JP3190791B2 - Control device of resistance welding machine - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a control device for a resistance welding machine.

[0002]

2. Description of the Related Art There is an apparatus which uses an inverter as a control apparatus for a resistance welding machine. FIG. 8 shows a conventional apparatus of this kind. The AC voltage of the AC power supply 1 is converted to DC by the rectifier 2, smoothed by the capacitor 3, and then converted to high-frequency AC of about 1 kHz by the inverter 4 composed of IGBTs (switch elements) 41 to 44. After being converted to a low AC voltage by the heater 5, it is converted to DC by the rectifier 7 and
To supply a direct current welding current. The stray inductance 8 exists in the wiring portion to the welding electrode, and effectively acts to smooth DC welding current.

[0003] The magnitude of the welding current is controlled by a current reference I ^* . That is, the welding current detects the primary current of the transformer 5 through the current transformer 6 and then simulates the DC welding current by the welding current simulator circuit 12 (the primary side of the transformer 5 has an internal rectifier of the welding current). 7 does not flow).
The current controller 13 outputs a current control signal a so as to reduce the error in comparison with the current reference I ^* . This signal a is compared with the triangular wave b output from the carrier generator 14 by the comparator 15 and becomes a PWM signal c. The carrier generator 14 outputs a signal d synchronized with the cycle of the triangular wave b, and the distribution circuit 16 activates one of the output signals e and f according to the signal d. This allows the driving circuit to be driven via AND circuits 17 and 18.
The PWM signal c is alternately applied to 21 and 22.

On the other hand, when the drive signal g is input from the starting circuit 19, the timer 20 operates and the energizing signal h is output only for the set time, and the drive circuits 21 and 22 are activated, whereby the switching signals j and k are changed. Output alternately, IGBT ₄₁ , I
Group of GBT ₄₄ and group of IGBT ₄₃ and IGBT ₄₂
The transformers are turned on alternately, and a high-frequency AC voltage is supplied to the primary side of the transformer 5 to control the welding current and the welding time.

[0005]

The conventional device has the advantage that the response of the current control is fast, the ripple of the current is small, and the inverter frequency is high, so that the transformer can be downsized. However, since a welding current of a direct current flows, the welding electrode is severely worn due to electrolysis, and if the welding process is robotized, the electrode must be replaced frequently, and the line must be stopped temporarily to reduce the operating rate. It is a factor.

Further, the power loss of the rectifier 7 for rectifying the current of tens of thousands of amps is increased from 10% to 20%, and the power loss is correspondingly generated, which causes an increase in the amount of cooling water. The present invention has been made to solve the above-mentioned problem, and an object thereof is to supply an AC square-wave welding current, thereby reducing the wear of a welding electrode, improving the operation rate, and reducing power loss. A control device for a resistance welding machine that has a high-precision current control function that improves efficiency, improves the welding quality by reducing fluctuations in the heat generated at the time of polarity reversal of the welding current, and reduces ripples in the welding current. To provide.

[0007]

Means for Solving the Problems As a first aspect of the present invention,
A DC voltage is converted into a square wave AC voltage and supplied to the primary side of the transformer to supply an AC welding current from the secondary side, and a current reference is compared with the output current of the inverter. A control unit that outputs a control signal that reduces the error,
A conductive PWM signal is output at a constant modulation cycle,
A pulse width modulator for outputting a non-conducting PWM signal according to a result of comparison between the control signal and the output current of the inverter; and a square wave signal for determining the polarity and frequency of the square wave AC voltage. A square wave generator and a drive unit for controlling the inverter according to the PWM signal and the square wave signal are provided.

According to a second aspect of the present invention, in the inverter, two switch elements each having a diode connected in anti-parallel are connected in series between the positive and negative sides of a DC voltage source, and an intermediate point thereof is connected to an AC. The drive section comprises one switch element of one half-bridge circuit and the other switch element of the other half-bridge circuit in response to the conducting PWM signal. To output a voltage having a polarity corresponding to the square wave signal, and to make only one switch element of one half-bridge circuit non-conductive in response to the non-conductive PWM signal. The current flowing through the transformer is circulated through a diode connected in anti-parallel to the other switch element of the circuit.

