JP2021053698A - Control device for resistance welding machine, conduction state monitoring method and normal/defective determination method of welded parts - Google Patents

Abstract

To lower the cost of monitoring changes in the total resistance value of the welding current path, and the cost of determining normal/defective states of welded parts and monitoring the electrode shape.SOLUTION: A control device 20 for a resistance welding machine (welding robot 10) comprises: an inverter circuit 42 that converts direct current into alternating current and supplies the alternating current to a welding transformer 15; a current detection unit 44 that measures a current value supplied from the inverter circuit 42 to the welding transformer 15; a current control circuit 43 that sets a pulse width W P on the basis of the current value measured by the current detection unit 44; and a recording unit 34 that records parameters related to the pulse width W P set by the current control circuit 43.SELECTED DRAWING: Figure 1

Description

The present invention relates to a control device for a resistance welder, a method for monitoring an energized state of a welded portion, and a method for determining quality.

For example, in the assembly process of an automobile body, resistance welding (spot welding) is performed by sandwiching a plurality of stacked steel plates between a pair of electrodes and energizing them to melt the contact parts between the steel plates by resistance heat generation to form a nugget. Is being done. As a means for inspecting the quality of the nugget, for example, a method of driving a chisel in the vicinity of the nugget and checking the peel strength between the steel plates is known. However, such an inspection method is time-consuming, and there are some parts where structural restrictions that can physically enter the chisel occur. There is also non-destructive inspection using ultrasonic waves or magnetism, but it takes time, so it is difficult to apply it to all of them on mass production lines. From the above, there is a need for a simpler method for inspecting the quality of nuggets.

For example, in Patent Document 1, the value of the current flowing between the pair of electrodes and the voltage value between the two electrodes are measured, the resistance value is calculated from these current values and the voltage values, and the change in the resistance value during energization is obtained. A method of determining the quality of welding quality (joint strength) based on this is shown. Specifically, immediately after the start of energization, the resistance value increases as the temperature of the contact portion between the metal plates rises, and once the nugget is formed, the resistance value decreases due to the expansion of the energization area as the nugget grows. As the growth of the nugget slows down, the resistance value becomes substantially constant (see FIG. 3 and the like in Patent Document 1). The quality of the nugget is determined based on the change (waveform) in the resistance value with respect to the energization time.

In addition, when resistance welding is performed on a large number of points, the electrodes are damaged by repeated energization and the tip is deformed, which makes it difficult for the resistance of the welded portion to increase or the timing of the increase to shift, resulting in welding quality. May have an adverse effect on. For example, in Patent Document 2 below, the current value and voltage value when energized with a test piece sandwiched between a pair of electrodes are measured, and the shape of the electrode is determined based on the resistance value based on these measured values. It shows how to monitor.

Japanese Unexamined Patent Publication No. 2011-240368 Japanese Unexamined Patent Publication No. 11-104848

In any of the above methods, the quality of the nugget is determined and the electrode shape is monitored based on the change in the resistance value between the electrodes (total resistance value of the welding current path). However, in order to measure the resistance value between electrodes, the sensor and wiring for detecting the voltage between electrodes, the conversion unit that converts the voltage value detected by this sensor into a digital signal, the detected voltage value, and the like are used. Since it is necessary to provide a calculation unit for calculating the resistance value, the cost is high. In addition, the wiring connecting the electrodes and the conversion unit may be cut due to unintended contact or the like accompanying the operation of the welding apparatus, so that management is also costly.

The present invention reduces the cost of the method of monitoring the energized state of the welded portion (change in the total resistance value of the welded current path), and inspects items correlating with this (for example, quality judgment of the welded portion and electrodes). The purpose is to reduce the cost of shape monitoring, etc.).

As shown in FIG. 8, a conventional general spot welding facility includes a welding robot 110 having a pair of electrodes 101 and 102, a timer contactor 120, and a robot control unit 130. The robot control unit 130 controls the operation of the robot arm 104 of the welding robot 110 and the pressing force of the pressurizing means 105. The timer contactor 120 includes a converter circuit 121 that converts alternating current supplied from the welding power supply 140 into direct current, an inverter circuit 122 that converts direct current into direct current, and a current control circuit 123 that controls the output current of the inverter circuit 122. It includes a sensor 124 that detects the current value output from the inverter circuit 122. A plurality of types of welding conditions (current value and energization time) are stored in advance in the current control circuit 123, and when a command for starting energization and a condition number are transmitted from the robot control unit 130, the welding conditions corresponding to the condition numbers are transmitted. Energize with.

