KR20050091165A - The method ans apparatus of laser weld monitoring using time variation of frequency components - Google Patents

Abstract

The present invention relates to a method and apparatus for monitoring a laser welding process, the object of which is to obtain a heat radiation signal generated at a welding pool at a sufficiently fast speed at a specific wavelength, and to determine the intensity of the signal as well as the time of the intensity of the frequency component. The present invention provides a welding monitoring signal processing algorithm using a change, and provides a method and apparatus for real time monitoring of a laser welding process using the same.

According to the present invention, the thermal radiation signal from the welding pool is externally acquired through a laser beam focusing lens and a laser transmission path such as an optical fiber, and then a frequency component is analyzed by a high-speed freeze transform, and the temporal The technical features of the method and apparatus for monitoring the laser welding state by comparing the change with a reference signal or by comparing the ratio of the intensity of specific frequency components with each other.

Description

The method ans apparatus of laser weld monitoring using time variation of frequency components

The present invention relates to a method and apparatus for monitoring a laser welding process, and in particular, a heat radiation signal emitted from a welding pool during laser welding is measured, and frequency analysis is performed to compare the signal strength of each frequency band with a reference signal to monitor the welding state in real time. It relates to a welding monitoring method and apparatus that can be.

In laser welding, when a laser beam is focused on a welding object by a lens, the welding object melts and welding is performed. A weld pool is created in the laser focused area, where thermal radiation is emitted. Since the heat radiation signal is affected by the laser output, the focus state, etc., the laser welding state can be monitored by acquiring / analyzing the signal, and an optical monitoring device that detects such a signal and extracts information about the welding state is provided. It has been developed a lot.

Optical monitoring method of laser welding state can be classified into two types. One is a method of acquiring and analyzing an image of a welding pool with a CCD camera or an infrared camera, and the other is a method of measuring and analyzing the intensity of a heat radiation signal emitted from the welding pool with a photodetector such as a photodide.

The method of acquiring and analyzing the image has an advantage of obtaining a lot of information, but on the other hand, a complicated and expensive device with fast information processing capability is required, and in the case of laser welding transmitting to an optical fiber, There is a disadvantage in that an image cannot be acquired by a transmitting optical fiber.

On the contrary, the method of measuring the thermal radiation signal has the advantages of a simple and inexpensive device, fast signal processing, and high stability in an industrial environment. However, there is a disadvantage in that the amount of information is limited compared to the method of acquiring an image.

For example, in the prior art, US Patent No. 6,479,786 (Laser welding system) installs an image sensor such as a CCD camera next to the welding head (head) to analyze the shape of the welding bead (bead) in real time welding state As a patent for a device for monitoring in real time, an image of a welded cross section is obtained so that an accurate state can be known.

In addition, U.S. Patent 5,659,479 (Method and apparatus for real-time control of laser processing of materials.) Uses a camera to monitor the welding section in real time to monitor the welding state and use fuzzy logic control. As a patent for the analysis by using an infrared (IR), sound wave signal, and ultraviolet (UV) signal at the same time proposed a method for improving the accuracy.

However, these methods require a camera device in the monitoring device and a device for processing image information. Therefore, the device must be expensive, and the configuration of the device is complicated because it cannot be obtained by transmitting an image using an optical fiber that transmits a laser beam. There is a disadvantage, and more monitoring methods using a single sensor are being devised.

Monitoring methods using such a single sensor are described in US Pat. No. 6,060,685 (Method for monitoring laser weld quality via plasma light intensity measurements), 5,728,992 (Apparatus and method for real time evaluation of laser welds especially in confined spaces such as within heat exchanger tubing), 5,155,329 (Monitoring method and system for laser beam welding), 6,188,041 (Method and apparatus for real-time weld process monitoring in a pulsed laser welding), 6,329,635 (Methods for weld monitoring and laser heat treatment monitoring) or Korean Patent No. 346090 ( Welding pool size monitoring and focus control method and apparatus in laser welding).

The US 6,060,685 or 5,329,091 (Weld monitoring device) is a method of obtaining the light generated in the plasma generated during laser welding and comparing the intensity or shape with a reference signal, the wavelength band obtained is visible light and ultraviolet light.

The US Patent 5,728,992, 5,155,329 and the like is a method of acquiring a heat radiation signal of the welding surface with an optical sensor such as a silicon detector, a lead sulfide detector, and monitoring the shape of the heat radiation signal from the welding pool in comparison with the reference form. The signals obtained in the normal welding state are recorded for each pulse, and the thermal radiation signal is compared with each other in the two laser sensors during the laser welding. Compare with

However, these patents all use one or two single light sensors and do not provide focus shift information of the focusing lens.