According to a third aspect of the present invention, the current control unit further integrates an error between a current reference of a DC amount and an absolute value of the output current of the inverter detected via a current detector. An integrator is provided, and a value obtained by adding an output of the integrator to the current reference is output as the control signal.

According to a fourth aspect of the present invention, the pulse width modulator further includes a pulse generator for outputting a pulse at a constant modulation cycle, and a function generator for outputting a sawtooth dither signal synchronized with the modulation cycle. And outputting a non-conducting PWM signal according to a difference value between the control signal and a value obtained by adding a dither signal to the output current of the inverter, while outputting a non-conducting PWM signal when the pulse is generated. A signal holding unit for performing the operation.

According to a fifth aspect of the present invention, there is further provided a reactor connected in parallel to the primary side of the transformer and having characteristics close to saturation characteristics of the magnetic flux density of the transformer, and an exciting current flowing through the reactor. A current reference correction unit for outputting the correction signal as a correction signal, and adding the correction signal to the current reference to correct the current reference.

According to a sixth aspect of the present invention, in the configuration of the third aspect, there is further provided an operation unit for obtaining an effective value of the output current of the inverter, and an amplifier for amplifying an error between the current reference and the effective value. The current reference is corrected by adding the output of the amplifier to the current reference.

According to a seventh aspect of the present invention, in the configuration of the second aspect, the driving unit is further made conductive by the immediately preceding PWM signal for a predetermined period from the time when the square wave signal changes. And a polarity switching control unit for circulating a current flowing through the transformer via a diode connected in anti-parallel to the other switch element of the half bridge circuit and the other switch element of one half bridge circuit.

According to an eighth aspect of the present invention, in the configuration of the fourth aspect, the pulse width modulating section includes a flip-flop that is set by a pulse output from a pulse generating section and outputs a conductive PWM signal, and the conductive PWM. A comparator whose output is reset to a predetermined value by a signal, and a differentiation circuit having a time constant longer than the modulation period for outputting a sawtooth dither signal by resetting the output of the comparator to a predetermined value. Setting the output of the comparator according to the difference between the control signal and the value obtained by adding the dither signal to the output current of the inverter, resetting the flip-flop and outputting a non-conductive PWM signal. .

[0015]

According to the first aspect of the present invention, the square wave generator outputs a square wave signal having a binary state at a predetermined frequency. The current control section outputs a current control signal that changes according to an error between the current reference and the output current of the inverter. The pulse width modulation section outputs a conductive PWM signal at a constant modulation cycle, and outputs a non-conductive PWM signal within the modulation cycle according to the current control signal. The drive unit controls the inverter according to the PWM signal and the square wave signal, and supplies a square wave AC welding current.

According to the second aspect of the present invention, the drive unit outputs one of the half bridge circuits so as to output a voltage having a polarity corresponding to the value of the square wave signal when the conductive PWM signal is supplied. When a switch element and the other switch element of the other half-bridge circuit are made conductive and a non-conductive PWM signal is given, only one switch element of one half-bridge circuit is made non-conductive and one half of the half-bridge circuit is made non-conductive. Tio connected in anti-parallel to the other switch element of the fbridge circuit
The current flowing to the primary side of the transformer is returned through the other switch element of the half-bridge circuit and the other half-bridge circuit.

According to the third aspect of the present invention, the current control unit comprises:
The integrator integrates the error between the current reference of the DC amount and the absolute value of the output current of the inverter detected via the current detector, and outputs a signal whose rate of change changes according to the error value. ,
This signal is added to the current reference and output as a current control signal.

According to a fourth aspect of the present invention, the pulse width modulation section outputs a pulse at a constant modulation cycle by the pulse generation section, and the function generation section generates a sawtooth waveform that monotonically increases or decreases within the modulation cycle from the output point of the pulse. And outputs a conductive PWM signal at the time of output of the pulse by the signal holding unit, and outputs a non-conductive PWM signal within a modulation period according to a difference value between the current control signal and the dither signal.
Output M signal.

According to a fifth aspect of the present invention, the exciting current flowing through the reactor simulates the exciting current flowing through the primary side of the transformer, and the current reference correction unit detects the exciting current flowing through the reactor and outputs it as a correction signal. The current reference is corrected by adding to the current reference.

According to a sixth aspect of the present invention, the arithmetic section outputs the effective value of the output current of the inverter, the amplifier amplifies the error between the current reference and the effective value, and adds the output to the current reference to correct the error. Obtained current reference.

