Abstract:
The invention relates to a device and method for determining parameters of a welding system. According to the invention, a welding area ( 18, 21, 22 ) is subjected to the action of ultrasonic waves, preferably to the action of shear waves, by using an ultrasound source ( 14 ). During a first welding process (n 1 ), a signal processing ( 30 )) determines a first ultrasonic permeability (Dn 1 ) from a received ultrasonic signal (UE). During at least one subsequent welding process (n 2 ), a second ultrasonic permeability (Dn 2 ) is determined from an ultrasonic signal (UE) that is received during a renewed exposure of the welding area ( 18, 21, 22 ) to ultrasonic waves. A display ( 32 ) and/or a diagnostic function and/or a correction of control quantities of the welding system is/are carried out as a function of the at least two ultrasonic permeabilities (Dn 1 , Dn 2 ).

Description:
BACKGROUND OF THE INVENTION  
         [0001]    The invention is based on a device and a method for determining parameters of a welding system according to the general class of the independent claims.  
           [0002]    A method for evaluating resistance-welded joints is made known in DE-A 43 25 878. In order to evaluate welding procedures on-line, the ultrasonic permeability of the welded joint is determined by acting upon it with distortional waves. To this end, the mean ultrasonic energy is determined—during each current half-wave of the welding current—from the output signal of the ultrasonic receiver within a time window that is delayed by a defined lag time as compared with the constant ultrasonic transmitted signal. Said mean ultrasonic energy is used as a measure of the quality of the welded joints. To regulate the welding process, the variation of the ultrasonic permeability can be compared with a prespecified sample trace in order to change the welding parameters, e.g., current intensity, accordingly if a deviation occurs, so that the subsequent ultrasonic permeability values can conform with the sample trace once more.  
           [0003]    The object of the invention is to determine further parameters relevant for the welding procedure that provide an indication of the condition of the welding system. It is desirable, in particular, to determine the wear of an electrode of the welding system exactly so that it can be displayed or so that maintenance intervals for the electrodes can be stated automatically. It will also be made possible for the closed-loop control system to take the aging process of the welding electrode into account.  
           [0004]    This object is attained by means of the features of the independent claims.  
         ADVANTAGES OF THE INVENTION  
         [0005]    The device according to the invention for determining parameters of a welding system uses an ultrasonic source to act on a welded region with ultrasonic waves, preferably distortional waves. In a first welding procedure, a signal processing determines—based on a received ultrasonic signal—a measure of a first ultrasonic permeability of the welded region. It also determines—based on an ultrasonic signal received in a further welding procedure—a measure of a further ultrasonic permeability of the welded region. The measure of the first ultrasonic permeability and the further ultrasonic permeability are stored in order to trigger a display and/or a diagnostic function and/or to correct control variables of the welding system. It has been demonstrated that the measure of the ultrasonic permeability changes characteristically as the number of welding procedures increases. This is due to wearing of the electrodes and/or the electrode caps. As the duration of welding increases, the ultrasonic permeability of the welded region increases. This understanding is taken into consideration by storing at least two corresponding ultrasonic permeability values and performing a subsequent evaluation. It is therefore possible, according to the invention, to determine the condition of the electrodes and/or the electrode caps immediately during the on-going welding procedure based on the changing ultrasonic permeability. Maintenance and/or inspection intervals during which the condition of the electrodes is typically examined can therefore be eliminated.  
           [0006]    In an advantageous further development, the ultrasonic permeability depending on the number of welds or a variable dependent thereon is compared with a limit value that, when exceeded, indicates to the operator that maintenance must be performed on the electrode and/or the electrode cap. For example, the electrode cap must be completely replaced or milled off. The welding device can therefore be monitored automatically by the signal detection. The device automatically indicates when the operator should intervene. Additionally, a control signal can be generated automatically with which an automatic maintenance function is activated. For example, an automatic milling system starts to mill the worn electrode caps and/or electrodes on its own. The production process can be further optimized by the fact that the maintenance function can be activated as needed.  
