METHOD AND APPARATUS FOR SPOT WELD CONTROL

Abstract: Ultrasonic transducers (28) spring biased (36) into seating engagement in a pair of opposed spot welding tips (24) transmit and receive ultrasonic waves and associated circuitry (16,18) measures the time of flight between the transducers which is a function of tip separation, workpiece temperature and weld nugget size which are indicators of the progress of a weld. The rate of change of flight time is measured and the weld current is adjusted early in the weld cycle to maintain a preset rate of weld growth. The weld current is terminated later in the cycle if the transit time rate falls below another preset value. Otherwise the weld current and termination time follow a standard schedule.

Description: This invention relates to a method and apparatus for controlling resistance spot welds and particularly to such a method and apparatus using ultrasonic transit time measurements which are related to weld progress. Many techniques have been attempted for monitoring or controlling resistance spot welds during the welding operation. These have included sensing weld resistance, acoustic emissions, weld tip displacement, and ultrasonic testing. Very few of the attempts have been commercially successful and any successes have been limited to narrow limits of operation or nearly ideal welding operations. Environments such as automotive manufacture provide less than ideal condition and varied parameters so that different metal thicknesses, number of layers, and metal coatings all challenge the ability of a given approach to weld control. Ultrasonics have been tried for weld monitoring with no real success prior to this invention. The usual ultrasonic method is to try to track the weld nugget growth by transmitting ultrasonic energy through the weld and analyzing the signal amplitude variations. U.S. Patent No. 3,384,733 describes an example of this type. Weld tip displacement has been used to track the progress of a weld and terminate the weld current when the displacement indicates that indentation has occurred. This is shown in an article by Steibel et al, entitled "Monitoring and control of spot Weld Operations", Society of Automotive Engineers, 1986. In this case the only control is the termination time, the current being controlled by a preset schedule. It is therefore an object of the invention to provide a method and apparatus for monitoring the progress of a weld and using a feedback control to alter the weld parameters during the weld process. To this end, a method of, and apparatus for, controlling a resistance spot weld is characterised by the features specified in the characterising portion of claims 1 & 4 respectively. The invention is carried out by the method of controlling a resistance spot weld comprising the steps of: transmitting ultrasonic pulses between spot weld tips during a weld cycle, measuring the transit time of the ultrasonic pulses to obtain weld nugget growth information, the transit time of each pulse being a function of weld nugget growth, determining the rate of transit time change from the transit times of at least two pulses, comparing the rate of transit time change to a preset range during the early portion of the weld cycle and, when there is a discrepancy, adjusting the weld current to conform the measured rate to the preset range, and comparing the rate of transit time change to a preset rate during a later portion of the weld cycle and, when the measured rate falls below the preset rate, terminating the weld current. The invention is further carried out by a resistance spot weld control having: monitoring means for monitoring weld progress including a coolant cavity in each weld tip, an ultrasonic transducer in each weld tip, a spring for biasing each ultrasonic transducer against the bottom of its respective coolant cavity, and a control circuit for activating the transducers and periodically measuring the transit time of ultrasonic signals between the ultrasonic transducers; and processor means programmed to a) store rate of change of transit time target values, b) calculate rate of change of transit time data values from the measured transit times, and c) compare the target values and data values to determine any substantial variation from target value and produce a current change signal to correct for such a variation. The present invention is now described, by way of example, with reference to the following description taken in conjunction with the accompanying drawings wherein like reference numerals refer to like parts and wherein: Figure 1 is a schematic diagram of a weld control apparatus according to the invention; Figure 2 is a cross-sectional view of welding electrodes for the weld control apparatus of Figure 1; and Figures 3 and 4 are graphs of the profile of transit times during welds on two different workpieces and illustrating the application of a control algorithm according to the invention. It has been found that the transit time of ultrasonic signals between electrode mounted ultrasonic transducers is determined by several parameters. The separation of the electrodes due to the thickness of the workpiece and portions of the electrodes establishes the nominal transit time and further separation changes due to thermal expansion and contraction effects changes in the transit time. More dominant influences in the change of transit time, however, are the effects of temperature and phase on the velocity of the ultrasonic signal. As the temperatures of the workpiece and the electrodes increases the velocity decreases; in addition, the velocity in the molten weld nugget is much lower than in the solid phase. Thus the rate of heating and melting of the workpiece is directly reflected in changes of the transit time. Thus transit time can be used to monitor the progress of a weld. The rate of change of transit time in the early portion of a weld cycle is useful as a measure of the heating rate and later, when the weld nugget is forming, is useful as a measure of the weld nugget growth. Referring to Figures 1 and 2, the weld control apparatus for monitoring the progress of a spot weld and controlling the progress in real time comprises a spot welder 10 having electrodes 12 for welding a workpiece 14, an ultrasonic control circuit 16, a computer or other processor means 