SYSTEM FOR CORRECTING A PATH OF AN AUTOMATIC WELDING MACHINE

Abstract: A system for correcting a path of an automatic welding machine has a means which differentiates the welding cur­ rent at the right and left positions of the torch during weav­ ing and a means which detects welding current when a value obtained from the above differentiation means is zero. The welding current is subtracted when the right and left differentiated values are zero. Based upon the value thus obtained, central position of weaving in which the torch proceeds is corrected.

Description: This invention relates to an automatic welding machine path correction system and, more particularly, to a path correction system in an automatic welding machine for performing welding while weaving a welding torch left and right with respect to a welding line. Background Art An automatic welding machine performs welding by impressing a voltage across a wire and a workpiece to produce an arc at the tip of the wire, and moving the wire tip along a welding path while the wire is successively paid out in small increments. Fig. 8 is a schematic view of such a welding machine. In the Figure, WR denotes the wire, which is paid out in small increments in the direction of the arrow by feed rollers FR so that its tip protrudes from the end of a torch TC via a guide member GB, with the amount by which the wire is fed being limited in such a manner that the tip comes to occupy a position spaced a predetermined distance from the surface of the workpiece WK. A welding power supply PS, which impresses the voltage across the wire WR and the workpiece WK, generates a continuous high-voltage at a predetermined period. The plus side is applied to the wire WR through the guide member GB, and the minus side is connected to the workpiece WK. In the torch TC, CO₂ gas from a gas supply unit, which is not shown, is supplied by a CO₂ source. Welding is performed while the CO₂ gas is jetted toward the portion being welded, thereby preventing oxidation of the welded portion. When a high voltage is generated continuously by the welding power supply PS while the CO₂ gas is fed from the gas supply unit, not shown, and the wire WR is paid out in small increments in the welding machine having the above construction, an arc is produced at the tip of the wire and both the wire and the portion being welded are melted to weld the fused portions of the workpiece together. It has recently become possible to perform such a welding operation automatically by robot. Specifically, the torch TC of the welding machine is grasped by a robot, which is caused to move the torch (the tip of the wire) along a welding path to weld the workpiece. When weaving is performed to the left and right of a cutting line CT, as shown in Fig. 6, the robot control unit moves the torch TC upon calculating the extreme points to the left and right from starting and end points, which have been taught, as well as amplitude and frequency. However, in cases where there are differences in the machined precision of the workpiece or where there is a shift in the attached position, welding cannot be performed accurately at the predetermined location of the workpiece when the torch TC is moved based on the taught points. Accordingly, as shown in Fig. 7, the conventional practice is to monitor the value of the integral of the welding current and effect a path correction upon comparing the integral value with a reference value when the difference between an integral value prevailing at a leftward swing and an integral value prevailing at a rightward swing in one cycle exceeds a stipulated value. With this method of correcting the path based on the integrated value of the welding current, sensitivity is very low, especially when the current is small, and the method is not suited to some welding modes as far as the welding characteristics are concerned. EP-A2-0 076 498 discloses a welding robot controlling system in which welding current or welding voltage is detected at both ends (i.e. extreme left and right sides) of a weaving operation of a welding torch carried by a welding robot. Deflection of the centre of the weaving from an actual weld line is determined on the basis of the difference between the detected values of welding current or welding voltage. Positioning of the robot arm is accordingly corrected. Weaving is carried out essentially as shown in Fig. 6 of this application. A sensor detects welding current or voltage at the ends of weaving, in response to measurement starting command produced, on the basis of a computation of the position of the torch, to indicate that a weaving end has been reached. An object of the present invention is to provide an automatic welding machine path correction system capable of correcting a welding path smoothly and accurately irrespective of the welding mode. Disclosure of the Invention According to the present invention, there is provided a path correction method in an automatic welding machine which performs welding while weaving a welding torch to the left and right of a welding line, which method has the following steps:- (a) a first step of calculating by interpolation, from a given starting point, end point, amplitude and frequency, a weaving path; (b) a second step of moving the torch on a trajectory calculated in the first step and sensing welding current when the torch is at left and right extreme points of its weaving path; and (c) a third step of performing a weaving path correction based on a difference between welding currents sensed at left and right extreme points in the second step, characterised in that the weaving path includes, at left and right extreme points, path portions which are parallel to the welding line, and welding current is sensed at such parallel path portions. According to the present invention, there is also provided a path correction system of an automatic welding machine which