           [0007]    The current regulation of the resistance-welding process can also be influenced based on the electrode wear curve. Preferably, as the number of welding processes increases, the current should be increased at the same rate as the increase in ultrasonic permeability. As a result, the current density through the welded region is kept constant, which contributes to consistent quality of the weld. This current adjustment can be carried out continuously, as a result of which a consistently high quality of the welds and the resultant welding spots is obtained automatically, even as the wear on the electrodes and/or electrode caps increases.  
           [0008]    In an advantageous further development, the determined ultrasonic permeability values are subjected to a certain smoothing procedure to determine a trend characteristic based on said values. This ensures that individual data points do not mistakenly activate the maintenance display.  
           [0009]    Additional advantageous further developments result from the further dependent claims and their description. 
       
    
    
       [0010]    An exemplary embodiment of the invention is shown in the drawings and is described in greater detail below.  
         [0011]    [0011]FIG. 1 is a block diagram of the device according to the invention,  
         [0012]    [0012]FIGS. 2 a ,  2   b  show the ultrasonic transmitted and received signals,  
         [0013]    [0013]FIGS. 3 a  through  3   c  show the associated trigger and current variations with associated ultrasonic permeability,  
         [0014]    [0014]FIG. 4 shows the characteristic ultrasonic permeability variations as a functionn of the number of welding spots and/or welds, and  
         [0015]    [0015]FIGS. 5 a  and  5   b  show variations in ultrasonic permeability, the ultrasonic permeability trend characteristic, and the current as a function of the number of welds. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0016]    A first welding electrode  11  is acted upon by a current  1 . An ultrasonic transmitter  14  is located at the first welding electrode  11 . An ultrasonic receiver  18  is placed on the outer wall of a second welding electrode  12 . A first electrode cap  19  is mounted on the end of the first welding electrode  11 , and a second electrode cap  20  is mounted on the end of the second welding electrode  12 . A first metal sheet  21  and a metal sheet  22  joined by a welding spot  18  are located between the two electrode caps  19 ,  20 . The ultrasonic transmitter  14  is acted upon by a transmitted signal US furnished by a transmitter control  24  as a function of a trigger signal Trig from a welding control  28 . The transmitted signal US is sent to the ultrasonic receiver  16  via the first electrode  11 , the first electrode cap  19 , the first and second metal sheet  21 ,  22 , the welding spot  18 , the second electrode cap  20 , and via the second electrode  12 .  
         [0017]    The ultrasonic receiver  16  emits a measuring signal UE to a signal detection  26 . The signal detection  26  forwards the detected measuring signal UE to a signal processing  30 .  
         [0018]    [0018]FIG. 2 a  shows the course of the measuring signal UE over time. At instant t 0 , the ultrasonic transmitter  14  emits a transmitted signal US that contains a sinusoidal vibration (FIG. 2 b ). After the transit time tl, the ultrasonic receiver  16  receives the measuring signal UE, the level of the amplitude of the sinusoidal vibration of which first increases and then decreases, and then dies away. The measuring signal UE is evaluated within a measuring window with the parameters TM 1  and TM 2 .  
         [0019]    In normal operation, the resistance-welding system is acted upon by a discontinuous current I comprising sinusoidal half-waves (FIG. 3 b ). As indicated using dotted lines, the current intensity I can be influenced by changing the control variable. The variation of the trigger signal Trig results depending on the current variation I according to FIG. 3 b . The trigger signal Trig is selected so that a measurement is started by emission of the transmitted signal US at the moment when no current I flows. The ultrasonic permeability D is shown in FIG. 3 c  as a function of time t. For a good weld, the ultrasonic permeability curve D has the fluctuation shown. Only those measured values that lie within the time window TM 1 , TM 2  contribute to the determination of the ultrasonic permeability D. The trigger signal Trig activates the emission of the transmitted signal US.  
         [0020]    The ultrasonic permeability curve D according to FIG. 3 c  changes as the number n of welding spots and/or welds increases. The ultrasonic permeability D increases as the number n of welding spots increases, as shown in FIG. 4.  
         [0021]    According to FIG. 5 a , the measures for the ultrasonic permeability values Dn are plotted as a function of the number n of welds and/or welding spots. The trend characteristic  40  is determined using a mathematical smoothing procedure based on these measured values. The variation of the current I as a function of the number n of welding spots and/or welds essentially conforms with the trend characteristic  40 , FIG. 5 b.    
