Welding power supply

Abstract

To provide a welding power supply device capable of reducing the occurrence of an output error.SOLUTION: The welding power supply device includes a switching element, a driver, an operation start point adjuster, and a target follower. The switching element outputs a current to a welding electrode in contact with an object to be welded by a switching operation between a conductive state and a non-conductive state. The drive unit outputs a drive signal for applying a predetermined voltage to the switching element to bring the switching element into a conductive state or a non-conductive state. The operation start point adjustment unit adjusts an operation start point at which the switching element enters a conductive state and a current starts to flow. The target follow-up unit compares a target value of the welding command with an actual output value, calculates a sum of three control terms of a proportional term, an integral term, and a differential term calculated from a deviation signal which is a comparison result, and outputs a calculation result as a control signal of the drive unit.SELECTED DRAWING: Figure 9

Description

[Technical Field]

[0001] The present invention relates to a welding power supply. [Background technology]

[0002] Resistance welding is one of the metal welding methods. Resistance welding is a welding method in which electrodes sandwich a material to be welded and current is passed through the material, generating heat due to the metal's electrical resistance, which then melts the material. Control methods for resistance welding include, for example, an adaptive control method in which a detected voltage value is periodically compared with a voltage value based on a preset reference voltage curve, the welding current is controlled according to the difference between the voltage values, and the voltage between the electrodes is changed in accordance with the reference voltage curve (see Patent Document 1). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Publication No. 57-127584 Summary of the Invention [Problem to be solved by the invention]

[0004] Good welding quality depends on the characteristics of the workpiece. Therefore, good welding quality is achieved by adjusting the welding conditions according to the characteristics of the workpiece. Therefore, in a welding power supply, it is necessary to make the actual output follow the welding command set based on the welding conditions. However, in a welding power supply, an error occurs in the actual output relative to the welding command. Therefore, from the perspective of ensuring good welding quality, it is desirable for the welding power supply to be able to reduce the occurrence of output error.

[0005] The technique of the present disclosure aims to provide a welding power supply that can reduce the occurrence of output errors. [Means for solving the problem]

[0006] The technology disclosed herein employs the following technical means to solve the above-mentioned problems. A welding power supply according to one aspect of the technology disclosed herein is a welding power supply used for resistance welding. The welding power supply includes a switching element, a drive unit, an operation start point adjustment unit, and a target tracking unit. The switching element outputs current to a welding electrode in contact with a workpiece by switching between a conductive state and a non-conductive state. The drive unit outputs a drive signal to the switching element to apply a predetermined voltage, bringing the switching element into a conductive state or a non-conductive state. The operation start point adjustment unit adjusts the operation start point at which the switching element becomes conductive and current begins to flow. The target tracking unit compares a target value of a welding command with an actual output value, calculates the sum of three control terms - a proportional term, an integral term, and a differential term - calculated from a deviation signal representing the comparison result, and outputs the calculation result as a control signal for the drive unit. [Effects of the Invention]

[0007] According to one aspect of the technique of the present disclosure, it is possible to provide a welding power supply that can reduce the occurrence of output errors. In other words, the technique of the present disclosure can improve the ability of the actual output to follow a welding command. [Brief explanation of the drawings]

[0008] [Figure 1] 1 is a schematic configuration diagram of a welding machine according to a first embodiment. [Figure 2] FIG. 2 is a diagram illustrating an example of a circuit configuration of a welding power supply. [Figure 3] FIG. 4 is a diagram showing an example of a welding command waveform. [Figure 4] 1A is a diagram illustrating an example of the configuration of a first feedback circuit, and FIG. 1B is a diagram illustrating an example of the configuration of a second feedback circuit. [Figure 5] FIG. 2 is a diagram showing an example of a circuit configuration of a welding power supply including a first feedback circuit. [Figure 6] 1A is a diagram showing an example of input / output characteristics before and after correction; FIG. 1B is a diagram showing an example of input / output characteristics after correction; [Figure 7] FIG. 10 is a diagram illustrating an example of output error correction when an integrating circuit (I circuit) is used. [Figure 8] FIG. 10 is a diagram illustrating an example of output error correction when a differentiation circuit (D circuit) is used. [Figure 9] 1 is a diagram showing an example of a circuit configuration of a welding power supply according to a first embodiment. [Figure 10] 5A and 5B are diagrams illustrating an example of adjusting the operation starting point of the power MOSFET according to the first embodiment. [Figure 11] FIG. 2 is a diagram illustrating an example of the configuration of a PID circuit according to the first embodiment. [Figure 12] 5A and 5B are diagrams illustrating an example of correction of an output error when a bias circuit and a PID circuit according to the first embodiment are used. DETAILED DESCRIPTION OF THE INVENTION

[0009] Hereinafter, embodiments of the technology of the present disclosure will be described with reference to the drawings. The same or equivalent components, parts, and processes shown in each drawing will be denoted by the same reference numerals, and redundant descriptions will be omitted as appropriate.

[0010] First Embodiment (resistance welding machine) FIG. 1 is a schematic diagram of a welding machine 100 according to this embodiment, illustrating an example of a resistance welding machine. The welding machine 100 according to this embodiment primarily includes a welding power supply 10 and a welding head 90. The welding power supply 10 controls the current (welding current) and voltage (welding voltage) required for welding. The welding power supply 10 is electrically connected to the welding head 90 via predetermined conductive wires Wa and Wb. The welding head 90 primarily includes a drive unit (not shown), a pressure unit (not shown), and two electrodes 91a and 91b. The welding head 90 is driven by the current and voltage supplied from the welding power supply 10 as follows: The welding head 90 brings the two electrodes (welding electrodes) 91a and 91b into contact with a workpiece M placed between them, applies pressure to the workpiece M with a constant force, and passes a welding current through the workpiece M. The workpieces M are two or more metal materials (welding materials) that are not yet joined. Therefore, in the workpieces M, resistance heat (Joule heat) causes some of the metal materials to melt and solidify. As a result, the two or more metal materials are joined.

