JPS58196185A - Controlling method and device for resistance welding - Google Patents

Abstract

PURPOSE:To control resistance welding with high accuracy by improving the disadvantage relating to the evaluation of the resistance component between electrode tips from the difference in the local max. value of both change curves between the max. welding current in a range where no welding nuggets are formed and the resistance between the electrode tips in the state of ordinary conduction of welding current. CONSTITUTION:The resistance mu between tips is measured 1 and is shaped, then the local max. value mu max. is detected in the mu max. detecting and holding circuit 2 in the next stage. On the other hand, the resistance nu between the tips in the stage of conducting max. welding current in the state in which no welding nuggets are formed under the same conditions as the conditions for the mu is beforehand measured and stored, and the voltage signal corresponding to the pattern after said nu max. is transmitted 3 simultaneously with the detection of the max. An arithmetic circuit 4 for (mu max. -mu) and an arithmetic circuit 5 for (nu max. -nu) is syncyronized exactly with time and the (mu max. -mu) and the (nu max. -nu) corresponding to the respective current conduction times are operated and are inputted respectively to the (c=a-b) operator of an arithmetic circuit 6 for (a-b) in the next stage. As a result, the (c) is supplied as the effective quantity relating to the growth of a nugget area to a control part 7 for welding nugget in the next stage.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method and apparatus for evaluating resistance or voltage between electrode tips for weld nugget control in lap resistance welding such as spot welding.

In recent years, in order to monitor or control nugget quality in spot welding, etc., the voltage drop during welding current is measured between electrode tips, and changes in the inter-tip voltage due to nugget formation, or the value is processed with welding current, to determine the welding current. Many methods have been published that can be used to detect changes in inter-resistance.

Figure 2: Changes in resistance or voltage between these chips
).

In other words, when the contact resistance at the tip of the tip or between the workpieces disappears at the poles of welding energization, the resistance between the tips increases due to the temperature rise of the workpiece (mainly the conductive steel plate).Next, when a weld nugget is generated between the workpieces, the resistance between the tips increases. Since the inter-work resistance in that part decreases, the inter-chip resistance decreases as the nugget grows, and the curve has a local maximum value.

This local maximum value, that is, the difference (a) in resistance (or voltage) between the value of (a) max in Figure 2 (a) and the same curve (a)
The value of or the area of the triangle containing (a) Σ((a) max-
The value of (a)) has a correlation with the area of the nugget for the reasons mentioned above, and is used as a parameter for monitoring or controlling the size of the nugget.

, 1・L However, regarding the correlation with the nugget area, Figure 2 (8
It has become clear that problems will occur if the value of (a) in ), that is, the value of (a) max - (a), is used as is.

If the welding current value is gradually decreased under the welding conditions that yield the pattern shown in Figure 2(a), it will eventually become (
A situation occurs in which weld nuggets are not generated even if the energization time is extended.

It has been known for a long time that this is because the amount of Joule heat generated by the welding current and workpiece contact resistance is balanced with the amount of heat conduction and radiation to the water-cooled electrode tip and the workpiece in the lateral direction.

However, the problem is the state of change in interchip resistance (or voltage) at this time. Figure 2(b) shows spc.

1. The characteristic curve of this state in the state where □mmt×2 sheets ■ is shown is shown.

Because the welding current is small, the time of occurrence is late (and Fig. 2).
Although the amount of change is smaller than in a), there is still a local maximum value (b) max, showing a pattern similar to the one in which a small nugget was generated.

At this time, the state of the joint between the works is that, in the case of the SPC board, white discoloration occurs, and the temporary comrades are lightly pressed together due to the strong pressure between the electrode tips.

In other words, since no nuggets are generated, there is almost no so-called welding strength, but the resistance (or voltage) between the electrode tips
It has a characteristic that after the local maximum value described above, it gradually decreases as the energization time increases.

Figure 1 shows a cross-section of a nugget during spot welding.Even when a nugget is generated with an appropriate welding current, the current density is uneven (as described above) in the surrounding so-called corona pound area and furthermore in the outer periphery. Although it has no effect on the welding strength, there is a part that affects the reduction of the resistance (or voltage) between the electrode tips.Also, under conditions where the welding current is small, the It has been experimentally confirmed that the invalid portion of (b) max - (b) has a value that cannot be ignored with respect to the control amount of (a) max - (a) in FIG. 2(a).

