DD300003A7 - Method for process control on plasma torches - Google Patents

Abstract

The invention relates to a process for Prozeszsteuerung at plasma torches for Schweiszen / cutting with transferred arc taking advantage of the electrical voltage of the plasma arc. Characteristic of the invention is that within a maximum of 20 ms long Meszzyklus the partial voltages between the electrode and Duese and between Duese and workpiece and the total voltage between the electrode and the workpiece measured and stored the respective power levels of the pilot and / or main arc are assigned, z the Prozeszsteuergroeszen "plasma gas quantity" and / or "discharge space pressure" and "feed" in their switching and Groeszenzustand erfaszt and stored as logical Groeszen, dasz the stored partial voltages between the electrode and Duese and between Duese and Werkstueck with each other and summarily and individually with the total voltage or voltage thresholds are compared by comparing the comparison results of a plurality of consecutive measuring cycles with each other and with the associated stored logical magnitudes of the process control vectors, and utilizing the output quantities by signaling or control commands et. {Meszzyklus; Partial voltages; Electrode; Jet; Workpiece; Total voltage; amperage; Pilot, main arc; Prozeszsteuergroeszen; Voltage limits; Comparison results; logical sizes}

Description

Object of the invention It is an object of the invention to increase the effectiveness of plasma torches. The essence of the invention

The invention has for its object to develop a method by which the process control of plasma torches can be designed so that in addition to changes in the height of the plasma torch above the workpiece other technological disturbances on the plasma torch can be detected and corrected by utilizing the electrical voltage of the plasma arc ,

According to the invention, the object was achieved by using plasma cutting torches known per se for welding, cutting and similar methods with transferred arc and evaluation of the electrical voltage of the plasma arc by means of electronic MeL and evaluation means that within a maximum 20ms long measuring cycle 'several times the partial voltages between Electrode and nozzle and between the nozzle and the workpiece and the total voltage between the electrode and workpiece measured and the respective current of the pilot and / or main arc associated stored that the process control variables "plasma gas amount" and / or "Entladungsraumdruck" and .Fort "in their switching and size state detected and stored as logical quantities that the stored partial voltages between the electrode and nozzle and between the nozzle and workpiece with each other and summarily and individually with the total voltage or Spannungsgre nzwerten be compared that the comparison results of several successive measuring cycles compared with each other and with the associated stored logical variables of the process control variables and the output variables used by signaling or control commands.

The invention is based on the new findings that

- The incineration or burning of the electrode to a much greater extent on the partial voltage between the electrode and nozzle as the total voltage between the electrode and the workpiece affects, with a temporally low negative voltage gradient to higher voltages towards normal burning behavior in the allowable wear range and an increasing time positive Voltage gradient to lower voltages out a life limit announces due to additional thermal electron emission from the electrode base material with unchanged plasma gas volume and unchanged discharge space pressure.

the burning out of the nozzle has a much greater effect on the partial voltage between the electrode and the workpiece than the total voltage between the electrode and the workpiece, whereby the nozzle burns out at short-term larger negative voltage gradients between electrode and nozzle and simultaneous positive voltage gradients at lower voltages between electrode and workpiece is registered and the wear limit can be registered with the falling below a threshold voltage after the occurrence of such a voltage gradient, wherein a decreasing discharge space pressure at a constant plasma gas amount is an additional hinwels on the nozzle wear.

- Changes in the height of the plasma torch in small temporal positive or negative voltage gradients Teilspa'.inung effect between nozzle and workpiece, however, are relatively larger than the simultaneous same directional voltage gradients of the total voltage with unchanged partial voltage between the electrode and nozzle and plasma gas volume and discharge space pressure.

- Participate in the preparation of metal and slag splashes near the nozzle mouth stochastic to a greater extent additional electrons from the thermal emission from the splash on the current flow, resulting in several temporally short but strong positive voltage gradient of the partial voltage between the nozzle and workpiece at the same time much lower positive voltage gradient between electrode and nozzle with unchanged plasma gas volume and discharge space pressure.

- When double arcs occur strong positive voltage gradients between the electrode and nozzle occur simultaneously with less strong positive Spannungsgradionten between the nozzle and workpiece and also less strong positive voltage gradients of the total voltage occur with unchanged plasma gas volume and reduced discharge space pressure.

Ausführungsbelspiel The invention will be explained in more detail below on an exemplary embodiment. By means of measuring means known per se are applied to a plasma torch with transferred plasma arc (direct polarity)

the following measured variables are determined:

UG: total voltage between electrode and workpiece, with lower limit voltage: X UT2: partial voltage between electrode and nozzle UT1: Partial tension between nozzle and workpiece V: plasma gas amount P: discharge space pressure of the plasma gas in the space around the electrode end) I: Current of the main arc I ₀ : current intensity of the pilot arc Y: Feed of the burner in the cutting direction

During the cutting process, the partial voltages UT1 and UT2 as well as the total voltage UG are measured several times within a 20 ms long measuring cycle and stored in accordance with the current intensity I. At the same time, the process control variables V, P and Y are detected in their switching and sizing state and stored as logical quantities. The stored partial voltages UT1 and UT2 are compared with each other and summarily and individually with the total voltage UG and their lower limit X, the comparison results of two successive measurement cycles are compared with each other and with the stored logical variables of the process control variables.

In terms of Boolean algebra, the logical quantities A, B, C, D, E, F, G and H are output and stored as follows by the process measuring and evaluation computer.

