Abstract:
A joining system head is provided for fixation to a movable frame, in particular to a robot having a holder for an element to be joined to a part. A joining drive moves the holder along a joining direction for joining. The holder is mounted on the joining system head and is rotatable about an axis running transverse to a joining direction.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application is a continuation of U.S. patent application Ser. No. 10/436,882 filed on May 13, 2003, which issued as U.S. Pat. No. 7,060,930 on Jun. 13, 2006, which claims the benefit of German Patent Application DE 10223147.8, filed May 16, 2002. The disclosure of the above application is incorporated herein by reference. 

   BACKGROUND AND SUMMARY OF THE INVENTION 
   The present invention relates to a joining system head for attachment to a movable frame, in particular to a robot, having
         a holding means for an element to be joined to a part, and   a joining drive means to move the holding means along a joining direction for joining.       

   The present invention relates further to a joining system having a robot movable on at least two coordinate axes and a joining system head attached to the robot. Lastly, the present invention relates to a method of feeding elements from a stationary unit to a movable joining system head and joining said elements to parts by means of the joining system head. 
   Such a joining system head, such a joining system and such a method of feeding and joining elements by means of a joining system head are generally known. The term ‘joining’ in the present context is intended to refer to all ways of connecting elements to parts, in particular connections of metal elements to metal parts, for example by bonding, forming, as for example riveting, or by union of matter, as for example welding, including short-time arc welding. Short-time arc welding is often referred to as bolt welding, even though it is not exclusively bolts that are welded. A current system of bolt welding in industrial use, in combination with a robot, is known in the brochure “Neue TUCKER Technologie. BolzenschweiBen mit System!,” Emhart TUCKER, September 1999. 
   Bolt welding finds application chiefly, but not exclusively, in vehicular technology. Here, metal elements such as metal bolts, with or without threads, eyes, nuts etc., are welded onto the sheet metal of the bodywork. The metal elements then serve as anchors, or fastening elements, to fix for example interior fittings, lines and the like to the sheet metal of the body. At the joining system head, disclosed in the above-mentioned Emhart TUCKER publication, the joining drive means is configured either as a linear electric motor or as a combination of a lift magnet and a spring. 
   The holding means is constituted by a one-piece tongs elastically expandable in radial direction. The elements are as a rule welding bolts comprising a head having a somewhat larger diameter than the shank of the bolt. In the known system, the bolts are fed to the welding head by way of suitable feeding conduits by means of compressed air. The bolts are thus fed ‘head first’ into the tongs from behind. Ordinarily the bolt will strike the tongs from the inside, but without passing through it. A loading pin provided coaxial with the tongs is then actuated to propel the bolt thus fed through the tongs. The tongs are elastically expanded radially when the head of the bolt passes through. Then the tongs snap closed elastically around the shank of the bolt and hold it fast in the position determined by the travel of the pin. 
   The joining drive means in the form of a linear motor (or lift magnet/spring combination) has a travel of a few millimeters. Also, the welding head is fixed at the end of an arm of the robot, usually by way of a pneumatic or hydraulic carriage. That is, the entire welding head is movable in a direction parallel to the welding axis by means of the carriage, which has a considerably greater travel than the linear motor. The welding head further comprises a control means to control the linear motor and the loading pin, provided spatially separate from the welding head, more specifically in a stationary feeder. 
   To perform a welding operation, first the robot is programmed so that it travels into a predetermined position in which the carriage and linear motor axes are perpendicular to the sheet metal onto which the bolt is to be welded. The bolt is prestressed so that it protrudes vis-à-vis a supporting foot. Then the carriage is actuated until the foot meets the sheet metal. The bolt held in the holding means then rests in contact with the sheet metal. Next comes a determination of the zero line of the holding means with respect to the sheet metal. Alternatively, however, there are methods of zero line determination that dispense with the supporting foot. 
   Then, in the case of welding with supporting foot, an electric pre-current is switched on, passing through the bolt and the part. The bolt is then lifted relative to the part by means of the linear motor (lifting means). An electric arc is set up. Then a switch is made to the welding current. By the high welding current, the opposed faces of bolt and part begin to be fused. The bolt is then lowered onto the part again, so that the respective melts will mingle. Upon attainment of the part and the short circuit of the arc, or just before, the welding current is switched off. The entire melt solidifies and the welded connection is complete. 
   Now the welding head is drawn off from the welded-on bolt, using the carriage. The carriage is necessary because, among other reasons, the drawing-off motion must take place exactly on the centerline of the welded-on bolt. Otherwise, owing to the one-piece tongs, there would be danger of damage to the bolt and/or the tongs. The robot arm alone is not capable of such a precise linear motion in an arbitrary direction of space. For owing to the superposition of the simultaneous regulation of several components of robot arm motion, as required for this purpose, such linear motions can be executed by the robot with a certain amount of undulation only. The known welding head comprises a comparatively great axial extent. Since moreover the welding head must be drawn off from the bolt in axial direction, use of the welding head in places of difficult access is possible only within limits. 