In order to overcome this disadvantage, US Patent 4,992,859 attempts to control the focal position of the lens by using the chromatic aberration of the lens, and US Patent 5,785,651 and 5,218,193 also used the chromatic aberration information of the lens for distance measurement and distance control.

U.S. Patents 6,329,635 and 5,674,415 devised a method of dividing the thermal radiation signal from the welding surface obtained by the optical sensor into AC and DC components and comparing the intensity with a reference signal. Surveillance accuracy can be improved, but the amount of information is still limited compared to the use of cameras or multiple light sensors, which limits the accuracy of the surveillance.

In US Pat. Nos. 5,486,677 and 4,121,339, and Japanese Patent 2002-321073, more advanced methods have been proposed for calculating frequency components by fast free-switching optical signals generated during laser welding and comparing these frequency values with reference intensities. However, the method proposed by these patents is a way to analyze the frequency component compared to the reference value, which is insufficient to quantify the welding signal by the change of the frequency component over time.

An object of the present invention for solving the above problems is to obtain a thermal radiation signal generated in the welding pool at a sufficiently fast speed at a specific wavelength, and to use the strength of this signal as well as the time-dependent change in the strength of the frequency component. The present invention provides a monitoring signal processing algorithm and provides a real-time monitoring method and apparatus for a laser welding process.

In addition, another object of the present invention is the real-time laser monitoring method using the frequency analysis of the existing optical signal, it is difficult to precisely monitor the laser welding process by the comparison / analysis of these absolute frequency signals in a manner using only the intensity of the frequency component. In order to solve the problem is to provide a more accurate laser welding monitoring method and apparatus through the time changes of various frequency components.

In the present invention to achieve the object as described above and to perform the problem to eliminate the conventional defects in the real-time laser welding monitoring method for optically monitoring the laser welding state,

 The heat radiation signal from the welding pool is obtained from the outside through the laser beam focusing lens and the laser transmission path such as the optical fiber, and then the frequency component is analyzed by the high-speed freeze transform, and the temporal change of the signal intensity of the specific frequency is compared with the reference signal. A method of monitoring a laser welding state is characterized by comparing or comparing ratios of intensities of specific frequency components with each other.

The method for monitoring the laser welding state is a step of sending a heat radiation optical signal emitted from the welding pool to the light detection device unit far away from the welding specimen by using a beam transmitting device such as a lens and an optical fiber for focusing and transmitting a laser beam. Wow;

Separating the laser beam reflected from the specimen and the emitted thermal radiation light signal with a color separation mirror;

Passing the thermal radiation optical signal only through an optical signal having a specific wavelength using a narrowband optical filter, and then detecting the narrowband thermal radiation optical signal by using an optical detector and converting the optical signal into a digital signal;

Divide the digitized photodetector signal into several sections that show the change of each frequency component over time, and obtain the intensity of each frequency component in each section by using the fast freeze transform method. The welding state is monitored by comparing the temporal change of the intensity and comparing the intensity of these frequency components with a reference value.

A method of evaluating a welding state by obtaining a temporal change in the ratio of the intensities of two or more frequency components, and monitoring the temporal change of these values and the magnitude of the absolute value.

In the above, the temporal change of common logarithm values of two or more frequency components is obtained, and the welding state is evaluated by comparing the temporal change of these values with an absolute value.

The device configuration of the present invention is a real-time laser welding monitoring apparatus for optically monitoring the laser welding state,

A laser 1 for oscillating a laser beam;

Enumeration beams 11 provided at the front end of the laser to reflect the oscillated laser beam;

A focusing lens 12 for focusing the laser reflected from the enclosing mirror;

An optical fiber 2 for transmitting the focused laser;

A welding head 21 which receives the laser beam focused from the optical fiber and focuses the welding specimen 22 to generate a welding pool 23;

The heat radiation signal emitted from the welding pool 23 is transmitted back through the welding head 21 and the optical fiber 2 to be collimated by the focusing lens 12, and then reflected by the enumeration beam 11 on the welding surface. A photo detector 14 which is separated from the laser beam and which is transmitted by the narrow band filter 13 to detect only a specific wavelength;

Means for interfacing with the photo detector and processing the output of the photo detector as a digital signal through an A / D converter to obtain and store the thermal radiation signal in digital form;

The digitized photodetector signal is monitored in real time for the laser welding condition. The digitized photodetector signal is divided into several sections to show the change of each frequency component over time. And a signal processing computer 3 having means for storing a program for comparing the temporal changes of the intensity of the frequency components of the low frequency and the high frequency and comparing the intensity of these frequency components with a reference value.

