Exploration of new technology for analog arc length feedback

1 Introduction
With the rapid development of modern inverter technology and the continuous improvement of power devices, inverter CO2 shielded welding technology has become increasingly perfect and its reliability has been greatly enhanced. Under this circumstance, stabilizing the wire speed and improving the quality of weld formation have become the focus of inverter shielded welding machine manufacturers. To this end, various manufacturers have racked their brains to find ways to solve this problem, among which arc length feedback technology is one of the key technologies to solve this problem. Our company has also conducted a lot of exploration in arc length feedback technology. In order to be compatible with the output interface of traditional shielded welding machines, we innovatively adopted analog arc length feedback technology and achieved good results.

2 Principle of arc length feedback The
so-called arc length feedback is to use the proportional relationship between arc length and voltage applied to the arc to sample arc length voltage, feed back the sampled signal to the wire feeding control circuit, adjust the wire feeding speed, thereby stabilizing the wire feeding speed, and then stabilize the arc length and welding current.
The principle of arc length feedback is shown in Figure 1. When the wire feeding speed of the wire feeder becomes faster, the welding current rises, the arc length becomes shorter, and the arc length voltage becomes lower, this sampled signal is fed back to the wire feeder control circuit, and added to the error signal of the previous PID, and the pulse width signal is adjusted together to make the thyristor output voltage lower and the wire feeder speed slower. On the contrary, the thyristor output voltage becomes higher and the wire feeder speed becomes faster, so as to adjust the wire feeding speed and achieve the purpose of stabilizing the arc length and welding current.

3 New technology of analog arc length feedback
Introducing arc length feedback in the wire feeding circuit can indeed stabilize the arc length and welding current, but it also brings a problem, that is, it is necessary to add a feedback line from the welding gun to the welding machine, which will be different from the traditional wire feeder cable interface, resulting in the wire feeding system of the traditional shielded welding machine cannot be used on the new inverter shielded welding machine. For this reason, we innovatively adopted the analog arc length feedback technology to solve this problem. Figure 2 is the principle design block diagram of the analog arc length feedback technology.

By comparing Figure 2 with Figure 1, it is found that Figure 2 does not sample the arc length at the welding gun, but directly samples the output voltage and current of the inverter CO2 welding machine, performs subtraction calculation after weighting, and uses the difference to simulate the change of arc length, so as to adjust the pulse width and the output of the thyristor, thereby achieving the same effect as arc length feedback.

4 Problems encountered and measures taken
The simulation of arc length feedback is feasible in principle, but there are also many problems in practice. For example, before the welding machine starts welding, the welding machine is in an unloaded state. At this time, the output voltage is large, and the output current is zero. Therefore, the difference between voltage and current is large, and controlling the wire feeding speed in this way will inevitably cause the wire feeding motor to rotate rapidly. At the beginning of welding, the load is in a short-circuit state, the output voltage is zero, and the output current increases rapidly to several hundred amperes. The difference between the two is a large negative value, which will cause the wire feeder to stop. If no measures are taken, the welding machine will not be able to be used.
To this end, after exploration, we took the following measures in the circuit: ① First limit the amplitude of the positive difference signal, and then superimpose it with the set value of the wire feeding speed, so that the problem of rapid rotation of the wire feeding motor when the welding machine is unloaded can be effectively solved; ② Filter out the negative difference signal to prevent it from affecting the wire feeding speed at arc starting; ③ Integrate the difference signal so that it can smoothly adjust the wire feeding speed.
In order to verify the feasibility of the above measures, the circuit was simulated and modified many times through circuit simulation software, and finally satisfactory results were achieved.
5 Experimental Improvement
In order to verify the practicality of the simulated arc length feedback theory, a new wire feeder control circuit was designed based on the principle design and simulation, and a new control board was made and installed on the shielded welding machine. At the beginning of the test, there were arc starting difficulties and large-scale current changes. Through repeated adjustments to the PID circuit parameters and improvements to the control circuit, the above problems were completely solved. After repeated welding tests, the new welding machine using simulated arc length feedback has stable arc starting, stable welding current, good weld formation, and small spatter, which is very practical.
Based on the principle prototype, we mass-produced 20 units of this new inverter CO2 shielded welding machine, which has been tried in the company for more than a year and achieved satisfactory results.

6 Conclusion
This design innovatively adopts the analog arc length feedback technology, which not only successfully solves the problem of welding current stability, but also improves the quality of weld formation. At the same time, it also solves the compatibility problem between inverter shielded welding machine and traditional shielded welding machine wire feeder. Whether it is the arc starting process or the welding process, the current is very stable and can well meet the use requirements. References [1] Zhang Libin, "Basics of Arc Welding Quality Automatic Control", first edition. Heilongjiang. Harbin Engineering University Press, 2007.10.224 pages.

[ 2] Huang Shisheng, "Arc Welding Power Supply and Its Digital Control", first edition. Beijing. Machinery Industry Press. 2007.1.287 pages. [3] Hu Shengsun, "Modern Arc Welding Power Supply and Its Control", first edition. Beijing. Machinery Industry Press. 2007.2.259 pages.