performs welding while weaving a welding torch to the left and right of a welding line, which system comprises:- a torch for welding a workpiece; a welding power supply for supplying said torch with current; current sensing means for sensing the welding current supplied to the torch by said welding power supply; means for calculating by interpolation, from a given starting point, end point, amplitude and frequency, a weaving path; means for moving the torch on a trajectory calculated by the calculating means; means for causing said current sensing means to sense welding current supplied to the torch by the welding power supply at left and right extreme points of its weaving path when weaving is performed; and means for obtaining a difference between welding current sensed at left and right extreme points and performing weaving path correction based on the difference; characterised in that the calculating means, and the means for moving the torch on the trajectory calculated by the calculating means, are operable to provide a weaving path which includes, at left and right extreme points, path portions which are parallel to the welding line, and the means for causing said current sensing means to sense welding current are operable to cause the current sensing means to sense welding current at such parallel path portions. The present invention makes it possible to accurately sense the extreme points of the weaving pattern and to correct the welding path smoothly irrespective of the welding mode. Brief Description of the Drawings Fig. 1 is a view for describing a weaving pattern used in the present invention, Fig. 2 is a view for describing a change in welding current corresponding to the weaving pattern shown in Fig. 1, Fig. 3 is a view for describing the manner in which a correction amount is decided, Fig. 4 is a block diagram of a welding robot for practicing the present invention, Fig. 5 is a flowchart for describing the operation of the present invention, Fig. 6 is a view for describing a conventional weaving pattern, Fig. 7 is a view for describing a conventional manner of correcting a path, and Fig. 8 is a schematic view showing the construction of a welding machine. Best Mode for Carrying Out the Invention An embodiment of the present invention will now be described in detail with reference to Figs. 1 through 3. In a case where a V- or L-shaped beveled portion is to be welded by a weaving pattern, the present invention monitors welding current to correct weaving in such a manner that the center of weaving conforms to the center (welding line CT) of the beveled portion. When it is judged that a path correction is necessary, the amount of the correction is reflected in the path calculation of the next cycle, whereby the correction is made during the next cycle. In order to effect the weaving path correction, a weaving frequency higher than 0.4 Hz is required, and the path correction is performed based on the symmetry of a change in welding current when welding motion is made to the left and right during weaving. Accordingly, the beveled shape is formed so as to be symmetrical within the range in which the weaving motion is made. Fig. 1 is a view for describing a weaving pattern used in the present invention. The weaving pattern defines a trapezoidal pattern having a path whose left and right extremes lie parallel to the welding line. Fig. 2 is a view for describing a change in welding current corresponding to the weaving pattern shown in Fig. 1. As is evident from Fig. 2, the welding current has a differential value (rate of change) of a certain magnitude up to a point 1 ○. However, when the torch TC begins to move along the side wall of the workpiece, namely when the torch is located between points 1 ○ and 2 ○ on the path parallel to the welding line, there is no change in the welding current and, hence, the differential value is zero. A control unit is capable of sensing the welding current at the extremes of the weaving pattern by taking a sampling of values at the extremes. Alternatively, the detection of the welding current at the extremes of the weaving pattern need not rely upon sampled values as mentioned above but can be accomplished by obtaining the average value of welding currents from extreme point 1 ○ to extreme point 2 ○ on the path, which lies parallel to the welding line, where the amount of change in the welding current is zero. Let us now describe a welding path correction. Let CE represent a current error for determining the amount of correction, RD the amount of welding current sensed on the right side of the welding line, and LD the amount of welding current sensed on the left side of the welding line. In general, then, we will have CE = LD - RD (1) In other words, in order to effect a path correction, it is necessary to constantly monitor the symmetry of the change in the welding current when welding motion takes place to the left and right at the time of weaving. An amount of bias to one side can be allowed for when necessary. For example, if it is desired to allow for an amount of bias on the right side, then we will have CE = (LD - BR) - RD (2) In this way it is possible to deal with a workpiece having left-right asymmetry. Determining the amount of path correction will be described next. Fig. 3 is a view for describing the manner in which the amount of path correction is decided. In the Figure, a, b, and c denote values of the abovementioned current error CE, and ℓ₁, ℓ₂ represent designated values of parameters serving as the basis of a correction value. Here it is appropriate to set changes in the current error value CE to steps of e.g. 0.1 A, and to set the parameter designation values, which serve as the basis of a correction value, to steps of 0.1 mm. Accordingly, if the current error CE is a, namely 0.1 A, then the control unit will correct the center of weaving by a correction amount ℓ₁, namely 0.1 mm. Note