DE4244834C2 - Capacitive distance measurement between workpiece and machining head e.g. of laser cutter or welding machine - Google Patents

Abstract

A sensor electrode mounted on a machining head produces a sensor signal corresp. to the distance between a workpiece and the machining head. A later sensor signal value is compared with a comparison value derived from an earlier sensor signal value. A distance value corresp. to the comparison value is at least approximately maintained if the later signal is smaller than the comparison value by more than a defined amount. In other cases the later value is used to determine the distance value.

Description

The invention relates to a device for machining a workpiece by means of a laser beam according to the preamble of patent claim 1.

Such a device is already known from DE 40 05 453 A1. This known device contains an imaging device for Fo kissing the laser beam on the workpiece, as well as on a holder set up the imaging device attached tube that down is led to the workpiece machining area and through the one Shielding gas is passed through.

In the known device, a distance measurement is made between egg ner laser processing nozzle and the workpiece by means of a measurement Laser-based distance measuring arrangement. To an optical decoupling tion between measuring laser, process laser beam and laser-induced plasma To achieve a shielding nozzle is provided, which on the active site facing side of a laser processing nozzle between the point of action of the Laser radiation and measuring point of the measuring laser is detachably attached. The Shielding nozzle has at least one with one in the jacket of the machining tion nozzle trained process gas supply hole in connection hole that opens towards the point of action of the laser radiation.

The invention is based, to set up the task mentioned type so that you can measure distance with it can also be carried out capacitively, especially also if cutting or welding is carried out with the help of the laser beam should be led.

The solution to the problem is in the characterizing part of the patent claim 1 specified. Advantageous embodiments of the invention are can be found in the subclaims.

According to the invention there is a for at the lower end of the tube Workpiece-pointing sensor electrode for capacitive measurement of the Distance between it and the workpiece.

The invention is na with reference to the drawing described here. Show it:

Figure 1 shows a laser processing head according to the invention with a protective gas tube, on which a sensor electrode for measuring distance is attached.

Fig. 2 is a removed from the sensor electrode sensor signal;

Fig. 3 shows a circuit arrangement for processing the sensor signal;

FIG. 4 shows a fault detector circuit for the circuit arrangement according to FIG. 3; and

Fig. 5 shows a further circuit arrangement for processing the sensor signal with the fault detector circuit shown in Fig. 4.

An embodiment of a device according to the invention is shown in FIG. 1. This is a so-called open system in which there is no nozzle cone. With the aid of an imaging device 18 , a focused laser beam 9 is generated, which is focused on a workpiece 2 in order to carry out a welding or cutting process. The direction of movement of the imaging device 18 is identified in FIG. 1 by the reference symbol B. The workpiece 2 is connected to earth potential via a connection 5 .

A tube 20 is fastened to the imaging device 18 via a holding device 19, which tube leads down to the welding or cutting area (workpiece processing area) and is bent there so that its inclination relative to the workpiece 2 is no longer so great. The lower pipe opening points to the welding or cutting area, so that the welding or cutting area can be supplied with a protective gas via the pipe 20 . The protective gas flows in the direction of arrow G out of the lower end of the tube 20 , that is, in a direction that is opposite to the direction of movement B. This can prevent a plasma P which arises during welding or cutting from reaching the region of the bent lower tube end. The actual function of the protective gas is to prevent oxygen from coming to the processing point.

At the lower and bent end of the tube 20 there is a sensor electrode 2 1, which consists for example of copper. The sensor electric de 21 points to the workpiece 2 and is fastened to the tube 20 via a holding device 22 . The holding device 22 can be a heat-resistant adhesive, for example. With the sensor electrode 21 , a shielded cable 23 is connected, for example a coaxial cable, the other end of which is connected to a socket 24 which is attached to the upper end of the tube 20 . About this connector 24 (coaxial connector se) can be transmitted on the one hand via the cable 23 sensor potential to the sensor electrode 21 , while active shielding potential is applied to the screen conductor ter of the cable 23 .

Due to the above arrangement of the sensor electrode 21 , the distance between the sensor electrode 21 and the workpiece 2 can also be measured in a capacitive manner, especially in laser welding, without this measurement being adversely affected by the plasma P produced during welding.

Another advantage of the above-mentioned design is that the sensor electrode 21 can be cooled simultaneously by the protective gas passed through the tube 20 . This extends their lifespan. The cable 23 can also be laid inside the tube 20 , where the shielding function can then take over the tube 20 . In this case, the tube 20 is subjected to an active shielding potential, while the cable laid in the tube 20 no longer requires an additional shielding conductor. The sensor electrode 21 , the z. B. can be circular plate-shaped, could also be surrounded by a further and lying in the sensor electrode plane shield electrode, which is in electrically conductive con tact with the tube 20 and thus comes to lie on the shield potential. In this way, the influence of the plasma P in the capacitive distance measurement between the sensor electrode 21 and the workpiece could be suppressed even further.

