The Droplet Transfer Control And Application In Gas Metal Arc Welding

Gas metal arc welding(GMAW) is suitable for the automatic welding and robot with high efficiency and energy saving. The continuously supplied wire and welding pool are heated and melt by the arc between them to form a welding seam. In the welding process, the welding area are protect by the shielding gas from the hazard action of the air.Spatter, poor appearance of welding seam and hard-matched welding parameters are main factors which impact on the promotion and application of GMAW. The welding quality is related to the droplet transfer methods. The GMAW process provides short circuit transfer and free transfer methods. The currently methods of control of droplet transfer is to eliminate the energy to droplet to reduce the spatter and keep the arc voltage to make the welding process steady. It is difficult to be suitable for different droplet transfer methods and welding parameters.The scheme of a power source with different control Strategies adapt to different droplet methods are designed to make the process of transfer steady and get a high quality of welding seam with lower spatter. The main research contributions of the thesis are as follows:(1)Based on the analysis of the forces acting on the droplet and the transfer process, a mathematical model describing the GMAW process is developed. The mathematical model includes a description of the droplet displacement and mass, electrical circuit, the arc length, and the melting rate. Simulation programs show the effectiveness of the model, based on which an adaptive contol algorithm is developed.(2)In the short circuit transfer mode, the droplet transfer process is divided into five periods, the corresponding current wave form that adapt to the states of the droplet in each period is output. A big current is applied on the period of arc burning to provide the enough energy to ensure the good appearance of welding seam. The principle of five periods wave form is dicussed and the simulation model of wave form droplet transfer control is developed to optimize the parameters of wave form. The duration of arc burning period is adjusted by the fuzzy neural network algorithm to control the arc length to the preset value. The membership function of fuzzy variable is optimized by experiment data with genetic algorithm. On the process of welding, the additional momentum method is used to adjust the weight of the neural network on the realtime.(3)To solving the problem of hard-matching of parameters in free transfer mode, the simple direct model reference adaptive control is applied to decouple the welding parameters. The simple direct model reference adaptive control algorithm is based on the the theory of command generator trace, Only the states of the reference model and the trace error are needed to design the controller that let the high order plant model trace the simple chosen low order reference model. A improved discrete simple direct model reference adaptive control algorithm is proposed to remove the steady state error. Current and arc length as being the process output are controlled by open circuit voltage and wire feed speed to trace the output of the reference model, which ensure the size and the transfer interval is uniform.(4)The application of droplet transfer control strategies depend on the rapid dynamic characteristics of the welding power source. The performance of the traditional welding power source is hard to satified the requirements. The soft switch technique is applied to improve the switching frequency of inverter which is the key to the dynamic response of the power source. A zero voltage and zero current full bridge soft switching topology is proposed, in which a dc blocking capacitor and a small saturable inductor is added in the primary side to achieve ZCS for the lagging-leg switches.The working states of soft switching circuit is analyzed. The way to eliminate the turn-on switching loss of the leading-leg switches is discussed. The conditions that achieve the soft switch in a wide range of load which is from off-duty to full duty in the welding process is hard to be satisfied, so a inductor is add on the secondary side to provide the additional energy. A power source based on the topology is designded and the successful tests have been carried out.(5)A welding system suiable for the different parameters and droplet transfer method is developed based on the module design. The system are consist of microprocessor controlled welding power source unit and wire feeder unit which exchang the welding parameters by communication. The complete protection of overheat, over current and undervoltage are included.The welding experiments show that the system can get high welding quality with lower spatter. The droplet transfer control methods and the system are valuable for the application of GMAW.