METHOD AND DEVICE FOR MONITORING THE QUALITY OF AN ULTRASONIC WELD

Abstract

A method for monitoring the quality of an ultrasonic weld, wherein the vibration behavior during the joining process with respect to an actual value of the vibration frequency and / or the vibration amplitude of at least one joining partner (10, 20) involved in the joining process and / or of a tool of an ultrasonic welding device (30) used in the joining process is detected by means of measurement with at least one measuring device (40) designed to quantify mechanical vibrations, the detected vibration behavior is analyzed by means of a Fourier analysis and compared with a predetermined target value as a reference value, wherein a frequency shift between an excitation frequency of the ultrasonic welding device (30) and the frequency measured at a measuring point during the joining process is detected and this is compared with a stored target frequency shift,to determine the degree of purity of the joining partners (10, 20) by means of the degree of frequency shift, wherein an eddy current sensor is used to quantify the mechanical vibrations during the joining process.

Description

Ultrasonic welding can be used for joining cable packages. In this case, modern ultrasonic welding systems enable precise repeatable methods on the basis of digital ultrasonic generator technology and the use of microprocessors. When setting parameters for different applications and materials, experience values are usually used. However, monitoring the joining process frequently presents problems, since a destructive test is usually carried out after the welding process.In the majority of the parameters to be determined for checking the quality of the ultrasonic welding, destructive checking of the parts is necessary. Furthermore, the evaluation of the connection point takes place only after the welding, that is to say when the process has already been concluded. Thus, with such methods, only detection of errors can be carried out.Production is likewise impaired during the destructive testing, since parts must be removed from the production.Further influencing factors, such as contamination or contamination of the joining partners, introduce further uncalculable parameters.For example, US 2003 / 0218050 A1 describes a device for connecting cables to a sonotrode and a measurement pickup, wherein the oscillations measured at the measurement pickup serve for determining connection errors of the cables during operation of the sonotrode.Furthermore, US 2019 / 0271669 A1 discloses a method for nondestructive vibration measurement of the quality of a weld seam or of an adhesive bond and characterization of the bond by means of a bond quality index value.It is an object of the invention to provide an advantageous concept for monitoring a quality of an ultrasonic weld.A measurement and evaluation of variables characterizing an oscillating process during the welding process make possible a comprehensive and meaningful assessment of the welding process. The evaluation by means of a Fourier analysis allows a temporal assessment of the quality of the welding process. The quality here comprises a purity and / or a bonding quality of the ultrasonic welding.According to a first aspect, the object is achieved by a method for monitoring a quality of an ultrasonic weld, wherein the vibration behavior during the joining process is detected with respect to an actual value of the vibration frequency and / or the vibration amplitude of at least one joining partner involved in the joining process and / or tool used in the joining process of an ultrasonic welding device by means of measurement with at least one measuring device which is designed for quantifying mechanical vibrations, the detected vibration behavior is analyzed by means of a Fourier analysis and compared with a predetermined setpoint value as a reference value.The application of a Fourier analysis can comprise applying a short-term Fourier analysis, from which the temporal dependence of the amplitude, frequency or other variables can be determined. Individual window sections of the short-time Fourier analysis can rely on a suitable window function, in particular a flattop window in the calculation of an amplitude, a rectangular window in the calculation of a frequency or a phase. Zeropadding and interpolation can also be applied, in particular in the calculation of a frequency in short-time Fourier analysis with small window sizes.In one embodiment, an optical measuring device is used for quantifying the mechanical oscillations during the joining process.In one embodiment, an eddy current sensor and / or a laser viscometer is used to quantify the mechanical oscillations during the joining process. A laser viscometer can measure at one or more measurement points and record at least one oscillation parameter such as the oscillation frequency or the oscillation amplitude.The measurement of the deflection can be carried out using various measurement methods. The deflection can also be measured, in particular, by an eddy current sensor. This makes it possible to avoid mechanical impairment during the welding process. In particular, the deflection, i.e. also the speed of the sonotrode, is measured with a device in which an eddy current sensor is brought as close as possible to a measurement point on the sonotrode surface which is orthogonal to the direction of movement and thus at right angles to the coupling surface.The measurement point can be located in particular as close as possible to the coupling surface in order to obtain a measurement of the movement in the joining zone as accurately as possible. The eddy current sensor is particularly well suited for this task, since it can be relatively easily attached to different locations of the vibration system. The restriction for the worker is thus also less than in the case of measurement using laser-optical systems, such as laser vibrometry. A combination of the two aforementioned measurement methods with one another or with further measurements can increase an accuracy of the measurement.In one embodiment, a plurality of measurement points are checked by the measuring device with respect to their oscillation behavior deviating from the setpoint values.In one embodiment, a measurement point on a sonotrode, an anvil, a first joining partner, in particular a first conductor and / or a second joining partner, in particular a second conductor, is used to quantify the mechanical oscillations thereof.In one embodiment, a pair of uncontaminated, in particular cleaned, joining partners is used as reference value of a specified pair of joining partners for determining the setpoint values and the quantification of the mechanical oscillations during the joining process is detected by means of the measuring device by the oscillation amplitude and / or oscillation frequency being detected at one or more reference points. A means can also be formed via a plurality of such reference measurements.In one embodiment, a measure of the relative deviation of the detected actual values, preferably of the detected oscillation amplitude and / or oscillation frequency from the respective setpoint value, is used as a measure for the purity and thus the connection quality of the joining partners. Furthermore, a display device can be provided which displays a reference curve of an uncontaminated pairing of joining partners on the display screen thereof and to this end overlays or also reproduces the current measurement curve on the same view, so that a deviation from a good weld can already be detected for a user from the deviation of the curves shown.In one embodiment, in the event of a relative deviation of the detected actual value from the corresponding setpoint value, which is greater than a predetermined limit value, the joining process is interrupted and / or the joining partners are detected as defective parts.In one embodiment, the limit value is a maximum of 10%, in particular a maximum of 5%.In one embodiment, a frequency shift between an excitation frequency of the ultrasonic welding device and a frequency measured at a measurement point during the joining process is detected and compared with a stored setpoint frequency shift in order to detect a degree of purity of the joining partners by means of the degree of the frequency shift.According to a second aspect of the invention, the output is achieved by an apparatus for carrying out a method according to the first aspect with an ultrasonic welding apparatus and a measuring apparatus which is designed to detect the vibration behavior during the joining process, to analyze the detected vibration behavior by means of a Fourier analysis and to compare it with a predetermined setpoint value as a reference value in order to monitor a quality of an ultrasonic weld. The measuring device can comprise an eddy current sensor and / or a laser viscometer.Further exemplary embodiments are illustrated in more detail with reference to the figures. The following are shown: FIG. 1 shows a schematic illustration of a device according to an exemplary embodiment; FIG. 2 shows a flow chart for a method according to an exemplary embodiment.FIG. 1 shows a schematic view of a device according to an exemplary embodiment with an ultrasonic welding device 30 with a tool with a sonotrode 31 and an anvil 32, between which joining partners 10, 20 are arranged in order to weld them by excitation of vibration of the sonotrode 31.The ultrasonic welding device 30 is provided with an optical measurement device 40 arranged so that a mechanical vibration behavior can be detected. In the described exemplary embodiment, the measuring device 40 is a laser viscometer. Wires of a cable harness are used as the joining partners 10, 20. In a further exemplary embodiment, other joining partners are used. In another embodiment, the measurement device 40 includes an eddy current sensor that is mounted close to the sonotrode surface.The measuring device 40 serves for detecting and quantifying the mechanical oscillations, in the present case the oscillation frequency that occurs at the sonotrode 31 or at one of the joining partners 10, 20, during the joining process.FIG. 2 shows a flow diagram 200 for a method according to an exemplary embodiment.In step 201, the measuring device 40 acquires the oscillation data, in particular frequency and amplitude, and analyzes these by means of a Fourier analysis. Subsequently, the measuring device compares this analyzed oscillation data with a setpoint value or with setpoint reference data in step 203. If a deviation is determined in step 205, the sample can be considered to be contaminated if the deviation is greater than the permitted limit value.If no deviation or a deviation which is smaller than the permitted limit value is determined in step 205, the weld can be evaluated as good.List of reference characters10, 20 Joining partner 30 Welding system 31 Sonotrode 32 Anvil 40 Measuring device 200 Flow diagram 201- 205 Method step

