Current status and problems of arc sensor research and application
The two main challenges in achieving welding automation are: weld tracking and weld quality control. Among them, weld seam tracking is the premise to ensure the quality of welding, so the automatic tracking of the weld seam becomes the first problem to be solved. A weld seam tracking system consists of two main aspects: the sensor and the controller. Sensors can be divided into two types of additional sensors and arc sensors depending on the sensing method. Traditional weld seam tracking sensors are mostly add-on. For example, contact sensors, electromagnetic sensors, and various optical sensors that attach an additional mechanical, electromagnetic, or optical device to the torch to detect the relative position of the weld. The common problem is the sensor and the arc. It is separate and has complicated attachments, which is inconvenient to apply and the effect is not ideal. The arc sensor uses the arc itself as a sensor to extract the current or voltage change amount in the welding process as a sensor signal according to the basic characteristics of the welding arc. Therefore, arc sensors have unique advantages over add-on sensors:
(1) The detection point is the welding point, there is no problem of the sensor first, and it is a completely real-time sensor.
(2) There is no need to equip other special devices around the welding head, and the accessibility of the welding torch is good.
(3) Since the arc itself acts as a sensor, it is not affected by arc deflection caused by wire bending and magnetic bias blowing.
(4) It can not only track the sensing, but also ensure the stability of the welding parameters, and also improve the forming effect of the weld.
(5) Anti-light, electromagnetic and thermal interference, long service life.
It is these advantages that make arc sensors have become a hot spot in the development of arc welding in recent years, but the practical use of arc sensors in China is still in its infancy.
First, the principle of the arc sensor
The welding arc is oscillated by electrical or mechanical means, and the change of welding current and voltage is detected to determine whether the center of the swing deviates from the center of the groove and is corrected. The methods for oscillating the arc are mechanical, electromagnetic, and jet. The oscillating trajectory can be divided into linear reciprocating motion, circular arc motion, and rotational motion. When using double wire parallel welding, it is also possible to do not swing.
From the above, when the arc is scanned along the vertical direction of the weld, the welding current will change with the change of the welding height caused by the scanning, thereby obtaining the information of the weld bevel and achieving the purpose of sensing.
Second, the research and application status of arc sensor
1. Type of arc sensor
At present, arc sensors are mainly used in the following types:
(1) Non-scanning double-wire parallel type This type uses the static characteristics of the arc. When the torch is misaligned, the difference in height between the two arcs will be reflected in the current (voltage) difference, thereby achieving weld tracking. However, since it is necessary to simultaneously perform the bevel welding by using two independent circuit power supplies with the same parameters, it is difficult to implement, so the utility is limited.
(2) Oscillating arc sensor Generally, the oscillating arc sensor is mechanically mostly. Due to the limitation of the mechanism, the scanning frequency is generally below 5 Hz, which makes the sensitivity lower, and the flow and filling of the liquid metal in the molten pool is also Weld bevel identification creates obstacles.
(3) Rotary Scanning Arc Sensor The principle of the rotating arc sensor is that, under the driving of the DC motor, the eccentric hole on the conductive nozzle is used to rotate the wire and the arc to achieve high-speed scanning of the arc. The general scanning frequency is 15 to 35 Hz. This design overcomes a series of problems caused by mechanical low scan frequencies (see Figure 2).
Figure 2 Rotating arc sensor structure
1. Electrode 2. Motor 3. Optical disc 4. Eccentric mechanism 5. Self-aligning bearing 6. Welding wire
Rotary scanning torch first appeared in NKK, Japan, for narrow gap welding literature to improve the formation of both sides of fusion and fillet welds and multi-pass welding. Later, it was found that high-speed rotating arc can also be used for welding tracking sensing, and the sensitivity is more High, is actively committed to the development of high-speed rotating arc in sensing tracking, and successfully developed a high-speed rotating arc sensor.
The principle is: under the driving of the motor, the conductive nozzle rotates around itself, and the eccentric hole on the conductive nozzle is used to rotate the wire end and the arc to realize high-speed rotation scanning of the arc.
In 1990, Tsinghua University gave a conical swing scheme. The conductive rod swings in a cone without rotating around itself, thus avoiding friction between the welding wire and the contact tip. Further development in 1993, the hollow motor structure design, the structure is simpler, the vibration and noise are reduced, and the volume is also greatly reduced. The designed outer diameter of the rotary scanning torch is only 80mm, which can be like a normal welding torch. Used the same and applied for a Chinese patent. Nanchang University has made further improvements on the basis of the structure, which makes the structure of the rotary scanning torch more compact. Its outer diameter is reduced to 45mm, the volume is reduced by 10%, and the weight is reduced by 20%, which makes the vibration damping performance better. It has been applied to arc welding robots and has taken a big step toward practical use.
