# A Digital Twin System for Adaptive Aligning of Large Cylindrical Components
## 1 Introduction  
Typical aerospace products, such as rockets, satellites, and missiles are usually composed of several large, elongated cylindrical components (hereinafter referred to as large cylindrical components). The large cylindrical component has the characteristics of a large size and many types, and the production mode of \"part assembly--final assembly\" is often adopted for it. The part assembly stage is mainly to complete the assembly of large component parts, and the final assembly stage is mainly to complete the aligning assembly between the large cylindrical components. The aligning of these large cylindrical components is a critical procedure in the final assembly, testing, and experimentation stages of aerospace products, with stringent technical and safety requirements, which belongs to high-precision assembly, and is one of the core processes of aerospace manufacturing and also belongs to the small-batch multi-batch production process, and reflects the overall level of spacecraft manufacturing technology [1; 2; 3; 4; 5]. During the aligning process of large cylindrical components, it is necessary to align the axes of the aligning cylindrical component and the target cylindrical component using a posture adjustment platform and then apply axial force to achieve the aligning assembly of the two large cylindrical components. These components are positioned using pins and connected with bolts, requiring high positioning accuracy. Currently, most large cylindrical parts in aerospace products still use traditional aligning methods, where the target cylindrical component is fixed on a specific aligning platform, and the component to be assembled is manually moved to the target posture using rails. This method is realized based on analog quantity transfer; it can effectively reduce the internal aligning stress, but the assembly process requires the repeated adjustment of large cylindrical components, which results in a lengthy process that is labor-intensive with a low automation level, heavily relying on the operator's experience and unable to ensure consistent aligning precision, leading to inefficiency [6; 7]. Thus, traditional manual methods cannot meet the needs of the high-precision, high-efficiency, and highly reliable production of spacecraft, as well as the demand for mass production. With the development of automation and digitalization technologies, automated aligning techniques have been introduced to complete the aligning process of large cylindrical components. Note that Figure 1 shows a typical automated aligning platform for large cylindrical components, which adopts a linear-guide-type CNC aligning platform as the aligning actuator, and its CNC docking platform has certain adjustment ability in six degrees of freedom, which can achieve the pose adjustment of large components in six degrees of freedom, as shown in Figure 1[8]. In addition, advanced measurement technologies, such as laser tracking, 3D scanning, and visual measurement, combined with automated posture adjustment systems, enable the precise positioning and aligning of large cylindrical components [9; 10; 11]. However, during the automated aligning process, installation and motion transmission errors can reduce the aligning accuracy. Furthermore, there is still a lack of effective monitoring methods, preventing operators from perceiving real-time changes in the posture and assembly forces of the large cylindrical components, leading to an inability to effectively control the aligning quality, which may ultimately affect the overall performance and quality of aerospace products. Digital twin (DT) technology [12; 13; 14] provides an effective solution to these problems. DT constructs a connection between physical entities and virtual models in a digital manner [15], simulating the characteristics, behaviors, processes, and performance of physical entities in the real environment through data interaction between the data model  
Figure 1: A typical automated aligning platform for large cylindrical components [8].