JPH07195188A - Work manufacture method by laser beam - Google Patents

Abstract

PURPOSE: To decide a measurement capacitance existing between a sensor electrode and a workpiece by analyzing a change in an alternative current signal due to the measurement capacitance. CONSTITUTION: A workpiece 2 is machined by a laser beam by using a sensor electrode 3 on which an alternative electric signal is imposed and whose position is decided with respect to the workpiece 2. In this case, a direct current signal is generated corresponding to a distance between the sensor electrode 3 and the workpiece 2 from the alternative current signal. A noise measurement signal is formed from the direct current signal through a process of a frequency filtering and a following rectification. Since the signal is compared with a threshold, a state signal for an additional controlling of the machining process is generated when the noise measurement signal passes over the threshold.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method of machining a workpiece using a laser beam.

[0002]

BACKGROUND OF THE INVENTION The machining of workpieces with a laser beam is already generally known. At that time, a welding process using a laser beam, a hole in a workpiece, or the like can be performed. In general, the distance provided between the laser machining nozzle and the workpiece for this machining process is adjusted as desired or maintained constant,
It is based on distance measurement. Therefore, a sensor electrode to which an electrical alternating current signal is applied is provided at the tip of the laser processing nozzle, and the change in the alternating current signal due to the measured capacitance is evaluated by that to determine the measured capacitance existing between the sensor electrode and the workpiece. You can ask. In that case, first, a DC signal corresponding to the distance between the sensor electrode and the workpiece is formed from the AC signal. This DC signal is then supplied to the controller, which maintains a constant distance between the laser machining nozzle or sensor electrode and the workpiece in accordance with the DC signal.

Recently, on-line process monitoring has become increasingly important when laser machining a workpiece. In particular, in that case, attention is paid to determining the quality of the welding operation, for example using a CO ₂ laser, or to determine the puncture time in the case of a laser ball, to name a few.

With regard to determining the point of puncture, it has already been described from EP 0344339A1 to use one or several detectors for the electromagnetic beam emitted from the workpiece during laser processing or the laser beam emerging through the puncture hole. It is known to measure.

On the other hand, the paper “Laser Weld Quality M
onitoring and fault diagnostic
sis, L .; Li, et al. Internet
onal Conference on Laser
Systems Applicaiton inInd
industry (Torino, Italy, 1990 1
Jan. 7-9) ”deals with the detection of process parameters by measuring the electrostatic charge generated during processing, eg in laser balls or laser welding.

[0006]

The object of the present invention is to improve the above-mentioned method, in particular by the noise existing by the uncharged metal droplets occurring during material processing and existing in the sensor electrode and machining. The signal used for controlling the distance between objects can be further processed and monitored to improve the efficiency of laser processing.

[0007]

The solution of the set problem is described in the characterizing part of claim 1. Preferred embodiments of the invention are apparent from the dependent claims.

According to the first aspect of the present invention, the measuring capacitance (CMess) existing between the sensor electrode (3) and the workpiece (2) is obtained by evaluating the change of the AC signal due to the measuring capacitance (CMess). A sensor electrode (3) that can be positioned with respect to the workpiece (2) and to which an electrical AC signal is applied
A method of processing a workpiece (2) with a laser beam by using, in which case an electrical DC signal corresponding to the distance between the sensor electrode (3) and the workpiece (2) is first calculated from the AC signal. Forming a noise measurement signal from the DC signal by frequency filtering and subsequent rectification, comparing the noise measurement signal with at least one predetermined threshold value, the noise measurement signal being the threshold value. It is characterized in that a status signal is generated when the signal passes through.

A second aspect of the present invention is characterized in that in the first aspect, the frequency filtering process is a high-pass filtering process.

A third aspect of the present invention is characterized in that, in the first aspect, the frequency filtering process is a bandpass filtering process.

A fourth aspect of the present invention is characterized in that, in any one of the first to third aspects, the frequency of the electrical AC signal can be switched.

According to a fifth aspect of the present invention, in any one of the first to fourth aspects, the electric alternating current signal is measured capacitance (CMe).
ss), and is an alternating voltage generated by an alternating current (i) applied at a constant effective value.

