JP2001105153A - Robot for spot welding - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a robot for spot welding with which a relative deviation in position between a work and a welding gun is prevented and the work is uniformly pressed. SOLUTION: The wrist 7 of a six-axis articulated type robot 2 is equipped with a welding gun 8. Each of axes J1 to J6 of the robot 2 is driven by a servo motor of M1 to M6 respectively, electric current value to each of the servo motors M1 to M6 is monitored with a monitoring means 23. When welding is performed, first, the welding gun 8 is positioned at a hitting point for welding of the work W, next, a movable electrode tip 13 is moved to the stationary electrode tip 14 side, and the work W is held between them. Then, if the positions of the welding gun 8 and the work W are relatively deviated, the work W is elastically deformed, and accordingly, the electric current values of the servo motors M1 to M6 vary. On the basis of the variation of the electric current values detected by the monitoring means 23, an evaluation value is calculated, and when the evaluation value is larger than the value A specified in advance, the hitting position for welding gun 8 is shifted to the moving direction of the movable electrode.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a spot welding robot which carries out spot welding by mounting a welding gun on a wrist of an articulated robot, for example.

[0002]

2. Description of the Related Art A welding gun mounted on the wrist of a robot includes a fixed electrode tip fixedly provided on the welding gun,
A movable electrode tip movably provided, the fixed electrode tip and the movable electrode tip hold a predetermined welding point of a workpiece to be welded with a predetermined pressing force, and spot welding is performed by flowing a welding current. Is performed.

[0003] The positions of the welding points of the welding gun are stored in advance for each of the welding points of the work. In the welding operation, the moving electrode and the fixed electrode are separated from each other and the welding gun is opened. The welding gun is positioned at the position of the welding point by the robot so that the welding point of the work is arranged therebetween.
At this time, the welding gun is positioned so that the fixed electrode abuts on the welding point of the work, that is, zero-touches. Thereafter, the movable electrode is advanced, and the welding point of the workpiece is sandwiched between both electrode tips to perform spot welding.

[0004] In addition, the positioning of the hit point of the welding gun is performed as follows.
By the online teaching, the fixed electrode tip is positioned so as to zero-touch the work, and the hitting position is taught.

[0005]

For example, during teaching, the electrode tip is displaced in the vertical direction with respect to the work, or the deflection of the gun arm at the time of pressurization causes the relative position of the electrode tip to be displaced with respect to the work. Then, the relative position between the workpiece and the electrode tip is shifted at the time of the repeat for reproducing the taught work operation program. When the work is pressed and sandwiched in such a state of being displaced, the work is elastically deformed, and a difference occurs in the pressing force between the fixed electrode tip and the movable electrode tip.

[0006] Even if the hitting position is accurately taught, if the work is displaced from a predetermined position during the repeat operation, the work is elastically deformed when the work is pressed and clamped by both electrodes. However, there is a difference in the applied pressure between the fixed electrode tip and the movable electrode tip.

If there is a difference in the pressing force of the electrode tip, the contact resistance between the electrode tip having a small pressing force and the work becomes larger than the contact resistance between the electrode tip having a large pressing force and the work, and the heat generated by the work when energized. The center shifts to the electrode tip side where the pressing force is small. As a result, when the center of heat generation is shifted from the joint surface of the two works, the shear welding strength of the works is reduced.

If the welding current is increased so that the shear welding strength does not decrease, the work on the electrode side, which is small due to the applied pressure, is excessively melted, causing problems such as spattering and welding of the electrode tip. Causes a decrease in

In order to prevent such a problem, it is necessary to pressurize the work evenly, and for that purpose, it is necessary to determine the hitting position of the welding gun so that the fixed electrode tip makes zero touch. For that purpose, for example, Japanese Patent Application Laid-Open No. 6-3976
A method is conceivable in which the control method described in Japanese Patent Publication No. 0 “Robot control device” is applied to a spot welding robot.

This publication describes a control method for moving the robot so that the external force is reduced when an external force is applied to the robot. Using this method, the control of the hitting position is performed such that the fixed electrode makes zero touch. A method of performing positioning is conceivable.

However, in the control method described in this publication, since the moving direction of the robot is not specified, when applied to a spot welding robot, the moving direction of the robot becomes unstable. It is not a valid solution.

An object of the present invention is to provide a spot welding robot capable of accurately positioning the hitting point of a welding gun so as to perform zero-touch and applying uniform pressure.

[0013]

According to a first aspect of the present invention, there is provided a fixed electrode tip and a movable electrode tip disposed opposite to the fixed electrode tip, the movable electrode tip being movable toward and away from the fixed electrode tip. A servo that drives each axis of the robot in a spot welding robot that has a welding gun on the wrist of the robot, advances the moving electrode tip, and sandwiches a predetermined welding point on the workpiece with both electrode tips to perform spot welding. Monitoring means for monitoring the current value supplied to the motor; storage means for storing a welding point position, which is the position of the welding gun when welding the welding point of the workpiece; and positioning the welding gun at the welding point position by a robot. When the workpiece is sandwiched between the two electrode tips, the change in the current value of the servomotor when the electrode tip contacts the workpiece and the workpiece elastically deforms. Calculating means for calculating an evaluation value based on the detected value by the monitoring means, and when the calculated evaluation value is larger than a predetermined value, a dot is applied along the moving direction of the moving electrode and on the side where the evaluation value decreases. A spot welding robot characterized by shifting the position.

