PROCEDURE AND MECHANISM FOR DETERMINING THE DISTANCE BETWEEN A SENSOR ELECTRODE AND A WORKPIECE

The invention concerns a procedure for determining the distance between a sensor electrode and a workpiece of a laser processing machine, with which the sensor electrode with the workpiece forms a Messkondensator, the part of a resonant circuit is, whereby from a seized resonant circuit frequency the distance between the workpiece and the sensor electrode is determined. A such procedure is for example from the DE 43 40 395 Cl admits become.

With capacitive spacer regulation mechanisms of a laser processing machine for example the Schneiddüse and the workpiece which can be cut form a condenser. This condenser is switched into a resonant circuit and the capacity of the condenser determines the frequency of the resonant circuit. The frequency depends on the distance between the Schneiddüse and the workpiece. The conversion of the frequency into a distance can be made by a characteristic.

Footstep with laser cuttings a plasma up, affects itself this like a parallel switched Ohm's resistance between the Schneiddüse and the workpiece. Thus a phase shift in the resonant circuit, which entails a frequency change and thus a change of the spacer measured value, develops. If the resistance becomes too small, this leads to an intensified absorption, which can lead up to suspending the resonant circuit. A distance measurement is not possible any longer.

From the DE 40 20 196 A1 a capacitive spacer measuring procedure is well-known, with which a Messkondensator with a constant alternating current is fed, so that the measuring voltage measured at the sensor electrode of the Messkondensators depends exclusively on the impedance of the Messkondensators. As long as during processing the workpiece no plasma between the sensor electrode and the workpiece exists, impedance is formed practically exclusively by the capacitive resistance of the Messkondensators, so that the measuring voltage is proportional to the distance between sensor electrode and workpiece. However if a plasma arises, then lies parallel to the capacity of the Messkondensators a Ohm's resistance, which affects the impedance of the Messkondensators. The plasma can lead thereby to a such reduction of the impedance of the Messkondensators that the sensor signal breaks down practically and is feigned a controlling mean a too small distance. With this procedure it is possible to fade out brief disturbances by a plasma on electronic way. With a kondinuierlich lining up plasma this is however not possible.

In order to overcome the problems resulting from a plasma is in the DE 199 06 442 A1 a procedure for measuring the distance between a sensor electrode and a workpiece descriptive, with which it is intended that from the measuring voltage their real part and their imaginary part are determined, in order the distance D which can be measured to determine and thus the disturbing influence of a plasma forming between sensor electrode and workpiece on the impedance of the Messkondensators eliminate themselves. This requires however a substantial circuit and evaluation expenditure.

From the DE 43 40 395 c1 it is well-known to form a resonant circuit in which an inductance in row is arranged to a measuring capacity and thus a resonant circuit is trained. The resonant circuit becomes lively with a measuring frequency. Frequency changes of the resonant circuit are transformed by a frequency/potential transformer into a Spannungsänderung, which correlates directly with a change of capacity. Thus also a change of the distance can be determined. A flag is produced, which describes the influence of the plasma developing during the Laserbearbeitung. For this purpose at the resonant circuit a measuring signal is uncoupled and supplied to an electric rectifier. The high frequency portions produced by the plasma in the equal signal are filtered by a filter and afterwards parallelly, in order to receive a plasma-referred breakdown measuring signal, and comparative with a threshold given in advance, in order to receive a flag. Dependent on the flag it can be intended that a laser beam is only moved relative to the workpiece after production of a flag or that a movement of the laser beam is stopped relative to the workpiece after production of the flag.

In order to minimize and/or exclude the influence of a plasma, was considered to develop special Schneiddüsen for example Schneiddüsen, which consist of two electrically isolated parts, whereby the internal part, which is affected by the plasma, is used not for the distance measurement. With such nozzles only the outside part of the Schneiddüse as well as the workpiece would form the measuring capacity. Such nozzles are however relatively expensive.

Task of the available invention is it to place a procedure for the spacer regulation and a spacer regulation mechanism ready with which a reliable and simple spacer regulation is made possible also with lying close plasma.

