METHOD FOR NON-DESTRUCTIVELY TESTING A QUALITY OF AN ULTRASONIC WELD

Abstract

A method for non-destructive testing of a quality of an ultrasonic weld from a welding process includes detecting of a time-dependent measurement value over a period of time, where the measurement value is characteristic of a mechanical or electrical vibration behavior of a welding process to be tested. The method includes evaluating a measurement-value course of the detected time-dependent measurement value by using a Fourier analysis. The method further includes comparing a result of the evaluation to a reference value in order to test the quality of the weld. A measuring device and an ultrasonic welding system are also included.

Claims

1. A method for non-destructive testing of a quality of an ultrasonic weld from a welding process, the method comprising: detecting a time-dependent measurement value over a period of time, wherein the time-dependent measurement value is characteristic of a mechanical or electrical vibration behavior of the welding process; evaluating a measurement-value course of the detected time-dependent measurement value by using a Fourier analysis; and comparing a result of the evaluation to a reference value in order to test the quality of the ultrasonic weld.

2. The method according to claim 1, wherein the period of time corresponds to a duration of the welding process.

3. The method according claim 1, wherein the time-dependent measurement value is characteristic of a mechanical vibration behavior of the welding process, the mechanical vibration behavior comprising a shear force and a deflection of a sonotrode.

4. The method according to claim 1, wherein the time-dependent measurement value is characteristic of an electrical vibration behavior of the welding process, the electrical vibration behavior comprising at least one of an electrical current and an electrical voltage for controlling a piezo actuator.

5. The method according to claim 1, wherein the detecting of the time-dependent measurement value comprises detecting a differential value and a flow value.

6. The method according to claim 1, wherein the evaluating comprises calculating of a phase relationship of at least one of two detected time-dependent measurement values, an operating frequency for operating a sonotrode, an amplitude of a phase of a detected time-dependent measurement value, and an amplitude of the operating frequency for operating a sonotrode.

7. The method according to claim 6, wherein the evaluating further comprises deriving of a power value from a calculated phase relationship of two detected time-dependent measurement values.

8. The method according to claim 7, wherein the power value is at least one of an apparent power, an active power, a reactive power, and an impedence value.

9. The method according to claim 8, wherein the impedance value is at least one of an apparent resistance, resistance, and reactance.

10. The method according to claim 1, wherein the evaluating the measurement-value course is based on at least one of a fundamental oscillation and a harmonic of the time-dependent detected measurement value.

11. The method according to claim 1, wherein the comparing the result of the evaluation to the reference value comprises comparing at least one of an operating frequency and a phase difference between an electrical current and an electrical voltage for controlling a piezo actuator.

12. The method according to claim 1, wherein a reference-value determination precedes the detecting and is based on at least one of a test series and a self-learning algorithm.

13. The method according to claim 12, wherein the self-learning algorithm is a neural network.

14. The method according to claim 1, further comprising regulating the welding process based on a result of a comparison of the result of the evaluation to the reference value.

15. The method according to claim 1, further comprising adapting a parameter of a welding system carrying out the welding process based on the result of the comparing.

16. The method according to claim 1, further comprising a vibration analysis for representing dynamics of the welding process based on the comparing, in order to detect an under- or an over-welding.

17. A measuring device for non-destructive testing of a quality of an ultrasonic weld, is the measuring device being configured: to detect a measurement value over a period, wherein the measurement value is characteristic of a mechanical or electrical vibration behavior of a welding process to be tested; to evaluate the detected measurement value by using a Fourier analysis; and to compare a result of the evaluation to a reference value in order to test the quality of the weld.

18. An ultrasonic welding system comprising: a sonotrode for applying a mechanical force to a workpiece; a piezo actuator configured to convert an electrical control signal into a mechanical vibration and to transmit the mechanical vibration to the sonotrode; an ultrasound generator configured to provide the electrical control signal; and a measuring device according to claim 17.

