JPH0320316B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to an improvement of an inverter type DC resistance welding machine.

(Conventional technology) In a conventional inverter type DC resistance welding machine,
The welding current is set to a certain value depending on the shape of the workpiece and gun, etc., and based on this current value, welding is carried out by an inverter with a certain frequency determined with priority given to minimizing the weight of the welding transformer.
Controlled by PWM (PULSE WIDTH MODULATION) method.

Therefore, if the workpiece to be welded changes, the shape of the gun changes, and the welding current value changes, it is generally necessary to create a new welding transformer based on the frequency of the above-mentioned inverter, and in that case, the original welding transformer The disadvantage was that the priority condition of minimizing the weight of the welding transformer could not be maintained, and the welding transformer could not be made smaller.

(Problems to be Solved by the Invention) Therefore, the present invention provides a structure in which the welding current can be controlled within a range where the welding transformer is not magnetically saturated so that the frequency of the inverter can be changed while keeping the weight of the welding transformer to a minimum. This is a technical problem to be solved.

Here, first, the main points of the configuration, operation, and effects of the present invention will be specifically explained.

That is, the electric circuit of the present invention shown in FIG. 2 can be represented by the equivalent circuit shown below (where E: voltage,
f: frequency, r: resistance, L: inductance), the formula for calculating the welding current ia in that case is In this state, the present invention converts the direction of welding current ia from + to - or from - to + when welding current ia reaches one of the set welding current values of +Ia and -Ia. Therefore, as shown in Fig. 4, when the set welding current value Ia is high, the time to change the direction of welding current ia is longer than when the set welding current Ia is low, as shown in Fig. 5, and the equation (1) above is satisfied. The welding current frequency f is automatically lowered, the reactance (2πfL) of the load circuit is reduced, and as a result, a sufficient welding current can be obtained.

Also, it can be obtained from the magnetic characteristic equation of the transformer (where N: number of turns of the transformer, φ: instantaneous magnetic flux number, e: instantaneous voltage) e=N: dφ/dt (here, Φ: number of magnetic flux, F: frequency ) φ=I/N・E・I/2F…(2) It is preferable that φ in this equation (2) is kept constant at a value just before saturation determined by the transformer, and in that case, the large welding current When the power is on,
Even if the power supply voltage in the factory decreases, Φ in equation (2) can be maintained at a value on the verge of saturation due to the decrease in the welding current frequency f due to the application of a large welding current, and large welding can be performed even in a state where the power supply voltage is reduced. Enough current can be obtained.

In addition, in the case of the present invention, the input and output waveforms of the rectifier in a large welding current energization state where the welding current frequency f is low and in a small welding current energization state where the welding current frequency f is high are as shown in the figure below. The ripple of the DC output is small in both the state and the state in which a small welding current is applied, and good welding can be performed even in the state in which a small welding current is applied.

In addition, with each existing welding transformer, the frequency of the welding transformer is kept constant to ensure a Φ on the verge of saturation when the MAX welding current is applied, so the welding control circuit also maintains a specific frequency that varies depending on the transformer. However, in the case of the present invention, because of the variable frequency control method, it is not possible to install transformers with different frequency characteristics in the same welding control circuit. Can be used efficiently.

(Means for solving the problem) The technical means for solving the above problem is to convert the AC waveform from an AC power source into DC using an AC power rectifier circuit, and then convert it to an AC waveform of an arbitrary frequency using an inverter. In an inverter-type DC resistance welding machine with a pulse width modulation method, the output from the welding transformer is rectified by a welding transformer output rectifier circuit to supply welding current to the workpiece. a welding current setting means for setting a welding current value corresponding to the type, etc., a welding current detection means for detecting an actual welding current value to detect magnetic saturation of the welding transformer, and the welding current setting means The set welding current value set by the inverter is compared with the actual welding current value detected by the welding current detection means, and when the actual welding current value reaches the set welding current value, the inverter The present invention provides an inverter-type DC resistance welding machine including frequency control means for changing the output frequency of the inverter by inverting the output.