This task is solved by a procedure of the kind initially specified, with which the elements of the resonant circuit are not selected in such a manner that the resonant circuit exhibits a quality, with which the oscillation ability of the resonant circuit with one between workpiece and sensor electrode adjusting plasma or insignificantly affect themselves. As previously mentioned, a main problem is with the occurrence of a plasma that a so high absorption in the resonant circuit develops that this suspends. This means that no more frequency can be measured and thus a spacer regulation becomes from the beginning impossible. By a suitable choice of the elements of the resonant circuit the resonant circuit can attain however a so high quality that suspending of the resonant circuit becomes almost impossible. This means that a resonant circuit frequency can be always measured, which can be consulted for a spacer regulation. In this surprisingly simple way a spacer regulation is always possible. Those while stationary the technology descriptive aufwändigen procedures, in particular a high technical circuiting expenditure or the determination of real and imaginary part of a measuring voltage, can be avoided. In addition a standard gumption nozzle can be further used. With the procedure according to invention in particular those can be further used so far while stationary the technology used one-piece nozzles.

With a preferential procedure variant it can be intended that as resonant circuit a LC-resonant circuit with a coil with a quality is preferably used ≥ 80, ≥ 100. In principle it is conceivable to increase the quality of the resonant circuit as the measuring capacity is changed. There this is affected by Schneiddüse, workpiece and medium between Schneiddüse and workpiece, is only limited possible changing the measuring capacity. The use of a coil has the advantage that this can be affected considerably the quality of the resonant circuit and be increased thus by the use of a coil with a high quality the quality of the entire resonant circuit easily and way.

Favourable way the coil and the Messkondensator are switched into row, so that a LC-Serienschwingkreis develops.

With a preferential procedure variant as natural frequency of the resonant circuit a frequency f is used and/or adjusted ≥ 18 MHz, in particular ≥ 20 MHz. The attitude can take place via suitable choice of the inductance of the coil. Preferably a smaller inductance is selected than so far while stationary the technology usually. In particular with natural frequencies over 20 MHz the plasma-conditioned spacer measuring errors are minimized or completely avoided.

Over the spacer regulation to be able is it would drive through favourably, if the resonant circuit frequency from the resonant circuit is uncoupled and frequency-modulated, in particular on the supply voltage of sensor electronics up-modulates, to an evaluation mechanism is conveyed.

With a preferential procedure variant it can be intended that the resistance of the medium between the sensor electrode and the workpiece is determined. The size of the electrical resistance of the medium between the sensor electrode and the workpiece permits a conclusion on the cut quality obtained in the gumption place. The electrical resistance takes relatively large values on with regular gumption enterprise, if the laser beam penetrates the workpiece problem-free. Difficulties the treatment of workpieces, in particular problems regarding the retention of the necessary cut depth of the laser beam, are accompanied by a reduction of the electrical resistance in relation to the values measured with regular gumption enterprise. In the case of a measurement of relatively low resistance values for example the cutting speed, in particular the feed speed of the laser cutter head can be lowered to the workpiece, over a mechanism relative to the controlling of the parameters of the cutting process. Alternatively or supplementing among other things also a varying of distance between nozzles, gumption gas pressure, gumption mass of gas and/or laser beam achievement presents itself. The evaluation mechanism already mentioned considers thereby on the one hand the result of measurement of the resistance test for the determination of the size of the electrical resistance of the medium between sensor electrode and workpiece. On the other hand the evaluation mechanism refers also the distance existing between the sensor electrode and the workpiece at the time of the resistance test. In this way a misguided policy of the machine is excluded. The distance measurement opens the possibility of the examination whether for example a measured electrical resistance value low by means of the resistance test is to be regarded actually than reference to a decreased cut quality, or whether this resistance value is based on it that the sensor electrode, in particular the laser cutter head, came inadvertently also from the Einstechvorgang caused beads or clips or with cinder into contact and by it a short-circuit was produced, consequently the measured electrical resistance likewise a low value takes. A controlling of the parameters of the cutting process leading to the enlargement of the measured resistance values only in the first mentioned case one arranges, in which the electrical resistance measured for the medium between sensor electrode and workpiece possesses a low value and the distance between sensor electrode and workpiece of zero is different.

In the context of the invention also spacer measuring instrument of a laser processing machine falls for the determination of the distance between a sensor electrode and a workpiece, with which the sensor electrode with the workpiece forms a Messkondensator, the part of a resonant circuit is, whereby from a seized resonant circuit frequency the distance between the workpiece and the sensor electrode is determined. The elements of the resonant circuit are thereby in such a manner dimensioned that the resonant circuit exhibits a quality, with which the oscillation ability of the resonant circuit with a plasma adjusting between workpiece and sensor electrode not or insignificantly affect themselves. If the quality of the resonant circuit is high enough, a distance can be always determined despite plasma-conditioned absorption.