The signal processing method and apparatus configuration of the present invention as described above by monitoring the signal obtained by one single optical sensor as well as the frequency component of the intensity, the change over time of the frequency component and the change over time of the ratio of the frequency component The accuracy of the laser welding condition monitoring can be improved.

Hereinafter, the configuration and the operation of the embodiment of the present invention will be described in detail with reference to the accompanying drawings. The following description gives an example of continuous oscillation Nd: YAG laser welding having a wavelength of 1.06 mu m, but the same principle can be applied to other laser welding.

1 is a block diagram of a welding signal monitoring device according to the present invention, and shows a configuration of a real-time welding process monitoring device using heat radiation emitted from a welding pool.

The laser beam oscillated from the laser 1 is reflected by the enumeration beam 11 at the front end of the laser and then focused by the focusing lens 12 and transmitted to the optical fiber 2 to the welding head 21.

The laser beam focused on the welding head 21 is focused on the welding specimen 22 to generate the welding pool 23.

The heat radiation signal emitted from the welding pool 23 is transmitted again through the welding head 21 and the optical fiber 2.

The transmitted thermal radiation signal is collimated by the focusing lens 12 and separated from the Nd: YAG laser beam of 1.06 占 퐉 wavelength reflected from the welding surface by the enumeration beam 11 and separated into the narrowband filter 13. By this, only a specific wavelength is transmitted and detected by the photodetector 14.

The output of the photo detector is connected to the signal processing computer 3 via an interface, which is processed into an digital signal through an A / D converter.

The resolution of the digital signal was 12 bits or more to keep the precision high.

In precision welding, the signal acquisition time interval must be small enough for the precise analysis of the acquired signal.

The signal acquisition time interval can be adjusted at the interface, but it can be processed at 100 kHz speed, so there is no problem in extracting signals of several hundred Hz.

The signal processing computer 3 acquires and stores the heat radiation signal in digital form and analyzes it in real time to monitor the laser welding condition.

The intensity of the heat radiation signal on the welding surface is displayed in real time. When each frequency component is specified, the spectrum of the frequency component is shown over time, indicating a more precise welding state monitoring signal.

It also has the ability to issue an "error" alarm if the change in the frequency signal is outside the predetermined range.

With the configuration of the present invention described above, the experiment was performed to obtain the welding signal during the actual laser welding experiment and to find out the effect of the frequency analysis method of the welding signal.

The laser used in this experiment is Trump's continuous oscillation CW Nd: YAG laser with a wavelength of 1.06㎛ average power of 2 ㎾ and HAAS laser system has a very compact structure.

The laser beam is transmitted through the optical fiber and the laser rod has a diameter of 10 mm and a length of 150 mm. The laser system is configured to control in conjunction with a CNC machine.

A silicon detector (Thorlab, DET 100) was used to measure the thermal radiation wavelength used in laser welding.

In addition, the response speed of the light detector is 10 ns, it is possible to detect a signal generated during welding more than 100kHz.

The photodetector signal data was acquired using a small data acquisition board for the PCMCIA socket (DAQ-112B, IO Tech.) And the analog signal generated during the laser welding process as a digital signal available to the computer. 2 is a signal processing flowchart of a welding signal observation and analysis program, and FIG. 3 is a screen of welding signal observation and analysis software.

Figures 4a and b overlap two SUS 316 plates with thicknesses of 1 mm and 2 mm, starting at -10 mm out-of-focus (left end) and passing the correct focus by +10 mm. This is the result of a welding experiment ending in an out-of-focus state (right end). The laser beam intensity used a laser power of 1.5 kHz. The picture below shows the front / back side of the specimen after welding. The welding condition of the part of the gown (the part in focus) is very good but the width is not large, and the side parts (the part out of focus) are almost welded. It can be seen that this is not. Figure 4a is a front view of the welding result of the welding specimen, Figure 4b is a photograph showing the back side.