In the invention according to claim 7, when the value of the square wave signal is changed by the polarity switching control unit, the driving unit is turned on by the immediately preceding PWM signal for a predetermined period from that time. The current flowing to the primary side of the transformer is circulated through a diode connected in anti-parallel to the other switch element of the half bridge circuit and the other switch element of one half bridge circuit, and a predetermined period elapses. Then, the inverter is controlled so as to output a voltage having a polarity corresponding to the value of the square wave signal.

In the invention according to claim 8, when the output of the comparator is reset to a predetermined value by the conducting PWM signal,
A sawtooth-shaped dither signal monotonically decreasing with the time constant of the differentiating circuit is obtained, and this dither signal acts so as to be added to the output current of the inverter. The control signal and the inverter
The comparator operates according to the difference value between the output current of the data and the dither signal, and when the output is set and changes, the flip-flop is reset and a non-conductive PWM signal is output.

[0023]

FIG. 1 shows an embodiment corresponding to claims 1 to 5 of the present invention. In FIG. 1, reference numeral 23 denotes a small-capacity reactor that simulates an exciting current of the transformer 5, and the magnetic flux saturation characteristic of the iron core is approximated to the characteristic of the transformer 5. 24 current transformer, 25 is a level detector absolute body value of the exciting current i ₀ which flows through the reactor 23 is for outputting a signal s ₁ when exceeding a predetermined value, 26 the polarity signal to determine the polarity of the exciting current i ₀ polarity detector which outputs a POL, 27 includes a clock pulse generator of a constant frequency, while conduction signal h is active, counted zero clears and outputs a pulse signal s ₂ every time the counting a predetermined clock pulse counter repeating, are zero cleared outputs forcibly pulse signal s ₂ by the signal s _1, the signal s ₄ is preset to a predetermined count. 28 is a pulse signal s ₂ while the energizing signal h is active.
There a square wave circuit which outputs a square wave signal s ₃ binary predetermined frequency value is inverted in every binary input (1,0),
The value of square wave initially output is determined by the signal s ₅ from the preset circuit. Reference numeral 29 denotes a latch circuit, which outputs a polarity signal PO at that time when the energization signal h becomes inactive.
Hold L. Reference numeral 30 denotes a preset circuit which outputs signals s ₄ and s ₅ for initializing the counter 27 and the square wave circuit 28 in accordance with the value of the polarity signal POL held in the latch circuit 29. 31 compensation circuit for outputting a compensation signal i ₀₁ obtained by inverting the polarity of the input signal i ₀ according to the value of square wave signal s _3, 32 is compensated by adding the compensation signal i ₀₁ (the welding) current reference I ^* The adder 33 outputs the output (output of the inverter) current reference I ₁ ^*. Reference numeral 33 denotes the absolute value of the current reference I ₁ ^* and the output current i ₁ of the inverter detected via the current detector 11. An integrator that integrates an error from i _1d to output a control signal a, 34 is an adder that adds the current reference I ₁ ^* and the control signal a and outputs a current control signal b, and 35 is a constant modulation period. A pulse generator which outputs a pulse signal c and outputs a rectangular synchronous signal d having a duty of 50%, and outputs a sawtooth dither signal e monotonically increasing within a modulation period in synchronization with the synchronous signal d. dither circuit, 37 is a control signal b and the inverter - plus dither signal e in absolute body value i _1d of data of the output current A comparator 38 outputs a pulse signal f when the polarity of the difference value is inverted. The comparator 38 outputs a PWM signal j which is set by the pulse signal c and becomes conductive, and is reset by the pulse signal f and becomes non-conductive. A flip-flop 39 for outputting a PWM signal j, and a distribution circuit 39 for outputting signals k and l for controlling driving circuits 40 and 41 for controlling the inverter 4 in accordance with the square wave signal s ₃ and the PWM signal j. is there. Others are the same as those in the conventional art and are indicated by the same reference numerals.

The operation of the level detector 25, the polarity detector 26, the counter 27, the square wave circuit 28, the latch circuit 29, and the preset circuit 30 has been previously proposed (Japanese Patent Application No. 6-14372).
9) Please refer to the detailed description. Here, the part related to the operation of the present invention will be described.

