## 3 Topological Diversity and Structural Design in Magnetic Gear Systems
### Conventional Non-Modulated Magnetic Gear  
Non-modulated magnetic gears (MGs) represent one of the most fundamental structural types in magnetic torque transmission. In these systems, the input and output rotors interact magnetically through directly opposing arrays of permanent magnets; there is no additional ferromagnetic modulator to direct or modulate the magnetic field. This simplified structure not only reduces design complexity but also lowers manufacturing costs. Torque transmission occurs through the alignment of magnetic poles on the rotors and the direct interaction of the magnetic fields. The number of poles on the input and output rotors is typically equal or configured according to a specific ratio. However, this limits the flexibility of the gear ratio. Additionally, due to the restricted magnetic flux paths, the torque density is generally lower compared to modulated structures. The torque capacity of these systems is directly related to rotor dimensions and the magnetic properties of the permanent magnets used, such as remanent flux density (Br), coercivity (Hc), maximum energy product (BHmax), and temperature resistance. Therefore, while they are not preferred in applications requiring high torque, they are commonly used in low-power and low-speed applications due to their simplicity and cost advantages. Non-modulatedmagnetic gears are further divided into different sub-topologies based on magnet orientation, placement geometry, and torque transmission mechanism. Figure 2 structurally illustrates eight fundamental topologies frequently encountered in the literature. Figure 2a illustrates the spur-type magnetic gear, which features the simplest structure among conventional magnetic gears. It is configured similarly to mechanical gears, but it transmits power through the face-to-face arrangement of permanent magnets rather than physical contact. This design minimizes drawbacks, such as wear, friction, and the need for lubrication [18]. In Figure 2b, the worm-type magnetic gear is shown, consisting of a helically magnetized rotor that magnetically interacts with a mating gear. Its ability to achieve high gear ratios without contact makes it attractive for robotics and precision control applications [21]. Figure 2c presents the perpendicular magnetic gear, which generates radial magnetic field interaction through magnets aligned orthogonally to one another. Its compact size and 1:1 rotation ratio enable high efficiency in confined spaces [22]. Figure 2d shows the magnetic screw gear, characterized by a helical magnet arrangement that converts rotary motion into linear motion or vice versa without physical contact. Its quiet operation and lack of lubrication requirements make it ideal for use in sensitive or sterile environments [23]. The planetary-type magnetic gear, shown in Figure 2e, is designed as a non-contact counterpart of the classic planetary system. It consists of sun, planet, and ring rotors, which interact synchronously through permanent magnets. Its  
Figure 1: Classification of magnetic gear. key advantages include high torque density and multi-stage torque transmission [24]. The cycloidal-type magnetic gear in Figure 2f operates based on the non-contact magnetic interaction between a cycloidal disc with curved magnet poles and fixed pin structures. This configuration is particularly suitable for defense and aerospace applications requiring high gear ratios and torque density [25; 26]. The harmonic magnetic gear, shown in Figure 2g, includes three main components: a wave generator, a flexspline, and a circular spline. Torque transmission is achieved through the periodic variation of the air gap enabled by elliptical magnet arrangements. This topology is notable in the literature for its ability to provide high speed ratios and torque density [27; 28]. Finally, Figure 2h illustrates the trans-rotary magnetic gear, which transforms rotational motion into linear motion through helical magnet arrays on the rotor and linear components.