According to a sixth aspect of the present invention, in any one of the first to fourth aspects, the electrical alternating current signal is measured capacitance (CMe
ss) and an inductance (L) are generated by the oscillation circuit (CMess, L), and the oscillation circuit (C
Mess, L) frequency is a frequency / DC voltage converter (15)
It is characterized in that it is supplied to a DC signal to generate a DC signal.

According to a seventh aspect of the present invention, in the sixth aspect, the direct current signal is generated after the amplitude of the alternating current signal of the oscillation circuit (CMess, L) is directly rectified.

According to an eighth aspect of the present invention, the second state signal generated on the basis of the amplitude of the AC signal is multiplied by a weighting coefficient and subtracted from the first state signal to obtain another state. A signal is formed.

A ninth aspect of the present invention is characterized in that, in any one of the first to eighth aspects, the laser beam is moved with respect to the workpiece (2) only after the status signal is generated.

A tenth aspect is characterized in that, in any one of the first to eighth aspects, the movement of the laser beam with respect to the workpiece (2) is stopped after the generation of the status signal.

Claim 11 is characterized in that in any one of claims 1 to 10 it is checked whether the noise measurement signal lies between the upper and lower threshold values.

[0019]

The method according to the invention provides that a noise measuring signal is generated from a DC signal corresponding to the distance between the sensor electrode and the workpiece by frequency filtering and subsequent rectification, and this noise measuring signal is predetermined. It is characterized in that it is compared with a threshold value and accordingly a status signal is generated if the noise measurement signal exceeds the threshold value.

This noise measurement signal is generated not only by the plasma generated during the laser machining, but also by the uncharged metal droplets which change the measured capacitance between the sensor electrode and the workpiece, which is generated at this time. Sufficient on-line process monitoring is performed for a number of uses, resulting in improved process safety and / or increased operating speed during laser processing.

For example, in the case of laser cutting a thin metal sheet, at the beginning of the cutting a hole is first formed by the laser beam, whereby the cutting can then be started.
During the drilling process, the noise measurement signal is generated by the metal splash already described. When the plate is thick, the punching process continues for many seconds, and in this case, even if the plate is thick, the piercing time does not necessarily have to be constant due to variations in the laser beam, differences in the surface structure, and the like. Therefore, in the past, it was necessary to always select a puncture time that was somewhat longer than the original one. On the other hand, according to the invention, during laser drilling, the noise measurement signal is monitored whether it exceeds or falls below a predetermined threshold, so that when the status signal is generated, the laser beam is already emitted. Is moved relative to the work piece, so that the cutting operation can be started immediately after the piercing of the thin plate.
This eliminates unnecessary time settings, which leads to a faster machining of the workpiece and the resulting reduction in costs.

Monitoring of the welding process is also done in this way. The noise measurement signal that normally occurs during welding remains below a predetermined threshold. However, if a material defect occurs in the region of the weld seam, for example due to a material defect, this leads to an increased formation of droplets, which causes the noise measurement signal to exceed a predetermined threshold value. In that case, a status signal is formed, which reveals that the workpiece being machined is a waste material. In that case, the welding process is aborted.

Furthermore, it is possible to check the noise measuring signal whether it lies between the upper and lower threshold values. If the noise measurement signal exceeds the upper threshold value during welding, for example, then material defects may be present, as already mentioned. If below the lower threshold, on the other hand, the laser beam may have dropped unacceptably, for example, or the laser may have been turned off. In that case as well, a status signal is generated which is used to stop the device. The same can occur, for example, when the welding beam accidentally punches the workpiece during welding.

Also in the case of laser cutting, the noise measurement signal is compared with the upper and lower threshold values. If it is below the lower threshold, a perfect cut is obtained, which is indicated by the appropriate status signal. On the other hand, if the noise measurement signal exceeds the upper threshold during laser cutting, this can be seen as a drop in the laser beam or a defect in the material. Exceeding the upper threshold suggests droplet formation, which means that a perfect cut is not made.

In accordance with the preferred embodiment of the present invention, the DC signal is high pass filtered to provide a relatively general noise measurement signal. This is because the frequencies are then added over a very large area. A general noise measurement signal of this kind often provides sufficiently perfect process monitoring.

According to another embodiment of the invention, the DC signal can be bandpass filtered to obtain a noise measurement signal based on the information in the frequency domain limited thereby. This kind of frequency domain can be determined empirically.

