JP2819618B2 - Power supply for arc machining - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial application field> The present invention relates to an improvement of an arc machining power supply device in which the output of a DC power supply is ON-OFF controlled by a switching element and the output is adjusted by the conduction ratio. is there.

<Prior art> In an arc processing power supply device in which the output of a DC power supply is adjusted by a switching element to obtain a desired output, an apparatus for adjusting a pulse time width (PWM method) or a pulse time width which is constant. There is one that changes the repetition frequency (PFM method). All of them detect an output voltage or an output current, obtain a difference signal by comparing with a reference signal, and adjust a pulse time width or a pulse repetition frequency in a direction in which the difference signal decreases. Has used analog signals for both the reference signal and the detection signal.

On the other hand, digitizing these controls for the purpose of improving control performance was considered and some experiments were performed. However, in arc welding, large currents are switched, so the noise generated during switching (ON and OFF) is large, and the ripple content in the output is also large. for,
After passing through a low-pass filter with a relatively large attenuation
The method of D conversion was adopted.

FIG. 5 shows an example of a conventional apparatus. In the figure, 1 is a DC power supply, 2a and 2b are diodes, 3a and 3b are switching elements, and transistors are used in the illustrated example. ,
4 is a transformer, 5a and 5b diodes, 6 is a reactor, 7 is an electrode, 8 is a work to be welded, and the diodes 2a, 2b to 5a, 5b constitute a forward converter. 9 is an output current detector, and 10 is an output current detector 9
Is a low-pass filter for removing high-frequency components of the output If, and 11 is a digital signal d
The A / D converter circuit converts the output of the low-pass filter 10 into a digital signal in synchronization with the output pulse of the clock pulse oscillator 12 oscillating at a constant frequency. Reference numeral 13 denotes a reference signal generation circuit that generates a digital signal Ir corresponding to the output of the A / D conversion circuit 11 for determining an output current value. A comparison circuit 14 compares the output Ir of the reference signal generation circuit 13 with the output d of the A / D conversion circuit and outputs an analog signal ΔI according to the difference signal. An oscillation circuit 15 generates a sawtooth wave having a constant frequency. An output circuit 16 compares the output ΔI of the comparison circuit 14 with the output c of the sawtooth wave oscillation circuit 15. When ΔI> c, the switching elements 3a and 3b Is a comparison circuit that outputs a signal s for conducting the current. The sawtooth wave oscillation circuit 15 and the comparison circuit 16 constitute a known PWM control circuit.

The operation of the apparatus shown in FIG. 5 will be described with reference to the waveform chart of FIG. 6A shows the output .DELTA.I of the comparison circuit 14 and the output c of the sawtooth wave oscillation circuit 15, and FIG. 6B shows the comparison circuit.
16 shows the output s. 2C shows the output If of the output current detector, FIG. 2D shows the output of the clock pulse oscillator 12, and FIG. 2E shows the digital amount of the output of the A / D conversion circuit 11. Is schematically shown by the height of a bar line. In the apparatus shown in FIG. 5, the output current detection value If is converted into a digital signal by the A / D conversion circuit 11 and compared with the output Ir of the reference signal generation circuit 13 by the comparison circuit 14 to obtain a difference signal (Ir
−If) is calculated. This difference signal is converted into an analog signal ΔI, and then converted into a signal s by a PWM control circuit including a sawtooth wave oscillation circuit 15 and a comparison circuit 16, and the conduction time width of the switching elements 3a and 3b is determined. At this time, since the PWM control circuit is configured so that the output pulse width changes in the direction of decreasing the difference signal ΔI, the output current is thereby maintained at a value corresponding to the set value of the reference signal generation circuit 13. You will be drowned. As described above, since the switching elements 3a and 3b form a forward converter together with the diodes 2a to 5b and the transformer 4, the conduction time ratio of the transformer 4 is 50% or less in order to prevent magnetic saturation of the transformer 4. Is limited to The output current is smoothed by the reactor 6 and the diode 5b and becomes a continuous DC current.

<Problems to be Solved by the Invention> In the above-described conventional device, a large energy noise is generated when the switching elements 3a and 3b are turned on and off in order to adjust the main current circuit by switching. As shown in FIG. 3C, the noise also mixes in the output current detection signal (If). When the detection signal If containing such noise is sampled by a clock pulse supplied at a constant cycle and converted into a digital signal,
An erroneous signal may be generated by this noise. That is, as shown in FIG. 6D, the clock pulse for sampling is determined by the oscillation cycle of the clock pulse oscillator 12 irrespective of the ON-OFF timing of the switching element. As shown at times t1, t2, and t3 in FIG. 9D, when the switching element happens to be on or off or coincident with the time immediately after that, the detection signal containing large noise at the time of switching is sampled and taken in as it is. That is, a value larger or smaller than the true output current value is recognized as shown by the hatched portion in FIG. 6 (e). For this reason, the PWM control circuit supplies an erroneous pulse width signal to the switching elements 3a and 3b, and a large disturbance occurs in the output current.

