Patent Application
20140312011
The present invention relates to a method of monitoring plasma arc welding that enables a high energy density, high speed and high quality welding, and a plasma arc welding device that enables a high energy density, high speed and high quality welding.
In general, the plasma arc welding has a higher energy density than other welding such as a gas metal arc (GMA) welding and a gas tungsten arc (GTA) welding. Thus, the plasma arc welding can perform keyhole welding, i.e., can cause the plasma arc to penetrate from a front face (upper face) of a welding base metal (matrix, mother material) to a back face (lower face) while the welding is performed. If the keyhole welding is possible, the welding from the back face of the base metal is unnecessary, and therefore the welding work efficiency is significantly improved. During the keyhole welding, however, the keyhole tends to take an unstable behavior because of various factors, such as the temperature increase of the base metal during the welding, the atmosphere temperature, and magnetic blow caused by grounding. Therefore, how the welding is going on should always be monitored when the welding is performed.
A conventional method of confirming an ongoing situation of welding is disclosed, for example, in Patent Literature 1 (Japanese Patent Application Laid-Open (Kokai) Publication No. 62-89570). In Patent Literature 1, the deviation angle, theta (θ), of the arc flame emitted from a keyhole of the welding workpiece is monitored to monitor the ongoing situation of the welding. Patent Literature 2 (Japanese Patent Application Laid-Open Publication No. 62-93072) teaches a back shield jig tool that is attached to the back face of the welding workpiece to shield the welding target area of the welding workpiece, and detects the voltage between the back shield jig tool and the base metal. Then, the detected voltage is compared with a reference voltage to check the discrepancy of the detected voltage from the reference voltage and confirm the welding situation.
PATENT LITERATURE 1: Japanese Patent Application Laid-Open (Kokai) Publication No. 62-89570
PATENT LITERATURE 2: Japanese Patent Application Laid-Open (Kokai) Publication No. 62-93072
The method, such as that disclosed in Patent Literature 1, however, has to attach the back shield jig tool on the back face of the welding workpiece along the welding line and provide the back shield jig tool with a plurality of light-receiving elements. This entails additional cost, time and labor. Because the arc flame changes with various factors, such as magnetic blow, it is difficult to accurately know (determine) the welding situation if the deviation angle theta of the arc flame is only monitored. The method, such as that discloses in Patent Literature 2, compares the voltage generated between the back shield jig tool and the base metal with the reference voltage, and determines the welding situation based on the discrepancy between the generated voltage and the reference voltage only. Thus, it is difficult to accurately detect presence/absence of abnormalities during the welding. In addition, neither Patent Literature 1 nor Patent Literature 2 can determine the quality (good or bad) of the penetration bead formed on the back face of the base metal, which in particular influences (decides) the decentness of the welding.
The present invention is proposed to address these problems, and an object of the present invention is to provide a novel method of monitoring plasma arc welding that can precisely determine whether or not a stable penetration bead having a constant height will be created without dripping (dropping) and irregularities when the keyhole welding is performed, and a novel plasma arc welding device that can precisely determine whether or not a stable penetration bead having a desired height will be created without dropping and irregularities when the keyhole welding is performed.
In order to overcome these problems, the inventors carried out intensive studies and experiments, and found that there was relationship between shaking behavior or oscillation (frequency) of the weld pool formed on (in) the back face of the base metal during the welding and the behavior (frequency) of the welding voltage output (applied) during the welding. The inventors arrived at the present invention based on such finding.
Specifically, when the above-described keyhole welding is carried out by the plasma arc, as shown in
To achieve the above-mentioned object, the first aspect of the present invention provides a method of monitoring welding that continuously welds a welding target area of a welding workpiece when forming a keyhole in the welding target area of the welding workpiece by a plasma arc. The method includes the step of measuring output (applied) voltages when a constant current is used for the welding. This step is an output voltage measuring step. The method also includes the step of analyzing the frequencies of those welding voltages, among the output voltages measured by the output voltage measuring step, which possibly correlate to the vibration of the weld pool formed on the back face of the base metal during the welding, to obtain the peak frequencies of the welding voltages (output voltages) and their distributions. This step is a welding voltage frequency analyzing step. The method also includes the step of identifying those peak frequencies which possibly correlate to the vibration of the weld pool, based on the frequency analysis results of the welding voltage frequency analyzing step. This step is a peak frequency identifying step. The method also includes the step of comparing the peak frequencies identified by the peak frequency identifying step with a preset frequency range to determine the quality (good or bad) of the welding. This step is a determination step.
