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The research and future prospects of the arc sensor
Weld seam tracking is to ensure that the premise of quality and welding automation, and sensors are the basis for achieving this prerequisite. Welding sensor according to the different ways of sensing additional sensors can be divided into two major categories and arc sensors. Most of the traditional seam tracking is an additional type of sensor, for example, touch sensors, electromagnetic sensors and a variety of optical sensors, such sensors is the sensor and the common problems is the separation of the arc, the sensor detection point arc with a certain distance away , a large arc in the welding seam will affect tracking performance. The arc sensor using the arc welding process, the current waveform or the change of arc voltage waveforms obtained ARC Centre has deviated from the seam as the sensor information and real time, tracking well. Arc sensor is its biggest advantage of the anti-arc, high temperature and strong magnetic fields are strong, and it is always consistent with the overall arc, simple structure, low cost, at present, arc welding sensor as a means of sensors much national attention, many foreign welding equipment research and manufacturing organizations are working to develop this area. Early start of the study of industrial countries, have developed a variety of arc scan forms (such as double wire tie, swing and rotation) of the arc sensor, suitable for submerged arc welding, TIG and MIG / MAG welding methods, such as different, and some have been used in welding production. Many countries on the production of arc welding robot equipped with swing arc sensor tracking device.
1 Classification of arc sensor
1.1 Swing scanning arc sensor
Swing arc scanning is currently the most widely used sensor of a welding arc sensor, which needs a swing arc sensor device, in the horizontal direction of the weld seam tracking back and forth and achieve. Used in arc welding robot sensors do not need to swing on a swing device, driven by the robot arm for horizontal swing to torch. However, the impact of structural factors by robots, robot beat frequency of 10Hz is generally less as shown in Figure 1a. In the high-speed arc welding and weld large cases, the tracking performance will be affected.
M KODAMA invented an electromagnetic high-speed swing arc sensor, the sensor on both sides of this arc has a permanent magnet and exciting coil, when the excitation coil of the DC current through a certain frequency, the conductive rod swing will produce a certain frequency to achieve seam tracking. This high-speed swing arc sensor generally can beat frequency adjustable between 0 ~ 40Hz, 0 ~ 4mm adjustable swing, the maximum welding speed 400mm / sec. It features small size, less weight 1Kg, the structure shown in Figure 1b.
1.2 Double wire parallel arc sensor
The arc sensor using two separate parallel arc welding on the workpiece, the welding wire around the two current (voltage) difference between the provision of two electric arc welding has deviated from the centerline of information, thus can be realized seam tracking. According to the two arc parameters and compare the difference between the reference value can also be achieved between the tip and the surface distance adjustment. This sensing method is the use of arc static characteristic parameters change as the sensor signal, use the same two parameters were independent loop power parallel groove welding, torch structure more complex, there are certain difficulties in implementation, so the utility on restricted.
1.3 high-speed rotating arc sensor scan
The arc sensor to rotate the arc instead of the swing arc, the rotation frequency up to 100Hz. Twentieth century, the eighties, NKK Corporation of Japan has invented a rotating arc sensor, and applied to the narrow gap weld, the principle shown in Figure 2: conductive rod as the cone of the bus, around the cone axis of rotation (revolution) , and not conducting rod around its axis of rotation (rotation), and the magnitude of cone movement at the top of a small diameter of this structure is to adjust the scanning cone angle adjustment, the sensor need to use a gear drive, the structure large, affecting the accessibility of welding torch. NKK Corporation of this technology in ships, boilers and structure to be applied in production, and achieved remarkable results.
CH Kim Korea produced a high-speed rotating arc sensor, shown in Figure 3, the sensor tip depend on the eccentricity to achieve the arc of the rotating tip of the eccentricity is the arc radius. Although it's rotating body is relatively simple, compact, but its high-speed rotation, the wire tip must be in the same rotational speed, which exacerbated the tip of the loss.
Germany U. Dilthey also done a lot of high speed rotating arc sensor research, production and made the arc seam tracking sensors.