In the above configuration, when the start signal g for starting the energization is input from the start circuit 19, the timer 20 activates the energization signal h for the set welding time. Thus the counter 27 outputs a pulse signal s ₂ at a predetermined period and starts counting of the internal clock pulse, the pulse signal s ₂
There square wave circuit 28 every time it is input and outputs a square wave signal s ₃ inverts the binary. The value of this square wave signal s ₃ is inverted - acts as a signal for determining the polarity of the AC output voltage of the motor 4, the distribution circuit 39 in accordance with the value of the square wave signal s ₃ inverter -
And outputs control signals k and l for turning on the corresponding switch elements of the switch 4. Driving circuit 40 and 41, while conduction signal h is Akuteibu, the function is enabled, the control signal k, to conduct the relevant switching element in accordance with the l, inverter - data 4 corresponding to the value of square wave signal s ₃ outputs a voltage v ₁ of the polarity that supplies a current i ₁ on the primary side of the transformer 5.

On the other hand, the pulse generator 35 has a constant modulation period T
_The pulse signal c is output at ₁ and the flip-flop 38
Although outputs a PWM signal is set conductive by a pulse signal c, inverter - the absolute body value signal the output current i ₁ of the motor is detected through the to the current detector 11 increases, i _1d
Is increased and reaches a target value, a pulse signal f is output from the comparator 37, the flip-flop 38 is reset and outputs a non-conductive PWM signal j, and the supply of power from the inverter 4 is stopped. Thereby, the current i ₁ on the primary side of the transformer 5 starts to decrease. The flip-flop 38 is set for each modulation cycle, and the inverter 4 returns to the original conductive state. Thus, as shown in FIG. 2A, so-called PWM control by so-called instantaneous value control is performed.

When the target value of the welding current is given as the current reference I ^* , the target value of the output current i ₁ of the inverter is determined by the signal b output from the adder 34. That is, a current reference I ₁ ^{* obtained} by adding the compensation signal i ₀₁ output from the compensation circuit 31 to the primary current reference I ^* corresponding to the welding current and compensating for the exciting current of the transformer 5 is output from the adder 32. Is output. The integrator 33 outputs a signal a obtained by integrating the error between the current reference I ₁ ^* and the absolute value i _1d of the output current of the inverter. The adder 34 outputs the current reference I ₁ ^* and the signal a. the adds, inverter - outputs as a target value of the output current i ₁ of the motor.

The square wave signal s ₃ and the PWM signal j distributed according to the circuit 39, inverter as shown in FIG. 2 (a) - by controlling the switch elements of the motor 4 41-44, the square wave signal s ₃ Is 1, the switch elements 41 and 44 are operated to set the positive voltage level v
Outputs _1, when the square wave signal s ₃ 0, the switch element 4
3 and 42 is operated to output the negative voltage v _1. Also, P
When the WM signal j is the conduction command (1), t ₁ −t ₂ , t ₃
−t ₄ or t ₆ −t ₇ , t ₈ −t ₉ ,
Switch elements 41 and 44 or both 43 and 42 conduct,
When the PWM signal j is a non-conduction command (0), t ₂ −t ₃ ,
t ₄ - ₅ or t ₇ -t _8, t ₉ switching element 41 as shown in -t ₁₀ and 44 or 43 and 42 one of the switching element only to the non-conductive alternately. By performing the switching control in this manner, when the PWM signal j is a non-conduction command, the current i ₁ flowing on the primary side of the transformer 5 is not regenerated to the DC power supply side but is returned via the inverter 4 to return. The decrease in i ₁ is reduced, and the ripple can be reduced.
/ 2. Incidentally, when the square wave signal s ₃ changes from 1 to 0, conduction of the switch element 42 as a DC short circuit is not caused by the operation delay of the switching element 44 is performed with a number μs delay called dead time. However, when switching control is performed in this way, the inverter
When the output current i ₁ of the data flows stably reached the target value, the variation width of deviation of the signal b and the i _1d is reduced, compared with a slight noise intrusion, as shown in FIG. 2 (b) vessel
The judgment time point of 37 is affected like t _2A , t _2B and PWM
The pulse width of the signal j changes significantly, inverter - there is a case where the output current i ₁ of the motor becomes unstable. In this embodiment, a dither circuit 36 is provided to reduce the influence of the noise. Dither circuit 36, the signal d is outputted in synchronization with the modulation period T ₁ outputs a dither signal e of the sawtooth wave as shown in FIG. 2 (c), the comparator 37, inverter the signal e - data Of the output current in addition to the signal b in addition to the absolute value i _1d . Thus, the effect of the expanded deviation of the apparent upon leaving the target time t ₂ to a non-conducting PWM signal j noise t _2A, can be relaxed as t _2B. The error between the current reference I ₁ ^* and the absolute value i _1d of the output current of the inverter caused by the introduction of the dither signal e is compensated by the signal a output from the integrator 33.