[0011] (Circuit configuration) Fig. 2 is a diagram showing an example of the circuit configuration of basic welding power supply 1 (comparison device). As shown in Fig. 2, welding power supply 1 has control circuit 11, welding command circuit 12, error amplifier 13, gate driver 14, power MOSFET (Power Metal-Oxide-Semiconductor Field-Effect Transistor) 15, current detection resistor 16, etc. Welding power supply 1 also has FB circuits (feedback circuits) 171-173, switching circuit 17s, etc.

[0012] Control circuit 11 is, for example, a CPU (Central Processing Unit). An output terminal of control circuit 11 is connected to an input terminal of welding command circuit 12. Control circuit 11 sets output conditions (welding conditions) for welding command circuit 12.

[0013] Welding command circuit 12 is provided on a path between error amplifier 13 and control circuit 11. An input terminal of welding command circuit 12 is connected to an output terminal of control circuit 11. An output terminal of welding command circuit 12 is connected to an input terminal of error amplifier 13.

[0014] Welding command circuit 12 generates a reference voltage in accordance with set output conditions, and outputs the generated reference voltage (welding command waveform) to error amplifier 13. Here, the welding command waveform will be described.

[0015] FIG. 3 shows an example of a welding command waveform SC. As shown in FIG. 3, the welding command waveform SC is a time series of values ​​of a welding command and indicates a target value (reference voltage) corresponding to the welding time. Preheat PH corresponds to a predetermined welding period from time t0 to time t1 and is a period during which preliminary welding is performed. Preheat PH also serves as a preparatory period for heating the joint before actual welding to prevent breakdown of insulating coatings between electrodes 91a, 91b and workpieces M, and between workpieces M themselves, and to smooth out surface irregularities. Interval ITV corresponds to a predetermined welding period from time t1 to time t2 and is a period during which output is stopped between preliminary welding and actual welding. Upslope US corresponds to a predetermined welding period from time t2 to time t3 and is a period during which output is increased from zero (0) to the maximum value required for actual welding. Main heat MH corresponds to a predetermined welding period from time t3 to time t4 and is a period during which actual welding is performed while maintaining the maximum output after the output has risen. The downslope DS is a period during which the output is decreased from the maximum value required for actual welding to zero (0).

[0016] The voltage, current, or power of the preheat PH and main heat MH are controlled by a control method. The waveform of the preheat PH is a square wave, while the waveform of the main heat MH is a trapezoidal wave. The waveform of the main heat MH is not limited to a trapezoidal wave. For example, the waveform of the main heat MH can be a square wave, a triangular wave, a sawtooth wave, or other waveforms depending on the setting of the energization time from the upslope US to the downslope DS.

[0017] Such a target value (a set value of a welding command) according to the current application time is stored as digital data inside the power source, for example.

[0018] Returning to the explanation of Figure 2, welding command circuit 12 has a DA converter (Digital-to-Analog Converter) that converts the welding command value from a digital value to an analog value and outputs it as a reference voltage. As a result, welding power supply 1 controls the actual output to follow the reference voltage based on the analog-converted reference voltage using error amplifier 13 and feedback circuits 171-173 located downstream of welding command circuit 12.

[0019] Error amplifier 13 is provided on a path between gate driver 14 and welding command circuit 12 and FB circuits 171 to 173. An input terminal of error amplifier 13 is connected to an output terminal of welding command circuit 12 and, via switching circuit 17s, to each output terminal of FB circuits 171 to 173. An output terminal of error amplifier 13 is connected to an input terminal of gate driver 14.

[0020] Error amplifier 13 is an error amplifier that compares a reference voltage (target value) generated in accordance with welding conditions with a feedback signal and amplifies the error resulting from the comparison. Specifically, when the control method is constant current control, error amplifier 13 receives a feedback signal from current feedback circuit 171 and outputs the amplified error to gate driver 14. When the control method is constant voltage control, error amplifier 13 receives a feedback signal from voltage feedback circuit 172 and outputs the amplified error to gate driver 14. When the control method is constant power control, error amplifier 13 receives a feedback signal from power feedback circuit 173 and outputs the amplified error to gate driver 14.

[0021] The gate driver 14 is provided on a path between the power MOSFET 15 and the error amplifier 13. The input terminal of the gate driver 14 is connected to the output terminal of the error amplifier 13. The output terminal of the gate driver 14 is connected to the input terminal of the power MOSFET 15.

[0022] The gate driver 14 is driven by a driving power supply V dd The gate driver 14 is a gate drive circuit that outputs a gate signal (drive signal) for applying a voltage between the gate (G: Gate) and source (S: Source) of the power MOSFET 15 and controls the ON / OFF of the MOSFET 15. The gate driver 14 outputs the gate signal based on the output signal from the error amplifier 13.

[0023] The power MOSFET 15 is provided in a path between the current detection resistor 16 and the gate driver 14. The input terminal of the power MOSFET 15 is connected to the output terminal of the gate driver 14. The output terminal of the power MOSFET 15 is connected to the input terminal of the current detection resistor 16.

[0024] The power MOSFET 15 is a field-effect transistor, and is a switching element that turns on (conductive state) between the drain (D) and the source (S) when a voltage is applied between the gate (G) and the source (S). The power MOSFET 15 also corresponds to an output driver. FIG. 2 shows an example in which an N-channel (N-Ch) type power MOSFET 15 is used as an output driver. The N-channel type power MOSFET 15 turns on when a positive voltage with respect to the source (S) is applied to the gate (G).

[0025] Power MOSFET 15 controls the output during welding. Welding power supply 1 controls the voltage applied between the gate (G) and source (S). When the voltage applied to the gate (G) is low, the resistance between the drain (D) and source (S) of power MOSFET 15 increases. When the voltage applied to the gate (G) is high, the resistance between the drain (D) and source (S) of power MOSFET 15 decreases. In this way, welding power supply 1 controls the actual output to follow the reference voltage by changing the resistance between the drain (D) and source (S) in accordance with the gate voltage.

