Abstract:
A welding system includes a device for evaluating an ultrasonic signal during a welding process. The device has a meassured value evaluation unit that, in normal operation, evaluates at least one measurement signal that is derived from an ultrasonic signal and is located inside a measurement window. A mechanism for establishing the measurement window is provided, which establishes the measurement window according to a measurement signal that is received in a calibration operation.

Description:
BACKGROUND INFORMATION 
     A method for assessing resistance-welded joints is made known in German Patent Application DE-A 43 25 878. In order to assess welds during the welding process itself, the ultrasonic permeability, or attenuation, of the welded joint is determined by acting upon said welded joint with shear waves. To accomplish this, the mean ultrasonic energy is determined from the output signal from the ultrasound receiver during each current half-cycle of the welding current within a time window that is delayed relative to the constant ultrasonic transmitted signal by a defined delay time. The mean ultrasonic energy is used as a measure of the quality of the welded joint. How the time window is selected and the time interval by which it should be delayed relative to the ultrasonic transmitted signal are left open, however. 
     The object of the present invention is to provide a device that allows the measurement window to be automatically adapted to different measurement situations. 
     SUMMARY OF THE INVENTION 
     The device, according to the present invention, for evaluating signals has a measured value evaluation unit that, in normal operation, further processes at least one measurement signal that is derived from an ultrasonic signal and is located inside a measurement window. According to the present invention, means for establishing the measurement window are provided that establish the measurement window according to a measurement signal that is received in a calibration operation. By taking the measurement signal into account immediately when selecting the measurement window, situations that are different in terms of environmental conditions can be taken into account automatically. In addition, the measurement window need not be manually adjusted anew each time. The measurement window is adjusted automatically before the beginning of the particular process, which is controlled by the measured value detection unit. As a result, less-qualified workers can also work with the corresponding measurement and controlling devices. In addition, the device can be used to indicate possible sources of error at an early point in time. When used for a resistance-welding system in particular, statements can be made at an early point in time about possible electrode wear. Additionally, welding tongs can be inspected during a pause in production. 
     In an advantageous further development, detection of elapsed time is provided that establishes the measurement window according to the elapsed time of the measurement signal. By taking into account the elapsed time based on a transmitted signal that effects the measurement signal, the measurement window can be matched to the particular process, since the process may influence the elapsed time. The accuracy of the evaluation method is increased as a result. 
     An advantageous further development provides a zero transition point determination of the measurement signal, with which the period interval of the measurement signal is determined, and that can also be used to establish the measurement window. The measurement window preferably begins and ends at a zero transition point. In addition, statements can also be made about whether the frequencies of the transmitted signal and the measurement signal that are capable of being determined in this manner approximately match. The measurement window is established such that those measured values are taken into account whose frequency and period interval approximately match those of the transmitted signal. As a result, only meaningful measured values are evaluated. 
     In an advantageous further development, an extreme value determination of the measured signal is provided, the output signal of which is used to establish the measurement window. The measurement window can now be selected such that the extreme values (maximum, minimum) of the measured values are located within the measurement window and are used for further processing. 
     In an advantageous embodiment, the measured value is an ultrasonic signal that is evaluated in order to assess the quality and/or control of a welded joint, in particular a resistance-welded joint. 
     Additional advantageous further developments result from further dependent claims and from the description. 
    
    
     
       BREIF DESCRIPTION OF THE DRAWINGS 
       An exemplary embodiment of the invention is shown in the drawing and will be described in greater detail hereinbelow. 
         FIGS. 1 and 2  show a block diagram of the device according to the present invention. 
         FIGS. 3   a ,  3   b ,  4   a  through  4   c  show characteristic time-dependent signal traces. 
         FIG. 5  shows a flow chart for operating the device. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A first welding electrode  11  is acted upon with a current i. An ultrasonic transmitter  14  is situated on the first welding electrode  11 . An ultrasound receiver  16  is situated on the outer wall of a second welding electrode  12 . A first piece of sheet metal  21  and a second piece of sheet metal  22 —that are joined by a weld point  18 —are located between the two electrodes  11  and  12 . The ultrasonic transmitter  14  is acted upon by a transmitted signal U S  that is made available by a transmitter control unit  24  according to a trigger signal Trig from a welding control unit  28 . The transmitted signal U S  is guided through the first electrode  11 , the first and second pieces of sheet metal  21 ,  22 , the welding spot  18  and through the second electrode  12  to the ultrasound receiver  16 . The ultrasound receiver  16  sends a measurement signal U e  to a signal detection unit  26 . The signal detection unit  26  forwards the detected measurement signal further to a signal processing unit  30 . 