18, and a conventional weld controller 20. The two electrodes 12 are the same and each comprise a shank 22 and a weld tip 24 coupled to the shank by a taper fit. Both elements are hollow to accommodate coolant flow in the conventional manner. The weld tip 24 differs from an ordinary tip in that its bore terminates in a flat surface 26 to facilitate good seating of an ultrasonic transducer 28. A high temperature couplant in the form of a film of grease between the ultrasonic transducer 28 and the flat surface 26 insures good ultrasonic transmission. A water flow tube 30 within the shank 22 carries cooling water into the weld tip 24 and also provides a passage for a transducer cable 32 which carries a driving signal to or a received signal from the ultrasonic transducer 28. Alternatively, the transducer cable 32 may be routed between the water flow tube 30 and the shank 22 and out through a port, not shown, in the side of the shank. The ultrasonic transducer 28 is hat-shaped with a radial flange 34. A coil spring 36, compressed between the end of the shank 22 and the ultrasonic transducer 28, surrounds the crown of the hat and one end presses against the radial flange 34 to urge the ultrasonic transducer 28 into firm contact with the flat surface 26. The other end of the coil spring 36 is soldered to a washer 37 which bears against the end of the shank 22. The weld tip/transducer assembly has two important advantages arising from the coil spring 36 arrangement. First, the coil spring 36 assures that the ultrasonic transducer 28 makes good contact with the flat surface 26 so that the ultrasonic signal will be effectively transferred to or from the weld tip 24. Prior attempts at instrumenting weld tips for ultrasonics included bonding the ultrasonic transducer to the inner surface of the weld tip. Such bonding proved to be temporary since the ultrasonic transducers came loose during the rigorous welding process. Moreover, the ultrasonic transducer, if bonded to the weld tip must be discarded when the weld tip is replaced. Since weld tip life is short, that practice would be economically prohibitive. Second, when the ultrasonic transducer 28 is spring biased against the weld tip 24, replacement of the weld tip is easy and, in fact, is essentially the same as replacing a conventional non-instrumented weld tip: the weld tip is simply knocked off the shank 22 and a new one is installed with a dab of couplant grease applied to the ultrasonic transducer. The ultrasonic transducer 28 will automatically be spring biased against the flat surface 26 of the new weld tip 24. In operation of the weld control apparatus the ultrasonic control circuit 16 energizes one of the ultrasonic transducers 28 with frequency bursts at the rate of 1000 to 3000 times per second. The energizing frequency is determined by the transducer crystal frequency which is about 5 MHz but may be in the range of 0.1 MHz to 15 MHZ. The ultrasonic signal is transmitted to the other ultrasonic transducer 28 and the time of flight (or transit time) is measured by the ultrasonic control circuit 16. Any change in temperature, tip spacing or weld nugget size will change the transit time accordingly so that weld progress is instantly and accurately measured. The transit time is then sampled by the computer 18 once each half-cycle for analysis by comparison of differences in transit time to programmed target values according to the method to be described, and the computer, in turn, signals to the weld controller 20 any change in the weld schedule needed to optimize the weld process. Traditionally, the weld controller 20 predetermines a weld current schedule to provide energy for weld nugget formation at a given rate intended to be the optimum rate for a given weld and also sets a termination time. The exact conditions encountered during a particular weld may differ from the ideal due to poor part fit-up, electrode wear, or other variables. To achieve adaptive control, the method of the invention uses the time of flight information to monitor the progress of the weld, compare the progress to an optimal model, and modify the scheduled current and termination time, if required, to form an acceptable weld. The preferred practice is to make one transit time measurement for each half-cycle but multiple measurements in each half-cycle may be made and averaged. The heating rate of the workpiece as reflected in the transit time rate of change is accurately determined and must be within prescribed limits in order to form an acceptable weld nugget within the allotted time. The weld duration is truncated if the transit time information indicates that the weld nugget growth in the line of travel of the signal has slowed to a preset rate. Figure 3 shows the transit time profile for a weld between two sheets of bare steel. The Y-axis is transit time expressed in arbitrary units Vt and the X-axis is weld time expressed as current half-cycles. The slope at any point on the rising curve indicates the rate of change of transit time and is measured by the difference in transit time (Delta) over a full cycle. During an early portion of the weld cycle a time increment is defined by T1 and T2 when Delta is measured to determine whether the slope (rate of heating) is proper. If Delta is below a minimum (DeltaMin) the current is increased and if Delta is above a maximum (DeltaMax) the current is decreased to compensate for the discrepancy. Later in the weld cycle when the weld might be expected to be completed, a pair of limits (Tmin and Tmax) are set and Delta is measured in every half-cycle in that range and compared to a value (DeltaEnd) which indicates that the weld nugget growth has slowed to a value which indicates that the weld is complete. Then the current is terminated. If the DeltaEnd is not found the current will be terminated at Tmax by the weld controller 20. The method can be described by the algorithm which forms the basis for programming the computer 18: Start weld at StartHeat: Record Vt at T1 and T2: Delta = V2 - V1: If Delta < DeltaMin increase heat: If Delta > DeltaMax decrease heat: After Tmin determine Delta for each half-cycle: If Delta < DeltaEnd stop the weld current: If Tmax is reached stop the weld current. For the particular case illustrated in Figure 3 the workpiece was two sheets of bare steel having thicknesses