Study On Real-time Detection And Control Of Full Penetration In GTAW Based On Laser Photoelectric Conversion

In the automation process of GTAW,an effective way of ensuring the welding quality is to detect the penetration information in welding process in real time and to design a reasonable control scheme so as to form a control loop,the key point of which is the choice of sensing methods and the building of a control model.There are lots of information showing welding quality on a weld pool surface.Whether the three-dimensional shape of a weld pool free surface or the oscillatory state of a weld pool surface after stimulated,they are related to the important parameter of welding quality,namely,the weld penetration directly.The quantitative relation between the dynamic behavior of a weld pool surface and the parameter of welding quality can provide the control model of full penetration in GTAW with data.Thus,because of the reflective characteristics of weld specular pool surface and the high efficiency as well as the good stability of the photoelectric converter,this thesis proposes the method based on laser photoelectrical conversion,which can realize the real-time detection and sense of the characteristic information of a weld pool surface.On the foundation of the sensing method,this thesis dose some research on the control of full penetration,which are as follows:Firstly,this thesis introduces the process of sensing photoelectric signals by the method based on laser photoelectrical conversion and then proposes the detection and control principles of full penetration under the conditions of DC-GTAW and Pulsed GTAW based on the method.Besides,this thesis also designs a software system which takes LABVIEW as the main part and MATLAB as a support to build a corresponding hardware platform.Secondly,according to analysis of photoelectric signals in DC-GTAW,it appears that when the penetration status changes from partial penetration to critical penetration,the weld pool surface changes from convexity to concavity and reflected laser stripes focus quickly,causing the rapid change of voltage signals of the photoelectrical conversion method.Using the characteristics of the phenomenon,based on the experimental modeling method,a single input single output(SISO)control model of full penetration is established in the classical control theory.In this model,the photoelectric signals are regarded as input signals,the backside width as output signals and the model is verified by simulation to have good dynamic performance and stability.Then the control scheme of full penetration welding is designed,through which the 304 stainless steels with different widths are tested respectively and finally gains beautiful welding lines with even welding width of two sides and the same weld penetration,which proves that the control experiment platform and the control scheme of full penetration have good real-time performance,reliability and stability.Thirdly,DC-GTAW is a passive mode for the sensing of the weld pool surface information,while Pulsed GTAW based on the laser photoelectrical conversion method is an active mode,which can reflect penetration status through sensing the oscillation frequency of weld pool.With the mechanism analysis method,SISO model of weld pool oscillation frequency and backside width is built under different penetration statuses in Pulsed GTAW and then experimental data and calculated values are compared.Results indicate that the output of the established model corresponding to the changing trend of the input conforms basically to the calculated values.This model can be applied to full penetration control under the condition of step-by-step weld.According to the mutation phenomena of weld pool oscillation frequency from partial penetration to full penetration,a control scheme of full penetration under the condition of step-by-step weld is designed,by which samples with different widths and footsteps are tested,all gaining even and beautiful welded joints.Fourthly,it requires that the full penetration control system based on the laser photoelectrical conversion method should have a strong self-adaptability when there are lots of interference factors in working conditions.Based on the interference factors of step pitch,gap clearance,weld offset,variational heat emission conditions and so on,the full penetration control experiments based on the laser photoelectrical conversion method are conducted respectively.Results show that this control system can be applied to the above factors well and the adoption and analysis of the sensing signals in the control process are not influenced basically.And the correspondence of the control signals keeps stable,finally getting even and stable welding lines.