0 Introduction Copper-aluminum tube butt welding machines are specialized equipment used for butt welding copper and aluminum tubes. Due to the use of a programmable logic controller (PLC) as the control system, its time control accuracy is high, reaching 0.01s. To achieve high-quality welding of copper and aluminum tubes and match the high control accuracy of the PLC, a high-precision actuator must be designed and manufactured. This paper analyzes the copper-aluminum tube welding process and its characteristics, decomposing the welding process into 6 actions, all of which involve linear motion. Therefore, hydraulic cylinders or pneumatic cylinders are considered as the main components of the actuator. After comparison, a pneumatic system was chosen to design the actuator of the copper-aluminum tube butt welding machine. The fixture is also one of the key components of the actuator; therefore, this paper also analyzes and designs the matching fixture for the copper-aluminum tube butt welding machine in detail. 1 Analysis of the Copper-Aluminum Tube Welding Process Since the overall shape of the copper and aluminum tubes is not limited, only the shape near the tube end is considered. The end of the copper tube to be welded is conical, while the aluminum tube is always cylindrical, as shown in Figures 1(a) and (b). During welding, the copper tube moves relative to the aluminum tube. When the copper tube is inserted into the aluminum tube and comes into contact with it, the welding circuit is connected, and a large current is generated between the copper and aluminum tubes, causing the two tubes to heat up. The temperature of the aluminum tube approaches the melting point, and the copper and aluminum begin to bond. When the copper tube moves to the position, the welding circuit is cut off. After the copper and aluminum tubes are firmly bonded, the welding process ends. Figure 1(c) shows a schematic diagram of the copper-aluminum tube joint after welding. This welding method has many advantages, such as a wide joint surface and high welding strength; it is easy to implement and has low production costs; it is not limited by the overall shape of the tube; it is well formed in one step and has a beautiful appearance. Practice has proven that this joint method is particularly suitable for the refrigeration industry. 2 Decomposition of the copper-aluminum tube welding process As mentioned above, the copper-aluminum tube welding process can be decomposed into 6 actions, as shown in Figure 2(a) to (f). (1) Loading and positioning: Since the overall shape of the tube is not determined, it is assumed that there is a loading mechanism or manual loading. After loading, the copper tube and aluminum tube need to determine 5 degrees of freedom, that is, 5 degrees of freedom other than axial rotation. Practice has shown that among the five degrees of freedom, the axial positioning accuracy has a smaller impact on the welding quality, while the radial and rotational positioning accuracy around the coordinate axis has a larger impact on the welding quality; (2) Clamping: After positioning, clamping is necessary to ensure that the copper and aluminum tubes are completely under the control of the control system during the welding process. Since the overall shape of the tube is not yet determined, and to ensure the welding quality, clamping is performed near the end face of the copper and aluminum tubes. (3) Advancing: After clamping, there is still a certain distance between the copper and aluminum tubes in the axial direction, and relative displacement is required during the welding process. The advancing action is completed by inserting the copper tube into the aluminum tube; (4) Welding: After the copper tube and aluminum tube come into contact, the welding circuit is connected, and a large current will be generated between the copper and aluminum tubes, causing the copper and aluminum tubes to heat up and the temperature to approach the melting point of aluminum. Until the aluminum tube completely covers the conical surface of the copper tube, the advancing is stopped and the welding circuit is cut off; (5) Releasing: After the temperature of the copper and aluminum tube welding joint drops and the connection is firm, the clamp is released and the material is unloaded; (6) Retraction: After unloading, the advancing mechanism is returned to its original position to prepare for the next welding. The decomposition of the copper-aluminum tube welding process provides a reliable basis for the design of the actuator. 3. Principle Design of the Actuator 3.1 Overall Structural Design of the Actuator As mentioned above, all actions of the actuator of the welding machine are linear motions. Considering that the clamping force and propulsion force are not very large during the welding process, the clamping force and propulsion force for welding φ8×1 copper-aluminum tubes are approximately 1000N. A pneumatic system is used to achieve the clamping and propulsion of the copper-aluminum tubes. The advantages of the pneumatic system are: the working medium is air, which is easy to obtain and discharge, does not pollute the environment, and realizes green production; it responds faster and acts more quickly than the hydraulic system; it is easy to achieve overload protection; the gas pipeline is made of plastic, which is easy to cut and connect, and can withstand a certain degree of deformation. Figure 3 shows the overall schematic diagram of the actuator of the copper-aluminum tube welding machine. 