Furthermore, the frequency of the electrical alternating signal is switched in order to be able to improve the sensitivity of the measuring method according to the respectively predicted noise.

The AC signal can be, for example, an AC voltage generated by an AC applied with a constant RMS value, which drops through the measuring capacitance. The AC voltage is first rectified and low-pass processed, so that the DC signal required for distance control can be obtained. This is then further frequency filtered, rectified and compared to a pre-adjusted threshold to produce a status signal.

Alternatively, an electrical alternating signal can also be generated by an oscillator circuit formed by the measuring capacitance and the inductance. In that case, the frequency of the oscillator circuit is supplied to the frequency / DC voltage converter in order to generate a DC signal, in which case the DC signal is further used for distance control. It is also possible to obtain a status signal by subjecting the same DC signal to another frequency filtering and subsequent rectification and thereby comparing it with a preset threshold value.

Furthermore, the amplitude of the oscillating circuit AC signal is rectified to directly produce a DC signal, which is then also high-pass or band-pass filtered, rectified and compared with a preset threshold value. , Thereby allowing other status signals to be obtained.

[0031]

Embodiments of the present invention will be described below in detail with reference to the drawings.

In the following, referring to FIG. 1, a first embodiment of a circuit implementing the method according to the invention will be described in detail.

A laser beam (not shown) is guided in the axial direction through the laser processing nozzle 1 and focused on the workpiece 2. A sensor electrode 3 made of, for example, copper is attached to the tip of the laser processing nozzle 1. In that case, the sensor electrode 3 concentrically surrounds the laser beam and is therefore ring-shaped. The sensor electrode forms with the workpiece 2 a measuring capacitance CMess related to distance.

The workpiece 2 itself is connected to earth potential and the sensor electrode 3 is a constant alternating current source 4 as well as a rectifier 5.
Of the rectifier, and the output of the rectifier is supplied to the low-pass filter 6. The output of the low-pass filter 6 is connected to the input of the distance control circuit 7, and the other filter 8
Is also connected to the input of, and this filter can be, for example, a high-pass filter or a band-pass filter. The output of this other filter 8 is fed to the comparison circuit 10 via another rectifier 9 and thereby the regulation circuit 11
Is compared with a preset threshold value. The status signal can be taken out from the terminal 12 connected to the output of the comparison circuit 10. Furthermore, the distance control circuit 7 is connected to the laser processing nozzle 1 via a mechanical adjustment mechanism,
Thereby, the distance of the laser processing nozzle to the work piece 2 is maintained at a desired value according to the DC signal supplied from the low pass filter 6.

For example, when welding, or the workpiece 2
When a workpiece is processed using a laser beam, such as when making a hole in a hole, an electrically neutral hot metal droplet is generated in the area between the workpiece 2 and the sensor electrode 3 in addition to the normal plasma. In particular, metal splashes change the measured volume CMess. The corresponding noise capacity is shown in FIG. 1 as Cst, and the plasma effect is illustrated by an equivalent series circuit consisting of a voltage generator UG and a plasma internal resistance Ri connected in parallel to the measuring capacity CMess. There is.

The sensor electrode 3 of the laser-working nozzle 1 forms a distance-related measuring capacitance CMess with the workpiece 2, which has already been explained, which is about 0.1 pF to about 10 pF.
between pF.

An alternating current having a frequency in the range of 10 kHz to about 100 kHz is supplied to the measuring capacitance CMess using a constant alternating current power supply which supplies an effective current Ieff of about 100 nA. If the measuring capacitor formed by the workpiece 2 and the sensor electrode 3 corresponds to an ideal plate capacitor, the effective value of the alternating voltage that drops there is directly proportional to the distance of the sensor electrode 3 from the workpiece 2, And measuring capacity CMe
Inversely proportional to ss.

The AC voltage (AC signal) that drops through the measuring capacitance CMess is rectified by the rectifier 5 and converted into a DC voltage (DC signal) by a low-pass filter (for example, 100 Hz to 3 kHz) in the subsequent stage. This DC voltage is supplied from the output of the low-pass filter 6 to the input of the distance control circuit 7, and the distance control circuit causes the distance between the laser processing beam 1 to the sensor electrode 3 and the workpiece 2 via the operating mechanism 13 according to the obtained DC voltage. Is adjusted, eg kept constant. This distance control is generally known and will not be described in detail here.

