Electric heating – Metal heating – By arc
Reexamination Certificate
1998-11-13
2001-02-13
Ryan, Patrick (Department: 1725)
Electric heating
Metal heating
By arc
C219S121640, C219S121830, C219S121850, C219S121600, C219S121720
Reexamination Certificate
active
06188041
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates in general to weld process monitoring techniques, and more particularly to an improved method and apparatus for real-time monitoring of thermal radiation of a weld pool to monitor a focus shift, a power variation and a weld depth for weld process control and a weld defect for weld process quality assurance, utilizing the chromatic aberration of focusing lens or lenses.
2. Description of the Prior Art
The application of high power Nd:YAG lasers for precision welding in industry has been rapidly growing quite recently in diverse areas such as for the automotive, electronic and aerospace industries. Nowadays, as much as 6 kW of average power is available in the market. Delivery of the power by fiber optics makes it useful for many remote applications. On the other hand, these diverse applications also require using the latest developments for precise control and reliable process monitoring. Due to the hostile environment during laser welding, remote monitoring is required and many acoustic and optical remote monitoring techniques have been developed. However, acoustic monitoring is not suitable for the application in a factory due to the acoustic interference from environmental noise including mechanical noise. Therefore, optical monitoring is preferred in industrial applications.
Two approaches for optical monitoring have been followed: one monitors the image of a weld pool with a CCD or an IR camera and the other monitors the radiation from a weld pool with one or more single-element detectors. The monitoring of an image requires fast data processing and is quite expensive and complicated to implement. Furthermore, the monitoring of an image is not applicable to laser welding with a laser delivery fiber because the image of a weld pool can not be transmitted through a single-core laser delivery fiber. On the other hand, the monitoring of radiation is simple and cheap to implement, and fast and robust for industrial applications. However, the information on a weld pool status is narrowed for radiation monitoring. Therefore, several spectral bands of radiation from UV to IR have been monitored with a plurality of detectors to broaden the information on a weld pool status. Examples of such a method or apparatus for weld monitoring can be found in U.S. Pat. Nos. 4,446,354, 5,272,312, 5,360,960, 5,506,386, 5,651,903, 5,674,415, 5,681,490 and 5,728,992.
Many focus shift and power variation monitoring techniques have been developed. However, interference between them has been neglected. Usually, the focus shift has been considered independently from the power variation. However, the focus shift and the power variation are closely related and can not be considered independently. For example, many focus shift monitoring techniques are affected by power variation. Usually, the power variation of a laser itself can be easily monitored. On the other hand, the power variation on a workpiece can be induced by the transmission loss of the delivery optics. In other words, many focus shift monitoring techniques can not be applied to laser welding due to power variation. However, the monitoring of power variation on the workpiece is quite important for precision welding and process automation. Sometimes, the laser has to be stopped before the failure in delivery optics brings a disaster. Therefore, both focus shift monitoring and power variation monitoring are required simultaneously for industrial precision laser welding. Furthermore, focus shift monitoring should not be affected by power variation monitoring and vice versa. For the forgoing reasons, there is a need to discriminate the focus shift from the power variation. The meaning of the discrimination between the focus shift and the power variation is described below in “Summary of the Invention”.
The importance of the simultaneous monitoring of power variation and focus shift for precision laser welding is explained above. However, the weld depth in laser welding depends not only on the focus shift and the power variation but also on environmental parameters such as the temperature of a workpiece, the flow rate of a shielding gas and the weld gap. For a precision laser welding, the variation of weld depth induced by the changes in the environmental parameters has to be compensated. To compensate the weld depth variation due to the changes in these environmental parameters, the laser power has to be adjusted to compensate the weld depth variation induced by these environmental changes. One example is the weld depth variation at the beginning of a seam welding. Therefore, there is a need for weld depth monitoring for precision laser welding. Furthermore, the monitoring of a weld defect such as a weld gap is also required to verify the weld quality and to determine the cause of bad welding. This kind of comprehensive weld process monitoring has never been demonstrated.
SUMMARY OF THE INVENTION
Hence it is the fundamental object of the present invention to provide a method and an apparatus whereby the simultaneous monitoring of the focus shift and the power variation can be obtained in a manner which is simple and suitable for industrial application. It is another object of the present invention to provide a method and an apparatus whereby the weld depth and the weld defects can be monitored for both weld process control and weld quality assurance.
These objects are satisfied by utilizing the chromatic filtering of the thermal radiation of a weld pool. A method and apparatus for real-time weld process monitoring are provided for a pulsed laser welding. The thermal radiation from a weld pool is measured at several spectral bands through an aperture with single-element detectors after splitting the spectral bands with dichromatic mirrors and beam splitters. The distal end of an optical fiber for laser delivery can be used as an aperture and each spectral band signal is measured with a single-element detector. Due to the chromatic aberration of an imaging optics, the field of view from a single-element detector through the aperture is varied by the wavelength of spectral band. The weld pool size contributing to the spectral band signal varies by the wavelength of the spectral band. The transmittance profile of each spectral band also depends on the focus shift of imaging optics. By processing the measured spectral band signals, the size of a weld pool, the power variation on a workpiece and the focus shift of imaging optics can be monitored simultaneously. Furthermore, the weld pool sizes at predetermined positions in time are correlated to the weld depth and the weld defect such as a weld gap for weld quality assurance.
In conclusion, the monitoring of weld pool size variation is achieved with a plurality of single-element detectors by utilizing the chromatic filtering of the thermal radiation from a weld pool. The use of a plurality of single-element detectors rather than a CCD or IR camera in monitoring the size of a weld pool makes it very fast to process the data and cheap to implement for industrial application. The monitoring of weld pool size variation can also be used to monitor the weld depth and the weld gap by utilizing the information on the weld pool size variation and the slope of weld pool size variation at predetermined locations in time during the cooling stage. Furthermore, the interference between power variation monitoring and focus shift monitoring has been minimized and discrimination between power variation monitoring and focus shift monitoring can be achieved by optimizing the chromatic filtering of the thermal radiation from a weld pool.
These and other features, aspects and advantages of the present invention will become better understood with preference to the following description and appended claims.
REFERENCES:
patent: 4446354 (1984-05-01), Kearney
patent: 4766285 (1988-08-01), Decailloz et al.
patent: 5155329 (1992-10-01), Terada et al.
patent: 5249727 (1993-10-01), Eberle et al.
patent: 5272312 (1993-12-01), Jurca
patent: 52834
Baik Sung-Hoon
Chung Chin-Man
Kim Cheol-Jung
Kim Min-Suk
Arent Fox Kintner & Plotkin & Kahn, PLLC
Elve M. Alexandra
Korea Atomic Energy Research Institute
Ryan Patrick
LandOfFree
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