   Then there are developments for employing robot technology to feed the bolt. Here a separate pick-up takes pre-sorted bolts and brings them to the welding location. This is disclosed in “BolzenschweiBen. Grundlagen und Anwendung” by Trillmich, Welz, Fachbuchreihe SchweiBtechnik, DVS Verlag, 1997, Chapter 9.3. It is there explained that this technology lends itself especially to headed bolts that, because of their size and shape, cannot be blown through hoses. This type is referred to as the “pick-up system.” 
   Further, a welding head by the firm of Nelson has been disclosed, in which a lift device moves a carrier projecting laterally arm-like up and down. At the terminal portion of the carrier, a holding means with tongs is rigidly mounted. The bolts are fed, as in the case of the TUCKER welding head described above, to the tongs from behind, by means of a compressed air hose extending through the carrier. The end portion of the carrier with holding device fixed thereto is more readily positioned at inaccessible locations. The lift device to move the projecting arm and the pertinent control means are arranged in the initial portion of the carrier. 
   Against this background, the object of the invention consists in specifying an improved joining system head, an improved joining system and an improved method of feeding and joining fed elements. This object is accomplished, in the case of the joining system head initially mentioned, in that the holding means is mounted rotatable at the joining system head about an axis extending transverse to the joining direction. 
   The joining system head according to the invention represents a completely novel concept. For joining, in particular for bolt welding, the joining operation of the prior art always takes place in a linear motion. In the prior art, consequently, it was the practice to mount the holding means slidable at least along a linear axis. For example, it is known that the joining system head may be mounted bodily on a carriage which in turn is fixed to the robot. 
   Owing to the rotatable mounting of the holding means on the joining system head, it is now possible to move the holding means along a circular or circular arc path. This creates the prerequisite for a number of fundamental changes in past concepts of joining system heads. The rotatability of the holding means is comparatively simple to realize as a matter of design. In particular, it is possible by means of the robot to turn the holding means on the circular path in order to reach various welding positions quickly and without extensive motion routines, for example a welding position for welding in vertical direction downward and then an overhead welding operation. The turnability as an additional degree of freedom at the joining system head is sufficient for many applications. Considering that also the carrier itself is rotatable about its longitudinal axis by means of the robot as a rule, and positionable at will in space, joining operations can be performed at very inaccessible locations indeed. The interference edge profile of the joining tool is here determined by the required radius of swing. In the joining system according to the invention, the above object is accomplished in that a joining system head according to the invention is attached to the robot. 
   The method according to the invention for feeding elements from a stationary unit to a movable joining system head and for joining said elements to parts by means of the joining system head, said joining system head comprising a holding means for an element, mounted rotatable about an axis extending transverse to the joining direction, includes the steps of feeding an element from the stationary unit to the transfer station at the joining system head, rotating the holding means towards the transfer station, taking over an element from the transfer station into the holding means, and joining the element taken over to the part. In the method of the invention, accordingly, there is a fundamental departure from the idea of feeding elements from the stationary unit directly to the holding means. Instead, the elements are fed to a transfer station at the joining system head, and the holding means is rotated towards the transfer station, to pick up the elements. The holding means thus ‘fetches’ the elements from the transfer station in each instance. As a result, there is an uncoupling between the feeding means comprised by the transfer station and the holding means. This is a prerequisite for a number of advantages about to be illustrated in detail. The object, then, has been wholly accomplished. 
   It is of especial advantage if the holding means and the joining drive means are mounted rotatable about the axis as a joining tool. In this embodiment, the holding means and the joining drive means form a rotatable unit of small dimensions. This is true especially if a control means to control the joining drive means is mounted at the welding head, but spatially separate from the joining tool. The joining tool can consequently be made with small dimensions and little relevant edge interference. 
   Here it is especially preferred if the joining tool is mounted rotatable about the axis at an end portion of a projecting elongated carrier. Owing to the arrangement of a joining tool of small dimensions at the end portion of an elongated carrier, it is possible to bring the joining tool to places difficult of access. Here no transmission of a lifting motion over long distances (no boom or the like) is required. Therefore the positioning and the actual joining or welding operation itself can be performed locally with high precision. At the same time, it is especially advantageous if the control means is provided in an initial portion of the carrier. The joining tool of small dimensions can then be brought to inaccessible locations through openings. 
   In an especially preferred embodiment, the elongated carrier comprises two arms running parallel, between which the joining tool is rotatably mounted. This embodiment has the advantage, firstly, that the mounting of the joining tool can be accomplished with high spatial precision. Besides, the space remaining between the arms of the carrier can be utilized for other functional units. These units as well as the joining tool are moreover protected between the arms of the carrier. 
   It is of especial advantage also if the axis of rotation is oriented transverse to the longitudinal axis of the carrier. In this embodiment, it is advantageously brought about that the circular path of the holding means can extend beyond the foremost ends of the carrier. Consequently, the carrier can be of comparatively short configuration. Secondly, it is brought about that the holding means can be swung as far as a midportion of the carrier, and can therefore be brought all the way to other functional units. 