In China, starting from the late eighties, to Tsinghua University, Academy of Pan Jiluan research group headed by the rotating arc sensor has done a lot of research work, and has made valuable scientific research. In 1993, Tsinghua University doctoral student in the doctoral student fees Liao sword jump agricultural research, based on the successful development of a hollow shaft motor-driven rotating scan sensors, and access to a national patent, shown in Figure 4. This high-speed rotating arc sensor scan design using the hollow shaft to the hollow motor as the prime mover, skew conductive rod through the hollow shaft motor. In the hollow shaft end, through the coaxial bearings installed aligning conductive rod, the conductive rod eccentric position is zero,-aligning bearings can be installed on the motor shaft or housing. The bottom of the hollow shaft, the eccentric sleeve installed in the shaft, the eccentric sleeve kit installed in the eccentric hole, aligning bearing kit installed in the eccentric hole, conductive rod installed in the bearing hole. By the eccentricity of the department conducting rod inside and outside the respective eccentricity eccentric sleeve and inner eccentric relative to the angle of the eccentric sleeve and decided to turn. When the motor rotates, the bearings will be struck down as a tapered conductive bus bar around the axis as the motor revolution, or as conical swing.
In recent years, Nanchang University in Jiangxi Province Key Laboratory of robotics and welding automation on the basis of this high-speed rotating arc sensor scan and vibration in small areas such as in-depth and meticulous research, and was further improved, produced a prototype, the prototype installed in the arc welding robot successfully carried out in real time tracking, Figure 5 for the installation of the hollow shaft in the robot rotating arc sensor.
2 arc welding performance sensor
2.1 weld arc sensor
Rotating arc welding and arc for the sensor or in a straight line swing weld when significantly different, the different movements of the welding arc weld under way have been studied, Figure 6 for three kinds of arc welding during exercise seam forming comparison chart. Rotation in the arc mode, the width of the weld movement pattern than the other two a slight increase in penetration has been reduced, weld reinforcement slightly reduced, due to high-speed rotation arc, droplet by the rotating centrifugal force the role of the pool to the surrounding fired, resulting in increasing the width of the pool; rotation frequency of the larger arc of rotation, a point arc in the reduced length of stay, and accordingly, the arc force on the role of pool at the bottom of shortening the time penetration decrease; the effect of centrifugal force and the scope of the expansion of the arc, weld reinforcement decreases.
Seam welding in the horizontal angle, high-speed rotating arc forming significantly improved, the protruding shape of the weld arc improved high-speed rotation. The horizontal angle of the high-speed rotating arc weld were studied welding at the horizontal angle, the rotating arc pressure acting on the molten pool on the surface of the effect of reduced dispersion and improved the smoothness of the weld. Rotating welding on both sides of the weld penetration in all of the increase, and tend to the web, it is because of the high-speed rotating arc and arc power to heat evenly scattered around, reducing erosion of the arc root of the weld pool the role of the rotation arc of the weld pool will produce stirring force, weakened the molten pool of metal gravity, Figure 7 is a straight line rotating arc welding and weld welding angle control diagram. In the actual use of rotating arc welding sensor, adjust the angle between the gun and the web to get good welds.
2.2 arc sensor research in high-speed welding
In the actual welding production in order to improve productivity, often require high-speed welding, other welding sheet metal, in order to avoid welding to wear, but also need high-speed welding. In order to study the high-speed rotating arc welding sensor performance, three kinds of arc movement pattern (ie, the arc in a straight line, swing movement and high-speed rotation) welding was studied. Figure 8 shows the movement pattern of three welding arc weld diagram. Linear motion in the high-speed welding torch when the weld is poor, there are undercut and weld there was a "hump" weld metal, which is due to higher welding current, welding speed faster, then, arc weld pool the role of liquid metal in the back of a strong, deep crater, and not enough liquid metal to fill the crater on both sides, thus creating a biting edge. As shown in Figure 8a; torch swinging motion in the high-speed welding of the weld as shown in Figure 8b, weld wavy, and the shape is poor, severely undercut; Figure 8c is a high-speed rotation welding torch in the weld when the map , weld, and no undercut, because the high-speed rotation arc to arc and dispersion forces on the molten pool, crater depth decreases, reducing the arc of the high-speed rotation arc on the back of the role of the liquid metal pool thus forming a good weld.
High-speed rotating arc welding robot tracking control of high speed, the sample for the thickness of 3.2mm, 500mm long, wavy lap joint, the welding current is 300A, the rotation frequency is 50Hz, the tracking speed of 120cm / min. M KODAMA developed high-speed electromagnetic drive swing arc sensor in the welding current 530A, when the oscillation frequency of 20Hz welding speed can reach 120cm/min.
