DE10244548A1 - Position determination method for laser beam relative to workpiece surface, comprises measuring process parameters during processing, and determining focus position by comparing measured values with function values - Google Patents

Abstract

Process parameters are measured during processing, and the focus position is determined by comparing the measured values with the function values of the previously measured process parameters. An Independent claim is also included for a computer program with a code device for carrying out the above process.

Description

The invention relates to a method to determine the position of the focus of a laser material processing used laser beam relative to the workpiece surface. Furthermore, the Invention a computer program for implementing the method.

Many processes of laser material processing, e.g. Cutting, drilling or welding are based on the action focused laser radiation on the workpiece to be machined. So be for example certain filters with a laser in single shot technique realized (up to 500 holes per second), either the all the energy necessary for piercing contained in a laser pulse or creates a hole with several individual laser pulses will (percussion). Furthermore, different lasers are used for cutting or welding different workpieces. For one efficient process flow, it is necessary to focus on Position the laser beam in a controlled manner on the workpiece surface. in this connection have to Accuracies of typically 50 μm and less are maintained.

That said accuracy of Focus also over longer periods be adhered to under the environmental conditions of industrial production experience has shown that it cannot solely through the passive stability of the processing machines to ensure. Rather, an occasional correction to the relative location of workpiece and focus of the laser beam necessary.

A measurement of the focus position using known methods laser beam analysis generally achieves the desired accuracy Not. The disadvantage here is that the focus position is measured relative to the corresponding beam analyzer and not, as desired, relative on the location of the surface of the workpiece to be machined.

The control is generally carried out the focus position depending from the machining result. If the result is not satisfactory (e.g. kerf width, borehole geometry, strength of the weld seam etc.) the focus position is corrected manually by systematic Variation of the focus position and subsequent evaluation of the processing result. This practice is unsatisfactory because it requires significant human resources and requires a significant production downtime. There one already done for the process as a critical deviation of the relative Focus position from its target position is not automatically recognized and readjusted there is also the risk of producing reject parts. Furthermore, it is with the aforementioned method for checking the focus position not possible, Focus position fluctuations safely from fluctuations of others, for the process to distinguish relevant parameters.

It is therefore desirable that Focus position of a processing laser beam relative to a workpiece reproducible to measure with high accuracy and readjust if necessary. The ongoing production should for not or only for this intervention be interrupted for a maximum of a few seconds so that determination and readjustment of the focus position without any significant reduction in production output at regular intervals or run continuously can be.

According to the invention during the machining process one or more process characteristics measured and compared the measured values with function values depending on them the current process features measured from the focus position in advance Focus position determined. This is done before the start of the processing process carry out a preliminary measurement under typical process conditions, who the elected Process characteristics depending be measured and determined from the focus position. That way The measurement curve obtained is used in the subsequent machining process for reference, so while Process characteristics measured corresponding to the machining process Function values can be assigned to this measurement curve which then gives the focus position.

It has been found that a multitude of, in particular directly accessible process features exist, whose measured values with the focus position of the used for the machining process Laser beam can be related. Especially has shown that the recorded measurement curves in the vicinity of the location characteristic of the focus on the workpiece surface curves (Extreme values, turning points etc.).

For the method according to the invention, the use of variables as process features that are directly accessible to the measurement and directly affect the machining process, such as the total power of the optical or acoustic emission, the spectrally resolved power of the optical or acoustic emission or the ratio, has proven to be particularly advantageous the spectrally resolved power at two different frequencies of optical or acoustic emission during the machining process. The quantities mentioned can be measured by known radiation or sound sensors. For example, it has been shown that the sound intensity has a local minimum in the area of the focus position on the workpiece surface. Sound intensity measurements at one or more times the repetition rate of the are particularly suitable Laser.

In addition to the sizes mentioned also come their first derivations after the focus position as preferred process features into consideration. The latter can be adjusted by modulating the focus position and in-phase demodulation of the measured quantity without essential technical Provide effort. An existing (local) extremum in the Measurement curve of a certain process characteristic would appear in the corresponding Express the first derivative after the focus position in the form of a zero crossing. deviations would therefore be from the zero defocusing the laser beam during material processing Clues.

The method according to the invention is suitable for Control and regulation of the focus position during laser material processing. After determining the actual focus position, this can be done with a predetermined target focus position are compared and any deviation is compensated for. For this Compensation, it is advantageous that the workpiece to be machined and / or to move the focusing lens along the laser beam direction, until the deviation of the actual and target focus position, for example below one predetermined threshold has been reduced.