   Over all, then, it is of advantage firstly if the joining system head comprises a feeding means with transfer station for the feeding of elements and if a loading drive means is designed to rotate the holding means and/or the joining tool all the way to the transfer station. Thus the elements are not fed, as in the prior art, all the way to the holding means. Rather, the feeding of the elements at first takes place only as far as the transfer station. Hence this step of the feed can take place while the joining-welding head itself is joining an already fed element to a part. This parallel processing serves to permit shorter periods over all. It is especially preferred if the transfer station is fixed to the carrier. Provided the transfer station is arranged on the elongated carrier, a fixed relative position of the transfer station can be achieved in relation to the holding means or the joining tool. Besides, it is advantageous that the cross section of the carrier is smaller as a rule than the cross section of the joining tool or the holding means, so that space is available for the transfer station. 
   According to one embodiment, the loading drive means comprises a rotary motor arranged at the end portion of the carrier. In this embodiment, a precise control of the joining tool can be achieved, with good response behavior. 
   In an alternative embodiment, the loading drive means comprises a rotary motor arranged in the initial portion of the holder and a gear to transmit the motions of the motor to the holding means. In this embodiment, an improved interference edge clearance results, since the interference-relevant end portion of the carrier has no motor of its own to move the holding means and/or the joining tool. Rather, the comparatively bulky motor is arranged in the initial portion of the carrier and transmits its motions to the holding means and/or the joining tool by way of a transmission. Also, a rotary motor will serve to execute motions with precision and high responsiveness. 
   It is especially preferred if the transmission is a transmission with tension means. The transmission with tension means will permit comparatively long distances between the initial portion of the carrier on the one hand and the final portion of the carrier on the other hand by comparatively simple design means. 
   In general, in a preferred embodiment, provision is made for the loading drive means and the joining drive means to consist of a single rotary drive means. In this embodiment, the rotatability of the holding means is used not only to swing the holding means all the way to a transfer station to ‘fetch’ an element. Rather, the holding means is moved to join a held element, not in a direction perpendicular to the axis of rotation, but along a circle around the axis of rotation. This embodiment has the special advantage that an axially prolonged linear drive in the region of the holding means, in particular the end region of the elongated carrier, is not required. Rather, the rotary drive means constituting the loading drive means and the joining drive means may be provided for example in the initial portion of the carrier, and their motions can be transmitted to the holding means in the end portion of the carrier by way of a transmission with tension means. In this embodiment, a sort of ‘reduced’ joining tool is formed at the anterior end portion of the carrier, consisting basically of the holding means alone. In this embodiment, consequently, an especially low interference edge relevance results, and hence the possibility of performing joining operations even in especially inaccessible locations. 
   In an alternative embodiment, the joining drive means comprises a linear drive means instead. In this embodiment, the holding means is consequently set in rectilinear motion for joining in conventional manner. The rotatability of the holding means about the axis of rotation is then preferably employed by means of development of the loading drive means to rotate the holding means or the joining tool into any welding position and/or ‘fetch’ elements from a transfer station of the feeding means. 
   Provided the linear drive means comprises a linear electric motor, only comparatively few lines are required for control. The holding means may then be regulated in either lift direction. In this embodiment, it is of especial advantage if the longitudinal axis of the joining drive means and the longitudinal axis of the holding means are spaced apart parallel to each other. Here it is possible to position the holding means so that even welding positions close to edges are attainable. The distance of the longitudinal axes may be within the range of a few centimeters, just enough to shift the holding means out of the joining drive direction projected into the joining direction. 
   In general, provision is made, in a preferred embodiment, for the holding means to comprise a plurality of jaws arranged distributed around the longitudinal axis of the holding means and movable away from each other so as to hold or release one element in each instance. It is especially preferred if the holding means comprises two jaws. The term ‘jaws’ is to be understood broadly in the present context. The jaws may for example refer to elongated fingers. With two jaws, rotationally symmetrical or approximately rotationally symmetrical parts in particular can be picked up conveniently and held securely. It is preferred for the jaws to be movable away from each other far enough so that the holding means can release the element by being drawn off from the element obliquely to the joining direction. 
   This embodiment makes it possible to accomplish the process of ‘running’ the joining system head away from the element joined to the part, by means of the robot alone. Then no carriage is required to establish a completely rectilinear reverse motion. Thus this embodiment also contributes to a small axial extent of the welding head. 
   However, it is especially preferred if the jaws are movable away from each other far enough so that the holding means can release the element by being swung away from the element about the axis of rotation. In this embodiment, the jaws can be moved far enough away from each other so that the joining tool need not be run away in the joining direction. Rather, it is possible to run the joining tool and/or the holding means away after the joining operation transversely, in particular perpendicular to the joining direction, the element passing between the jaws of the holding means. In this embodiment, therefore, no axial motion is required. In this way it is possible to pass the carrier with joining tool and/or holding means arranged at the anterior end portion through even extremely small means, and execute joining operations inside of cavities. The carrier, after the joining position has been reached, can remain positioned almost without change. After the joining operation, the joining tool is swung away transversely to the joining direction, and then the carrier can be run out of the cavity again along its longitudinal axis. 
   Also, this embodiment makes it possible for the elements to be picked up especially simply from the transfer station. The joining tool in this embodiment is swung in one step so that the holding means is oriented with an element at the transfer station with jaws released. Then the element can be grasped by the jaws and taken out of the transfer station by an ensuing swinging motion. In general, it is here preferred for a jaw actuator to be provided, actively opening and/or closing the jaws. In this embodiment, the jaws are usually configured as rigid fingers. The jaw actuator ensures that the jaws are either actively opened, to release an element, or else actively closed to hold the element. 
