Electric heating – Metal heating – By arc
Patent
1993-08-19
1996-01-23
Evans, Geoffrey S.
Electric heating
Metal heating
By arc
B23K 2600
Patent
active
054866774
DESCRIPTION:
BRIEF SUMMARY
CROSS REFERENCE TO RELATED APPLICATIONS
This application is a national phase of PCT/DE 92/00115 filed 19 February 1992 and based, in turn, upon German national phase application P 41 06 007.5 filed 26 February 1991 under the International Convention.
Field of the Invention
The invention relates to a method of machining workpieces with a laser beam wherein the machining process, in particular the weld penetration depth or the degree of throughwelding, is monitored by detecting an optical and/or acoustical signal created by an unshielded laser-induced plasma or vapor.
Background of the Invention
A method of cutting workpieces, in particular the joining of plates with a laser beam, is known from DE 38 20 848 A1 having the above-given characteristics. It makes possible a determination of the degree of through welding in that there is a sudden fluctuation of the plasma light and/or of the plasma noise which can be reduced to an optical and/or acoustic signal indicating the degree of through welding. This known process is critical with respect to evaluation of the signals. In particular there are variations that are impossible to account for that are due to the geometry and/or the process parameters so that use of the known method in a wide range of applications is difficult, that is it cannot be applied to a wide range of processes.
In addition it is known from SPIE Vol. 801, 1987, p. 340 to divide the radiation from a cutting process with a radiation splitter into the visible range and the infrared range. Both parts are detected separately by photodiodes. The relationship of the steady part of the visible signal is calculated with respect to the infrared signal. It grows as the temperature increases at the cutting front. The calculation can be used to influence the cutting speed. Even here there are the above-mentioned dependencies on the geometry of the workpiece and/or of the process parameters which has a bad effect on the use of the method.
Object of the Invention
It is an object of the present invention so to improve a process with the above-mentioned features that generally valid criteria are obtained for proceeding with the machining and for machining characteristics.
Summary of the Invention
This object is obtained in that the signals are subjected to a frequency analysis and the average amplitudes of two different frequency bands of the analyzed frequencies are used with a predetermined calculation function to determine a rating or evaluation value.
Thus with this process the signals or their average values are not themselves used in order to obtain a monitoring characteristic, but instead they are subjected to a frequency analysis. Therefore a particular value of the light intensity, of the sound amplitude, or of the frequency of a particular signal are not used directly in order to determine the degree of throughwelding. Instead one or more values are subjected to a frequency analysis so that the results of this frequency analysis produce a band of analyzed frequencies which are evaluated. The evaluation is done such that the average amplitudes of two different frequency bands are drawn on to derive a rating value. This derivation is done with a calculation function that must be determined in advance. The derived rating value is then used to interpret the machining process and/or the machining results. In particular the following can be detected: influencing of the laser-induced plasma and of the plasma absorption (e.g. plasma shielding), or
It has been found to be particularly advantageous for the predetermined calculation function to be a quotient formation.
In order to get evaluation results as soon as possible and to introduce these results in particular into the process control, the method provides that the frequency analysis is done by a fast Fourier transformation. This is the optimum calculation in a digital computer for reducing the calculation time. A particular advantage is obtained by using the process of real-time analyses, e.g. in the neighborhood of less than a second.
In
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Abels Peter
Beyer Eckhard
Drenker Alexander J.
Maischner Dora
Dubno Herbert
Evans Geoffrey S.
Fraunhofer-Gesellschaft zur Forderung der Angewandten Forschung
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