Further, according to the present invention, the DC voltage appearing at the output of the low pass filter 6 can be supplied to the other filter 8, and the other filter can be, for example, a 50 Hz high pass filter. By this means, the frequency components associated with the metallic splash activity in the DC signal can be filtered and thereby converted by the rectifier 9, for example, into a DC signal corresponding to the magnitude of the splash activity. Rectifier 9 in some cases
Another low-pass filter (not shown) can be provided in the subsequent stage. This DC voltage at the output of the rectifier 9 represents the noise measurement signal, which is fed to the comparison circuit 10 (comparator). When this DC voltage exceeds or falls below the threshold value set by the adjustment circuit 11, the comparison circuit 10
Provides a status signal to terminal 12.

For example, when the laser beam is used for drilling, the noise measurement signal appears at the output of the rectifier 9 until the puncture is completed. After that, no metal splashes occur anymore, so that the noise measurement signal falls below the set threshold value. The status signal generated by it is
It is used to start the driving of the laser processing nozzle 1, whereby the laser processing nozzle 1 is moved a certain distance with respect to the workpiece 2 and the cutting process is carried out.

In the case of welding, on the other hand, the comparison threshold is adjusted such that the noise measurement signal initially does not exceed this threshold. The material defect causes a very strong splash activity that causes the threshold to be crossed only when the noise measurement signal becomes large and a status signal is generated, which is used to abort the welding process.

FIG. 2 shows another circuit implementing the method according to the invention.

In this case as well, the sensor electrode 3 at the tip of the laser processing nozzle 1 forms with the grounded workpiece 2 a measuring capacitance CMes related to distance. This measuring capacity CMess
Form an oscillator circuit with a fixed inductance L connected in series with the measuring capacitance CMess. The end of the inductance L on the side opposite to the measurement capacitance CMess is connected to the exciter circuit 14, and further connected to the frequency / DC voltage converter 15. At the output of the frequency / DC voltage converter, the DC signal already described is generated. The output of the frequency / DC voltage converter 15 is connected to the component units 8, 9, 10, 11 and 12 and 7 and 13 as in the circuit shown in FIG. These will not be described again here. However, it is further mentioned that the inductance L can be arranged inside the laser machining nozzle 1.

Another rectifier 16 is connected to the end of the inductance L opposite to the measuring capacitance CMess, and the measuring signal taken out from the oscillation circuit is directly supplied to this rectifier. On its output side, a sequential configuration unit 8 ', 9', 10 ', an attached adjusting circuit 11' and a terminal 1 are provided.
2'is provided, in which case these constituent units are the constituent units 8, 9, 10, 11 and 1 of the circuit of FIG.
2 and are connected to each other as in the case of FIG.

In the circuit shown in FIG. 2, both the noise capacitance CSt generated by neutral metal splashes and the equivalent circuit representing plasma are parallel to the oscillator circuit. For example, the capacitance change of the oscillator circuit caused by the noise capacitance CSt causes a frequency change of the oscillator circuit here. The measurement frequency is in the region of many MHz, for example, and can be optimized for the laser processing process. For example, the measurement frequency can be 10.7 MHz. The frequency change of the oscillation circuit is changed by the frequency / voltage converter 15.
It is directly converted into a voltage change related to the capacitance change. Therefore, a DC voltage (DC signal) is generated at the output of the frequency / voltage converter 15, which is supplied to the distance control circuit 17 and reaches the input of another filter 8. Here, another high-pass filter process or band-pass filter process is performed at a limit frequency of 50 Hz, for example, according to a desired observation method. Optimal filter characteristics are process-related and must be determined empirically. Therefore, the other filter 8 can filter the capacitance change caused by the metal droplets generated during the laser machining of the workpiece from the measurement signal. Rectifier 9
Another rectification or low-pass filtering in the converter converts this change into a DC voltage corresponding to the magnitude of the metal splash activity. This DC voltage again forms the noise measurement signal. The noise measurement signal is then sent to the comparison circuit 1
0, where it is compared with the threshold set by the adjusting circuit 11. If the noise measurement signal is preconditioned and exceeds or falls below a previously empirically determined threshold value, a status signal is output at terminal 12.

