Sung-Hoon Baik

Auto-focus control and weld process monitoring of laser welding using chromatic filtering of thermal radiation

The thermal radiation from a weld pool is focused on an aperture and the transmitted thermal radiation is monitored at two wavelengths with high-speed single-element detectors. Due to the chromatic aberration introduced in the focusing optics, the transmittance curve of thermal radiation varies by the wavelength. Likewise, the detector field of view varies by the wavelength. Owing to this difference in the transmittance and in the field of view, the local variation in a weld pool can be monitored by processing the two spectroscopic signals from two detectors. In this paper, the algorithms to monitor the weld pool size and the focus shift are presented and the performances of weld pool size monitoring and auto-focus control are shown for a pulsed Nd:YAG laser welding. The size variation monitoring has been applied to the weld depth and weld defects monitoring. The effects of laser power change and weld defects on the weld pool size variation are also shown.

New centroid detection algorithm for the Shack-Hartmann wavefront sensor

A new center position detecting algorithm of the spot image for the Shack-Hartmann wavefront sensor was developed. The new algorithm is a modified center of weight algorithm, which uses some power of the grey level intensity of the spot images instead of the grey level intensity itself of the spot images. From experiments, the repeatability and accuracy of the center position detection of the spot images of the Shack-Hartmann wavefront sensor which used the new algorithm were improved compared with the conventional center position detection algorithm using the center of weight. Applications of the algorithm to measurement of the displacement of the spot images and the Shack-Hartmann wavefront sensor for measuring wavefront distortion and the experimental results of closed-loop wavefront correction are described in this paper.

Performance optimization of COIL using the visualization of supersonic flow

This paper describes the results achieved in the development of a chemical oxygen iodine laser (COIL) for industrial applications. The COIL was designed to obtain kW class output power with jet type singlet oxygen generator (SOG). The output power with subsonic flow was 230 W at 15 mmole/sec chlorine flow rate. The real-time holographic interferometer with a digital high-speed camera was applied to the visualization of mixed gas flow in COIL. The visualization of gas flow shows the profile of the variation of refractive index and can be applied to the measurement of the variations in the pressure and the temperature of the gas.

Application of chromatic filtering to focus control and kerf width monitoring in pulsed Nd:YAG laser cutting

The optical monitoring using chromatic filtering of the thermal radiation from a molten zone could be applied to the pulsed laser cutting. The variation of transmittance profiles of a molten zone for different wavelengths made the chromatic filtering successful in laser cutting. Using this optical monitoring method, we could control the focus shift within the required depth and maintain the uniform kerf width for cutting of an arbitrarily shaped workpiece. The effect of power variation during the laser cutting was investigated and the focus control and the kerf width were not affected by the power variation of up to 10 %.The optical monitoring using chromatic filtering of the thermal radiation from a molten zone could be applied to the pulsed laser cutting. The variation of transmittance profiles of a molten zone for different wavelengths made the chromatic filtering successful in laser cutting. Using this optical monitoring method, we could control the focus shift within the required depth and maintain the uniform kerf width for cutting of an arbitrarily shaped workpiece. The effect of power variation during the laser cutting was investigated and the focus control and the kerf width were not affected by the power variation of up to 10 %.

Laser ultrasonic testing system using a confocal Fabry-Perot interferometer with a fringe stabilization module

A laser ultrasonic testing system using a confocal Fabry-Perot interferometer and a pulsed Nd:YAG laser is developed for the fatigue test of materials. To stabilize the fringe pattern of the confocal interferometer, an adaptive stabilization fringe control system is developed using two photodiode signals. The closed-loop fringe control system is operated automatically. The optical system is composed of many polarization components, such as a half-wave plate, quarter-wave plates and polarization beam splitters to improve the signal to noise ratio. The laser ultrasonic system carried out performance test. The optical configuration of the interferometer system and the stabilization module are investigated in this paper. The experimental results of the basic experiments are also described.

Adaptive optics system with a hierarchical algorithm for measuring and correcting wavefront distortions

A robust measuring technique for the wavefront is one of the key parts for a stable adaptive optics system in the practical fields. Also, the measurement resolution of the wavefront is important for improving the performance of an adaptive optics system. In this paper, we propose a robust measuring technique for the wavefront using a Shack-Hartmann wavefront sensor for an adaptive optics system. The proposed measuring technique for the wavefront uses an iterative center of mass algorithm with the hierarchical sizes of a searching window and the hierarchical threshold values. The measurement accuracy and stability are investigated using the proposed hierarchical algorithm and compared with the conventional algorithm of the wavefront in the experiments. Also, we describe the hardware configuration of the adatptive optics system operating in our laboratory.

