Study On Digital Gmaw-P Welding Power Source And Modeling Of Metal Transfer Signature

Pulsed gas metal arc welding (GMAW-P) becomes more and more important with the application and development of semi-automatic and automatic industrial production. GMAW-P has high efficiency and good quality with wide process scope and good environmental protection effect. Now the wider use of aluminium and aluminium alloy is brought the new process of welding equipment, such as a higher demand. It is important to further improve GMAW-P welding control performance and corresponding process performance by digital control design in welder power source research.One of the problems of GMAW-P application is the arc stability. Since the switch of current waveform from base to pulse, the arc length is fluctuated, too. Metal transfer process is affected by control performance of welding power source, so it presents higher requirement for output characteristic of welding power source. Moreover droplet formation, dimension and transfer form are affected by pulse parameters. The arc length stability and the metal transfer are two urgent problems for GMAW-P.The application environment of GMAW-P welding power source is bad, which are located mostly in workshop or field. The failure of welding power source based on microcontroller is resulted easily from all kinds of disturbances of high frequency signal. There some problems with digital welding power source: 1) A single CPU: the signal is highly concentrated; 2) Resource-exclusive: the program design becomes complicated and low reliable; 3) Lower reliability: one part's error may lead to the paralysis of the whole system.Considering all above problems that exist in current GMAW-P welding power source, it is a good solution to use the module method. Module division of GMAW-P welding power source system is separated to main circuit module and control module. There are three parts of control module which are information interactive module, process control module and accessorial module. Two CPUs respectively for two modules, one is MCU for information interactive module and the other is DSP for process control module. The communication between DSP and MCU is based on RS485, and the communication between MCU and pc is based on can bus. Dual CPU design makes more rational allocation of the algorithm and management for optimization GMAW-P technology. The module entropy (?) IS discussed to value the system modular degree.The fault tree of GMAW-P welding power source system is built by the hierarchical control technology. Modules of fault trees can be used to reduce the computational cost of basic operation on fault trees. Graph theory (directed graph) which reflects the relation of fault-dissemination obviously divides the fault-nodes to each level use the matrix to mean the relation of the fault-nodes and top-fault node, and calculate the length of the directed side from one node to the top-fault node, we can assurance the weight of each thoroughfare with the key-importance calculated with the inefficiency of the bottom affairs, then we can know which node to be first checked and which one is last. This test matrix is easy to be processing for the computer, avoiding the difficulty of finding the minimums combination and improve the efficiency of diagnosis. The ideal metal transfer mode in GMAW-P is ODOP (one-droplet-one-pulse) which is related to pulse frequency as the function: f = k * v_f/D. But in one pulse period, there two parts of the melton metal, one is the droplet and the other is the melton string at the end of the electrode. So the function is corrected: f =εk * v_f/D, (?). A new method of arc length control is discussed which has double control parameters: static characteristic of welding power source and frequency. The results show that the frequency fluctuation is within 10%.Metal droplet transfer mode which are influenced by the pulse parameters such as pulse current ( I_p), pulse time ( t_p), base current ( I_b) and pulse frequency ( f ) etc. has play an important role in the weld quality. An experimental system has been developed to sample the transient electrical parameters and record the images of droplet transfer process simultaneously which is based on the Lab VIEW virtual instrument and high speed photography technology. Use one trigger signal to start the electrical parameters sample and at the same time the high speed camera records the droplet transfer process. After the welding, all the data can copy into the PC. By this system, how pulse welding parameters affect on metal transfer form was studied on. The system established in this paper provided found support for controlling the droplet transfer form and optimizing the welding parameters.A transition voltage U d in the pulse voltage drop edge of electrical waveform which has been discovered according to the analysis of synchronous electric and image signals. The relationship between the metal transfer mode and the slope of the transition voltage has been proposed. The logistic regression model is designed: (?)The result of experiments is shown that the model can be used in closed-loop controlling of one droplet per pulse transfer mode and also can help to optimize the welding parameters.