The present invention aims to address the shortcomings associated with the evaluation of the inter-chip resistance (or voltage) reduction and provide a system that eliminates the unreasonableness regarding nugget monitoring or control all at once.

Figure 2 (C) shows the concept of the present invention, and Figure 2 (a)
)ノ(()ma!-(()ni included in Figure 2(b) (7
) to) max-io), and calculate the deviation of the resistance (or voltage) between curves (b) and (a) in Figure 2 (C) or its integral value as the nugget area, In other words, this method is characterized by treating it as an effective amount for welding strength.

Moreover, FIG. 3 shows an embodiment of the present invention as an electrical block diagram, and (a), (a) max, (b),
(b) max and the symbols such as (a), (b), (C1, etc.) match the symbols shown in FIG.

The inter-chip resistance or voltage (A) corresponding to welding current is measured by the inter-chip resistance (voltage) detection circuit (1).

The local maximum value (A) max is detected by the next stage (A) max*output, hold, and output times holding circuit, and that value is held.

On the other hand, the inter-chip resistance or voltage (b) when the maximum welding current is passed under the same conditions as (a) and no weld nuggets are generated is measured and stored in advance, and its (o) m
Voltage signals corresponding to patterns after ax start being transmitted by the pattern voltage signal circuit (3) in FIG. 3 at the same time as (a) max is detected.

(A1max-(A) Arithmetic circuit (4) and (B) in Figure 3
The max-(b) calculation circuit (5) calculates (a) max-(b) and (1max-(b)) corresponding to the respective energization times in complete temporal synchronization, and calculates the function voltage ( f of the next stage (a)-(b) arithmetic circuit (6) as a) and (b)
c) -(a)-(b) Give input to the arithmetic unit.

This calculation result (C) is supplied to the next stage welding nugget control section (7) as an effective amount regarding nugget area growth.

This control may be carried out, for example, by adjusting the energization time so that the ratio of the value (C) to the value (A) max becomes a predetermined constant value, or by adjusting the energization time so that the ratio of the value (C) to the value (B) max in FIG.
Generation of a nugget with a predetermined welding strength is controlled by adjusting the current application time so that the area of the triangular-like part formed by (b) and (a) becomes a predetermined constant value.

By adopting the control method or control device of the present invention,
Conventional deviation amount (a) max - (a) or its integral value Σ
((A) max = (A))Instead of error-free control that uses the resistance between the electrode tips or the change in voltage as the measurement quantity, which includes inactive components, only the effective components that are truly related to the growth of nuggets are used. As a result, highly accurate adaptive control can be achieved.

[Brief explanation of the drawing]

Figure 1 is a cross section of the nugget part of spot welding, Figure 2 is:
A graph showing changes in inter-chip resistance (or voltage), and FIG. 3 is an electrical block diagram showing an embodiment of the present invention. (1) - Chip-to-chip resistance (voltage) detection circuit (2)...
・(A 1max detection holding circuit (3)...Pattern voltage signal circuit (4)...(A) max-(A) Arithmetic circuit (5)-(
ol+nax −(ol performance J1 circuit (6)...(al
- (bl operation circuit (7)...Welding nugget control unit inventor Masahiro Kato (two others)

Claims (1)

[Claims] 1. Measure the change curve of the resistance (or voltage) between the electrode tips with respect to the current application time under normal welding current, and calculate the amount of decrease from the local maximum value of the curve for the welding nugget. When using it as a control parameter, measure the change curve of resistance (or voltage) between similar electrode tips in advance under the same conditions and flowing the maximum welding current within the range where weld nuggets are not generated, and Nugget control in lap resistance welding, characterized in that when the local maximum values of the curves are superimposed, the difference in resistance value (or voltage furnace) of both curves is defined as the amount of decrease from the local maximum value. Method. 2. A measurement circuit for detecting the change curve (a) of resistance (or voltage) between electrode tips corresponding to welding current application, and a circuit for detecting its local maximum value (a) max and holding that value. , Under the same conditions as in (a) above, the local maximum value (lo ) maw, and a transmitting circuit of a functional voltage signal (b) equivalent to the following pattern, and at the same time when the change curve (a) reaches the local maximum value (a) max point, fa) corresponding to the energization time. -(A) max -(A) (b) -f port 1max -(B) (C) -fa)-(b) A nugget control device for lap resistance welding, characterized in that nugget is handled as a decrease from a local maximum value in .