JPH0677851B2 - Resistance welder - Google Patents

Description

TECHNICAL FIELD The present invention relates to an inverter resistance welding machine, and is devised so as to obtain particularly stable welding quality.

(Prior Art) Recently, a resistance welding machine using an inverter for a power supply circuit has appeared on the market and is showing signs of spread. This inverter type resistance welding machine has the following features as compared with the most frequently used single-phase AC resistance welding machine.

(1). Since a high frequency alternating current is passed through the welding transformer, the transformer can be made smaller. Therefore, for example, when applied to robot welding, it is possible to mount a welding transformer on the tip of the robot arm and eliminate the need for a secondary cable,
This not only raises the cable cost, but also saves power by eliminating power loss due to the cable.

(2). Since it is a DC welding current, it has a high heat generation efficiency. Therefore, the welding current can be made relatively small, or the energization time can be made relatively short, and power consumption can be saved and the life of the welding electrode can be extended.

(3). A three-phase commercial AC power supply can be used, in which case a three-phase balanced load will be obtained and the power factor will be good.

By the way, in the conventional inverter type resistance welding machine, a constant current timer is incorporated to perform control such that the effective value of the welding current is constant during the energization period. This is based on the idea of constant current phase control in the single-phase alternating current system, and the welding current is the main welding condition, and it is intended to obtain stable welding quality with little variation by keeping this constant. is there.

(Problems to be Solved by the Invention) However, in the single-phase AC type, the AC welding current is used, whereas in the inverter type, the DC welding current is used. May occur. For example, if the applied pressure is insufficient, the resistance between the welding electrodes that sandwich the material to be welded increases, but since the welding current is kept constant, the welding current becomes relatively large with respect to the applied pressure and scatters. The rate of burrs and burr is high. Also, when the tip of the welding electrode wears, the contact area between the welding electrode and the material to be welded increases, but since the welding current is kept constant even in such a case, the current density at the welded part decreases. There is a risk that resistance heating will decrease and welding will be defective.

The present invention has been made in view of the above problems, and an object of the present invention is to provide an inverter-type resistance welding machine that suppresses the generation of dust and burrs and obtains stable welding quality.

(Means for Solving the Problems) A configuration of the present invention that achieves the above object is to rectify a commercial alternating current into a direct current, convert the direct current into a pulsed high frequency alternating current having a constant frequency by an inverter, and convert the high frequency alternating current into the high frequency alternating current. In a resistance welding machine in which a direct current is passed through a welding transformer and then a rectifier, and this direct current is supplied to the material to be welded through a welding electrode, the DC voltage applied to the welding electrode is detected and its level is detected. A voltage detecting means for measuring the voltage; a voltage setting means for giving a constant voltage that can be set arbitrarily; a comparison for comparing the measured value of the DC voltage with the constant voltage and generating an error signal indicating an error between them. Means; responsive to the error signal,
And a means for controlling the pulse width of the high frequency alternating current in synchronization with the constant frequency so that the direct current voltage applied to the welding electrode is maintained at a constant level.

(Operation) The operation of the present invention will be described with reference to FIGS. 3 and 4.

FIG. 3 shows a structure between welding electrodes. As shown, a pair of welding electrodes 10a and 10b are in contact with each other so as to sandwich the two materials to be welded 12a and 12b, and a pressing force is applied between the welding electrodes and a DC welding current I is passed. In such a configuration, R1, R5 are the resistances of the welding electrodes 10a, 10b, R2, R4 are the resistances of the contact surfaces between the electrodes 10a, 10b and the welded materials 12a, 12b, and R3 is the welded materials 12a, 12b. It is resistance. When the welding current I flows, resistance heating (Joule heat) occurs at each of these resistances.

In the present invention, the voltage detecting means, the comparing means and the pulse width controlling means form a feedback loop for constant voltage control. Therefore, the voltage V between the welding electrodes 10a and 10b is expressed as follows.

