JPH11104849A - Welding control device for spot welding equipment - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a welding control device for spot welding equipment which prevents a weld defect caused by an initial clearance at low cost. SOLUTION: When there is no (or small) clearance between members to be welded, they are welded in a short time by a usual single-stage energization. When a clearance between members to be welded is large, it is automatically detected and compatibility between the contact surfaces of the members to be welded is previously improved by preliminary energization of two-stage energization before the members to be welded are jointed by the main energization.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a welding control apparatus for a spot welding apparatus which can prevent welding defects due to an initial gap at low cost.

[0002]

2. Description of the Related Art There are two types of energization methods for spot welding: a one-stage energization method in which energization is performed only once (see FIG. 6) and a two-stage energization method in which preliminary energization is performed before main energization (see FIG. 7).
Normally, a single-stage energization system is used, which consumes less power and has a shorter cycle time than the two-stage energization system.

[0003]

However, in a normal single-stage energizing method, if there is a large gap between the plates (weld materials) to be joined, the contact area between the materials to be welded becomes small and the current density decreases. Because it is too large, there is a possibility that spatters and bubbles will be generated. Further, when the plate thickness is large, there is a possibility that the joining strength of the workpiece is insufficient. However, as a matter of course, this energization method has the advantages that it does not require complicated control, has a shorter cycle time, and requires less power consumption than the two-stage energization method.

On the other hand, in the two-stage energization method, the main energization is performed after the preliminary energization is performed to improve the conformity of the contact surfaces between the plates, so that even if there is a gap in the workpiece, there is a gap. While the above welding defects (insufficient bonding strength, spatter generation, bubble generation, etc.) can be prevented, the current is conducted twice with a suitable cool time,
The timing control of energization is complicated, and the cycle time and power consumption may be increased.

For example, taking spot welding of a body of an automobile as an example, recently, as a safety measure in the event of a collision, the thickness of each piece to be welded is increased, or a two-piece set is used as a three-piece set. Attempts have been made to increase the thickness of the body by, for example, reducing the thickness of the body. In this case, it is required to ensure sufficient welding quality at the lowest possible cost. In particular, if there is a gap in the plate set to be welded, it is likely to cause poor welding. Therefore, when thickening the plate thickness, it is necessary to further consider this point.

The present invention has been made in view of the above problems in spot welding, and has as its object to provide a welding control device of a spot welding device which can prevent welding failure due to an initial gap at low cost. And

[0007]

In order to achieve the above object, the invention according to claim 1 is directed to a spot welding apparatus for welding a local portion by pressing an object to be welded by an electrode tip and applying an electric current to the object for an appropriate time. In the welding control device, an electrode position detecting means for detecting a position of the electrode tip during a pressing operation, a pressing force detecting means for detecting a pressing force applied to the workpiece by the electrode tip, and the pressing force detecting means A gap estimating means for estimating the size of the gap of the workpiece at the hitting point from the output of the electrode position detecting means at the time when the output reaches a predetermined value and the reference total plate thickness at the hitting point set in advance. The output of the gap estimating means is compared with a preset reference gap. If the estimated gap is larger than the reference gap, the energization mode is set to the two-stage energization method, If during or less is characterized by having a energization method setting means for setting the energization mode to stage energization method.

According to the structure of the present invention, the electrode position detecting means detects the position of the electrode tip during the pressing operation, and the pressing force detecting means detects the pressing force applied to the workpiece by the electrode tip. The gap estimating means is configured to calculate, based on the output (the position of the electrode tip) of the electrode position detecting means at the time when the output (the pressing force) of the pressing force detecting means reaches a predetermined value, and the reference total plate thickness at the preset hitting point position, Estimate the size of the gap between the workpieces to be hit. The energization method setting means compares the output (estimated gap) of the gap estimating means with a preset reference gap, and, if the estimated gap is larger than the reference gap, performs preliminary energization in advance to perform a preliminary energization on the contact surface of the workpiece. Set the energization mode to the two-stage energization method in order to perform the main energization after improving the familiarity. If the estimated gap is less than the reference gap, it is determined that there is no gap that requires two-stage energization. Then, the energization mode is set to the normal one-stage energization method.

