DE19857694A1 - Laser beam focussing, in laser machining, comprises producing workpiece surface tracks at different laser head stand-off distances and determining the distance corresponding to maximum plasma intensity - Google Patents

Abstract

Laser beam focal point adjustment, by producing workpiece surface tracks at different laser head stand-off distances and determining the distance corresponding to maximum resulting plasma intensity, is new. The focal point of a laser beam (3) is adjusted by (a) using the laser beam to produce several spaced tracks in a workpiece surface under identical conditions except for different distances between the laser processing head (1) and the workpiece (2); (b) separately measuring the resulting plasma intensity for each track; and (c) determining the distance corresponding to maximum plasma intensity. An Independent claim is also included for equipment for carrying out the above process.

Description

The invention relates to a method and a device for adjusting according to the focus position of a laser beam directed at a workpiece claims 1 and 9.

It is when machining a workpiece using a laser beam important that the focus of the laser beam in the area of the surface surface of the workpiece comes to good machining results achieve.

It has already been thought of for this purpose in the surface of the Workpiece some tracks at different focus positions of the machining burn in the laser and then examine these traces men. After that, the focus position for the track with the most intense should Penetration selected and for focus position calibration of the laser processing be used. However, it turned out that the visual assessment of such traces is difficult and often not leads to the desired result.

The invention has for its object a method and a Vorrich tion of the type mentioned so that the location of the Focus of the machining laser more precisely relative to the workpiece surface can be adjusted. The aim of the invention is also this setting to be carried out automatically, i.e. without human assistance.

A procedural solution to the task can be found in the kenn Drawing part of claim 1. In contrast, is a device-side Solution to the problem stated in claim 9. Beneficial Embodiments of the invention are in the respective subordinate claims marked.

A method for setting the focus position of a laser beam directed onto a workpiece, which emerges from a processing head with focusing optics for the laser beam, is characterized by the following steps:

• - In the surface of the workpiece with the help of the laser beam several separate tracks with different Ab are located between the machining head and the workpiece and otherwise branded same conditions;
• - the strength of a resulting plasma between the machining The measuring head and workpiece are measured separately for each of the tracks; and
• - It is the distance between the machining head and the work piece selected (focus position) for which the maximum plasma strength is ge has been measured.

With this method it is possible to switch between the respective Spu to differentiate ren assigned plasma strengths very precisely, so that egg ne clear allocation between plasma strength on the one hand and focus la ge can be performed on the other hand. The most intense one Brand-appropriate focus position can therefore be clearly selected. At high The number of tracks used can be selected quite well by the focus position can be optimized, because then the focus position within a desired Fo area can be divided very finely or in smaller steps can. Even the smallest changes in the focus position can be made with this Prove the procedure with the help of the burned-in traces sen, so that here too the association between plasma strength and focus la ge remains clear.

According to a development of the invention, all tracks are of equal length trained, the plasma strength measured per track integrated becomes. By integrating the plasma strength along the respective tracks Fluctuations in plasma strength can be compensated for, leading to even clearer results between plasma strength and focus position leads.

In an embodiment of the invention, the Plas measured in each case can be mastärke be stored in a memory, after completion of the Plasma strength measurements of all traces of the plasma stored in the memory  strengths are compared with each other in order to find out that trace along which the maximum plasma strength has been determined. The too focus position belonging to this track (distance between laser processing head and workpiece) is then ge for further calibration of the system chooses. The distance between the laser processing head and the workpiece can changed track by track by the system itself in very small steps be, the respective storage locations for the plasma strength per The associated focus positions or distances are assigned. The system is therefore able to automatically close the entire process control, for example based on a suitable machine control signal, the before, during or after a normal laser machining of the workpiece can be generated.

In yet a further embodiment of the invention, the plasma can be measured for example, the intensity of a plasma emitted Radiation can be used. For this purpose, a radiation sensor for Use that on the area between the tip of the laserbear processing head and the workpiece is aligned, such as on the area between a nozzle tip and the workpiece, where usually a Plasma is generated during the machining of the workpiece, such as Welding or cutting the workpiece.

Alternatively, for example, in the case of a capacitive measurement solution of the distance between the machining head and the workpiece using an egg nes LC generator, the capacity of which by a processing Head and workpiece existing measuring capacity is formed, the amplitude de the generator output signal to measure the plasma strength pull. It is advantageous here that one and the same generator signal both the distance signal for determining the distance between machining head and workpiece as well as the plasma strength signal can be generated. The procedure for determining the focus position can therefore be carried out particularly easily.

