Arc welding robots are playing an increasingly important role in the automotive manufacturing industry, and their bodies are a crucial component, significantly impacting their performance. Arc welding robots are human-like, automatically controlled, and reprogrammable mechatronic automated production equipment, playing a vital role in high-quality and efficient welding production in the automotive industry. The arc welding robot body is one of the key components affecting its performance. With the widespread application of arc welding robots in the automotive industry, understanding their mechanical structure and working principles can lay the foundation for future application research and secondary development. Field Application Chery Automobile Co., Ltd. has a diverse and complex range of vehicle models. Previously, on manual production lines, when mixing different types of the same model, the manual welding speed was significantly reduced due to differences in workpieces and tools, resulting in inconsistent product quality. Therefore, Chery adopted arc welding robots to replace manual operation, stabilizing and improving welding quality, improving workers' working conditions, increasing labor productivity, and clarifying product cycles. The following section uses a chassis arc welding robot system as an example to introduce the robot's application. This system consists of an arc welding robot, a rotary fixture, and a PLC centralized control system. The arc welding robot is responsible for the welding work, controlled by a supporting control system. A rotary fixture, as a peripheral device of the robot, transfers workpieces for processing through position changes (0°, 180°, and 360° on the worktable), with manual loading and unloading. A PLC acts as the host computer, coordinating and controlling the rotary fixture and exchanging real-time information with the robot control system to ensure the normal operation of the entire arc welding system. With the adoption of arc welding robots, only several different motion and welding programs need to be pre-programmed to adapt to the production of different car models. The robot will automatically call upon the corresponding work programs and the tools required for welding different car bodies according to the work instructions, thus automatically adapting to the complex changes in car models. The widespread application of arc welding robots has brought unprecedented vitality to Chery Automobile's flexible automated production. Currently, many automobile manufacturers have adopted arc welding robots for automated production; however, the purchase cost of arc welding robots is high, and the results of some field applications are not ideal. During the arc welding process, if the welding conditions are relatively stable, the robot can guarantee the welding quality. However, due to various factors, the actual welding conditions often change. Intense arc radiation, high temperatures, fumes, spatter, bevel conditions, machining errors, fixture clamping accuracy, surface condition, and workpiece thermal deformation can all cause the welding torch to deviate from the weld seam, leading to decreased welding quality or even failure. In process design, there is often a lack of in-depth research on equipment selection, with the assumption that robots are "omnipotent." The selected weldments and weld seam distributions are complex, making it difficult for robots to adapt. For a single weldment with dozens or even hundreds of weld passes, even with a robot possessing starting point finding and tracking capabilities, deviations in the weld passes can cause the welding torch to deviate from the weld pass after only 20%–30%, or even 40%–50%, of the welding completed. [Intrinsic Analysis] Enterprises compete by equipping themselves with robots to improve product quality while simultaneously reducing equipment investment and lowering product costs. Furthermore, from a technical perspective, robot purchases must be aligned with the application environment; failure to meet on-site requirements will lead to robot application failure. Therefore, only by strengthening talent training and gaining a deeper understanding of robots can correct selection and rational application be achieved. The MOTOMAN arc welding robot is a highly representative model. The MOTOMAN-YR-K6 arc welding robot has an end-effector load of 15kg and six degrees of freedom. Its mechanical body mainly consists of a base (1 axis), an arm (2 and 3 axes), and a wrist (4, 5, and 6 axes), driven by a drive system through a transmission mechanism to achieve the required position and posture of the robot's end effector in space. 1. Base and Arm: The robot's arm consists of powered joints and connecting rods, used to support and adjust the position of the wrist and end effector. The arm components generally have 2-3 degrees of freedom, including the drive unit, transmission mechanism, and support connectors. The robot arm is mounted on the base, which enables the arm's overall rotation or lifting. The MOTOMAN-YR-K6 robot's base adopts a rotating base structure to accommodate the entire weight and working load of the robot body, achieving overall rotation of the robot body. As a special type of arm, it uses high-strength materials to ensure sufficient rigidity, strength, and load-bearing capacity. The arm section, based on its degrees of freedom, motion form, load-bearing capacity, and motion accuracy requirements, mounts the forearm drive motor and reducer on the upper arm, transmitting motion and power to the forearm via a parallel