Of course, it is also possible to switch the laser beam 9 on and off periodically in all of the exemplary embodiments shown, in order to carry out the distance measurement in a capacitive manner during the switch-off phases of the laser beam.

FIG. 2 shows an enlarged representation of a sensor signal M which is taken from the sensor electrode or which is z. B. is obtained during a welding process. The time axis is stretched in FIG. 2 in order to be able to recognize the sensor signal M better. As a result of the plasma generated during the welding process between the sensor electrode and the workpiece, the sensor signal M breaks down at irregular time intervals, so that only a sequence of signal peaks Sp is obtained as the sensor signal M. The sensor signal M can be digitized so that it can be further processed by software measures, as will be described first below. But it can also be processed as an analog signal, which is carried out with reference to FIGS. 3 and 4.

A time grid is placed over the sensor signal M for digitization. In other words, the sensor signal M in periodic periods Δt₁, Δt₂,. . . , sampled in order to detect the maximum signal value for the respective period Δt. The maximum signal value is referred to below as the sensor signal value, so that a sensor signal value M₁ for the period Δt₁, a sensor signal value M₂ for the period Δt₂, and the like, are obtained. The length of the respective time periods Δt _i is determined in accordance with the expected disturbances in the sensor signal M, in the present case, for example, to 50 milliseconds. The most suitable length of time should be determined beforehand for a specific machining process during a trial run.

After sampling the sensor signal values M₁, M₂,. . . are these between saved for further processing. This two Storage can only affect a subset of the sensor signal values to enable online operation.

In the following it is assumed that in the period Δt₂ as the maximum value Sensor signal value M₂ is sampled, based on which the distance between sensor electrode and workpiece. This sensor signal value M₂ remains during the next period Δt₃ ten, by the end of this period Δt₃. Within the period Δt₃ the sensor signal value M₃ is then sampled, this sensor signal value M₃ is compared with a comparison value V, the z. B. the Sensor signal value M₂ or the one maintained over the period Δt₂ Sensor signal value M₁ can be. The comparison value can also be done by Averaging of previous sensor signal values has been obtained be, for example by averaging the signal values M₁ and M₂. It is found at the end of the period Δt₃ that the sensor signal value M₃ is not less than the Ver by more than a predetermined amount S equals value V, this sensor becomes Δt₄ for the subsequent period maintain signal value M₃, etc.

In the period Δt₈, the previously determined sensor signal value M₇ applies. wherein the sensor signal value M₈ is detected in the period Δt₈. He is tall ßer than the sensor signal value M₇, so that for the next period Δt₉  this sensor signal value M₈ applies. In other words, the time has passed dream Δt₈ for the period Δt₉ the distance between the sensor electrode and Workpiece enlarged. So it becomes the new value M₈ in the period Δt₉ taken over, since this also by no more than the predetermined amount S smaller than the comparison value V (e.g. the sensor signal value M₇) was.

Another situation should be written based on the periods Δt₁t, Δt₁₅ and Δt₁₆ be. In the period Δt₁₅ only a relatively small sensor signal value M₁₅ is obtained, for example as a result of a plasma phenomenon. This sensor signal value M₁₅ (later sensor signal value) is compared with the sensor signal value M₁₄ (earlier sensor signal value), it being established that the sensor signal value M₁₅ is more than the predetermined amount S smaller than the sensor signal value M₁₄, which forms the comparison value V here. This has the consequence that the sensor signal value M₁₄, which was already valid during the period Δt₁₅, is also maintained during the period Δt₁₆, as can be seen in FIG. 2. For the period Δt₁₆ but the predetermined amount can now be increased to the value 2S in order to be able to drive the control device as quickly as possible to a distance that has actually become smaller between the sensor electrode and the workpiece. For example, in the period Δt₁₆ the sensor signal value M₁₆ is obtained, which falls in the area 2S shown there. This sensor signal value M₁₆ is therefore not more than the predetermined amount 2S smaller than the comparison value M₁₄, so that it now applies to the next period Δt₁₇. The increase in the predetermined amount S can be linearly related to the number of periods Δt _i over which the sensor signal is maintained. Would z. B. the sensor signal value M₁₆ not reach into the area 2S, the sensor signal value M₁₄ would also apply in the period Δt₁₇. However, the predetermined amount would be increased to 3S for the period Δt₁₇.