According to the present invention, a welding gun is provided at the wrist of the robot, and the welding gun is positioned by the robot at a hitting position stored in advance. For example, when the workpiece is relatively displaced toward the movable electrode, the workpiece is elastically deformed toward the fixed electrode when the movable electrode tip is advanced to clamp the workpiece. The elastic restoring force due to the elastic deformation of the workpiece acts as an external force on the robot via the moving electrode tip and the welding gun.

Each axis of the robot is controlled by a servomotor. When an external force acts on the robot when it is moved to a designated position, such as when it comes into contact with an object to be welded, the robot is moved to the designated position. The servo motor current value changes. The current value of the servomotor is monitored by the monitoring means, and an evaluation value is calculated by the calculating means based on a change in the current value of the servomotor detected by the monitoring means.

In the case where the fixed electrode tip makes zero touch on the workpiece, when the welding gun is positioned at the hitting position,
And when the moving electrode tip is advanced and the workpiece is clamped, the workpiece does not deform and the current value of the servo motor does not change, but the workpiece is relatively displaced as described above. When the welding gun is positioned or when the workpiece is sandwiched, the workpiece is deformed, and the current value of the servomotor changes.

In the present invention, an evaluation value is calculated based on the change in the current value, and when the calculated evaluation value is larger than a predetermined value, it is determined that the workpiece and the welding gun are relatively displaced. , Shift the position of the welding gun. As a result, the relative displacement between the welding gun and the workpiece is eliminated, and the workpiece can be evenly pressed by both electrodes.
The deterioration of welding quality can be prevented. Further, since the shift direction of the hitting point position is determined in the moving direction of the moving electrode tip, it is possible to prevent the moving direction from becoming unstable as in the above-described related art. At the time of welding, the welding gun is arranged so that the moving direction of the electrode and the welding surface are perpendicular to each other, and the effect of the relative displacement is most apparent in the displacement of the moving electrode in the moving direction. Therefore, it is most preferable to set the shift direction to the direction in which the electrodes move.

According to a second aspect of the present invention, the robot is
A multi-joint robot that controls each joint by a corresponding servo motor, and a change in current value of the servo motor based on which an evaluation value is calculated is selected based on a posture of the robot at the time of welding. .

According to the present invention, the robot is an articulated robot, and its posture is controlled by a plurality of servomotors. The posture of the robot at the time of welding is different, and the servomotor whose current value changes when the work to be welded is elastically deformed differs according to the posture of the robot. Therefore, the relative displacement between the workpiece and the welding gun is calculated by appropriately selecting one or a plurality of servomotors having large changes in the current value and calculating the evaluation value in accordance with the posture of the robot at the time of welding. An evaluation value responsive to the above can be obtained.

According to the third aspect of the present invention, the change in the current value includes the average value of the current for a predetermined time before the elastic deformation of the workpiece and the average of the current for a predetermined time after the elastic deformation of the workpiece. It is characterized by the difference with the value.

According to the present invention, the change in the current is calculated as the difference between the average values of the respective predetermined times before and after the deformation of the work to be welded. Can be calculated.

According to a fourth aspect of the present invention, the evaluation value is a reaction force applied to the robot from the elastically deformed workpiece.

When the position and direction of each axis of the robot and the change of the motor current value of each axis are known, the reaction force applied to the robot from the elastically deformed workpiece can be calculated. As described above, the reaction force is calculated based on the change in the current value of each axis, and is one type of the evaluation value.
It can also be used as an evaluation value.

According to a fifth aspect of the present invention, the evaluation value is based on a value obtained by calculating a change in a current value for each servomotor and adding an absolute value of the change, or Is divided by the current value after the elastic deformation of the work to be welded and based on the value obtained by adding these absolute values, or for each servomotor, the change in the current value is calculated after the elastic deformation of the work to be welded. It is characterized in that it is based on a value obtained by dividing by the current value, squaring, and adding these.

According to the present invention, the change in the current value, that is, the difference between the current value before and after the deformation of the workpiece is calculated for each servomotor, and the evaluation value is calculated based on the sum of these values. By doing so, it is possible to calculate an evaluation value in consideration of the current values of a plurality of servomotors whose current values change,
It is possible to obtain an evaluation value satisfactorily responding to the relative displacement between the workpiece and the welding gun.

Further, by calculating the evaluation value by dividing the change in the current value by the current value after deformation of the workpiece, the rate of change in the current value can be obtained, and the relative displacement can be more appropriately determined. Can be evaluated. Further, the change in the current value may be divided by the current value after the deformation of the work to be welded, and this value may be squared to calculate the evaluation value.

According to a sixth aspect of the present invention, the shift amount of the hitting position is a predetermined fixed amount.