The current control circuit 123 controls the output current of the inverter circuit 122 by adjusting the pulse width (time for turning on the switching element) of the output signal to the inverter circuit 122, that is, so-called PWM (Pulse Width Modulation) control. It is something to do. Conventionally, the value of the pulse width itself has not been used for inspection of the welding process, but the present inventors have used the pulse width set by the control device (current control circuit) to use the secondary side of the transformer. We focused on the fact that it is possible to estimate the energized state of the weld (change in the total resistance value of the weld energized path) without measuring the current value and voltage value of.

Specifically, the signal of the current value detected by the sensor 124 is fed back to the current control circuit 123, the pulse width is set based on this signal, and the switching element is switched on / off by the waveform of the pulse width. The inverter circuit 122 is driven into the drive. For example, as shown in FIG. 9A, when the energized area A1 (that is, the size of the nugget) of the welded portion at a certain time t1 is smaller than the planned energized area A0, it becomes difficult for current to flow through the welded portion. Therefore, the current value detected by the sensor 124 tends to be smaller than the planned current value. Current control circuit 123 which has received the signal, a (see dotted lines in FIG. 9 (B)), the amount of power supplied to the welding transformer 103 by increasing the pulse width _{W P} in the subsequent time t2, t3 · · · By increasing, the planned current value is maintained. On the other hand, as shown in FIG. 10A, when the energizing area A1 of the welded portion at a certain time t1 is larger than the planned energizing area A0, the current easily flows through the welded portion, so that the current detected by the sensor 124 The value tries to be greater than the expected current value. Current control circuit 123 which has received the signal, reduce the amount of power supplied to the welding transformer 103 (see dotted lines in FIG. 10 (B)) to reduce the pulse width _{W P} in the subsequent time t2, t3 · · · By doing so, the planned current value is maintained. As described above, the inverter circuit 122 is controlled so that the planned current value is supplied to the welding transformer 103.

In this way, when performing PWM control, the pulse width is changed to adjust the amount of electric power supplied to the welding transformer 103, so that the current value is controlled to be a predetermined value. In other words, the pulse width is monitored, and if the pulse width is large as shown by the dotted line in FIG. 9B, it means that electricity does not easily flow in the welding current path at that time (specifically, the time immediately before that). If the pulse width is smaller than planned as shown by the dotted line of 10 (B), it means that electricity is likely to flow in the welding current path at that time (specifically, the time immediately before that).

From the above, the present invention measures with the inverter circuit that converts direct current into alternating current and supplies it to the welding transformer, the current detection unit that measures the current value supplied from the inverter circuit to the welding transformer, and the current detection unit. A current control circuit that sets a pulse width, which is a time for turning on the switching element of the inverter circuit, and a recording unit that records parameters related to the pulse width set by the current control circuit are provided based on the current value. Provided is a control device for an inverter.

In this way, by recording the parameters related to the pulse width set by the current control circuit in the recording unit, the energized state of the welded part (change in the total resistance value of the welded energized path), and by extension, this It is possible to estimate changes in the current-carrying area and changes in the electrode shape that have a correlation. In this case, it is not necessary to provide wiring, a converter, or the like for measuring the voltage on the secondary side of the transformer, so that the cost can be reduced. Further, in PWM control, since the current value is maintained to be constant by changing the pulse width, the pulse width may change even when the current value is substantially constant. Therefore, by monitoring the parameters related to the pulse width as described above, it is possible to detect even a slight change in the energized state that hardly appears in the change in the current value.

By the way, in order to realize stable welding, instead of energizing with a constant current value, a device is made to gradually increase the current value at the initial stage of energization or to energize while gradually increasing the current value. There are times. In this way, when the current value is changed during energization, the pulse width fluctuates as the current value changes. Therefore, even if the pulse width fluctuates, it is difficult to determine whether the fluctuation is due to a change in the energized area of the welding current path (that is, growth of the nugget) or a change in the current value. There is.

