METHOD AND DEVICE FOR DETECTING AN IMPENDING INCOMPLETE CUT OR AN INCOMPLETE CUT WHICH HAS ALREADY OCCURRED WHEN THERMALLY SEPARATING A WORKPIECE

Abstract

In order to be able to detect a possible loss of cut during the thermal separation of a workpiece as early as during the separation, the invention proposes a method for detecting an impending loss of cut or a loss of cut that has already occurred, in which energy is input into a cutting region and which comprises the following method steps: a) applying a first alternating signal to the workpiece, b) identifying a second alternating signal caused by the first alternating signal in a measurement electrode spaced apart from the workpiece, c) ascertaining the phase shift between the first and second alternating signal by outputting a phase shift signal, d) comparing the phase shift signal with a prescribed upper limit value and a prescribed lower limit value for the phase shift signal,
wherein, when the phase shift signal exceeds the upper limit value or undershoots the lower limit value, the energy input is changed, for example, by stopping the thermal separation of the workpiece.

Claims

1. A method for detecting an impending loss of cut or a loss of cut that has already occurred during the thermal separation of a workpiece, in which energy is input into a cutting region, said method comprising the following steps: a) applying a first alternating signal to the workpiece, b) identifying a second alternating signal caused by the first alternating signal in a measurement electrode spaced apart from the workpiece, c) ascertaining a phase shift between the first alternating signal and the second alternating signal by outputting a phase shift signal, d) comparing the phase shift signal with a prescribed upper limit value and a prescribed lower limit value for the phase shift signal, wherein, when the phase shift signal exceeds the upper limit value or undershoots the lower limit value, the energy input into the cutting region is changed.

2. The method as claimed in claim 1, wherein the thermal separation is effected at a separation rate and the energy input into the cutting region is changed by reducing the separation rate.

3. The method as claimed in claim 2, wherein the separation rate is reduced in steps.

4. The method as claimed in claim 2, wherein, after the reduction of the separation rate, the separation rate is increased again when the phase shift signal is back in a range between the lower and upper limit value.

5. The method as claimed in claim 1, wherein the energy input is changed by stopping the thermal separation of the workpiece.

6. The method as claimed in claim 5, wherein, after the stopping of the thermal separation of the workpiece, a separation process is started again from a loss of cut point.

7. The method as claimed in claim 1, wherein, during thermal separation, a distance of the measurement electrode from the workpiece is kept at a prescribed distance setpoint value using distance regulation and, when the phase shift signal exceeds the upper limit value or undershoots the lower limit value, the measurement electrode is set to a prescribed fixed height position.

8. The method as claimed in claim 7, wherein the prescribed fixed height position is ascertained from height values or distance values of the measurement electrode with respect to the workpiece surface in a time interval before the upper limit value is exceeded or the lower limit value is undershot.

9. The method as claimed in claim 1, wherein, when the phase shift signal exceeds the upper limit value or undershoots the lower limit value, a warning signal is output.

10. An apparatus for detecting an impending loss of cut or a loss of cut that has already occurred during the thermal separation of a workpiece, in which energy is input into a cutting region, said apparatus comprising: an alternating signal generator generating a first alternating signal, a measurement electrode, which is spaced apart from the workpiece, identifying a second alternating signal caused by the alternating signal in the measurement electrode, a phase discriminator ascertaining a phase shift between the first alternating signal and the second alternating signal, wherein the phase discriminator outputs a phase shift signal, and an electronic circuit comparing the phase shift signal with a prescribed upper limit value and a prescribed lower limit value of the phase shift signal, and wherein the electronic circuit is configured so as to change the energy input into the cutting region when the upper limit value is exceeded or the lower limit value is undershot.

11. The apparatus as claimed in claim 10, wherein the electronic circuit is designed in such a way that it stops the thermal separation of the workpiece when the upper limit value is exceeded or the lower limit value is undershot.

12. The method as claimed in claim 3, wherein, after the reduction of the separation rate, the separation rate is increased again when the phase shift signal is back in the range between the lower and upper limit value.