With such method, it is possible to determine the quality of the keyhole welding by simply comparing the peak frequencies which correlate to the vibration of the weld pool formed on the back face of the base metal during the welding, with the preset frequency range. In other words, if the peak frequencies which correlate to the vibration of the weld pool are compared with the preset frequency range, it is possible to precisely determine whether a stable penetration bead having a constant height (desired height) can be obtained without dropping and irregularities, when the keyhole welding is performed with a constant current.
The second aspect of the present invention provides another monitoring method for the plasma arc welding defined by the first aspect of the invention, wherein the determination step determines that the quality of the welding is good if those peak frequencies which correlate to the vibration of the weld pool, identified by the peak frequency identifying step, fall in the preset frequency range, and determines that the quality of the welding is bad if the peak frequencies identified by the peak frequency identifying step do not fall in the preset frequency range.
By simply checking the specific relationship between the identified peak frequencies which correlate to the vibration of the weld pool and the preset frequency range, it is possible to precisely determine whether or not a stable penetration bead having a desired height is obtained without dropping and irregularities when the keyhole welding is performed with a constant current.
The third aspect of the present invention provides another monitoring method for the plasma arc welding defined by the second aspect of the invention, wherein the determination step determines that the quality of the welding is bad if there is any peak value equal to or greater than a predetermined level, in addition to the peak value in the frequency range.
By checking the presence/absence of an additional peak that is equal to or higher than the predetermine level, outside the frequency range, it is possible to further precisely determine whether a stable penetration bead having a desired height can be obtained without irregularities.
The fourth aspect of the present invention provides another method of monitoring welding that continuously welds a welding target area of a welding workpiece when forming a keyhole in the welding target area of the welding workpiece by a plasma arc. The method includes the step of measuring output (applied) voltages when a pulse current is used for the welding. This step is an output voltage measuring step. The method also includes the step of analyzing the frequencies of those welding voltages, among the output voltages measured by the output voltage measuring step, which possibly correlate to the vibration of the weld pool formed on the back face of the base metal during the welding, to obtain the peak frequencies of the welding voltages (output voltages) and their distributions. This step is a welding voltage frequency analyzing step. The method also includes the step of identifying those peak frequencies which possibly correlate to the vibration of the weld pool, based on the frequency analysis results of the welding voltage frequency analyzing step. This step is a peak frequency identifying step. The method also includes the step of comparing the peak frequencies identified by the peak frequency identifying step with a pulse frequency of the pulse current to determine the quality (good or bad) of the welding. This step is a determination step.
With such method, it is possible to determine the quality of the keyhole welding with the pulse current by simply comparing the peak frequencies which correlate to the vibration of the weld pool with the pulse frequency of the pulse current. In other words, if the peak frequencies which correlate to the vibration of the weld pool are compared with the pulse frequency of the pulse current, it is possible to precisely determine whether a stable penetration bead having a constant height (desired height) can be obtained without dropping and irregularities, when the keyhole welding is performed with the pulse current.
The fifth aspect of the present invention provides another monitoring method for the plasma arc welding defined by any one of the first to fourth aspects of the invention, wherein when the determination step determines that the welding quality is good under the above-mentioned criteria, the determination step further checks the welding quality based on variations in the welding voltage per unit time and the discrepancy from the reference voltage.
If the welding is determined to be good under the above-described criteria, then it is further determined whether the welding quality is good or bad based on variations in the welding voltage per unit time and the discrepancy of the welding voltage from the reference voltage. This makes it possible to further precisely determine whether the penetration bead having a gentle shape and a desired height can be obtained without dropping and irregularities when the keyhole welding is performed.