According to the present invention, when the calculated evaluation value is larger than the predetermined value, the welding gun is shifted by a predetermined amount. By determining the shift amount in advance in this way, the relative displacement between the workpiece and the welding gun can be easily corrected.

According to a seventh aspect of the present invention, the shift of the hit point position is repeated until the calculated evaluation value becomes smaller than a predetermined value.

According to the present invention, the shift of the welding gun is repeated until the evaluation value becomes smaller than a predetermined value, so that the relative displacement between the workpiece and the welding gun can be reliably set within a predetermined tolerance. It can fit within the range.

The present invention according to claim 8 is characterized in that, based on the relationship between the shift amount obtained in advance and the evaluation value, the shift amount at the hit point where the evaluation value is equal to or less than a predetermined value is calculated.

According to the present invention, for example, the relationship between the thickness of the workpiece to be welded and the evaluation value, the relationship between the shift amount of the welding gun and the evaluation value, and the like are determined in advance by experiments, or the actual The relationship between the shift amount and the evaluation value is obtained in the work. Then, based on the calculated evaluation value and the relationship between the shift amount and the evaluation value obtained in advance, a shift amount for reducing the evaluation value to a predetermined value or less is derived. In this way, it is possible to easily obtain the shift amount at which the evaluation value becomes equal to or less than the predetermined value without performing a complicated calculation, thereby keeping the relative shift amount within an allowable range in one shift. Can be.

According to a ninth aspect of the present invention, when the work operation program is taught, the hitting position is a previously stored teaching position, and when the calculated evaluation value is larger than a predetermined value, the teaching position is shifted. The teaching position is corrected.

The method of shifting the welding gun based on a change in the current value of the servomotor and correcting the relative displacement between the welding gun and the workpiece can be applied when teaching the spot welding robot. . That is, when the evaluation value is larger than a predetermined value, the teaching position stored first is shifted and rewritten to correct the teaching position.
Thereby, the teaching of the welding point in the moving direction of the moving electrode (the direction perpendicular to the welding surface of the workpiece) can be performed with high accuracy. By teaching the hitting position with high precision in this manner, it is possible to press evenly with both electrode tips during welding, and to improve the welding quality.

According to a tenth aspect of the present invention, when reproducing the work operation program, if the calculated evaluation value is larger than a predetermined value, the welding position of the welding gun is shifted to perform spot welding. .

The above-described method of shifting the welding gun based on a change in the current value of the servomotor to correct the relative displacement between the welding gun and the workpiece is reproduced by reproducing the work operation program of the robot. It can also be applied at the time of repeating work. That is, an evaluation value is calculated for each welding point, and when the evaluation value is larger than a predetermined value, the welding position is shifted to perform welding. Accordingly, even when the workpiece is displaced, the relative displacement between the workpiece and the welding gun can be eliminated, and spot welding can be performed with uniform pressing.

According to the eleventh aspect of the present invention, when a plurality of welding points are present in the same workpiece, only the first welding point among the plurality of welding points is evaluated, and the evaluation value is calculated. The spot position is shifted when the value is larger than a predetermined value, and the evaluation value is not calculated for the welding points after the first time, and the spot position is shifted by the same amount as the shift amount of the first welding point to perform spot welding. And

According to the present invention, when a plurality of welding points are present on an object to be welded having the same shape, for example, when an object to be horizontally arranged is displaced in a vertical direction, All the welding points are shifted in the vertical direction. In such a case, in the present invention, the evaluation value is calculated by sandwiching the work to be welded between both electrodes only at the first welding point, and when the evaluation value is larger than a predetermined value, the position of the welding point is shifted. Then, at the next welding point on the same workpiece, the evaluation value is not calculated, and the spot position is shifted by the same amount as the initial shift amount to perform spot welding. In this way, when the workpiece is displaced, the relative displacement of all the welding points is efficiently corrected,
The welding operation can be performed continuously.

According to a twelfth aspect of the present invention, when a plurality of workpieces having the same shape are sequentially conveyed and a plurality of workpieces having the same shape are welded, only the first welding point of the workpiece is provided for each welding point. An evaluation value is calculated, and when the evaluation value is larger than a predetermined value, the hitting position is shifted for each welding hitting point.

According to the present invention, when the workpieces having the same shape are sequentially conveyed and the welding operation is sequentially performed, for example, when the welding points are taught to be relatively displaced, the conveyed workpieces are conveyed. All the corresponding welding points of the workpiece will be shifted. In such a case, in the present invention, an evaluation value is calculated by sandwiching the work to be welded only at the first welding point of the work to be welded, and the tapped position taught based on the calculated evaluation value is shifted, that is, the taught position. Shift. As a result, at the corresponding welding point of the work to be welded from the next time on,
Since the teaching position is corrected, it is possible to correct the relative displacement between the workpiece and the welding gun without newly calculating an evaluation value.

[0041]

FIG. 1 is a perspective view showing the appearance of a spot welding robot 1 according to an embodiment of the present invention. The spot welding robot 1 is configured by equipping a wrist 7 of the robot 2 with a welding gun 8.

