Introduction Equipment remanufacturing engineering is an effective way to conserve resources, protect the environment, upgrade and transform old equipment, and accumulate experience for new equipment. It is an inheritance, development, and deepening of maintenance engineering and surface engineering, as well as their theorization and systematization. It is also an important technical approach to creating a resource-saving society, building a circular economy, and adhering to the sustainable development strategy. Due to the unprecedented brutality of modern warfare and the increasingly severe damage to equipment, precise support is urgently needed. At the same time, due to the complexity, high quality, variety, small batch production, short cycle requirements, and high repair difficulty of modern weapons and equipment, traditional mass production technologies can no longer meet the needs of modern warfare in terms of the quality, efficiency, precision of remanufactured products, as well as in reducing labor intensity and environmental pollution. Therefore, developing automated, flexible, and intelligent rapid manufacturing technologies has become an important issue facing militaries worldwide. Adopting automated, flexible, and intelligent rapid manufacturing technologies can also reduce inventory backlog, lower support costs, and improve support levels. It also has significant strategic and military importance for the rapid repair and precise support of parts in long-distance, high-temperature, and highly toxic environments such as those used in maritime, land, air, and space operations. Meanwhile, in today's highly competitive economic society, this technology can significantly improve the flexibility of enterprises in manufacturing small batches and diverse products, enhancing their competitiveness and market responsiveness. Therefore, the development of automated, flexible, and intelligent rapid production technologies has broad research, development, and application prospects in both national defense and the national economy. Robotic arc spraying flexible rapid prototyping technology based on remanufacturing is one such example. This technology refers to the rapid manufacturing of arbitrary-shaped equipment parts (mainly thin-walled parts) under robot control, directly driven by CAD models, using arc spraying. It is a product of the interdisciplinary integration and crossover of mechanical engineering, CAD, automatic control, arc spraying, and materials science. It is a technology that automatically, quickly, and accurately transforms design ideas into functional prototypes or directly manufactures parts. This paper preliminarily explores the composition, characteristics, working process, and path planning methods of robotic arc spraying flexible rapid prototyping technology and provides an outlook on its future development. 1. System Composition The robotic arc spraying flexible system mainly consists of a robot body and controller, arc spraying equipment, positioner, peripheral devices, and a control system. Industrial robots used for spraying are generally six-axis articulated robots with a payload of 6kg or 16kg. The lower three axes (arms) move the spray gun to different spatial positions, while the upper three axes (wrists) handle the spray gun's posture. The joint movements are driven by AC servo motors, exhibiting good dynamic characteristics, strong load capacity, low failure rate, and rapid acceleration and deceleration of each axis. The robot controller is the central hub of the entire system, comprising computer hardware, software, and dedicated circuits. The software includes controller system software, robot language, robot kinematics and dynamics software, robot control software, robot simulation software, and robot self-diagnosis and sub-protection software. The controller is responsible for all information and controlling all robot actions during operation. Arc spraying equipment generally consists of a spray gun, spraying power supply, wire feeding mechanism, and auxiliary mechanisms. To ensure the robot controller can control and program spraying parameters, the interface protocol between the robot and the spraying equipment must be consistent. The positioner primarily coordinates with the robot during the spraying process, positioning itself appropriately to obtain parts with good forming quality. Peripheral devices mainly include workpiece fixtures and safety anti-collision devices. The control system employs a high-speed I/O bus control system centered on a series of programmable logic controllers (PLCs). It mainly consists of a control box and a teach pendant. The teach pendant is equipped with a programmable terminal (touchscreen), capable of performing all operations, providing various instructions, and inputting parameters. In particular, adjusting fixtures or operating programs can be easily achieved through the programmable touchscreen terminal without replacing hardware. 