Here, the following relational expression (1) holds between the current value I, the current density J, and the energization area A.
I = JA ・ ・ ・ (1)

Further, the current density J is a unit area and the amount of electricity (electric charge) flowing per unit time, and can be defined by the following equation. Incidentally, alpha is a coefficient, W _P is the pulse width.
_{J = α · W P ··· (} 2)

From the above equations (1) and (2), the following equations (3) and (4) are established.
_{I = α · W P · A} ··· (3)

From the above equation (4), the energized area A is proportional to _{the ratio (I / W P) of the pulse width and the current value.} Thus, as the parameter relates to a pulse width, if the recording parameters including the ratio of the current value I and the pulse width W _P supplied from the inverter circuit to the welding transformer (I / W _P), regardless of the change in the current value I , The change in the energized area A can be monitored.

When the current state of the weld changes and the current value is about to change, the current value is maintained constant by increasing or decreasing the current supply amount by making the pulse width larger or smaller than the predetermined range. .. Therefore, if the pulse width is out of the predetermined range many times, it can be estimated that the energized state of the welded portion is unstable. Therefore, by recording the number of times the pulse width is out of the predetermined range as a parameter related to the pulse width, it is possible to monitor the energized state of the welded portion.

As the nugget grows during energization, the energized area expands and the total resistance value of the welding current path decreases. Therefore, the timing and amount of change in the total resistance value of the welding current path differ depending on how the nugget grows. .. Therefore, the quality of the welded portion (nugget) can be determined by monitoring the change in the energized area of the welded portion that correlates with the total resistance value of the welding current path by the above parameters related to the pulse width.

In addition, when welding is performed on a large number of welding points, the tip of the electrode is worn due to damage caused by energization. Therefore, when welding is performed using a new (immediately after polishing) electrode and when welding is performed using a worn electrode. The timing and amount of change in the total resistance value of the welding current path are different. Therefore, it is possible to monitor the shape of the electrode that correlates with the change in the total resistance value of the welding current path by the above parameters related to the pulse width.

As described above, by recording the parameters related to the pulse width, based on these parameters, the energized state of the welded part (total resistance value of the welding current path) can be achieved at low cost without providing a separate sensor or wiring. Changes) can be monitored. This makes it possible to reduce the cost of inspecting items that correlate with changes in the total resistance value of the welding current path (for example, determining the quality of the welded portion, monitoring the electrode shape, etc.).

It is a schematic diagram of a spot welding equipment. It is a graph which shows the relationship between the time and the current value supplied from the inverter circuit to the welding transformer. It is a graph which shows the relationship between a pulse width and time, (A) shows the case where a good weld part was formed, and (B) (C) show the case where welding was poor. (A) is a graph which shows the relationship between the time and the current value supplied from the inverter circuit to the welding transformer which concerns on another example. (B) is a graph showing the relationship between the pulse width and the time when energized in the pattern of (A). _{It is a graph which shows the relationship between the ratio (I / W P} ) of the current value supplied from the inverter circuit to the welding transformer, the pulse width, and time, and (A) is (B) when a good weld is formed. (C) shows the case where welding is defective. It is a schematic diagram of the spot welding equipment which concerns on other embodiment. It is a side view which shows the tip shape of an electrode. It is a schematic diagram of the conventional spot welding equipment. (A) is a cross-sectional view showing a state in which the welded portion is smaller than planned, and (B) is a graph showing the pulse width at that time. (A) is a cross-sectional view showing a state in which the welded portion is larger than planned, and (B) is a graph showing the pulse width at that time.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.

The resistance welding equipment shown in FIG. 1 includes a welding robot 10 as a resistance welding machine and a control device 20.