[0026] Current detection resistor 16 is provided in a path between the conductive wire Wa, which passes a current through workpiece M, and power MOSFET 15. An input terminal of current detection resistor 16 is connected to an output terminal of power MOSFET 15. An output terminal of current detection resistor 16 is connected to a connection terminal of the conductive wire Wa, which passes a current through workpiece M.

[0027] Current detection resistor 16 is a resistor for detecting the welding current. The welding current is output from power MOSFET 15 to current detection resistor 16. Power MOSFET 15 outputs the welding current by switching between a conductive state and a non-conductive state. The resistance value of current detection resistor 16 is set to be smaller than the contact resistance of workpiece M.

[0028] The feedback circuits 171 to 173 are circuits that perform target control by returning an output signal as an input signal. In FIG. 2, three feedback circuits 171 to 173 corresponding to constant current control, constant voltage control, and constant power control are provided. Specifically, a current feedback circuit 171 corresponding to constant current control, a voltage feedback circuit 172 corresponding to constant voltage control, and a power feedback circuit 173 corresponding to constant power control are provided. Note that the feedback circuits 171 to 173 for constant current control, constant voltage control, and constant power control are collectively referred to as "FB circuit 17n."

[0029] The FB circuit 17n inputs the output current, the output voltage, or the output power to the error amplifier 13 to perform target control of the current, the voltage, or the power.

[0030] Current feedback circuit 171 is provided on a path between the input terminal of error amplifier 13 and current detection resistor 16. The input terminal of current feedback circuit 171 is connected to the connection terminal of current detection resistor 16. The output terminal of current feedback circuit 171 is connected to the input terminal of error amplifier 13. Current detection resistor 16 outputs the detected welding current to current feedback circuit 171. Current feedback circuit 171 inputs the welding current (output current) detected by current detection resistor 16 to error amplifier 13.

[0031] The voltage feedback circuit 172 is provided in a path between the input terminal of the error amplifier 13 and the connection terminals of the conductive wires Wa, Wb that pass current through the workpiece M. The input terminal of the voltage feedback circuit 172 is connected to the connection terminals of the conductive wires Wa, Wb. The output terminal of the voltage feedback circuit 172 is connected to the input terminal of the error amplifier 13. The voltage feedback circuit 172 inputs the welding voltage (output voltage) to the error amplifier 13.

[0032] The power feedback circuit 173 is provided on a path between an input terminal of the error amplifier 13 and each output terminal of the current feedback circuit 171 and the voltage feedback circuit 172. The input terminal of the power feedback circuit 173 is connected to each output terminal of the current feedback circuit 171 and the voltage feedback circuit 172. The output terminal of the power feedback circuit 173 is connected to an input terminal of the error amplifier 13. The power feedback circuit 173 inputs welding power (output power) based on the output current of the current feedback circuit 171 and the output voltage of the voltage feedback circuit 172 to the error amplifier 13.

[0033] The circuits located after the FB circuits 171 to 173 are provided with feedback signal switching circuits 17s.

[0034] The switching circuit 17s is a switching circuit that switches between three control methods, namely, constant current control, constant voltage control, and constant power control, in accordance with a control method switching signal. The control method switching signal is input from the control circuit 11. Therefore, the control method is set in the switching circuit 17s based on the input signal from the control circuit 11, and the switching circuit 17s switches the control method in accordance with the set control method.

[0035] When the set control method is the constant current control method, switching circuit 17s connects the output terminal of current feedback circuit 171 to the input terminal of error amplifier 13, and switches the circuit connection so that a feedback signal of the output current is input to error amplifier 13. When the set control method is the constant voltage control method, switching circuit 17s connects the output terminal of voltage feedback circuit 172 to the input terminal of error amplifier 13, and switches the circuit connection so that a feedback signal of the output voltage is input to error amplifier 13. When the set control method is the constant power control method, switching circuit 17s connects the output terminal of power feedback circuit 173 to the input terminal of error amplifier 13, and switches the circuit connection so that a feedback signal of the output power is input to error amplifier 13.

[0036] (Application of feedback circuit) As mentioned above, welding power supply 1 usually requires that the actual output follow the welding command set based on the welding conditions. Therefore, welding power supply 1 employs a configuration that uses a feedback circuit to cause the actual output to follow the welding command.

[0037] 4A is a diagram showing an example of the configuration of a first feedback circuit Ca. The first feedback circuit Ca corresponds to a basic negative feedback circuit. The first feedback circuit Ca mainly includes an adder, an amplifier, a feedback circuit, etc.

[0038] FIG. 4A shows a configuration example in which a combination circuit of the error amplifier 13, gate driver 14, and power MOSFET 15 corresponds to an amplifier, and FB circuits 171 to 173 for current, voltage, and power correspond to feedback circuits.

[0039] When the gain Av of the combination circuit of the error amplifier 13, gate driver 14, and power MOSFET 15 is large, the input / output gain of the circuit is determined by the reciprocal of the feedback ratio β. For example, if the feedback ratio β is 1 and the control method is constant voltage control, and the reference voltage according to the welding command is 1 [V], the output voltage will also be 1 [V].

[0040] The input of the feedback circuit Ca is V IN If the output is Vo, the transfer function of the input and output of the feedback circuit Ca is Vo / V IN =Av / (1+β*Av). In this case, if the gain Av of the combinational circuit is large, Vo / V IN =1 / β. Therefore, when the feedback ratio β is 1, Vo=V IN This becomes:

[0041] When the configuration of first feedback circuit Ca is applied to welding power supply 1 in this way, local feedback loop 18a including power MOSFET 15 can be configured as shown in Fig. 5. Fig. 5 is a diagram showing an example circuit configuration of welding power supply 1 equipped with first feedback circuit Ca. Specifically, local feedback loop 18a having an overall feedback factor β of 1 can be configured by a combination circuit of error amplifier 13, gate driver 14, and power MOSFET 15. With the above configuration, since gain Av of the combination circuit includes input / output characteristic gm of power MOSFET 15, it is possible to cancel the threshold voltage (threshold of the reference voltage) at which power MOSFET 15 turns ON and current begins to flow.