In addition to this first status signal, further status signals can be generated at terminal 12 '. This second status signal characterizes the effect of the plasma generated during laser processing.

Therefore, the measurement signal is taken out in the oscillator circuit and supplied to the rectifier 16. The generated DC signal corresponds here to the amplitude of the vibration. It has been found that the resistance of the plasma varies with the process conditions, respectively, and thus the amplitude of the oscillation is more or less damped. The high frequency components generated by the plasma in the DC signal are filtered as desired by another filter 8 '(high-pass or band-pass filter) and then rectified by a rectifier circuit 9', whereby plasma-related noise A measurement signal is obtained, which signal is then compared inside the comparison circuit 10 'with a threshold value set by the adjustment circuit 11', whereby another status signal is supplied to the terminal 1.
Obtained in 2 '.

This further status signal at terminal 12 'can be fed to the multiplication stage 17 and multiplied by a weighting factor smaller than 1, for example. This other status signal multiplied by the weighting factor is then applied to the subtractor 18 at terminal 12
From the first status signal at, resulting in a further status signal at the output 12 "of the subtractor 18 which is substantially plasma-free.

Of course, instead of the circuit elements described above, it is also possible to generate the overall status signal by means of a processor circuit with the corresponding software.

Further, in the circuit shown in FIG. 1 as well, by superimposing a constant direct current voltage on the applied alternating current, the circuit shown in FIG.
Other evaluations can be performed which generate other status signals related to the resistance of the plasma described in connection with the circuit shown in FIG. For this purpose, it is necessary to connect the input of the rectifier 16 of FIG. 2 to the sensor electrode of FIG. 3 and the input of the subtractor 18 of FIG. 2 that is not connected to the multiplication stage to the terminal 12 of FIG. . In that case the remaining component units 8 ',
9 ', 10', 11 ', 17 and 18 need only be inherited as shown in FIG. Furthermore, a constant DC voltage must be supplied to the sensor electrodes via the generator U shown in dotted lines in FIG.

[0051]

As is apparent from the above description, according to the present invention, the measured capacitance between the sensor electrode and the workpiece is generated not only by the plasma generated during the laser processing but also in general at that time. This noise measurement signal produced by uncharged metal droplets changing the temperature provides sufficient on-line process monitoring for a number of uses, thereby improving process safety and / or working speed during laser processing. Can be obtained.

For example, in the case of laser cutting a thin metal sheet, at the beginning of the cutting a hole is first formed by the laser beam, whereby the cutting can then be started.

According to the invention, during laser drilling, the noise measurement signal is monitored whether it exceeds or falls below a predetermined threshold, so that when the status signal is generated, the laser beam is already emitted. Is moved relative to the work piece, so that the cutting operation can be started immediately after the piercing of the thin plate. This eliminates unnecessary time settings, which leads to a faster machining of the workpiece and the resulting reduction in costs.

Monitoring of the welding process is also done in this way. The noise measurement signal that normally occurs during welding remains below a predetermined threshold. However, if a material defect occurs in the region of the weld seam, for example due to a material defect, this leads to an increased formation of droplets, which causes the noise measurement signal to exceed a predetermined threshold value. In that case, a status signal is formed, which reveals that the workpiece being machined is a waste material. In that case, the welding process is aborted.

Furthermore, it is possible to check the noise measurement signal if it lies between the upper and lower threshold values. If the noise measurement signal exceeds the upper threshold value during welding, for example, then material defects may be present, as already mentioned. If below the lower threshold, on the other hand, the laser beam may have dropped unacceptably, for example, or the laser may have been turned off. In that case as well, a status signal is generated which is used to stop the device. The same can occur, for example, when the welding beam accidentally punches the workpiece during welding.

Also in the case of laser cutting, the noise measurement signal is compared with the upper and lower threshold values. If it is below the lower threshold, a perfect cut is obtained, which is indicated by the appropriate status signal. On the other hand, if the noise measurement signal exceeds the upper threshold during laser cutting, this can be seen as a drop in the laser beam or a defect in the material. Exceeding the upper threshold suggests droplet formation, which means that a perfect cut is not made.