     The design of the welding control unit  28  and the signal processing unit  30  is shown in greater detail in  FIG. 2 . The measurement signal U e  detected by the signal detection unit  26  reaches the following four functional units. An elapsed-time determination unit  35  determines—depending on the trigger signal Trig and the measurement signal KE—the elapsed time t L  for a measurement window determination unit  41 . Additionally, a zero transition point determination unit  37  is provided, as well as an extreme value determination unit  39 , each of which sends their output signals to the measurement window determination unit  41 . This measurement window determination unit establishes the measurement window and its parameters t m1 , t m2 , t S1, t   S2 . Depending on the measurement window, a measured value evaluation unit  43  evaluates the output signal from the signal detection unit  26  and, from this, determines the actual value of the attenuation, or permeability D. The attenuation or permeability D is an input variable for the welding control unit  28 . The difference between the determined attenuation or permeability D and the target attenuation or permeability D target  is calculated at a first summing point and forwarded to an ultrasonic controller  45 . Based on this, the ultrasonic controller  45  determines a current target value I target . The difference between the current target value I target  and the current actual value I is calculated at a second summing point. A current controller  47  receives the difference as an input variable and uses it to generate a triggering signal for an actuator  49 . The actuator  49  effects the desired current flow I through the electrodes  11 ,  12  to generate a weld point  18  that joins the pieces of sheet metal  21 ,  22 . Additionally, a trigger generator  33  is provided in the signal processing unit  30 , that evaluates the current flow I in order to generate a trigger signal Trig. The trigger generator  33  could also be integrated in the welding control unit  28 . 
       FIG. 3   a  shows the course of the measurement signal U e  over time. At the instant t=0, the ultrasonic transmitter  14  emits a transmitted signal U s  that contains a sinusoidal oscillation ( FIG. 3   b ). After the elapsed time t L , the ultrasound receiver  16  detects the measurement signal U e , whose amplitude of sinusoidal oscillation first increases but then decreases in terms of amount, and dies out. The interference window is placed within the elapsed time t L , the interference window being established by parameters t S1  and t S2 . The same applies for the measurement window with parameters t m1  and t m2 . 
     In normal operation, the resistance welding system is acted upon with a discontinuous current I having sinusoidal half-waves ( FIG. 4   b ). The current intensity I is capable of being influenced by changing the variable, as shown with the dashed line. Depending on the current trace I according to  FIG. 4   b , the course of the trigger signal Trig results. The trigger signal Trig is selected such that a measurement is started by the emission of the transmitted signal U S  specifically when no current I flows. The attenuation or permeability D as a function of time is shown in  FIG. 4   c . The attenuation or permeability curve of a good weld has the shape shown in the illustration. Only those measured values contribute to the determination of attenuation or permeability that are located within the measurement window t m1 , t m2 . The trigger signal Trig activates emission of the transmitted signal U S . 
     The establishment of a measurement and/or interference window t m1 , t m2 , t S1 , t S2  is described with reference to the flow chart in  FIG. 5 . The automatic calibration of the ultrasonic measurement device is activated by the start of the welding process (Step  101 ). Thereupon, the welding control unit closes the welding tongs formed by the two electrodes  11 ,  12  (Step  103 ). The transmitter control unit  24  then triggers the ultrasonic transmitter  14  to output a transmitted signal U S  that has the shape shown in  FIG. 3   b  (Step  105 ). The ultrasonic transmitter  14  preferably emits shear waves that propagate in the walls of the electrodes  11 ,  12  and that the ultrasound receiver  16  receives and forwards in the form of the measured signal U e  to the signal detection unit  26  (Step  107 ). The signal detection unit  26  smooths the measurement signal U e  using appropriate filters (Step  109 ). The course of the amplitude of the measurement signal U e  over time is measured and stored, so that the signal trace of the measurement signal U e  shown in  FIG. 3   a  is available to devices  35 ,  37 ,  39 ,  43  (Step  111 ). In query  113 , it is determined if the amplitude of the measurement signal U e  exceeds a threshold in terms of amount within a specifiable time period (Query  113 ). If this is not the case, an error is determined to have occurred, because a measurement signal U e  did not arrive at the ultrasound receiver  16 . An appropriate error message is output (Step  125 ). 
     Otherwise, the elapsed time t L  between the transmitted signal U S  and the measurement signal U e  is determined by the elapsed time determination unit (Step  115 ). The trigger point Trig and, therefore, the start of the transmitted signal U S , is known. The trigger signal starts a counter that serves to detect time. The counter is not stopped until the amplitude of the measurement signal U e  exceeds a certain threshold value in terms of amount. This threshold value is selected such that interfering signals are not detected. The elapsed time t L  determined in this manner is shown in  FIG. 3   a . If the ultrasonic transmitter  14  and the ultrasound receiver  16  are located 110 mm apart, for example, the (theoretical) elapsed time t L  is of the magnitude of 50 μs. Depending on this variable that is determined by calculation, a limit value can be established with which the elapsed time t L  determined in Step  115  is compared (Step  117 ). The measurement signal U e  should be within the limit value or exceed the specifiable amplitude threshold value, otherwise an error exists (Step  125 ). 