(Function) Next, the function of the above configuration will be explained. The AC power source is full-wave rectified by an AC power rectifier circuit to become a direct current, which is then inversely converted by an inverter to become an alternating current again, which is input to the primary coil of the welding transformer and generates a secondary current in the secondary coil. This secondary current is rectified again to become a direct current welding current that flows between the two poles of the gun to weld the workpiece.

The above is the action of the welding main circuit. Since the inverter is equipped with a variable frequency control device, the frequency of the inverter's output current input to the primary coil of the welding transformer can be adjusted to an arbitrary welding current setting value. control so that

That is, if the set welding current value set by the welding current setting means is high in accordance with the type of workpiece, etc.
Due to the impedance of the load circuit, the time it takes for the actual welding current value to reach the set welding current becomes longer, and the period of inverter output reversal increases accordingly, so the welding frequency becomes lower when a large current is applied. This allows a large current to be applied to the workpiece without magnetically saturating the welding transformer, and when the set welding current value is low, the time it takes for the actual welding current value to reach the set welding current value is shortened. As the period of inverter output reversal becomes shorter, the welding frequency becomes higher when applying a small current, and the welding transformer is efficiently used near magnetic saturation to apply a small current to the workpiece. be able to.

(Effects of the Invention) As a result, the present invention detects the magnetic saturation of the welding transformer by detecting the excitation current that rapidly increases due to the magnetic saturation of the welding transformer, and increases the inverter frequency beyond that. There is an effect that a welding current of a large or small magnitude can be applied to the workpiece while the inverter output current is stabilized by stopping the change.

(Example) The configuration of one embodiment of the present invention will be described below.

Figure 1 is a block diagram of the inverter-type resistance welding machine, which includes a rectifier circuit 1, an inverter 2, a welding transformer 3, a main controller 4, a magnetic saturation detection circuit 5, a current sensor 6, and a current transformer CT in this embodiment. It is composed of a current sensor 6 and a gun 7,
The object to be welded 9 is welded by being supplied with power from an AC commercial power source 8.

Figure 2 is the electrical circuit diagram of the main welding circuit, rectifier circuit 1.
are three sets of full-wave rectifiers that rectify the commercial AC power supply 8,
The inverter 2 is an inverse converter consisting of bridge-connected transistor switching elements Tr1 to Tr4, and the welding transformer 3 has a secondary winding of a one-turn coil with a winding ratio of, for example, 50:1, and rectifies at the end of the winding. This is a transformer that has built-in connected elements and outputs a DC welding current, and the above constitutes a main welding circuit.

The control circuit for the above welding main circuit is a welding current setting device 1, as shown in the block diagram of Figure 3.
0, comparator 11, T-type flip-flop circuit 12, turn-on turn-off time setting circuit 1
3. The primary welding current conductor 16 that connects the main control device 4 consisting of AND logic elements 14 and 15, the magnetic saturation detection circuit 5, the output terminal of the inverter 2, and the primary coil input terminal of the welding transformer 3. It is composed of a current sensor 6 of a current transformer CT as a secondary coil.

Next, the operation of the above embodiment will be explained.

The commercial AC power source 8 is full-wave rectified by the rectifier circuit 1, then reversely converted by the inverter 2, and becomes an alternating current again, which flows to the primary coil of the welding transformer 3.
A secondary current is induced in the next coil. The secondary current is output as a DC welding current by a rectifier connected to the end of the secondary coil, and the object to be welded 9 is spot-welded by the gun 7.

The above is the operation of the welding main circuit.