Figure 5 shows the original of the optical signal on the welding surface obtained by the photodetector in the welding experiment. The actual welding time is about 2.5 seconds, the acquisition speed of the welding signal is 10㎑, and the total number of data including the data before and after welding is about 31,000. In FIG. 5, a large portion of the signal represents a moment when the actual laser beam comes out and is welded. As shown in the welding cross-sectional view of Figs. 4A and 4B, the welding signal of the part of the gown (the part in focus) shows a stable signal having a small high frequency component, and the side part (the part of the out of focus part) shows the shape of a signal with a large swing. see.

Frequency analysis of the photodetector signal of FIG. 5 was performed. When Fourier transformation is performed for frequency analysis of a signal on the time axis as shown in FIG. 5, a graph is drawn on the frequency axis. In the present invention, since a magnitude change of the frequency is known over time, the frequency division is divided into regions. Was done. In other words, the total 30,000-32,000 signals are grouped by 512 and divided into about 60 zones, frequency analysis is performed on each of 512 data, and then a specific frequency signal is extracted to analyze the change over time of a specific frequency.

The frequency analysis results are shown in FIGS. 6, 7, 8, 9, 10, and 11.

6 and 7 show the intensity of each frequency on a linear scale, and FIGS. 8 and 9 show the logarithmic scale. That is, FIG. 6 shows a linear scale of 10 kHz and 20 kHz frequency components, and FIG. 7 shows a linear scale of 240 kHz and 250 kHz frequency components. Is the log scale of the 10 Hz and 20 Hz frequency components over time, and FIG. 9 is the log scale of the 240 Hz and 250 Hz frequency components over time.

10 is the ratio of two frequency components over time (I _{(10 Hz)} / I _{(250 Hz)} , I _{(20 Hz)} / I _{(250 Hz)} ), and FIG. 11 is the ratio of two frequency components over time ( I _{(250 Hz)} / I _{(10 Hz)} and I _{(250 Hz)} / I _{(20 Hz)} ) are shown, and FIGS. 10 and 11 show the ratios of the two frequency components. In FIG. 10, the well welded portion corresponds well to the ratio of 2 or less, and the larger the value, the poorer the weld. In FIG. 11, the opposite aspect is shown. In this case, the better the welded part, the larger the value.

As described above, it can be seen that monitoring the welding state from the change of time of each frequency component of the welding signal is advantageous in quantifying it than monitoring by the signal itself generated at the welding site. In actual construction of the welding monitoring device, it is thought that more effective real-time monitoring of welding is possible by introducing not only the ratio of the frequency components but also the sum and difference or the physical quantity based on the appropriate equation.

Although a specific laser welding monitoring example has been described above, it can be applied in almost all laser welding experiments such as laser welding using different lasers or using different specimens, and different frequencies and different frequencies depending on the material characteristics of each laser or specimen. Good results can be obtained by finding and optimizing the equations of the frequency components.

The present invention is not limited to the above-described specific preferred embodiments, and various modifications can be made by any person having ordinary skill in the art without departing from the gist of the present invention claimed in the claims. Of course, such changes will fall within the scope of the claims.

As described above, the present invention obtains a thermal radiation optical signal generated in the welding pool during laser welding with a single optical sensor such as a photodiode, analyzes the frequency of the optical signal, and calculates a change in a specific frequency signal over time. By comparing the ratio and the difference of these signals, the laser welding state such as penetration depth and laser output abnormality is monitored. The method using the temporal variation of the frequency component can compare a greater amount of information than the method of using the relative intensity of the thermal radiation signal generated by laser welding or comparing the intensity of the frequency component with the reference signal. This feature allows for more accurate welding condition monitoring.

In addition, in the present invention, when the laser beam is focused on the welding object by a lens, the welding object is melted and welded. A weld pool is created in the laser focused area, where thermal radiation is emitted. Since the heat radiation optical signal is affected by the output of the laser, the focus state, and the like, the laser welding state can be monitored by acquiring / analyzing the signal.

In addition, the present invention obtains a heat radiation optical signal generated in the welding pool during laser welding with a single optical sensor such as a photodiode, and analyzes the frequency of the optical signal intensity to calculate the change of a specific frequency signal over time, these signals By comparing the ratio and the difference, it is possible to monitor the laser welding state such as penetration depth and laser output abnormality.

Therefore, the method using the temporal variation of the frequency component of the present invention can compare a greater amount of information than the method of using the relative intensity of the thermal radiation signal generated during the conventional laser welding or comparing the intensity of the frequency component with the reference signal. It is a useful invention that has the advantage of being able to monitor the welding state more precisely because of its characteristics, and is an invention which is expected to be greatly used in industry.