2. Technical Features (1) High speed, high flexibility, and high technology density realize the integration of design and manufacturing; (2) Free forming manufacturing, not limited by the complexity of parts, realizes flexible production of sprayed parts; (3) No need for traditional tools, fixtures and molds, maintains the stability of spraying parameters, and improves the consistency of part forming; (4) Short production cycle, fast market response, high production efficiency, and strong product competitiveness; (5) Low production cost, high part quality, cost is only 1/3 to 1/5 of traditional processing, outstanding economic benefits, material saving, energy saving, environmental protection, and in line with the concept of green production; (6) It can realize zero inventory of spare parts for weapon equipment and rapid and accurate support on the battlefield, with significant military significance. 3. Working Process Robotic arc spraying flexible rapid prototyping manufacturing technology is different from traditional forced forming (such as forging) and removal forming (such as cutting), and is a forming process based on discrete-stacking. First, using 3D CAD or reverse engineering, information on the geometric shape, structure, and material combination of the parts is obtained to acquire a concept of the target prototype, and a digital descriptive model is established based on this. Then, this information is output to a computer-controlled mechatronics manufacturing system, where the material is "3D stacked" point-by-point and surface-by-surface to form the prototype. After necessary processing, it meets design requirements in terms of appearance, strength, and performance. The general workflow is as follows: 1) Establishing a 3D CAD data model; 2) Discretization processing; 3) Layer information processing; 4) Determining the powder material and using arc spraying for layer processing; 5) Spraying and stacking manufacturing of the prototype or part; 6) Post-processing of the prototype or part. 4. Path Planning Methods Robot trajectory planning aims to move a robot from an initial state to a specified target state within a specified time, at a certain speed and acceleration. The main path planning methods are the Cartesian coordinate space method and the joint space method. The advantages of the joint space method are low computational cost, high efficiency, and limitation only by joint velocity and acceleration, avoiding singularity issues. It is particularly suitable for robot end effectors that leave an object without requiring a specified path and move rapidly within a large empty travel range. The disadvantage is that it's difficult to visualize the actual trajectory of the end effector in space before the robot executes the end effector command, especially when there are obstacles in the robot's workspace, which can easily lead to danger. The advantage of Cartesian coordinate space trajectory planning is that the motion between segment points can be well determined, making it very suitable for defined function trajectories. The disadvantage is that it requires not only spatial interpolation but also conversion to joints, and it's difficult to estimate the motion time, joint velocity, and acceleration limits. 4.1 In the rapid prototyping process of arc spraying, the Cartesian coordinate space method requires not only accurate positioning of the robot at the endpoint but also a certain precision in the motion trajectory, i.e., continuous path control of the arm. To ensure reliable gripping and unloading of the spray gun, a preparatory action is required, thus necessitating the addition of two poses: approach and disengagement. At the same time, in order to avoid unnecessary jitter and pauses, it is often required that the actuator moves continuously when passing through these spatial points. CP motion modes include spatial linear motion, spatial circular arc motion and spatial elliptical motion. (1) Spatial linear motion At this time, the robot only needs to complete a spatial linear trajectory during the motion. Its trajectory control adopts the linear interpolation method in the rectangular coordinate space. As long as the pose information of the starting point and ending point of the trajectory in the rectangular coordinate space is given, the pose information of a series of points on the straight line segment determined by the two points can be calculated. The pose of this series of points can be obtained by kinematic inverse solution to obtain the joint angle required for each joint corresponding point. (2) Spatial circular arc motion In addition to simple straight lines, circular arcs and ellipses, three-dimensional spatial trajectories also include complex curves such as parabolas, hyperbolas and spirals. Suppose a robot starts at pose T06, passes through an intermediate point T16, and arrives at end point T26 in a fixed coordinate system. The starting point position in the X, Y, Z directions is p*0 = (p*0x, p*0y, p*0z)T, with attitude angles α0, β0, γ0. The intermediate point position is p*1 = (p*1x, p*1y, p*1z)T, with attitude angles α1, β1, γ1. The