In accordance with the period T ₂ of the square wave signal s ₃ , the inverter 4 outputs an AC voltage v ₁ as shown in FIG. 3 and applies it to the primary side of the transformer 5. welding current i ₂ of the square wave is supplied. The primary side of the transformer 5 flows a current i ₁ of the sum of the excitation current of the transformer 5 and current corresponding to the welding current i _2. An exciting current i ₀ similar to the exciting current of the transformer 5 flows through the reactor 23. The excitation current i ₀ of the reactor 23 detected via the current transformer 24 generates a compensation signal i ₀₁ whose polarity is inverted every half cycle by the compensation circuit 31 in accordance with the value of the square wave signal s ₃ , and the welding current added to current reference I ^* defining the, inverter - a current reference I ₁ ^* to determine the output current i ₁ of the motor.

When the welding current i ₂ is large (close to 100%), the exciting current of the transformer 5 can be neglected, but when the welding current is small (20% or less), the exciting current of the transformer 5 becomes smaller than the welding current. On the other hand, the error is increased to 5 to 10% or more, and according to this embodiment, the welding current can be supplied with high accuracy by eliminating this error.

The integrator 33 also has a function of compensating for a decrease in welding power during a transition period when the polarity of the welding current i ₂ is inverted.
That is, when there is no integrator 33, as shown in FIG. 4 (a), in the transition period T ₃ in which the polarity is reversed, the welding current i
₂ changes at a predetermined current change rate determined by a circuit constant, and is controlled to be constant when the target welding current is reached. Accordingly, the welding power P (square of i ₂ ) decreases by the amount indicated by the hatched portion of the transition period T ₃ , and the temperature of the weld decreases. Case of providing the integrator 33, the welding current i ₂ is only for the period T ₄ immediately after the transition period T ₃ as shown in FIG. 4 (b) O - Ba - shoe -
To a stable value. The O - Ba - shoe - action of TMG, current reference I ₁ ^* and inverter during the transition period T ₃ -
This is performed by integrating the error of the output current of the data from the absolute value i _1d by the integrator 33 and adding the integrated value (signal a) to the current reference I ₁ ^* . Thus, welding power P can be restored at an early stage reduced minute temperature drop immediately complement a crack weld period T ₄ in the period T _3.

FIG. 5 shows an embodiment corresponding to claim 6 of the present invention.
Shown in In this embodiment, an effective value operation circuit 51 obtains an effective value i _rms of the output current of the inverter from an output current i ₀₁ of the inverter detected via the current detector 11 and obtains a reference I ^{* of the} welding current ^. The difference between the output current and the effective value i _rms is amplified by an amplifier 52 and added to I ^* to obtain a reference I ₁ ^{* for} the output current of the inverter. According to this embodiment, it is possible to compensate for the decrease in the effective value during the transition period when the polarity of the welding current is reversed, and to control the heat value of the welded portion to be constant.

