JPH01107979A - Method for controlling welding action of welding robot - Google Patents

Abstract

PURPOSE:To enable accurate welding when pressure welding is carried out with plural arms of a robot by returning an electrode tip shifted by repulsive force produced by the pressing of the tip to the original position by a uniaxial change in the angle of the wrist of the robot. CONSTITUTION:The electrode tip 42 of a pressure welding jig is pressed on the objective position of works W held on a fixed electrode 43, and the tip 42 shifted by repulsive force produced from the works to be welded by the pressing is returned to the original position only by a uniaxial change in the angle of the wrist 34 of a robot. By this method, accurate welding can be stably carried out because the tip 42 can be accurately positioned at the welding position.

Description

[Detailed Description of the Invention] [Field of Industrial Application] The present invention relates to a method for controlling welding operations in a welding robot, and in particular, a method for controlling welding operations in a welding robot. By reducing the positioning loop gain of the robot wrist, we can prevent the electrode tip from shifting from the target position due to the pressure reaction force acting on the electrode tip when welding by applying pressure to the electrode.
This invention relates to a welding position control method for a welding robot so that the angular displacement of the wrist can be absorbed and welding can be performed at the correct target position.

[Prior art]

Here, to explain in general the basic structure and function of the welding robot/nod that performs stud gun welding, as shown in Figure 1, the welding robot is a robot that has a robot arm and a robot wrist. The robot includes a main body and a pressure welding tool attached to the tip of the robot wrist of the robot main body. The robot main body has a degree of freedom of movement around 5 or 6 axes for boat fishing, and in this example, a 6-axis robot,
7 is illustrated. That is, the robot main body includes a robot trunk 31 that can horizontally rotate over a predetermined angle around a vertical axis (theta axis) with respect to a robot base 30, and a robot trunk 31 that can rotate back and forth via a joint 31a at the upper end of this robot trunk 31. A robot upper arm section 32 that can rotate around an axis (W axis), a robot forearm section 33 that can rotate up and down around a horizontal axis (U axis) via a joint 32a at the tip of this robot upper arm section 32, and this robot forearm. 3 which are pivotally attached to the tip of the part 33 and are orthogonal to each other.
It has joints around the axes (α-axis, β-axis, γ-axis), so the three
The robot wrist 34 has a degree of freedom of movement around the axis. In this case, the operating elements of the robot wrist 34 around the α-axis form the tip of the wrist, and the five-axis robot apparatus is configured such that the operating elements of the wrist 34 around the γ-axis are omitted. Therefore, the robot wrist 34 has a degree of freedom of movement around at least the α-axis and the β-axis. A stud gun 41 as a pressure welding tool is attached to the tip of the robot wrist 34, and a welding electrode 42 is provided at the tip of the stud gun 41. 42 to desired target positions one after another according to a program taught and stored in a storage unit of a robot control device (not shown), and movement for the positioning is performed by the robot trunk described above. 31, the robot upper arm 32, the robot forearm 33, and the robot wrist 34, the configuration is achieved by a positioning and speed control loop having a position detector as well as a speed reduction device from each drive motor that starts the movement around the respective rotation axes. It has become. At this time, the desired target position in the case of the welding robot is each welding position of the welding target W, and the stud gun 41 is placed at each welding position.
positioning the tip of the welding electrode 42, and on the other hand, positioning the welding target W
is fixed, for example, on a fixed base 44, and this fixed base 44
When the fixed electrode 43 provided on the stud gun 41 and the electrode 42 of the stud gun 41 are pressurized and energized at a position facing each other, the object W to be welded is welded.

Now, FIG. 2(a) is a mechanical conceptual diagram schematically showing the welding robot shown in FIG. 1, and parts corresponding to each operating part in FIG. 1 are indicated by the same reference numerals. Moreover, FIG. 2(b) is a partially enlarged view of FIG. 2(a). Figure 2 (
To explain the operation of the welding robot with reference to a) and (b), the operator controls the welding electrode 42 of the stud gun 41 via the teaching panel (not shown) on the robot control device side.
The position indicated by the solid line in FIGS. 2(a) and 2(b), that is, facing the fixed electrode 43 across the workpiece W to be welded,
In addition, teaching/memory operations are performed and programmed in advance so that the welding point is positioned at a position away from the workpiece W to be welded. According to this taught program, the robot control device (not shown) moves the welding electrode 42 of the stud gun 41 to the above-mentioned taught position to control the robot torso 31, robot upper arm 32, robot forearm 33, and robot wrist 34 of the robot main body. Moved and positioned by motion. Welding electrode 42
Once positioned, the robot controller energizes the stud gun 41. That is, pressurized air is supplied to the stud gun 41. As a result, the welding electrode 42 of the stud gun 41 is pressurized and pushed toward the workpiece W, and
and presses the workpiece W in cooperation with the fixed electrode 43. That is, as shown in FIG. 3, when the welding electrode 42 is positioned at the teaching position (P,), the welding electrode 42 is pressed by the stud gun 41 toward the intended welding position. FIG. 3 shows this pressurizing process. When the pressurizing force reaches a desired value at time T, the robot controller starts applying current between the welding electrode 42 and the fixed electrode 43, and as a result, the welded object is welded. The object W is electrically welded while being pressed between the picture electrodes 42 and 43. This state continues for a certain period of time, and when pressure welding is sufficiently established, the power supply is stopped, the welding electrode 42 of the stud gun 41 is pulled away from the target welding position, and moved toward the next target welding position. be done.