     Subsequently, the zero transition point determination unit  37  determines the zero transition points of the amplitude of the measurement signal U e  (Step  119 ). The instants at which the amplitude of the measurement signal U e  assumes the value “zero” are therefore known. Based on the instants at which the zero transition points occur, the associated period intervals and frequencies can be determined and stored (Step  121 ). With a sinusoidal measurement signal U e , the three first measured values form the first period interval, the reciprocal value of which corresponds to the frequency of the measurement signal U e . The second period interval results from the third to fifth zero transition point with associated frequency. In this manner, it is possible to associate frequencies with the particular positive and negative half-waves. The frequency of the transmitted signal U S  is known as well. The frequencies determined in Step  121  are compared with the transmit frequency of the transmitted signal U S  (Query  123 ). If the transmit frequency and measurement signal frequency deviate from each other only slightly, a meaningful measurement signal U e  was obtained. Otherwise an error message is output in Step  125 . 
     In Step  127 , “half-wave measured values” are then calculated from the course of the measurement signal U e  over time that was determined in Step  115 . The root-mean-square value, the arithmetic mean or another measure of the energy content of a half-wave of the measurement signal U e  is determined as the half-wave measured value. The appropriate half-wave measured value is therefore available for every half-wave (positive or negative) of the measurement signal U e . 
     In subsequent Step  129 , the two first greatest half-wave measured values in succession are determined by the extreme value determination unit  39  by the fact, for instance, that the root-mean-square value of the measurement signal U e  exceeds a specifiable threshold. In the signal trace according to  FIG. 3   a , they are the two half-waves that each enclose a shaded area. The area enclosed by the particular half-wave is a measure of the corresponding half-wave measured value and/or the root-mean-square value. Then, a periodic measured value is determined from the sum of the two determined first greatest half-wave measured values with the associated period interval. The period interval can be determined based on the zero transition points determined in Step  119 . This period interval establishes the width of the measurement window (Step  131 ). The starting point t m1  of the measurement window t m1  is now selected such that the two first greatest (in terms of amount) half-wave measured values in succession are located within this measurement window t m1 , t m2 . With the existing signal trace according to  FIG. 3   a , the start of the measurement window t m1  is set at the third zero transition point. The end of the measurement window t m2  results from the sum of the starting point t m1  of the measurement window and the period interval determined in Step  131  (Step  135 ). 
     The width of the interference window t S1 , t S2  also matches the width of the measurement window t m1 , t m2 . The starting point of the interference window is selected such that the interference window t S1 , t S2  is located within the elapsed time t L  of the measurement signal U e , in order to prevent the interference window t S1 , t S2  and the measurement window t m1 , t m2  from overlapping. The end t S2  of the interference window is preferably located temporally ahead of the first zero transition point. The positions of the measurement and interference window t m1 , t m2 , t S1 , t S2  are stored (Step  141 ). In addition, the limit values for monitoring the measurement signal U e , for example, are established (Step  143 ). Based on the maximum value of the measurement signal U e  located in the measurement window, a first limit value can be used for the monitoring of the measurement signal U e  that occurs in the interference window, e.g., the first limit value is 20% of the extreme value. If the measurement signal U e  in the interference window exceeds the limit value, this is an indication of an error. The program sequence for automatically establishing the measurement window is therefore ended (Step  145 ). 
     The settings of the interference window t S1 , t S2  and the measurement window t m1 , t m2  are retained for the subsequent welding process. In on-going operation, the ultrasonic transmitter  14  is always activated in the non-energized phase (I=0) when the trigger signal Trig appears. The measured values that are located within the measurement window t m1 , t m2  are then evaluated to determine the attenuation or permeability D of the welded joint. To accomplish this, the energy content is determined, e.g., via the root-mean-square of the two half-waves, as described hereinabove in conjunction with Step  131 . A first attenuation or permeability D 0  occurs at the first trigger point T 0 , a second attenuation or permeability D 1  occurs at the second trigger point T 1 , and so on. By selecting the measurement window t m1 , t m2  in purposeful fashion and evaluating the temporal course of the measurement signal U e  located only within this measurement window t m1 , t m2 , it is ensured that only suitable measurement signals U e  are used in the determination of the attenuation or permeability curve D. 
     In the welding control unit  28 , the attenuation or permeability curve D according to  FIG. 4   c  determined in this fashion is compared with a target attenuation or permeability curve D target  that is representative of a good weld, and the weld is constantly corrected during on-going operation via the current I. This is achieved with the controller shown in  FIG. 2 . The current I is therefore adjusted such that the target attenuation or permeability curve D target  is reliably achieved.