JPH0349666B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a welding current control device for providing constant current characteristics to an arc welding power source.

Arc welding power sources used for TIG welding, etc., detect the welding current, compare it with the set value, and use the error signal to operate the current control section of the welding current circuit.Feedback control allows the current to be maintained even if the welding voltage changes. A constant current characteristic is obtained in which the current is kept almost constant. FIG. 1 is a circuit diagram showing a conventional example of an AC arc welding power source with feedback control. In the figure, 1 is a power transformer, 2 is a current control unit configured by bridge-connecting thyristors, 3 is a reactor, and 4
5 is a current detection section, 5 is a reference voltage generation section for current setting,
6 is an error amplifier that amplifies the difference between the feedback signal from the current detection section and the reference voltage, and 7a and 7b are firing signal generation sections that convert the error amplifier output signals into thyristor firing signals. By applying firing signals to the two sets of gate terminals a, d and b, c, and alternately firing the two sets of thyristors, an AC output current I whose effective value is controlled to be approximately constant can be obtained. I can do it.

FIG. 2 is a detailed diagram of the current detection unit 4, which includes a current transformer 8 for detecting the welding current, a resistor 9 for converting the secondary current of the current transformer into voltage and outputting it as a current detection signal Vi, and a rectifier 10. It is made up of

FIG. 3 shows the welding current waveform and the waveform of the current detection signal by the above circuit. I ₁ is the welding current waveform when positive and negative balance is achieved, and at this time, the current detection signal Vi ₁ also has a substantially uniform waveform with relatively small ripples.

However, in AC TIG welding, for example, in order to control the cleaning effect on the aluminum base material, the positive and negative AC energization periods are sometimes changed to make the welding current waveform unbalanced. In this case, the welding current waveform becomes as shown in FIG. 3 _I2 , and accordingly, the current detection signal _Vi2 also becomes a waveform with large ripples.

Conventionally, when performing constant current control, this Vi ₂ waveform was smoothed with a capacitor and used as a feedback signal, but this required a smoothing capacitor with a considerable capacity (for example, about 1 μF for a rated output current of 300 A). This smoothing capacitor becomes a delay element in the control system, making it impossible to quickly adapt to transient load fluctuations caused by changes in arc length at the start of welding or during welding, resulting in lash current or insufficient current. This can cause the arc to become unstable and sometimes cause the base metal to burn through.

The present invention has been made in view of the above points, and includes a peak detection circuit that detects the peak value of a current detection signal, and a sampling hold circuit that periodically repeats sampling and holding of the detected peak value. By using the output voltage of this sampling and holding circuit as a feedback signal, the ripple of the feedback signal is reduced without impairing the responsiveness of the control system, and stable constant current control can be performed.

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 4 is a diagram showing an embodiment of the present invention, in which parts corresponding to those in FIG. 1 are denoted by the same reference numerals. Although shown in a simplified manner in this figure, the output of the power transformer 1 is controlled by a current control unit 2 consisting of a thyristor, and an AC output current I having a substantially constant effective value is supplied to an arc load (not shown). do. The secondary output of the current transformer 8 constituting the current detection section is converted into a voltage by a voltage signal conversion section 11 consisting of a resistor 9 and a rectifier 10 shown in FIG. 2, and output as a current detection signal Vi. 12 is a current detection signal amplification section.

13 is a diode 14, a holding capacitor a15, a reset switch 16, and a discharge resistor 1
A peak detection circuit including a charging/discharging circuit consisting of 7, 1
8 is a sampling and holding circuit consisting of a sampling _switch 19 and a holding capacitor b20;
In addition, the voltage _VB corresponding to the peak value of the current detection signal Vi obtained by peak holding is obtained by periodically sampling the voltage of this holding capacitor a15 by the sampling switch 19,
This voltage is obtained by being held by the holding capacitor b20, and the output voltage _VB of this sampling and holding circuit is negatively fed back to the error amplifier 6 as a feedback signal. The current setting reference voltage generation section 5 applies a positive signal to the error amplifier 6, and the first
The firing signal generating section 7 corresponding to 7a and 7b in the figure converts the signal from the error amplifier 6 into a thyristor firing signal for phase control, and transmits the signal to the current control section 2.

21 is a sampling signal that turns on the sampling switch 19 and turns on the reset switch 16.
This is a signal generating section which generates a reset signal for the ignition signal generating section 7, and in this embodiment, it consists of a delay circuit which operates based on the signal from the ignition signal generating section 7. The limit switch 16 and the sampling switch 19 are analog switches composed of transistors and the like.

