Study On The Calibration Of 3-D Weld Pool Surface Measurement System In TIG Welding

As an important manufacturing technology, Gas Tungsten Arc Welding (GTAW) is widely used in industry. Sensing and measuring the information of welding quality exactly and reliably is the prerequisite for realizing GTAW welding process automation and making it intelligent. Weld pool is the important parameter affecting welding quality. It's meaningful to sensing and measuring the 3-D surface shape of weld pool exactly and reliably for studying the welding quality and welding penetration control.The Fourier Transform Profilometry is mainly for duffuse surface objects, it has encountered problems in recovering the surface of weld pool or mirror like object. Hence, the dynamic specular weld pool surface makes height observation very difficult. This paper presents a method to solve the problem of phase height mapping. In this paper, the laser grating pattern is projected onto the area under the torch electrode and covers the whole possible weld pool surface. During the welding process, the molten specular weld pool surface is just like a mirror and it can reflect the majority of the incident laser light. A 4f system is placed in the reflected light axis, and the weld pool surface is the input plane of the 4f system. In order to intercept the reflected laser grating pattern, a white panel paralleled to the reference plane is placed at the output plane. In the meantime, a CCD sensor is used to detect the output images on the white panel. The output images are the same as the input images according to the Fourier transform lens characteristics, and the diffuse reflective surface of the white panel changes the propagation direction of laser, which make the specular reflection to diffuse reflection, and the camera imaging on the imaging plane can be seen as imaging on the weld pool in real-time. Then the whole measuring system can be simplified as the traditional triangle measuring system. Establishing the relationship between the h(x, y) in the Z direction and the phase differenceΔφ(x,y) according to the principle of FTP. Sampling a series of heights and the corresponding phases value along the Z direction and calculating the relationship by the least square method. The camera is calibrated based on the pinhole camera model, and the affection factors of calibration error are analyzed. Building the relationship between the pixels coordinates within the X-Yplane and the phase-height in the Z direction, according to the vertical and horizontal calibration results. The 3-D surface shape of weld pool is obtained based on calibration results.According to the concept of the equivalent wavelength, the basic requirement for measurement system and the selection of projection grating pitch are discussed in this paper to avoid shadows and frequency spectrum alias in a crossed-optical-axes system in FTP. The influence of CCD sampling to the FTP is also discussed to obtain CCD sampling condition. Finally, the range of the grating pitch is determined in our experimental conditions, and the optimal grating pitch is obtained after the experiments.