The sixth aspect of the present invention provides a plasma arc welding device for continuously welding a welding target area of a welding workpiece while forming a keyhole in the welding target area of the welding workpiece by use of a welding torch. The welding torch is configured to generate a plasma arc. The plasma arc welding device includes an output voltage measuring unit configured to measure an output voltage when a constant current is used for the welding. The welding device also includes a welding voltage frequency analyzing unit configured to analyze the frequencies of those welding voltages, among the output voltages measured by the output voltage measuring unit, which possibly correlate to the vibration of the weld pool formed on the back face of the base metal during the welding, to obtain the peak frequencies of the welding voltages (output voltages) and their distributions. The welding device also includes a peak frequency identifying unit configured to identify those peak frequencies which possibly correlate to the vibration of the weld pool, based on the frequency analysis results of the welding voltage frequency analyzing unit. The welding device also includes a determination unit configured to compare the peak frequencies identified by the peak frequency identifying unit with a preset frequency range to determine the quality (good or bad) of the welding.
With the welding device having such configuration, it is possible to determine the quality of the keyhole welding performed with the constant current, by simply comparing the peak frequencies which correlate to the vibration of the weld pool with the preset frequency range, like the first aspect of the present invention. In other words, if the peak frequencies which correlate to the vibration of the weld pool are compared with the preset frequency range, it is possible to precisely determine whether a stable penetration bead having a constant height can be obtained without dropping and irregularities, when the keyhole welding is performed with the constant current.
The seventh aspect of the present invention provides another plasma arc welding device for continuously welding a welding target area of a welding workpiece while forming a keyhole in the welding target area of the welding workpiece by use of a welding torch. The welding torch is adapted to generate a plasma arc. The welding device includes an output voltage measuring unit configured to measure an output voltage when a pulse current is used for the welding. The welding device also includes a welding voltage frequency analyzing unit configured to analyze the frequencies of those welding voltages, among the output voltages measured by the output voltage measuring unit, which possibly correlate to the vibration of the weld pool formed on the back face of the base metal during the welding, to obtain the peak frequencies of the welding voltages (output voltages) and their distributions. The welding device also includes a peak frequency identifying unit configured to identify those peak frequencies which possibly correlate to the vibration of the weld pool, based on the frequency analysis results of the welding voltage frequency analyzing unit. The welding device also includes a determination unit configured to compare the peak frequencies identified by the peak frequency identifying unit with a pulse frequency of the pulse current to determine the quality (good or bad) of the welding.
With the welding device having such configuration, it is possible to determine the quality of the keyhole welding with the pulse current by simply comparing the peak frequencies which correlate to the vibration of the weld pool with the pulse frequency of the pulse current, as in the fourth aspect of the present invention. In other words, if the peak frequencies which correlate to the vibration of the weld pool are compared with the pulse frequency of the pulse current, it is possible to precisely determine whether a stable penetration bead having a constant height can be obtained without dropping and irregularities, when the keyhole welding is performed with the pulse current.
According to the present invention, it is possible to precisely determine whether a stable penetration bead having a constant (desired) height can be obtained by the keyhole welding without dropping and irregularities, by simply comparing the peak frequencies which correlate to the vibration of the weld pool formed on the back face of the base metal during the welding with the preset frequency range.
A method of monitoring plasma arc welding and a plasma arc welding device according to embodiments of the present invention are now described with reference to the accompanying drawings.
As shown in
As shown in
The welding power source 30 feeds a predetermined voltage to provide a necessary current to generate the plasma arc 16 between the welding torch 10 and the base metal 15. The current value and the voltage value are precisely controlled by the welding controller 50. The welding power source 30 may supply a pulse current having, for example, a rectangular waveform, at a predetermined frequency as shown in
The welding controller 50 includes a central control unit 51, a storage unit (database) 52, an output voltage measuring unit 53, a welding voltage frequency analyzing unit 56, an input unit 54 and an output unit 55. The central control unit 51 has information processing devices (e.g., CPU, ROM, RAM, and input/output interface) for the computer system and other components. The central control unit 51 controls the above-mentioned components 10-40 and other components based on the operation instructions entered from the input unit 54 and/or appropriate control programs.
The storage unit (database) 52 is a storage device including HDD and semiconductor memories, which enables the data writing and reading. The storage unit 52 stores not only various control programs but also, at least, various welding conditions as well as data about the different natural frequencies of the weld pool to be formed on the back side of the keyhole for the respective welding conditions. The programs and data in the storage unit are writable and readable.