Claims

Method for monitoring a quality of an ultrasonic weld, wherein the oscillation behavior during the joining process is detected with respect to an actual value of the oscillation frequency and / or the oscillation amplitude of at least one joining partner (10, 20) involved in the joining process and / or tool of an ultrasonic welding device (30) used in the joining process by means of measurement with at least one measuring device (40) which is designed for quantifying mechanical oscillations, the detected oscillation behavior is analyzed by means of a Fourier analysis and compared with a predetermined setpoint value as a reference value, wherein a frequency shift between an excitation frequency of the ultrasonic welding device (30) and the frequency measured at a measurement point during the joining process is detected and this is compared with a stored setpoint frequency shift in order to detect a degree of purity of the joining partners (10, 20) by means of the degree of the frequency shift, wherein an eddy current sensor is used for quantifying the mechanical oscillations during the joining process.Method according to claim 1, wherein an optical measuring device (40) is used for quantifying the mechanical oscillations during the joining process.Method according to one of the preceding claims, wherein a plurality of measurement points are checked by the measuring device (40) with respect to their oscillation behavior deviating from the setpoint values.Method according to claim 3, wherein a measurement point on a sonotrode (31), an anvil (32), a first joining partner, in particular a first conductor and / or a second joining partner, in particular a second conductor, is used for quantifying the mechanical oscillations thereof.Method according to one of the preceding claims, wherein a pair of non-soiled, in particular cleaned, joining partners (10, 20) is used for determining the setpoint values as a reference value of a specified pair of joining partners (10, 20), and the quantification of the mechanical oscillations during the joining process is detected by means of the measuring device (40) by the oscillation amplitude and / or oscillation frequency being detected at one or more reference points.Method according to one of the preceding claims, wherein a measure of the relative deviation of the detected actual values, preferably of the detected oscillation amplitude and / or oscillation frequency from the respective desired value is used as a measure for the purity and thus the connection quality of the joining partners.Method according to Claim 6, wherein, in the case of a relative deviation of the detected actual value from the corresponding setpoint value which is greater than a predetermined limit value, the joining process is interrupted and / or the joining partners are detected as missing parts.Method according to claim 7, wherein the limit value is a maximum of 10%, in particular a maximum of 5%.Device for carrying out a method according to one of Claims 1 to 8, having an ultrasonic welding device (30) and a measuring device (40) which is designed to record the oscillation behavior during the joining process, to analyze the recorded oscillation behavior by means of a Fourier analysis and to compare it with a predetermined setpoint value as a reference value in order to monitor a quality of an ultrasonic welding, wherein a frequency shift between an excitation frequency of the ultrasonic welding device (30) and the frequency measured at a measurement point during the joining process is recorded and this frequency is comparable to a stored setpoint frequency shift in order to record a degree of purity of the joining partners (10, 20) by means of the degree of the frequency shift, wherein an eddy current sensor is used for quantifying the mechanical oscillations during the joining process.

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