## 3 Methods
### AR Configuration Software  
#### 3.2.1 Three-Dimensional Animation Configuration Module Design  
The 3D animation configuration module is mainly used to plan the disassembly animation of the assembly and guide workers to complete disassembly operations. This module includes two parts: the assembly structure design unit and the assembly disassembly animation planning unit. As for configuring 3D animation, the 3D animation model needs to be lightweight to improve the rendering speed. The main goal of lightweight processing is to make the assembly CAD model lightweight and output a lightweight assembly mesh model. Generally, CAD models have the characteristics of a large amount of information, massive redundant data, and complex structural relationships, which are not conducive to achieving rapid AR display on mobile devices. Therefore, lightweight processing of CAD models is necessary. In this study, the main steps of making the assembly model lightweight are as follows:  
1. Model format conversion: The CAD models of mechanical assemblies are mostly established by 3D modeling software, and most of the parts are solid models. In this study, the CAD models are converted into triangular mesh files and stored in the STL format;
2. Mesh model simplification: under the premise of not affecting the appearance characteristics and display accuracy of the assembly model, the mesh model in the STL format is simplified, redundant patches are eliminated, and the accuracy of relatively smooth areas is reduced, thereby reducing the number of patches in the model and improving model rendering speed. The assembly structure design unit is to add Degree of Freedom (DOF) nodes to the assembly model for motion planning of each part in the assembly model. This study converts the mesh model in the STL format into an OpenFlight format, adds DOF nodes to each part in the assembly in this file format, and establishes a local logical coordinate system to define the motion of the DOF node and its sub-nodes in this coordinate system. The assembly disassembly animation planning unit is used to interactively plan the disassembly steps and the disassembly motion route of each part of the assembly in the virtual scene. Figure 2 illustrates the assembly disassembly animation planning interface designed in this study. The assembly disassembly animation planning unit loads the assembly file in the OpenFlight format, extracts, and displays the DOF nodes in the assembly file. Each node is used as the logical coordinate system of the local motion of the model. The planning of disassembly steps can be completed by setting the motion type, motion direction, and motion speed of different nodes and their sub-nodes. Each step consists of multiple operations, and each operation involves motion planning for multiple parts. Figure 3 shows the flow chart of the assembly disassembly animation planning unit. The planning process of specific disassembly steps can be divided into the following steps. 1. The assembly model file in OpenFlight format is imported into the virtual scene for display. All DOF nodes in the assembly model are extracted and displayed;
2. Planning assembly animation or planning disassembly animation is selected according to actual needs. When the former is selected, the display state of all nodes is \"hidden\" by default; when the latter is selected, the display state of all nodes is \"displayed\" by default;
3. According to the disassembly steps, the corresponding DOF node is selected. This node is used as the local coordinate system. Meanwhile, the motion types, motion parameters (speed, range, etc. ), and description text of the node and its sub-nodes are set. The motion types include translation, rotation, and scaling. The display state properties of nodes include highlighted display, outline display, hidden, semi-transparent display, and normal display. After a node is set up, its planning information is saved and displayed in the disassembly step planning list on the right side of the software;
4.