JPH065027Y2 - Inverter resistance welding machine power supply - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial application] The present invention relates to a power supply device of an inverter resistance welding machine having a constant current control circuit.

[Prior Art] An inverter-type resistance welding machine rectifies commercial alternating current into direct current, converts this direct current into pulsed high frequency alternating current of a constant frequency by an inverter, and passes this high frequency alternating current through a welding transformer to convert it back into direct current. The direct current is supplied to the material to be welded through the welding electrode to perform resistance welding. Since high-frequency alternating current is passed through the welding transformer, the size of the transformer can be reduced, it can be easily incorporated into the robot, and the heat generation efficiency is high and the power consumption can be saved.

In order to control the welding current constant in such an inverter type resistance welding machine, a feedback control type constant current control circuit is used. This type of constant current control circuit detects the welding current or the primary current (inverter output current) of the welding transformer, compares the detected value with a set value, and controls the switching operation of the inverter according to the comparison error. Therefore, when the current exceeds the set value, the output of the inverter, that is, the pulse width of the pulsed high frequency alternating current is narrowed, and when the current is smaller than the set value, the pulse width of the pulsed high frequency alternating current is widened. Done.

[Problems to be Solved by the Invention] By the way, there is a case where the contact between the materials to be welded or the contact between the material to be welded and the welding electrode is not good at the initial stage of energization in resistance welding. In particular, if dust, oil, etc. are present, the contact resistance becomes very high, and it becomes difficult for current to flow.
Then, the feedback type constant current control circuit controls the switching operation of the inverter in the direction in which the current flows.

This will be described with reference to FIG. This figure shows the waveform of a pulsed high frequency alternating current output from the inverter, where T is constant in a half cycle and ΔT is variable in pulse width. When the voltage between the welding electrodes and the resistance value are V and R, respectively, the effective current Irms is expressed by the following equation.

Irms = V · ΔT / R · T The welding voltage V is substantially constant without being affected by the welding current or the resistance value between electrodes. The period T is also fixed. Then, as described above, when the material to be welded or the welding electrode is in a poor contact state at the initial stage of energization and the interelectrode resistance value R is too high, the constant current control circuit widens the pulse width ΔT so as to raise Irms to the set value. .

Then, unless the capacity of the power supply device is particularly small, the pulse width ΔT increases up to the half cycle T, and the high-frequency AC voltage is in a full-wave state. However, at this point, there is no or little current. When the condition of contact between the material to be welded or the welding electrode is improved, a high-frequency alternating current suddenly flows out as a full wave, and a large direct current flows in the material to be welded. As a result, splash (bombing) may occur or the material to be welded may be destroyed.

The contact failure at the initial stage of energization as described above is temporary, and a normal AC resistance welding machine performs constant current control at a low speed of 50 Hz or 60 Hz, so that the current does not reach a full wave state. However, in the case of the inverter type, 1
Since the constant current control is performed at a high speed such as KHz, a full wave is reached immediately, and as a result, the above-mentioned problems occur.

The present invention has been made in view of the above problems, and prevents the inverter output from becoming a full wave state when the resistance value between welding electrodes is very high due to a contact failure at the beginning of energization, etc. It is an object of the present invention to provide a power supply device of an inverter type resistance welding machine which is capable of performing stable welding with no occurrence of electric shock.

[Means for Solving the Problems] In order to achieve the above object, the present invention provides a constant current control circuit for controlling the switching operation of an inverter by a feedback control method so that a substantially constant current is passed through a material to be welded. In the power supply device of the inverter type resistance welding machine equipped with, the pulse width upper limit value setting means for setting the upper limit value for the pulse width of the control pulse for controlling the switching operation of the inverter, and the pulse width of the control pulse has the set upper limit value. The pulse width limiting means is provided so as not to exceed the limit.

[Operation] When the current is difficult to flow due to the poor contact condition of the welded portion at the initial stage of energization, feedback control works to increase the current to the set value, and the pulse width of the control pulse and thus the pulse width of the inverter output is increased. Since the inverter type is high-speed control, the pulse width becomes large immediately. However, the pulse width of the control pulse is suppressed below the preset upper limit value by the function of the pulse width limiting means, so that the pulse width of the inverter output is also suppressed below a certain value, and the full wave state is not reached. As a result, even if the contact condition of the welded portion is improved and a current starts to flow, there is no possibility of splashing because the current is not a sudden large current in the full wave state.

The upper limit of the pulse width is set to an appropriate value by the pulse width upper limit setting means according to the material, shape, thickness, etc. of the material to be welded.

[Embodiment] FIG. 1 shows a circuit configuration of a power supply device according to an embodiment of the present invention.

First, the circuit for performing resistance welding is as follows.

The input terminal of the rectifier circuit 12 is connected to the three-phase commercial AC power supply terminal 10, and DC is obtained at the output terminal of the rectifier circuit 12. This direct current is smoothed by a smoothing circuit composed of the coil 14 and the capacitor 16, and then input to the inverter circuit 18. The inverter circuit 18 is a well-known one composed of power transistors or FETs, and outputs pulsed (rectangular) high frequency AC by chopping the input DC by high frequency switching. This high-frequency alternating current is supplied to the primary side of the welding transformer 20, and a decompressed high-frequency alternating current is obtained on the secondary side of the welding transformer 20.
It is rectified into a direct current by the rectifier circuit 24 composed of a and 22b. Then, this direct current is supplied to the materials to be welded 28, 30 via the welding electrodes 26a, 26b.

The constant current control circuit in this power supply device is as follows.