A study on 3D measurement for the composition of the human bust by using digital projection moire

Moire topography method is a well-known non-contacting 3-D measurement method. Recently, the automatic 3-D measurement by moire topography has been required since the method was frequently applied to the engineering and medical fields. 3-D measurement using projection moire topography is very attractive because of its high measuring speed and high sensitivity. In this paper, using two-wavelength methods of projection moire topography was tested to a measuring object with 2π-ambiguity problems. Experimental results prove that the proposed scheme is capable of finding absolute fringe orders, so that the 2π-ambiguity problems can be effectively overcome so as to treat large step discontinuities in measured objects.

A study on a 3-D profilemeter using dynamic shape reconstruction with adaptive pattern clustering of the line-shaped laser light

One of the main problems in machine vision inspection systems that use the triangulation of line-shaped laser light is finding the exact laser light center positions. We describe a 3-D surface profilometer that improves the measurement efficiency. Reflection light intensity from a line-shaped laser light on the object surface is different from the colors and shapes of objects. The proposed 3-D surface profilometer can measure a 3-D surface shape remotely with enhanced measurement resolution than the conventional optical systems by using the dynamic shape reconstruction with adaptive pattern clustering of the line-shaped laser light. This proposed 3-D profilometer can be easily applied to practical situations involving 3-D surface shape measurements by extracting the exact center positions of the line-shaped laser light and compact hardware compositions.

Application of chromatic filtering to process monitoring of CW laser welding

The chromatic filtering of thermal radiation has been applied to the process monitoring and the auto-focus control in pulsed laser welding. The transmittance of thermal radiation from a weld pool depends on a position of radiation source in a weld pool and on the distance of the weld pool from a focusing lens. Its dependence has been analyzed to monitor the variations in weld pool size and to control the focus shift in a pulsed laser welding. The chromatic filtering has many advantages if the thermal radiation is measured when the weld pool size is reduced to the laser spot size. This requirement is easily satisfied in a pulsed laser welding by measuring the thermal radiation between the laser pulses during the cooling stage. However, it can not be satisfied for CW laser welding where the weld pool size is always larger than the laser spot size except at the beginning of laser welding.To apply the chromatic filtering to a CW laser welding, the algorithm measuring the size of a weld pool is analyzed and the requirements to obtain a reference size in measuring the increment or the decrement of weld pool size from that reference size in absolute value are shown. This weld pool size variation is used in monitoring the power variation at a workpiece or the weld depth in a CW laser welding. However, this weld pool size monitoring depends on the focus shift. Therefore, to make it useful in industry, it is essential to keep the focus position in some range required in the weld pool size monitoring. The optimization of optical parameters to minimize the induced error from a power variation in the focus shift monitoring is shown. It turns out that the focus shift can be controlled in the required range at the weld pool size of up to 1.5 times the laser spot size. In conclusion, the weld pool size variation can be monitored and the focus shift can be controlled in the required range in a CW laser welding as well as in a pulsed laser welding.The chromatic filtering of thermal radiation has been applied to the process monitoring and the auto-focus control in pulsed laser welding. The transmittance of thermal radiation from a weld pool depends on a position of radiation source in a weld pool and on the distance of the weld pool from a focusing lens. Its dependence has been analyzed to monitor the variations in weld pool size and to control the focus shift in a pulsed laser welding. The chromatic filtering has many advantages if the thermal radiation is measured when the weld pool size is reduced to the laser spot size. This requirement is easily satisfied in a pulsed laser welding by measuring the thermal radiation between the laser pulses during the cooling stage. However, it can not be satisfied for CW laser welding where the weld pool size is always larger than the laser spot size except at the beginning of laser welding.To apply the chromatic filtering to a CW laser welding, the algorithm measuring the size of a weld pool is analyzed and th...

Remote optical power and focus monitoring in pulsed Nd:YAG laser welding

The thermal radiation from the weld-pool is measured at two wavelengths through the laser delivery fiber between Nd:YAG laser pulses. The chromatic aberration of delivery optics has been optimized to detect the variation of weld bead width. The design of optics and the signal processing algorithm for optical monitoring is described. Furthermore, the applications of optical monitoring to the detection of power variation and focus shift are shown.