(UG> X) λ (AUG

((UG> X) a (AUG «0) -» A-1

(UG> X) a (AUG> 0) - »B- 1

(UG> X) λ (AUG <0) - »S-O

AUT2 * 0 - ♦ C -0

AUT2 -0 -> C = 1

AUT2> 0 - ♦ D = 1

AUT 2 <0 - »D - 0

AV = 0 -> E = 1

AV * 0 - ♦ E = 0

ΔΡ = 0 - »F = 1

AP * 0 - ♦ F -0

AY - 0 - ♦ G - 1

AY * 0 - * δ - 1

AUT1> 0 -> H - 1

AUT1 <0 - ♦ R -0

The evaluation of the memory sizes takes place, expressed in Boolean algebra, with the following output variables:

EaFaAaD - »Electrode wear FaEaCaAaB - ♦ Nozzle wear EaF a_B a_H a C - ♦ Nozzle clearance becomes larger EaFaB_aH_aC - * Nozzle clearance becomes smaller EaFaHaDaA * Splatter (slag) on nozzle EaFaHaDaB -> Double arc

E - »Gas supply fault

5 - * feed disturbance

Di * * 'aregabegöße are utilized by signaling or corresponding control commands.

Claims (1)

Method for process control on plasma torches for welding, cutting and similar processes with transferred plasma add sheet under evaluation of the electrical voltage of the plasma arc, characterized in that within a maximum of 20ms long measuring cycle 'times the partial voltages between the electrode and nozzle and between nozzle and workpiece and the total voltage between Measured electrode and workpiece and the respective current strength of the pilot and / or main arc are stored, that the process control variables "plasma gas quantity" and / or "discharge space pressure" and "feed" detected in their switching and sizing state and stored as logical quantities, in that the stored partial voltages between the electrode and the nozzle and between the nozzle and the workpiece are compared with each other and in summary as well as individually with the total voltage or voltage limit values, that the comparison results of several successive the measuring cycles are compared with each other and with the associated stored logical variables of the process control variables and the output variables are utilized by signaling or control commands. Field of application of the invention The invention relates to a method for process control of plasma torches for welding and cutting with transferred arc taking advantage of the electrical voltage of the plasma arc. Characteristic of the known state of the art It is known that the plasma arc voltage can be evaluated to control the distance between the burner and the workpiece surface. The dependence of the plasma arc voltage on the burner height is described by a monotonically increasing function, being used for welding and cutting mainly with transferred arc and composite electrodes. In DE-OS 26 59188 it is proposed that at a time in which the plasma arc burns at the predetermined target distance, the size of the plasma arc voltage is entered as a setpoint in a Sezu storage and that thereafter the arc voltage is further measured and compared with this setpoint. The signal generated by comparing the two voltages is input to a burner motor height control circuit for controlling the distance between the burner and the workpiece surface, the torch height. A similar proposal is made in DE-OS 2706232. Here, the voltage to be stored is digitized via an analog-to-digital converter and entered into a corresponding memory. The comparison of nominal value and actual value is performed in digital form by a microprocessor. Using this microprocessor as a comparator between the stored setpoint value and the instantaneous actual value of the arc voltage, it is also possible to generate a shutdown signal which arises when the burner is moved beyond the workpiece. This shutdown signal, which is triggered by the extinction of the main arc and the associated large change in the plasma arc voltage to zero or the high level of open circuit voltage, then causes the burner is raised to a predetermined end position. This measure ensures that the burner support does not sag down when passing over the edge of the workpiece ". In DE-OS 2906751 is compared to the above solution, the analog-to-digital converter and the memory replaced by an oscillator and a counter. In the aforementioned solutions, a comparison between a predetermined desired value and the instantaneous actual value of the plasma arc voltage is carried out and with the optionally amplified differential voltage a Brennerhöhenyerstellmotor is driven. In order to reduce the resulting from the high technical complexity of these solutions failure frequency and technological uncertainty was proposed in DD-PS 244 674 to reduce the analog arc voltage in a conventional manner to a fraction of its value and oppose this voltage fraction a constant compensation voltage, so in that a difference voltage of fixed polarity with lower limit close to zero is formed. The compensation voltage is thus about the same size as the said voltage fraction. The differential voltage near zero is then amplified, and then the amplified analog differential voltage is stored as a fixed value in one of a few possible different binary voltage signals at a time, in particular sheet performance, burner distance, working speed and welding gas supply a setpoint and immediately in one of the few possible reconverted analogously sized analog signals and compared this in a conventional manner during the entire welding or cutting process with the instantaneous amplified analog differential voltage, and that takes place with the resulting voltage from the comparison voltage signal with known electrical and mechanical means, a distance control between the burner and the workpiece. With the same solution should also be possible, while keeping constant this Brennerao state another process variable, which obviously refers to the above-mentioned sheet performance or the operating speed or the welding gas supply to regulate. The disadvantage of the known solution for the automatic control of plasma cutting systems utilizing the electric plasma arc voltage is that although a sufficient control of a process variable, in particular the torch distance is possible, however, that technological disturbances are not recognized as such, but their impact on the plasma arc voltage than Changes in distance between the burner and workpiece surface are sensed and controlled, which corresponds to a misinterpretation and usually leads to the destruction of the burner. Such technological disturbances occur when z. B. a so-called double arc occurs, odor electrode and / or nozzle beyond a certain level wear, i. burn out if the cooling of the electrode and / or you-decreases, if the plasma gas volume falls below a permissible limit, or if settle at the nozzle in increased metnil or slag splash.