Magneto-optical Imaging Mechanism Of Micro-gap Weld Seam And Detection Of Weld Position

Since the mid-1980s, as laser welding has many excellent characters, it is gradually used in many different fields such as automobile industry, manufacturing industry, powder metallurgy, electronic industry and biomedicine.At present, the structure light way is mainly used in laser welding seam detection, but this detection way can only be used to detect groove weld seam or rather wide weld seam, it can not be used to detect micro weld seam, not to mention on zero weld seam, so it is difficult to acquire the location information of the ultramicro weld seam in the visible spectrum. Closed butt joint or no gap is recommened for laser welding, but existent detection measure can not fit the demand of laser welding need. Aimed at the technical requirement of laser welding, this thesis discusses the mechanisms of using magneto-optical(MO) sensor to detect laser welding seam and the micro weld seam tracking problems which based on using MO sensor from the following aspects:(1)Firstly the current utilization statuses of laser welding was discussed, then the usage of the magneto-optical sensor at home and abroad. On the basic of further studying the magneto-optical imaging detection theory, a magneto-optical detection device was set up to detect the line weld seam and triangle weld seam and the corresponding magneto-optical images were acquired. By comparing the actual weldment’s weld seam shape with the images acquired, the conclusion is that the transition region of the magneto-optical image, which is ranged from bright region to dark region, is the corresponding region of actual weld seam region. This conclusion sets up experiment foundation for further studying in the magneto-optical imaging applying theory.(2)By using the optical theory and Faraday magneto-optical effecting theory, it was made sure that the mechanism of magneto-optical imaging:if the light emited by brightness LED lights is polarized by polarizer to be a linearly polarized light, when this light goes through a magneto-optical medium, the linearly polarized light’s rotation angle will rotate an angle as the surround magnetic field makes the polarized light produce a Faraday magnetic rotation effect. If the phase difference angle between the polarizing angle and the analyzing angle is tuned to 450, at the same time, if the surround magnetic field’s strength changes a little on the MO sensor detecting plane, the corresponding linear relationship MO image is produced by magneto-optical sensor and this mechanism theory can be used to detect the leakage magnetic field on the weld seam.(3)The distribution model of micro weld seam leakage magnetic field was set up and the near-surface’s verticle magnetic field distribution of weld seam was captured by magneto-optical sensor. If the weld gap width is kept constant, the transitional zone in the magneto-optical image will become wide as the distance of magneto-optical sensor above the weld increases, The transitional zone in the MO image will become narrow as the distance of magneto-optical sensor above the weld decreases. If the distance of magneto-optical sensor above the weld is kept constant, the transitional zone in a magneto-optical image will become narrow as the weld gap width increases, the transitional zone in a magneto-optical image will become wide as the weld gap width decreases.(4)To further discuss the reliability of the micro weld seam’s leakage magnetic field distribution model which was built up by magnetic charge theory, a test model was set up by means of the finite element method to test the model of weld seam’s leakage magnetic field and ANSYS sofeware was used to do the simulated analysis. The effectiveness using magnetic charge theory to analyse the near-surface magnetic field distribution of the weld seam was ensured and the way which the magneto-optical sensor was used to detect the leakage magnetic field of the weld seam was valid.(5)A variety of image processing algorithms had been discussed for the weld seam digital image obtained by the magneto-optical sensor, and the weld seam edge was acquired. The transitional zone centre was confirmed by using the weld seam edge to calculate. The mechanism of forming weld seam MO image was that different polar magnetic field had different rotation angle on polarized light in MO sensor to form weld seam MO image. It was confirmed that the gray value of weld seam MO image has direct proportional linear correlation with magnetic flux density, and the correspondence relation between theirs was made sure. The transitional zone’s center was the actual weld seam’s geometric center correspondingly.(6) In the aspect of seam tracking, the following works have been done. The micro weld seam can be detected effectively by using magneto-optical imaging method, when the weld deviation occurs, the weld seam measured value can reflect the actual deviation value and this measured value can be used for weld seam tracking. The method using magneto-optical imaging to detect the deviation of the weld seam has a high measurement accuracy. Using analysis of regression for analyzing the weld seam deviation, the weld seam deviation detecting precision was more accurate than detecting by using MO image directly. A magneto-optical imaging detecting weld seam model based on Kalman filtering algorithm was set up, a higher tracking accuracy was gained by using Kalman filtering algorithm than the direct detecting method, it can be illustrated that using Kalman filtering algorithm can get more tracking accuracy for tracking micro weld seam.