Of course, there can be another relationship between the above agreed amount S and the number of periods are used via which the sensor signal is kept constant. However, it is important that the predetermined amount S for successive ones of the plurality Ab tactile periods gradually increased to actually as quickly as possible to be able to record smaller distance values.  

Is used for a specified number of periods, for example for six consecutive periods, no change in the sensor signal received after the decision has been made, the sensor signal value to maintain, it is assumed that a collision between Sensor electrode and workpiece is present, which usually leads to shutdown facility leads.

Is used in the scanning signal during the period .DELTA.t _i, a larger Sen sorsignalwert M _i is determined as the period .DELTA.t _i-1, this larger sensor signal value M _i may also be applied immediately and .DELTA.t _{i + 1} immediately ge be started. This larger sensor signal value M _i then applies during the period Δt _{i + 1} . The time grid for sampling the sensor signal M is not constant here, rather the individual time periods of z. B. 50 milliseconds always restarted when the genann te higher sample value has been obtained. This allows the distance control to be carried out more quickly.

FIG. 3 shows a distance measuring measurement and control device.

According to FIG. 3, the distance measuring and regulating device contains a sensor unit 25 which is connected on the input side to a sensor electrode 26 (nozzle electrode) via a coaxial cable 25 a. The sensor unit 25 outputs the sensor signal M at its output, that is, a measuring voltage, the size of which depends on the distance (or capacity) between the sensor electrode 26 and the workpiece. This sensor signal M is fed to the input of a distance control circuit 27 , which compares the sensor signal M, which serves as an actual value signal, with a desired value signal. A comparison-dependent control signal is supplied from the output of the distance control circuit 27 via a switch 28 and a servo amplifier 29 to a motor 30 , which in turn changes the position of a processing head, not shown, to which the sensor electrode 26 is attached, depending on the control signal. In this way, z. B. From the sensor electrode 26 from the workpiece was kept constant.

The switch 28 has three switch contacts 28 a, 28 b and 28 c. A movable switching element 28 b is constantly connected to the switching contact 28 a, which in turn is connected to the input of the servo amplifier 29 . The switch contact 28 b is connected to the output of the distance control circuit 27 , while the switch contact 28 c is grounded. In other words, the switching element 28 d can be moved back and forth between the switching contacts 28 b and 28 c.

The switching element 28 d is displaced via an actuating device 31 which is controlled by a fault detector 32 , which in turn receives the sensor signal M at the input.

The fault detector 32 monitors the sensor signal 25 issued by the sensor unit M. After detection of a fault, the input of the servo amplifier 29 is switched to ground, for which purpose the fault detector 32 controls the actuating device 31 such that the switching element 28 d with the switching contact 28 is activated by it c is connected. The motor or the drive for the machining head controlled by it then maintains its current position until the switch 28 is switched again. If there is no fault, the switching element 28 d is connected to the switching contact 28 b. In this case, the distance control is carried out.

FIG. 4 shows the precise structure of the fault detector 32. It contains a delay line 33 which receives the sensor signal M at its input which appears at the output of the sensor unit 25 . The output of the delay line 33 is connected to the input of a subtraction stage 34 , the second input (subtraction input) of which also receives the sensor signal M without delay. The output of the subtractor stage 34 is connected to the positive input of a comparator 35 (differential amplifier), the negative input of which receives a reference voltage. A monoflop 36 is driven via the output of the comparator 35 , the output signal of which is used to switch over the switch 28 .

With the help of the fault detector shown in Fig. 4, the sensor signal M supplied by the sensor unit 25 is subtracted from a time-delayed sensor signal M '. In the event of signal dips, positive signals are produced at the output of the subtracting stage 34 . If these signals exceed a certain value, the monoflop 36 is triggered. From the output of the monoflop 36 generates a "freeze signal" through which the switch 28 is switched so that the switching element 28 d comes into contact with the switching contact 28 c to put the input of the servo amplifier 29 to ground. In other words, the input of the motor output stage is switched to ground for an adjustable time. This adjustable time depends on the duration of the typically occurring faults and can be determined beforehand in a trial run. The same applies to the delay time of the delay line 33 .

Occurs in the sensor signal M during the delay time of the delay circuit 33 to a signal breakdown, so the sensor signal corresponding to M 'at the output of the delay line 33 the sensor signal M at the input thereof. The subtractor 34 therefore outputs only a very small signal at its output, so that the output of the comparator 35 also remains below a certain threshold if its negative input Ref z. B. is on ground. In this case, the monoflop 36 is not actuated, so that the switching element 28 d remains connected to the switching contact 28 b.

