WO2009143882A1

WO2009143882A1 - Arc welding robot

Info

Publication number: WO2009143882A1
Application number: PCT/EP2008/056481
Authority: WO
Inventors: Görgen Johansson
Original assignee: Abb Technology Ab
Priority date: 2008-05-27
Filing date: 2008-05-27
Publication date: 2009-12-03

Abstract

The invention relates to an arc welding robot having a welding gun (2) a welding power source (3), a robot controller (4), a current sensor (5) sensing the current supplied to the arc, a position measurement system (7) sensing the position of the welding gun (2) and processing means processing data related to the sensed current and the sensed position and delivering robot commands based on the processed data. According to the invention the processing means is the micro processor (8) of the robot controller (4). The invention also relates to a corresponding method for controlling an arc welding robot.

Description

ARC WELDING ROBOT

Field of invention

The present invention in a first aspect relates to an arc welding robot 5 including a welding gun.

In a second aspect the invention relates to a method for controlling an arc welding robot, the method being of the kind performed by an arc welding robot as set out above.

i o Background of invention

In arc welding performed by an arc welding robot it is important that the welding gun properly tracks and follows the joint. When joining the pieces together with a seam the robot is programmed to follow the joint which might be a straight line or have another profile. The robot is also pre-programmed to move the

15 welding gun in the direction perpendicular to the joint in a pattern e.g. sinus shaped spanning the joint. The robot is pre-programmed to move the welding torch along the seam in its nominal position.

Normally the robot also is programmed to roughly localize the joint. It is of course important that the welding gun exactly follows the joint such that the neutral

20 axis of the movement is in the middle of the joint.

For this reason it is required to have a tracking system such that the above mentioned condition can be met. Known tracking system include the use of a video camera or a laser beam for tracking the joint and to provide the robot controller with the obtained information for governing the welding gun to be

25 positioned accordingly. This, however, entails costs for the video camera or the laser, and further raises parallax problems. This equipment might also hinder the access to the work pieces.

Another known solution is based on the fact that the impedance across the arc is proportional to the arc length. For example in a V-shaped groove it has a

30 maximum at the middle of the joint and decreases to a minimum at each side of the joint. Thereby a special measuring device is provided that measures the current such that the impedance can be derived for the gap, either by referring the current to the preset voltage or by also measuring the actual voltage across the arc gap. The curve representing the varying impedance is compared in a micro processor with information about the actual movements of the welding gun. Any mismatching between the two information sources is processed in the micro processor through an adequate algorithm to provide robot commands to move the welding gun to a path where they match each other. An arc welding robot that is provided with this kind of tracking system has shown to operate satisfactory. However the auxiliary equipment required for this, in particular the voltage and current measuring device and the external micro processor represent a substantial extra cost. Also the interaction with these components and the robot requires particular measures. An example of a joint tracking system based on this principle is disclosed in US 5 343 016.

The object of the present invention is to reach a tracking system of the varying impedance based kind where the drawbacks of known such system are overcome or at least reduced, in particular to reduce the cost for a tracking system based on the varying impedance.

Summary of invention

This object is according to the first aspect of the invention achieved in that an arc welding robot having a welding power source, a robot controller, a voltage sensor arranged for sensing the voltage across the arc gap, a position measurement system arranged for sensing the position of the welding gun and processing means arranged for processing data and deliver robot commands based on the input data, the input data including the voltage sensed by the voltage sensor, the voltage across the arc gap and the position data sensed by the position measurement system includes the specific feature that the processing means is the micro processor of the robot controller.

Thereby the need for the auxiliary equipment of an external micro processor is eliminated, with a corresponding cost reduction. Since a micro processor normally is present in the robot controller for other purposes, there will be practically no extra costs to use this micro processor for processing the data related to the tracking of the joint.

According to a preferred embodiment the current sensor forms a part of the welding power source. Thereby also the need for an external current sensor for the joint tracking is eliminated. This will further reduce the cost for auxiliary equipment.

According to a further preferred embodiment, the robot further includes a voltage sensor arranged for sensing the voltage across the arc gap, which voltage sensor forms a part of the welding power source, and the input data includes the voltage sensed by the voltage sensor.

Although a normally sufficient value of the impedance can be obtained by sensing only the current and base the impedance calculation on the sensed current and the pre-set voltage, there might due to various circumstances be some deviations between the pre-set voltage and the actual voltage. By using the actually sensed voltage the accuracy when calculating the impedance will be increased which allows a more precise tracking of the joint.

