PLASMA CUTTING MACHINE COMPRISING A PROTECTION DEVICE, AND METHOD FOR OPERATING SAID PLASMA CUTTING MACHINE

Abstract

Known plasma cutting machines comprise a plasma burner that can be moved by means of a movement unit, a distance sensor that can be actuated at a low voltage level and has an electronic control unit for determining the distance between a workpiece to be machined and said plasma burner, an ignition device which can be actuated at a high voltage level and by means of which an ignition process can be triggered, and a protection device which moves with said plasma burner for the purpose of protecting the electronic control unit from interference voltage resulting from said ignition process. According to the invention, in order to provide a plasma cutting machine based thereupon which satisfies high demands on interference immunity and operational safety, it is suggested that the protection device contains an electronic circuit which comprises said electronic control unit for the distance sensor, a protection switch having a plurality of protection levels for the purpose of reducing the interference voltage, and an interface to a machine control system.

Claims

1. A plasma cutting machine comprising: a plasma burner that is configured to be moved by a movement unit, a distance sensor that is configured to be actuated at a low voltage level and has an electronic control unit determining the distance between a workpiece to be worked and the plasma burner, an ignition device that triggers an ignition process when actuated at a high voltage level, and a protection device that is moved with the plasma burner, said protection device protecting the electronic control unit against interference voltage resulting from the ignition process, wherein the protection device contains an electronic circuit that includes comprises the electronic control unit of the distance sensor, a protection switch having a plurality of protection levels reducing the interference voltage, and an interface to a machine control.

2. A plasma cutting machine according to claim 1, wherein a first protection level has an electrical connection to the workpiece and is configured as a high-voltage protection level reducing interference voltage with more than 100 V, the electrical connection comprising a diode and a high-frequency filter.

3. A plasma cutting machine according to claim 1, wherein an impedance in the form of a current-compensated coil is provided between the first protection level and a second protection level.

4. A plasma cutting machine according to claim 3, wherein the second protection level has a potential separation with at least one DC/DC converter and an optocoupler.

5. A plasma cutting machine according to claim 3, wherein a supply line electrically supplying components of the protection levels is provided between the second protection level and a third protection level, and the supply line has inserted therein a low-pass filter, and a protection diode.

6. A plasma cutting machine according to claim 1, wherein the protection circuit comprises one or plural protection levels against overvoltage and interference frequencies starting from a ground pole of an electrical voltage supply.

7. A plasma cutting machine according to claim 6, wherein a ground line electrically connected to the ground pole is provided between the second protection level and a third protection level, and the ground line has inserted therein a low-pass filter and a protection diode.

8. A plasma cutting machine according to claim 7, wherein a first ground potential is generated at the second protection level and a second ground potential at the third protection level, which jointly form a floating ground.

9. A plasma cutting machine according to claim 8, wherein the third protection level is a ground-potential protection wiring that comprises plural impedances in the form of frequency-dependent coils and fast diodes and capacitors.

10. A plasma cutting machine according to claim 1, wherein a fourth protection level that is equipped with plural impedances is provided and connected to the interface to the machine control.

11. A plasma cutting machine according to claim 1, wherein the interface is connected to the electronic control unit providing distance measurement.

12. A method for operating the plasma cutting machine according to claim 1, said method comprising: actuating with the electronic control unit of the distance sensor the movement unit for the plasma burner such that said burner is mounted on the workpiece to be worked, recording the mounting as an electrical signal by the control unit and generating a reference point for the distance measurement, wherein the actuating of the movement unit is such that the plasma burner is moved into a work position above the workpiece, and triggering an ignition process using the ignition device at a high voltage level, wherein an interference voltage resulting from the ignition process is reduced using the protection device moved with the plasma burner, wherein the electronic circuit is used as the protection device, said electronic circuit comprising the electronic control unit for the distance sensor and a protection circuit comprising a plurality of protection levels reducing the interference voltage, and the interface to the machine control.

13. A plasma cutting machine according to claim 5, wherein the low-pass filter is configured as an LC member.

14. A plasma cutting machine according to claim 7, wherein the low-pass filter is configured as an LC member.

Description

EMBODIMENT

[0053] The invention will now be explained in more detail with reference to embodiments and a drawing. In detail,

[0054] FIG. 1 shows an embodiment of the plasma cutting machine according to the invention, in a schematic illustration with a plasma burner and a protection device with distance regulation, and

[0055] FIG. 2 is a schematically simplified block diagram with the protection levels of the protection device and the distance regulation.

[0056] FIG. 1 schematically shows a plasma burner 1 of a plasma cutting machine. The burner 1 is movable by means of a schematically illustrated transverse carriage 2 in the standard way in a horizontal plane and in vertical direction by means of a height adjustment device 3. The detection of the distance between the plasma burner 1 and the workpiece 4 to be worked and the control of the height adjustment device 3 take place via sensor electronics which is accommodated together with a protection circuit in a protection device 5. This device is mounted on the transverse carriage 2 in direct vicinity of the plasma burner 1 and is moved along with said burner. The protection device 5, which will be explained in more detail further below with reference to FIG. 2, is connected via a measurement and control line 14 to the height adjustment device 3.

[0057] A central machine control 6 which is fixedly installed in a control cabinet on the wall next to the plasma cutting machine is provided for adjusting, monitoring and controlling all of the other machine functions.

[0058] The plasma burner 1 comprises a cutting nozzle 7 which defines an exit opening for an ionization gas and which surrounds an inner electrode 8. The cutting nozzle 7 simultaneously serves as an electrode of the capacitive distance measurement. The counter electrode is formed by the workpiece 4 which is of the same potential as the electrical ground.