Since the quality of a resonant circuit can be particularly simply affected, if this exhibits a coil, it is preferably intended that the resonant circuit is designed as LC-Serienschwingkreis. A sufficient quality of the resonant circuit for a spacer regulation without or with only small influence of an adjusting plasma is reached, if the coil of the resonant circuit exhibits a quality ≥ 80, preferably ≥ 100.

Additional elements can be saved, if as sensor electrode a Schneiddüse, in particular a laser nozzle of a laser cutter head is intended. It is placed by this measure in particular surely that a plasma causes only one absorption of the resonant circuit which can be neglected, so that a resonant circuit frequency is determinable always and the resonant circuit does not fail.

The influence of the plasma during the spacer regulation can be minimized, if the resonant circuit exhibits a natural frequency f ≥ 18 MHz, preferably f ≥ 20 MHz. The resonant circuit frequency can be seized by sensor electronics.

During a preferential arrangement of the invention an evaluation mechanism is intended, which is connected by a shielded cable with sensor electronics, in particular. The evaluation mechanism exhibits thereby favourable-proves a frequency regulation mechanism, in particular a digital counter. In the evaluation mechanism thus the distance between the sensor electrode and the workpiece can be determined on the basis of the seized resonant circuit frequency with the help of a characteristic. The evaluation mechanism can stand with a control in connection, which steers the cutting process of the laser processing machine. The measured distance and possibly a resistance measured by additional resistance measuring instrument for the measurement of the resistance of the medium between sensor electrode and workpiece can be considered by the control, in order to intend appropriate parameters for the controlling of the cutting process.

The invention is described in the following on the basis the design. Show:

Fig. 1

strongly a spacer regulation mechanism schematizes;

Fig. 2

a representation of a laser cutting machine; and

Fig. 3

a sectional view by a laser cutter head and a workpiece.

The Fig. 1 shows a Messkondensator 1, which is formed by a sensor electrode 2 and a workpiece 3. The sensor electrode 2 can be arranged thereby in the proximity of a laser cutter head. In particular it can be intended that the sensor electrode 2 is the laser nozzle of the laser cutter head. The Messkondensator 1 is switched in row to a coil 4, which can be arranged in the proximity of the laser nozzle, in particular in the laser cutter head. By the coil 4 and the Messkondensator 1 a series resonant circuit is formed. Further elements of the resonant circuit are contained in sensor electronics 5. Sensor electronics 5 is supplied via the line 6 with a tension. The noise of supply voltage is sufficient, around the resonant circuit anzuschwingen. The resonant circuit is thus selbstanschwingend. The suggestion is produced from the oscillation by fed back reinforcement.

By a frequency uncoupling mechanism 5a of sensor electronics 5 a resonant circuit frequency adjusting in the resonant circuit is uncoupled. This frequency is up-modulated on supply voltage and conveyed over a shielded line 6, in particular a coaxial cable, to an evaluation mechanism 7.

In the evaluation mechanism 7 a digital counter is intended, which determines the resonant circuit frequency. From this the distance of the sensor electrode 2 to the workpiece 3 can be determined on the basis a given or before taken up characteristic. From the distance in the evaluation mechanism 7 correcting variables, like e.g. a speed desired value, are determined, which are handed over to the control 8. The control 8 ensures for example for the fact that is moved the axle of the laser processing machine determining for the distance between workpiece 3 and Schneiddüse.

Under certain circumstances, for example during a change of direction with laser cuttings, a plasma cloud between the workpiece 3 and the sensor electrode 2 can develop, so that a Ohm's resistance between the sensor electrode 2 and the workpiece 3 is formed, which is suggested by the resistance 9. This bypass resistor 9 leads to a ill-will of the resonant circuit. Thus develops a higher absorption, which can lead to suspending the resonant circuit. In addition the phase condition and the frequency change, without the distance between the sensor electrode 2 and the workpiece 3 changed. However a situation can be produced, in which “by the plasma one sees”. This taken place, as on the one hand the quality of the coil 4 is increased, in particular on a quality ≥ 100. Further a natural frequency of the resonant circuit is stopped of more than 18 MHz. Under these conditions the effect of the plasma-conditioned Ohm's resistance 9 is negligible for the spacer regulation.