[0042] However, with the above configuration, the output terminal of the power supply is in an open state (high impedance state), which makes it susceptible to ambient noise. As a result, the power supply becomes unstable. For example, a problem occurs in which a current is mistakenly output when there is no welding command and it should not be output. Because the output terminal is in an open state, no current flows. However, if the workpiece M is brought into contact with the electrodes 91a and 91b in this state, there is a risk of sparks occurring.

[0043] In view of the above, welding power supply 1 prioritizes power supply stability, and employs a configuration in which local feedback loop 18b is formed by error amplifier 13 and gate driver 14, and power MOSFET 15 is separated from the loop, as shown in FIG. 2.

[0044] Fig. 4B is a diagram showing a configuration example of the second feedback circuit Cb, in which a combination circuit of the error amplifier 13 and the gate driver 14 corresponds to an amplifier, and the power MOSFET 15 is separated from the combination circuit.

[0045] The input of the feedback circuit Cb is V IN If the output is Vo, the transfer function of the input and output of the feedback circuit Cb is Vo / V IN =Av*gm / (1+β*Av*gm). Dividing the numerator and denominator on the right side of the equation by Av*gm gives Vo / V IN = 1 / (1 / Av*gm+β). Therefore, 1 / Av*gm remains as the error term.

[0046] In this configuration, the gain Av of the combination circuit of the error amplifier 13 and the gate driver 14 does not include the input / output characteristics gm of the power MOSFET 15. Therefore, the threshold voltage of the power MOSFET 15 cannot be canceled. If the output of the combination circuit is not equal to or higher than the threshold voltage, the power MOSFET 15 will not turn on, resulting in an output error. The error becomes larger when the welding command itself is small or when the output waveform of the welding command upslope US or the like changes in a ramp shape.

[0047] FIG. 6(A) is a diagram showing an example of input / output characteristics before correction. The vertical axis represents the actual output value, and the horizontal axis represents the set value of the welding command. In the input / output characteristics of the combination circuit of error amplifier 13 and gate driver 14, the solid line represents the actual input / output characteristics LO, and the dotted line represents the ideal (target) input / output characteristics LR. The output of the combination circuit can be expressed as (reference voltage-feedback voltage)*gain Av. As shown in FIG. 6(A), in the linear regions of the input / output characteristics LO and LR, in region 1 where the output of the combination circuit is smaller than the threshold voltage of power MOSFET 15, there is no output during welding. Therefore, in region 1, an error occurs between the actual output and the ideal output.

[0048] The error can be reduced by increasing the gain Av of the combination circuit. However, if the output during welding is large or the load resistance is high, setting the gain Av of the combination circuit to a large value makes it more likely to oscillate.

[0049] (output error correction) Typically, the resistance welding welding power supply 1 shown in Figure 2 undergoes the following adjustments during inspection before shipping. In each of the constant voltage, constant current, and constant power control methods, the welding power supply 1 adjusts the output voltage, output current, and output power actually generated at an arbitrary reference resistance (e.g., 1 mΩ) so that they fall within a certain error range compared with the set value of the welding command (e.g., the set value of the reference voltage). The welding power supply 1 performs the above adjustments by changing the gain Av of the combination circuit of the error amplifier 13 and the gate driver 14. At this time, the change in gain Av is kept within a range in which the output does not oscillate.

[0050] Welding power supply 1 also sets a bias (offset) to the set value (digital value) of the welding command, and corrects the threshold voltage of power MOSFET 15.

[0051] 6B is a diagram showing an example of input / output characteristics after correction. The dashed line indicates the input / output characteristics LA after correction. Correction 1 corresponds to the correction by bias setting described above. Adjustment 1 corresponds to the adjustment of the feedback factor β of the second feedback circuit Cb.

[0052] As shown in Figure 6(B), if the bias is simply set, the maximum value of the welding command (maximum digital value) will be saturated in the corrected input-output characteristic LA. Therefore, the conversion coefficient used to convert the welding command set value to an analog value is reduced. This adjustment reduces the error in the maximum value. However, it increases the error in the intermediate and minimum values ​​of the welding command. Therefore, the feedback factor β of the second feedback circuit Cb is adjusted to reduce the error in the entire input-output characteristic.

[0053] (Results of the inventors' study) As described above, in general, adjustments in pre-shipment inspections of resistance welding welding power supply 1 are performed based on a fixed reference resistance value. However, in actual resistance welding, the load resistance value changes dynamically before and after welding. In addition, the initial resistance itself before welding also changes depending on the pressure applied by two electrodes 91a, 91b that contact workpiece M.

[0054] The circuit configuration and adjustment method of welding power supply 1 described above can reduce the output error for load resistance values ​​close to the reference resistance value. However, the circuit configuration and adjustment method of welding power supply 1 increases the output error for load resistance values ​​that differ significantly from the reference resistance value.

[0055] There are two causes for this error. The first cause (Cause 1) is that the upper limit of the output voltage is determined by the value of the gain Av of the combination circuit of the error amplifier 13 and the gate driver 14. The second cause (Cause 2) is the adjustment method that changes the reference voltage and feedback factor β so that the error relative to the reference resistance is reduced.

[0056] (Cause 1) During constant current control or constant power control, if the load resistance is large (five times or more the reference resistance), the value of gain Av is finite, and it is not possible to increase the voltage applied to the threshold voltage of power MOSFET 15. Therefore, the actual output value is smaller than the welding command value (set value).

[0057] Without feedback of the output value, the combination circuit of error amplifier 13 and gate driver 14 increases its output to follow the set value of the welding command. However, the maximum value of output voltage Vo is limited to the value calculated by gm*Av* (reference voltage). Therefore, the upper limit of output voltage Vo is determined to a specific value. Therefore, if the load resistance value Rw [mΩ] is large, it may not be possible to flow the output current Io [A] (=Vo / Rw) set as the welding command.

[0058] As a result, welding power supply 1 experiences a large output error in constant current control that feeds back the output current and in constant power control that feeds back the product of the output voltage and the output current.

