Electric heating – Metal heating – By arc
Reexamination Certificate
2001-12-10
2003-04-29
Heinrich, Samuel M. (Department: 1725)
Electric heating
Metal heating
By arc
C219S121830
Reexamination Certificate
active
06555780
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 improved methods for real-time monitoring of thermal radiation of a weld pool to monitor a size variation and a focus shift of the weld pool for weld process control, 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 growing quite fast these days in diverse areas such as the automobile, the electronics and the aerospace industries. Nowadays, a Nd:YAG laser with as much as 10 kW of average power is available in the market and the fiber delivery of it makes it useful for many remote applications. On the other hand, these diverse applications also require the new developments for the precise control and the reliable process monitoring. Due to the hostile environment in laser welding, a remote monitoring is required and man 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. Therefore, optical monitoring is preferred in industrial applications.
In a laser welding, a laser beam is focused on a workpiece by a focusing lens or lenses. The focusing lens or lenses image an aperture liming the size of the laser beam on the workpiece and the size of focused laser beam is the image size of the aperture on the workpiece at the wavelength of the laser. A weld pool is generated by the interaction of the focused laser beam and the workpiece. Due to the thermal conduction of the workpiece, the size of the weld pool is generally not the same as the size of the focused laser beam and varies with the power of the laser or with the focus shift of the focusing lens or lenses. The weld pool radiates a thermal radiation. Many optical monitoring methods have been developed measuring the thermal radiation from a weld pool.
As to the optical monitoring, two approaches have been followed: one monitors the image of a weld pool with a CCD or an infrared camera and the other monitors the thermal radiation from a weld pool with one or more single-element detectors. The monitoring of image requires the fast data processing and is quite complicated and expensive to be implemented. Furthermore, the monitoring of image is not applicable to a 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 thermal radiation is simple and cheap to be implemented, and fast and robust for industrial applications. However, the information on a weld pool status in the thermal radiation monitoring is limited compared to the image monitoring. Therefore, several spectral bands of thermal radiation from ultraviolet to infrared have been monitored with a plurality of detectors to widen the information on a weld pool status. Examples of such method for weld monitoring can be found in U.S. Pat. Nos. 4,446,354, 5,155,329, 5,272,312, 5,360,960, 5,506,386, 5,651,903, 5,674,415, 5,681,490 and 5,728,992.
However, these methods could provide some information on the change in the status of a weld pool, but could not provide the information on the focus shift of a focusing lens or lenses to maintain a uniform laser welding. On the other hand, chromatic aberration of a lens or lenses has been used in the focus control of a lens or lenses as shown in U.S. Pat. No. 4,992,859 and has also been used in the distance measurement or in the distance control as shown in U.S. Pat. No. 5,785,651 or in U.S. Pat. No. 5,218,193. In the monitoring of a laser welding, U.S. Pat. No. 5,850,068 could provide the information on the focus shift of a focusing lens or lenses using the chromatic aberration of the focusing lens or lenses by subtracting one spectral band signal from the other spectral band signal. However, U.S. Pat. No. 5,850,068 could not provide the information on the size variation of a weld pool. Furthermore, the information on the focus shift of a focusing lens or lenses provided by U.S. Pat. No. 5,850,068 would be incorrect if the power of a laser is varied because the subtraction of one spectral band signal from the other spectral band signal is doubled if the thermal radiation intensity is doubled by the increase of laser power even if the focus position of a focusing lens or lenses is not shifted. The information on the focus shift of a focusing lens or lenses provided by U.S. Pat. No. 5,850,068 would also be incorrect if the size of a weld pool is varied during a laser welding because the focus position of the highest intensity of each spectral band signal shifts as the size of a weld pool varies and the error signal changes even if the focus position of a focusing lens or lenses are not shifted.
The variation of a laser power can be easily monitored at the laser unit itself, but it is not easy to monitor the variation of a laser power at a workpiece due to the absorption by mirrors or lenses in the path of laser delivery. Therefore, a focus shift monitoring independent of the size variation of a weld pool is required for industrial laser welding applications.
As to the image monitoring of a weld pool using a CCD or an infrared camera, the size of a weld pool provides important information on the status of a weld pool. It can provide the information on the change of a laser power at a workpiece and also the information on the weld depth. If a variation of weld pool size is measured, the variation of weld pool size can be compensated by adjusting the laser power. However, the size of a weld pool measured with a CCD or an infrared camera depends on the focus shift of a workpiece. If the focus position of a workpiece is shifted during a laser welding, the size of a weld pool measured with a CCD or an infrared camera varies and the information on the status of a weld pool becomes incorrect. Therefore, a size monitoring of a weld pool independent of the focus shift of a workpiece is required for industrial laser welding applications.
U.S. Pat. No. 5,875,026, Korean Pat. No. 0193,276 and Japanese Pat. 2,895,021 disclosed a method measuring the size variation and the focus shift of an extended radiation source using the chromatic filtering wherein the chromatic aberration of imaging optics is used to provide the information on the size variation and the focus shift of an extended radiation source by measuring spectral band signals of thermal radiation through the imaging optics and through an aperture. The variations of the transmittances of the spectral band signals through the imaging optics and through the aperture are used in providing the information on the size variation and the focus shift of an extended radiation source.
U.S. Pat. No. 6,188,041 used the chromatic filtering disclosed in U.S. Pat. No. 5,875,026, Korean Pat. No. 0193276 and Japanese Pat. 2895021 to provide the size variation and the focus shift of a weld pool in a pulsed laser welding wherein the size of a weld pool could be reduced between the laser pulses due to the thermal conduction cooling through a workpiece. In U.S. Pat. No. 6,188,041, an algorithm was disclosed wherein a function obtained from the transmittances of the spectral bands for the size measurement of a weld pool provided a reference size which is usually the same as the size of a focused laser beam so that the size of a weld pool could be measured from the reference size by comparing a value obtained from the processed spectral band signals with another value of the function obtained from the transmittances of the spectral bands for the size measurement and another algorithm was disclosed wherein another function obtained from the transmittances of the spectral bands for the focus shift measurement of a weld pool provided the information on the focus shift of a weld pool by comparing a value obtained from the processed spectral band signals with an
Baik Sung-Hoon
Chung Chin-Man
Kim Cheol-Jung
Kim Min-Suk
Arent Fox Kintner Plotkin & Kahn
Heinrich Samuel M.
Korea Atomic Energy Research Institute
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