The welding robot 10 includes a welding gun 14 having a pair of electrodes 11 and 12, a welding transformer 15 that supplies a welding current between the pair of electrodes 11 and 12, and a robot arm 16 having a welding gun 14 attached to the tip. Be prepared. The welding gun 14 is a welding gun for direct spot welding in which a pair of electrodes 11 and 12 are arranged coaxially and one of the electrodes 11 is driven by the pressurizing means 13. As the pressurizing means 13, for example, a servomotor can be used. By passing the current supplied from the welding transformer 15 between the pair of electrodes 11 and 12 in a state where the overlapping portions of a plurality of metal plates (for example, steel plates) indicated by chain wires are pressure-sandwiched by the pair of electrodes 11 and 12. , The contact portion between the two metal plates melts due to resistance heat generation to form a nugget. The welding gun is not limited to the above, and may be a welding gun for series spot welding or a welding gun for indirect spot welding. Further, as the resistance welding machine, a stationary resistance welding machine fixed at a fixed position or a portable resistance welding machine that the operator holds and moves by hand may be used.

The control device 20 includes a robot control unit 30 and a timer contactor 40. In the present embodiment, the timer contactor 40 is built in the control panel (control device 20) having the robot control unit 30.

The robot control unit 30 includes a robot CPU board 31, a robot control circuit 32, and a timer board 33. The robot CPU board 31 incorporates in advance a program for controlling the motors of the joints of the robot arm 16 and the pressurizing means 13 of the welding gun 14. A command based on this program is transmitted to the robot control circuit 32, and the motor and the pressurizing means 13 of each joint of the robot arm 16 are driven.

The timer contactor 40 includes a converter circuit 41 that converts alternating current supplied from the welding power source 50 into direct current, an inverter circuit 42 that converts direct current into high-frequency alternating current, and a current control circuit 43 that controls the output current of the inverter circuit 42. And a sensor 44 (for example, a CT coil) as a current detection unit for detecting the current value output from the inverter circuit 42. The current control circuit 43 controls the output current of the inverter circuit 42 by adjusting the time (that is, the pulse width) for turning on the switching element (for example, the IGBT) of the inverter circuit 42. Several types of welding conditions (current value and energization time) are stored in advance in the current control circuit 43, and when a command to start energization and a condition number are transmitted from the timer substrate 33, energization is performed under the welding conditions corresponding to the condition number. I do. In this embodiment, as shown in FIG. 2, a case where a constant current value is supplied from the inverter circuit 42 to the welding transformer 15 is shown.

The current control circuit 43 performs so-called PWM control that controls the output power by changing the pulse width. Specifically, the signal of the current value detected by the sensor 44 is fed back to the current control circuit 43, and the current control circuit 43 automatically adjusts the pulse width based on this current value to the welding transformer 15. The supplied current value is kept constant. For example, as the energization progresses, the welded portion (nugget) formed at the contact portion between the metal plates becomes larger, so that the area of the energizing path between the electrodes 11 and 12 increases. As a result, the current easily flows, so that the current value on the secondary side of the welding transformer 15 including the welded portion, and by extension, the current value supplied from the inverter circuit 42 to the primary side of the welding transformer 15 tends to increase. When the sensor 44 detects this increasing tendency of the current value, it feeds it back to the current control circuit 43 to reduce the pulse width, thereby suppressing the increase in the current value supplied to the primary side of the welding transformer 15 and causing the current. The value is kept constant. Therefore, if the nugget of the welded portion is continued to grow steadily, as shown in FIG. 3 (A), gradually decreases the pulse width W _P that is set by the current control circuit 43.

In conventional spot welding equipment, the pulse width is not recorded, only the resulting current value (see FIG. 2) is monitored. On the other hand, in the present invention, as shown in FIG. 1, the timer board 33 of the control device 20 is provided with a recording unit 34 for recording the history of the pulse width set by the current control circuit 43. This recording unit 34 is recorded a history of the pulse width W _P as shown in FIG. 3 (change over time), based on the history, is possible to estimate the changes in the energizing area of the weld during energization it can. For example, as shown in FIG. 3 (A), if the pulse width W _P is gradually decreased with time, the total resistance value gradually decreases the welding current path increasing current area of the welded portion is gradually It can be estimated that what is being done, that is, the nugget is growing steadily. On the other hand, and when the pulse width W _P is substantially constant as shown in FIG. 3 (B), when the pulse width W _P as shown in FIG. 3 (C) becomes constant after dropped halfway is energized area It can be estimated that the growth of the nugget, that is, the growth of the nugget is insufficient.