V = IR = I (R1 + R2 + R3 + R4 + R5) (1) Then, even if any of the resistors Ri changes, the voltage V is kept constant, so that the current I changes so as to compensate for the change in resistance. . As a result, the following operational effects are exhibited.

． Scatter and burrs are suppressed during welding.

． Even if the welding electrode is worn, the welding quality does not deteriorate.

． Applicable to surface treated steel sheet.

． Excellent welding quality for small metals and thin metal plates.

First of all, the main factors of scattering and burrs are insufficient pressurization force, poor adhesion between the materials to be welded, and the electrode diameter being too small. All of these factors cause an increase in the resistances R2 and R4. However, according to the present invention, the current I decreases as much as the resistances R2 and R4 increase, and as a result, the current I does not become relatively large with respect to the resistance R, and scattering and burrs are suppressed.

Next, when the welding electrode is worn, the contact area between the welding electrode and the material to be welded increases, and the current density decreases as it is. However, since the resistances R2 and R4 decrease at this time,
The current I increases correspondingly and the decrease in current density is prevented, and stable welding quality is secured.

Regarding the materials to be welded 12a and 12b, which are surface-treated steel sheets, the resistance of the above formula (1) varies, which is relatively low in galvanized steel sheets and turn sheets, and in chemical conversion treated steel sheets and zinc-plated steel sheets. On the contrary, although relatively high, according to the present invention, the welding current is automatically increased for the former to prevent insufficient welding strength, and for the latter, the welding current is automatically decreased to cause dispersion. Such defects are prevented.

Regarding the materials to be welded 12a, 12b are small metals or very thin metal plates, the energization time is usually very short such as 10 ms, 20 ms, etc. During this period, the metal is deformed and the welding current conduction path is likely to change. For example, as shown in FIG. 4 (A), the small metal 14a and the thin metal plate 14b are in line contact with each other at the beginning of energization, but at the end of energization, as shown in FIG. Are in surface contact. When a constant current is supplied to such a material to be welded as in the conventional case, scattering occurs at the beginning of energization and the welding current often becomes insufficient at the end of energization. However, in the present invention, since the constant voltage control is finely performed in synchronization with the high frequency, the welding current is reduced in the initial stage even during a short energization period to suppress the dispersion, and the welding current increases rapidly as the end period is approached. And good welding quality can be obtained.

According to the present invention, a constant welding strength is guaranteed even if the power supply voltage changes. That is, the fluctuation of the power supply voltage is reflected in the comparison error, and in response to the error, the pulse width of the high frequency alternating current is quickly adjusted in synchronization with the high frequency to compensate the voltage fluctuation, and the welding voltage is kept constant.

(Examples) Examples of the present invention will be described below with reference to FIGS. 1 and 2.

FIG. 1 shows the main structure of an inverter type resistance welding machine according to an embodiment of the present invention.

The input terminal of the rectifier circuit 22 is connected to the three-phase commercial AC power supply terminal 20, and DC is obtained at the output terminal of the rectifier circuit 22. This direct current is smoothed by a smoothing circuit composed of the coil 24 and the capacitor 26 and then input to the inverter circuit 28. The inverter circuit 28 is a well-known one composed of a power transistor or a FET, and outputs a pulsed (rectangular) high frequency AC by chopping the input DC by high frequency switching. The switching of the inverter circuit 28, and thus the pulse width of its high frequency AC output, is variably controlled by the pulse width control circuit 44 via the inverter drive circuit 48.

The high frequency alternating current output from the inverter circuit 28 is supplied to the primary side of the welding transformer 30, and the reduced side high frequency alternating current is obtained on the secondary side thereof, which is rectified into direct current by the rectifying circuit 34 including the diodes 32a and 32b. To be done. Then, this direct current is supplied to the materials to be welded 12a and 12b via the welding electrodes 10a and 10b.