that when the current error CE exceeds ±0.3 A, an alarm is issued and the control unit stops the welding operation. In other words, if the current error becomes too large, a path correction cannot be made in a normal manner. This is indicated by an alarm and the welding operation is halted. A welding robot for practicing the present invention will now be described in brief. Fig. 4 is a block diagram schematically illustrating a welding robot. In the Figure, numeral 1 denotes a welding machine, and numeral 2 represents a welding robot having a hand which grasps the torch TC of the welding machine and moves the torch TC along the workpiece WK in accordance with taught data. Numeral 3 denotes a numerical control unit for controlling the welding machine 1 and the welding robot 2. Numeral 4 represents a welding power supply for supplying the torch TC with a welding current. The numerical control unit 3 has a central processor (CPU) 31 for controlling the overall numerical controlunit and for executing processing, a read-only memory (ROM) 31 storing a control program, a random-access memory (RAM) 32 for temporarily storing various data such as the data the robot has been taught, an operator's panel 33 for entering various welding conditions and for teaching the welding robot as well as a welding activity, a display device 34 for displaying the present position of the torch TC, a program list and data, and an digital/analog converter 35 for converting digital command values received from the CPU 31 into analog command values and for delivering these analog command values to the welding machine 1. It should be noted that the welding current supplied from the welding machine 1 to the torch TC is measured by a current sensor 1a, which applies the measured value to the numerical control unit 3. The operation of the invention will now be described using the flowchart shown in Fig. 5. Let us assume that, prior to the execution of welding, the welding robot has been taught the necessary data, such as welding line (CT) position data and a weaving command format for commanding a weaving operation, by means of a teaching operation, that initial motion of the torch TC has ended, that the torch TC is at a point P in Fig. 1 in the course of operation, and that the torch is to be moved from this position. The torch TC starts to be moved from point P to the left side. This motion is effected by using two items of data, namely a frequency f and an amplitude value, commanded earlier in the weaving command format [step (1)]. Meanwhile, the welding current supplied to the torch TC from the welding machine 1 is constantly being sensed by the current sensor 1a, which sends the resulting signal to the numerical control unit 3. Motion of the torch TC is monitored at all times by the CPU 31. When the torch TC reaches the positions 1 ○ and 2 ○ in Figs. 1 and 2, between which positions the change in welding current ceases, the current value LD prevailing at this time is stored in the RAM 33 [steps (2) and (3)]. It should be noted that monitoring the cessation of the change in current can be performed at the position of the torch TC, as described above, or by differentiating the welding current. As the movement of the torch TC proceeds, the torch moves to the right side of the welding line CT [step (4)]. In the meantime, however, the welding current supplied to the torch TC by the welding machine 1 is constantly sensed by the current sensor 1a, whose output signal is applied to the numerical control unit 3, and motion of the torch TC is constantly monitored by the CPU 31. When the torch TC reaches the positions (3) and (4) in Figs. 1 and 2, between which positions the change in welding current ceases, the current value RD prevailing at this time is stored in the RAM 33 [steps (5) and (6)]. The calculation (LD - RD) = CE is performed during one cycle of the welding operation [step (7)]. If the result of the calculation is zero, i.e. the error current CE has a value of zero, there is no left-right positional correction with respect to the center of the weaving pattern along which the torch TC advances. If the result of the calculation is a positive (+) value, the position of the center of the weaving pattern along which the torch TC advances is too far to the left. In the next cycle of the welding operation, therefore, the position is corrected to the right side by the magnitude of the current error CE. If the result of the calculation is a negative (-) value, the position of the center of the weaving pattern along which the torch TC advances is too far to the right and, hence, in the next cycle of the welding operation, the position is corrected to the left side by the magnitude of the current error CE [step (8)]. Thus, as set forth above, the present invention monitors the amount of change in the welding current at the time of weaving, senses the welding current at the left and right extremes where the amount of change is zero, and corrects the weaving path based on the difference between the current values at the left and right extremes. Accordingly, the extreme points of the weaving pattern can be accurately sensed and the welding path can be corrected smoothly irrespective of the welding mode. This makes it possible to reduce the labor and energy involved in the welding operation and to enhance both the efficiency and reliability of the operation. Though the present invention has been described based on an embodiment, the invention is not limited to this embodiment but can be modified in various ways such modifications being within the scope of the invention as defined by the claims. Industrial Applicability As described above, the automatic welding machine path correction system according to the present invention enables the path of a welding torch to be corrected automatically and therefore is well-suited for application to a welding robot having a soft weaving function.