In accordance with the present invention, the DC signal is high pass filtered thereby providing a relatively general noise measurement signal. This is because the frequencies are then added over a very large area. A general noise measurement signal of this kind often provides sufficiently perfect process monitoring.

According to the present invention, the DC signal is bandpass filtered and the noise measurement signal can be determined based on the information in the frequency domain limited thereby. This kind of frequency domain can be determined empirically.

Furthermore, the frequency of the electrical alternating signal is switched in order to be able to improve the sensitivity of the measuring method according to the respectively predicted noise.

The AC signal may be, for example, an AC voltage generated by an AC applied with a constant effective value, which drops through the measuring capacitance. The AC voltage is first rectified and low-pass processed, so that the DC signal required for distance control can be obtained. This is then further frequency filtered, rectified and compared to a pre-adjusted threshold to produce a status signal.

Alternatively, an electrical alternating signal can also be generated by an oscillator circuit formed by the measuring capacitance and the inductance. In that case, the frequency of the oscillator circuit is supplied to the frequency / DC voltage converter in order to generate a DC signal, in which case the DC signal is further used for distance control. It is also possible to obtain a status signal by subjecting the same DC signal to another frequency filtering and subsequent rectification and thereby comparing it with a preset threshold value.

Further, the amplitude of the AC signal of the oscillator circuit is rectified to directly generate the DC signal, which is then also high-pass or band-pass filtered, rectified and compared with a preset threshold value. , Thereby allowing other status signals to be obtained.

[Brief description of drawings]

1 is a block diagram showing a first circuit for implementing the method according to the invention.

FIG. 2 is a block diagram showing a second circuit for implementing the method according to the invention, having an oscillator circuit.

[Explanation of symbols]

 1 Laser Processing Nozzle 2 Workpiece 3 Sensor Electrode 4 Constant Current Source 5 Rectifier 6 Low Pass 7 Distance Control Circuit 8, 8'Filter 9, 9 'Rectifier 10, 10' Comparison Circuit 11, 11 'Control Circuit 12, 12' Terminal 14 Exciter 15 Frequency / DC voltage converter 18 Subtractor

Claims (11)

[Claims] 1. A measuring capacitance (CMess) existing between a sensor electrode (3) and a workpiece (2) is measured by the measuring capacitance (CMess).
To determine by assessing the change in the AC signal due to the use of a sensor electrode (3) positionable with respect to the workpiece (2) and to which an electrical AC signal is applied,
A method of machining a workpiece (2) by a laser beam, in which case an electrical DC signal corresponding to the distance between the sensor electrode (3) and the workpiece (2) is first formed from an AC signal. In, a noise measurement signal is formed from a DC signal by frequency filtering and subsequent rectification, and this noise measurement signal is compared with at least one predetermined threshold value, and when the noise measurement signal passes the threshold value, A method of machining a workpiece using a laser beam, which comprises generating a status signal. 2. The method of claim 1, wherein the frequency filtering is high pass filtering. 3. The method of claim 1, wherein the frequency filtering is bandpass filtering. 4. The method according to claim 1, wherein the frequency of the electrical alternating signal is switchable. 5. An electrical AC signal is measured capacitance (CMes).
5. A method as claimed in any one of the preceding claims, characterized in that it is an alternating voltage generated by an alternating current (i ~) falling through s) and applied at a constant effective value. . 6. An electrical AC signal is measured capacitance (CMes).
s) and an inductance (L) are generated by the oscillation circuit (CMess, L), and the oscillation circuit (C
Mess, L) frequency is a frequency / DC voltage converter (15)
5. A method as claimed in any one of the preceding claims, characterized in that it is supplied to a DC signal. 7. Method according to claim 6, characterized in that the direct current signal is generated after direct rectification of the amplitude of the alternating current signal of the oscillator circuit (CMess, L). 8. A further status signal is formed by multiplying the second status signal generated based on the amplitude of the AC signal by a weighting coefficient and subtracting from the first status signal. The method according to claim 7, wherein 9. Method according to claim 1, characterized in that the laser beam is moved relative to the workpiece (2) only after the generation of the status signal. 10. Method according to claim 1, characterized in that the movement of the laser beam with respect to the workpiece (2) is stopped after the generation of the status signal. 11. Method according to claim 1, characterized in that it checks whether the noise measurement signal lies between an upper and a lower threshold value.