According to a second aspect of the present invention, there is provided a welding control apparatus for a spot welding apparatus for welding a local portion by pressing an object to be welded by applying a proper time to the object by moving an electrode tip by rotating a servomotor. Electrode position detecting means for detecting the position of the electrode tip at the time of pressure operation, motor current detecting means for detecting a motor current of the servomotor, and a work to be welded by the electrode tip based on a change in output of the motor current detecting means. From the output of the electrode position detecting means at the time when it is detected that a predetermined pressing force is applied to the workpiece and the reference total thickness at the preset hitting position, the size of the gap of the workpiece at the hitting position is estimated. The gap estimating means to be compared with the output of the gap estimating means is compared with a preset reference gap. Set de the two-stage energization method, if the estimated clearance is less than the reference gap and having a current supply method setting means for setting the energization mode to stage energization method.

According to the structure of the present invention, the electrode position detecting means detects the position of the electrode tip during the pressing operation, and the motor current detecting means detects the motor current of the servo motor. The gap estimating means is set in advance to the output of the electrode position detecting means (position of the electrode tip) at the time when it is detected that a predetermined pressure is applied to the workpiece by the electrode tip based on a change in the output of the motor current detecting means. The size of the gap between the workpieces at the hitting point is estimated from the reference total plate thickness at the hitting point. The energization method setting means compares the output (estimated gap) of the gap estimating means with a preset reference gap. In order to perform the main energization after improving the familiarity, set the energization mode to the two-stage energization method, and if the estimated gap is less than the reference gap,
It is determined that there is no gap enough to require two-stage energization, and the energization mode is set to the normal one-stage energization method.

According to a third aspect of the present invention, there is provided a welding control device for a spot welding apparatus for welding a local portion by pressurizing an object to be welded with an electrode tip and applying a current to the object for an appropriate time. Electrode position detecting means for detecting a position, pressing force detecting means for detecting a pressing force applied to the workpiece by the electrode tip, and detecting the electrode position when the output of the pressing force detecting means reaches a predetermined value A gap estimating means for estimating the size of the gap between the workpieces at the hitting point from the output of the means and the preset total thickness of the hitting point, and a preset reference gap for the output of the gap estimating means. If the estimated gap is larger than the reference gap, the welding current is set to correspond to the estimated gap in the optimal welding current value set in advance for each size of the gap. And having a welding current changing means for changing the optimum welding current value.

According to the structure of the present invention, the electrode position detecting means detects the position of the electrode tip during the pressing operation, and the pressing force detecting means detects the pressing force applied to the workpiece by the electrode tip. The gap estimating means is configured to calculate, based on the output (the position of the electrode tip) of the electrode position detecting means at the time when the output (the pressing force) of the pressing force detecting means reaches a predetermined value, and the reference total plate thickness at the preset hitting point position, Estimate the size of the gap between the workpieces to be hit. The welding current changing means compares the output (estimated gap) of the gap estimating means with a preset reference gap, and when the estimated gap is larger than the reference gap, for example, determines the welding current in the one-stage energization by the size of the gap in advance. The optimum welding current value corresponding to the estimated gap in the optimum welding current value set for each is changed.

According to a fourth aspect of the present invention, there is provided a welding control device for a spot welding apparatus for welding a local part by pressing an object to be welded by applying a proper time to the object by moving an electrode tip by rotating a servomotor. Electrode position detecting means for detecting the position of the electrode tip at the time of pressure operation, motor current detecting means for detecting a motor current of the servomotor, and a work to be welded by the electrode tip based on a change in output of the motor current detecting means. From the output of the electrode position detecting means at the time when it is detected that a predetermined pressing force is applied to the workpiece and the reference total thickness at the preset hitting position, the size of the gap of the workpiece at the hitting position is estimated. The gap estimating means to be compared with the output of the gap estimating means is compared with a preset reference gap, and if the estimated gap is larger than the reference gap, welding is performed. And having a welding current changing means for changing the optimum welding current value corresponding to the estimated gap in the flow advance gap size set optimum welding current value for each.