[0059] One way to resolve this error is to add an integrating circuit to the second feedback circuit Cb. Figure 7 shows an example of output error correction when an integrating circuit (I circuit) is used. The vertical axis represents the actual output value, and the horizontal axis represents the welding time. The solid line represents the output waveform OW, and the dotted line represents the welding command waveform SC.

[0060] The integrator circuit accumulates the deviation of a preset integral time and feeds it back. As a result, the error between the welding command setting and the actual output value is eventually eliminated. Because the integrator circuit adds the integration result, the upper limit of the output voltage is not set to a specific value.

[0061] However, as shown in waveform portion A of the output waveform OW in Figure 7, the integrator circuit may overshoot depending on the magnitude of the integral gain. Also, as shown in waveform portion B of the output waveform OW in Figure 7, in a configuration using an integrator circuit, the fall of the actual output corresponding to the downslope DS of the welding command may be delayed and unable to follow.

[0062] Furthermore, the integrator circuit's output error correction is not reflected in the actual output until the integration time has elapsed. Therefore, a configuration using an integrator circuit cannot track ramp-like changes in the output waveform, such as the upslope US and downslope DS of the welding command, in real time. The actual output can track the welding command in the final stage of the welding period, but there is a delay in the initial stage.

[0063] Therefore, another solution is to further improve the tracking ability by adding a differential circuit to the second feedback circuit Cb. Figure 8 is a diagram showing an example of output error correction when a differential circuit (D circuit: Differential Circuit) is used. The vertical axis represents the actual output value, and the horizontal axis represents the welding time. The solid line represents the output waveform OW, and the dotted line represents the welding command waveform SC.

[0064] As shown in waveform portion C of the output waveform OW in Figure 8, the output of a differentiation circuit may become unstable midway through the upslope US of the welding command, depending on the magnitude of the differentiation gain. Also, as shown in waveform portion D of the output waveform OW in Figure 8, in a configuration using a differentiation circuit, even if the differentiation gain is increased, if the set value of the welding command is small, the rise of the actual output corresponding to the upslope US of the welding command may be delayed and unable to follow.

[0065] When the welding command waveform SC changes in a ramp pattern, the reference voltage immediately after the welding command waveform SC starts to change approaches zero (0). The current circuit of welding power supply 1 does not have the function to distinguish between a state in which the reference voltage is close to zero (0) and a state in which the reference voltage itself does not exist (no welding is occurring). Therefore, even if a differentiating circuit is added, it is not possible to improve the delay in the rise of the actual output (the rise of the output relative to the reference voltage) corresponding to the upslope US of the welding command.

[0066] In addition, an integrator circuit and a differentiator circuit are added to the current circuit of welding power supply 1, and a bias is set for the voltage between the gate (G) and source (S) of power MOSFET 15. This configuration has a certain effect in improving the delay in the rise of the actual output corresponding to the upslope US of the welding command. However, applying a bias by software means correcting the welding command itself and the feedback factor β from the set values, which may lead to errors when the load resistance Rw changes.

[0067] (Cause 2) In a feedback circuit, the "inverse of the feedback ratio β (1 / β)" of the feedback signal is multiplied by the input value V IN If these values ​​deviate from the set value in the welding command, the error will be large for resistance values ​​that differ significantly from the reference resistance.

[0068] Furthermore, when welding power supply 1 for resistance welding is mounted on an automatic welding machine and used, the conductive wires Wa, Wb (strand wires) leading to welding electrodes 91a, 91b and welding electrodes 91a, 91b deteriorate over long periods of operation, increasing the load resistance Rw of welding power supply 1. As the load resistance Rw increases, the output error relative to the set value of the welding command also increases. Therefore, as the conductive wires Wa, Wb and welding electrodes 91a, 91b deteriorate, the set value of the welding command must be changed.

[0069] In consideration of the above-described results, welding power supply 10 according to this embodiment is configured to improve the delay in the rise of the actual output corresponding to the upslope US of the welding command. Furthermore, welding power supply 10 is configured to reduce the occurrence of an output error relative to the set value of the welding command even when the load resistance value Rw changes.

[0070] Fig. 9 is a diagram showing an example of the circuit configuration of welding power supply 10 according to this embodiment. In the explanation of Fig. 9, parts that are common to the basic configuration shown in Fig. 2 are given the same reference numerals, and explanations of those parts will be omitted. In the following explanation, only the points that are different from the basic configuration will be explained.

[0071] Welding power supply 10 according to this embodiment further includes bias circuit 21, PID (Proportional-Integral-Differential) circuit 22, and second error amplifier 23 to achieve the above configuration.

[0072] (Bias circuit) The bias circuit 21 is provided on a path between the control circuit 11 and the second error amplifier 23. The input terminal of the bias circuit 21 is connected to the output terminal of the control circuit 11. The output terminal of the bias circuit 21 is connected to the input terminal of the second error amplifier 23.

[0073] In bias application by software, if a bias is set to the set value of the welding command itself, the reference voltage of the tracking target changes. To prevent an increase in output error when the load resistance value Rw changes, it is necessary to reduce the influence of the threshold voltage of power MOSFET 15 without changing the reference voltage of the tracking target. Therefore, bias circuit 21 according to this embodiment is not configured to set a bias (offset) to the set value of the welding command itself.

[0074] The bias circuit 21 is an operation start point adjusting circuit that sets a bias and adjusts the operation start timing (hereinafter referred to as the "operation start point") at which the power MOSFET 15 turns on and current starts to flow. That is, the bias circuit 21 corrects the threshold voltage of the power MOSFET 15 and adjusts the operation start timing.

[0075] FIG. 10 is a diagram showing an example of adjusting the operation start point of power MOSFET 15 according to this embodiment. Also, FIG. 10 shows the output characteristics (I D -V GS This is a graph of the characteristic. D is the drain current [A], and V GS is the gate (G)-source (S) voltage [V]. Tc is the case temperature during operation [°C].

