JPH0212672B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a constant current control circuit for a resistance welding machine that controls welding current to a set effective value.

Conventionally, in resistance welding machines where the power source is connected to the welding gun via an inverter, a thyristor is mainly used as the inverter's current control element. The welding current was controlled by controlling the .

Therefore, in order to control the welding current to a predetermined effective value, the welding current of one or more previous cycles is sampled and held, and the sample and hold value is fed back to determine the firing angle of the next cycle. It had been decided.

Thus, when the power supply voltage, welding load, etc. change during the cycle, the effect appears as a change in the welding current, resulting in disadvantages such as a limit in terms of accuracy.

The present invention aims to eliminate such inconveniences, and provides a resistance welding machine in which a power supply circuit is connected to a welding gun via an inverter. An arithmetic circuit that calculates the effective value or a value corresponding to the effective value of the current connected to the connected circuit, an effective value setting circuit, and an output of the arithmetic circuit and a setting value set by the effective value setting circuit. a comparison circuit that generates an output signal when the output matches a set value, a maximum pulse width setter, an integrator that integrates the output of the setter, and the set value set by the effective value setting circuit. an inverter element maximum energization pulse width setting circuit comprising a comparison circuit that compares the output of the integrator and generates an output signal when the output of the integrator matches the set value; an overcurrent setting device; and the setting device. an overcurrent setting circuit connected to the inverter, comprising a comparator that compares the output of the current detector with the output of the current detector and generates an output signal when the output of the current detector matches the output of the setting device; a pulse width control circuit for controlling an inverter, and the pulse width control circuit is configured to control an inverter by an output signal generated by a comparison circuit connected to the effective value setting circuit and a comparison circuit of the maximum energization pulse width setting circuit; The present invention is characterized in that the current flowing through the connection circuit is cut off by an output signal of a latch circuit connected to a comparison circuit of the overcurrent setting circuit.

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

FIG. 1 shows a block diagram of one embodiment of the invention. In the figure, reference numeral 1 denotes a power supply circuit, and the power supply circuit 1 includes a rectifier circuit 3 and a smoothing circuit 4 connected to a commercial power supply terminal 2. Reference numeral 5 denotes an inverter composed of transistors, etc., and the AC output of the inverter 5 is applied to the welding gun 8 via a welding transformer 6 and a rectifier circuit 7.

A current detector 9 such as a current transformer is inserted into the input circuit of the inverter 5 (or the secondary circuit of the welding transformer 6) in the connection circuit that connects the power supply circuit 1 to the welding gun 8 via the inverter 5.

10 is a squarer 1 connected to the current detector 9
1 and an integrator 12, which calculates a value corresponding to the effective value of the current flowing through the connection circuit; 13 is an effective value setting circuit; The output correction circuit 16 is configured to output a value corresponding to the effective value of the current set by the current setting device 14.

The effective value setting circuit 13 and the arithmetic circuit 10 were connected to a comparator 17 to form a comparison circuit that generates an output signal when the output of the arithmetic circuit 10 matches the output of the effective value setting circuit 13.

The overshoot correction circuit 16 is composed of, for example, an attenuator, and detects that the current flowing through the connected circuit matches a set value, and then controls the inverter 5.
This circuit is for correcting the time delay until the transistors 5 ₁ to 5 ₄ are turned off. Since this time delay has a correlation with the current value, in order to perform accurate correction, the overshoot correction circuit 6 should be configured with an attenuator, a differential amplifier, etc., and one input terminal of the amplifier should be connected as shown by the chain line in the figure. is connected to the squarer 11, and the square value of the current is also used as a determining factor for the correction amount.

Further, instead of the current setting device 14, the current setting input may be applied to the squarer 15 from the terminal 18 as shown by the chain line.

19 is a control circuit for controlling the output pulse width of the inverter 5; Constituent transistors 5 ₁ , 5 ₂ , 5 ₃ ,
5 ₄ base drive circuits 23 ₁ , 23 ₂ , 23 ₃ ,
When a low level signal is input to the pulse width control circuit ₂₁ , a high level signal is output that operates the inverter 5, and when a high level signal is input, the inverter 5 is disabled. It now outputs a low-level signal that activates and cuts off the current. In the figure, 24 is a setting circuit for setting the maximum energizing pulse width of the transistors 5 ₁ to 5 ₄ to ensure the operation of the inverter 5, and the setting circuit 24 includes a maximum pulse width setter 25, an integrator 26, and a It consists of a comparator 27 that compares the output of the integrator 26 and the output Vref ₂ of the squarer 15, and is connected to the pulse width control circuit 21 via the NAND gate 20.
When the output of 0 matches the output Vref ₂ of the squarer 5 and reaches the set maximum pulse width, the pulse width control circuit 21 is activated and the inverter 5 is deactivated by the signal output. Reference numeral 28 denotes an overcurrent setting circuit, which includes an overcurrent setting device 29 and a comparator 30 that compares the output of the overcurrent setting device 29 with the output of the current detector 9. When the set overcurrent is exceeded, the pulse width control circuit 21 is activated, the inverter 5 is deactivated, and the transistor 5 is activated.
₁ to 5 ₄ are now protected.

FIG. 2 shows an example of the pulse width control circuit 21. In the same figure, 32, 33, 34, 35 are
36 and 37 are flip-flops, 38 is an OR gate, and 39 is a monostable multivibrator whose terminal output is at a low level for a short time with a pulse. Then for each input
High level signals are alternately output from the AND gates 34 and 35 with a short period of low level between them, and when the output of the NAND gate 20 is input, the outputs of both AND gates 34 and 35 are output. Both operate at low levels.