The robot 2 is a 6-axis vertical articulated robot and has a base 4 fixed to the floor, a lower arm 5, an upper arm 6, and a wrist 7. The lower arm 5 is attached to the base 4 so that the lower end can pivot about a vertical first axis J1 and can be displaced in the longitudinal direction about a horizontal second axis J2. The base end of the upper arm 6 is attached to the upper end of the lower arm 5 so as to be vertically displaceable about a horizontal third axis J3. A wrist 7 attached to the tip of the upper arm 6 is attached so as to be angularly displaceable around a fourth axis J4 parallel to the axis of the upper arm 6,
It is attached so as to be angularly displaceable about a fifth axis J5 perpendicular to the axis of the upper arm 6. The spot welding gun 8 attached to the wrist 7 is provided so as to be angularly displaceable about a sixth axis J6 perpendicular to the fifth axis.

The spot welding gun 8 has a base 9 attached to the wrist 7 and a U-shaped gun arm 12 fixed to a lower portion of the base 9, and a fixed electrode tip 14 is provided at an end of the gun arm 12. Fixed. The movable electrode tip 13 is opposed to the fixed electrode tip 14 and is movable in the direction of approaching / separating from the fixed electrode tip 14.
Is provided. The electrode tips 13 and 14 are rod-shaped and arranged coaxially with each other. The moving electrode tip 13 is driven by a welding servomotor Mw provided on the welding gun 8. In addition, each joint axis J1 of the 6-axis articulated robot 2
J6 is driven by servo motors M1 to M6, respectively.

Next, a control configuration of the welding robot 1 will be described. FIG. 2 is a block diagram showing a control configuration of the servomotor M1 of the first axis J1. The other servo motors M2 to M6, Mw have the same configuration as the servo motor M1, and are not shown.

The storage means 22 stores the teaching data of the welding robot 1. The teaching data includes a work operation program for sequentially spot-welding each welding point of the workpiece W to be welded, and a welding point position which is a position and a posture of the welding gun 8 when welding each welding point. The hitting position is given by a position in the absolute coordinate system, and the attitude is given by the inclination of the electrode (the moving direction of the movable electrode 13) during welding in the absolute coordinate system.

The position controller 20 changes the posture of the robot 2 by giving the angle command values P1 to P6 of the axes J1 to J6 to the servo controller 21 based on the teaching data.
The welding gun 8 is positioned at the designated hitting position. The servo controller 21 supplies current values I1 to I6 corresponding to the angle command values P1 to P6 to the servo motors M1 to M6 so that the axes J1 to J6 have the commanded angles.

Similarly, when a movement command value Pw to the moving electrode chip 13 is given from the position controller 20 to the servo controller 21, the servo controller 21 applies a current Iw corresponding to the movement command value Pw to the welding servo motor Mw. give. Thus, the work W is sandwiched and pressed between the electrode chips 13 and 14.

An encoder E1 is attached to the servomotor M1, and a rotation angle of the servomotor M1 around the first axis J1 is detected by the encoder E1. The detected encoder value is returned to the servo controller 21.
The servo controller 21 performs feedback control of the servo motor M1 based on the angle command value P1 from the position controller 20 and the encoder value from the encoder E1 so that the first axis J1 is at the designated angular position. Such a configuration is the same for the servo motors M2 to M6 and Mw.

At the time of teaching to teach the work operation to the welding robot 1, the absolute coordinates of the hitting position and the inclination of the electrode are stored in the storage means 22 as the teaching position based on the encoder values of the axes J1 to J6 of the robot 2. You.

Since the servomotors M1 to M6 and Mw are feedback-controlled, when the welding robot 1 is moved to a designated position, for example, when the posture of the robot 2 is changed or the moving electrode 13 is advanced. When an external force acts on the robot 1 due to contact with the work W or the like, the current supplied to each of the servomotors M1 to M6 and Mw is increased or decreased so as to move to a designated position against the external force. The current values of these servo motors M1 to M6, Mw are detected and monitored by the monitoring means 23.

Next, an outline of spot welding performed by the welding robot 1 will be described. First, the welding gun 8 is opened by moving the movable electrode 13 away from the fixed electrode 14, and the welding gun 8 is moved to the hitting position. At this time, the fixed electrode 14 of the welding gun 8 is positioned so as to abut on the welding point of the work W, that is, zero touch. Further, the posture of the welding gun 8 at this time is arranged such that the inclination of the electrode (the moving direction of the moving electrode 13) is perpendicular to the welding surface of the workpiece W.

Next, the movable electrode tip 13 is moved to the fixed electrode tip 14 side, the welding gun 8 is closed, the welding point of the work W is sandwiched between the two electrode tips 13 and 14, and a predetermined pressing force is applied to the welding tip. Electric current is applied to perform spot welding.

When spot welding is performed as described above,
The work W is sandwiched between the two electrode tips 13 and 14 and pressurized. When the work W and the welding gun 8 are displaced relative to each other in the direction in which the electrodes are inclined, the two electrode tips 1 and 2 are pressed.
The pressures of the pressures 3 and 14 are not equalized, thereby deteriorating the welding quality.