FIG. 6 shows an embodiment according to claim 7 of the present invention.
(A). In this embodiment, in the transition period when the polarity of the welding current is reversed, a device is added to reduce the magnetic flux density of the transformer 5 after the polarity is reversed. That is, to output the pulse signal P of the square wave signal s ₃ values one-shot circuit 45 from a predetermined width every time changes (T _5), a predetermined time period from the square wave signal s ₃ from the delay circuit 46 by the signal P ( T ₅ ), and outputs a square wave signal s _3A that changes with a delay of
The gate of the AND circuit 47 is closed to forcibly turn off the PWM.
Generate a signal. Thus the distribution circuit 39 t ₁ ~t ₂ between the zero voltage output (a value of 1 in the figure) value before the value of the square wave signal s ₃ is changed mode - de (primary current of the transformer 5 is invar The switching element of the inverter 4 is controlled so as to be in a state of reflux through the inverter, and the output voltage of the inverter 5 becomes zero voltage as shown by v ₁ (2) in FIG. Become. Gate of the AND circuit 47 with the pulse signal P is a square wave signal s _3A output from the delay circuit 46 when disappears at time t ₂ is changed to the same value as the square wave signal s ₃ of the normal (the state in FIG. 0) - Is opened, the normal PWM signal j is output, and the distribution circuit
39 performs normal switching control. Therefore,
As shown in v ₁ (2), the output voltage of the data 4 outputs a negative voltage corresponding to the value of the square wave signal s _3A and reaches the target value.
Between ₂ ~t ₄ follower - becomes constant at value supplied welding current after outputting a single voltage. For comparison, when reversing the polarity of the output voltage, the output voltage and welding current without the zero voltage output mode are represented by v ₁ (1) and i ₂ (1).
It was shown to.

According to the present embodiment, the time during which the welding current i ₂ (2) is reversed becomes slightly longer, but the time product of the voltage applied to the transformer 5 becomes zero during the period from t ₁ to t ₂ . t _{1 to} t ₄
It becomes smaller when compared between them. This indicates that the magnetic flux density is low, and that the transformer 5 can be downsized.

FIG. 7 shows an embodiment according to claim 8 of the present invention.
(A). This embodiment simplifies the generation of a dither signal. That is, when the conduction PWM signal j is output, the amount of bias applied to the input of the comparator 37 via the inverting circuit 63 and the resistor 62 is set to zero, and the output n of the comparator 37 is set to a predetermined value (in FIG. 7, (High level state).
When the output n changes to a high level, the capacitor 64,
Displacement current flows through a differentiating circuit composed of resistors 65 and 66 to obtain a sawtooth-shaped dither signal e. This dither signal e is applied to the in-phase input terminal of the comparator 37, and as a result, is added to the absolute value i _1d of the output current of the inverter. When the output n of the comparator 37 changes to a low level (-15 V) due to the difference between the control signal b and the value obtained by adding the dither signal e to the output current i _1d of the inverter, the flip-flop is turned on via the inverting circuit 68. 38 is reset to output a non-conducting PWM signal j, a bias amount is again applied to the comparator 37, and the capacitor 64 is charged with a voltage having the illustrated polarity.

[0036]

According to the present invention, an AC square wave welding current is supplied, so that the wear of the welding electrode is reduced, and the frequency of electrode replacement is reduced, so that the operation rate can be improved. In addition, the response characteristics of the current control can be improved, the noise resistance can be improved, and the ripple of the welding current can be reduced. In addition, it is possible to compensate for a decrease in the temperature of the welded portion when the polarity of the welding current is reversed, and to compensate for the exciting current of the transformer. Good welding can be performed. Further, it is possible to provide a control device for a resistance welding machine capable of improving the magnetic flux utilization rate of a transformer and reducing its size.

[Brief description of the drawings]

FIG. 1 is a configuration diagram of an embodiment corresponding to claims 1 to 5;

FIG. 2 is a waveform chart for explaining the operation of the embodiment.

FIG. 3 is a waveform chart for explaining the operation of the excitation current compensation of the embodiment.

FIG. 4 is a waveform chart for explaining the operation of the integrator 33 of the embodiment.

FIG. 5 is a configuration diagram of a main part of an embodiment corresponding to claim 6;

FIGS. 6A and 6B show an embodiment corresponding to claim 7, wherein FIG.

FIGS. 7A and 7B show an embodiment corresponding to claim 8, wherein FIG.