         [0022]    According to the invention, the ultrasonic permeability D is now evaluated with different welding procedures to determine the wear of the electrodes  11 ,  12  or the electrode caps  19 ,  20 . The ultrasonic permeability D increases as the duration of welding increases, and/or as the number of welds increases, due to flattening of the electrode caps  19 ,  20 .  
         [0023]    The way in which the ultrasonic permeability curve D is determined for a weld will now be described with reference to FIGS. 1 through 3. The measuring procedure of the welding process starts at instant t 0 . At instant t 0 , the welding control  28  emits a trigger impulse Trigg to the transmitter control  24  which then triggers the ultrasonic transmitter  14  to emit the transmitted signal US shown in FIG. 2 b . The ultrasonic transmitter  14  produces distortional waves, preferably transverse ultrasonic waves or torsional sound vibration. The signal US emitted by the transmitter  14  travels via the welded region  18 ,  21 ,  22  to the electrodes  11 ,  12 , as well as the electrode caps  19 ,  20  to the ultrasonic receiver  16 , which receives the measured signal UE and forwards it to signal detection  26 . The signal variation of the measured signal UE is shown in FIG. 2 a . Based on the measured signal UE, the signal detection  26  and the signal processing  30  determine the ultrasonic permeability D at the (trigger) instant t 0 . To determine the ultrasonic permeability D of the welded region during each current half-wave of the welding current, the mean ultrasonic energy of the measured signal UE is determined within a suitable time window TM 1 , TM 2 . A measure of the ultrasonic energy is the area enclosed by the measured signal UE, shown as shaded areas in FIG. 2 a . The root-mean-square value or the arithmetic mean of the curve trace of the measured signal UE lying within the measurement window TM 1 , TM 2 , for example, could be calculated as a measure of the ultrasonic permeability D at instant t 0 . This is repeated for a single weld often enough to obtain the curve trace shown in FIG. 3 c . The welded region  18 ,  21 ,  22  is acted upon by the current variation I shown in FIG. 3 b . During this one welding procedure, the ultrasonic permeabilities are determined once more at instants t 1 , t 2 , t 3 , etc. using the method just described. When a trigger signal arrives, a transmitted signal US is emitted once more, as shown in FIG. 2 b , which is followed by the ultrasonic permeability determination described in conjunction with FIG. 2 a . For a proper spot weld, the characteristic variation of ultrasonic permeability shown in FIG. 3 c  therefore results. As the welded region melts further, the ultrasonic permeability D increases to a maximum value. If the welded region now becomes fluid, the distortional waves become weaker, so that the ultrasonic permeability decreases once more. Reference is made to DE-A 43 25 878 for a more detailed description.  
         [0024]    The ultrasonic permeability curves Dn 0 , Dn 1 , Dn 2 , Dn 3  plotted against the number of welds are now shown in FIG. 4. As the number n (n 0 &lt;n 1 &lt;n 2 &lt;n 3 ) of welds increases, the corresponding ultrasonic permeability amplitude also increases at matching instants t 0 , t 1 , when the number n of welds is carried out with the same electrode caps  19 ,  20  and/or electrodes  11 ,  12  subject to wear. Essentially, it is observed that the amplitude of the ultrasonic permeability curve D increases as the number n of welds increases, and it is observed that the maximum of the ultrasonic permeability curve D shifts.  
         [0025]    This change in the ultrasonic permeability D as the number n of welds increases is therefore a measure of the wear of the electrode  11 ,  12  or the electrode caps  19 ,  20 . Wearing electrode caps  19 ,  20  become increasingly wider as the number n of welds increases, which is why it is easier for the ultrasonic waves to pass through the welded region. This phenomenon can now be used in order to detect wear on the welding system and to implement suitable countermeasures.  