If the welding current Ia is now set in the welding current setting device 10, the welding current is sent to the above-mentioned main welding circuit.
ia flows. Then, the current sensor 6 detects the welding current.
A signal voltage Ew corresponding to ia is outputted and inputted to the non-inverting terminal of the comparator 11 via the magnetic saturation detection circuit 5. A comparator 11 is connected to a signal voltage Es corresponding to a set welding current value Ia set by the welding current setting device 10 input to the inverting terminal and an actually flowing welding current ia input to the non-inverting terminal. Compare the corresponding signal voltage Ew, and when the actual welding current ia reaches the set welding current value Ia, a constant voltage signal is generated as shown in Figures 4 and 5.
Output Ed. The signal Ed is input to the input terminal T of the T-type flip-flop circuit 12, which outputs flip-flop signals alternately from the Q and Q terminals, and is also simultaneously input to the turn-on/turn-off time setting circuit 13, so that the opposite switching element Tr1 of the inverter 2 is output. , Tr4 or Tr2, Tr3
A certain waiting time td is set between the off period and the on period.

The AND logic element 14 has two input terminals A and B. A flip-flop signal is input from the output terminal Q of the flip-flop circuit 12 in the previous stage, and one input terminal B has a turn-on/turn-off time setting circuit. Similarly, the AND logic element 15 has two input terminals A' and B', and A' receives the flip-flop signal from the output terminal Q of the flip-flop circuit 12 in the previous stage. However, a waiting time signal is inputted from the turn-on/turn-off time setting circuit 13 to one input terminal B'.

Therefore, after the waiting time td elapses, the input terminals A and B or both A' and B' of the AND logic signal 14 or 15 turn on, and the output terminal X or Y of the AND logic signal 14 or 15 receives an on signal. The ON signal is input to the bases of switching elements Tr1, Tr4 or Tr2, Tr3 of the inverter 2, and the switching elements Tr1, Tr3 are output.
Tr4 or Tr2 and Tr3 are brought into conduction.

Therefore, the output current from the inverter 2 flows from the input terminal U of the welding transformer 3 to the other input terminal V via the primary coil. When the welding current ia flowing through the primary coil increases and the welding transformer 3 approaches magnetic saturation, the excitation current component begins to increase rapidly as shown in FIG. The current sensor 6 outputs a detection signal corresponding to the welding current ia to the magnetic saturation detection circuit 5, and the magnetic saturation detection circuit 5 outputs the signal voltage Ew as information data of the welding current ia which has started to rapidly increase to the comparator 11. Output to the non-inverting terminal of The comparator 11 compares this signal voltage Ew with a signal voltage Es corresponding to the set welding current value Ia inputted to the inverting terminal.

Here, switching element Tr of inverter 2
When Tr1 and Tr4 or Tr2 and Tr3 become conductive, the output current of the inverter 2 flows from the input terminal V or U of the welding transformer 3 to the input terminal U or V via the primary coil. The above has described the action of one cycle, and if it is expressed as a flowchart, it will be as shown in Fig. 7.

That is, when the set welding current value Ia set by the welding current setting device 10 in accordance with the type of workpiece, etc. is high, the actual welding current value ia reaches the set welding current value Ia due to the impedance of the load circuit. Since the period of reversal of the inverter 2 output becomes longer, the welding frequency becomes lower when applying a large current, and the welding transformer 3 is not magnetically saturated and the welding material is greatly affected. If current can be applied and the set welding current value is low, the time it takes for the actual welding current value ia to reach the set welding current value Ia will be shortened, and the inverter 2 output reversal will be reduced by the shortened time. Since the period becomes shorter, the welding frequency becomes higher when a small current is applied, and the welding transformer 3 can be efficiently used near magnetic saturation to apply a small current to the workpiece.

As described above, if the welding current setting value Ia is small, the welding transformer 3 operates at a high frequency, and conversely, if the welding current setting value Ia is set high, the welding transformer 3 operates continuously at a low frequency within the range where the iron core is not saturated. Driven.

The frequency control of the welding current can be controlled by a computer according to the flowchart shown in FIG. 7, as well as by the electric circuit shown in FIG.

