[0001] The present invention relates to a method for joining studs to work- pieces, in particular for the purpose of stud welding, and to a joining appliance for executing such a method.

[0002] Various methods for joining fastening elements, such as studs, to a workpiece, such as a plate, are known in the prior art. There is known, for example, the practice of adhesive-bonding or thermoplastically welding fastening elements to component parts. Further, so-called stud welding is an established process for welding metallic fastening elements to metallic workpieces.

[0003] These types of joining method are used extensively in the automobile industry, for example, wherein studs are joined to workpieces such as vehicle- body plates or other vehicle-body portions. The studs then frequently serve as anchors for fastening mount-on parts or for clipping-on plastic clips to which, in turn, other component parts can be fixed, such as electrical lines, brake hoses, etc.

[0004] In industrial application, these types of joining method are usually automated, a joining head usually being guided in an automated manner by a robot. Further, there is known the practice of automated feeding of the studs to the joining head, for example by means of compressed air. In the joining head, the studs are then usually each held singly by a holding device or a gripper, before they are joined to the workpiece.

[0005] Known in this case is the practice of acquiring a dimension of a stud to be joined, before the actual joining operation, through application of a sensing element to the stud and through acquisition of a travel distance of the sensing element required for this purpose. It can thereby be ensured, for example, that studs of the correct shank diameter are joined. In the case of the sequential processing of studs of differing dimensions, in particular, such a measuring operation before the actual joining operation is of particular importance. In the event of an incorrect stud being erroneously present in the joining head, it can still be removed before the actual joining operation is performed. Elaborate repair works can thereby be avoided. In this case, the dimension of the stud is generally compared with an allowable tolerance range.

[0006] Against this background, it is an object of the invention to specify an improved method for joining studs to workpieces, in particular for the purpose of stud welding, and to provide an associated joining appliance.

[0007] The above object is achieved by a method for joining studs to work- pieces, in particular for the purpose of stud welding, comprising the steps of: acquiring at least one dimension of a stud to be joined, through application of a sensing element to the stud and through acquisition of a travel distance of the sensing element required for this purpose; and joining the stud to a workpiece, if the dimension of the stud is within an allowable tolerance range, the allowable tolerance range being adapted in dependence on the travel distance of the sensing element in the case of at least one preceding method step.

[0008] Further, the above object is achieved through an appliance for executing such a method.

[0009] It has been shown that, in execution of a multiplicity of joining operations, the acquisition of the dimension of the stud to be joined is rendered difficult in that either the sensing element becomes abraded with time, or deposits accumulate thereon. Consequently, in the prior art, it is necessary either to replace the sensing element at cyclic intervals or, alternatively, manually to recalibrate the allowable tolerance range in order to compensate the change in the length of the sensing element. In the case of the manual calibration operation, manual intervention becomes necessary, in the case of successive joining operations, for example, when one or more studs are outside of the tolerance range. In the case of this manual intervention, the variation of the sensing element is then measured, for example, on the basis of a supplied stud, and the tolerance range is reset manually.

[0010] The present invention, by contrast, renders possible an automatic adaptation of the allowable tolerance range, this being in dependence on the travel distance of the sensing element in the case of at least one preceding method step. This can either be effected in the case of each joining operation or, alternatively, cyclically, after a defined number of joining operations. It thereby becomes possible for the creeping change in the length of the sensing element (whether due to abrasion or deposits) to be corrected continuously during operation.

[0011] Whereas, in the prior art, recalibration of the tolerance range is effected whenever one or more studs are outside of the originally set tolerance range, according to the invention the tolerance range is adapted automatically to the change in the length of the sensing element. Manual interventions can therefore usually be avoided, up to a point in time at which the sensing element must be replaced completely. Consequently, there is absolutely no need, or at least there is no longer such a frequent need, for manual intervention in order to recalibrate the sensing element for the purpose of compensating the wear on the sensing element.

[0012] The dimension of the stud can be a diameter, such as a shank diameter or a flange diameter of a stud. However, the dimension of the stud to be joined can also be a length such as, for example, a shank length, a stud length or a flange length.