   Alternatively, it is possible for the jaws to be elastically configured or elastically mounted, in such manner that they are passively movable towards and/or away from each other. Here the jaws may either be made of an elastic material, in which case other elastic means may as a rule be dispensed with, or alternatively the jaws may be configured as rigid elements and elastically mounted. It is also possible within the scope of this embodiment for the jaws to be elastically pre-stressed in holding or in releasing direction. In that case, as a rule an actuator is provided that moves the jaws actively in the respective other direction. 
   In the joining system according to the invention, it is of advantage if a stationary individualizing means conveys individual elements to the feeding means of the joining-welding head. This embodiment serves generally to enhance the degree of automation. Such stationary individualizing and feeding means are known per se in the prior art. They convey individual elements in one step, however, all the way to the holding means, whereas in the joining system according to the invention, a conveyance occurs only as far as the feeding means (transfer station). Thence the holding means ‘fetches’ an element conveyed thither. It will be understood that the features named above and the features yet to be illustrated below may be employed not only in the combination given in each instance, but also in other combinations or by themselves, without departing from the scope of the present invention. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Embodiments of the invention are represented in the drawing by way of example and will be illustrated in more detail in the description to follow. In the drawing, 
       FIG. 1  shows a schematic view of a joining system according to the invention; 
       FIG. 2  shows an alternative conformation of a joining system according to the invention; 
       FIG. 3  shows a longitudinal section of an embodiment of a holding means; 
       FIG. 4  shows a cross-section at the line IV-IV in  FIG. 3 ; 
       FIG. 5  shows the end portion of a joining system head according to the invention, with an alternative conformation of a joining tool; 
       FIG. 6  shows a view of the end portion of the joining system head of  FIG. 5  from below; 
       FIG. 7  shows another alternative embodiment of a joining tool of a joining system head according to the invention; 
       FIG. 8  shows a schematic representation of a transfer station of a joining system head according to the invention; 
       FIG. 9  shows a schematic representation of an alternative transfer station of a joining system head according to the invention; 
       FIG. 10  shows a schematic representation of still another alternative embodiment of a transfer station of a joining system head according to the invention; 
       FIG. 11  shows a schematic sectional view at the line XI-XI in  FIG. 10 ; and, 
       FIG. 12  shows a schematic side view of an alternative embodiment of a joining system head according to the invention. 
   

   DETAILED DESCRIPTION 
   In  FIG. 1 , a joining system according to the invention is generally designated  10 . The joining system  10  comprises a robot  12 . The robot  12  contains a stationary base  14  from which two arms  16 ,  18  extend, articulately connected to each other. At the end of the arm  18 , a flange  20  is provided. 
   To the flange  20 , a joining system head is attached, generally designated  22  in  FIG. 1 . The joining system head  22  comprises a baseplate  24  attached to the flange. From the baseplate  24 , an elongated carrier  26  extends. The elongated carrier  26  comprises a first short carrying segment  28  and an adjoining second elongated carrying segment  30 . The second carrying segment  30  is bent off from the first carrying segment  28  by an angle α of 120°. The angle α is preferably between 60° and 80° or between 100° and 120°. In general, however, it is also conceivable that the first carrying segment  28  and the second carrying segment  30  may be oriented on an axis with each other. The axis of the second carrying segment  30  is designated  27  in  FIG. 1 . 
   At the end of the second carrying segment  30 , a joining tool  32  is mounted rotatable about an axis  34 . The axis of rotation  34  extends perpendicular to the axis  27  of the second carrying segment  30  and, in the embodiment shown, is oriented about parallel to the baseplate  24 . 
   The joining tool  32  serves to weld an element, in particular a welding bolt  36 , to a part, in particular a metal sheet  38 . Although the joining system may be employed for numerous kinds of joining as a matter of design, a conformation of the joining system as a bolt-welding system, or short-time arc-welding system with lift ignition, is especially preferred. In the following, therefore, without loss of generality, the joining system will be referred to as a bolt welding system and the joining system head  22  as a bolt welding head. The joining tool  32  will be referred to as a welding tool  32 . The welding tool  32  welds the bolts  36  to the part  38  in a linear motion (joining direction  40 ). 
   The welding head  22  further comprises a control means  42 . The control means  42  is provided at the initial portion of the elongated carrier  26  and, in the embodiment shown, is mounted on the first carrying segment  28 , to wit next to the baseplate  24 . The control means  42  serves to drive the joining tool  32  and as intersection with superordinate control devices. 
   The welding head  22  further comprises a feeding means  44 . The feeding means  44  serves to pick up bolts by the shank in advance from a feed hose and place them in readiness at a transfer station  46 . The feed means  44  is consequently configured essentially as a tube or hose and extends along the elongated carrier  26 . The transfer station  46  is located in a mid-portion of the second carrying segment  30 . In it, one element at a time is placed in readiness for transfer to the welding tool  32 . This element is designated  36 ″ in  FIG. 1 . 
