## 2 Construction of Digital Twin Model of Planetary Gearbox  
Traditional modeling is divided into numerical modeling and physical modeling. Numerical modeling is difficult to analyze theoretically and renders deriving mathematical formulas complex, while physical modeling is relatively simple but often does not contain all entities of the simulation object. For example, in the traditional physical modeling method for planetary gearboxes, the modeling of the drive motor is omitted, resulting in the physical model not accurately reflecting the real dynamics of the simulation object. Based on this, this paper uses the digital twin method to build the physical model of the planetary gearbox system and then models the drive motor based on the planetary gearbox model, so that the planetary gearbox system model is more complete and the simulation results are closer to reality. The digital twin model of a planetary gearbox is a digital representation of a physical planetary gearbox, a carrier of scenario-aware data for a planetary gearbox, and an important basis for predictive maintenance algorithms. However, the planetary gearbox is located in a complex environment, and the construction of its model requires a comprehensive consideration of the multi-domain characteristics of the planetary gear. Therefore, based on the principles of object-oriented, incremental, and multi-domain unified modeling, the mechanical model of a planetary gearbox and the coupling of multi-domain models were studied to realize the digital representation of the complex characteristics of planetary gearbox, so that the digital twin model could reflect the operation state of planetary gearbox gear. In the planetary gear system, the key transmission components are the sun gear, the planetary gear, the ring gear, etc. In order to make the simulation results more accurate later, in the process of modeling the components of the planetary gear system, and in order to prevent inaccurate simulation results due to modeling errors, the professional CAD modeling software SOLIDWORKS was used to establish a high-precision geometric model of the components containing teeth in the gear model. The 3D model of other components was also established. Through the data interface between the software and other simulation software, the 3D assembly or part model created in SOLIDWORKS could then be directly imported into the simulation software for the subsequent model simulation and validation work. The parameters of the constructed gears are given in Table 1 below. After the modeling of the components of the planetary gearbox was completed by SOLIDWORKS, the overall assembly work was performed. The complete mating diagram is shown in Figure 1 below, and the failure modes of each tooth-containing part are given in Figure 2.  
In this paper, ADAMS and MATLAB were selected as the joint simulation platform for the digital twin. In the ADAMS multi-body dynamics modeling environment, the part model in X_T format or the part assembly constraints in ADAMS format was read, the properties of the part entities were modified, the new part coordinate systems were added to connect other entities and constraints, and the contact forces and torques in the model were defined. The modification and addition of kinematic pairs according to the actual application purpose could then be enacted to realistically reflect the actual connection relationship between the planetary gearbox components. The physical properties of the kinematic subsets, such as stiffness and damping, and the friction coefficient were also defined. The physical entities were structural steel and the contact forces were simulated using the IMPACT function. Considering that the pure rigid body dynamics simulation can only simulate the dynamics map of the model's center of mass, and cannot extract