DE19857694C2 - Method and device for adjusting the focus position of a laser beam directed at a workpiece - Google Patents

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 7.

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.

DE 40 28 974 A1 describes a method for determining the focus position of a pulsed laser beam in which the amplitude profile of a Measurement signal that shows the course of the power density of a machining glow, i.e. a plasma and / or surface glow above the Represents time for a laser pulse is evaluated. The characteristic The characteristic for determining the focus position is the time between the impact of the laser beam on a workpiece surface and the first maximum in the amplitude course of the machining lights.

This time between the first laser pulse hitting the factory piece surface and reaching the first maximum is used for ver different distances between laser processing head and workpiece surface determined so that there is a curve, the minimum of which op maximum distance between processing head and workpiece surface and thus describes the position of the laser beam focus.  

Another method for determining which is known from DE 40 28 974 A1 The focus position uses the power density of machining lights at a fixed time before the first maximum, which is for different ne distances are measured and evaluated.

DE 44 23 409 A1 describes a method for regulating the distance between a laser processing head and a workpiece where one existing measurement between a sensor electrode and a workpiece capacity is detected and evaluated in order to deliver a control signal.

In particular, an electrical alternating signal is applied to the measuring capacitance placed, the change as a result of changes in measuring capacity, ie due to changes in distance between the sensor electrode and the factory piece is detected. For this purpose, the alternating signal is first an elec tric DC signal generated, the distance between the sensor electro de and workpiece corresponds.

The invention has for its object a method and a Vorrich tion of the type mentioned so that the location of the Focus the machining laser exactly relative to the workpiece surface can put. 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 7. Beneficial Embodiments of the invention are in the respective subordinate claims marked.

By the method of claim 1, it is possible between the each to differentiate very closely between the plasma strengths associated with the traces the one, so that a clear assignment between plasma strength on the one hand and focus position can be carried out on the other hand. The intense The most suitable focus position can therefore be clearly selected.  

If the number of tracks used is high, the focus position can be selected be optimized quite well, because then the focus position within a ge desired focus position area very fine or in smaller steps can be divided. Let even the smallest changes in the focus position with this procedure with the help of the burned-in traces still prove, so that here too the assignment between plasma strength and focus position remains clear.

By integrating the plasma strength along the respective tracks Fluctuations in the plasma strength can be compensated for, which leads to leads to clearer results between plasma strength and focus position, because you can work practically independent of fluctuations.

To find out the trace along which the maximum plasma Strength has been determined after the plasma strength has ended measure the plasma strengths stored in the memory with each other like. The focus position belonging to this track (distance between La machining head and workpiece) is then used for further calibration system selected.

The distance between the laser processing head and the workpiece can be determined by the system itself can be changed in very small steps, track by track the, the respective storage locations for the plasma strength per track the associated focus positions or distances are assigned. The System, it is therefore possible to control the entire process automatically, for example, based on a suitable machine control signal that was selected generated or after a conventional laser machining of the workpiece can be.

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 carrier fre quenz or the aforementioned intensity measurement signal can both before being rectified.

According to a development of the device according to the invention set the focus position of a laser beam directed at a workpiece according to claim 1, the measuring device for measuring the intensity radiation emitted by the plasma in a suitable wavelength be trained. But it is also possible that the measuring device device has an LC generator, the capacity of which by a between Machining head and workpiece lying measuring capacity is formed where with the output signal of the LC generator, information about the Plas contains strength.

The LC generator can use a filter to filter out the carrier sequence of the LC generator, so as not to determine the focus position  to suppress suitable signals, such as noise. 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, the storage device, the comparison device, the machining head or its drive and a device for loading the distance between the machining head and workpiece can be adjusted NEN all connected to a machine control to a full automatic focus position selection or setting before, during or to be able to perform after an ordinary laser processing process nen.

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 a laser arranged in the interior of the Bear beitungskopfs 1 focusing optics. 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 can be 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 height or the maximum integration value of the integrator 12 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 , that is to say no plasma is formed, the measuring capacitance C _M 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 T2, 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 the amplitude of which 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 (12)

1. 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 ), in which:

• - In the surface of the workpiece ( 2 ) with the aid of the laser beam ( 3 ) several separate tracks at different distances from the machining head ( 1 , 19 ) and workpiece ( 2 ) and otherwise are burned in under the same conditions, all tracks being the same are long;
• - The strength of a resulting plasma ( 5 ) for each track is measured separately by integrating the plasma strength measured in each track;
• - The plasma strength measured per track is stored in a memory ( 13 );
• - After completion of the plasma strength measurements of all traces, the plasma strengths stored in the memory ( 13 ) are compared with one another; 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 it is on Command of a machine control signal is automatically executed. 3. The method according to any one of claims 1 or 2, characterized in that the intensity of a radiation emitted by the plasma ( 5 ) is used to measure the plasma strength. 4. The method according to any one of claims 1 or 2, characterized in that in the case of a capacitive measurement of the distance (h) between processing head ( 1 , 19 ) and workpiece ( 2 ) by means of an LC generator (G), the Capacity is formed by a measuring capacity (C _M ) between the processing head ( 1 , 19 ) and workpiece ( 2 ), the amplitude of the generator output signal ( 22 ) is used for measuring the plasma strength. 5. The method according to claim 4, characterized in that the carrier frequency of the LC generator (G) is filtered out of the generator output signal ( 22 ). 6. The method according to claim 3 or 5, characterized in that the intensity measurement signal or the filtered out carrier frequency is the same be judged. 7. 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 ), the measuring device being an integrator ( 12 ) for integrating the measured Has plasma strength;
• - 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.

8. The device according to claim 7, characterized in that the measuring device ( 9 ) for measuring the intensity of a plasma from the abge ( 5 ) radiation is formed. 9. The device according to claim 7, characterized in that the measuring device (G, 24 , 25 ) has an LC generator (G), the capacity of which lies between a processing head ( 19 ) and workpiece ( 2 ) de measuring capacity (C _M ) is formed, and the output signal ( 22 ) contains an information about the plasma strength. 10. The device according to claim 9, characterized in that the LC generator (G) is followed by a filter ( 24 ) for filtering out the carrier frequency of the LC generator (G). 11. The device according to claim 8 or 10, characterized in that the measuring device is followed by a rectifier ( 25 ). 12. The device according to one of claims 8 to 11, 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 .