The carrier frequency of the LC  Generator are filtered out based on the amplitude of the filtered carrier frequency signal a conclusion on the plasma strength to draw. For easier assessment of the filtered signal or of the aforementioned intensity signal, both can previously be the same be tested.

In accordance with the invention, a device for adjusting the focus position of a laser beam directed at a workpiece contains the following:

• A processing head having focusing optics, through which the laser beam passes to be focused;
• - A device for determining a distance between machining tion head and workpiece;
• - A measuring device for measuring the strength of a laser beam generated plasma along a track on the workpiece;
• - A storage device for storing each along ge separated traces of measured plasma strengths; and
• - A comparison device to search for that Ab stands for which a maximum plasma strength was measured along a track has been.

The measuring device for measuring the intensity of one of Plasma emitted radiation in a suitable wavelength range be trained. However, it is also possible for the measuring device to have an LC Generator, whose capacity is determined by an intermediate machining head and workpiece lying measuring capacity is formed, the off output signal of the LC generator provides information about the plasma strength contains.

The LC generator can use a filter to filter out the carrier sequence of the LC generator, so as not to be used for the focus la suitable signals, such as noise, to be suppressed.

Furthermore, the measuring device according to a further embodiment of the  Invention a rectifier to be a simple and to enable more precise recording of the measured values.

The measuring device can also be an integrator for integration included in the measured plasma strength to avoid fluctuations to be able to work more efficiently. The measuring device, the storage device, the comparison device, the processing head or its drive and a device for determining the distance between machining The head and workpiece can all be controlled by a machine control be bound to a fully automatic focus position selection or setting development before, during or after an ordinary laser processing pro to be able to perform.

The invention is described below with reference to the drawing in described. Show it:

Figure 1 is a block diagram of a laser processing system according to the invention with the possibility of optical measurement of the plasma strength.

Fig. 2 is a sketch for explaining the relationship between the laser processing head and the workpiece in the capacitive distance measurement in the presence of a plasma between the laser processing head and the workpiece;

Fig. 3 is a laser processing system with the possibility of Bestim mung the plasma strength by measuring the resistance of the plasma between the laser processing head and the workpiece; and

Fig. 4 shows a LC-generator for the circuit of FIG. 3.

A laser processing system according to a first exemplary embodiment of the invention has a laser processing head 1 , through which a laser beam 3 to be focused on a workpiece 2 passes. The focus tion of the laser beam 3 is performed by means of an arranged in the interior of the laser focusing optics Bear beitungskopfs 1. 4 With the help of the laser beam 3 focused on the workpiece 2 , welding or cutting work can be carried out, for which purpose the laser processing head 1 and the workpiece 2 are moved relative to one another. A plasma 5 is created between the tip of the laser processing head 1 and the workpiece 2 . The height h between the tip of the laser processing head 1 and the workpiece 2 can be adjusted or kept constant by suitable means in order to keep the focus of the laser beam 3 in the region of the surface of the workpiece 2 . For this purpose a height adjustment device 6 is connected to the laser processing head 1, which equips a displacement of the laser machining head 1 perpendicular to the surface of the workpiece 2 ge. This height adjustment mechanism 6 is a machine controls tion of 7 driven which provides via a line 8, a suitable height adjusting signal to the height adjustment. 6 The height h between the laser processing head 1 and workpiece 2 can be measured in a suitable way, for example capacitively. To maintain contact with the height h, the height setting signal on line 8 can be regulated accordingly.

A measuring device 9 is provided for measuring the thickness of the plasma 5 between the laser processing head 1 and the workpiece 2 . This Meßeinrich device 9 consists of a provided with an input optics 10 radiation detector 11 , which emits an output voltage U corresponding to the measured radiation intensity at its output. The input optics 10 is directed towards the area between the laser processing head 1 and the workpiece 2 and in this way measures the radiation emitted by the plasma 5 in a desired or predetermined wavelength range. The radiation detector 11 is coupled to the laser processing head 1 or fixedly connected, so that it takes part in its movement and always remains focused on the plasma area 5 . The stronger the plasma 5 , the stronger the radiation intensity emitted by it and the greater the output voltage U of the radiation detector 11 .

The measuring device 9 also includes an integrator 12 , the input of which is connected to the output of the radiation detector 11 . The integrator 12 integrates the radiation intensity measured by the radiation detector 11 or the output voltage U output by the radiation detector 11 and provides a corresponding integration value at its output.

The integrator 12 is followed by an electronic memory 13 , which serves to store the integration values made available by the integrator 12 .