FIG. 8 is a configuration diagram of a conventional device.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... AC power supply 2 ... Rectifier 3 ... Capacitor 4 ... Inverter 5 ... Transformer 6 ... Current transformer 8 ... Floating inductance 9 ... Welding electrode
11… Current detector 19… Start-up circuit 20… Timer 23… Reactor 24… Current transformer 25… Level detector 26… Polarity detector 27… Counter 28… Square wave circuit 29… Latch circuit
30 ... Preset circuit 31 ... Compensation circuit 32,34 ... Adder
33… Integrator 35… Pulse generator 36… Dither circuit 37
… Comparator 38… Flip-flop 39… Distribution circuit 40
41 ... Drive circuit 45 ... One-shot circuit 46 ... Delay circuit
47… AND circuit 51… Effective value calculation circuit 52… Amplifier 60
~ 62,65,66,69 ... resistor 63,68 ... inverting circuit 64 ... capacitor 67 ... diode

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-4-333380 (JP, A) JP-A-4-4977 (JP, A) JP-A-60-137581 (JP, A) JP-A-1- 293985 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) B23K 11/24 H02M 9/00

Claims (8)

(57) [Claims] 1. An inverter for converting a DC voltage into a square wave AC voltage and supplying it to a primary side of a transformer and supplying an AC welding current from a secondary side, a current reference and an output of the inverter. A control unit for comparing a current and outputting a control signal for reducing the error, outputting a conductive PWM signal at a constant modulation cycle, and responding to a comparison result between the control signal and the output current of the inverter; A pulse width modulator that outputs a non-conducting PWM signal, a square wave generator that outputs a square wave signal that determines the polarity and frequency of the square wave AC voltage,
A control device for a resistance welding machine, further comprising a drive unit for controlling the inverter according to the PWM signal and the square wave signal. 2. The control device for a resistance welding machine according to claim 1, wherein said inverter has two switch elements each having a diode connected in anti-parallel to each other in series between a positive terminal and a negative terminal of a DC voltage source. The drive unit comprises one half-bridge circuit and one half-bridge circuit in response to the conductive PWM signal. The other switch element of the half bridge circuit is turned on to output a voltage having a polarity corresponding to the square wave signal, and only one switch element of one half bridge circuit is turned off in response to the non-conductive PWM signal. And a control device for the resistance welding machine, wherein the current flowing through the transformer is circulated through a diode connected in antiparallel to the other switch element of one half bridge circuit. 3. The control device for a resistance welding machine according to claim 1, wherein the control unit is configured to determine a reference value of a DC current and an absolute value of an output current of the inverter detected via a current detector. A control device for a resistance welding machine, comprising: an integrator that integrates an error with the current reference; and outputting a value obtained by adding an output of the integrator to the current reference as the control signal. 4. The control device for a resistance welding machine according to claim 1, wherein the pulse width modulation section outputs a pulse with a constant modulation cycle, and a sawtooth dither synchronized with the modulation cycle. A function generator for outputting a signal,
A conducting PWM signal is output at the time of the generation of the pulse, and a non-conducting PWM signal is output according to a difference value between the control signal and a value obtained by adding a dither signal to the output current of the inverter.
A control device for a resistance welding machine, comprising a signal holding unit for outputting a signal. 5. The control device for a resistance welding machine according to claim 1, further comprising a parallel connection to a primary side of the transformer,
A reactor having characteristics similar to the saturation characteristics of the magnetic flux density of the transformer, and a current reference correction unit that detects an exciting current flowing through the reactor and outputs the correction signal as a correction signal, and adds the correction signal to a current reference. A control device for a resistance welding machine, wherein a current reference is corrected. 6. The control device for a resistance welding machine according to claim 3, further comprising: an operation unit for obtaining an effective value of the output current of the inverter; and an amplifier for amplifying an error between the current reference and the effective value. Wherein the current reference is corrected by adding the output of the amplifier to the current reference. 7. The control device for a resistance welding machine according to claim 2, wherein the drive unit is further turned on by a PWM signal that is turned on immediately before the time when the square wave signal changes for a predetermined period. And a polarity switching control unit for circulating a current flowing through the transformer through a diode connected in antiparallel to the other switch element of the other half bridge circuit and the other switch element of the one half bridge circuit. A control device for a resistance welding machine. 8. The control device for a resistance welding machine according to claim 4, wherein the pulse width modulation unit is a flip-flop that is set by a pulse output from a pulse generation unit and outputs a conductive PWM signal; A comparator whose output is reset to a predetermined value by a PWM signal, and a differentiating circuit having a time constant longer than the modulation period for outputting a sawtooth dither signal by resetting the output of the comparator to a predetermined value. And setting the output of the comparator according to the difference between the control signal and the value obtained by adding the dither signal to the output current of the inverter, resetting the flip-flop, and generating a non-conductive PWM signal. A control device for a resistance welding machine characterized by outputting.