The temporal relationship between reset and sampling by these switches is as shown in Figure 5.The sampling switch 19 is turned on, and after the holding capacitor b20 holds the sampled voltage _VA , the sampling switch 19 is turned on.
OFF, and then immediately turn on the reset switch 16 for a short time to reset the holding capacitor A15.
Reset the voltage V _A of

The waveforms of Vi, V _A and V _B in this embodiment are as shown in FIG. 7, for example. In this case, Figure 3
Since the positive and negative magnitude of the AC welding current is changed as shown in I ₂ , the current detection signal Vi has a waveform with large ripples, but the peak detection circuit 13 and sampling hold circuit 18 cause the feedback signal to is converted into a small ripple signal like _VB .

FIG. 6 shows another embodiment of the present invention, in which the signal generating section 21 generates a sampling signal and a reset signal in synchronization with the power supply frequency.
In Fig. 4, the reset signal is generated based on the thyristor firing signal, but since the thyristor firing timing of the current control section 2 is not constant, the capacitor charging time of the peak detection circuit 13 changes, As a result, variations may occur in the current value I, which is the controlled amount. In the embodiment shown in FIG. 6, since the reset switch 16 is operated in synchronization with the power supply frequency, a circuit for detecting the zero-crossing point of the power supply voltage is required, but the capacitor charging time of the peak detection circuit 13 is always constant. This allows more accurate constant current control. An example of the waveforms of Vi, V _A and V _B at this time is shown in FIG.

Figure 9 shows Vi when the sampling and reset repetition frequency is twice the power supply frequency.
The waveform diagram of V _A and V _B shows the current detection signal V _i in this case
corresponds to a state in which the AC welding current is balanced in positive and negative polarity.

Although the above embodiments have described the control of AC welding current, the present invention can be similarly applied to the case where the average value of DC welding current or pulsed welding current is controlled to be substantially constant. A shunt resistor may be used instead of a current transformer to detect current.

If, as in the present invention, the output voltage of the sampling and holding circuit which detects the peak value of the current detection signal and periodically repeats sampling and holding of the detected peak value is used as the feedback signal, the current detection signal can be compared with the conventional smoothing capacitor. Much smaller capacity (1/
The ripple of the feedback signal can be sufficiently reduced by using a capacitor of 10 or less), and the response speed of the control system can be more than doubled compared to the conventional example. This improves the responsiveness and stability of constant current control, and prevents latch current from flowing at the start of welding due to a delay in response, causing burn-through of the base metal, and instability of the arc due to camera shake of the torch, etc. It can be prevented.

[Brief explanation of drawings]

Fig. 1 is a circuit diagram showing a conventional example of a welding current control device, Fig. 2 is a detailed view of the current detection section, Fig. 3 is a diagram showing the welding current waveform and current detection signal waveform of the conventional example, Fig. 4 is a circuit diagram showing an embodiment of the present invention,
FIG. 5 is an operational waveform diagram of the sampling switch and reset switch, FIG. 6 is a circuit diagram showing another embodiment of the present invention, and FIGS. 7 to 9 are signal waveform diagrams in the present invention. 1: Power transformer, 2: Current control section, 5: Current setting reference voltage generation section, 6: Error amplifier, 7: Starting signal generation section, 8: Current detection current transformer, 11: Voltage signal conversion section , 12: Current detection signal amplification section, 13:
Peak detection circuit, 15: Holding capacitor a, 16: Reset switch, 18: Sampling hold circuit, 19: Sampling switch, 20: Holding capacitor b, 21:
Sampling signal and reset signal generator.

Claims (1)

[Scope of Claims] 1. A welding current control device that maintains a welding current constant through feedback control, comprising: a holding capacitor a that maintains the peak value of a signal for detecting the welding current; and a reset switch that discharges the holding capacitor a. a peak detection circuit including a sampling switch and a holding capacitor b that periodically repeats sampling and holding of the peak value from the peak detection circuit; and a signal generating section for generating a signal to a reset switch of the peak detecting circuit and a sampling switch of the sampling and holding circuit, and sampling the periodically generated output of the peak detecting circuit according to the signal from the signal generating section. A welding current control device comprising a control circuit that performs feedback control by using a held signal as a feedback signal.