As such, the storage unit (database) 52 stores, at least, a plurality of welding conditions and the information about the natural frequencies of the weld pool P, which correspond to the respective welding conditions, in the form of database. Each (each set) of the welding conditions uniquely decides the natural frequency of the weld pool P. The welding conditions may include conditions related to the welding workpiece 14 and conditions related to the welding work. The conditions related to the welding workpiece 14 may include the material (type of the base metal), the plate thickness t (see
The output voltage measuring unit 53 measures, always or at desired timing, the output voltage of the welding power source 30 and sends the measured output voltage to the welding voltage frequency analyzing unit 56 and the central control unit 51. The welding voltage frequency analyzing unit 56 analyzes the frequency of that welding voltage which correlates to the frequency of the weld pool P, among the output voltages measured by the output voltage measuring unit 53, so as to obtain the peak frequency and the distribution thereof. The welding voltage frequency analyzing unit 56 sends the analysis results to the central control unit 51. The input unit 54 may have various types of input devices such as a keyboard and a mouse. The welding conditions and operation commands/instructions are entered from the input unit 54. The output unit 55 may have various types of output devices such as a monitor device (e.g., CRT and LCD) and a speaker. The output unit 55 displays the welding conditions entered from the input unit 54 to confirm the accurate entering of the welding conditions. The output unit 55 also displays information such as various situations of the ongoing welding. It should be noted that the output unit 55 may have a touch panel or the like in its monitor screen, which provides the output unit with an additional function, i.e., input function. Then, the output unit 55 may also be able to function as the input unit 54.
One exemplary method of monitoring the plasma welding performed by the plasma arc device 100 having the above-described structure will be described with reference to
As the keyhole welding starts, the welding controller 50 (central control unit 51) firstly obtains the welding condition which is entered from the input part 54 (Step S100), and then accesses the storage unit 52 to obtain the natural frequency of the weld pool P which is uniquely decided under the obtained welding condition (Step S200), as shown in
At Step S503, it is determined how far the welding voltage is from the reference voltage, i.e., the discrepancy of the welding voltage from the reference voltage is detected. It is determined whether the discrepancy is in a predetermine range, measured from the reference voltage. Specifically, as shown in
At Step S505, a peak frequency is identified (specified) which correlates to the vibration of the weld pool P, based on the frequency analysis result of the preceding step, i.e., Step S400. Then, the controller proceeds to Step S507. At Step S507, it is determined whether the identified peak frequency is in the predetermined frequency range. Because this peak frequency corresponds to the actual frequency of the weld pool P, this step determines whether this actual number of vibrations (frequency) substantially matches the natural frequency of the weld pool P, which is already read.
As shown in
At Step S509, it is determined whether there is any peak, other than the identified peak frequency, that is equal to or greater than the predetermined level due to noises or disturbance, although it is already determined that the identified peak frequency is in the predetermined frequency range. If there is a large peak that is equal to or greater than the predetermined level, other than the identified peak frequency, then an accurate determination becomes difficult under the influences of that large peak. For example, as shown in
At Step S511, it is determined that the good welding is performed such that the penetration bead does not have irregularities as shown in
As described above, the monitoring method of the present invention can precisely determine in real time whether it is possible to obtain a penetration bead having a stable and gentle height without dropping and irregularities when the keyhole welding is performed with a plasma arc. Consequently, it is possible to accurately make a determination on whether to continue the welding or stop the welding. Even if the welding deficiency occurs, a repair work to the welding deficiency or other after-treatments can be minimized or greatly reduced.
In the processing for determining the welding quality of this embodiment (Step S500), as shown in
Referring now to
If the determination of this step indicates that the only identified peak frequency is the preset pulse frequency, as shown in
If the pulse current is used as the welding current as described above, it is possible to more precisely determine the quality of the keyhole welding by determining the relationship between the peak frequency of the welding current and the pulse frequency of the pulse current. It should be noted that the order of the determination steps is not limited to the above-described order of this embodiment. For example, as shown in
Among the units or steps that constitute the present invention described in the “SOLUTION TO OVERCOME THE PROBLEMS” section, the output voltage detection unit (or step) for detecting the output (applied) voltage when the welding is performed with a constant current or a pulse current corresponds, for example, to the output voltage measuring unit 53 shown in
| Number | Date | Country | Kind |
|---|---|---|---|
| 2011-251739 | Nov 2011 | JP | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/JP2012/079747 | 11/16/2012 | WO | 00 | 5/15/2014 |