13. The method as claimed in claim 2, wherein the energy input is changed by stopping the thermal separation of the workpiece.

14. The method as claimed in claim 3, wherein the energy input is changed by stopping the thermal separation of the workpiece.

15. The method as claimed in claim 4, wherein the energy input is changed by stopping the thermal separation of the workpiece.

16. The method as claimed in claim 12, wherein the energy input is changed by stopping the thermal separation of the workpiece.

17. The method as claimed in claim 2, wherein, during thermal separation, a distance of the measurement electrode from the workpiece is kept at a prescribed distance setpoint value using distance regulation and, when the phase shift signal exceeds the upper limit value or undershoots the lower limit value, the measurement electrode is set to a prescribed fixed height position.

18. The method as claimed in claim 3, wherein, during thermal separation, a distance of the measurement electrode from the workpiece is kept at a prescribed distance setpoint value using distance regulation and, when the phase shift signal exceeds the upper limit value or undershoots the lower limit value, the measurement electrode is set to a prescribed fixed height position.

19. The method as claimed in claim 4, wherein, during thermal separation, a distance of the measurement electrode from the workpiece is kept at a prescribed distance setpoint value using distance regulation and, when the phase shift signal exceeds the upper limit value or undershoots the lower limit value, the measurement electrode is set to a prescribed fixed height position.

20. The method as claimed in claim 5, wherein, during thermal separation, a distance of the measurement electrode from the workpiece is kept at a prescribed distance setpoint value using distance regulation and, when the phase shift signal exceeds the upper limit value or undershoots the lower limit value, the measurement electrode is set to a prescribed fixed height position.

Description

EXEMPLARY EMBODIMENT

[0049] The invention is described in more detail below with reference to exemplary embodi-ments and two drawings. In detail and illustrated schematically:

[0050] FIG. 1 shows a schematic circuit diagram of a loss of cut detection apparatus according to the invention, and

[0051] FIG. 2 shows a graph in which a phase shift DC voltage signal is illustrated as a function of time.

[0052] FIG. 1 shows a section A of a schematic circuit diagram of a loss of cut detection apparatus according to the invention, the whole of which is assigned the reference number 20. The apparatus 20 comprises an alternating signal generator 200, a measurement electrode 207, an inverter 201, a phase discriminator 202, a monitoring unit 203 and three independent electronic circuits 204, 205, 206.

[0053] The apparatus 20 is part of a laser cutting machine (not illustrated), as is used, for example, for cutting a planar workpiece 208 from metal, preferably from stainless steel, aluminum, copper or brass.

[0054] The laser cutting machine comprises a workbench having a contact face (not illustrated) for holding the workpiece 208 and a movable laser processing unit (likewise not illustrated) having a laser cutting head 209. The measurement electrode 207 is fas-tened to the laser cutting head 209. To set a prescribed distance of the laser cutting head 209 from the workpiece surface, a height sensor system (not illustrated) that determines the position of the laser cutting head 209 and hence of the measurement electrode 207 is provided.

[0055] In the following text, the method according to the invention is explained with reference to the laser cutting machine described above.

[0056] First, an alternating voltage signal U.sub.1 (t) is applied to the workpiece 208. To this end, the alternating signal generator 200 generates the alternating voltage signal U.sub.1 (t), which is applied to the workpiece 208 and is subsequently used as the reference signal.

[0057] The alternating voltage signal U.sub.1 (t) causes an alternating current signal I.sub.1,φ(t) in the measurement electrode 207. Both alternating signals U.sub.1 (t) and I.sub.1,φ(t) have identical periods; however, they differ in phase, wherein the alternating current signal I.sub.1,φ(t) is phase-shifted with respect to the first alternating voltage signal U.sub.1 (t) by the angle φ. In this case, the magnitude of the phase shift depends on the distance of the measurement electrode 207 from the workpiece 208, among other things. The alternating current signal I.sub.1,φ(t) is identified by means of the measurement electrode 207.