Description

DRAWINGS

[0065] In order that the disclosure may be well understood, there will now be described various forms thereof, given by way of example, reference being made to the accompanying drawings, in which:

[0066] FIG. 1 shows a schematic representation of a welding system in accordance with the teachings of the present disclosure;

[0067] FIG. 2 shows a flow diagram for a method in accordance with the teachings of the present disclosure;

[0068] FIG. 3a shows a graph of a frequency course according to the teachings of the present disclosure;

[0069] FIG. 3b shows a graph of a frequency course according to the teachings of the present disclosure;

[0070] FIG. 3c shows a graph of a current course according to the teachings of the present disclosure;

[0071] FIG. 4 shows a graph of a current amplitude course according to the teachings of the present disclosure; and

[0072] FIG. 5 shows a graph of an amplitude course of a sonotrode deflection according to the teachings of the present disclosure.

[0073] The drawings described herein are for illustration purposes only and are not intended to limit the scope of the present disclosure in any way.

DETAILED DESCRIPTION

[0074] The following description is merely exemplary in nature and is not intended to limit the present disclosure, application, or uses. It should be understood that throughout the drawings, corresponding reference numerals indicate like or corresponding parts and features.

[0075] FIG. 1 shows a schematic representation of a welding system 100 according to one form of the present disclosure. The welding system 100 is an ultrasonic welding system. The welding system 100 includes an ultrasound generator 101. The ultrasound generator 101 is a signal generator that is configured to generate an electrical signal. The electrical signal is regulated for example according to a voltage and reproduces an ultrasound vibration.

[0076] The welding system 100 includes a piezo actuator 103. The piezo actuator 103 is electrically coupled to the ultrasound generator 101 and configured to convert the electrical signal of the ultrasound generator 101 into a mechanical vibration. For this purpose the welding system 100 uses the piezoelectric effect.

[0077] A sonotrode 105 is mechanically coupled to the piezo actuator 103. In one form, the sonotrode 105 comprises the piezo actuator 103.

[0078] The mechanical ultrasound vibration of the piezo actuator 103 is transmitted to the sonotrode 105 by the mechanical coupling between sonotrode 105 and piezo actuator 103.

[0079] A workpiece, in particular a cable packet 107, can be inserted into the sonotrode 105. Here, stripped free ends of the individual cables of the cable packet 107 are inserted into the sonotrode 105.

[0080] When the welding system 100 is taken into operation, then the ultrasound generator 101 generates the electrical signal. The piezo actuator 103 converts the electrical signal into a mechanical vibration that is transmitted to the cable packet 107 using the sonotrode 105.

[0081] Here, the individual cables of the cable packet 107 are welded to one another.

[0082] The welding system 100 comprises a measuring device 109. The measuring device 109 is configured to measure the electrical signal of the ultrasound generator 101. The measuring device 109 is configured to measure a mechanical vibration value on the sonotrode 105. In the electrical signal the measuring device 109 detects a voltage and a current. The current is measured non-invasively via a transformer. In one form, the current is measured via a current-measuring resistance. The mechanical deflection is measured via an eddy current sensor.

[0083] In order to obtain reference curves, i.e., a reference value or a target value, cable packets 107 prepared with interferants or without additional measures are welded, and the measurement values are recorded. Subsequently the quality of the weld is verified via an additional test, in particular it is destructively tested.

[0084] In this way, a supply of reference curves results that can be associated with a good or bad measurement.

[0085] The values measured over the time are subjected to a Fourier analysis, in particular a short-term Fourier analysis. In this evaluation the time behavior of the welding system 100 can be detected in operation.

[0086] Unacceptable welds and system states can be detected with this vibration analysis. In this case “unacceptable” means that the welds resulting therefrom have a breakage force below the tolerance limit. The breakage force is prescribed for the reference curves. In operation, it can then be determined using a comparison of the evaluated measurement values to the reference curves, i.e., reference values or target values, whether the welding is good or poor, without having to carry out a breakage test.

[0087] It can be difficult to define global characteristic values that apply system-wide and durably as differentiation criterion between good and poor welds for different welding systems 100. Instead, with each test process, in particular after a batch change, a setup process, or a layer change, samples must be welded and tested for their breakage force. Good welds can thereby be determined. From the curve courses of these good welds, a tolerance band is defined that serves for the classification. Welds outside the band are classified as poor welds.

[0088] Such a procedure by repeated experiment stems from the fact that the curves differ after each setup, even though the quality of the welds has not changed. The current amplitudes, as well as all derived electrical values also differ depending on the setup process. Only the voltage amplitude remains unchanged in all of the examples.