end point position is p*2 = (p*2x, p*2y, p*2z)T, with attitude angles α2, β2, γ2. If we use the spatial circle equation: (x-x0)*2 + (y-y0)*2 + (z-z0)*2 = r*2 to plan the trajectory, writing the parametric equations for the trajectory is extremely difficult, and interpolating the above equations is also very complex. In order to facilitate the calculation of the trajectory equation of the circle, the trajectory equation of the circle must first be transformed from the fixed coordinate system, and then the trajectory planning interpolation is introduced into the new coordinate system, and then transformed back to the reference coordinate system. (3) The elliptical motion of the welding robot is similar to that of the circular motion, except that it is planned by an ellipse. Based on any three points on the trajectory and the ratio of the major and minor axes k=a/b, the trajectory planning of the ellipse can be carried out. When a=b, the trajectory planning of the ellipse is the trajectory planning of the circle. 4.2 Joint space method First, the path points are converted into joint vector angle values ​​by inverse kinematics, and then a smooth function is fitted to each joint. From the initial point, all path points are passed in sequence to reach the target point and the movement time of each joint is the same for each path. The joint trajectory must also satisfy a set of constraints, such as pose, velocity, acceleration and continuity. Under the condition of satisfying the constraints, different types of joint interpolation functions can be selected. Commonly used interpolation functions include: cubic polynomial interpolation, higher-order polynomial interpolation and linear interpolation with parabolic transition. Each joint function is independent of the others and there will be no singularity problem in the mechanism. (1) PTP motion without intermediate points PTP motion without intermediate points refers to the movement of the robot's hand from a certain pose at the starting point to another pose at the end point without the hand passing through any intermediate pose points. Let the robot's initial pose be T*06, and the joint angles of the pose at this time be q*0i (i=1, 2, ..., 6). After time tf, the robot's hand directly reaches the target pose T*16 without passing through any other intermediate points, and the corresponding joint position is q*1i (i=1, 2, ..., 6). In order to make the robot's movement fast and coordinated, the smooth function qi(t) must be found. Cubic polynomial interpolation: qi(t) = a0 + a1t + a2t*2 + a3t*3 The joint movement starts from the static starting point and ends at the static ending point. The four coefficients can be obtained from the constraints of the starting point and the ending point. (2) PTP motion with intermediate points In general, the robot's motion needs to pass through some intermediate points, and it is hoped that the robot can pass through the intermediate points smoothly so that the robot can quickly reach the target position. Suppose the robot starts at pose T*06, passes through intermediate points T*16, T*26, ..., T*m-16, and finally reaches the termination point T*m6. The joint angles at each pose point are q0*i, q1*i, q2*i, ..., qm*i (i=1, 2, ..., 6), and the time interval for each movement segment is t1, t2, tm. If each segment is represented by a cubic polynomial, then: q*ji(t) = a*ji0 + a*ji1t + a*ji2t*2 + a*ji3t*3 (i=1, 2, ..., 6; j=1, 2, ..., m; 0 ≤ j ≤ 0). To facilitate calculation starting from zero for each time interval, there are a total of 4m equations, each with 4 coefficients, for a total of 4m coefficients. The coefficients can be obtained from the velocity, acceleration, continuity at the intermediate points, and the constraints at the starting and ending points. 5 Conclusion Compared to other manufacturing technologies, robotic arc spraying flexible rapid prototyping technology has unique advantages in manufacturing small batches of thin-walled parts. It can significantly reduce costs and shorten production time, enabling zero-inventory of spare parts for weaponry and equipment, rapid and precise battlefield deployment, and greatly enhancing enterprise competitiveness. Therefore, this technology has significant research, development, and application potential in both military and civilian fields, with a very broad development prospect. In particular, the rise of information and intelligent technologies in recent years has provided new opportunities for its development. However, research on this technology is still in its early stages. Further in-depth and systematic research is needed on how to improve the control precision and production efficiency of rapid prototyping products, reduce the cohesive strength of parts, and how to actively apply its latest research results to the military and national economy. It is believed that with the joint efforts of relevant scientific and technological personnel, this technology will gradually develop and improve, and play a role in national defense and the national economy.