Furthermore, a comparison device 14 is connected downstream of the memory 13 , with the aid of which the integration values stored in the memory 13 can be compared with one another.

The integrator 12 and the memory device 13 are connected to the machine control 7 via a line 15 , via which the machine control 7 supplies an integration start command or memory start command to the integrator 12 or to the memory device 13 .

The machine controller 7 is connected to the memory device 13 and the comparison device 14 via a line 16 in order to transmit a read / compare command to them. Finally, the machine control 7 is connected to the memory device 13 and the device 14 for comparison, for example via three address lines 17 . Via these address lines 17 it gives addresses to the memory device 13 , under which integration values supplied by the integrator 12 are to be stored. On the other hand, the comparison device 14 supplies via the address lines 17 that address to the machine control 7 which corresponds to the memory space on which the greatest integration value has been stored. An assignment between the respective memory location addresses and the assigned distances h between the laser processing head 1 and workpiece 2 is found in a table memory 18 of the machine control 7 , so that the latter, based on the address provided by the comparing device 14 , corresponds to the maximum integration value of the integrator 12 or Can read focus position h from this table 18 . The machine controller 7 then sets the laser processing head 1 finally via the height adjustment device 6 to the focus position (height h) determined in this way. In the following, this is kept constant by suitable regulation.

In the method according to the invention, for example, eight adjacent tracks can be burned into the surface of the workpiece 2 at different heights h. All eight tracks have the same length. After a track has been burned in, the sum of the plasma intensity of this track is stored on a memory location in the memory device 13 , the address of which is specified by the machine control via the line 17 . The assigned height h is stored in the memory table 18 . After the for example eight total values of the plasma intensity have been stored by burning in the eight tracks with different focus positions on the associated memory locations of the memory device 13 , the comparison device 14 is activated by activating a read signal by the machine controller 7 . The comparison device 14 then compares the integration values of the storage device 13 stored under the respective addresses and supplies, via the address lines 17, that address to the machine controller 7 under which the greatest integration value is stored. The read signal is transmitted via line 16 to both storage device 13 and comparison device 14 , as already mentioned. Finally, with the aid of the address transmitted to the machine control 7 via the line 17, the associated focus position (distance h) is found there from the memory table 18 .

By simultaneously activating the signals on lines 15 and 16 , the entire system can be reset to its original state.

According to another embodiment of the method according to the invention, in the case of a capacitive measurement of the distance between the machining head and the workpiece by means of an LC generator, the capacity of which is formed by a measuring capacitance C _M between the machining head and the workpiece, the amplitude of the generator output signal for measuring the Plasma strength can be used.

An advantage is that an already known capacitive distance sensor can be used not only to determine the distance between the workpiece and the machining head but also to monitor the plasma strength between the machining head and the workpiece. Electrically, the plasma can be described as a resistance Z _P, for example with a noise source Q _R connected in parallel. This resistance Z _P for example with a noise source connected in parallel then lies as a parallel circuit to the distance-dependent capacitance C _{M '} between, for example, a copper tip of the machining head and the workpiece.

With this method, the distance-dependent capacitance C _{M '} between the machining head and the workpiece can still be determined independently of the plasma resistance. For this purpose, the measuring capacitance C _{M is integrated} in an LC generator. The LC generator can be, for example, a generally known emitter-coupled LC oscillator which contains the measuring capacitance C _M as a capacitive element.

If there is no machining of the workpiece by means of the laser beam, only the distance-dependent capacitance C _{M '} between the machining head and the workpiece is integrated into the LC generator. Only with the formation of the plasma occur parallel to the distance-dependent capacitance C _{M '} of the plasma-dependent resistance Z _P and the noise source, which, however, have practically no effect on the frequency of the generator output signal generated by the LC generator. The stronger the plasma 5 , the lower the resistance Z _P and the smaller the amplitude of the generator output signal.

By integrating the measuring capacitance C _M into the LC generator, the water thus generates a generator output signal, the frequency of which is determined solely by the position-dependent capacitance C _{M '} between the machining head and the workpiece. In contrast, the resistance and the parallel noise source of the plasma act on the amplitude of the generated generator output signal. The amplitude of the generator output signal and the noise source can thus serve as plasma strength indicators. The carrier frequency of the LC generator can also be filtered out or passed through from the generator output signal, and in this way even more characteristic amplitude values with respect to the resistance of the plasma are obtained from the generator output signal, as a result of which an even more meaningful plasma strength signal is obtained, from which then draw more precise conclusions about the focal position.