Sensor electronics 5 exhibits further to resistance measuring instrument 5b, which on the basis a direct current measurement a value for the plasma resistance 9, which accomplishes is drawn in again. Also the resistance measuring instrument 5b is connected by the shielded line 6 with the evaluation mechanism 7. Additional requirements of electric current due to a small Ohm's resistance of the plasma resistance 9 and/or intensified requirements of electric current can be conveyed thus to the evaluation unit 7.

In the gumption enterprise of the laser processing machine over the resistance measuring instrument 5b sequentially the electrical resistance of the medium between the sensor electrode 2, in particular the laser nozzle, and that is determined workpiece 3. With regular gumption conditions, on which by means of a laser beam in the workpiece 3 a separation cut with the desired quality is manufactured, the measured resistance accepts a constant and against infinitely going value. If it comes in the range of the gumption place, for example with driving along curves of the laser cutter head with strong curve curvature with given parameters of the cutting process, to difficulties during the maintenance of the necessary cut depth of the laser beam, then at the same time the electrical resistance determined with the help of the resistance measuring instrument 5b decreases in relation to the starting situation substantially. This resistance reduction and/or the arising reduced resistance value is seized by means of the evaluation mechanism 7.

At the same time the evaluation mechanism 7 seizes the distance from sensor electrode 2 and workpiece 3., determined by means of the resonant circuit and the frequency uncoupling mechanism 5a, is different the distance between the sensor electrode 2 and the workpiece 3 of zero, is beabstandet thus the laser nozzle of the workpiece 3 and assumes the electrical resistance of the medium between the sensor electrode 2 and the workpiece 3 a relatively low value, determined by means of the resistance measuring instrument 5b, then this can indicate a being present of processing conditions, under which on a decreased gumption quality is depending upon process for the evaluation mechanism 7 to be counted, since the laser beam does not intersperse the workpiece 3 any more and/or any longer constantly along the cut line. On the basis of the measured resistance values and/or the resistance change the evaluation mechanism produces 7 control signals for the control 8 for the controlling of the parameters of the cutting process, due to those the control 8 the relative velocity of laser cutter head and workpiece 3 lowers, until the electrical resistance determined over the resistance measuring instrument 5b takes values, which correspond to the value measured with regular gumption enterprise or are this value at least closely neighbouring.

To the descriptive way the parameters of the cutting process are stopped in such a manner that in the gumption place always working on conditions prevail, which ensure an optimal working on success.

The Fig. exemplarily a perspective opinion of a laser processing machine 10, which is trained as laser flat bed machine in the remark example, shows 2. The concept according to invention is in principle applicable with each laser processing machine. A workpiece 3 rests upon a workpiece edition 11. The workpiece 3 can be worked on over a laser cutter head 12, which exhibits a laser nozzle 13. A control 8 is intended for the controlling of the laser cutting process. The control 8 stands with a drive unit not shown 14 for the movement of the laser cutter head 12 along the different machine axles in connection. The laser beam is led by the laser resonator 14 across deflecting mirrors 15 by a lens 16 to the working on place 17.

The Fig. 3 shows a laser cutter head 12 with a laser nozzle 13 manufactured from an electrically leading material, which is electrically in an isolated manner attached in relation to further parts of the laser cutter head 12. At a nozzle delta 20 a laser beam withdraws 21, which meets the laser cutter head 12 oppositely a workpiece 3 in form of an electrically leading sheet metal, in order this, to be penetrated as broken suggested, under cutting treatment from the laser cutter head 12. Over a Gaszuführung 22 gumption gas, in the represented case of example nitrogen, is given up into the laser cutter head 12. A gumption gas jet 23 withdraws likewise over the nozzle delta 20 from the inside of the laser cutter head 12 in the direction of the workpiece 3. During the cutting treatment the workpiece 3 is kept stationary, the laser cutter head 12 relative to the workpiece 3 is beabstandet by this and shifted parallel to this.

A supply terminal 24 is on the one hand connected with the laser nozzle 13 and on the other hand with the workpiece 3 leading. Resistance measuring instrument 5b is switched between the supply terminal 24 and the workpiece 3. In the broader sense are also the laser nozzle 13, the supply terminal 24 and the workpiece 3 components of the resistance measuring instrument 5b. The frequency uncoupling mechanism 5a is likewise connected with the laser nozzle 13.

As suggested in the illustration broken, both the resistance measuring instrument 5b and the frequency uncoupling mechanism 5a with a computer-aided evaluation mechanism 7 stand in connection. This again is with a control 8 the parameter of the cutting process, speaks: coupled with press control.