2. Arc signal processing and its control technology
How to get the position of the torch from the arc sensing signal, that is, the left and right, high and low deviation information of the weld, which is the most critical part of the actual weld rectification. In 1978, the integral difference method was used to solve the identification of V-shaped and fillet welds, and the current-noise problem in CO2 welding was solved by the method of wave-eliminating. In 1990, the method of computer simulation was firstly used to study the identification method of arc sensing system, and important results were obtained. In 1996, using computer simulation technology, the information input and output of the sensing system were deeply studied. The information space was first introduced, the method of identifying the groove by the characteristic harmonic method, and the digital filtering technology was proposed to make the information processing of the arc sensor A breakthrough in progress. In 1993, JWKim and SJNa in South Korea were fitted to a curve by using a quadratic curve fitting method, in which the welding current signal value is inversely proportional to the distance from the torch to the workpiece. At the change of the swing direction, the curve fits the welding current signal, that is, the welding current value, and thus can be used as a control signal for the weld seam tracking. In 1998, Huadong Shipbuilding Institute also used fuzzy control to achieve automatic tracking of welds.
3. Application of arc sensor
After many years of research and development, the arc sensor has greatly reduced the volume and weight, and the weld seam recognition ability has been enhanced. At the same time, with the development of computer technology and fuzzy mathematics and other related disciplines, the rotary arc sensor has entered a practical stage. Tsinghua University has developed Two automatic welders. The first is to add a cross slider to the ordinary automatic welder, install the arc sensor on the cross slider, and correct the deviation by controlling the motor of the cross slider. The second is a full-degree-of-freedom automatic welder that does not require a fixed track like the first, and is a free-tracking automatic tracking welder. Nanchang University installed a high-speed rotating arc sensor on the welding robot to achieve automatic tracking of the weld. In 1993, JWKim and SJNa designed an adaptive fuzzy controller, which applied fuzzy control technology to weld seam tracking and achieved good tracking results. In the industry, arc sensors have a good application in arc welding machines. CLOOS in Germany uses arc-oscillating arc sensors for arc welding robots. In Japan, arc welding robots equipped with high-speed rotating arc sensors have been used. Used in the shipbuilding industry. Huadong Shipbuilding Institute uses the single-chip microcomputer to realize the weld seam tracking of the rotary scanning torch. All of these indicate that arc sensors have broad application prospects.
Third, the problems faced
Although there has been a lot of research on arc sensors, the application of arc sensors, especially in domestic applications, has not been satisfactory, and is currently limited to weld tracking in some special occasions. First, it is known from the principle of the arc sensor that it requires the welding gun to oscillate. For this reason, there is a device for controlling the swing, and in some cases, it is not suitable to use the swing. Satisfactory results cannot be obtained for the I-shaped groove or the gapless butt welding. In existing industrial applications, successful examples are also done on V-grooves and fillet welds. These limits the range of applications for arc sensors. Second, the processing of the arc sensor signal is also one of the difficulties, because the arc welding process has many unfavorable factors for signal acquisition and processing, such as short-circuit current interference, molten metal liquid metal fluctuation or flow interference, etc. The bevel shape signal brings great difficulty. How to remove these interferences to get a good signal, although there have been a lot of research, but there is no better solution. Third, the choice of control methods, traditional PID control can not meet the complex, non-linear welding process, and the use of self-organized fuzzy control is a better solution, but it will encounter problems such as large computational complexity. Real-time control is not possible for simple processors. The use of PCs increases the cost and fails to meet the economic efficiency requirements of enterprises.
The characteristics of the arc sensor are very beneficial to the realization of the welding intelligent control, and the intelligent control is very important to ensure the welding quality. Solving the above three problems will be a hot research topic in welding automation and intelligence in the future.
Fourth, the conclusion
The arc sensor and the optical sensor are called arc welding automatic tracking two sensors, wherein the arc sensor is different from any other existing sensor, it is not independent of the welding arc welding power system, but with the arc characteristics and The characteristics of the power supply are closely related to the part. Although its applicability is limited in some cases, its application prospects are still very broad.
With the development of intelligent control technology and the solution of a series of problems such as weld identification and real-time control, arc sensors will play a more important application value in welding automation.