FIG. 2 again shows a laser processing head 19 with a sensor electrode 20 formed on its tip, for example a Cu electrode. The sensor electrode 2 serves to form a measuring capacitance C _M between the laser processing head 19 and the workpiece 2 .

In addition, Fig. 2 shows an equivalent circuit diagram of the measured capacitance C _M currency rend the plasma formation. The measuring capacitance C _M is formed by a distance-dependent capacitance C _{M '} , to which a resistance Z _{P representing} the plasma and a noise source Q _R caused by the plasma lie in parallel. If there is no machining of the workpiece 2 , ie no plasma is formed, the measuring capacitance cm between the laser machining head 19 or the sensor electrode 20 and the workpiece 2 is equal to the distance-dependent capacitance C _{M '} . When machining of the workpiece 2, a leaking out of the machining head 19 laser beam strikes (not shown here) on the workpiece 2, whereby this beitungspunkt at Bear is strongly heated so that material Material of oxygen burns at to drove (plasma formation). The measuring capacitance C _M present during machining of the workpiece 2 can thus be represented by the distance-dependent capacitance C _{M '} with an impedance Z _P connected in parallel and a noise source Q _R connected in parallel.

While C _{M 'is} a measure of the distance h between laser processing head 19 or sensor electrode 20 on the one hand and workpiece 2 on the other hand, the resistance Z _P together with the noise source Q _R serves as an indicator of the intensity or strength of the plasma.

Fig. 3 shows a block diagram of another embodiment of the inventive device. The equivalent circuit of the measuring capacitance C _M shown in FIG. 2 is integrated in an LC generator G in FIG . The LC generator G is, for example, an emitter-coupled LC oscillator, the remaining components of which are symbolized by the block 21 . The circuit of the LC generator G with the built-in measuring capacitance C _M will be explained in more detail later in the description of FIG. 4.

By integrating the measuring capacitance C _M into the LC generator G, this produces at its output a generator output signal 22 , the frequency of which is determined by the distance-dependent capacitance C _{M '} and the amplitude of which is determined by the resistance Z _P or the noise source Q _R. More precisely, the amplitude is proportional to the resistance Z _P.

By means of a frequency / voltage converter 23 connected downstream of the LC generator G, the generator output signal 22 can be used in a generally known manner for measuring the distance between the laser processing head 19 and workpiece 2 , since the resistance Z _P and the noise source Q _{R have} practically no influence on the frequency of the LC generators G have.

As shown in Fig. 3, a filter 24 is connected on the output side to the LC generator G, which filters out the carrier frequency of the LC generator G from the generator output signal 22 or lets it pass through. On the output side, a rectifier 25 is also connected to the filter 24 in order to generate a rectified signal.

To determine the plasma strength, the signal appearing at the output of the rectifier 25 can then be fed to the integrator 12 already shown in FIG. 1 as an input signal, in order then to be processed in accordance with the further process shown in FIG. 1. The signal coming from the frequency / voltage converter 23 is fed to the machine control 7 shown in FIG. 1 so that the height h per track is kept constant in this way and stored in the table memory 18. This in turn takes place via the control of the height adjustment device 6 . Otherwise, the process flow is the same as that which has already been described with reference to FIG. 1, so that it need not be explained again at this point. Only he mentions that in the case of plasma formation, the generator output signals obtained at the respectively set heights h or their amplitudes are compared with the amplitude of the generator output signal that is obtained when there is no plasma 5 present. For example, if the amplitude of the generator output signal without plasma formation is 2.0 volts, the amplitudes of the generator output signal continue to decrease with increasing plasma formation, for example from 1.5 volts to 1.0 volt to 0.5 volt (with three tracks). The smallest amplitude stands for the strongest plasma, in which case the mentioned voltage difference (1.5 volts) is greatest. The respective voltage differences based on the assigned heights h are then stored in the memory 13 and the comparison device 14 then looks for the address in the memory 13 under which the greatest voltage difference is stored. This address is then transmitted to the machine controller 7 , which then searches the assigned height h (focus position) from the table memory 18 and calibrates the system with this height h below.

Fig. 4 shows an LC generator G used in the device according to Fig. 3. This contains a transistor T ₁ , the collector connection is connected to ground and the base connection is coupled to an inductance L and the measuring capacitance C _M in a common connection point is. The other connections of the inductance L and the network capacitance C _M are also grounded. The LC generator G also contains a transistor T ₂ , the base connection of which is connected to ground and the collector connection of which is connected to the common connection point of the inductance L and the measurement capacitance C _M. The emitter connections of the transistors T ₁ and T ₂ are directly connected to one another, a resistor R _{E being} coupled to the emitter connections of T ₁ and T ₂ via a common connection point.