[0013] In the present case, the term joining is to be understood in a wide sense, and comprises both stud welding and methods for adhesive-bonding of studs to workpieces and methods for thermoplastic welding of studs to workpieces. Consequently, both the studs and the workpieces can be made of any materials such as, for example, metal, plastic, composite materials, etc.

[0014] Also, in the present case, the term stud is to be understood in a wide sense, and comprises both rotationally symmetrical component parts and irregularly shaped component parts, such as plate pieces that are joined at an end face to a workpiece ("Weldfast"®).

[0015] The allowable tolerance range can be matched to the respective application, and can be relatively wide. However, the tolerance range can also be relatively narrow, and in extreme cases can be a single, defined value.

[0016] In order to prevent the joining of non-fitting studs, the acquisition of the at least one dimension of a stud to be joined is preferably effected before a respective joining operation. Generally, however, it is also possible for such an acquisition operation to be performed after the joining operation.

[0017] The travel distance of the sensing element that is used for determining the dimension of the stud can be acquired either directly or, also, indirectly, for example through an angle of movement of the sensing element, through the speed of the sensing element, etc.

[0018] The allowable tolerance range can be an individual setpoint value, which is adapted in dependence on the travel distance of the sensing element in the case of at least one preceding joining operation. In this case, it is possible for a tolerance region to be adapted indirectly, in that the tolerance region is coupled to the setpoint value for the necessary travel distance. The tolerance range can thus also be adapted automatically.

[0019] Overall, it is possible to dispense with cyclic, manual calibration of the sensing element. The periods between interventions by the plant personnel can be extended. Disruptions are minimized, in that incorrect stud diameters are detected at an early stage. This is because the setpoint value, or the allowable tolerance range, is adapted to the wear on the sensing element.

[0020] The above object is thereby achieved in full.

[0021] According to a particularly preferred embodiment, the travel distances of the sensing element are acquired during application to a stud to be joined, in the case of at least one preceding joining operation, for the purpose of adaptation of the allowable tolerance range.

[0022] In the case of this embodiment, automatic adaptation of the tolerance range in dependence on the stud dimension of at least one preceding joining operation is preferably effected in the case of each joining operation or, alternatively, cyclically.

[0023] In the case of this embodiment, the automatic adaptation of the tolerance range can be effected without any structural change to the joining tool. The method according to the invention can be realized by means of software. [0024] It is particularly advantageous if the allowable tolerance range is adapted in dependence on the travel distances of the sensing element in the case of a plurality of preceding joining operations.

[0025] The adaptation of the tolerance range can thereby be effected with greater accuracy. In particular, it is possible to equalize certain dimensional variations. This can be effected, for example, through appropriate filtering of the preceding measurement values of the travel distances. It is also possible in this case for more recent measurement values to be given a greater weighting than older measurement values.

[0026] It is particularly preferable if the allowable tolerance range is adapted in dependence on a sliding mean value of the travel distances of the sensing element in the case of the plurality of preceding joining operations. A relatively accurate adaptation of the tolerance range can be effected without need for a high computing capacity.

[0027] In the case of a sufficiently high number of preceding joining operations being included in the formation of the sliding mean value, the adaptation of the allowable tolerance range can be effected even if the dimension, for example of one of the plurality of joined studs, was outside of the allowable tolerance band.

[0028] Preferably, however, the formation of the sliding mean value is effected on the basis of a relatively small number of preceding joining operations, for example fewer than 10. It is preferred, in particular, if the adaptation of the allowable tolerance range is effected only if the dimension of the stud in the preceding joining operation was within the allowable tolerance band.

[0029] This prevents the dimensions of incorrect studs from being included in the adaptation of the tolerance range. [0030] According to a further preferred embodiment, for the purpose of adaptation of the additional tolerance range the travel distance of the sensing element is acquired during application to a reference mandrel.