[0058] Under normal cutting conditions, the distance between the measurement electrode 207 and the workpiece 208 is kept as constant as possible by the height sensor system—apart from regulation deviations. Although the alternating current signal I.sub.1,φ(t) resulting therefrom has a certain level of noise, it exhibits an almost constant phase shift over time compared to the reference U.sub.1 (t).

[0059] To ascertain the phase shift, the reference signal U.sub.1 (t) is first inverted, that is to say is phase rotated by 180°, by means of the inverter 201. The inverter 201 delivers a phase-rotated alternating current signal I.sub.1,inv (t) as output signal.

[0060] Both the phase-rotated alternating current signal I.sub.1,inv(t) and the phase-shifted alternating current signal I.sub.1,φ(t) are applied at the phase discriminator 202 as input signals. The phase discriminator 202 also contains a rectifier. If the alternating current signals I.sub.1,φ(t) and I.sub.1,inv(t) are not phase-shifted with respect to one another, they cancel each other out completely at the same amplitude level. In the case of a phase shift, however, a positive or a negative phase shift signal in the form of the DC voltage signal U.sub.DC results depending on whether I.sub.1,φ(t) I.sub.1,inv(t) is leading or lagging. The magnitude of the signal is a measure for the phase angle Δφ, in which the phases of the signals differ. In order to make it possible to easily compare the signals, at least one of the signals applied at the phase discriminator 202 is optionally preamplified (not illustrated) in order to adjust the amplitude level of the two signals to one another.

[0061] The phase shift signal U.sub.DC is then compared with a prescribed upper and lower limit value by the monitoring unit 203.

[0062] In normal cutting operation, the limit values are regularly not exceeded or undershot. However, if a loss of cut occurs, a plasma capsule 210 is produced on the top side of the workpiece 208. Said plasma capsule 210 is produced essentially by the coupling of high power peaks into the workpiece 208.

[0063] Section B shows the laser cutting head 209, the workpiece 208 and the plasma capsule 210 in the event of a loss of cut.

[0064] The plasma capsule 210 causes a change in the capacitance between the measurement electrode 207 and the top side of the workpiece 208. Moreover, detached workpiece components are accelerated in the direction of the nozzle or the measurement electrode 207 on account of the cutting kerf no longer penetrating the material. This results in a changed phase shift of the signals I.sub.1,φ(t) and I.sub.1,inv (t). Since the capacitance between the measurement electrode 207 and the top side of the workpiece 208 changes and fluctuates over the course of time on account of the changing plasma, a fluctuating phase shift signal U.sub.DC is also obtained as the output signal of the phase discriminator 202, said phase shift signal being used to detect the loss of cut. To this end, the phase shift signal is monitored by the monitoring unit 203 for the exceeding of an upper limit value or the undershooting of a lower limit value. In the case of the exceeding or undershooting of the respective limit value: [0065] the separation rate is reduced by means of the electronic circuit 204, [0066] the measurement electrode is set at a prescribed fixed position by means of the electronic circuit 205, and [0067] an optical and acoustic warning signal is output by means of the electronic circuit 206.

[0068] FIG. 2 shows by way of example a time profile of the phase shift voltage signal U.sub.DC in the case of a good cut (section I), an impending loss of cut (section II) and after a loss of cut has occurred (section III). The phase shift signal is denoted by the reference number 1.

[0069] Before the loss of cut, the phase shift signal 1 has a conventional level of noise during the cutting process. Nevertheless, the phase shift signal 1 in section I is substantially constant and fluctuates with only a small deviation about a mean value. An impending loss of cut leads to oscillation of the phase shift signal 1 in section II up to full deflec-tion in section III.

[0070] In order to be able to counteract an impending loss of cut successfully and to prevent a loss of cut as a result, it is important to detect a loss of cut that is beginning at as early a stage as possible. The use of the phase shift signal makes it possible to detect a loss of cut at an early stage, in particular in section II. The upper limit value U.sub.lim,1 and the lower limit value U.sub.lim,2 are selected in such a way that they make detection at an early stage possible.