As further shown in FIG. 4, an amplifier unit 27 is connected upstream of the output 26 of the LC generator G.

The LC generator G shown in Fig. 4 is realized as a differential amplifier. Since the base potential of T _{1 is} in phase with the collector potential of T ₂ , the positive feedback can be generated by direct connection. The loop gain is proportional to the slope of the transistors. It can be set within wide limits by changing the emitter current. At the output 26 of the LC generator G, the generator output signal is then obtained, the frequency of which depends on the distance-dependent capacitance C _{M '} and whose amplitude depends on the resistance of the plasma and on the noise generated by the plasma. The output 26 of the LC generator G practically provides the generator output signal 22 shown in FIG. 3.

Claims (15)

1. A method for setting the focus position of a laser beam ( 3 ) directed onto a workpiece ( 2 ), which emerges from a processing head ( 1 , 19 ) with a focusing device ( 4 ) for the laser beam ( 3 ), characterized by the following steps:

• - In the surface of the workpiece ( 2 ) with the help of the laser beam ( 3 ) several separate tracks at different distances between the processing head ( 1 , 19 ) and workpiece ( 2 ) and otherwise burned in under the same conditions;
• - The strength of a resulting plasma ( 5 ) is measured separately for each track; and
• - The distance between the processing head ( 1 , 19 ) and workpiece ( 2 ) is selected for which the maximum plasma strength has been measured.

2. The method according to claim 1, characterized in that all Tracks are of equal length and the plasma strength measured for each track is integrated. 3. The method of claim 1 or 2, characterized in that each abge in a memory (13) per track measured plasma strength lays is and that of all the tracks stored in the memory (13) plasma strengths to each other aligned ver after completion of the plasma strength measurements. 4. The method according to claim 1, 2 or 3, characterized in that it is automatically executed on command of a machine control signal. 5. The method according to any one of claims 1 to 4, characterized in that the intensity of a radiation emitted by the plasma ( 5 ) is used to measure the plasma strength. 6. The method according to any one of claims 1 to 4, characterized in that in the case of a capacitive measurement of the distance (h) between the machining head ( 1 , 19 ) and the workpiece ( 2 ) by means of an LC generator (G), its capacity is formed by an existing between the processing head ( 1 , 19 ) and workpiece ( 2 ) measuring capacity (C _M ), the amplitude of the generator output signal ( 22 ) for measuring the plasma strength is used gene. 7. The method according to claim 6, characterized in that the carrier frequency of the LC generator (G) is filtered out of the generator output signal ( 22 ). 8. The method according to claim 5 or 7, characterized in that the intensity or the filtered signal are rectified. 9. Device for adjusting the focus position of a laser beam ( 3 ) directed onto a workpiece ( 2 ), comprising:

• - A processing head ( 1 , 19 ) having focusing optics ( 4 ) through which the laser beam ( 3 ) passes;
• - A device for determining a distance (h) between the machining head ( 1 , 19 ) and the workpiece ( 2 );
• - A measuring device ( 9 ; G; 24 ; 25 ) for measuring the strength of a plasma ( 5 ) generated by the laser beam ( 3 ) along a track on the workpiece ( 2 );
• - a storage device ( 13 ) for storing plasma strengths measured along separate tracks in each case; and
• - A comparison device ( 14 ) for selecting that from stand (h) for which a maximum plasma strength has been measured along a track.

10. The device according to claim 9, characterized in that the measuring device ( 9 ) for measuring the intensity of a plasma from the abge ( 5 ) is formed radiation. 11. The device according to claim 9, characterized in that the measuring device (G, 24 , 25 ) has an LC generator (G), the capacitance by a between the processing head ( 19 ) and the workpiece ( 2 ) lie de measuring capacity (C _M ) is formed, and the output signal ( 22 ) contains an information about the plasma strength. 12. The apparatus according to claim 11, characterized in that the LC generator (G) is followed by a filter ( 24 ) for filtering out the carrier frequency of the LC generator (G). 13. The apparatus of claim 10 or 12, characterized in that a rectifier ( 25 ) is connected downstream of the measuring device. 14. Device according to one of claims 9 to 13, characterized in that the measuring device has an integrator ( 12 ) for integrating the measured plasma strength. 15. The device according to one of claims 9 to 14, characterized in that a drive ( 6 ) for the machining head ( 1 , 19 ) and the named devices ( 9 , 13 , 14 ) with a machine control ( 7 ) are connected ver .