On Study Of The Mechantronic Dynamic Model In Constant Current Saw And The Mechanism Of Arc Stability

Submerged arc welding (SAW) is a popular welding method used in many industrial fields because it has the property of high productivity. The technique of multi-electrode welding, or multi-wire welding, is commonly used in submerged welding processing. Multi-electrode welding can also be classified into two groups, single-power-supplied (SPS) multi-electrode welding and multi-power-supplied (MPS) multi-electrode welding. In SPS multi electrode welding system, the heat supplied from one single power, the total heat input being limited in a definite value, wires are used for heat distribution in welding processing. In MPS multi-electrode system, two or more welding arcs burning in one molten pool, the heat input is several times of single power supply. It greatly incases the welding productive because the amount of melting metal in a unit time is increased. In this paper, we refer to this kind of welding scheme as multi-electrode welding.In recent years, a new type of electronic controlled welding power supplies, switch-mode welding power is emerged and used in industrial field. The output character of this power can be well controlled ether in constant voltage (CV) mode or in constant current mode (CC). The dynamic model and the stability of welding arc under this power supply in submerged arc welding are studied in this paper.Generally, a submerged welding system consists of three parts, a power supply for heat input, a bogie for welding speed control, and a control unit for welding arc adjusting. In the welding processing, wire is burned in the arc, and is sent into the arc by the wire feeder at same time. A smooth welding seam could be obtained only if a balance were reached between the speed of wire burning and the speed of wire sending. In the period of arc initiation, a transient is existed from the time of arc igniting to the time of arc stable. Taking welding arc voltage as major a parameter, cooperated with power supply and wire feeder, an electron-mechanical dynamic model of submerged arc welding in constant current condition is established. Under this model, the balance relationship between wire burning and wire sending is depicted. A compound time constant, which is derived from control circuit, wire feeder, and welding arc, is obtained. It is the theoretical foundation for the further designing of numerical controller. The stability of the system is analyzed in S domain through Lap lace’s transformation.Due to the defference of electronic poles in defferent power supplies and the electron-magnetic interference (EMI) in multi-electrode system, the isolation schemes among powers and the coordination method among arcs become the major problems which need to be solved in multi-electrode welding. According to the character and request above, optical isolation technique is used for the transmission of signals or instructions among equipments besides the traditional relay- and transformer-isolation schemes. The ability of anti-EMI of the system is increased. And it also enhances the adaptability and reduces the bulk of the system. Furthermore, the trend today goes towards numerical equipments in mechanical field. A task/job/procedure program structure for several devices working coordinately is reported in the paper. Under this structure, the method of numerical synchronization for several devices becomes easy and clear.Based on the theory, a module structured multi-electrode welding coordinate control circuit, which is placed on a 20cm*30cm printed circuit board (PCB), is designed. Two DC-DC converters, which derived from Buck converter but power ground and gate drive signals ground are common earthed, are designed on it. One is designed for the bogie speed control, and another is for the balance of welding arc and wire feeder control. The principle of a pulse width modulation (PWM) generator and its adjusting method based on integrated circuit SG3525 is reported. The method of the parameters selection of operation amplifier, estimate and design of time constant, are reported as well. Some of the specific questions, such as the method of series connection of several sub-systems, soft start, over current protection, and the prevention of wire inserting into the molten pool during wind up are discussed.According to the experimental results, the welding speeds reached to 1.4-2.0(m/min.) in double-wire welding, and to 2.5-2.8(m/min.) in three-wire welding. The results tally with the theoretical analysis, as well as the reported results in some related references.