## 3 Topological Diversity and Structural Design in Magnetic Gear Systems
### Modulated Magnetic Gear  
With the development of high-energy permanent magnets (PMs) and the increasing use of advanced magnetic circuit designs, modulated magnetic gears (MGs) have emerged as a powerful alternative, particularly in linear and rotary systems requiring high torque density and efficiency [4, 30]. The key feature of the modulated design is the incorporation of ferromagnetic modulators placed between the input and output rotors. These modulators reshape the magnetic flux lines, guiding the magnetic path and thereby enhancing the system's torque transmission capability. Furthermore, the involvement of all magnets in active torque production significantly improves magnetic utilization efficiency. This design approach offers ideal solutions for applications demanding high torque density and wide gear ratio ranges.  
Figure 3 presents structural representations of eight fundamental modulated magnetic gear topologies commonly discussed in the literature. These topologies exhibit different performance characteristics depending on the modulator arrangement, magnet configuration, and system geometry.  
Figure 3a presents the coaxial magnetic gear, composed of three main components: an inner rotor, an outer rotor, and a stationary modulator ring. The permanent magnet arrangements on the inner and outer rotors are synchronized through the modulator to enable torque transmission. This configuration has been further developed in various forms, such as surface-mounted variants and dual-stack coaxial arrangements to enhance torque density and mechanical stability. The spoke-type magnetic gear (Figure 3b) employs internal permanent magnet (IPM) structures to reduce eddy current losses within the magnets. The rotor's integration with cage-like steel poles improves torque density while offering an alternative to the manufacturing complexities of surface-mounted configurations [31, 32]. Figure 3c shows the IPM-type magnetic gear, which includes two counter-rotating rotors and fixed stator pole pieces. Based on the internal permanent magnet concept, this structure eliminates the need for magnet holders in high-speed applications, offering structural advantages [33]. The reluctance-type magnetic gear (Figure 3d) features permanent magnets only on the low-speed rotor, with the high-speed rotor comprised solely of salient iron structures. Operating on the Vernier principle, this design enables torque transmission, even without permanent magnets, providing a unique solution [34, 35]. The L-shaped axial-flux magnetic gear (Figure 3e) uses L-shaped ferromagnetic pole pieces to reduce flux leakage and improve torque density. However, due to its inherently complex three-dimensional structure, it presents significant manufacturing challenges [21, 36]. The T-shaped axial magnetic gear (Figure 3f) maximizes torque production by employing both axial and transverse magnetic paths. T-shaped soft magnetic components reduce saturation and minimize flux leakage, thereby enhancing system efficiency [37]. Figure 3g illustrates the hybrid-flux magnetic gear, which features a triple permanent magnet excitation (TPME) topology combining both transverse and axial flux paths. Iron segments enable both magnetic flux modulation and additional torque generation, resulting in higher torque capacity compared to traditional designs [38]. Finally, the HTS linear magnetic gear (Figure 3h) utilizes high-temperature superconducting (HTS) bulks for flux modulation. The Meissner  
effect significantly reduces flux leakage while boosting linear torque output. This topology shows great promise for precision applications requiring linear motion [39].  
These advanced modulated magnetic gear topologies, with their innovative structural features and high-performance potential, are poised to play a key role in future applications, particularly in electric drive systems, robotic mechanisms, and compact mechanical transmission solutions.  
Figure 3: Modulated MG topologies.