The low-pass filter 10 is provided to remove such switching noise.However, if the attenuation factor of the low-pass filter is too large, a response delay occurs between the input signal and the output signal, and a sudden change in the load occurs. And does not respond fast enough to sudden changes in command values.

On the other hand, the frequency of the clock pulse oscillator 12 may be increased to obtain an average value of a plurality of sampling results.However, for this purpose, the sampling cycle is several minutes with respect to the switching cycle of the switching elements 3a and 3b. It is necessary to have a short cycle of 1 or less. However, switching element 3
In general, it is desirable to operate the switching frequencies of a and 3b at a frequency sufficiently higher than the audio frequency, for example, 30 KHz to 50 KHz, in order to improve the output control function and prevent generation of harsh operating noise. Therefore, in the method of shortening the sampling period, it is necessary to set the oscillation frequency of the clock pulse oscillator 12 for sampling to a high frequency of 100 KHz or more, and to respond to such a high frequency clock pulse. In order to sample the input signal and obtain an average value of several sampling results, the configuration of the A / D conversion circuit 11 becomes complicated and expensive.

<Means for Solving the Problems> According to the present invention, when the output detection signal is sampled and converted into a digital signal, the sampling timing is shifted from the switching timing of the switching element, and the output is desirably output at an intermediate time point of the switching. The detection signal is sampled and converted into a digital signal to solve the above-mentioned drawbacks of the conventional device.

<Operation> According to the present invention, as described above, the output detection signal is sampled while avoiding switching at which large noise occurs, so that the low-pass filter can be made small with no response delay, and the sampling can be performed. Can be made the same as the switching cycle, so that an inexpensive and stable device can be obtained.

<Example> FIG. 1 shows an example of the present invention. In this figure, reference numerals 1 to 10 and 13 to 16 denote elements having the same functions as those of the conventional apparatus shown in FIG. Reference numeral 11 denotes an A / D conversion circuit similar to the A / D conversion circuit 11 in FIG. 5, but the sampling timing is obtained by the output of the delay circuit 17.
The delay circuit 17 outputs a pulse when the output c of the saw-tooth wave oscillating circuit 15 is delayed by a predetermined time from the falling point. 2A and 2B are diagrams for explaining the operation of the apparatus shown in FIG. 1, wherein FIG. 2A shows changes in the output c of the sawtooth wave oscillation circuit 15 and changes in the output ΔI of the comparison circuit 14, and FIG. Comparison circuit 16
The output s of (c) shows the output If of the detector 9. (D) shows the output of the delay circuit 17, and (e) schematically shows the size of the output (d) of the A / D conversion circuit 11 by the height of a bar.

In the apparatus shown in FIG. 1, the delay circuit 17 is activated from the time when the output signal c of the sawtooth wave oscillation circuit 15 falls, and the set time td
After the pulse. This pulse is output from the A / D conversion circuit 11
Is supplied as a sampling pulse. A / D conversion circuit 1
Reference numeral 1 denotes a sampling circuit for sampling the signal of the output If of the output current detector 9 through the low-pass filter 10 for each sampling pulse, converting the signal into a digital signal d,
To supply. In the comparison circuit 14, the output of the reference signal generation circuit 13
Ir is compared with the output d of the A / D conversion circuit 11 to obtain a difference signal, which is converted into an analog signal and output as an error signal ΔI. This error signal ΔI is the output c of the sawtooth wave oscillation circuit 15.
And a comparison circuit 16 to output a signal s for turning on the switching elements 3a and 3b only during a period of ΔI> c.

Here, the delay circuit 17 is activated from the time when the output c of the sawtooth wave oscillation circuit 15 falls, and generates a pulse with a delay of the set time td. As shown in FIG. 1, when the main circuit is constituted by the forward converter, the ON duty of the switching elements 3a and 3b is set to 50% in order to prevent magnetic flux saturation of the core of the output transformer 4 as shown in FIG. %, The time td may be set to a switching period, that is, a range of not less than 1/2 and not more than 1 of the oscillation period T of the saw-tooth wave oscillation circuit 15. By doing so, the A / D conversion circuit 11 samples and takes in the output current detection signal at a time different from the switching time of the switching elements 3a and 3b, thereby preventing noise from being mixed. The most desirable time of the delay time td is td ≒ 3/4 · T.

As described above, the sampling timing needs to be after the noise generated by the ON or OFF of the switching elements 3a and 3b has disappeared. However, sampling is performed once in one switching cycle to obtain a correct detection value. For this purpose, the sampling time may be determined to be a time that is substantially equal to the average value of the output current.