[Problems to be solved]

In the above, the operation process of pressure welding by the welding robot has been explained, but in the process of pressurizing the welding electrode 42 to press the workpiece W to be welded, the pressurizing force of the stud gun 41 is It exhibits a large value of 300 to 800 Kg/cm", and moreover, pressurization and pushing are performed instantaneously.

As a result, the welding electrode 42 of the stand gun 41 receives a large reaction force from the workpiece W to be welded. Due to the reaction force, the robot main body was bent as shown by the broken line in both FIGS. 2(a) and 2(b), and therefore the welding electrode 42 of the stud gun 41 itself was also bent. A positional shift occurs from the position facing the fixed electrode 43, and the electrode 42a is displaced to the position shown by the broken line. This amount of displacement varies depending on the magnitude of the pressing force and also varies depending on the posture of the robot main body, and the amount of displacement is approximately several ntn+. That is, there is a problem in that there may be a non-negligible error between the intended welding position and the actual welding position.

Moreover, if the stud gun is energized after positioning to the taught position P1 is completed, the positional displacement will occur as described above, but it will take some time for the positioning and pressing force to stabilize. After stabilization, the current is applied for electric welding, so the welding time at each target welding position will be slightly longer, and in the process of sequentially welding many target welding positions, the cumulative This has the disadvantage that the working time becomes longer and the welding work efficiency decreases. Therefore, it is an object of the present invention to provide a method for controlling the welding operation of a welding robot while eliminating such inconveniences.

[Means of solution and action]

According to the present invention, a plurality of robot arms are pivotally connected via a plurality of joints, and the robot arm is connected to the frontmost arm of the plurality of robot arms and can rotate around at least two axes orthogonal to each other. When performing pressure welding at each target position of a fixedly installed welding target using a welding robot having a robot wrist with a degree of freedom and a pressure welding tool attached to the tip of the robot wrist, The tip of the electrode of the pressure welding tool is pressed against the target position of the welding object with a pressure force, and the displacement of the tip of the electrode of the pressure welding tool due to the reaction force from the object of welding against the pressure is controlled by the robot wrist. The present invention provides a welding operation control method for a welding robot that absorbs only angular displacement around one axis and prevents displacement of the electrode tip of the pressure welding tool.

In addition, in order to absorb it only by the above-mentioned angular displacement around one axis of the robot wrist, when pressing the pressure welding tool with a pressure force to each target position of the welding target, almost immediately before applying the pressure welding tool to each target position of the welding target, The angular displacement of the robot wrist may be caused by reducing the control loop gain around the axis, thereby reducing the system stiffness. In this way, if the reaction force against the pressing action during pressure welding is absorbed by the rotation and angular displacement of the robot wrist, the robot arm mechanism will not be bent. It can be held in place permanently.

〔Example〕

Now, the welding operation control according to the present invention is as shown in FIG.
It can be applied to welding lofts with axial degrees of freedom of motion as well as well-known welding robots with 5-axis degrees of freedom of motion, and the structure of the robot body and pressure welding tool has been specially modified. There's no need to add it.

Therefore, in the following description of the embodiments, the mechanical structure of the welding robot will be explained with the 6-axis motion degree-of-freedom type robot shown in FIGS. 1 and 2 in mind.

Now, as mentioned above, in the process of positioning the tip of the welding electrode 42 at the tip of the stud gun 41 as a pressure welding tool to the teaching position P1 close to the desired target welding position, the robot body Operating parts such as the robot torso 31, robot upper arm 32, robot forearm 33, and robot wrist 34 are driven based on positioning commands from the robot control device. In this case, each operating part is equipped with a drive motor (usually It is well known that the system is equipped with a position command type servo system that includes an electric servo motor), a reduction gear, a transmission mechanism such as a belt/pulley mechanism, a position detector, etc., and is configured to drive its operating parts in accordance with the positioning command. It is as follows. Therefore, the position command type and speed command type servo systems of each operating part have a control loop gain, and this control loop gain can be increased or decreased by adjusting the constant of the transfer function on the robot controller side. It has become possible. It is also known that when the control loop gain is large, the system stiffness of the operating section becomes high, and when the gain is reduced, the system stiffness becomes low. Therefore, if the control loop gain of each operating section is maintained large, the rigidity of the robot main body can be maintained high.