[0031] Such an adaptation operation can be effected, for example, cyclically, in such a way that the tolerance range is always adapted at a sufficiently early stage, before there is a risk of a correct stud falling out of the tolerance range. For example, such an adaptation operation can be performed after a number of joining operations, which number is less than 10,000, in particular less than 5,000, in particular less than 2,000. The reference mandrel can be a specially produced standard part or, alternatively, a "normal" stud.

[0032] Further, it is advantageous overall if the stud is gripped by a mechanical gripper before being joined to the workpiece.

[0033] It is particularly advantageous in this case if the gripper has at least one gripper arm, which constitutes the sensing element.

[0034] This enables the method to be performed with a small amount of structural resource.

[0035] It is understood that the above mentioned features and those to be explained in the following can be applied, not only in the respectively specified combination, but also in other combinations or singly, without departure from the scope of the present invention.

[0036] Exemplary embodiments of the invention are represented in the drawing and described more fully in the following description. In the drawing:

Fig. 1 shows a schematic representation of a joining appliance according to the invention; Fig. 2 shows a representation of a measured stud diameter and of an associated allowable tolerance range, as obtained over time, or over the number of joining operations performed, in the performance of the method according to the invention;

Fig. 3 shows a representation comparable to that of Fig. 2, wherein the tolerance ange is redefined through a manual intervention; and

Fig. 4 shows a representation comparable to that of Fig. 2, wherein the tolerance range is effected through automatic calibration on a reference mandrel.

[0037] In Fig. 1, a joining appliance is denoted in general by 10. The joining appliance 10 serves to join studs 12 to workpieces 14, for example through stud welding, stud adhesive-bonding, etc. The joining appliance 10 has a robot 16, on the arm of which there is arranged a joining head 18. Arranged on the joining head 18 is a gripper 20, for gripping one stud in each case. Further, the joining appliance 10 comprises a magazine 22, in which studs 12', 12" that are subsequently to be joined are made available. It is further shown in Fig. 1 that a stud 12'" has already been joined to the workpiece 14. After the stud 12 held by the gripper 20 in Fig. 1 has been joined to the workpiece 14, the joining head 18 can be moved, by means of the robot 16, towards the magazine 22, in order to grip a further stud 12' by means of the gripper 20 and thereby supply it to a further joining operation. A stud, denoted by 12" in Fig. 1, is then joined.

[0038] In the present case, the studs 12 are represented schematically as flanged studs, which have a shank diameter D.

[0039] In the case of the appliance represented in Fig. 1, it is provided that, before a joining operation, this shank diameter D is measured in order to determine whether the stud 12 held by the gripper 20 is the correct stud type for the joining operation to follow.

[0040] It is understood, however, that means can also be provided in the joining head in order to measure, as an alternative or in addition thereto, the flange diameter, the stud length, the shank length or other dimensions of the stud.

[0041] The dimension of the stud, in the present case the shank diameter D, is measured in that a sensing element is applied to the stud and in that the travel distance of the sensing element required for this purpose is acquired. In the present case, the acquisition of the dimension is effected by means of a displacement sensor 24, which is represented schematically in Fig. 1 and which is designed to acquire the travel distances of a first gripper arm 26 and a second gripper arm 28 of the gripper 20.

[0042] Thus, Fig. 1 shows that the first and the second gripper arm 26, 28, before gripping a stud 12, are in a zero position, denoted in Fig. 1 by Poi and P ₀ 2, respectively. For the purpose of gripping the stud 12, the gripper arms 26, 28 are moved towards one another, wherein they travel a first and a second travel distance Pi and P ₂ , respectively.

[0043] As soon as the gripper arms 26, 28 bear on the shank of the stud 12, its diameter D can be calculated by means of the formula

[0044] This manner of calculating the dimension of the stud is to be understood as being merely an example. It is conceivable, in other embodiments, for gripper elements to move rotationally onto the stud, such that a rotational angle of the gripper arms is acquired. It is also conceivable for one of the gripper arms to be held rigid, and only a second gripper arm to be moved onto the rigid gripper arm. It is also possible to provide more than two gripper arms. In the present case, the diameter D of the stud shank is calculated through acquisition of the travel distances of the gripper arms 26, 28. Alternatively, however, it is also possible for the acquisition of the dimension of a stud to be joined to be performed by a measuring device that is realized so as to be separate from the gripper 20. Such a separate measuring device can be arranged on the joining head 18, but can also be arranged in the region of the magazine 22.