According to the structure of the present invention, the electrode position detecting means detects the position of the electrode tip during the pressing operation, and the motor current detecting means detects the motor current of the servo motor. The gap estimating means is set in advance to the output of the electrode position detecting means (position of the electrode tip) at the time when it is detected that a predetermined pressure is applied to the workpiece by the electrode tip based on a change in the output of the motor current detecting means. The size of the gap between the workpieces at the hitting point is estimated from the reference total plate thickness at the hitting point. The welding current changing means compares the output (estimated gap) of the gap estimating means with a preset reference gap. If the estimated gap is larger than the reference gap, for example, the welding current in the welding current is determined in advance by the size of the gap. The optimum welding current value corresponding to the estimated gap in the optimum welding current value set for each is changed.

[0015]

Therefore, according to the first or second aspect of the present invention, when there is no (or small) gap between the workpieces, the normal one-stage energization is performed, and only when the gap between the workpieces is large, the second energization is performed. Since stepwise energization is performed, welding failure due to an initial gap can be prevented, and power consumption and cycle time can be reduced as compared with the case where two-stage energization is performed each time.

According to the third or fourth aspect of the present invention, when the gap between the objects to be welded is large, the welding current is changed to an optimum welding current value corresponding to the size of the estimated gap to perform predetermined welding. For example, even if there is a gap, single-stage energization may be used as long as the welding current is increased, that is, when there is no problem in quality, without increasing the cycle time and power consumption without performing complicated two-stage energization. It is possible to prevent poor welding when there is a gap.

[0017]

Embodiments of the present invention will be described below with reference to the drawings. Here, an example in which the present invention is applied to spot welding of an automobile body will be described.

First Embodiment FIG. 1 is a schematic diagram showing a main part of an overall configuration of a spot welding apparatus to which the present invention is applied. The spot welding apparatus includes a welding gun, a welding robot (both not shown), a control unit 30, and the like. The welding gun is mounted on the wrist of the welding robot. The opening / pressing operation of the welding gun and the movement of the welding robot are controlled by the control means 30.

The welding gun has a frame (not shown), and a pair of electrode tips 11 and 12 are mounted on the frame via a holder (not shown) so as to face each other. A servo motor 13 is mounted on the upper part of the frame. The electrode tip 12 is configured to move in the axial direction by rotation of the servomotor 13 via an appropriate transmission mechanism 14. The transmission mechanism 14 includes, for example, a shaft coupling, a ball screw, a bearing, a nut, and the like. The position of the electrode tip 12 can be detected by an encoder 15 as an electrode position detecting means attached to the servomotor 13.

In this embodiment, the servo motor 1
Although the position of the electrode tip 12 is indirectly detected by the amount of rotation of 3, the electrode position detecting means for directly detecting the moving distance of the electrode tip 12 may be used.

The electrode tip 12 is electrically connected to a welding transformer (not shown) fixed to a welding robot, for example, via a welding cable (not shown). The welding transformer is connected to a controller 4 described later having a timer function.
0 is connected to a welding power source (not shown). The controller 40 is connected to the control means 30.

