I. Overview With the continuous development of modern manufacturing technology, various new ideas and technologies are constantly emerging, such as Computer Integrated Manufacturing (CIM), Concurrent Engineering (CE), Agile Manufacturing (AM), Design for Manufacture (DFM), and Design for Assembly (DFA). These new manufacturing and design concepts place more comprehensive demands on modern CAD technology at a higher level. Integration, intelligence, and visualization have become inevitable trends in the development of modern CAD technology. How to address this inevitable trend in the research and development of CAD systems in the electrical field has become an urgent and arduous task. Solid State Relays (SSRs) are contactless electronic switches that utilize discrete electronic components, integrated circuits, and hybrid microelectronics technology to achieve electrical isolation and signal coupling between the control circuit and the load circuit. The load switching function is achieved by solid-state devices, with no internal mechanical moving parts. They are a relatively new type of switch developed in the last decade or so. Due to its wide application, diverse types, and excellent performance, CAD systems are receiving increasing attention, and the development of virtual assembly systems for solid-state relays reflects this trend. II. Assembly Model Research Research on assembly models (AM) began in the late 1970s and has a history of over twenty years. The earliest attempt was AUTOPASS, a geometric modeling system developed by Liberman and Wesley. Currently, most research on assembly modeling focuses on meeting the needs of specific application scenarios, such as assembly process planning. Although it is still in its early stages in China, related research is becoming increasingly active. An assembly model is a product information model that can completely and accurately convey the design parameters, assembly levels, and assembly information of different assemblies. The purpose of establishing an assembly model is to create a complete representation of product assembly information. This information includes: management information, such as the names, materials, technical specifications, technical requirements, designers, and design versions of each component; geometric information, including the geometry, dimensions, final position, and orientation of the assembly; topological information, including the hierarchical structure of the product assembly and the geometric constraints between assemblies; engineering semantic information, i.e., information related to the product's engineering applications; assembly process information, referring to information related to the product's assembly and disassembly processes and their specific operations; and assembly resource information, referring to the sum of assembly resources related to the specific implementation of the product assembly process, mainly the composition and control parameters of the assembly system equipment. In short, the assembly model must not only process the input information of the design system but also the intermediate and final information of the design process. Therefore, the assembly model information should be continuously enriched and improved as the design process progresses. An assembly model should possess the following characteristics: It should be able to completely express product assembly information, describing not only the information of the components themselves, but also the assembly relationships and topology between them; it should support parallel design, not only completely expressing product information, but also describing the inheritance relationship of product design parameters and their variation constraint mechanisms to ensure the consistency of design parameters, thus supporting parallel product design; it should meet rapidly changing market demands, meaning that when product requirements change, the assembly model can easily modify the product design to adapt to new product requirements; and it should have a certain degree of independence. The core issue of an assembly model is how to express and store the interrelationships between the components of an assembly in a computer. Currently, data structures representing assembly information can be summarized into two categories: directly storing the mutual positional information between assembly components; and storing assembly information such as the fit and connection between assembly components, with the homogeneous transformation matrix determining the mutual positions of the assembly components calculated based on this information. In AUTOPASS, developed by Liberman and Wesley, parts and assemblies are represented as nodes in a graph structure. Branches in the graph represent assembly relationships between parts, and each branch stores a spatial transformation matrix to determine the relative positions of parts and other non-geometric information. De Fazio and Whitney proposed a method called Priority Relationship Graph, which defines a set of priority rules to obtain an assembly sequence by sorting the graph. Homem de Mello and Sanderson proposed an AND-OR graph to describe assemblies, where each leaf node represents the lowest-level component of the assembly, and the root node represents the final product, somewhat similar to a CSG structure. Lee and Gossard proposed a truly hierarchical modeling method based on the AND-OR graph, decomposing assemblies layer by layer into a tree structure composed of components. Components can be either parts or subassemblies. The top of the tree is the finished assembly, the bottom is the indivisible part, and the remaining parts are subassemblies determined by the conceptual design. Lee introduced the concept of virtual connections; the entire assembly tree is connected by virtual connections, each virtual connection being a collection of related information, thus enabling hierarchical storage of assembly information. Each of the aforementioned data structures or storage models has its advantages and disadvantages, and can be selected according to specific development requirements. The main development trend of this problem is the shift from graph-based topological structures to tree-based hierarchical structures. III. Virtual Reality Technology As an emerging interdisciplinary field, Virtual Reality (VR) technology is one of the most widely discussed topics in the modern computer science community. Virtual reality utilizes computer systems to generate a simulated environment (such as an aircraft cockpit or assembly line), using various sensing devices to create an immersive experience for the user and enabling direct and natural interaction between the user and the environment. It is essentially an advanced human-computer interface, providing users with intuitive and natural real-time perceptual interaction methods to maximize user convenience and present a simulated or emulated effect of a specific theme. A virtual reality system typically