Some or all of the steps mentioned inventive method (Measurement of the process characteristics in advance or during the machining process, Comparison of the measured values with recorded function values, Allocation of the respective focus positions from this comparison, readjustment the focus position on a target focus position to be maintained) can be expediently by means of a computer program which is based on a suitable Computer unit executed becomes. This computer unit is advantageously in a device integrated or connected to laser material processing, so that measurement and control can be carried out continuously or at certain times the focus position can be. The Computer program can be stored on suitable data carriers, such as EEPROMs, flash memories, but also CD-ROMs, Floppy disks or hard drives are stored.

The invention is in one embodiment based on the attached Figures closer explained.

1 shows schematically a device for laser material processing and

2 shows function values of a process feature measured depending on the focus position.

Preferred embodiment

1 shows schematically a structure for laser material processing, here for laser drilling.

The laser processing head is shown 1 with a focusing lens 8th and a deflecting mirror 2 , As can be seen from the beam path of the laser, the laser beam 3 on the workpiece to be machined 4 focused. In this example, the energy applied to the workpiece surface is used to create a borehole through the workpiece 4 , The workpiece 4 is from the recording 5 held and positioned. With the recording 5 the axis system is firmly connected 7 , which enables a spatial shift in all three spatial directions within specified areas.

In order to make the drilling process as efficient as possible, the focus position of the laser beam is necessary 3 to control exactly. This can result in a warming up and melting of the workpiece caused by the focusing 4 be avoided.

According to the invention by means of a sensor system, here a sound sensor 6 , captured a certain spatial area during the machining process so that the spectrally resolved sound power can be measured.

Of course, others can, from the focus position dependent Process features, such as the optical intensity in one certain spectral range can be detected.

2 now shows an example of the dependency of the spectral power density of the acoustic emission as a measured variable of a process feature in a laser drilling process at a frequency of 4420 Hz, 26 times the repetition rate of the laser. The normalized sound intensity is plotted as a function of the focus position, the zero point of the focus position corresponding to the position of the focus on the workpiece surface. The deviations from this are given in millimeter units. For this process, the desired focus position (target focus position) is clearly visible in the minimum of the process feature shown. Already with deviations of 0.1 mm from the target focus position, there are clear increases in sound intensity in relation to the minimum shown.

In the 2 The measurement curve shown is stored in a memory, for example in the form of a look-up table. During the laser drilling process, the sound sensor either continuously or at certain intervals 6 the sound intensity measured at a frequency of 4420 Hz. The measured value is assigned to a function value of the stored measurement curve and from this the corresponding focus position is determined as the actual value. If the actual focus position deviates from the desired value (0 mm), the workpiece to be machined can 4 or the focusing lens 8th are shifted along the laser beam direction until the deviation of the actual and target focus positions is largely compensated.

For this purpose, threshold values can be defined that must be undershot in order to abort the readjustment of the focus position.

The invention leads to an increase in process reliability and availability the manufacturing plant. About that can out Fluctuations in the machining result and scrap production be reduced.

Claims (7)

Method for determining the position of the focus of a laser beam used for laser material processing ( 3 ) relative to the workpiece surface, characterized in that one or more process features are measured during the machining process and the focus position is determined by comparing the measured values with functional values of these previously measured process features depending on the focus position. A method according to claim 1, characterized in that are directly accessible as process features of the measurement variables related to the machining process, such as the total output the optical or acoustic emission, the spectrally resolved power the optical or acoustic emission or the ratio of the spectrally resolved Performance at two different frequencies of optical or acoustic Emission during editing. A method according to claim 1 or 2, characterized in that as process characteristics, the first derivations based on the focus of the directly measurable quantities used become. Method according to one of claims 1 to 3, characterized in that that after determining the actual focus position, this with a predetermined target focus position compared and a deviation is compensated. A method according to claim 4, characterized in that to compensate for the deviation of the target and actual focus position, the workpiece to be machined ( 4 ) and / or the focusing lens ( 8th ) is shifted along the laser beam direction. Computer program with program code means for carrying out a Method according to one of the claims 1 to 5 if the computer program on a computer or a corresponding computer unit, which or in a device for Laser material processing is integrated, is executed. Computer program product with program code means, which are stored on a computer-readable data carrier, to carry out a Method according to one of the claims 1 to 5 if the computer program product on a computer or a corresponding computer unit, which or in a device integrated for laser material processing is executed.