear motion device  54  is retracted until the support foot  86  contacts the component  34 .  
         [0038]    When the relative position is determined without the support foot  86 , said foot is in the rest position  86 ′. The position determination can then be performed in that the holder is first retracted fully by means of the linear motion device. Further operation is then identical to the operation described above for the embodiment in FIG. 1. Alternatively, it is also possible to have the welding head  50  approach the element  34  with the holder  52  maximally extended. Determination of the relative position can then take place as described in the aforementioned WO 96/05105, for example.  
         [0039]    In both embodiments, the element  32  can approach the component  34  with high speed and high precision. It is then preferable for the combination of control unit  46  or  68 , position sensor  44  or  70 , and linear motion device  36  or  54  to be used with or without the support foot  31  or  86  to determine the relative position of the element  32  to the component  34 . Consequently, sequential welding operations can be executed with uniformly high quality, independent of the precision of the positioning of the welding head  22  or  50 . As already mentioned, the determination of the relative position without support foot  31  or  86  is preferred.  
         [0040]    However, insofar as the component  34  is elastic or elastically mounted, determination of relative position with the support foot  31  or  86  is more favorable, since in this case a type of “bracing” takes place between the component and the welding head  20  or  50 , as is described above. Information is normally stored in the control unit  46  or  68  for each welding operation as to whether it is to be performed with or without the support foot  31  or  86 . Based on this information, the support foot  31  or  86  is moved either to the operating position or the rest position  31 ′ or  86 ′ before a welding operation.  
         [0041]    Yet another alternative embodiment is indicated by dashed lines in FIG. 1, wherein the component  34 A can be positioned along a stroke  28 A by means of a schematically indicated positioning device. This embodiment represents an alternative to the arrangement of a carriage  21  on the welding head base  20 . As a result, if such a positioning device for the component  34 A is present it is easier to rigidly attach the welding head to the welding head base  20 . Moreover, in some cases, it may suffice to move the end of the robot arm directly; hence, with no carriage  21  or movable component, into a position such that the element  32  is in the close position labeled  32 ′ in FIG. 1. In particular, this is the case when the holder  30  or  52  is elastically preloaded in a rest position with respect to the welding head  22  or  50  by elastic means such as the spring  40  or  72 . In an alternative embodiment, the robot  12  and the carriage  21  can also be replaced by a simple automatically controlled linear guide.