Short arc welding system

Abstract

A system for controlling a weld-current in an arc welding apparatus for short arc welding comprising a current regulator included in a voltage feedback loop from a power supply to a welding electrode and a ramp generator arranged to provide current ramps during a short circuit phase at said welding electrode.

Claims

1. A system for controlling a weld-current in a direct current arc welding apparatus for short arc welding comprising: a current regulator included in a voltage feedback loop from a power supply to a welding electrode; an open circuit detector included in the voltage feedback loop, the open circuit detector being configured to detect that an open circuit state is present and to suppress a feedback error when an open circuit state is detected by the open circuit detector; a node configured to receive an output voltage of the power supply and to generate a voltage signal derived from the output voltage; a switch connected between the node and the current regulator; and a ramp generator arranged to provide current ramps during a short circuit phase at the welding electrode, wherein the open circuit detector comprises a comparator that receives an open circuit voltage threshold and a signal derived from the output voltage, the open circuit voltage threshold comprising a threshold level above which only the open circuit state is possible, the comparator having an output directly connected to the switch, wherein the switch is configured to, based on the output of the comparator, connect or disconnect the current regulator to receive the voltage signal from the node.

2. A system for controlling a weld-current according to claim 1, wherein the node is a subtraction node, wherein the subtraction node is configured to calculate a difference between the output voltage and a reference voltage.

3. A system for controlling a weld-current according to claim 1, wherein the current regulator and the ramp generator are connected in parallel and provide a reference current for the power supply to the welding electrode.

4. A system for controlling a weld-current according to claim 3, wherein the reference current is a sum of an output current from the current regulator and a current ramp provided from the ramp generator.

5. A system for controlling a weld-current according to claim 1, wherein the current regulator is a PI regulator.

6. A system for controlling a weld-current according to claim 1, wherein the system further includes a short circuit state detector having a threshold level for detection of a short circuit state, wherein the threshold level is dependent on a magnitude of a reference current supplied to the power supply.

7. A direct current short arc welding system including a power source and a welding torch connected to a power source, wherein the power source is controlled by the system for controlling a weld-current according to claim 1.

Description

BRIEF DESCRIPTION OF FIGURES

(1) An embodiment of the invention will be described with references to appended drawings, where:

(2) FIG. 1 shows a general description of a state of the art short arc welding process,

(3) FIG. 2 is a schematic drawing of a system for controlling a weld-current in an arc welding apparatus for short arc welding according to the invention, and

(4) FIG. 3 shows a schematic drawing of a welding system.

EMBODIMENT OF THE INVENTION

(5) FIG. 2 shows a system 12 for controlling a weld-current in an arc welding apparatus 20 (FIG. 3) for short arc welding.

(6) A voltage feedback loop 13 controls the voltage at a welding electrode connected to a constant current power source.

(7) The constant current power source includes a current feedback loop 5a which compares an output current with a reference current provided from a system 12 for controlling a weld-current in the arc welding apparatus 20 (FIG. 3). A current regulator included in the constant current power supply 5 controls the output current of the power supply in dependence of a regulation error between the reference current and the output current.

(8) The voltage feedback loop 13 includes a subtraction node 1 where the output voltage Uarc is subtracted from the reference voltage Uref in the node 1. The difference between the output voltage Uarc and the reference voltage Uref constitutes a regulation error E, which is delivered to the PI regulator 3 as an input signal via a switch 2.

(9) The voltage feedback loop 13 furthermore includes a current regulator, preferably a PI regulator 3, a switch 2 and an open circuit state detector 6.

(10) The switch 2 is driven by the signal from an open circuit state detector 6, which comprises low-pass filter 6a and the comparator 6b. The open circuit voltage threshold Uo.c. is the threshold level above which only the open circuit state is possible. The current regulator 3 has an input 3a connected to the switch 2. The switch 2 is opened when an open circuit state is sensed by the open circuit state detector 6. Therefore the input voltage to the PI regulator, which input voltage corresponds to the regulation error, will be set to zero.

(11) Hence, the PI regulator 3 will assume that the voltage over the electrode and working piece is appropriate during the open circuit state. When the input signal to the PI regulator is suppressed during the open circuit state, the PI regulator will remain in its present state and neither internal charges nor the output current Ireg from the PI regulator will be changed. In this way voltage regulating feedback loop will not take account of the increased feedback voltage during non-arc phases.

(12) In a summing node 4, delivering a reference current signal Iref to the power supply 5, the signal from the PI regulator 3 is added to the signal from the ramp generator 11.

(13) The state of the short circuit is sensed by a short circuit detector 7, which comprises low-pass filter 8 and the comparator 10. When a detected voltage Uarc is below a threshold value Uth, which may be dependent on the reference current, the ramp generator is connected to generate a current ramp.

(14) The threshold of the comparator 10 may be current dependant. The threshold signal is created in the summing node 9 by adding a signal proportional to the current reference Iref and zero-current threshold voltage Us.c.o. When the short circuit detector detects a short circuit state, the ramp generator 11 will create an additional ramp current ramp with predefined up- and down-slopes.

(15) FIG. 3 schematically represents an arc welding apparatus 20 for short arc welding. The short arc welding system 20 includes a power source 21, a welding control system 22, a wire reel 23, a wire feed motor 24, a shielding gas supply 25 and a welding torch 26. The system in the figure is set up for performing a welding operation on a welding piece 27.

(16) A system for controlling the weld-current 28 according to the invention is connected to the power source for control of the weld-current during operation. The system for controlling the welding current 28 may conveniently be integrated in the casing of the power source 21.

(17) The output voltage of the power source and the weld-current may be controlled by the circuit described in FIG. 2. One power cable 29a from the power source is connected to the working piece 27 and the other 29b is connected to the electrode 32 of the welding torch 26, optionally via the welding control system 22.

(18) The shielding gas supply 25 is connected to the welding torch 2 by a tube system. The amount of gas can be regulated via the welding control system 22.

(19) Further, cooling channels may be present in the welding torch. Cooling liquid inlet and outlet channels 30, 31 may be connected to the cooling channels in the welding torch.

(20) The wire feed motor 24 controls the feeding of the welding electrode during operation.