Now, in the present embodiment, the voltage detection circuit 38 is connected between the rectification circuit 34 and the welding electrodes 10a, 10b via the induced voltage removal circuit 36. The induction voltage removing circuit 36 is composed of a low-pass filter, and induces voltage (noise) due to high frequency components of welding current.
Get rid of. The voltage detection circuit 38 may be a known DC voltage detection circuit and outputs a voltage signal SV representing the level of the DC voltage V applied to the welding electrodes 10a and 10b. This voltage signal SV is supplied to one input terminal of the voltage comparison circuit 40. The other input terminal of the voltage comparison circuit 40 is supplied with a voltage signal SP corresponding to a constant set voltage from a voltage setting circuit 42 including, for example, a potentiometer, and the voltage comparison circuit 40 compares the both input voltage signals SV and SP. Generates an error voltage ER indicating the comparison error between them, and supplies this to the pulse width control circuit 44.

The pulse width control circuit 44 controls the high frequency alternating current output from the inverter circuit 28 by pulse width modulation (PWM). The frequency generator 46 outputs a triangular wave signal having a constant frequency f as shown in FIG. 2 (A). A pulsed PWM with "H" and "L" levels as shown in Fig. 2 (B) by inputting WT as a modulated wave and taking the comparison logic with the error voltage.
A signal is generated, and on / off control of each transistor of the inverter circuit 28 is performed based on the signal. The sequence circuit 50 instructs the start and end of energization, and operates the pulse width control circuit 44 and thus the inverter circuit 28 only during the energization time.

In the above configuration, when the DC voltage V changes, the voltage signal SV changes and the error voltage ER changes. For example, when the DC voltage V rises, the voltage signal SV rises accordingly and the error voltage ER also rises as shown by the dotted line in FIG. 2 (A). Then, the pulse width of the PWM signal decreases, and in response to this, the pulse width of the high frequency AC output of the inverter circuit 28 decreases and the DC voltage V decreases. In this way, the DC voltage V is finely controlled at a constant frequency for each cycle of the constant frequency f, so that even if the resistance of the welding electrodes 10a, 10b fluctuates, the current rapidly increases and decreases, resulting in As a result, stable welding quality can be obtained.

(Effects of the Invention) As described above, according to the present invention, since the high-frequency AC output of the inverter is controlled by the pulse width by the constant voltage control, it is possible to improve the suppression of scattering, burrs, etc. Welding quality is obtained.

[Brief description of drawings]

FIG. 1 is a block diagram showing a main configuration of an inverter type resistance welding machine according to an embodiment of the present invention, FIG. 2 is a diagram showing an example of constant current control in the resistance welding machine of FIG. 1, and FIG. FIG. 4 and FIG. 4 are views for explaining the operation of the present invention. 10a, 10b …… Welding electrode, 12a, 12b …… Welding material, 20 …… Commercial AC power supply terminal, 22,34 …… Rectifier circuit, 28 …… Inverter circuit, 30 …… Welding transformer, 36 …… Induction voltage Removal circuit, 38 ...... voltage detection circuit, 40 ... voltage comparison circuit, 42 ... voltage setting circuit, 44 ... pulse width control device, 46 ... frequency generator, 48 ... inverter drive circuit.

Claims (1)

[Claims] 1. A commercial alternating current is rectified into a direct current, the direct current is converted into a pulsed high frequency alternating current with a constant frequency by an inverter, and the high frequency alternating current is passed through a welding transformer and then passed through a rectifier to be converted into direct current again. In a resistance welding machine adapted to supply the material to be welded through a welding electrode, a voltage detecting means for detecting a DC voltage applied to the welding electrode and measuring the level thereof, and a presettable constant value. Voltage setting means for applying a voltage, comparing means for comparing the measured value of the DC voltage with the constant voltage and generating an error signal indicating an error between the two, and a voltage response means for responding to the error signal and applying to the welding electrode Means for controlling the pulse width of the high-frequency alternating current in synchronism with the constant frequency so that the direct-current voltage is maintained at a constant level.