FIG. 3 shows another embodiment of the present invention.
In the embodiment shown in the figure, the delay circuit 17 of the embodiment shown in FIG.
Instead, the delay time td = Ton + 1/2 (T-Ton) from the fall of the output signal of the sawtooth wave oscillating circuit 15 using the output c of the sawtooth wave oscillating circuit 15 and the output ΔI of the comparison circuit 14 as inputs. (1) (where Ton is the conduction time of the switching elements 3a and 3b and is proportional to the signal ΔI. T is the switching cycle and the length of one cycle of the signal c). A delay time calculating circuit 18 for generating a pulse after the delay time is provided, and the other components are the same as those in the apparatus shown in FIG. FIG. 4 is a diagram for explaining the operation of the apparatus of the embodiment shown in FIG. 3, and FIGS. 4 (a) to 4 (c) and 4 (e) show (a) to (c) of FIG. ) And (e), and (d) shows the output of the delay time calculation circuit 18. In the apparatus shown in FIG. 3, the value of the expression (1) is obtained by the delay time calculation circuit 18 as the sampling timing of the A / D conversion circuit 11, so that the sampling of the output detection signal is always performed during the OFF period of the switching elements 3a and 3b. It will be performed at an intermediate position. Therefore, the A / D conversion circuit 11 can obtain a substantially average value of the output current detection values. As a result, a correct value can be obtained by sampling once in one ON-OFF cycle of the switching element.

In FIGS. 1 and 3, the delay time is determined so that the switching element is in the OFF period as the sampling timing of the A / D conversion circuit. However, in the present invention, when the output detection value is A / D converted, The timing at which the detection signal is sampled depends on whether the switching element in the main circuit is ON or OF.
This may be set so as not to coincide with the time of F switching, and this may be performed in the middle of the ON period of the switching element. In this case, for example, in the embodiment of FIG. 3, a circuit that calculates td = 1/2 · Ton is used as the delay time calculation circuit 18, and 1/2 · Ton ≒ 1 from the moment when the switching element is turned on. Sampling may be performed with a time delay corresponding to / 2 · ΔI.

Further, the present invention provides a switching circuit of a known type such as a series chopper type, a bridge type inverter, a half-bridge type inverter or a push-pull type inverter other than the forward converter type shown in FIGS. 1 and 3. Applicable to In these cases, during the ON period of the switching element or during OF
Any method may be used as long as the sampling timing is determined in the middle of the F period, or the ON period and the OFF period may be compared, and switching may be performed so that sampling is performed in the middle of the longer period. Also, in order to prevent a short circuit of the power supply as in the case of the bridge inverter type, the switching element is not connected during the conduction period of the switching element of each of the forward and reverse half waves.
In the case of a switching method that requires an OFF period (dead time), sampling may be performed during this dead time.

Furthermore, the present invention can be applied to the case where the output voltage is used as the output detection value, and the conduction time width (pulse width) is constant for any of these detection values instead of the PWM control shown in the embodiment. It is also applicable to a method in which the conduction time interval is made variable (pulse frequency control: PFM method).
In this case, in the embodiment of FIGS. 1 and 3, a pulse width setting device for setting a pulse width for a fixed time is used instead of the sawtooth wave oscillation circuit 15 and the input signal Δ
A voltage-controlled oscillator (V / F conversion circuit) that generates a pulse having a frequency corresponding to I may be used.

<Effect of the Invention> Since the device of the present invention operates as described above, a low-pass filter for noise absorption inserted between the output detector and the A / D converter can be omitted or simplified, so that a response delay occurs. Instead, the effect of digitizing the feedback system can be fully exhibited. Since the sampling cycle of the output detection value may be equal to the switching cycle of the main circuit, the circuit is simplified, and the A / D is cheaper than the method of shortening the sampling cycle and finding the average value.
A conversion circuit can be used.

[Brief description of the drawings]

FIG. 1 is a connection diagram showing an embodiment of the present invention, FIG. 2 is a diagram for explaining the operation of the apparatus of the embodiment of FIG. 1, and FIG. 3 is a connection showing another embodiment of the present invention. FIG. 4 is a diagram for explaining the operation of the embodiment of FIG. 3, FIG. 5 is a connection diagram showing an example of a conventional device, and FIG. 6 is a diagram explaining the operation of the conventional device of FIG. FIG. 1 DC power supply, 3a, 3b switching element, 4
Transformer, 9 Output current detector, 10 Low-pass filter, 11 A / D converter circuit, 13 Reference signal generation circuit, 14
…… Comparison circuit, 15… Sawtooth wave oscillation circuit, 16 …… Comparison circuit, 17… Delay circuit, 18… Delay time calculation circuit

Claims (2)

(57) [Claims] 1. An arc machining apparatus wherein an output of a DC power supply is adjusted by a switching element, an output is detected and compared with a reference value, and a conduction ratio of the switching element is determined by a difference signal to obtain a predetermined output. In a power supply device, a detector for detecting an output current or an output voltage as an analog signal, and an output signal of the detector is converted into a digital signal by synchronizing with a switching cycle of the switching element and sampling at a time different from the switching time. A power supply device for arc processing, comprising: an A / D conversion unit for performing the switching operation; and a switching element control unit for controlling a conduction ratio of the switching element based on a difference signal between an output of the A / D conversion unit and a reference signal. 2. The arc machining apparatus according to claim 1, wherein the switching element control section sets a sampling time of an output signal of the detector at substantially a middle of an ON period or an OFF period of the switching element. Power supply.