[0089] After each setup process, a new repeated-experiment process can therefore be carried out. Likewise after certain time intervals. With 10 welding samples that have been manufactured 24 hours later, wherein the system has not been reconfigured and has not been used, the curve remains unchanged for the operating frequency; however, the phase difference can decrease significantly.

[0090] If, after a setup process or certain time interval, a repeated experiment with 20 welding samples takes place, which welding samples all achieve the prescribed breakage force, then a tolerance band can be defined wherein or in the proximity of which all curves of future good welds should be located. Instead of repeated-experiment curves this can be graphically represented as a repeated-experiment band. Such bands are depicted in FIGS. 3a to 3c.

[0091] In one form, only 10 samples have been used. In another form, this number can vary. If all welds were classified as “poor” whose curves are located outside the band, this would mean a too-low selectivity, i.e., many good welds incorrectly classified as poor welds. Therefore the tolerance band that is actually used for classification must be wider. Determining this band can be effected using statistical methods, in particular a widening at each point in time by one or two standard deviations of the 10 measurement values.

[0092] For the repeated-experiment welds other physical conditions prevail in part than for the subsequent welds of the series manufacturing. In one form, the line lengths of the cable packets 107 are significantly shorter during the repeated experiments, and the cycle frequency and thus the tool temperature can also differ. It must be provided that these differences have no influence on the measured curves.

[0093] Even with large fluctuations the cycle frequency has little influence on any of the measured curves. The operating frequency is most affected.

[0094] FIG. 2 shows a flow diagram 200 for a method according to one form of the present disclosure.

[0095] In a first step 201, measurement values are detected. Here an electrical and mechanical characteristic vibration value is detected over a period of time, in particular the entire welding process. In one form, only electrical or only mechanical measurement values are detected.

[0096] In a step 203, the detected measurement values are evaluated. Here the detected time courses for the measurement values are evaluated using a Fourier analysis.

[0097] In step 205, the evaluated results are compared to a reference value, in particular a target curve. The reference value here represents a band wherein the curve course should be located.

[0098] In order to determine the influence of the line lengths, an investigation with 3-line lengths, each with 2 samples, has been carried out. Generally over different lengths the operating frequencies are located in a band of at most 30 Hz width, which corresponds approximately to the width of the bands in the experiment with respect to the setup process. In addition, the curves of the short line length of 13 cm differ from the others, but 30 cm lines, whose curves hardly differ from those longer lines, are used in the process control in manufacturing. It is thus not assumed that the line length has a significant influence.

[0099] After each batch change, breakage tests are carried out in manufacturing for process control. These welds can also be used for a repeated-experiment process. This is necessary since the curves can change significantly due to the batch change without a change of the breakage forces necessarily occurring as well. Using the example of three different batches of nomically identical lines with 10 weld samples each, significantly separated tolerance bands arise with operating frequency, current amplitude, and phase difference, as well as all derived values. The difference is also clearly visible with the harmonic distortion.

[0100] FIG. 3a shows a graph of a frequency course according to one form of the present disclosure. Time is plotted on the abscissa axis, and frequency is plotted on the ordinate axis, here in the range of 20250 Hz below and 20450 Hz above.

[0101] A realistic example of the reduction of the weld quality is contamination of the weld surface by hand cream. For this experiment a surface proportion of the weld surface has been contaminated with hand cream in a targeted manner through a template.

[0102] A curve 301 corresponds to an application of 0% hand cream, a curve 303 corresponds to an application of 5% hand cream, a curve 305 corresponds to an application of 20% hand cream, and a curve 307 corresponds to an application of 30% hand cream.

[0103] At the operating frequency it can be clearly seen that the operating frequency decreases with increasing contamination by hand cream. Other values do not allow any change to be seen. Only with very specific values and sections of higher harmonics can a trend be recognized, for example, with the second harmonics in the phase difference during the compression vibration and in the voltage amplitude during the weld vibration, which, however, are not suitable as reliable classifiers, at best as support of the classification by the operating frequency.

[0104] The experiment with controlled contamination with hand cream also makes possible the verification that some curves have a correlation with the breakage force. If the weld samples are grouped according to their breakage force, it can be seen that the operating frequency is a good classifier in order to separate, by vibration analysis, very poor samples that have a breakage force under 100N from good samples that have a breakage force over 109N. In this way critical welds can be detected.