[0059] The free edge of the cutting-nozzle exit opening further serves as an auxiliary electrode for the ignition of an auxiliary arc. For this purpose the cutting nozzle 7 is connected via a shielded high-voltage line 15 to a high-voltage source 9 for 20 kV alternating voltage.

[0060] An electric auxiliary arc is thereby producible between the cutting nozzle 7 and the workpiece 4, with the arc in an introductory phase of the cutting operation promoting the formation of the plasma cutting beam proper. The plasma cutting beam burns in the stationary operative state between the inner electrode 8 and the workpiece 4. For this purpose the inner electrode 8 is also connected to the high-voltage source 9 which provides an operating voltage of 300 V with 30 kHz during the cutting operation.

[0061] The circuit of the protection device 5 is on the one hand of the same potential as the electrical ground (via the electrical line 10 to the workpiece 4) and is on the other hand connected via a separable useful-signal line 11 to the plasma burner 1. The useful signal line 11 can here extend over a certain distance with the high-voltage line 8 in a common cable assembly 12, which is only schematically illustrated in FIG. 1. Moreover, the protection device 5 is connected via a further useful-signal line 13 for the transmission of 24-V DC voltage to the central machine control 6.

[0062] Hence, the protection device 5 is connected between the useful signal line 11 coming from the plasma burner 1 and the useful signal line 13 leading to the machine control 6. This device serves not only to measure and adjust the work distance, but for the machine control 6 and the electronics of the distance sensor it is also operative as a protection switch in case of overvoltages and high-frequency interference voltages from the lines 10 and 11, as will be explained hereinafter in more detail with reference to FIG. 2 and in combination with an example of the method according to the invention.

[0063] In a first phase the cutting nozzle 7 is placed on the surface of the workpiece 4 for the initial value finding of the distance regulation. For this purpose the height adjustment device 3 is actuated by the sensor electronics 21 of the protection device 5 in a corresponding way. The switch 22 is here closed, so that the contact between cutting nozzle 7 and workpiece 4 can be detected as an electrical signal by the sensor electronics 21. Owing to the contact signal the plasma burner 1 is moved by means of the height adjustment device 3 into a predetermined work position above the workpiece 4.

[0064] In this position an ignition process is initiated in a second phase by means of the machine control 6. An ignition voltage of 20 kV is here applied by means of the high-voltage generator 9 between the inner electrode 8 and the workpiece 4, resulting in a discharge in the form of an ignition arc.

[0065] Due to the ignition arc, the plasma gas flowing out of the nozzle exit opening is activated to such an extent that a stable plasma cutting beam is formed between the inner electrode 8 and the workpiece 4 in the third phase at an operational voltage of 300 V and a frequency of 30 kHz. Thereupon, the cutting process is started, with the distance between burner 1 and workpiece 4 being measured and regulated by means of the sensor electronics 21, which is fed with a low voltage of 24 V.

[0066] The high voltage in the ignition process can cross-couple into the lines 10 and 11, respectively, and must be reduced within a short period of time and over a short route in the protection device 5 to avoid damage to electronic components and also process malfunctions. For this purpose the protection device 5 has a protection circuit 23 with a total of four protection levels 24, 25, 26 and 27.

[0067] At the first protection level 24, diodes and high-frequency filters are substantially connected in parallel. These are functional elements for the reduction of high voltage of more than 100 V and high-frequency interference potentials. At the output side of the first protection level 24 the interference potential is not more than 36 V.

[0068] An impedance which comprises current-compensated coil and two capacitors, each with a capacitance of 47 nF, and which serves to further reduce the energy of non-eliminated transients of the first protection level is provided between the first protection level 24 and the second protection level 25.

[0069] The second protection level 25 forms a potential separation between the machine potential and the sensor electronics. A DC/DC converter and an optocoupler are the essential functional elements. Moreover, the optocoupler is connected to the sensor electronics 21. Upon contact of the plasma burner with the workpiece the optocoupler detects a current flow and gives this information to the machine control (for triggering the above-explained ignition process).

[0070] A supply line coming from the machine control is provided between the second protection level 25 and the third protection level 26. An LC member is inserted as a low-pass filter into said supply line. The LC member is additionally provided with a protection diode, whereby the voltage is limited to 24V/DC and an undisturbed power supply of the electronic components of the protection device is ensured.

[0071] Since transients may also enter via the ground line 10 into the electronics, the same protection assembly, i.e. an LC member as the low-pass filter with a protection diode for voltage limitation, is also inserted into the ground line 10 between the second protection level 25 and the third protection level 26.

[0072] The protective effect of the electronic components which is thereby aimed at is ensured in that the power supply is configured at the second protection level 25 with a floating ground potential. In a floating ground the ground potential is separated from the fixed reference potential (zero), so that it can freely adjust, depending on the interference potential and the electrical state. Owing to this floating ground, interference potentials coming from the ground side can be mitigated and possible transients can be converted into a defined voltage state.

[0073] The third protection level 26 thereby represents a ground-potential protection wiring and it serves on the other hand to convert possible transients into a defined voltage state to achieve a clean 24V/DC useful signal and thus a reliable function of the electronic components of the protection device. Impedances are particularly provided for this purpose, said impedances including fast diodes and capacitors. In other words, by ensuring a clean supply voltage one can ensure a defined and reproducible state of the useful signals and an operationally safe function of the protection device.

[0074] At the fourth protection level 27 the incoming signals are prepared for transmission to the machine control 6. An undisturbed 24 V control voltage is provided. This protection level has impedances contributing to the provision of the above-explained floating ground. The fourth protection level 27 is connected to an interface 28 to the machine control 6 (line 13), which is also part of the sensor electronics 21 and through which the height adjustment device 3 is addressed (line 14).