[0076] In the power MOSFET 15 shown in the graph of FIG. 10, when a voltage of about 1.24 [V] is applied between the gate (G) and the source (S) in an environment where the case temperature Tc is 100 [°C], the drain (D) and the source (S) become ON (conductive state), and current begins to flow. Therefore, the voltage of 1.24 [V] is the threshold voltage V of the power MOSFET 15 in that environment. GS1 That is, the voltage of 1.24 [V] is the operation start point of the power MOSFET 15 in an environment where the case temperature Tc is 100 [° C.].

[0077] When the power MOSFET 15 is placed in an environment where the case temperature Tc is 25°C, if a voltage of about 1.5V is applied between the gate (G) and the source (S), the drain (D) and the source (S) are turned on (conductive), and a current starts to flow. Therefore, the voltage of 1.5V is the threshold voltage V of the power MOSFET 15 in that environment. GS2 That is, the voltage of 1.5 [V] is the operation start point of the power MOSFET 15 in an environment where the case temperature Tc is 25 [° C.].

[0078] When the power MOSFET 15 is in an environment where the case temperature Tc is −55° C., if a voltage of about 1.9 V is applied between the gate (G) and the source (S), the drain (D) and the source (S) are turned ON (conductive), and a current starts to flow. Therefore, the voltage of 1.9 V is the threshold voltage V of the power MOSFET 15 in that environment. GS3 That is, the voltage 1.9 [V] is the operation start point of the power MOSFET 15 in an environment where the case temperature Tc is −55 [° C.]. In the following description, each threshold voltage V GS1 ~V GS3 When referring to these collectively, the term "threshold voltage V GSn " he says.

[0079] Assume that the maximum output current required for welding power supply 10 used for resistance welding is 3000 [A]. This maximum output current cannot be output by a single power MOSFET 15. Therefore, welding power supply 10 needs to be configured to operate several tens of power MOSFETs 15 in parallel. For example, in the case of a configuration in which 60 power MOSFETs 15 are operated in parallel, a drain current I of 50 [A] can be output from one power MOSFET 15. D According to the output characteristics shown in the graph of Figure 10, a drain current of 50 [A] I D To allow current to flow, the voltage V between the gate (G) and source (S) GS However, it turns out that a voltage of around 3.3V is sufficient.

[0080] Based on the output characteristics of power MOSFET 15 and the assumed temperature (case temperature Tc) of the environment in which welding power supply 10 is used, in welding power supply 10 with a maximum output current of 3000 [A], bias circuit 21 adjusts the operation start point of power MOSFET 15 as follows: Specifically, in a configuration in which 60 power MOSFETs 15 are operated in parallel, bias circuit 21 adjusts the operation start point of power MOSFET 15 (threshold voltage V GSn ) is set to a bias between 0[V] and 1.5[V].

[0081] The control circuit 11 shown in FIG. 9 controls the bias circuit 21 to supply a bias voltage (threshold voltage V GSn The bias circuit 21 outputs a bias voltage to set the gate voltage of the power MOSFET 15 in accordance with the set bias voltage. When the gain Av of the local feedback loop 19 by the second error amplifier 23 and the gate driver 14 in the subsequent stage is applied to the output of the bias circuit 21, the output value after application is adjusted to the threshold voltage V of the power MOSFET 15. GSn The value is configured to be slightly smaller than the value of

[0082] Bias circuit 21 operates only when welding command waveform SC is output from welding command circuit 12. Specifically, bias circuit 21 outputs only during the current application period between times t0 and t5 shown in FIG.

[0083] According to the above configuration, the welding power supply 10 according to this embodiment applies a bias voltage (threshold voltage V GSn The bias setting (threshold voltage correction value) can be set. Welding power supply 10 is configured to move the operation start point of power MOSFET 15 in response to the welding command (reference voltage) to an appropriate position by the bias setting (threshold voltage correction). As a result, in welding power supply 10, power MOSFET 15 turns ON with a slight change in the reference voltage that is the tracking target. Therefore, in welding power supply 10, the rise of the actual output corresponding to the upslope US of the welding command is not delayed, and the welding power supply can track the welding command.

[0084] As described above, welding power supply 10 according to this embodiment includes bias circuit 21. This allows output adjustment to be performed using only the gain Av of second error amplifier 23 and the bias voltage. Therefore, there is no need to correct the reference voltage itself or the feedback factor β of second feedback circuit Cb. Therefore, in welding power supply 10, the output error does not increase even if the load resistance value Rw changes.

[0085] (PID circuit) PID circuit 22 is provided on a path between welding command circuit 12 and FB circuit 17n and second error amplifier 23. Input terminals of PID circuit 22 are connected to output terminals of welding command circuit 12 and FB circuit 17n. Output terminals of PID circuit 22 are connected to input terminals of second error amplifier 23.

[0086] PID circuit 22 shown in FIG. 9 is provided in the circuit configuration of welding power supply 1 shown in FIG. 2 as a means for improving the steady-state deviation with respect to main heat MH and the ability to follow the ramp-like change in welding command waveform SC.

[0087] Fig. 11 is a diagram showing an example of the configuration of the PID circuit 22 according to this embodiment. As shown in Fig. 11, the PID circuit 22 is a target tracking circuit mainly composed of a proportional circuit (P circuit) 221, an integrating circuit (I circuit) 222, and a differentiating circuit (D circuit) 223.

[0088] PID circuit 22 shown in FIG. 11 compares the FB value from FB circuit 17n with the set value (target value) of the welding command from welding command circuit 12. PID circuit 22 obtains the comparison result (e.g., the difference between a reference voltage and an actual output voltage) as deviation signal e(t). PID circuit 22 inputs the obtained deviation signal e(t) to proportional circuit 221, integral circuit 222, and differential circuit 223. PID circuit 22 generates control signal u(t) based on the outputs of proportional, integral, and differential circuits 221 to 223. As a result, PID circuit 22 outputs the generated control signal u(t) to second error amplifier 23 as the actual output value.