four-bar linkage. The advantage of this design is that mounting the forearm drive motor at the lower end of the upper arm reduces the forearm's weight, thus reducing the load on the upper arm and improving the robot's motion flexibility. However, this structure limits the robot's working range. Currently, mainstream robot bodies adopt an open-chain structure, significantly improving the robot's spatial mobility. The robot's base and arm sections improve positioning accuracy by adding buffers and limiting devices. Reliable connections ensure motion precision and rigidity, reducing motion errors between the base and arm (as shown in Figure 1). [align=center] Figure 1 Arc welding robot 1—Rotating base; 2, 3—AC servo motors; 4—Upper arm; 5—Link; 6—Forearm[/align] Working principle: Generally, AC servo motors are connected in series with reducers, brakes, and encoders to increase torque, improve posture accuracy, and enhance control performance. In Figure 1, the servo motor inside the rotary base 1 drives the entire base to rotate, causing each arm to rotate with the base. Servo motor 3 drives the upper arm 4 to swing back and forth, and servo motor 2 directly drives the linkage 5, indirectly driving the lower arm 6 to pitch according to the four-bar linkage principle. 2. The wrist of the wrist robot is the component connecting the arm and the end effector. Its main function is to realize the three position coordinates of the end effector in the workspace after the arm and base have achieved the three position coordinates of the end effector in the workspace, that is, to achieve three rotational degrees of freedom. It connects and supports the end effector through a mechanical interface. The rotary axis of the MOTOMAN-YR-K6 robot wrist adopts a hollow thin-walled rectangular frame structure according to the load and structural characteristics of the entire wrist, which improves the bending stiffness and torsional stiffness and reduces its own weight. The swing axis adopts a separate transmission according to the load and movement flexibility of the wrist, placing the servo motor inside the hollow structure of the wrist rotary axis, reducing its own weight and volume. The entire wrist part is made of high-strength, lightweight alloy material, which improves the transmission stiffness and reduces the moment of inertia. Because the entire wrist structure is compact, motion errors caused by gaps are reduced, improving the accuracy of wrist movements (as shown in Figure 2). [align=center] Figure 2 Wrist structure with three rotational degrees of freedom (4, 5, 6 axes) 1, 3, 4—servo motors; 2, 5—transmission belts; 6, 7—harmonic reducers; 8—wrist housing; 9, 13—rotation shafts; 10—pulleys; 11—swing shafts; 12—bevel gears[/align] Working principle: Servo motor 1 drives rotary shaft 9 to rotate (4-axis motion) through reducer 2 mounted on wrist housing 8; servo motors 3 and 4 realize wrist rotation; servo motor 4 drives swing shaft 11 to reciprocate through pulleys, transmission belt 5 and reducer 6, realizing wrist swing (5-axis motion); servo motor 3 drives rotary shaft 13 to rotate through pulley 10, transmission belt, bevel gear 12 and reducer 7, realizing wrist twisting (6-axis motion). Analysis of the MOTOMAN-YR-K6 robot body reveals the following principles in its design and selection: (1) Minimum Moment of Inertia Principle: Due to the large number of moving parts in the robot body and the frequent changes in motion state, impacts and vibrations are inevitable. Adopting the minimum moment of inertia principle and minimizing the mass of moving parts can increase the stability of the robot body's motion and improve its dynamic characteristics. (2) Size Optimization Principle: When the design requirements meet certain workspace requirements, size optimization is used to select the minimum arm size, which will help improve the body's stiffness and further reduce the moment of inertia. (3) High-Strength Material Selection Principle: Since the robot body acts as a load sequentially from the wrist, forearm, upper arm to the base, it is essential to select high-strength materials to reduce the mass of parts, reduce the dynamic load and impact during operation, reduce the load on the drive device, and improve the response speed of moving parts. (4) Principles of stiffness design: To maximize stiffness, it is necessary to properly select the cross-sectional shape and size of the rods, improve the support stiffness and contact stiffness, and rationally arrange the forces and moments acting on the arm to minimize the bending deformation of the rods. (5) Principle of reliability: Due to the complexity of the mechanism and the large number of links, the reliability of the robot body is particularly important. Generally speaking, the reliability of components should be higher than that of parts, and the reliability of parts should be higher than that of the whole machine. (6) Principle of manufacturability: The robot body is a high-precision, highly integrated automatic mechanical system. Good processing and assembly manufacturability is one of the important principles to be reflected in the design. Arc welding robots are widely used in China and abroad, and are also widely used in Chery Automobile Co., Ltd., such as body welding and chassis welding. Through the analysis of the MOTOMAN-YR-K6 robot body, we can gain a deeper understanding of the robot's mechanical structure and working principle, lay the groundwork for future application research and development and secondary development, and provide assistance in mastering robot technology.