As described above, the movement of the welding robot is controlled by the control means 30. The control means 30 teaches a plurality of welding positions, and the welding robot positions the welding gun at the taught welding position. It has become.
The position information of each axis of the welding robot is input to the control means 30, and based on this, the position information of the welding gun moved by the welding robot is grasped. Control means 3
To 0, the position information of the electrode tip 12 is input from the encoder 15. Based on the position information from the encoder 15, the control means 30 controls the rotation of the servomotor 13 and positions the electrode tip 12. Electrode tip 1
The positioning of 2 is completed when it is detected that a predetermined pressure is applied to the workpiece 20 by the electrode tips 11 and 12. The application of the predetermined pressure to the workpiece 20 is detected by monitoring the motor current of the servomotor 13 as described later.

The overall flow of the spot welding operation according to the above configuration is as follows. In an automobile assembly line,
The vehicle body is intermittently conveyed by the conveyor, and spot welding is performed when the vehicle body stops. When the vehicle body is positioned at a predetermined position, the welding robot is moved toward the welding position by the welding robot in the standby state. When the welding gun is positioned at a predetermined position, the electrode tip 12 is moved by the rotation of the servomotor 13, and the workpiece 20 is pressed by the electrode tips 11, 12. When the applied pressure reaches a predetermined value, the electrode tip 1
A welding current is applied for an appropriate time between the first and the second, and the workpiece 20 is welded.
Is performed. That is, welding of the workpiece 20 is performed while controlling the current value, the conduction time, and the pressing force, which are the three major conditions of spot welding.

FIG. 2 is a block diagram showing a schematic configuration of the welding control device of the spot welding device described above. here,
Only a configuration for realizing an energization control suitable for preventing poor welding due to an initial gap (insufficient bonding strength, generation of spatter, generation of bubbles, etc.) is shown.

The welding control apparatus includes a CPU 31 for performing various arithmetic processing, a motor current detecting means 32 for detecting a motor current of the servo motor 13, an electrode position detecting means 15,
And the controller 40. CP
U31 is a gap calculating unit 33 for estimating the size of the gap between the workpieces 20 at the hitting positions, and a total thickness storing unit 3 for storing data of a preset reference total thickness for each hitting position.
4. It has a gap determination unit 35 that determines the size of the estimated gap and sets the energization mode, and a reference value storage unit 36 that stores a reference value (reference gap) to be compared with the estimated gap.

More specifically, the gap calculating section 33 functions as a gap estimating means, and the change of the output of the motor current detecting means 32 causes the electrode tips 11 and 12 to change the work piece 20 to be welded.
The output (position information of the electrode tip 12) of the electrode position detecting means (encoder) 15 at the time when it is detected that a predetermined pressing force is applied to the electrode and the reference total of the hitting positions stored in the total thickness storage unit 34 It has a function of calculating the estimated gap of the workpiece 20 at the hitting point from the sheet thickness. The estimated gap is obtained by subtracting the reference total thickness from the measured total thickness (corresponding to the distance between the electrode tips 11 and 12) at the hitting position based on the actual position information of the electrode tips 12. The reference total plate thickness at the hitting point position on the workpiece 20 is grasped in advance by a combination of the plate thicknesses of the plurality of workpieces 21a and 21b.

The gap judging section 35 functions as an energizing method setting means, compares the estimated gap of the workpiece 20 at the hitting point with a preset reference gap, and if the estimated gap is larger than the reference gap, sets the energizing mode. Is set to the two-stage energization system, and when the estimated gap is smaller than the reference gap, the energization mode is set to the one-stage energization system.

The motor current detecting means 32 functions as a pressing force detecting means, and is constituted by, for example, a servo amplifier. The servo amplifier detects the current flowing through the servomotor of each axis of the welding robot, controls the load current of the servomotor based on the difference between the fed-back current value and the command value, and controls the load tip of the welding gun electrode tip 12. Is detected, and the motor current of the servo motor 13 is controlled based on the difference between the fed-back current value and the command value. When the electrode tips 11 and 12 come into contact with the workpiece 20 and pressurization is started, the motor current of the servo motor 13 greatly changes due to an increase in the load. It is possible to detect that a predetermined pressure is applied to 20.