         [0026]    Hereinbelow it will be assumed that a certain number n of welds, preferably spot welds, will be carried out with the same electrodes  11 ,  22  and the corresponding electrode caps  19 ,  20 . A measure for the ultrasonic permeability D will be determined first. To this end, the associated ultrasonic permeability DnO(t 0 ), Dn(t 1 ) will be determined, for example, starting with the first weld n 0  at a previously determined instant t 0  or t 1 , as described above. This determination is also carried out with the subsequent welds n 1 , n 2 , n 3  with the same electrode  11 ,  12  and/or the same electrode caps  19 ,  20  and, in fact, at the same instant t 0 , t 1  as with the previous measurement. This results in measured values of ultrasonic permeability as shown in FIG. 5 a . The measured values of ultrasonic permeability Dn determined in this fashion as a function of the number n of welds are now subjected to a smoothing procedure. This can be the method of least squares, for example, whereby a trend characteristic  40  results based on the formula y=c x b  (y represents the trend characteristic, x is the ultrasonic permeability Dn, and c and b are certain process parameters). This trend characteristic  40  is also shown in FIG. 5 a . Based on the trend characteristic  40 , it is apparent that the ultrasonic permeability D increases slowly as a function of the number n of welds. An updated trend characteristic  40  is always determined based on the new measured values that keep coming.  
         [0027]    In addition to this instant-based determination of a measure of the ultrasonic permeability Dn, the maximum of the respective ultrasonic permeability Dn could also be stored in order to determine the trend characteristic. The area enclosed by the respective permeability curve could also serve as a measure of the ultrasonic permeability Dn, which said area can be determined using certain mathematical methods. Another measure of the ultrasonic permeability Dn, for example, could be the four amplitudes of the ultrasonic permeability Dn(t 0 ), Dn(t 1 ), Dn(t 2 ), Dn(t 3 ) according to FIG. 3 c , which said amplitudes are added or determined arithmetically, for example. The same method for determining a measure of the ultrasonic permeability should be used for each weld in order to ensure comparability of the measures of the ultrasonic permeabilities Dn as a function of the number n of welds. The trend characteristic  40  is then processed as described hereinabove.  
         [0028]    This trend characteristic  40  is constantly compared with a specifiable limit value G. If the trend characteristic  40  exceeds the limit value G, this indicates that the electrodes  11 ,  12  and/or the electrode caps  19 ,  20  have a great deal of wear, and intervention is required. The signal processing  30  performs the appropriate processing of the ultrasonic permeability D and generates the trend characteristic  40 . It triggers a display  32  accordingly. If the trend characteristic  40  exceeds the limit value G, a warning is activated. In this manner the operator is informed that the electrode caps  19 ,  20  or the electrodes  11 ,  12  must be replaced or handled in another fashion. For example, the electrode caps  19 ,  20  could be re-milled in order to use them for further welding procedures.  
         [0029]    If the trend characteristic  40  exceeds the limit value G, the signal processing  30  generates a control signal. This control signal can be used, for example, to activate an automatic maintenance function. For instance, an automatic milling system starts the procedure to mill the worn electrode caps and/or electrodes. Automatic replacement of the electrodes or electrode caps could be activated as well. Control of such functions on an as-needed basis is therefore made possible.  
         [0030]    In order to obtain consistent quality of the welds, the current density through the welded region  18 ,  21 ,  22  should be kept constant. Since the tips of the electrode caps  19 ,  20  become wider, the current density—given a constant current I—would drop as the number n of welds increases. Since statements regarding the wear of the electrode caps  19 ,  20  are now available in the form of the trend characteristic  40 , however, the current I can be changed as a function of this trend characteristic  40 . The current variation I should have a trace that is essentially parallel with the trend characteristic  40  in order to keep the current density constant in the welding region  18 ,  21 ,  22 . To this end, the current I is adjusted accordingly.  
         [0031]    If the new current In 2  is reset for the number n 2  of welds, for example, this could take place using the following equation: In 2 =F×Dn 2 /Dn 1 ×In 1 , whereby Dn 1 , Dn 2  are the corresponding ultrasonic permeability values of the trend characteristic  40  for the respective number n 1 , n 2  of welds, and In 1  is the current value with which the system was acted upon at the number n 1  of welds, and F is a proportionality factor. In this manner, the new current value to be set could be adjusted in steps.