   The welding system  10  further comprises a stationary base station  50 . The base station  50  serves to furnish energy for welding to the welding head  22  and serves as superordinate control device. The base station  50  is connected to an individualizing device  52 . The individualizing device  52  serves to individualize bolts, as a rule supplied in bulk, and convey them to the feeding means  44  individually by way of a hose  54 . For this purpose, the individualizing device  52  as a rule comprises a compressed air unit to convey the elements  36  pneumatically. 
   Further,  FIG. 1  shows a line  56  connecting the base station  50  to the welding head  22 . The line  56  is generally embodied as a system of lines, and includes lines to carry the welding current, control lines etc. Further,  FIG. 1  shows a line  58  connecting the welding head  22  to the base  14  of the robot  12 . The line  58  is optionally provided and contains one or more control lines. By means of the control lines  58 , the motions of the robot  12  can be matched with those of the welding tool  32 . 
   Alternatively or additionally, the base  14  of the robot  12  is connected to the base station  50  by way of a line  60 . Hence it is possible also for the matching to take place between robot  12  and welding head  22  by way of lines  60 ,  56 . The lines  56 ,  58  are passed to the control means  42 , whence some are looped to the welding tool  32  (for energy supply to unit there), others are utilized directly. 
   The welding tool  32  comprises a housing  62  rotatably mounted on the axis  34 . At the housing  62 , a joining drive means  64  is provided in the form of a linear motor  64 . The linear motor  64  serves to move a holding means  66  projecting from the housing  62  perpendicular to the axis of rotation  34  for holding one bolt  36  at a time. The linear motor  64  therefore constitutes a lifting means for executing lift and dip motions in the course of a bolt welding operation, as described in the introduction. 
   Further, at the end portion of the second carrying segment  30 , a rotary drive  68  is provided, serving to rotate the welding tool  32  under control into any angular positions in relation to the second carrying segment  30 . The rotational range is typically at least 270°, commonly 360°. The rotary drive  68  serves firstly to rotate the welding tool  32  into a suitable welding position in each instance, one of which welding positions is shown in solid lines in  FIG. 1 . An alternative welding position is indicated by dot-dash lines at  32 ′. In the further welding position, the welding tool  32 ′ is employed along a welding direction  40 ′ to weld a bolt  36 ′ to a part not explicitly shown. 
   Further, the rotary drive  68  serves as loading drive means. For this purpose, the welding tool  32  is turned into a position shown dotted in  FIG. 1 . In this position, the holding means  66 ″ is oriented flush with the transfer station  46 , and is able in that position to grasp a bolt  36 ″ there held in readiness and take it over for a subsequent welding operation. 
   Although, in the embodiment represented, the loading drive means is constituted by the rotary drive  68  alone, for example an electric motor, modifications of this are conceivable. Thus the loading drive means may for example be constituted in that the—non-rotatable—welding tool  32  is shifted in lengthwise direction on the carrier  26 , to mention one example. It will be understood that then the transfer station  46  would have to be arranged correspondingly in a different place. 
   It is easily seen that the welding tool  32  may be configured with very small dimensions. In the first place, the welding tool  32  is spatially separated from the control means  42 . In the second place, the welding tool  32  is decoupled from the pneumatic bolt-feeding means. So no pneumatic or hydraulic lines need be flanged to the welding tool  32 . The supply of electricity to the linear motor  64  and/or the rotary drive  68  is comparatively easy to arrange. The same applies to the actuation of the holding means  66 , insofar as it is actively actuated electrically. 
   Since the bolts  36  are put into the holding means  66 , not from behind but from in front, no loading pin is required as in the prior art. Therefore the welding tool  32  can be compact in axial direction. It will be understood that instead of a linear motor as joining drive means  64 , alternatively a combination of a spring and a solenoid may be provided. Further, it will be understood that the rotary drive  68  may be configured as an electric step motor having a precision of &lt;1°, better yet 0.5°. 
   The parameters assigned to the rotary motion relate firstly to a welding program and secondly to a program of robot motion. Each welding position has its own welding program and its own robot motion program. By referring the parametric data to the several welding and robot motion programs, it is ensured that firstly the bolt  36  will always be perpendicular to the surface of the part  38 , and secondly the welding tool  32  will be in a position in the robot motion affording the robot maximal freedom of motion on the way to the welding position. The control of the rotary motion of the welding tool  32  may be effected by way of the base station  50  and/or by way of the base  14  of the robot  12 . 
   The oblique angling of the second carrying segment  30  with respect to the first carrying segment  28  offers, firstly, an improved interference edge clearance. Secondly, the feeding means  44  is easier to construct, since the bolts, as shown, are held at the transfer station  46  by gravity and/or blown air. 
     FIG. 1  further shows that the part  38  has the conformation of an angle part having a relatively small aperture  70 . Viewed from the robot  12 , the desired welding position is located inside of a cavity  72 . It is easily seen that the bolt welding system  10  according to the invention is quite especially well-suited to accomplish this object. To introduce the second carrying segment  30  through the opening  70 , the welding tool  32  can be turned into a position in which it is largely flush with the second carrying segment  30 , for example the position  32 ″ in  FIG. 1 . 
   After introduction into the cavity  72 , the welding tool  32  is turned into the welding position indicated by solid lines. Before that, a bolt  36  is picked up from the transfer station  46 , so that it is located in the holding means  66 . Then, in per se conventional manner, a bolt welding operation is carried out, as explained in the introduction. 
