JPH0367474B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field of the Invention] The present invention relates to a control device for an inverter-type resistance welding machine that can freely set the energizing current and energizing time.

[Technical background of the invention and its problems]

A conventional example of a control circuit for a DC resistance welding machine is shown in Part 3.
As shown in the figure.

In FIG. 3, a DC welding current flowing from a commercial power supply 1 to a welding electrode 5 via a transformer 3 and rectifiers 4a and 4b is controlled by phase control of a thyristor 2.

In the transformer 3, the primary voltage is stepped down to several tenths and supplied to the secondary side, so the welding current flowing to the secondary side ranges from several kiloamperes to several tens of kiloamperes and cannot be directly detected. Since this is difficult, the primary current is detected by a current transformer 6 and converted by a current simulator 11 into an output V _I corresponding to the secondary current.

FIG. 4 shows an example of the circuit configuration of the current simulator 11, in which the primary current _I6 detected by the current transformer 6 is full-wave rectified by a diode 61 and then passed through a resistor 62.
The voltage drop V _I becomes, and the resistors 63, 64,
An output voltage V _I is obtained through a peak charging circuit consisting of diodes 66 and 67, an operational amplifier 65, and a capacitor 68.

The discharge time constant of the peak charging circuit is determined by the product of the resistor 64 and the capacitor 68, so if this time constant is matched to the DC side time constant of the welding machine, the output voltage V _I
is a value corresponding to welding current _I5 .

FIG. 5 shows the relationship between V ₁ and _VI in FIG. 4.

Returning to FIG. 3, the energization time for welding is set by the energization cycle setter 7, and after pressing the start switch 9, the counter 8 counts the power cycles of the commercial power source 1, and the interlock circuit continues until the count value reaches the set value. 14 is turned on and the output of the phase shifter 13 is sent to the drive amplifier 15, thereby controlling the energization time.

On the other hand, the energizing current is set by a current setting device 10, and the difference between the welding current and the welding current detected by a current simulator 11 is amplified by an amplifier 12. The firing phase of the thyristor 2 is controlled by the welding current, thereby controlling the welding current to a set value.

Recently, large-capacity self-extinguishing devices such as high-power transistors and GTOs have become economically available, and there has been a trend to use voltage-source inverters instead of using thyristors to turn on and off the commercial power supply.

In this case as well, conventionally, the energization time has often been set by the number of cycles of the commercial power supply, but this has the following problems.

(1) Since the energization time is set based on the cycle of the commercial power supply, it is difficult to match welding conditions in 50-cycle and 60-cycle regions, and it is necessary to establish two types of welding standards to accommodate the difference in energization time. There is.

(2) The inverter frequency must be an integral multiple of the commercial power supply frequency, making it impossible to optimize and variable control the inverter frequency.

(3) When using an inverter frequency synchronized with the commercial power supply, when operating multiple welding machines in parallel,
The chance of starting and stopping from the fixed phase of the power supply voltage increases, leading to an increase in power supply waveform distortion.

[Purpose of the invention]

The present invention provides an inverter that eliminates restrictions due to power cycles by setting the energization period in terms of time, and also makes it possible to select the optimal frequency for the welding system by making the inverter frequency independent of the commercial power frequency. The purpose of the present invention is to provide a control device for a resistance welding machine of this type.

[Summary of the invention]

The present invention provides a control device for a resistance welding machine that controls the conduction period and conduction current of the primary AC power source of a transformer to control the resistance welding current on the secondary side. In addition to using an inverter circuit that obtains a modulated high-frequency alternating current output, an energization time control circuit that starts and stops the inverter circuit in accordance with a set energization time in synchronization with the inverter frequency, and the resistance welding current A welding current control circuit is installed that detects the primary current of the transformer via a current simulator and controls it according to the set current value.This allows the welding conditions to remain the same even if the frequency of the commercial power source changes. In addition, the inverter frequency is selected to the optimum value according to the welding conditions, reducing energy loss and downsizing the device.

[Embodiments of the invention]

An embodiment of the present invention is shown in FIG.

In Fig. 1, a commercial power supply 1 is converted into a smooth DC voltage via a rectifier 30 and a capacitor 31, and is further converted to a smooth DC voltage by an inverter bridge 32 using transistors and diodes to convert the commercial power supply frequency to a voltage higher than the commercial power supply frequency (generally 500Hz). ~1KHz) is converted into AC power, and the transformer 3 and rectifier 4a,
It is supplied to the electrode 5 via 4b.

The energization time is set by the energization time setting device 71, and when the start switch 9 is pressed, the timer counter 81
starts counting the energization time and sends an energization command signal to the synchronous circuit 19 until the count value reaches the set value.
v You are entering ₈₁ .

The triangular wave generator 16 generates a carrier frequency that pulse width modulates (PWM) the inverter bridge 32, the phase of which is detected by the phase detection circuit 18,
A phase detection signal _v18 is output.

