Study On Measurement Method Of Instantaneous Temperature Field Of Non-axisymmetric Arc

Fowler–Milne method is an important theory of spectral temperature measurement for arc plasma.With the development of high-speed visual technology,high-speed camera and narrow-band filter are combined to gain spectral information of arc plasma.Based on the principle of Fowler–Milne method,the temperature distribution of palsma can be gianed by measured monochromatic imaging.Because of its good temporal and spatial resolution,this method is widely used in welding arc temperature measurement.However,in the current related research mainly focus on fixed static arc,and the main purpose of them is to enhance the basic understanding of arc plasma.Also,this method is not easy to realize on-line measurement of arc temperature in welding process at present,which needs further improvement in two aspects.On the one hand,the measurement of non-axisymmetric arc plasma is to be realized.For the dynamic arc in welding process or the novel multi-electrode arc,the shape of them has been separated from the cylindrical symmetry distribution,and the traditional Fowler–Milne method can not be used.On the other hand,it is a challenge for Fowler–Milne method to realize automatic measurement,because the emission coefficient of the spectral line does not correspond to the temperature.The arc temperature can only be calculated only by assuming the position of the high temperature zone of the arc artificially.So the calculation process is not easy to run automatically.In this papert,to realize the on-line measurement of temperature field of welding arc,these two problems of Fowler–Milne method will be studied and improved.To realize the automatic measurement of Fowler-Milne method,a modified Fowler-Milne method based on two spectrum line is presented,which combining the ArI spectral line emitted by Ar atoms in an outer low temperature region of arc,and the ArII spectral line emitted by Ar ionization ion in high-temperature area of arc,to determining the different temperature range of the arc plasma,and then the whole temperature is calculated by the temperature corresponding to the intensity of the ArI spectral line in low-temperature area and high-temperature area,eliminating the adverse effects of single ArI spectral line emission coefficient field.Also a acquisition device of two spectral line is designed,based on this algorithm.Two CMOS cameras with a synchronous trigger device are enabled to simultaneously collect the spectral line information of the arc through two narrow-band filters.Taking the TIG arc at 150 A as measurement target,the feasibility of modified Fowler-Milne method based on two spectrum line and the measurement system of two spectral line is proved through the analysis of experimental data.For the measured monochromatic imaging of arc plasma is the projection of the internal emission coefficient field in the direction of lens,the study of nonaxisymmetric arc emission coefficient field,needs to collect the projection information under multiple azimuths,and then converted into three-dimensional non-axisymmetric emission coefficient field.Therefore,based on CT reconstruction technology,3D reconstruction of non-axisymmetric arc emission coefficient field is studied in this paper,and a 3D reconstruction algorithm is proposed.According to the relevant arc plasma boundary characteristics and internal emission coefficient distribution characteristics,a modified condition is added in the iterative process of the algorithm to reconstruct the emission coefficient field under the condition of sparse projected data and high noise energy.The reconstruction results of arc plasma model indicated that our algorithm was a significant improvement over the traditional method,as it can reconstruct sections with projections of limited views and low SNR.We also studied the influence of the number of views on the reconstruction.A multi-angle image acquisition module based on reflector is established,and the reflectors is used to converge the projections of different angles on the same plane at the same time.So in the case of low cost and high precision,the acquisition of arc multiangle projection is realized.This measurement system is built by using reflector group,right angle prism coated on the surface,optical support and optical platform.After a acquisition device of two spectral lines line information is designed and integrated with multi-angle image acquisition device,two CMOS cameras simultaneously collect two spectral line information at different angles of the arc through two narrow-band filters.A complete set of arc projection extraction process is designed,including image filtering,background noise reduction,arc image segmentation and extraction.In experiment,TIG arc plasma at DC 80 A,120A and 150 A was measured.After analyzing the collected data by measurement system,we found that the lighting area of ArII spectral line is about 2/3 of ArI spectral line,and also he emission coefficient of ArII spectral line varies greatly with the influence of temperature.With the increase of welding current,the maximum temperature inside the arc rises obviously,and the temperature field of the arc gradually expands outward.At the same time,this measurement experiment also proves that this measuring method we present solves the two problems of non-axisymmetric arc measurement and automatic temperature measurement,and provides a complete solution for online arc temperature measurement.