   As remains to be set forth in detail below, the holding means  66  is preferably of such configuration that it can release the welded-on bolt  36  in a direction transverse to the welding direction  40 . Consequently, it is possible to turn the joining tool  32  immediately after welding back into the flush position  32 ″, with no need for the second carrying segment  30  to execute a motion in the welding direction  40 . As soon as the flush position  32 ″ has been reached, the second carrying segment  30  can be withdrawn again through the opening  70 . The robot  12  then carries the welding head  22  to the next welding position. The axis of rotation  34  constitutes an additional axis of rotation for the robot  12 . Hence the positioning in a welding position can be accomplished in simpler manner. This the more so as the additional axis of rotation is located near the welding position. 
   Another advantage of the welding system  10  according to the invention results as follows. In the prior art, the welding head as a whole was interference-edge relevant. In the prior art, therefore, no pneumatic valves were provided on the welding head. But this occasioned very complicated cabling between the base station  50  and the welding head  22 . 
   Owing to the spatial separation of the control means  42  from the welding tool  32  at the welding head  22 , the control means  42  itself is not interference-edge relevant. Consequently valves can be integrated into the control means  42  at the welding head  22 , so that the number and complexity of the supply lines can be reduced. Since the control means  42  is provided at the welding head  22 , no great outlay of electric cabling is needed between welding head  22  and base station  50 . For example, it is possible for the supply lines  56  in a hose pack to contain only a welding cable, two auxiliary voltage supplies for the linear motor and a 24-volt supply for the control means, two light guides for serial transmission of measurement and control data and the feed hose  54 . In an enlarged version, the hose pack might be supplemented by a protective gas supply line and/or a jet-suction line, for example for color marking. Hence the hose pack can be lighter in weight, torsionally less rigid and therefore more secure. 
   Besides, the uncoupling of supply means  44  and welding tool  32  makes it possible for the bolts  36  to be fed to the transfer station  46  parallel with the bolt welding operation. In the prior art, bolt feeding and bolt welding are strictly serial. Therefore cycle periods of &lt;1 second are attainable only with great difficulty and under special boundary conditions. 
   According to the invention, immediately after removal of a bolt from the transfer station  46  to initiate a bolt welding operation, another bolt can be conveyed from the individualizing device  52  by way of hose  54  and feed means  44 , to the transfer station  46 . This can be accomplished while the welding tool  32  is performing a bolt welding operation. 
   Also, as the carrier  26  moves from one welding position to the next, the welding tool  32  can be swung to the transfer station  46  and then swung into the right setting for the new welding position. This parallelism also generally ensures that welding cycle periods of definitely less than 1 second are attainable. Although the elements to be welded may basically be of any shape, yet elements feedable by means of compressed air, in particular rotationally symmetrical elements, are especially suitable for processing by the joining system according to the invention. The further welding position  32 ′ may for example be an overhead position, like the position  32 ′ shown. This can be attained without need to rotate the carrier  26 . This avoids overstraining the supply cable and hoses. 
   In the following description and details and modifications of the joining system shown in  FIG. 1 , like or similar elements are designated by the same reference numerals. Identical designation generally implies like or similar mode of operation, unless expressly otherwise noted below. Where individual elements of the joining system are discussed, it may be assumed that the function is otherwise identical or similar to the function of the joining system  10  of  FIG. 1 . Further, it will be understood that subsequent references to welding systems, heads or tools are intended to refer generally to such elements for joining, including for example riveting or bonding processes. 
     FIG. 2  shows an alternative embodiment of a welding head  22 . In contradistinction to the welding head  22  of  FIG. 1 , a rotary drive  68 ′ is provided to rotate the welding tool  32 , not in the end portion of the second carrying segment  30 , but in the region of the control means  42 . The rotary motions of the rotary drive  68 ′ are transmitted to the welding tool  32  by means of a belt drive  80 . The belt drive  80  runs along the elongated carrier  26 . The elongated carrier  26  is formed in the representation of  FIG. 2  by two parallel arms, between whose end portions the welding tool  32  is rotatably mounted. 
     FIGS. 3 and 4  represent an embodiment of a holding means  66 . The holding means  66  comprises a housing  84 , in turn comprising an opening  86  facing downward in the joining direction. The holding means  66  comprises two jaws  88 A,  88 B mounted with limited swingability on the housing  84  and made of an essentially inelastic material. The jaws  88 A,  88 B form a tongs, an element  36  being grasped between the ends of the jaws  88 A,  88 B with a predetermined force. 
   The jaws  88 A,  88 B are each connected in one piece with a lever segment  92 A,  92 B. With respect to axes  90 A,  90 B on which the jaws  88 A,  88 B are mounted, the lever segments  92 A,  92 B extend in the respective other direction. The lever segments  92 A,  92 B are here bent off relative to the joining direction  40 , so that they overlap. By pressure on the lever segments  92 A,  92 B from above (in the representation of  FIG. 3 ), the jaws  88 A,  88 B are consequently moved away from each other, releasing the bolt  36 . This is shown for the jaw  88 A in  FIG. 3 . It may be seen that the jaw  88 A releases the bolt  36  completely in the direction transverse to the joining direction  40  (that is, in  FIG. 3 , out of the plane of the paper). Consequently the holding means  66  with opened jaws  88 A,  88 B can be moved transverse to the joining direction  40  and perpendicular to the plane of the jaws  88 A,  88 B without touching the bolt  36 . The direction of motion of the jaws  88 A,  88 B in this operation is designated  93  in  FIG. 4 . 