Next, to explain the operation of the circuit shown in the first diagram, when the inverter 5 is activated and welding current is supplied to the welding gun 8, the current flowing in the connecting circuit is detected by the current detector 9, and the arithmetic circuit 10 is activated. The squarer 11 outputs a current obtained by squaring the current as shown in FIG. 3A. This current is integrated by an integrator 12, and a voltage having a waveform as shown in FIG. 3B is output. This voltage corresponds to the effective value of the current flowing in the connected circuit, and is the voltage corresponding to the effective value of the current value set by the current setting device 14 in the comparator 17.
When it is compared with Vref ₁ and matches this voltage Vref ₁ ,
The comparator 17 outputs a low level signal as shown in FIG. 3D.

Thus, this signal controlled the pulse width control circuit 21 via the NAND gate 20, rose in synchronization with the output pulse of the oscillator 22, and flowed to the transistors 5 ₁ , 5 ₂ or 5 ₃ , 5 ₄ of the inverter 5. The pulse current is cut off when the output of the pulse width control circuit 21 becomes low level.

Since the voltage Vref ₁ is a value corrected by the overshoot correction circuit 6 as described above,
As shown in FIGS. 3A and 3B, even if the current flowing through the connection circuit continues to flow slightly after the point in time when the output voltage of the integrator 12 matches Vref ₁ , the current setting stops when the current is cut off. This becomes the effective value of the current set by the device 14.

Thus, the squared current waveform shown in Figure 3A,
, even if the amplitude changes depending on the power supply voltage and the value of the load resistance, the effective value of each current remains the set value.

When the load impedance becomes particularly high due to some abnormality, the amplitude of the squared current waveform decreases as shown in FIG. 3A, and the integrated voltage of the integrator 12 decreases as shown in FIG. When Vref ₁ is not reached even when the longest energizing pulse width of the transistors 5 ₁ to 5 ₄ set in 24 is reached, the output of the integrator 26 (FIG. 3C) matches the output Vref ₂ of the squarer 15,
Since the comparator 27 outputs a low level signal as shown in FIG. 3E, this output activates the pulse width control circuit 21 via the NAND gate 20 and cuts off the current.

In addition, when the squared current waveform V exceeds the overcurrent detection level Vref ₃ as shown in FIG. 3A due to an overload or a short circuit in the welding gun arm, the comparator 3
0, a low level signal is output as shown in FIG. 3F, and therefore the output of the latch circuit 31 prevents current flow until reset. Incidentally, in the above embodiment, the arithmetic circuit 10 is configured with the squarer 11 and the integrator 12 to calculate the value corresponding to the effective value of the current, but as is clear from the definition of the effective value,
Of course, an averaging circuit and a square root circuit may be further provided to calculate the effective value, and the effective value setting circuit 13 may be configured in the same manner.

In this way, according to the present invention, an arithmetic circuit is provided that calculates the effective value or a value corresponding to the effective value of the current flowing in the circuit that connects the power supply circuit to the welding gun via the inverter, and the output of the arithmetic unit is set. The pulse width control circuit is activated by the signal output when the value matches the value, and the current flowing through the inverter's transistors and other elements is cut off, so even if the power supply and load fluctuate, settings can be made more accurately and at higher speeds. It has the effect of being able to control the effective value.

In addition, the pulse width control circuit is operated by the output signal generated by the comparator of the inverter element maximum energization pulse width setting circuit and the output signal of the latch circuit connected to the overcurrent setting circuit to cut off the current flowing to the inverter elements. Therefore, if the load impedance becomes particularly high due to some abnormality,
Even when the welding current does not reach the effective value set by the effective value setting circuit, the operation of the inverter is ensured, and even if the welding current becomes excessive, the inverter elements are not damaged.

[Brief explanation of drawings]

FIG. 1 is a block diagram of one embodiment of the present invention, and FIG.
The figure is a circuit diagram of the main part thereof, and FIG. 3 is a diagram showing waveforms of each part. DESCRIPTION OF SYMBOLS 1... Power supply circuit, 5... Inverter, 8... Welding gun, 10... Arithmetic circuit, 13... Effective value setting circuit, 17... Comparator, 19... Control circuit, 21
... Pulse width control circuit, 22 ... Oscillator, 23 ₁
~23 ₄ ...Base drive circuit, 24...Setting circuit, 27...Comparator, 28...Overcurrent setting circuit.

Claims (1)

[Claims] 1. In a resistance welding machine in which a power supply circuit is connected to a welding gun via an inverter, a current detector inserted into the connection circuit and the effective value or effective value of the current connected to the detector and flowing through the connection circuit. An arithmetic circuit that calculates the value to be set, an effective value setting circuit, and an output signal that compares the output of the arithmetic circuit and the set value set by the effective value setting circuit, and generates an output signal when the output matches the set value. The output of the integrator is compared with the setting value set by the maximum pulse width setting device, the integrator that integrates the output of the setting device, and the effective value setting circuit, and the output of the integrator is An inverter element maximum energization pulse width setting circuit consisting of a comparison circuit that generates an output signal when the set value matches the overcurrent setting device, and the output of the setting device and the output of the current detector are compared, and the current an overcurrent setting circuit comprising a comparator that generates an output signal when the output of the detector matches the output of the setting device; and a pulse width control circuit connected to the inverter and controlling the inverter, the pulse width control circuit comprising: The circuit is activated by the output signals generated by the comparison circuit connected to the effective value setting circuit and the comparison circuit of the maximum energization pulse width setting circuit, and by the output signal of the latch circuit connected to the comparison circuit of the overcurrent setting circuit. A constant current control device for a resistance welding machine, characterized in that the constant current control device is configured to interrupt the current flowing through the connection circuit.