The energization command signal _v81 is generated by the synchronization circuit 19,
The signal v 19 is converted into a signal v ₁₉ synchronized with the phase detection signal v ₁₈ and input to the interlock circuit 14 .

On the other hand, the set current set by the current setting device 10 is compared with the welding current signal V _I obtained by the current simulator 11, and the difference is determined by the amplifier 12.
The amplifier is v ₁₂ , and the triangular wave generator 1
The phase detection signal is compared with the output signal vΔ of 6.
The PWM output signals v _{17a and} v _17b are input to the drive amplifier 15 through the interlock circuit 14, thereby controlling the inverter bridge 32 with PWM during the energization period. The welding current _I5 flowing through the electrode 5 is controlled to the current set value.

FIG. 2 is a time chart of the signals of each part showing the above operation. At time _t1 , start switch 9 is pressed,
This shows the case where the timer counter 81 starts counting and counts up at time _t3 .

In this case, the set value of the energization time is t ₃ − t ₁ ,
Usually set in units of 10ms.

vΔ is a carrier triangular wave voltage, and the phase detection signal _v18 is a signal synchronized with this, and the above counter signals _t1 to _t3 are synchronized with the rising edge of _v18 .
The energization time of the inverter bridge t ₂ to t ₄ by v ₁₉
is determined.

The rise of v ₁₈ appears every two cycles of vΔ, and corresponds to one cycle of the output V ₃ of the inverter bridge 32, so the voltage applied to the transformer always has the same number of positive and negative pulses, and the voltage applied to the transformer always has the same number of positive and negative pulses. 3 does not cause DC bias magnetism.

The PWM generator 17 generates a PWM signal divided every half cycle of v ₁₈ based on “1” and “0” of the phase detection signal v ₁₈ obtained by comparing the output v 12 of the amplifier ₁₂ and the triangular wave voltage vΔ. Output signals v _17a and v _17b are obtained,
By alternately activating the transistors of the inverter bridge 32 connected in H-shape, an alternating current output V ₃ is obtained, and a welding current is obtained via the transformer 3 as in the case of FIG.

〔Effect of the invention〕

As explained above, according to the present invention, the energization time of the welding machine can be set independently of the inverter frequency, so unlike setting based on the number of cycles, it can be set independently of the power supply frequency and the inverter frequency. It becomes possible to unify the welding conditions in the 50 cycle area and the 60 cycle area.

In addition, since the inverter bridge energizes alternately every cycle, the transformer is not subject to DC bias, and the energizing time and inverter frequency can be separated, so the inverter frequency can be set to the characteristics of the transformer and secondary rectifier. It becomes possible to select the optimum value in consideration of the

In other words, in general, increasing the inverter frequency will reduce the size of the transformer, but since the switching loss of the rectifier will increase, there is an optimal value that includes both, and the present invention can freely select the optimal inverter frequency corresponding to this. Therefore, it is advantageous in terms of energy saving and resource saving.

This is particularly effective for resistance welding machines in which the robot is equipped with a welding gun and a transformer, as it is possible to minimize the load on the robot by appropriately selecting the frequency of the inverter.

In addition, since the inverter frequency can be selected independently of the commercial power supply frequency, it is possible to reduce power distortion with respect to the commercial power supply when a large number of welding machines are operated.

[Brief explanation of drawings]

Figure 1 is a circuit diagram showing one embodiment of the present invention, Figure 2 is a circuit diagram showing an embodiment of the present invention.
The figure is a signal time chart of each part showing the operation of Fig. 1, Fig. 3 is a circuit diagram showing an example of a conventional resistance welding machine control device, Fig. 4 is a circuit diagram showing an example of the current simulator in Fig. 3, FIG. 5 is a time chart showing the operation. 1... Commercial power supply, 2... Thyristor, 3... Transformer, 4a, 4b... Rectifier, 5... Electrode, 6...
... Current transformer, 7 ... Current cycle setting device, 8 ... Counter, 9 ... Start switch, 10 ... Current setting device, 11 ... Current simulator, 30 ... Rectifier, 31 ... Capacitor, 32 ... Inverter bridge, 71... Energization time setting device, 81... Timer counter.

Claims (1)

[Claims] 1. In a control device for a resistance welding machine that controls the conduction period and conduction current of the primary side AC power source of a transformer to control the secondary side resistance welding current, a pulse width modulated high voltage source from a commercial power source is used as the primary side AC power source. In addition to using an inverter circuit that obtains an alternating current output of the frequency, an energization time control circuit that starts and stops the inverter circuit in synchronization with the inverter frequency in accordance with a set energization time, and a current simulator that controls the resistance welding current. A control device for a resistance welding machine, comprising a welding current control circuit that detects the primary current of a transformer and controls it in accordance with a set current value.