[0105] FIG. 3b shows a graph of a frequency course according to one form of the present disclosure. The time is plotted on the abscissa axis and the frequency is plotted on the ordinate axis, here in the range of 20250 Hz below and 20450 Hz above.

[0106] The curves 309, 311, and 313 show a distribution of the welding samples according to their breakage force. In the curve 309, breakage force over 200 N has been used to destroy the sample. In the curve 311, breakage force over between 100 N and 200 N has been used to destroy the sample. In the curve 313, breakage force under 100 N has been used to destroy the sample.

[0107] The vibration analysis can detect defects on the welding system 100. If damage occurs to the transducer, this damage is very clearly visible on the curves. The current amplitude decreases strongly, and the phase difference increases. As a result, the transducer is no longer in the position to introduce sufficient power into the joining zone, whereby the weld time also increases significantly, and no materially-bonded connection arises.

[0108] FIG. 3c shows a graph of a current course according one form of the present disclosure. The time is plotted on the abscissa axis, and the current is plotted on the ordinate axis, here in the range of 0 A below and 5 A above.

[0109] The curve 315 shows the current course of a new transducer, the curve 317 shows the current course of an intact, used transducer, and the curve 319 shows the current course of a defective, used transducer. It can be clearly seen that the curve 319 drops sharply. The state of the transducer can thus be detected.

[0110] Setup errors that cause a change of the boundary conditions for the vibration can also be detected. If, for example, during setup it is forgotten to fixedly screw the height stop, in the course of multiple vibrations the height stop can slip upward and during the welding the sonotrode 105 is still in motion when hitting the stop, and should exert further normal pressure on the weld nodes. In part this cannot be proven using weld-height monitoring and weld-time monitoring, even though the breakage forces have already dropped significantly.

[0111] FIG. 4 shows a graph of a current course according to one form of the present disclosure. The time is plotted on the abscissa axis, and the current is plotted on the ordinate axis, here in the range of 0 A below and 5 A above.

[0112] Here 10 curves 401 to 410 are plotted over time. The curves 402 to 406 are close to each other, and the current increases. However, the curves 407 to 410 fall sharply. It can be derived from this that in these runs the height stop has been incorrectly set.

[0113] Using the mechanical vibration measurement, i.e., using the measuring of the deflection of the sonotrode 105, a system defect in the welding system 100 can be detected. After the checking of the system thereby bumped, a spring breakage can be detected.

[0114] FIG. 5 shows a graph of an amplitude course of a sonotrode deflection according to one form of the present disclosure. The curves 501 show a course with new springs. The curves 503 show a course with broken springs. A decrease of the amplitude is clearly visible.

[0115] Also with the mechanical vibration analysis it is to be noted that different welding systems 100 show different curves, thus for analysis a system-specific reference curve is advantageous. As with the electrical vibration analysis, a repeated-experiment process is advantageous for detecting poor weld quality. Different line batches already lead to significantly changed curves. However, contact-part batches have no influence in this experiment.

[0116] Unless otherwise expressly indicated herein, all numerical values indicating mechanical/thermal properties, compositional percentages, dimensions and/or tolerances, or other characteristics are to be understood as modified by the word “about” or “approximately” in describing the scope of the present disclosure. This modification is desired for various reasons including industrial practice, material, manufacturing, and assembly tolerances, and testing capability.

[0117] As used herein, the phrase at least one of A, B, and C should be construed to mean a logical (A OR B OR C), using a non-exclusive logical OR, and should not be construed to mean “at least one of A, at least one of B, and at least one of C.”

[0118] The apparatuses and methods described in this application may be partially or fully implemented by a special purpose computer created by configuring a general-purpose computer to execute one or more particular functions embodied in computer programs. The functional blocks, flowchart components, and other elements described above serve as software specifications, which can be translated into the computer programs by the routine work of a skilled technician or programmer.

[0119] The description of the disclosure is merely exemplary in nature and, thus, variations that do not depart from the substance of the disclosure are intended to be within the scope of the disclosure. Such variations are not to be regarded as a departure from the spirit and scope of the disclosure.