   To actuate the lever segments  92 A,  92 B, an actuator  94  is provided, preferably triggered electrically. The actuator  94  opens and closes the jaws  88 A,  88 B actively in each instance. It will be understood that for this purpose the actuator  94  must be configured as a two-directional drive. 
   Active actuation of the jaws  88 A,  88 B has the advantage that the bolt  36  can be held with a defined force (for example 20 newtons). The derivation of the holding force from the elasticity of the several fingers of the tongs, as in the prior art, is dispensed with. Consequently a definitely longer service life can be attained. The direction of actuation of the actuator  94  is shown at  96  in  FIG. 3 . At their ends, the jaws  88 A,  88 B are of such conformation that they can securely grasp the bolt  36  in question. For this purpose, it may be appropriate to place suitable adapters on the jaws  88 A,  88 B, in order to fit different bolts  36 . 
   From the under side of the housing  94 , as shown in  FIG. 3 , a positioning pin  98  extends. The positioning or contact pin  98  is rigidly connected to the housing  84 . It serves, when a bolt  36  is picked up from the transfer station  46 , to ensure that the bolt  36  will occupy a defined position in relation to the holding means  66 , and as a stop to assume the axial forces in welding. 
   The two-directional active actuator can consist of a pneumatic or hydraulic drive. Preferably, however, it consists of a combination of two electromagnets, or of an unregulated linear motor on the ‘moving coil’ or ‘moving permanent magnet’ principle. Further, it is possible to configure the actuator  94  as semi-active. Then the opening of the jaws  88 A,  88 B is effected for example by an electromagnet. When this is switched on, suitably arranged springs serve to ensure that a bolt  36  will be grasped by the jaws  88 A,  88 B with a defined force. 
   For welding, the jaws  88 A and/or  88 B are supplied with welding current, conducted to the bolt  36 . The defined force provides for a secure, low-wear passage of current. For this reason, it will be understood that the jaws  88 A,  88 B will be made of a conductive metal. The positioning pin  98 , however, should be non-conductive, or insulated from the housing  84 . 
   Alternatively to an active or semi-active holding means  66 , it is possible also to provide jaws of elastic configuration, permitting a lateral introduction of the bolt  36  between them (in the direction  93 ) and releasing them without substantial exertion of force upon motion transverse to a welded-on bolt  36 . In  FIG. 3 , the longitudinal axis of the holding means  66  is designated  100 . 
   In  FIGS. 5 and 6 , an additional alternative embodiment of a welding tool  32  is shown. The welding tool  32  comprises a tool housing  102  to which a linear motor  104  of a joining drive means  64  is fixed. The axis or centerline of the linear motor  104  is shown at  105 . It is represented that the axis  100  of the holding means  66  and the axis  105  of the linear motor  104  are spaced at a distance d from each other. In this way the holding means  66  is shifted out of the projection of the linear motor  104  in joining direction. This makes it possible to position the holding means  66 , and therefore a held bolt  36 , closer to an interfering wall or edge. Upon the whole, this enhances the flexibility of the welding head  22 . 
   The linear motor  104  comprises an armature segment  106  connected to a guide plate  108  extending transverse to the joining direction. From the guide plate  108 , two guide rods  110 ,  112  extend, arranged diagonally in relation to the linear motor  104 . The guide rods  110 ,  112  ensure that the guide plate  108  is guided free from tilt. From the under side of the guide plate  108 , the holding means  66  extends. An actuator to actuate the hold means  66  may for example be configured on top of the guide plate  108  or integrated therein. 
   In  FIG. 6 , it is shown that the carrier  26  is made up of a comparatively massive carrying arm  116  and a less massive tension arm  118  extending parallel thereto. The welding tool  32  is mounted between the arms  116 ,  118  along the axis of rotation  34 . In  FIG. 6 , current cables  120 A,  120 B to supply current to the jaws  88 A,  88 B are also indicated. 
   Another alternative conformation of a joining tool  32  is shown in  FIG. 7 . The welding tool  32  comprises a linear motor housing  122 . At the tops of the guide rods  110 ,  112 , flanges  123  are provided in each instance. Between the flanges  123  and the linear motor housing  122 , compression springs  124  are arranged, configured around the guide rods  110 ,  112 . The linear motor  104  is consequently so pre-stressed by the compression springs  124  that the guide plate  108  moved thereby is located in the retracted, to wit not extended, position. In addition to the compression springs  124  or alternatively thereto, an additional compression spring  126  may be provided inside of the linear motor housing  122 . 
   Further, it is shown that on top of the guide plate  108 , a hinged magnet  128  is articulated to an axis  130 . The magnet  128  serves to press the lever segments  92 A,  92 B downward to open the jaws  88 . In general, however, the lever segments  92  are pre-stressed towards the closed position of the jaws  88  by means of a tension spring  132 . 