Since also the voltage sensor is integrated in the welding power source the need for external equipment also for this function is eliminated which further contributes to achieve a low-cost tracking system. Normally also a voltage sensor is present in the welding power source. This function thereby does not require any additional components within the welding power source.

According to the second aspect of the invention the object is met in that a method for controlling an arc welding robot of the kind specified above includes the specific measure that the input data are processed in the micro processor of the robot controller.

Preferred embodiments of the invented method correspond to the preferred embodiments of the invented arc welding robot as set out above. The invented method and the preferred embodiments thereof have advantages corresponding to those of the invented arc welding robot as explained above.

The invention will be further explained by the following detailed description of an example of the invention and with reference to the accompanying drawings.

Brief description of the drawings

Fig 1 illustrates the track of a welding gun across the joint.

Fig 2 is a schematical side view of an arc welding robot according to the invention. Fig. 1 illustrates joining two work pieces 101 and 102 to each other along a V-shaped joint 103. When welding the two pieces together the welding gun travels in the direction of the joint, the x-direction and in a reciprocating movement perpendicular thereto in the y-direction. The reciprocating movement bridges the V-shaped joint. The two motions in this case add up to sinusoidal travelling path P of the welding gun. The impedance across the arc gap is proportional to the length of the arc. When the welding gun travels in the y-direction there will thus be a maximum of the impedance when the welding gun is in the middle of the joint and go down to a minimum when it reaches the upper edge of the respective work piece. The impedance thereof will vary according to a sinus function.

The robot is programmed to follow the joint which can be a straight line or follow a certain curve. The robot is also programmed to make the lateral motion corresponding to the width of the joint. For attaining a proper seam it is necessary that this pre-set travelling pattern matches the actual location of the seam. The robot is pre-programmed to move the welding torch along the seam in its nominal position. For various reasons the actual position of a part including the seam may be misaligned with its nominal position. It is therefore necessary to take measures so that the programmed travelling pattern of the welding gun exactly matches the joint. The shape of the impedance curve and its correlation with the pendulum pattern is used to generate path correlations perpendicular to the joint. The mean calculation of the impedance is used to generate corrections of the arc length. The joint tracking system described so far belongs as such to prior art. Fig. 2 is a schematic side view of an arc welding robot according to the invention. The welding gun 2 operates to join the work pieces 101 , 102 along a joint 103 and is carried by the robot frame 1. The required voltage is received from the welding power source 3 and the operation of the robot is governed by the robot controller 4.

The current sensor 5 and the voltage sensor 6 normally present in the welding power source are used to provide the data required for obtaining the impedance across the arc gap. The position measurement system 7 of the robot controller provides the information represented by the curves B and C of figures 2 and 3. This information together with the impedance data received from the welding power source via the bus 9 are processed in the micro processor 8 of the robot controller 4 and governs the movements of the welding gun. All equipment required for the joint tracking thus is integrated in the welding power source and the robot controller, eliminating the need for external equipment traditionally used for this.

Claims

1. An arc welding robot including a welding gun (2), a welding power source (3), a robot controller (4) having a micro processor (8), a current sensor (5) arranged for sensing the current supplied to the arc, a position measurement system (7) arranged for sensing the position of the welding gun (2) and processing means arranged for processing input data and delivering robot commands based on the processed input data, the input data including the current sensed by the current sensor (5), the voltage across the arc gap and the position data sensed by the position measurement system (7), characterized in that said processing means is the micro processor of the robot controller (4).

2. An arc welding robot according to claim 1 , characterized in that said current sensor (5) forms a part of the welding power source (3).

3. An arc welding robot according to claim 1 or 2, characterized in that it further includes a voltage sensor (6) arranged for sensing the voltage across the arc gap, which voltage sensor (6) forms a part of the welding power source (3) and in that said input data includes the voltage sensed by the voltage sensor (6).

4. A method for controlling an arc welding robot having a welding gun, a welding power source and a robot controller, whereby the current supplied to the arc is sensed, the position of the welding gun is sensed, input data are provided , which input data includes the sensed current, the sensed position and the voltage across the arc gap, said input data are processed and robot commands are delivered based on the processed input data characterized in that the input data are processed in the micro processor of the robot controller.

5. A method according to claim 4, characterized in that current is sensed by a current sensor that forms a part of the welding power source.

6. A method according to claim 4 or 5, characterized in that the voltage across the arc gap is sensed by a voltage sensor that forms a part of the welding power supply and that this sensed voltage is included in the input data.