1 is a block diagram of the present invention welding signal monitoring device,

2 is a signal processing flowchart of the present invention welding signal observation and analysis program,

3 is a screen configuration diagram of the present invention welding signal observation and analysis software,

Figure 4a, b is a welding result of the welding specimen of the present invention (front and back photo),

5 is an original signal obtained from the photodetector when welding the present invention,

6 is a linear scale of the frequency components of the present invention 10Hz and 20Hz,

7 is a linear scale of the 240Hz and 250Hz frequency components of the present invention,

8 is a log scale of the frequency components of the present invention 10Hz and 20Hz.

9 is a log scale of the 240Hz and 250Hz frequency components of the present invention over time;

10 is the ratio of two frequency components (I _{(10 Hz)} / I _{(250 Hz)} , I _{(20 Hz)} / I _{(250 Hz)} ) over time of the present invention,

11 is the ratio of two frequency components (I _{(250 Hz)} / I _{(10 Hz)} , I _{(250 Hz)} / I _{(20 Hz)} ) over time of the present invention.

<Description of the symbols for the main parts of the drawings>

(1): laser (2): optical fiber

(3): signal processing computer (11): enumeration

(12): focusing lens (13): narrowband filter

14: photo detector 21: welding head

(22) Weld Specimen (23) Weld Pool

Claims (5)

In the real-time laser welding monitoring method for optically monitoring the laser welding state,  The heat radiation signal from the welding pool is obtained from the outside through the laser beam focusing lens and the laser transmission path such as the optical fiber, and then the frequency component is analyzed by the high-speed freeze transform, and the temporal change of the signal intensity of the specific frequency is compared with the reference signal. A method of monitoring a laser welding using a change in a frequency signal, the method comprising monitoring a laser welding state by comparing or comparing ratios of intensity of specific frequency components with each other. The method of claim 1, The method of monitoring the laser welding state includes the steps of sending a heat radiation optical signal emitted from the welding pool to a photodetector unit located far from the welding specimen by using a beam transmitting device such as a lens and an optical fiber for focusing and transmitting a laser beam, and ; Separating the laser beam reflected from the specimen and the emitted thermal radiation light signal with a color separation mirror; Passing the thermal radiation optical signal only through an optical signal having a specific wavelength using a narrowband optical filter, and then detecting the narrowband thermal radiation optical signal by using an optical detector and converting the optical signal into a digital signal; Divide the digitized photodetector signal into several sections that show the change of each frequency component over time, and obtain the intensity of each frequency component in each section by using the fast freeze transform method. A method of monitoring a laser welding using a change of a frequency signal by monitoring a welding state by comparing a temporal change of the intensity and comparing the intensity of these frequency components with a reference value. The method according to claim 1 or 2, A method of monitoring a laser welding using a change in a frequency signal characterized by a method for evaluating a welding state by obtaining a temporal change in the ratio of the strengths of two or more frequency components and monitoring the temporal change of these values and the magnitude of the absolute value. . The method according to claim 1 or 2, Laser welding monitoring method using the change of the frequency signal, which is a method of evaluating the welding state by obtaining the temporal change of common logarithm values of two or more frequency components, and comparing the temporal change of these values with the magnitude of the absolute value. . In the real-time laser welding monitoring device for optically monitoring the laser welding state, A laser 1 for oscillating a laser beam; Enumeration beams 11 provided at the front end of the laser to reflect the oscillated laser beam; A focusing lens 12 for focusing the laser reflected from the enclosing mirror; An optical fiber 2 for transmitting the focused laser; A welding head 21 which receives the laser beam focused from the optical fiber and focuses the welding specimen 22 to generate a welding pool 23; The heat radiation signal emitted from the welding pool 23 is transmitted back through the welding head 21 and the optical fiber 2 to be collimated by the focusing lens 12, and then reflected by the enumeration beam 11 on the welding surface. A photo detector 14 which is separated from the laser beam and which is transmitted by the narrow band filter 13 to detect only a specific wavelength; A digital photodetector interfaced with the photo detector to process the digital detector's output through an A / D converter to obtain and store the thermal radiation signal in digital form and to monitor and analyze in real time for laser welding conditions. The signal is divided into several sections that show the change of each frequency component over time, and the strength of each frequency component in each section is obtained by the fast free-transform method, and the temporal change of the intensity of the frequency components of the low frequency and the high frequency is obtained. And a signal processing computer (3) comprising means for storing a program for comparing and comparing the intensity of these frequency components with a reference value.