     FIG. 8  shows a first embodiment of a transfer station  46  of the feeding means  44 . At the transfer station  46 , two opposed sensors  136  (for example a light barrier) are provided, detecting whether there is a bolt  36  in the transfer station  46  or not. 
   The feeding means  44  consists essentially of a tube or hose  138 , bent off inward in the region of the transfer station  46 . The bolts  36  are fed shank first from the individualizing device  52  through the feeding means  44 . Consequently the head of the bolt  36  will strike the rolled edge of the tube  138  and remain so in the transfer station  46 . Thus the shank of the bolt  36  protrudes from the tube  138 . 
   The holding means  66  can now be run with opened jaws  88 A,  88 B all the way to the bolt  36  and grasp it. Then the holding means  66  is swung back again, out of the plane of the paper in the representation of  FIG. 8 . It will be understood that at the transfer station  46 , a suitable lateral recess must be provided in the tube  138 , though not explicitly shown in  FIG. 8 . 
   An alternative embodiment of a transfer station  46 ′ is shown in  FIG. 9 . In this embodiment a tube  138 ′ of the feeding means  44 ′ is open towards the end. At a transfer housing  140 , two clamping jaws  142 A,  142 B are rotatably mounted. The jaws  142  are pre-stressed by means of two springs  144  into a position where their inner sides block the exit of a bolt  36  from the tube  138 ′. The bolt  36  is braked thereby upon being fed. Here a positioning lever  146  is swung laterally out of the representation shown in  FIG. 9  to let the bolt  36  through. Then the positioning lever  146  is swung, as indicated at  147 . Thus the bolt  36  presses the jaws  142 A,  142 B apart and is shifted away from the tube  138 ′ until the head of the bolt  36  snaps into an annular recess  148 . The annular recess  148  is formed by the inner sides of the jaws  142 A,  142 B. In this position, the bolt  36  is definitely held with a certain force. The holding means  66  may, as in  FIG. 8 , grasp the shank of the bolt  36  and pull it laterally out of the annular recess  148 . 
   Over the embodiment of  FIG. 8 , this embodiment has the advantage that the bolt  36  is in a defined position in the transfer position  46 ′, and is held with a defined force, so that a secure hold on the bolt  36  by the holding means  66  is ensured. It will be understood that at the transfer station  46 ′ also, suitable sensors may be provided to detect a bolt  36  in the transfer position. 
   A third embodiment of a transfer station  46 ″ is shown in  FIGS. 10 and 11 . In this embodiment, the bolts  36  are conveyed by way of a tube  138 ″ into a bolt receptacle  154  of a swingable rotational segment  152 . The segment  152  is rotatable about a transfer housing  140 ″ about an axis  153  oriented transverse to the axis of the tube  138 ″ and transverse to the orientation of the bolt  36  in the transfer position. 
   In  FIGS. 10 and 11 , the segment  152  is in a transfer position. In this position, a pneumatic cylinder  156  serves to push the bolt  36  by means of a plunger  158  between two tension jaws  160 , between which the bolt  36  is then held in a defined manner. Then the segment  152  is turned back about transfer housing  140 ″ 0  to pick up another bolt  36  in the receiving position shown dotted, flush with the tube  138 ″. This embodiment has the advantage that the bolts  36  can be conveyed at high speed through the tube  138 ″. Hence short cycle periods can be achieved. 
   Another embodiment of a welding head according to the invention is generally designated  170  in  FIG. 12 . The welding head  170  comprises, at the anterior end of the carrier  26 , a welding tool  171  comprising only a housing rotatably mounted on the carrier  26  and a holding means  172  fixed thereto. The welding tool  171  is not provided with a welding drive motor, in particular not a linear motor. 
   The holding means  172  comprises two jaws  174  between which a bolt  36  is so held that it is oriented tangential to a circumference around the axis of rotation  34 . In other words, a joining operation does not occur along a rectilinear motion, but along a circular path. The corresponding direction of guidance is indicated in  FIG. 12  as a partial circle  176 . 
   In this embodiment, a rotary drive  175  serves as joining drive means, arranged in the region of the control means  42 . Rotary motions of the drive  175  are transmitted by a belt drive  80  to the welding tool  171 . It will be understood that the rotary drive  175  is preferably an electric precision step motor with which the difficult movements of the bolt  36  can be executed during a bolt welding operation. The rotary drive  175  thus serves simultaneously also as loading drive, being swung so as to pick up one new bolt  36  at a time from a transfer station  180  of a feeding means  178 . 
   In the feeding means  178 , the bolts  36  are not fed successively but side by side, in such manner that the holding means  172  can grasp the bolts  36  transverse to their own extent. It will be understood that the feeding means  178  may either comprise suitable means of converting the lengthwise motion out of the individualizing device  52  into the transverse orientation shown in  FIG. 12 , or alternatively it is possible to feed the bolts  36  out of the individualizing device  52  already in transverse position. 
   Further, in  FIG. 12  schematically a magazine  186  provided on the carrier  26  is provided. The magazine  186  may serve as supply magazine for a plurality of bolts  36 , then to be transferred by means of a suitable integrated individualizing device to the feeding means  178 , or to the transfer station  180 . It will be understood that such a magazine may also be employed in the embodiments of  FIGS. 1 to 11  instead of a stationary individualizing device  52  or in addition thereto.