However, if a signal breakdown occurs in the sensor signal M during the delay time of the delay line 33 , the delayed sensor signal M M (earlier sensor signal) is significantly larger than the later sensor signal M. The subtractor stage 34 therefore delivers a relatively high output signal to the positive input of the comparator 35 so that its output signal exceeds the threshold of the monoflop 36 and the mo noflop 36 is driven. With the help of the output signal of the monoflop 36 , the switching element 28 d of the switch 28 is now connected to the switching contact 28 c in order to freeze the motor position or position of the machining head. The monoflop 36 and thus the switch 28 are automatically switched back after the adjustable time mentioned, since the distance control can be continued with.

The delay time of the delay line 33 is set so that only rapid changes in the measurement signal due to interference such as z. B. generated by plasma or splashes, trigger the monoflop 36 , that is, the motor 30 stops . In welding operations, the delay time is z. B. 10 ms.

By changing the reference voltage of the comparator 35, the threshold of the comparator 35 can be adjusted so that the difference flops of the current and delayed measurement signal with interference for triggering the monostable 36 (height or thickness of the interference) results. As a result, an equal to special disturbances of the measurement signal is possible.

Another circuit arrangement is shown in FIG. 5. The same elements as in Fig. 3 are provided with the same reference numerals. In particular, the fault detector 32 in FIG. 5 can also have the structure shown in FIG. 4.

In contrast to the circuit of Fig. 3, the output of the distance control device is in the TIC arrangement of FIG. 5 27 directly connected to the input of the servo amplifier 29. In contrast, the switch 28 is located at the input of the distance control circuit 27 . More specifically, the switch contact 28 a of the switch 28 is connected to the input of the distance control circuit 27 , while the switch contact 28 c is connected to the output of the sensor unit 25 . The switching contact 28 b of the switch 28 is connected to the output of a filter circuit 37 , the input of which is connected to the output of the sensor unit 25 . The movable switching element 28 d is moved via the actuating device 31 in order to switch back and forth between the switching contacts 28 b and 28 c. The actuating device 31 is in turn controlled by the fault detector 32 .

The filter circuit 37 has an extremely low cutoff frequency, e.g. B. may be 2 Hz.

Is by the disturbance observer 32, a fault in the sensor signal M de tektiert as described above, the zusätzli che filter circuit 37 is switched after the sensor unit 25 for a selectable time to the Symptom in the sensor signal M to be bridged. For this purpose, the movable switching element 28 d of the switch 28 is switched from the switching contact 28 c to the switching contact 28 b. The preselectable time is again in accordance with the duration of the typically occurring faults and can be determined in advance.

The circuit arrangement according to FIG. 5 is used especially when an intervention in the distance control as shown in FIG. 3 is not possible. In the circuit arrangement according to FIG. 5, the interference compensation acts much more directly on the sensor signal M or the distance measuring voltage.

Claims (9)

1. Device for processing a workpiece ( 2 ) by means of a laser beam ( 9 ), with

• - An imaging device ( 18 ) for focusing the laser beam ( 9 ) on the workpiece ( 2 ); and
• - A on a holding device ( 19 ) of the imaging device ( 18 ) attached tube ( 20 ), which is guided down to the workpiece processing area, and through which a protective gas is passed,
  characterized in that a sensor electrode ( 21 ) pointing towards the workpiece ( 2 ) for capacitive measurement of the distance between it and the workpiece ( 2 ) is attached to the lower end of the tube ( 20 ).

2. Device according to claim 1, characterized in that the sensor electrode ( 21 ) is at a bent pipe end, the sen inclination relative to the workpiece ( 2 ) is no longer as large as the inclination of the rest of the tube. 3. Device according to claim 1 or 2, characterized in that its direction of movement relative to the workpiece ( 2 ) is selected so that the sensor electrode ( 21 ) leads the workpiece machining area. 4. Device according to one of claims 1 to 3, characterized in that the sensor electrode ( 21 ) is surrounded by a flat in the sensor electrodes shield electrode which is at shield potential. 5. Device according to one of claims 1 to 4, characterized in that the sensor electrode ( 21 ) with a shielded by shield potential cable ( 23 ) is connected to gene to receive sensor potential. 6. Device according to one of claims 1 to 4, characterized in that the sensor electrode ( 21 ) is verbun with an insulated cable which runs inside the tube ( 20 ), and that the tube ( 20 ) is at screen potential. 7. Device according to claim 4 and 6, characterized in that the shield electrode with the tube ( 20 ) is in electrically conductive contact. 8. Device according to claim 4, 5, 6 or 7, characterized net that the screen potential is active screen potential, which he will keep that the sensor potential via an amplifier with a ge desired degree of gain is directed. 9. Device according to one of claims 1 to 7, characterized in that the sensor electrode ( 21 ) is fastened to the tube ( 20 ) by means of a heat-resistant adhesive.