[0089] The PID circuit 22 calculates the sum of three control terms, a proportional term, an integral term, and a differential term, calculated from the deviation (deviation signal e(t)), and outputs the calculation result as a control signal u(t). Specifically, the control signal u(t) is expressed as (K P e(t))+(K I ∫e(τ)dτ)+(K D It can be expressed as de(t) / dt).

[0090] Proportional term (K P e(t)) is a simple gain K P The proportional circuit 221 is a circuit that calculates the proportional term. I ∫e(τ)dτ) is a control term proportional to the integral value (accumulated value) of the deviation. The integration circuit 222 is a circuit that calculates the integral term. The differential term (K D de(t) / dt) is a control term proportional to the differential value (rate of change) of the deviation. Differentiation circuit 223 is a circuit that calculates the differential term.

[0091] In the feedback loop 19, the proportional term has the effect of reducing the deviation in proportion to the instantaneous value. The integral term has the effect of bringing the deviation closer to zero. The derivative term has the effect of improving the responsiveness to the proportional term by predicting the future output value based on the rate of change of the deviation. Furthermore, the derivative term also has the effect of improving the stability of the feedback loop 19 by compensating for the delay of the integral term.

[0092] 11 is configured to calculate an input proportional to the deviation signal e(t). The proportional circuit 221 calculates a proportional gain (proportional constant) K P The proportional circuit 221 adjusts the control response by changing the proportional gain K P Increasing the proportional gain K P If becomes too large, overshoot etc. occurs, eventually resulting in undamped oscillation.

[0093] 11, the PID circuit 22 according to this embodiment includes an integrating circuit 222. The integrating circuit 222 multiplies the cumulative value of the deviation signal e(t) (cumulative deviation: ∫e(τ)dτ) by an integral gain (integral constant) K IIn the integration circuit 222, even if the deviation signal e(t) becomes zero (0), the integrator stores the past value.

[0094] The integrator circuit 222 has an integral gain K I If the integral gain K is increased, the response speed increases and the deviation can be reduced, and finally the deviation can be made zero (0). Therefore, the integral circuit 222 can solve the above problem of the proportional circuit 221, and can make the deviation as close to zero (0) as possible. However, the integral circuit 222 has a phase delay of 90 degrees due to the frequency characteristics of the integrator. This delay affects the tracking ability of the integral circuit 222. Furthermore, the integral circuit 222 also has an integral gain K I If becomes too large, overshoot etc. occurs, leading to oscillation.

[0095] The PID circuit 22 according to this embodiment further includes a differentiation circuit 223. The differentiation circuit 223 calculates the differential value (de(t) / dt) of the deviation signal e(t) by applying a differential gain (differential constant) K D The input is configured to calculate the multiplication result of the input.

[0096] The differentiator 223 can predict the future output value based on the rate of change of the deviation. This allows the differentiator 223 to increase the stability of the feedback loop 19 and to adjust the proportional gain K P and integral gain K I It also reduces the overshoot that occurs when the differential gain K is large. D is adjusted to obtain critical damping responsiveness of the output to changes in the welding command set value (target value) or FB value. Critical damping responsiveness means responsiveness to critical damping at which the output waveform OW does not oscillate. Therefore, by predicting changes in deviation and adjusting the critical damping responsiveness, differentiating circuit 223 can make the actual output follow the welding command even when the load resistance value changes dynamically before and after welding or when the welding command waveform SC changes in a ramp shape.

[0097] According to the above configuration, welding power supply 10 according to this embodiment has the above-described characteristic PID circuit 22. Welding power supply 10 can reduce the steady-state deviation with respect to main heat MH by increasing the response speed using proportional circuit 221. Furthermore, welding power supply 10 can make the steady-state deviation zero (0) by using integrator circuit 222.

[0098] Additionally, welding power supply 10 uses differentiating circuit 223 to predict future output values ​​based on the rate of change of the deviation, and adjusts the output to obtain critical damping responsiveness to changes in the set value (target value) or FB value of the welding command. This allows welding power supply 10 to make the actual output follow the welding command even when the load resistance value changes dynamically before and after welding or when the welding command waveform SC changes in a ramp shape.

[0099] (2nd error amplifier) 2 compares a reference voltage (target value) with a feedback signal and amplifies the error resulting from the comparison. In this embodiment, a PID circuit 22 compares the reference voltage (target value) with the value of the feedback signal (actual output value). Thus, in this embodiment, the PID circuit 22 functions as a first error amplifier corresponding to the error amplifier 13 shown in FIG. 2, and is configured to include a second error amplifier 23 in the subsequent stage to which a control signal u(t) is input.

[0100] (Effectiveness verification results) 12 is a diagram showing an example of output error correction when bias circuit 21 and PID circuit 22 according to this embodiment are used. The vertical axis represents the actual output value, and the horizontal axis represents the welding time. The solid line represents the output waveform OW, and the dotted line represents the welding command waveform SC.

[0101] As can be seen from a comparison of the correction example in Fig. 12 with the correction examples in Fig. 7 and Fig. 8, the occurrence of output errors can be reduced by the correction method of welding power supply 10 according to this embodiment. In a circuit configuration using only integrator circuit 222 or a circuit configuration using only differential circuit 223, the falling edge of the actual output corresponding to the downslope DS of the welding command is delayed and cannot be tracked, as shown in the output waveform OW in Fig. 7 or Fig. 8.

[0102] In contrast, in the welding power supply 10 according to this embodiment, the bias circuit 21 generates a bias voltage (threshold voltage V GSn The operation start point of power MOSFET 15 is adjusted by setting a correction value for the feedback factor β (i.e., the feedback factor β) to the welding command signal β. As a result, in welding power supply 10, the rise delay is improved as shown in waveform portion E of output waveform OW in Fig. 12. In other words, in welding power supply 10, the rise delay can be improved without correcting the welding command itself and feedback factor β from their set values.

[0103] In a circuit configuration using only integrator circuit 222, as shown in the output waveform OW in Figure 7, adjustment of the integral gain can result in overshoot. Furthermore, this configuration cannot follow ramp-like changes in the output waveform, such as the upslope US and downslope DS of the welding command, in real time. Furthermore, in a circuit configuration using only differential circuit 223, as shown in the output waveform OW in Figure 8, adjustment of the differential gain can result in the output becoming unstable midway through the upslope US of the welding command.