Claims: A path correction method in an automatic welding machine which performs welding while weaving a welding torch (TC) to the left and right of a welding line (CT), which method has the following steps:- (a) a first step of calculating by interpolation, from a given starting point, end point, amplitude and frequency, a weaving path; (b) a second step of moving the torch (TC) on a trajectory calculated in the first step and sensing welding current (LD, RD) when the torch (TC) is at left and right extreme points of its weaving path; and (c) a third step of performing a weaving path correction based on a difference (CE) between welding currents (LD, RD) sensed at left and right extreme points in the second step, characterised in that the weaving path includes, at left and right extreme points, path portions ( 1 ○ - 2 ○ ; 3 ○ - 4 ○ ) which are parallel to the welding line (CT), and welding current is sensed at such parallel path portions. A path correction method in an automatic welding machine according to claim 1, characterised in that, in said second step, a predetermined bias value (BR) for applying a bias to a left or right extreme point of weaving is added in advance to the welding current (LD, RD). A path correction method in an automatic welding machine according to claim 1, characterised in that the weaving path has trapezoidal path portions on both left and right. A path correction method in an automatic welding machine according to claim 1, 2 or 3, characterised in that, in said second step, the welding current is differentiated and cessation of change in the welding current is detected to indicate when a parallel path portion is reached. A path correction system of an automatic welding machine (1) which performs welding while weaving a welding torch (TC) to the left and right of a welding line (CT), which system comprises:- a torch (TC) for welding a workpiece (WK); a welding power supply (4) for supplying said torch (TC) with current; current sensing means (1a) for sensing the welding current supplied to the torch (TC) by said welding power supply (4); means (3) for calculating by interpolation, from a given starting point, end point, amplitude and frequency, a weaving path; means (1, 2) for moving the torch (TC) on a trajectory calculated by the calculating means (3); means for causing said current sensing means (1a) to sense welding current (LD, RD) supplied to the torch (TC) by the welding power supply (4) at left and right extreme points of its weaving path when weaving is performed; and means (32, 33, 31) for obtaining a difference between welding current (LD, RD) sensed at left and right extreme points and performing weaving path correction based on the difference; characterised in that the calculating means (3), and the means (1, 2) for moving the torch (TC) on the trajectory calculated by the calculating means (3), are operable to provide a weaving path which includes, at left and right extreme points, path portions ( 1 ○ - 2 ○ ; 3 ○ - 4 ○ ) which are parallel to the welding line (CT), and the means for causing said current sensing means (1) to sense welding current are operable to cause the current sensing means to sense welding current at such parallel path portions. A path correction system of an automatic welding machine according to claim 5, characterised in that the path correction system of the automatic welding machine is used in a welding robot (2).

Inventors: ISHIKAWA, HARUYUKI, KISHI, HAJIMU HINOHIRAYAMADAI-JUTAKU 1104, SAKAKIBARA, SHINSUKE
Applicants: FANUC LTD.
Application year: 1985
Main classification: B23K9
IPC codes: B23K9