3.2 Pneumatic System Design The copper-aluminum tube welding machine is controlled by a PLC to achieve coordinated actions of various parts and complete the welding process. The control of the cylinder must be completed through a solenoid valve. This requires the design of a corresponding pneumatic system to meet the control requirements. Figure 4 shows the schematic diagram of the pneumatic system of the copper-aluminum tube welding machine. The PLC controls the movement of four cylinders through two 2-position 4-way valves. Solenoid valve PQ1 controls the clamping and positioning cylinders. Solenoid valve PQ2 controls the propulsion cylinder, and a throttle valve adjusts the propulsion speed. An overflow valve limits high air pressure at the pneumatic system's air source, a pressure switch limits low air pressure, and a pressure gauge displays the current air pressure value. The pressure switch can feed back a low air pressure signal to the PLC, indicating that the clamping force and propulsion force are insufficient to complete the copper-aluminum tube welding. [ALIGN=CENTER] [/ALIGN] 3.3 Fixture Design The copper-aluminum tube welding machine fixture consists of three parts: positioning, clamping, and the fixture body. Because the fixture body is in direct contact with the workpiece, the fixture's accuracy directly affects the welding quality and must be handled with care. This paper only considers straight tubes. During the welding process, five degrees of freedom of the copper and aluminum tubes must be restricted, as shown in Figure 5, excluding the rotational degree of freedom around the Y-axis. If the overall shape of the welded pipe is considered, six degrees of freedom need to be restricted, requiring the design of a dedicated fixture to achieve complete positioning. Based on the characteristics of the copper-aluminum pipe and welding requirements, the fixture is designed as an upper and lower split-mold elongated hole shape, as shown in Figure 5. This restricts four degrees of freedom, meaning the fixture itself can achieve positioning for four degrees of freedom. A positioning plate is placed at the end face of the copper-aluminum pipe, allowing the end face to contact the positioning plate, thus restricting the fifth degree of freedom. Practice has shown that the axial positioning accuracy of the copper-aluminum pipe has a relatively small impact on welding quality; in contrast, the other four positioning accuracies have a greater impact. The design focus should be on the upper and lower split-mold fixture, as its machining accuracy directly affects welding quality. Regarding dimensions and tolerances, since the upper and lower split-mold fixture serves a clamping function, its cylindrical inner diameter should be slightly smaller than the outer diameter of the copper-aluminum pipe; that is, the upper tolerance limit should not exceed the lower tolerance limit of the pipe's outer diameter. Conversely, the lower tolerance limit of the fixture's inner diameter should not be too small, otherwise, indentations will appear on the surface of the pipe with a larger outer diameter. Therefore, the tolerance of the inner diameter of the cylindrical surface of the upper and lower mold clamping bodies must be limited to a small range, i.e., a narrow tolerance zone, to achieve clamping without indentation. Regarding form and position tolerances, firstly, for a single pipe, the upper and lower mold clamping bodies should maintain surface contact with the pipe after clamping. Therefore, the cylindricity requirement after clamping must be considered. Since slight deformation occurs after clamping, the inner cylindrical surface of the upper and lower mold clamping bodies is designed as a slightly elliptical cylindrical surface, which precisely meets the requirements after clamping. Secondly, regarding the welding of copper and aluminum pipes, the radial positioning error of the two pipes has a significant impact on the welding quality. Therefore, the straightness of the inner cylindrical surface of the two clamping bodies after clamping should be considered during assembly. The clamping body designed in this paper can achieve the above requirements by being processed on an online cutting machine through programming. 4. Implementation of the Actuator Design Scheme As described above, through high-precision machining, the copper-aluminum pipe butt welding machine, under the high-precision control of the PLC, finally achieves high-quality welding of copper and aluminum pipes. Figure 6 shows a front view of the copper-aluminum pipe butt welding machine. The clamp body is machined from copper and bolted to the clamping seat; the clamp body seat is machined from steel and threaded to the piston rod head of the clamping cylinder. The cylinder seat is made of copper, and the cylindrical guide rail is made of steel. The fixed cylinder seat and guide rail have an interference fit, while the sliding cylinder seat and guide rail have a small clearance fit and are oil-lubricated, ensuring both smooth movement and high accuracy. The propulsion cylinder uses an extended rod, with two sets of double nuts determining its advance and retraction positions to meet different welding requirements. The positioning cylinder has a large stroke but almost no load; it only moves the positioning plate up and down, providing positioning when in the upper position and not obstructing the advance of the sliding cylinder seat when in the lower position. The air passages in the pneumatic system are made of plastic air tubes, which have high flexibility and can withstand large deformations during movement, simplifying the overall actuator design. However, practice has shown that the length of the air passages for each cylinder has a certain impact on its operation; this should be carefully considered during PLC programming, and appropriate delay functions should be added. 5 Conclusions (1) By analyzing the copper-aluminum tube welding process, an actuator that can achieve high-quality copper-aluminum tube welding was designed; (2) Under the high-precision control of PLC, the actuator can accurately complete each action of the copper-aluminum tube welding process; (3) The interchangeable fixture makes the copper-aluminum tube welding machine have a certain flexibility and can weld copper-aluminum tubes of φ6～12mm; (4) The high precision of the actuator matches the high precision of the PLC control system, so that the overall performance level of the copper-aluminum tube welding machine reaches the international advanced level. References 1 Zhang Xichuan, Zhang Yue, Ma Yuqiang. Design of copper-aluminum tube welding machine based on functional analysis [J]. Modern Manufacturing Engineering, 2006(3):120～122 2 Han Rongdi, Zhang Yue, Zhang Xichuan. Application of programmable logic controller (PLC) in copper-aluminum tube welding machine [J]. Welding, 2004(7):20～23 3 Zhang Xichuan, Zhang Yue. PLC-controlled copper-aluminum tube welding machine [J]. Pipeline Technology and Equipment. 2005(6):29-31 4 Hou Zhenxiu, ed. Mechanical System Design [M]. Harbin: Harbin Institute of Technology Press, 2001 5 Feng Xin'an, ed. Mechanical Manufacturing Equipment Design [M]. Beijing: Machinery Industry Press, 1999