The timer substrate 33 of the control device 20 is provided with a monitoring unit 35 that monitors items that correlate with changes in the total resistance value of the welding electrode path based on the pulse width history stored in the storage unit 34. In the embodiment, a monitoring unit 35 for monitoring the quality of welding is provided. For example, in the graph of FIG. 3 (A) ~ (C) , if the rate of change with respect to the energizing time of the pulse width W _P (slope) is negative, the monitoring unit 35, the nugget is growing at that time to decide. Thus, for example, time rate of change for the energizing time of the pulse width W _P is negative, if the predetermined ratio for the entire weld time (e.g., 80%) or more, the monitoring unit 35, nugget fully grown It is determined that a good welded portion is formed. Alternatively, the monitoring unit 35, FIG. 3 (A) ~ and history of the pulse width W _P of (C), the pulse width W _P of the reference history (e.g., a pulse width when a good weld is formed comparing the W _P history), it is determined that these differences may be within the allowable range, a good weld is formed.

The present invention is not limited to the above embodiments. Hereinafter, other embodiments of the present invention will be described, but duplicate description will be omitted with respect to the same points as those of the above embodiments.

In the above embodiment, the case where the current value during welding is a constant value (see FIG. 2) is shown, but the present invention is not limited to this, and the current value during welding may be changed. For example, in the energization pattern shown in FIG. 4 (A), the current value is gradually increased. Specifically, this energization pattern has an upslope I0 that gradually raises the current value, a first energization section that energizes with a current value I1, a second energization section that energizes with a current value I2 higher than I1, and slightly lower than I2. A third energized section energized with a current value I3, a fourth energized section energized with a current value I4 higher than I3, a fifth energized section energized with a current value I5 slightly lower than I4, and energized with a current value I6 higher than I5. It has a sixth energization section to be energized and a seventh energization section to energize with a current value I7 slightly lower than I6.

If energized while changing the current value as described above, when changing the current value (upslope I0, and, when switching current interval), the current control circuit 43 changes the pulse width W _P. Thus, the history of the pulse width W _P, as shown in FIG. 4 (B), a complex shape affected by the change in the current value. In this case, from the history of the pulse width W _P, it is difficult to estimate the current area of the weld.

Therefore, when energized while changing the current value computing unit provided in the timer board 33 (not shown), and a current value I at each time, the pulse width at each time recorded in the recording unit 34 W _P calculating a parameter comprising the ratio (I / W _P) and, based on this parameter, it is preferable that the monitoring unit 35 monitors the current area a of the weld. The current value I at this time may be the current value (measured value) detected by the sensor 44 or the current value (command value) under the specified conditions. _{Since I / W P} = A · α holds from the above equation (4), the energized area A of the welded portion can be directly monitored by _{the I / W P.}

For example, FIG. 5 (A) shows the history _{of I / W P when a good weld is formed.} In this graph, the I / W _P (that is, the energization time A) continues to gradually increase with the passage of time. Thus, the history of I / W _P during welding was recorded, if continued to rise I / W _P gradually, current area A has continued to rise, it can be estimated that the nugget is continuing to grow.

On the other hand, FIGS. 5 (B) and 5 (C) show the history _{of I / W P} when the welded portion is defective (the nugget diameter does not reach a predetermined value). In these examples, I / W _P is an initial energizing only (see FIG. 5 (B)), or elevated in energizing the first half only (see FIG. 5 (C)), then is substantially constant. In these cases, it can be estimated that the nugget is growing in the initial stage of energization or the first half of energization, but the nugget is not growing after that.

As described above, based on the history of the parameter I / W _P, by monitoring unit 35 monitors the current area A of the welded portion, it is possible to determine the quality of the weld. For example, calculation unit provided in the timer board 33 (not shown), surrounded by the regions I / W _P is gradually increased, the graph of the maximum value and the I / W _P of I / W _P region The area of (the area shown by the dotted line in FIGS. 5A to 5C) is calculated, and if this area is equal to or more than a predetermined value, it is determined that the welded portion is good, and if it is less than the predetermined value, the welded portion is determined. Judge as defective. In addition, the proportion or for the entire I / W _P energization time period change rate for the current time is positive, the comparison result of the actual history and reference history of I / W _P, may be good or bad criterion ..