3, a mathematical model of arc sensor
3.1 The mathematical model of the static arc sensor
Static model refers to the atmosphere, welding, power supply parameters, wire feed welding torch and the workpiece speed and distance are constant, stable arc burning conditions, the relationship between the various physical quantities. The results show that the external characteristic features to ease the power down, the torch height (H) and the current average value (I) the relationship between the range can be used as a large linear system to deal with.
Work in the arc basic fixed (fixed wire feed speed and the external characteristic), the arc sensor static model is:
H = - Kst I + C
Where: Kst for the welding torch height and the relationship between welding current factor; I for the current sample value; C is the maximum theoretical value of the welding torch height. If Kst, C is known, then I can calculate based on current actual value of the welding torch height H, and then compared with the given value, the difference is the amount of welding torch height adjustment.
3.2 The dynamic mathematical model of arc sensor
Dynamic mathematical model of arc sensor quantitatively describes the relationship between input and output. Studies suggest that: For the dynamic response with excellent welding power, its dynamic characteristics can be considered as the proportion of outside links, the dynamic model is first-order model; when the external characteristic for the inertia, the arc for the second-order dynamic model of the sensor model.
Through theoretical and experimental research that in arc welding power supply has a good dynamic quality, the mathematical model is zero and a pole with a first-order system; but will change the dynamic characteristics of arc welding power source is first-order system, the mathematical model is zero and two poles with a second order system.
Filaments submerged arc sensor on the research, has been a dynamic model of the arc sensor, and the second order model; power dynamic corner frequency the greater the amplitude frequency characteristics of arc sensor transition frequency to high frequency movement. Therefore, the power dynamic quality will directly affect the sensor performance.
3.3 arc weld the mathematical model of the sensor
Established a high-speed rotating arc sensor mathematical model of weld. Selected four major welding parameters, namely welding voltage U, welding current I, K and spin rotation frequency diameter D. Taking into account the process parameters (U, I, K and D) and the interaction of an item have a major impact on the weld, using a regression orthogonal design, with two-level orthogonal table L16 (215) conducted 16 tests, I got high weld penetration depth and weld width and arc welding parameters between the sensor model. The mathematical model for the rotating arc welding sensor parameter selection provides a theoretical basis.
3.4 Other mathematical model
Korea's C-H Kim on rotating arc sensor wire melting characteristics of the dynamic simulation. Ignoring the radiation, consider the heat source for the Gaussian heat source, rotating around the rotational arc sensor, the average heat input to the pool as a quasi-steady temperature field of heat input, the establishment of a welding torch at a constant speed of the temperature field under the workpiece the mathematical model, and simulate a typical pool shape.
4 arc the applications of sensor technology
Over the years, many foreign workers in the welding arc sensor on an intensive study and research applied to practical production. Currently, the vast majority of arc welding robots are equipped with swing arc sensor, such as Germany CLOOS the ROMAT 76SW robot and Japan, Panasonic Pana-Robo robot on the installation of the swing arc sensor. Rotating arc sensor applications are increasingly common, such as rotating arc sensor developed by Tsinghua University, Dongfeng Motor Company used car storage pump automatic girth welding, South Korea HAN GIL Autowelding produced by rotating arc sensor can be used for arc welding and automatic welding robot shown in Figure 9. Panasonic's YA-11KMR51 type arc welding robot also installed a rotating arc sensor, shown in Figure 10.
5 Conclusion
Arc sensor as a real-time sensing devices and other types of sensors, compared with a structure is relatively simple, low cost, fast response characteristics, is an important sensor welding direction, with a strong vitality and prospects. Mainly used in two aspects: one is mainly used in arc welding robot, on the other hand is mainly used in automatic welding with Oldham.
In the future should focus on the arc sensor and three-dimensional information extraction performance of welding technology. Space in the welding seam, the welding torch position and orientation to be adjusted as can be satisfied with the weld. Arc current up and down and around the sensor can only capture two-dimensional information, before and after the extraction of information still to be in-depth research in order to adjust the posture for robot arc welding in all position welding. Through the efforts of welding science and technology workers, the intelligent tracking ability will be stronger.