In view of this point, in the present invention, whether the welding gun 8 and the work W are relatively displaced based on the monitoring means 23 for monitoring the current values of the servomotors M1 to M6 and Mw. Is determined, and if the position is shifted, the hit position is shifted in the direction in which the electrode is inclined to eliminate the relative position shift.

That is, when the workpiece W and the welding gun 8 are relatively displaced, the robot 2 is moved to position the welding gun 8 at the hitting position, and the movable electrode tip 13 is moved to move the workpiece W. When sandwiched, the workpiece W is elastically deformed, whereby an external force acts on the robot 2. When an external force acts, as described above, the servo motors M1 to M6
Current value changes. The monitoring means 23 monitors the current values of the respective servo motors M1 to M6, and the calculating means 24 calculates an evaluation value based on a change in these current values. If the evaluation value is larger than a predetermined value A, It determines that the welding gun 8 and the workpiece are relatively displaced, and outputs a predetermined shift amount ΔL. Then, the hit point position is corrected by shifting the hit point position by the shift amount ΔL in a direction in which the evaluation value decreases in the direction in which the electrode tilts.

Such a correction of the hit point position is performed by correcting, that is, rewriting the hit point position, which is pre-stored teaching data, during the teaching of the welding robot 1. Also,
At the time of the repetition in which the work operation program is reproduced and the welding work is actually performed, since the work W is a temporary displacement such as being displaced, the teaching data is not rewritten, and the welding of the displaced work W is performed. As a correction only for the operation, the welding operation is performed by shifting the hitting position of the welding gun 8.

Next, correction of teaching data during teaching will be described.

In the teaching of the hitting position of the welding gun 8, the positioning in a plane parallel to the welding surface of the work W is relatively easy, and can be accurately taught by, for example, an operator or off-line teaching. In the direction perpendicular to the welding surface of W, that is, in the direction in which the electrode is inclined, the fixed electrode tip 14 must be positioned so as to zero-touch the work W as described above,
Have difficulty.

Therefore, the position of the hit point in a plane parallel to the welding surface is accurately taught in advance by an offline teach or an operator, and the positioning of the electrode in the tilt direction is roughly taught.
As described above, the hit point position is shifted in the direction in which the electrode is inclined, and the hit point position, which is the teaching position stored first, is corrected.

Next, a method of correcting the hit point position will be described in more detail with reference to FIGS. FIG. 3 is a flowchart showing a method of correcting the hitting point position, FIG. 4 is a time chart of the current value of the welding servomotor Mw, and FIG. 5 is a time chart of each current value of the servomotors M2, M3, M5. FIG. 6 is a view showing a state when the welding gun 8 is positioned.

In this example, it is assumed that the work W is disposed horizontally, and the electrode chips 13 and 14 sandwich the work W in the vertical direction. Further, it is assumed that the approximate hit point position of the teaching data stored first is shifted vertically downward relative to the work W, and the hit position stored first is indicated by a reference numeral Pf in FIG. Show.

Since the relative displacement between the workpiece W and the hit point occurs in the vertical direction, if the electrode is in the vertical direction, the external force acting on the robot 2 from the elastically deformed workpiece W is mainly 2nd axis J2, 3rd axis J3, 5th axis J5
Act on. Therefore, in this example, the monitoring unit 23
The current value monitored by the second axis J2, the third axis J3,
The servo motors M2, M3, and M5 of the fifth axis J5 are used, and the evaluation value is calculated based on the current values of these servo motors M2, M3, and M5.

First, in step a1, the welding gun 8 is opened, and then, in step a2, the welding gun 8 is moved to the strike point based on the teaching data. At this time, as shown in FIG. 6A, first, a sufficient predetermined distance D, for example, 5 mm, which does not come into contact even if the workpiece W and the hit point position are relatively displaced, is first stored. The welding gun 8 is arranged at a position where the fixed electrode tip 14 separates downward from the hit point position Pf. The time at this time is defined as t1.

Next, in step a3, as shown in FIG. 6B, the welding gun 8 is raised by the predetermined distance D, and the welding gun 8 is arranged at the hitting position Pf stored first. In this example, it is assumed that the workpiece W is relatively displaced upward, so when the welding gun 8 is positioned at the hitting position Pf based on the teaching data in step a3,
The fixed electrode chip 14 does not perform zero touch on the work W, and the work W is disposed above the fixed electrode chip 14.

At time t2, when the welding gun 8 is arranged at the hitting position based on the teaching data, the process proceeds to the next step a4, in which the movable electrode tip 13 is moved to the fixed electrode tip 14 side to close the welding gun 8, and the work gun is closed. Hold W.

As described above, the work W is connected to the fixed electrode 14.
At a position farther away than the time t2.
When the moving electrode tip 13 is lowered from
The moving electrode tip 13 comes into contact with the upper surface of the work W at the time. Further, as the moving electrode tip 13 moves downward, the work W is pushed down, and the work W is elastically deformed downward.

Therefore, as shown in FIG. 4, the current value of the welding motor Mw starts to increase from the time t3 when it comes into contact with the workpiece W. When the moving electrode tip 13 is further depressed,
The work W comes into contact with the fixed electrode tip 14 at time t4. When the pressing force is further applied, the current value of the welding motor Mw further increases, and when the predetermined pressing force is reached at time t5, the increase in the current value of the servo motor Mw stops.