Further, as a parameter related to the pulse width, the number of times the pulse width becomes the maximum value or the minimum value may be recorded in the recording unit 34. For example, in each energization section of the energization pattern shown in FIG. 4, when the energization area of the welded portion is smaller than planned, the current value should be kept constant by increasing the pulse width beyond the predetermined range and increasing the current supply amount. And. Further, when the energized area of the welded portion is larger than planned in each energized section, the pulse width is made smaller than the predetermined range to reduce the current supply amount to maintain the current value constant. Therefore, if the number of times the pulse width is out of the predetermined range in each energization section is larger than the preset upper limit value, it can be estimated that the energization area of the welded portion is unstable. In this way, it is possible to monitor the energized state of the welded portion based on the number of times the pulse width is out of the predetermined range in each energized section.

Further, as shown in FIG. 6, a robot control unit 30 that controls the positions and pressing forces of the electrodes 11 and 12 and a current control unit that controls the welding current may be provided separately. In this case, the current control unit functions as the "control device 20". The current control unit (control device 20) includes a timer contactor 40 and a timer board 33. The timer board 33 is provided with a recording unit 34 that records parameters related to the pulse width set by the current control circuit 43, and can determine the quality of the welded portion based on the parameters recorded in the recording unit 34. ..

By the way, the tip shapes of the electrodes 11 and 12 are usually tapered, and for example, as shown in FIG. 7, a minute flat portion 17 is provided at the tip of the spherical surface. When spot welding is performed on a large number of points using such electrodes 11 and 12, the tips of the electrodes 11 and 12 are worn as shown by the dotted lines, and the size (diameter) of the flat portion 17 becomes large (D1 → D1'). In this case, the contact area between the electrodes 11 and 12 and the metal plate increases, and the resistance value of this contact portion decreases, but the area of the pressurized region formed by the electrodes 11 and 12 increases, and the contact portion between the metal plates becomes large. As a result, the total resistance of the welding current path remains high because the current is not concentrated and the nugget is difficult to grow. In this way, the total resistance value of the welding current path is different between the case where the welding is performed with the non-wearing electrode (see the solid line in FIG. 7) and the case where the welding is performed with the worn electrode (see the dotted line in FIG. 7). The mode of change is different.

Therefore, on the basis of parameters related to the pulse width W _P of each RBI, by monitoring the total resistance of the welding current path of each weld point, it is possible to monitor the electrode shape. Specifically, for example, to record the parameters relating to the pulse width W _P for a large number of welding points on the recording unit 34, a monitoring unit 35 for monitoring the electrode tip shape based on the parameters recorded in the recording unit 34, It can be provided on the timer board 33.

10 Welding robot (resistance welding machine)
11, 12 Electrodes 13 Pressurizing means 14 Welding gun 15 Welding transformer 16 Robot arm 20 Control device 30 Robot control unit 31 Robot CPU board 32 Robot control circuit 33 Timer board 34 Recording unit 35 Monitoring unit 40 Timer contactor 41 Converter circuit 42 Inverter Circuit 43 Current control circuit 44 Sensor (current detector)
50 welding power _{W P} pulse width

Claims (4)

Based on an inverter circuit that converts DC into alternating current and supplies it to the welding transformer, a current detection unit that measures the current value supplied from the inverter circuit to the welding transformer, and a current value measured by the current detection unit. A control device for a resistance welding machine including a current control circuit that sets a pulse width that is a time for turning on the switching element of the inverter circuit, and a recording unit that records parameters related to the pulse width set by the current control circuit. .. The control device for a resistance welder according to claim 1, wherein the parameter includes a ratio of a current value supplied from the inverter circuit to the welding transformer to the pulse width. A method for monitoring the energized state of the weld when performing welding while controlling the pulse width, which is the time to turn on the switching element of the inverter circuit, based on the current value supplied from the inverter circuit to the welding transformer. There,
A method for monitoring the energized state of a welded portion, which monitors the energized state of the welded portion based on the parameters related to the pulse width. A method for determining the quality of a welded portion, which determines the quality of the welded portion based on the energized state of the welded portion acquired by the method according to claim 3.

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