In this embodiment, the servo motor 1
Although the pressing force applied to the workpiece 20 indirectly by the motor current of No. 3 is detected, a pressing force detecting means for directly detecting the pressing force using a pressure sensor or the like may be used.

The controller 40 is connected to the welding transformer as described above, and has a function of controlling welding current, welding time, pressurizing time, and the like. Controller 40
The instruction of the power supply mode to is output from the CPU 31 of the control means 30.

FIG. 3 is a flowchart showing the operation of the above-described welding control device.

In step S11, when the welding gun is positioned at a predetermined position, the electrode tip 12 is moved by the rotation of the servo motor 13, and the gap calculation unit 33 controls the motor current detection means (servo amplifier) 32 to perform servo control. The motor current of the motor 13 is monitored to detect that the positioning of the electrode chips 11 and 12 is completed. That is, the fact that the electrode tips 11 and 12 are in contact with the workpiece 20 and apply a predetermined pressure is detected by a change in the motor current. At this point, the rotation of the servo motor 13 is stopped.

In step S12, the electrode tips 11, 1
The position of the electrode tip 12 at the time when the positioning of the workpiece 2 is completed is detected by the electrode position detecting means (encoder) 15, and the gap calculating unit 33 calculates the position of the workpiece 2 based on the positional information.
The total plate thickness at the 0 point is measured. This measured total thickness is
This corresponds to the distance between the electrode tips 11 and 12 at that time.

In step S13, the gap calculating unit 33 subtracts the reference total thickness at the hitting point position stored in the total thickness storing unit 34 from the measured total thickness at the hitting position obtained in step S12. Thus, the estimated gap of the superimposed workpiece 20 at the hit point position is calculated.

In step S14, the gap determination section 35 determines whether the estimated gap obtained in step S13 is larger than the reference gap stored in the reference value storage section 36.

Here, in the case of YES, that is, when it is determined that the estimated gap is larger than the reference gap, it is considered that the adhered state between the materials to be welded 21a and 21b is bad, and preliminary energization is performed in advance to perform welding. The energization mode is set to two-stage energization (see FIG. 7) in order to perform main energization after improving the familiarity of the contact surfaces between the materials (step S1).
5).

On the other hand, in the case of NO, that is, when it is determined that the estimated gap is smaller than the reference gap, it is considered that there is no gap that requires two-stage energization, and the energization mode is set for a short time. (Step S16).

When the energization mode is set, the command is given to the controller 40, and spot welding of the workpiece 20 at the hitting position is executed according to the set energization mode (step S17).

Therefore, according to the present embodiment, when there is no (or small) gap between the workpieces 20, welding is performed in a short time by ordinary one-step energization, and when the gap between the workpieces 20 is large, This is automatically detected, and pre-energization is performed in advance by two-stage energization, and the workpieces 21a and 21
b. After improving the familiarity of the contact surfaces between the contact surfaces, the workpieces 21a and 21b are joined by the main energization, so that the welding failure due to the gap between the workpieces 20 (insufficient joining strength, generation of spatter, generation of bubbles, etc.). When there is a risk of occurrence, two-stage energization is performed, and the above-described poor welding due to an initial gap can be prevented. In addition, since the two-stage energization is performed only when necessary, that is, when the gap between the workpieces 20 is large, the power consumption and the cycle time can be reduced as compared with the case where the two-stage energization is performed every time.

<< Second Embodiment >> FIG. 4 is a block diagram showing another schematic configuration of the welding control device of the spot welding device shown in FIG. Here, too, only a configuration for realizing an energization control suitable for preventing poor welding due to an initial gap (insufficient bonding strength, generation of spatter, generation of bubbles, etc.) is shown. In FIG. 4, the same parts as those in FIG. 2 are denoted by the same reference numerals, and description thereof will be omitted.

In the present embodiment, when the gap between the workpieces 20 is large, the welding current is increased in accordance with the calculated estimated gap and one-stage energization is performed to prevent poor welding. Like that.

