Research On Position Location And Welding Stability Of Robotic Welding For Large-scale Tubesheet Circular Seam

Tube-to-tubesheet heat exchanger,condenser,steam generator and other pressure vessels are the main industrial products to realize heat exchange.They play an important role in petrochemical industry,nuclear,ship,metallurgy and military industry,which are related to the national economy.Due to the need for sealing,pressure and medium flow,pressure vessels have high requirements on welding quality.However,most of the domestic petrochemical equipment manufacturing enterprises are still dominated by all-position tube-to-tubesheet welding special machine,with high labor intensity,low production efficiency and low welding quality.Especially,the tube joints of tubesheet heat exchanger are all circular seam,and the number of circular seam is large,densely distributed and large thermal deformation.It is difficult to finish the welding task by robot.Circular seam can be located by the teach-playback of robot;The common template matching method is difficult to identify the center of circular seam.The method of thermal deformation on-line detection and compensation needs to be supplemented and improved.Arc length control of circular seam in direct current pulse arc welding is a complex dynamic process with multivariable coupling,and it is more difficult to control the arc length of circular seam welding.In review of this,this thesis focuses on largescale circular seam detection and tracking control of tube-to-tubesheet welding robot,and carries out related research work,including the following aspects:Considering the teach-playback of robot difficult to meet the welding needs of large-scale circular seam,a robot autonomous positioning method under human-machine interaction is studied.The characteristics of all-position welding of large-scale tubesheet circular seam are analyzed.A tube-to-tubesheet welding robot system is constructed.A visual-assisted hand-eye calibration method based on object-image geometric relation is proposed.The coordinate transformation method based on drawing parsing is studied.The matrix relationship between the drawing coordinate system of the tubesheet and the robot coordinate system is established.It can realize the coordinate of any tube in the drawing coordinate system can be converted into the coordinate in the robot coordinate system.Meanwhile,the special welding software for tube-to-tubesheet welding is developed to realize man-machine interactive operation,so as to achieve the purpose of only calibrating the position of workpiece and teaching not necessary.Considering the problem that the matching rate of the conventional template matching method is not stable for large-scale circular seam,a visual locating method for the circular seam of the tubesheet based on the mask symbiosis is studied.A two-peak binarization algorithm based on Otsu' method is proposed to accurately separate the foreground image and the background image of the circular seam.Based on the connected domain minimum enclosing rectangle marking algorithm,the foreground image is located.The template is selfgenerated under the mask symbiosis competition criteria.A gaussian search algorithm for mask neighborhood template matching is proposed to achieve fast matching while preserving the accuracy.The image processing results show that this method in this paper has a strong robustness than common template matching,and provides the accurate coordinate based on the location of visual information.Aiming at the difficulty of locating the welding height of circular seam due to axial thermal deformation of tubesheet welding,a laser vision online detection and compensation method is studied.The image processing algorithm based on laser stripe center is used to calculate the intersection coordinate of cross-lines laser.And then,the online detection axial deformation mathematical model of tubesheet is established based on the cross-lines laser triangulation principle and least squares regression analysis method.Moreover,the model is regarded as the compensation feedback loop for circular seam welding height in tube-totubehsheet welding.The static detection error,the dynamic detection error and the compensation error of the thermal deformation are investigated by experiments.The overall error meets the technological requirements of tube-to-tubesheet welding.Considering the problem that the welding arc length of circular seam fluctuates greatly under the action of multivariable coupling,a fuzzy control method of real-time arc voltage for circular seam is studied.The relative position relationship between tungsten electrode and circular seam is analyzed by double factor and double level method.The idea of real-time adjusting welding height of circular seam by arc voltage fuzzy control method is designed.A real time arc voltage tracking algorithm based on pulse square wave is proposed to realize the calculation of arc voltage at the pulse peak stage,and the adjustment of arc length at the pulse base stage.A Mamdani type two-dimension three-segment fuzzy control structure is established.The arc voltage error and arc voltage error rate serve as the input of fuzzy controller,and the feed step serve as the output of fuzzy controller.Meanwhile,the control object belongs to three different arc voltage intervals,so as to realize robust control without model information.The tube-to-tubesheet welding robot is used as the test platform to realize the several validation experiments.For example,the matching tests of circular seam are finished,the matching rate of this method in this paper are above 0.9.In the visual locating accuracy test,the locating deviation of robot is from 0.21 to 0.35 mm.The axial thermal deformation tests show that the axial thermal deformation forces the tubesheet presenting a shape of “concave”.Meanwhile,the tubesheet with each row of circular seam show the characteristics of “Vshaped parabola”.The comparative test of arc voltage tracking shows that the arc voltage fuzzy control method in this thesis can reduce the arc voltage fluctuation.The voltage fluctuation of the test circular seam is changed from 1.84 V to 1.15 V.The internal mechanism of arc voltage fuzzy control to suppress welding defects is expounded.The reason for the eccentricity of arc voltage curve is revealed.The research results of this paper play an important role in accurately locating the position of circular seam,adjusting the welding arc and achieving high welding quality for large-scale circular seam.