Under such conditions, as described above, the welding electrode 42 of the stud gun 41 is positioned at the taught position P1, and then the stud gun 41 moves the electrode 42 to the target welding position of the workpiece W to be welded. If you push it toward the robot, the reaction force will bounce back to the robot body.

As a result, each robot arm 32.
33, etc., is bent due to its long arm length, and this bending causes the stand gun 41 attached to the tip of the robot wrist 34 to deviate from the taught position P1. Therefore, in the present invention, in the robot wrist 34 to which the stud gun 41 is attached, first, the reaction force during welding passes through the center of the β axis, which is the swinging motion axis of the robot wrist 34, at each target welding position. Corresponding teaching position P,
This is set in advance at the stage of teaching. Further, under such conditions, at the stage of the pressurization operation by the stud gun 41, the above β of the robot wrist 34 is
The rigidity around the axis is reduced, so to speak, the wrist is loosened, and when a reaction force is applied, the bending that occurs in each moving part of the robot body, especially the robot arms 32 and 33, and their joints 31a and 32a occurs. The angular displacement of the robot wrist 34 around the β-axis, that is, the movement of the wrist, is used to absorb the angular displacement of the robot wrist 34, and the tip of the welding electrode 42 of the stud gun 41 is maintained at the target welding position of the workpiece W. It is something to do. This can be easily achieved by reducing the previously described control loop gain in the β-axis of the robot wrist 34 on the robot (control device side) during pressure welding. Specifically, the robot control device may perform an operation of changing an appropriate constant among the transfer functions in the control loop. FIG. 4 shows the β-axis of the robot wrist 34 according to the present invention. Fig. 4 shows a timing chart for performing a control loop gain reduction operation.As can be understood from Fig. 4, first, the robot main body is operated and the timing is set to the teaching position P corresponding to each target welding position. When the arrival and positioning is completed at T, the pressurizing action by the stud gun 41 is started at timing T2.This pressurizing action continues for the time interval necessary to perform welding at each target welding position. This is the same as before.Thus, due to the pressurizing operation, energization is started at time T3 when the welding electrode 42 stops with the object W to be welded between it and the opposing fixed electrode 43, and this energization action is also constant. continues over time.

In the present invention, the above-mentioned positioning timing T.

At a time T4 between the timing T2 of the start of the pressurizing action, the control loop gain around the β axis of the robot wrist 34 is reduced, and this is adjusted to the end timing T2 of the welding action.
It is designed to continue until just before 5. End timing T of welding action.

After that, the stud gun 4 attached to the robot wrist 34 is moved toward the next welding target position by the movement of the robot body.
1 is performed.

The method for controlling the welding operation of a welding robot according to the present invention has been described above with respect to the control operation for performing the welding operation in the correct position at the target position. It goes without saying that the present invention is equally applicable to other industrial robots in which the end effector at the tip performs pressurizing and pushing operations.

〔Effect of the invention〕

According to the present invention, when the welding robot performs pressurization and welding using a stand gun, the displacement of the stud gun welding electrode caused by the reaction force is absorbed only by the robot operating part, especially the wrist. The desired welding effect can be obtained by pressurizing and energizing the welding electrode while it is correctly positioned at the desired welding position of the welding object.

[Brief explanation of the drawing]

Figure 1 is a front view showing the structural outline of the welding robot, Figures 2 (a) and (b) are mechanical diagrams explaining positional deviations that occur during conventional welding operations, and Figure 3 is a diagram showing the conventional welding operation. FIG. 4 is a graph showing the pressurizing process, and is a timing chart of the welding action according to the present invention. 30... Robot base, 31... Robot torso, 32... Robot upper arm, 33... Robot forearm, 34... Robot wrist, 41... Stand gun, 42... Welding electrode , 43... fixed electrode, W... object to be welded (welding target).

Claims (1)

[Scope of Claims] 1. A plurality of robot arms pivotally connected via a plurality of joints, and a robot arm connected to the frontmost arm of the plurality of robot arms and capable of rotating around at least two axes orthogonal to each other. Pressure welding is performed at each target position of a fixedly installed welding target by a welding robot comprising a robot wrist with a degree of freedom of movement and a pressure welding tool attached to the tip of the robot wrist. At this time, the electrode tip of the pressure welding tool is pressed against the target position of the welding object using a pressure force, and the displacement of the electrode tip of the pressure welding tool due to a reaction force from the welding object against the pressure force is detected by the robot. A welding operation control method for a welding robot, characterized in that the welding operation is absorbed by only the angular displacement around one axis of the wrist, and the displacement movement of the electrode tip of the pressure welding tool is prevented. 2. When pressing the pressure welding tool to each target position of the welding target with pressure, reduce the control loop gain around a predetermined axis of the robot wrist almost immediately before, thereby reducing system rigidity. 2. The welding operation control method for a welding robot according to claim 1, wherein angular displacement of the robot wrist is caused by causing the welding robot to move.