When the welding robot 1 is in the posture shown in FIG. 1, the largest force among the servomotors M2, M3, M5 is the third one that supports the horizontally extending upper arm 6.
The servomotor M3 of the axis J3, the servomotor M2 of the second axis J2 of the lower arm 5 extending vertically, and the third are the servomotors M of the fifth axis J5 around the wrist.
5 Therefore, each servo motor M2, M3, M
5, the current value of the servo motor M3 is the largest, and the current supplied to the servo motor M5 is the smallest, as shown in FIG.

In this state, when the movable electrode tip 13 is moved downward and the movable electrode tip 13 contacts the work W, a force acts on the robot 2 in an upward direction. Since the servomotors M2, M3, and M5 are respectively feedback-controlled, when an upward external force acts on the robot 2, the servomotors M2, M3, and M5 act to lower the welding gun 8, respectively. That is, the current supplied to each of the servomotors M2, M3, and M5 is reduced, and the welding gun 8
Weaken the force to hold upwards. Therefore, as shown in FIG. 5, the current values of the servomotors M3, M2, and M5 decrease from time t3 when the movable electrode tip 13 contacts the work W to time t4 when the work W contacts the fixed electrode tip 14.

Next, an evaluation value is calculated from a change in the current value of each of the servomotors M2, M3, and M5 monitored in step a5 and a predetermined evaluation function.

Here, assuming that the current value of the ith axis before contact with the workpiece W is Ini and the current value of the ith axis after the end of pressurization is Ipi, the evaluation function is

[0072]

(Equation 1)

Is given by the following three equations. Note that K1i,
K2i and K3i are constants on the i-th axis in the evaluation functions (1) to (3), respectively.

The evaluation function (1) is the absolute value of the difference between the current value Ini of each axis i before the workpiece W abuts and the current value Ipi of each axis i after the pressurization is completed and the workpiece W elastically deforms. Constant K
The constant K1i is a constant determined for each axis. In addition, as described above, in the present embodiment, the evaluation value is not calculated based on the current values of all the servo motors M1 to M6. In the example shown in FIG. 1, the evaluation value is calculated using three current values of the third axis J3, the second axis J2, and the fifth axis J5.

The constant K1i is set so as to increase the constant K1i of the axis i where the influence of the relative displacement appears most, and so to speak, weighting is performed. This makes it possible to obtain an evaluation value that appropriately reflects the magnitude of the relative displacement. Here, the closer to the electrode chips 13 and 14, the greater the relationship between the change in the current value and the magnitude of the relative shift, so in the present embodiment, K1 ₂ <K1 ₃ <K1
_It is chosen to be ₅ .

As the evaluation function (2), an evaluation value is calculated based on a value obtained by dividing a difference between current values for each axis by a current value Ini after pressurization. This makes it possible to obtain an evaluation value in which the rate of change of the current value is considered in comparison with the evaluation function (1).

The evaluation function (3) is a value obtained by dividing the difference between the current values for each axis used in the evaluation function (2) by the current value after pressurization.
Square to calculate the evaluation value. The evaluation function is such a 3
The function is not limited to the above and may be any function whose evaluation value changes according to a change in the current value.

In this embodiment, the evaluation value is calculated using the evaluation function (2). Further, in the above description, it is assumed that the hitting position of the welding gun 8 is displaced downward relative to the workpiece W, so that the current value changes after the moving electrode tip 13 is lowered. However, even in the case where the hit point position is relatively displaced upward with respect to the workpiece W, the welding gun 8 is disposed below the welding position in the above-described step a2 as described above, and thereafter, When the welding gun 8 is raised and positioned at the hitting position, the fixed electrode tip 14 contacts the work W from below, and at this time, the current values of the servo motors M2, M3, and M5 change.

Therefore, the work W used for calculating the evaluation value
The current value Ini before deformation is the current value at time t1 when the welding gun 8 is arranged below the welding position or after that. The current value after the deformation of the work W is the current value at time t5 at the end of pressurization or after that. In the present embodiment, the current value Ini before deformation is an average current value after a predetermined time Δt (second) from the time t1, and the current value Ipi after the deformation is a predetermined time Δt (second) from the time t5. It is the average current value of the time of. This can prevent a change in the current value due to the influence of disturbance or the like.

Next, it is determined whether or not the evaluation value calculated in step a6 is larger than a predetermined value A. If the evaluation value is not larger than the predetermined value A, the welding point of the welding gun 8 based on the teaching data stored in advance is determined. It is determined that the relative displacement between the positions is small and within the allowable range, and step a7 is performed.
The teaching data is not corrected and the currently stored teaching data is determined as the hitting position.

When the evaluation value is larger than the predetermined value A,
The hit point position is shifted by the shift amount ΔL in the pressing direction along the pressing direction and on the side where the evaluation value decreases, that is, upward in the present embodiment, and this position is stored in the storage means as a new hit point position. Is corrected. The shift amount ΔL is
In the present embodiment, the predetermined amount is assumed to be a predetermined amount.