[0045] During operation of the joining appliance 10, wear occurs on the bearing contact surfaces of the gripper arms 26, 28. Usually, this is a certain amount of abrasion, but could also be a deposit of materials.

[0046] As a result, for the example of the abrasion of the gripper arms 26, 28, there arises the situation whereby, after a first calibration, the measured shank thickness D becomes ever smaller, since the travel distances Pi, P ₂ to be traveled by the gripper arms 26, 28 becomes ever greater.

[0047] Fig. 3 shows an example wherein, in the case of a calibration operation ii, a measurement value DMESS is determined as being in accordance with an actual shank diameter, and wherein a tolerance range ΤΊ is defined around this calibrated value of DMESS. With the performance of a plurality of joining operations and the consequential abrasion of the gripper arms 26, 28, the value of D _ME SS reduces more and more, until finally it reaches the edge of the tolerance range T A manual intervention operation M is used to recalibrate the measuring system, such that, at an instant i ₂ , a new value of DMESS is defined as a correct measurement value and, accordingly, a new allowable tolerance range T ₂ is defined.

[0048] Fig. 2 shows that the allowable tolerance range T can be adapted automatically, during operation, to the abrasion of the gripper arms 26, 28. This can be effected, for example, in that the allowable tolerance range is adapted in dependence on a sliding mean value of the travel distances Pi, P ₂ of the gripper arms 26, 28 in the case of a plurality of preceding joining operations. Consequently, there is no need for the cyclic, manual recalibration M of the acquisition system. This enables the intervals of intervention by the plant personnel to be extended. Further, it is possible to minimize disruptions, which can occur as a result of incorrect stud diameters being identified. This is because the setpoint value, and the therewith associated allowable tolerance range T, are adapted to the wear on the gripper arms 26, 28.

[0049] In the present case, for the allowable tolerance range T in Fig. 2 a linear drop is obtained, which corresponds to a linear wear. If the wear is non-linear, the allowable tolerance range, correspondingly, will also be adapted in a non-linear manner.

[0050] A further embodiment of a method according to the invention is shown in Fig. 4. In the case of this method, a reference mandrel 30, as represented in Fig. 1, is used for the automatic adaptation of the tolerance range T. The reference mandrel 30 is located in the gripper region of the joining head 18, and can be approached by the joining head 18, between two joining operations, in order to perform an automatic recalibration by means of the gripper arms 26, 28. The diameter of the reference mandrel 30 is the standard diameter D of a stud 12.

[0051] Fig. 4 shows that, at an instant ii, for example before a first joining operation, calibration on the basis of the reference mandrel 30 is effected with new gripper arms 26, 28, and a first tolerance range Ti is then set.

[0052] At a second instant i ₂ , which, expectedly, is significantly before an instant at which the value of DMESS falls out of the first tolerance range Ti owing to the wear on the gripper arms 26, 28, a second measuring step is effected automatically, through use of a reference mandrel 30. Owing to the abrasion of the gripper arms 26, 28 that has occurred in the interim, there is a displacement of the tolerance range, as shown at T ₂ . After a defined number of joining operations (the number being, for example, <10,000, in particular <5,000, and particularly preferably <2,000), there is again an automatic operation to adapt the tolerance range through use of the reference mandrel 30 (at i ₃ , ...)·

[0053] In the case of this embodiment, owing to the use of a precise reference mandrel 30, there is no need to form a sliding mean value for the purpose of automatic adaptation of the tolerance range. The structural resource requirement for setting up such an automatic tolerance adaptation method is comparatively small, and consists, substantially, in a software adaptation and in the provision of a reference mandrel 30 in the gripper region of the joining head 18.