A current sensor 32 such as a toroidal coil or a current transformer is provided on the wiring (primary conductor) between the output terminal of the inverter 18 and the primary coil of the welding transformer 20. The output signal of the current sensor 32 is input to the current effective value arithmetic circuit 34, and the arithmetic circuit 34 outputs the current detection signal Vi representing the effective current value of the high frequency alternating current. The comparator circuit 36 inputs the current detection signal Vi and the reference value signal Vs representing the current setting value from the current setting circuit 38, compares both signals Vi and Vs, and outputs the error signal Er representing the comparison error. Output. This error signal Er is given to the pulse width control circuit 40 through the limiter circuit 48.

The pulse width control circuit 40 controls the high frequency AC output of the inverter circuit 18 by pulse width modulation (PWM),
The sawtooth wave signal SF having a constant frequency as shown in FIG. 2 (A) is input from the frequency generator 42 as a modulation wave, and the comparison logic between the signal SF and the error signal Er is taken to obtain the signal shown in FIG. 2 (B). Pulsed PWM signal SPWM having "H" and "L" levels as shown in FIG. Then, based on the PWM signal SPWM, each transistor in the inverter circuit 18 is controlled to be turned on / off via the inverter drive circuit 46. Therefore, the inverter circuit 18
The pulse width of the output of corresponds to the pulse width of the PWM signal SPWM. The sequence circuit 44 instructs the start and end of energization, and operates the pulse width control circuit 40 and thus the inverter circuit 18 only during the energization time.

The configuration (32 to 46) of such a constant current control circuit is the same as the conventional one, and the primary current of the welding transformer (DC welding current when the current sensor 32 is provided on the output side of the rectifier circuit 24) is set as the set value. The feedback control type constant current control is performed so as to make them coincide with each other. However, according to such constant current control, when the condition of contact between the materials to be welded 28, 30 or the condition of contact between the materials to be welded and the welding electrodes 26a, 26b is poor at the beginning of energization, the current is difficult to flow, The negative feedback control works strongly to rapidly widen the pulse width of the PWM signal SPWM and thus the pulse width of the inverter output, and the full-wave waveform state is immediately established. Then, as described above, when the contact condition of the welded portion is improved, a large current suddenly flows out, and splash is likely to occur.

In order to prevent such a situation, in this embodiment, a limiter circuit 48 is provided between the comparison circuit 36 and the pulse width control circuit 40.
To provide. The limiter circuit 48 prevents the error signal ER given to the pulse width control circuit 40 from exceeding the upper limit value EU, and thereby prevents the pulse width of the PWM signal SPWM from exceeding a predetermined upper limit value TU.

The operation of the limiter circuit 48 will be described with reference to FIG. When the current becomes difficult to flow, the error signal Er output from the comparison circuit 36 increases, and the pulse width of the PWM signal SPWM and thus the pulse ΔT of the inverter output increases accordingly. Since the inverter type is high-speed control, the pulse width ΔT
Grows up quickly. However, since the error signal ER given from the limiter circuit 48 to the pulse width control circuit 40 is suppressed below the upper limit value EU, the pulse width ΔT is limited to the upper limit value T.
It is suppressed to U or less, and therefore the full wave state (ΔT
= T) is not reached. As a result, even if the contact condition of the welded portion is improved and a current starts to flow, since the current is not a large current suddenly, splash does not occur and the materials to be welded 28 and 30 are not likely to be destroyed.

The pulse width upper limit value setting circuit 50 is a circuit for setting the upper limit value EU of the limiter circuit 18 corresponding to a desired pulse width upper limit value TU. For example, when the upper limit value EU is defined by the Zener diode built in the limiter circuit 48, the setting circuit 50 may be configured as a switching circuit that switches the number of connected Zener diodes. In addition, the pulse width upper limit value TU is the material to be welded 28, 3
It is selected according to the material, shape, thickness, etc.

As described above, in the present embodiment, the limiter circuit 48 limits the error signal ER to limit the PWM signal SPWM and thus the pulse ΔT of the inverter output. However, pulse width limiting means of other types or configurations may also be used. Is possible, for example, in the pulse width control circuit 40 or at a subsequent stage, P
A circuit that directly limits the pulse width of the WM signal SPWM is also possible.

[Effects of the Invention] The present invention has the following effects by having the above-described configuration.

Since the pulse width upper limit value setting means and the pulse width limiting means limit the pulse width of the control pulse to the desired upper limit value or less, the constant current of the feedback control method when the current is difficult to flow due to poor contact of the welded portion. Even if the control circuit operates to widen the pulse width of the control pulse and thus the pulse width of the inverter output, it does not exceed the upper limit value and reaches the full wave state, preventing the occurrence of splash and the destruction of the welded material, Stable and high quality welding results can be obtained.

[Brief description of drawings]

FIG. 1 is a block diagram showing a circuit configuration of a power supply device according to an embodiment of the present invention, and FIG. 2 is a signal waveform diagram for explaining respective operations of a pulse width control circuit and a limiter circuit of the embodiment, And FIG. 3 is a signal waveform diagram for explaining the operation of the constant current control circuit based on the feedback control method. 18 ... Inverter circuit, 36 ... Comparison circuit, 40 ... Pulse width control circuit, 48 ... Limiter circuit, 50 ... Pulse width upper limit value setting circuit.

Claims (1)

[Scope of utility model registration request] 1. A power supply device for an inverter resistance welding machine, comprising a constant current control circuit for controlling a switching operation of an inverter by a feedback control method so that a substantially constant current is passed through a material to be welded, the switching operation of the inverter. Pulse width upper limit value setting means for setting an upper limit value for the pulse width of the control pulse for controlling the pulse width, and pulse width limiting means for preventing the pulse width of the control pulse from exceeding the set upper limit value. A power supply device characterized by the above.