[0104] In contrast, in welding power supply 10 according to this embodiment, proportional circuit 221 of PID circuit 22 reduces the deviation in proportion to the instantaneous value, integrator circuit 222 brings the deviation closer to zero, and differential circuit 223 predicts future output values, thereby improving responsiveness. As a result, welding power supply 10 can obtain a stable output even during the upslope US of the welding command without generating an overshoot, as shown in output waveform OW in Figure 12. Furthermore, in welding power supply 10, the actual output follows the welding command from the early stage of the welding period, reducing the output error relative to the set value of the welding command.

[0105] Table 1 shows the verification results that show that the output error (actual output value relative to the welding specification set value) was improved when the load resistance value Rw was changed. Table 1 shows the maximum value of the output error [%] for each control method when the reference load resistance value Rw was 1 [mΩ], and the maximum value of the output error [%] for each control method when the load resistance value Rw was 5 [mΩ] after the load resistance was changed. [Table 1]

[0106] As shown in Table 1, welding power supply 10 according to the present embodiment has improved output error compared to a comparative device (welding power supply 1) that does not include bias circuit 21 and PID circuit 22, even when the load resistance value Rw varies. When the load resistance varies, welding power supply 10 according to the present embodiment has a smaller maximum output error value for each control method than welding power supply 1. Specifically, when the load resistance value Rw is 5 mΩ, the output error during constant voltage control is 53% for welding power supply 1 and 9% for welding power supply 10. Furthermore, the output error during constant current control is 15.6% (absolute value) for welding power supply 1 and 5% (absolute value) for welding power supply 10. Furthermore, the output error during constant power control is 60.48% for welding power supply 1 and 5.54% for welding power supply 10.

[0107] (Variation) In this embodiment, the power MOSFET 15 is used as the switching element, but the present invention is not limited to this. The switching element may be a semiconductor switch such as an IGBT (Insulated Gate Bipolar Transistor).

[0108] The present invention is not limited to the above-described embodiments and modifications, and various modifications are possible within the scope of the gist thereof. Furthermore, the configurations shown in the embodiments and modifications can be combined in any manner. That is, although the present invention has been described based on the embodiments, it is understood that the present invention is not limited to the embodiments, structures, etc. The technology of the present disclosure encompasses various modifications and modifications within the scope of equivalents. In addition, various combinations and forms, as well as other combinations and forms including only one element, more than one element, or less than one element, are also within the scope and concept of the technology of the present disclosure. [Explanation of symbols]

[0109] 10: Welding power supply; 11: control circuit; 12: Welding command circuit; 13: Error amplifier; 14: Gate driver; 15: current sensing resistor; 171~173,17n: FB circuit; SC: welding command waveform; OW: Output waveform

Claims

1. A welding power supply device used for resistance welding, a switching element that outputs a current to a welding electrode that is in contact with the workpiece by switching between a conductive state and a non-conductive state; a drive unit that outputs a drive signal for applying a predetermined voltage to the switching element to bring the switching element into the conductive state or the non-conductive state; an operation start point adjusting unit that adjusts an operation start point, which is the timing at which the switching element is brought into the conductive state and the current starts to flow; a target tracking unit that compares a target value of a welding command with an actual output value, calculates the sum of three control terms, i.e., a proportional term, an integral term, and a differential term, calculated from a deviation signal that is the comparison result, and outputs the calculation result as a control signal for the drive unit.

2. The switching element is a threshold voltage at which the conductive state is established and the current begins to flow; The operation start point adjustment unit 2. The welding power supply according to claim 1, wherein the threshold voltage is corrected by setting a predetermined bias voltage, and the operation start point is adjusted.

3. The operation start point adjustment unit 3. The welding power supply according to claim 2, wherein the bias voltage is set so that the threshold voltages of the plurality of switching elements operated in parallel to realize a maximum output current of the welding power supply are set to a predetermined voltage value.

4. Further, a welding command unit that outputs the welding command is provided. The operation start point adjustment unit 3. The welding power supply according to claim 2, which operates only when the welding command is output from the welding command unit.

5. The target tracking unit In the proportional term, an input is calculated by multiplying the deviation signal by a proportional constant, and the proportional constant is changed to adjust the response of the control; In the integral term, an input is calculated by multiplying the cumulative value of the deviation signal by an integral constant, and the deviation is approximated to zero by changing the integral constant; 2. The welding power supply according to claim 1, wherein, in the differential term, an input is calculated by multiplying a differential value of the deviation signal by a differential constant, and the differential constant is changed to adjust a critical damping response at which the actual output value does not oscillate with respect to the target value of the welding command.

6. a feedback unit that feeds back, as an input signal, an output signal corresponding to each of the control methods of constant current control, constant voltage control, and constant power control; The target tracking unit 2. The welding power supply according to claim 1, wherein a target value of the welding command is compared with the value of the input signal fed back as the actual output value.

7. A welding power supply device used for resistance welding, a field effect transistor that outputs a current to a welding electrode that is in contact with a workpiece by switching between a conductive state and a non-conductive state; a gate drive circuit that outputs a drive signal for applying a predetermined voltage to the field effect transistor, thereby bringing the field effect transistor into the conductive state or the non-conductive state; an operation start point adjusting circuit that adjusts an operation start point, which is the timing at which the field effect transistor is turned on and the current starts to flow; a PID circuit that compares a target value of a welding command with an actual output value, calculates the sum of three control terms, i.e., a proportional term, an integral term, and a differential term, calculated from a deviation signal that is the comparison result, and outputs the calculation result as a control signal for the gate drive circuit; a welding command circuit that outputs the welding command; a control circuit that sets an output condition for the welding command circuit and sets a predetermined bias voltage for the operation start point adjustment circuit, the operating point adjusting circuit is provided in a first circuit path between the control circuit and the field effect transistor; The PID circuit is provided in a second circuit path between the welding command circuit and the field effect transistor.

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