Next, the process returns to step a2 again based on the corrected hitting position, and the same operation as described above is repeated.
That is, the welding gun 8 is positioned at the corrected hitting position.
Is closed, the evaluation value is calculated again, and the evaluation value is a predetermined value A
If it is larger, the hit point is shifted again to correct the previous teaching data. In this way, the process is repeated until the evaluation value becomes equal to or less than the predetermined value A. When the evaluation value becomes equal to or less than the predetermined value A, the process proceeds to step a7 to determine the hitting position. As a result, it is possible to ensure that the relative displacement of the hit points is within an allowable range.

The positions and directions of the axes J1 to J6 of the robot 2 can be easily derived, and the monitoring means 23 monitors changes in the current values of the servo motors M1 to M6 due to the deformation of the work W. Therefore, the welding robot 1 is converted from the work W elastically deformed based on these.
Can be calculated.

Therefore, the evaluation functions (1) to
The above reaction force is calculated instead of the evaluation value based on (3),
The hit point position may be corrected by repeating the shift of the hit point position so that the reaction force becomes zero or less than a predetermined value.

Further, for example, by an experiment in advance,
A relationship between the evaluation value and the shift amount ΔL for making the evaluation value equal to or less than the predetermined value A is obtained for each thickness and type of the workpiece W to be welded, and a table is created and stored in the storage unit 22. The shift amount ΔL at which the evaluation value becomes equal to or less than the predetermined value A may be derived from the stored table based on the work W and the calculated evaluation value. Thus, it is possible to obtain a shift amount ΔL such that the evaluation value becomes equal to or less than the predetermined value A in one shift, and it is not necessary to repeat the shift for each welding point.

In addition, the relationship between the shift amount ΔL and the evaluation value is obtained in an actual work at the time of teaching, not in an experiment, and based on this, the evaluation value is set to a predetermined value A or less in one shift. The following shift amount ΔL may be derived.

The method of shifting the welding position based on the change in the current value described above can be applied not only to the correction of the teaching data at the time of teaching, but also to the case of correcting the welding position at the time of repeat.

For example, even if the hitting position of the teaching data is accurate, when the workpiece W is relatively displaced during the repeat operation, the workpiece W and the welding gun 8 are relatively displaced. . In such a case, the above-described method of correcting the hit point position can be applied. That is, at the time of the repeat, the welding gun is closed at the welding point to calculate the evaluation value and the shift amount, and the welding position is corrected by the calculated shift amount ΔL, that is, the welding gun 8 is shifted to perform the spot welding. At the time of the repeat, a change in the deviation of the positioning of the work W is assumed. Therefore, basically, the teaching data is not rewritten, and only the corresponding welding point is corrected.

By correcting the relative positional deviation in this way, the difference in the pressing force due to the positional deviation of the work W can be eliminated, the pressure can be evenly applied, and the deterioration of the welding quality can be prevented.

At the time of the repeat, as described above, the welding gun 8 is closed and the evaluation value is calculated for each welding point, and the position of the welding point is not corrected. May be used to correct the hitting point position, and from the next welding point, the correction may be made in the same manner as the correction at the first welding point.

For example, in the case where the workpieces W are arranged horizontally and there are a plurality of welding points on the same workpiece W, if the workpieces W are displaced in the vertical direction, all the locations of the welding points are set in the vertical direction. Need to shift. In such a case, for example, the work W is clamped only at the first welding point, the evaluation value is calculated, and the shift amount ΔL is calculated.
, And the spot welding position is shifted by the shift amount ΔL to perform spot welding. Then, at the subsequent welding points of the workpiece W arranged with the positional shift, the evaluation value is not calculated, and the spot welding position is shifted by the shift amount ΔL to perform the spot welding similarly to the correction of the initial welding point position. Do. This makes it possible to correct all welding points shifted by ΔL.

As described above, when the workpiece W is relatively displaced, it is not necessary to calculate the shift amount at all the welding points, so that the calculation time can be reduced and the working efficiency can be improved. it can.

When works W of the same shape are sequentially conveyed by, for example, a shuttle conveyor and are sequentially welded, for example, when teaching data is relatively shifted, the corresponding welding of each work W is performed. It is necessary to shift the hit point position by the same shift amount at all hit points.

In such a case, the shift amount ΔL is calculated only at the welding point of the first workpiece W, and the teaching data of the welding point is corrected. That is, in the following work W, since the teaching data is corrected at the corresponding welding point, it is not necessary to calculate the evaluation value, and the relative displacement between the work W and the welding gun 8 can be prevented. Thus, work efficiency can be improved.

In this embodiment, the work W is arranged horizontally, so that the servomotors for monitoring the welding current are only M2, M3 and M5.
Alternatively, according to the posture of the robot 2, a servo motor to be monitored may be appropriately selected by a predetermined number, for example, from the motor having the largest rate of change of the current value. A wrist shaft having a small frictional force may be preferentially selected, or the evaluation value may be calculated based on all the current values of the six servo motors M1 to M6.