Next, the effects of this embodiment will be explained.

(Effect of the invention) In conventional inverter-type DC resistance welding machines, the frequency of the inverter is fixed, so when welding is performed with a fixed value of welding current, it is not difficult to reduce the weight of the welding transformer, but it is necessary to use a dedicated With the exception of welding machines, there are generally a wide variety of objects to be welded, and the guns used are also of various shapes, so it is technically difficult to obtain a welding result of satisfactory quality with a constant welding current. There was a limit to this.

On the other hand, in the inverter type DC resistance welding machine according to the present invention, by automatically varying the inverter frequency according to the set welding current value Ia, the welding current is maintained at the design maximum of the welding transformer until just before the core is magnetically saturated. In addition to the effect of allowing the welding current to flow up to the set welding current value, when the welding current reaches the set welding current value, the direction of welding current is reversed and the frequency is varied, so the same set welding current value When the load increases, the rise of the current slows down, the frequency decreases accordingly, and the welding current flows more easily, preventing uneven welding due to insufficient energization due to load fluctuations. There is an effect that can be done.

In addition, when designing a welding transformer, frequency constraints can be excluded, so the maximum welding current value and weight required by the welded object can be prioritized as design conditions, which is useful when installing the welding transformer on a welding robot, etc. This also has the effect of increasing the degree of freedom in design, such as making it easier to design a robot that accommodates the robot's available weight.

Furthermore, unlike PWM (pulse width modulation) control, since the frequency is changed continuously, a welding current with less ripple can be obtained, which is expected to improve the quality of the workpiece.

On the other hand, in the conventional fixed frequency PWM control method, in order to miniaturize the welding transformer, the frequency of the inverter is set relatively high. However, the present invention indirectly solves this drawback.However, the present invention indirectly solves this drawback.

[Brief explanation of the drawing]

Figure 1 is a block diagram showing the overall control system of the inverter type DC resistance welding machine, Figure 2 is an electrical circuit diagram of the main welding control circuit, Figure 3 is a block diagram of the control circuit, and Figure 4 is the set welding current. Figure 5 is an operation diagram of the control circuit when the value Ia is increased, Figure 5 is an operation diagram of the control circuit when the set welding current value Ia is decreased, and Figure 6 is the operation showing the relationship between magnetic saturation and excitation current. 7 is a flowchart of welding current control. 2...Inverter, 3...Welding transformer, 4...
...Main controller, 5...Magnetic saturation detection circuit, 6...
Current sensor, 7...Gun, 10...Welding current setting device, 11...Comparator, 12...T-type flip-flop circuit, 13...Turn-on/turn-off time setting circuit, 14, 15...AND logic element.

Claims (1)

[Claims] 1. After converting the AC waveform from an AC power source into DC using an AC power rectifier circuit, converting it into an AC waveform of an arbitrary frequency using an inverter and supplying it to a welding transformer, the output from the welding transformer is Welding transformer output rectifier circuit rectifies the welding current to the workpiece in an inverter-type DC resistance welding machine that uses a pulse width modulation method to set the welding current value according to the type of workpiece. current setting means; welding current detection means for detecting an actual welding current value in order to detect magnetic saturation of the welding transformer; and a set welding current value set by the welding current setting means and the welding current detection. Frequency control means for changing the output frequency of the inverter by comparing the actual welding current detected by the means and inverting the output of the inverter when the actual welding current reaches a set welding current value. An inverter-type DC resistance welding machine characterized by being equipped with the following. 2 The frequency control means uses a comparator to compare the set welding current value set by the welding current setting means and the actual welding current value detected by the welding current detection means, and determines the actual welding current value set. The inverter-type DC resistance welding machine according to claim 1, characterized in that the inverter type DC resistance welding machine is a frequency control means that inverts the output of the inverter every time the comparator output is generated when a welding current value is reached.