As a configuration for this purpose, the CPU 31a constituting the control means 30 includes the gap calculating section 33, the total thickness storage section 34, the gap determining section 3 for determining the degree of the estimated gap.
5a, the reference value storage unit 36, an optimal welding current determining unit 37 for setting an optimal welding current value, and an optimal welding current value (in the case of single-step energization) corresponding to the size of each gap. It has a value storage unit 38.

The optimum welding current value stored in the optimum value storage unit 38 corresponds to each gap by changing the size of the gap with a predetermined plate set in advance and conducting experiments, etc., on the premise of one-step energization. The optimum welding current value was determined. The optimum welding current determination unit 37 determines that the gap determination unit 3 is in production.
When it is determined in 5a that the estimated gap exceeds the reference value, the controller has a function of reading out the optimum welding current value corresponding to the estimated gap from the optimum value storage unit 38 and instructing the controller 40 to perform one-step energization based on the value. ing. The welding current changing means includes a gap determination unit 35a and an optimum welding current determination unit 37.
It consists of.

FIG. 5 is a flow chart showing the operation of the above welding control device. Here, each process of steps S21 to S23 is exactly the same as each process of steps S11 to S13 in FIG.

In step S24, the gap determination unit 35a determines whether the estimated gap obtained in step S23 is larger than the reference gap stored in the reference value storage unit 36.

Here, in the case of YES, that is, when it is determined that the estimated gap is larger than the reference gap, it is considered that the state of close contact between the materials to be welded 21a and 21b is poor, and the optimum welding current determining unit 37 determines. Then, the optimum welding current value corresponding to the estimated gap is read from the optimum value storage unit 38, and the welding current in the one-stage energization is changed to the optimum welding current value (step S25).

On the other hand, in the case of NO, that is, when it is determined that the estimated gap is equal to or smaller than the reference gap, it is assumed that there is no gap enough to increase the welding current, The process immediately proceeds to step S26 without changing the welding current value.

In step S26, an instruction from the optimum welding current determination section 37 is given to the controller 40, and spot welding of the workpiece 20 at the hitting point is executed in accordance with the instruction. That is, when the estimated gap is larger than the reference value, one-step energization is performed using the optimum welding current value corresponding to the estimated gap, and when the estimated gap is equal to or smaller than the reference value,
One-stage energization is performed at a preset normal welding current value.

Therefore, according to the present embodiment, even if there is a gap in the workpiece 20, if the welding current is increased, a constant welding quality can be ensured even in a single-step current application. In such a case, when the sheet thickness is small, in the case of aluminum welding, etc., it is possible to prevent poor welding when there is a gap without incurring an increase in cycle time and power consumption without performing complicated two-stage energization.

The prevention of poor welding due to an increase in welding current corresponding to the gap can also be applied to the case of two-stage energization. That is, for example, when spot welding is performed on a work piece having a larger thickness, in order to reliably prevent poor welding, the main current welding in the two-step current operation performed when the gap exceeds the reference value. It is also possible to change the current to an optimum welding current value corresponding to the size of the gap.

[Brief description of the drawings]

FIG. 1 is a schematic view showing a main part of an overall configuration of a spot welding apparatus to which the present invention is applied.

FIG. 2 is a block diagram illustrating a schematic configuration of a welding control device of the spot welding device according to the first embodiment of the present invention.

FIG. 3 is a flowchart illustrating an operation of the welding control device of FIG. 2;

FIG. 4 is a block diagram illustrating a schematic configuration of a welding control device of a spot welding device according to a second embodiment of the present invention.

FIG. 5 is a flowchart showing an operation of the welding control device of FIG. 4;

FIG. 6 is a characteristic diagram showing a one-stage energization method.

FIG. 7 is a characteristic diagram showing a two-stage energization method.