[0096]

As described above, according to the present invention, when the welding gun and the workpiece are relatively displaced, the relative displacement can be eliminated by shifting the hitting position. As a result, uniform pressing can be performed, and a decrease in welding quality can be prevented.

Such a correction of the hitting position is performed when the hitting position, which is the teaching position stored first at the time of teaching, is corrected, and when the work to be welded is displaced during repeat. Can be applied to both the correction of the hitting point position in.

Further, the shift direction of the hitting point position is defined by the moving direction of the moving electrode, thereby preventing the shift direction from becoming unstable.

[Brief description of the drawings]

FIG. 1 is a perspective view showing an appearance of a spot welding robot 1 according to an embodiment of the present invention.

FIG. 2 is a block diagram showing a control configuration of a servo motor M1.

FIG. 3 is a flowchart illustrating a method of correcting teaching data.

FIG. 4 is a time chart of a current value of a welding motor Mw.

FIG. 5 is a time chart showing current values of servo motors M3, M2, M5.

FIG. 6 is a diagram showing a state when the welding gun 8 is positioned at a hitting position.

[Explanation of symbols]

 Reference Signs List 1 spot welding robot 2 6-axis articulated robot 4 base 5 lower arm 6 upper arm 8 welding gun 13 moving electrode tip 14 fixed electrode tip 20 control device 21 robot controller 22 storage means 23 monitoring means 24 arithmetic means M1 to M6 Mw servo motor

Claims (12)

[Claims] 1. A mobile electrode tip comprising: a fixed electrode tip; and a welding gun having a movable electrode tip disposed opposite to the fixed electrode tip and having a movable electrode tip movable toward and away from the fixed electrode tip. Move forward,
In a spot welding robot for performing spot welding by sandwiching a predetermined welding point of a workpiece with both electrode tips, monitoring means for monitoring a current value supplied to a servomotor driving each axis of the robot, Storage means for storing the welding point position, which is the position of the welding gun when welding the welding points, and positioning the welding gun at the welding point position by a robot so that when the workpiece is sandwiched by both electrode tips, the electrode tip is covered. Abutting on the workpiece, the change in the current value of the servomotor when the workpiece is elastically deformed is detected by the monitoring means,
And calculating means for calculating an evaluation value based on the calculated value. When the calculated evaluation value is larger than a predetermined value, the hit point position is shifted along the moving direction of the moving electrode to a side where the evaluation value decreases. And a spot welding robot. 2. The robot according to claim 1, wherein the robot is an articulated robot that controls each joint by a corresponding servo motor, and a change in current value of the servo motor based on which an evaluation value is calculated is based on a posture of the robot at the time of welding. 2. The spot welding robot according to claim 1, wherein the spot welding robot is selected from the following. 3. The change in the current value is a difference between an average value of a current for a predetermined time before elastic deformation of the workpiece and an average value of a current for a predetermined time after elastic deformation of the workpiece. The spot welding robot according to claim 1, wherein: 4. The spot welding robot according to claim 1, wherein the evaluation value is a reaction force applied to the robot from an elastically deformed workpiece. 5. The evaluation value is based on a value obtained by calculating a change in current value for each servomotor and adding an absolute value thereof, or a change in current value for each servomotor is calculated based on the change in current value. Divided by the elastically deformed current value, and based on the value obtained by adding these absolute values, or for each servo motor, divide the current value change by the elastically deformed current value of the workpiece. The welding robot according to any one of claims 1 to 3, wherein the welding robot is based on a value obtained by squaring and adding them. 6. The spot welding robot according to claim 1, wherein the shift amount of the hitting position is a predetermined fixed amount. 7. The spot welding robot according to claim 1, wherein the shift of the hit point position is repeated until the calculated evaluation value becomes smaller than a predetermined value. 8. A shift amount at a hit point where an evaluation value is equal to or less than a predetermined value is calculated based on a relationship between a shift amount and an evaluation value obtained in advance. The spot welding robot according to one. 9. At the time of teaching a work operation program,
9. The teaching position according to claim 1, wherein the hitting position is a teaching position stored in advance, and when the calculated evaluation value is larger than a predetermined value, the teaching position is shifted and the teaching position is corrected. The spot welding robot described in (1). 10. The method according to claim 1, wherein, when reproducing the work operation program, when the calculated evaluation value is larger than a predetermined value, the welding position of the welding gun is shifted to perform spot welding. The spot welding robot according to any one of the above. 11. In the case where a plurality of welding points are present in the same workpiece, an evaluation value is calculated only for an initial welding point among the plurality of welding points, and when the evaluation value is larger than a predetermined value, the evaluation point is calculated. 11. The method according to claim 10, wherein the position is shifted, and the evaluation value is not calculated for the welding point after the first welding, and the welding position is shifted by the same amount as the shift amount of the first welding point to perform the spot welding. Spot welding robot. 12. An object to be welded having the same shape is sequentially conveyed,
When welding a plurality of workpieces having the same shape, an evaluation value is calculated for each welding point only at the first welding point of the workpiece, and when the evaluation value is larger than a predetermined value, the welding position is determined for each welding point. The spot welding robot according to claim 10, wherein the robot shifts.