of 1.85 mm and 1.12 mm (0.073 and 0.044 inch) respectively. The parameters for the algorithm were: As can be seen from the graph the Delta was within the prescribed limits or target range and the current was not adjusted. The second target value, DeltaEnd, was passed at half-cycle 32 and the current stopped. Another example is shown in Figure 4 for a workpiece comprising two galvanized sheets each having a thickness of 1.32 mm (0.052 inch). The weld controller 20 is set to provide welding current in two stages separated by a cooling period. The transit time profile shows the workpiece heating and cooling as the current is applied or not. The current waveforms are superimposed on the graph to show each stage. In this case the heating rate was quite low when sampled at T1-T2 so that the Delta was below DeltaMin. The current was increased resulting in larger waveform amplitudes and an increased slope of the profile. As Delta sampling resumed after Tmin, a decreasing slope near the peak allowed the detection of a Delta below DeltaEnd to terminate the current. The half-cycles are numbered according to the actual count of current pulses so that the number does not continue to increment during the cooling period. The algorithm given above is directly applicable to this weld schedule and the parameters were as follows: A particular advantage of this control is that it terminates current at or near the optimum weld strength when the weld nugget becomes so large that its growth in the region directly between the electrodes 12 becomes slow. This is in contrast to those methods which are able to only recognize a large change in workpiece thickness which occurs at metal expulsion. The expulsion sensing systems can provide useful weld control on the basis that expulsion is indicative of a strong or good weld. However it is generally recognized that the weld has passed its peak strength by the time expulsion occurs so that an earlier detection can result in even stronger welds. Moreover, the splash of molten metal which can accompany expulsion is avoided. Repeated testing of the method and apparatus of the present invention under varying conditions and for different types of welds and two or more sheets in the workpiece has yielded acceptable welds. The computer control can be readily adapted to scheduling for multiple welds of differing types with the proper parameters programmed for each weld. It will thus be seen that the invention provides a practical way of accurately tracking weld progress and controlling weld parameters in real time to assure acceptable welds.

Claims: 1. A method of controlling a resistance spot weld comprising the steps of transmitting (16) ultrasonic pulses between spot weld tips (24) during a weld cycle, and measuring (18) the transit time of the ultrasonic pulses to obtain weld progress information, the transit time of each pulse being a function of temperature and weld nugget growth, characterised by the steps of determining (18) the rate of change of transit time from the transit times of at least two pulses, comparing (18) the rate of change of transit time to a preset range during the early portion of the weld cycle and, when there is a discrepancy, adjusting (20) the weld current to conform the measured rate to the preset range and comparing (18) the rate of change of transit time to a preset rate during a later portion of the weld cycle and, when the measured rate falls below the preset rate, terminating the weld current. 2. A method as claimed in Claim 1, wherein the adjusting step comprises increasing the weld current when the measured rate is below the preset range and decreasing the current when the measured rate is above the preset range. 3. A method as claimed in Claim 1, including the step of initiating the weld cycle according to an established weld current schedule, wherein the adjusting step can override the established weld current schedule by adjusting the weld current in a sense to compensate for any discrepancy, and wherein the terminating step can override the established weld current schedule to terminate the weld current. 4. Apparatus for resistance spot weld control comprising a pair of electrodes (12); a weld controller (20) for controlling current flow to the electrodes; and monitoring means for monitoring weld growth progress including a coolant cavity in each electrode (12), an ultrasonic transducer (28) in each electrode, and a control circuit (16) for activating the ultrasonic transducers and periodically measuring the transit time of ultrasonic signals between the ultrasonic transducers; characterised in that the monitoring includes a spring (36) for biasing each ultrasonic transducer against the bottom of its respective coolant cavity; and by processor means (18) programmed to a) store rate of change of transit time target values, b) calculate rate of change of transit time data values from the measured transit times, and c) compare the target values and data values to determine any substantial variation from target values and produce a current change signal to correct for such a variation. 5. Apparatus as claimed in Claim 4, wherein each electrode (12) comprises a shank (22) and a weld tip (24) which fits over the shank to define the coolant cavity in the shank and the weld tip, wherein the spring (36) is compressed between the end of the shank and the ultrasonic transducer (28), wherein the processor means (18) stores first and second rate of change of transit time target values, calculates first and second rate of change of transit time data values from the measured transit times, compares the first target values and the first data values to determine any substantial variation from target rate of change of transit time to produce the current change signal to correct for such a variation, and compares the second target values and the second data values to determine whether the second data value falls below the second target value to produce a current termination signal, and wherein the weld controller (20) is responsive to the signals produced by the processor means for adjusting weld current and terminating the weld.

Inventors: BILGE, UMIT, ROSS, JOHN J., SCARPELLI, AUGUST F., SCHWARTZ, RONALD E.
Applicants: GENERAL MOTORS CORPORATION
Application year: 1988
Main classification: G01N29
IPC codes: B23K11, G01N29