Process monitoring method for monitoring the joining of at least two workpieces by means of a self-piercing riveting device having joining tongs, and device having a control unit designed to carry out such a method

Abstract

A process monitoring method for monitoring the joining of at least two workpieces by means of a self-piercing riveting device having a joining gun including a) a total sheet metal thickness, a rivet length and a bending constant of the joining gun are detected for a plurality of joining operations for each operation before joining; b) a punch path for a certain force, an end force, a path end value for the end force, a setting path of the rivet, and a rivet head end position are detected for the plurality of joining operations; c) a force/path window for the joining operations is determined; and d) a determined force/path window of one additional joining operation is compared to the force/path window of the joining operations determined in step c), and the additional joining operation is monitored so that any joining-related defects are identified.

1 . A process monitoring method for monitoring the joining of at least two workpieces by means of a self-piercing riveting device having joining tongs, wherein the at least two workpieces are arranged between a punch and a counter-holder, and wherein a rivet arranged between the punch and a workpiece facing the punch is pressed by means of the punch into the at least two workpieces when a force F in the direction towards the counter-holder is applied to the punch, wherein
 a) a total thickness, a rivet length, and a bending constant of the joining tongs are recorded for a plurality of joining operations before joining,   b) a punch displacement at a specific force, an end force, a displacement end value at the end force, a setting displacement of the rivet, and a rivet head end position are recorded for the plurality of joining operations during and/or after joining,   c) in dependence on the parameters recorded in step a) and step b), an absolute force/displacement window and/or a relative force/displacement end window of the plurality of joining operations is/are determined, and   d) a determined absolute force/displacement window and/or a relative force/displacement end window of at least one further joining operation is/are compared with the absolute force/displacement window and/or the relative force/displacement end window determined in step c) of the plurality of joining operations, wherein the at least one further joining operation is monitored so that a fault in joining is identified.   
     
     
         2 . The method as claimed in  claim 1 , wherein relevant quality-determining regions of the joining operation are determined, wherein the process monitoring is reduced to the relevant quality-determining regions. 
     
     
         3 . The method as claimed in  claim 2 , wherein the relevant quality-determining regions are determined time-dependently and/or displacement-dependently. 
     
     
         4 . The method as claimed in  claim 1 , wherein, in step d), a fault in joining is identified above a specific threshold value of a difference between the determined absolute force/displacement window and the relative force/displacement end window of the further joining operation and the determined absolute force/displacement window and/or the relative force/displacement end window of the plurality of joining operations. 
     
     
         5 . The method as claimed in  claim 4 , wherein the threshold value is determined by comparison with at least one standard component and/or by learning on the basis of a plurality of joining operations. 
     
     
         6 . The method as claimed in  claim 1 , wherein the parameters in step a) and in step b) are determined in at least one specific time and/or displacement range during pressing in of the punch. 
     
     
         7 . The method as claimed in  claim 1 , wherein a gradient is not used for identifying a fault in joining. 
     
     
         8 . The method as claimed in  claim 1 , wherein the recorded parameters in step a) and b) for determining the absolute force/displacement window and/or the relative force/displacement end window are weighted differently. 
     
     
         9 . The method as claimed in  claim 1 , wherein at least one of a force/displacement profile and/or a force/time profile of the joining operation, is used for the process monitoring. 
     
     
         10 . A device having a control unit designed to carry out a method as claimed in  claim 1 , wherein the device further has a self-piercing riveting device having joining tongs, a punch and a counter-holder, wherein a force F in the direction towards the counter-holder can be applied to the punch so that a rivet can be pressed into at least two workpieces arranged between the punch and the counter-holder, wherein the device has sensors for recording the parameters, and at least one evaluation unit for determining an absolute force/displacement window and a relative force/displacement end window of a plurality of joining operations and comparing it with a determined absolute force/displacement window and/or a relative force/displacement end window of at least one further joining operation. 
     
     
         11 . The method as claimed in  claim 1 , wherein the parameters in step a) and in step b) determined in at least one time and/or displacement range in which, based on experience, a fault is to be expected.