requires high-performance computers, visual display devices, head-eye-hand positioning tracking devices, three-dimensional spatial localization devices, and tactile and force feedback devices, among other hardware and software support, to generate a sense of immersion and direct, natural real-time perceptual interaction. These devices often require substantial financial investment, which to some extent limits the application areas of virtual reality technology. However, different applications in different fields have varying requirements for hardware and software support. If the immersion requirement is appropriately reduced, and the emphasis is shifted from naturalness to real-time interactive operation, then interactive methods and display effects, including conventional input/output devices, can be considered. This is both the development philosophy of Desktop PC-based VR Systems (DPCVRS) and the initial intention behind our design of a solid-state relay virtual assembly system. Graphical support platforms, represented by AutoCAD, provide powerful 3D modeling capabilities, basically meeting the display and simulation requirements of virtual environments. However, the assembly design of most systems is not perfect, with problems such as insufficient constraints and difficulties in assembly management. Nevertheless, based on a thorough study of the assembly model, and according to the different assembly constraints in various fields, determining the relevant assembly tree, and utilizing the high openness of the graphical platform and its provided 3D modeling capabilities and powerful secondary development tools, creating a realistic professional assembly simulation environment to achieve functions such as translation, rotation, hiding, perspective, rendering, explosion, interference checking, material property calculation, parts list, defining assembly datum, and assembly process management is feasible. IV. Solid State Relay Assembly Relationship Model An assembly is an organic combination of multiple parts and sub-assemblies. To obtain a correct assembly, the correct and complete relationships between its components must be established. The assembly structure tree shows the membership between components, reflecting their hierarchical relationships. When two components have one or more connections, we say they have a constraint relationship, and assembly relationships are the basis for establishing constraint relationships. The assembly relationships of general product components are shown in Figure 1. Positioning relationships describe the spatial positional and mating relationships between components, representing a direct interrelationship; connection relationships include rigid connections and elastic connections. Rigid connections include threaded connections, keyed connections, pin connections, couplings, etc., while elastic connections mainly refer to spring connections. Connecting parts are generally standard parts; motion relationships are divided into transmission and relative motion. The former refers to gear transmission, belt transmission, etc., while the latter includes rotary motion, planar motion, cylindrical motion, etc. Due to their inherent characteristics, the vast majority of assembly relationships related to solid state relays are positioning and connection relationships, with very few motion relationships. Product design is the foundation of assembly. Design for Assembly (DFA) is an important component of concurrent engineering. Currently, two commonly used design processes are bottom-up and top-down. Bottom-up design involves designing parts of various shapes, inputting geometric constraints between them, and then assembling the designed parts into a product. Top-down design considers the constraints and positioning relationships between parts from the early stages of part design, and then implements the detailed design of individual parts after completing the overall product design. Each design has its advantages. Top-down design reflects the actual design process, saves unnecessary repetitive design, and improves design efficiency, while bottom-up design is simpler, faster, and more convenient, and is understood and accepted by most designers. The solid-state relay virtual assembly system adopts a bottom-up design approach. V. System Design ObjectARX is a new generation of secondary development tools launched by Autodesk. It provides a C++-based object-oriented development environment and application programming interface, enabling users to customize and extend the functionality of AutoCAD. ObjectARX provides a rich library of classes and functions, allowing users to quickly and directly access AutoCAD's graphics database and geometric modeling core to obtain the graphic elements that make up components, such as insertion points, areas, volumes, and centers of gravity. Based on this, the constraints of each component are determined according to the hierarchical structure revealed by the assembly tree. Different member functions are called according to different situations to achieve the positioning and combination of graphic elements, completing the simulated assembly of components. Collision detection is a crucial aspect of virtual assembly systems, specifically requiring the detection of collisions and the calculation of their locations. Real-time performance and accuracy are two important constraints for collision detection. Real-time performance is a specific requirement for real-time interaction in the system, while accuracy requires precise detection of collisions and calculation of their locations. Many collision detection methods exist, each with its own advantages, disadvantages, and applicable scope. For the components involved in the virtual assembly system, which are rigid objects represented by solid models, we designed a collision detection algorithm based on intersection operations, which satisfies the constraints well. Based on the above research, we adopted AutoCAD as the graphics support platform and ObjectARX as the development tool, designing a solid-state relay virtual assembly system based on a bottom-up design philosophy. VI. Conclusion The development and research of the solid-state relay virtual assembly system involves solid modeling technology, assembly technology, and virtual reality technology. Integrating these emerging related technologies into a high-performance assembly simulation system is a considerable challenge. Our work is only a preliminary attempt, and due to various constraints, it cannot yet provide sensory effects beyond visual ones. However, meeting the higher-level requirements of modern manufacturing technology for electrical CAD systems is an unavoidable responsibility and challenge. Based on the experience and lessons learned from the practical application of this system, we will continuously improve its design level and make unremitting efforts for the continuous development of electrical CAD technology and the advancement of industry technology.