[Explanation of symbols]

11, 12: Electrode tip 13: Servo motor 15: Encoder (electrode position detecting means) 20: Workpiece 30: Control means 31: CPU 32: Servo amplifier (motor current detecting means, pressing force detecting means) 33: Gap calculation Section (gap estimating means) 34: total sheet thickness storage section 35: gap determining section (energization method setting means) 35a: gap determining section (welding current changing means) 36: reference value storage section 37: optimum welding current determining section (welding) Current changing means) 38: optimum value storage unit 40: controller

Claims (4)

[Claims] An electrode position for detecting the position of the electrode tip during a pressing operation in a welding control device of a spot welding apparatus for welding a local part by pressing an object to be welded by an electrode tip and applying an electric current to the object for an appropriate time. Detecting means, pressing force detecting means for detecting the pressing force applied to the workpiece by the electrode tip, and presetting the output of the electrode position detecting means at the time when the output of the pressing force detecting means reaches a predetermined value. From the reference total plate thickness at the hitting point, a gap estimating means for estimating the size of the gap between the workpieces at the hitting point, and comparing the output of the gap estimating means with a preset reference gap, the estimated gap Is larger than the reference gap, the energization mode is set to the two-stage energization method, and if the estimated gap is smaller than the reference gap, the energization mode is set to the one-stage energization method. Welding control apparatus of the spot welding apparatus characterized by comprising: a setting means. 2. A welding control device for a spot welding device for welding an object by pressing an object to be welded by rotating an electrode tip by rotation of a servomotor and energizing the object for an appropriate time. An electrode position detecting means for detecting a position of the tip; a motor current detecting means for detecting a motor current of the servomotor; and a change in an output of the motor current detecting means whereby a predetermined pressure is applied to the workpiece by the electrode tip. A gap estimating means for estimating the size of the gap of the workpiece at the hitting position from the output of the electrode position detecting means at the time of detecting that the addition has been performed and the reference total plate thickness at the preset hitting position; The output of the gap estimating means is compared with a preset reference gap, and if the estimated gap is larger than the reference gap, the energizing mode is set to the two-stage energizing method. Set, welding control apparatus when the estimated clearance is less than the reference gap spot welding apparatus characterized by having a energization method setting means for setting the energization mode to stage energization method. 3. A welding control apparatus for a spot welding apparatus for welding a local part by pressurizing an object to be welded by an electrode tip and energizing the object for an appropriate time, wherein an electrode position for detecting a position of the electrode tip during a pressing operation. Detecting means, pressing force detecting means for detecting the pressing force applied to the workpiece by the electrode tip, and presetting the output of the electrode position detecting means at the time when the output of the pressing force detecting means reaches a predetermined value. From the reference total plate thickness at the hitting point, a gap estimating means for estimating the size of the gap between the workpieces at the hitting point, and comparing the output of the gap estimating means with a preset reference gap, the estimated gap Is larger than the reference gap,
A welding current changing means for changing a welding current to an optimum welding current value corresponding to the estimated gap among the optimum welding current values set in advance for each size of the gap, Welding control device. 4. A welding control device for a spot welding device for welding an object by pressurizing an object to be welded by energizing the object to be welded by moving an electrode tip by rotation of a servomotor, and applying a pressure to the electrode tip during a pressing operation. An electrode position detecting means for detecting a position of the tip; a motor current detecting means for detecting a motor current of the servomotor; and a change in an output of the motor current detecting means whereby a predetermined pressure is applied to the workpiece by the electrode tip. A gap estimating means for estimating the size of the gap of the workpiece at the hitting position from the output of the electrode position detecting means at the time of detecting that the addition has been performed and the reference total plate thickness at the preset hitting position; Compare the output of the gap estimating means with a preset reference gap, if the estimated gap is larger than the reference gap,
A welding current changing means for changing a welding current to an optimum welding current value corresponding to the estimated gap among the optimum welding current values set in advance for each size of the gap, and a spot welding apparatus comprising: Welding control device.