## 3 Novel Rotating Motion Mechanism
### Joint Transmission Mechanism
#### 3.3.7 Noncircular Gear Five-Bar Mechanism  
[PERSON] et al. [126] proposed an exoskeleton knee joint robot based on differential noncircular gears and a five-bar mechanism. By constraining the degrees of freedom of the mechanism through the arbitrary transmission ratio characteristics of noncircular gears, the complex structure of traditional multi-bar mechanisms is simplified, as shown in the three-dimensional model in Figure 65 [126]. The noncircular gear system consists of a sun gear and two planetary gears, which drive a five-bar mechanism through the variable transmission ratio characteristics of the noncircular gear to achieve complex motion trajectories. The noncircular gear drives the planetary gear to rotate, driving the linkage mechanism to move, and the end effector reproduces the flexion and extension trajectory of the knee joint. By changing the transmission ratio of noncircular gears, the degrees of freedom of the five-bar mechanism are constrained to ensure that the trajectory matches the natural motion of the human body. This mechanism has a compact structure and better transmission stability than traditional linkage mechanisms. Figure 64: The snapshots of the reference group (**a**) and the assistance group (**b**). The research status of joint rotation transmission mechanisms presents a trend of deep integration of biomimetic design, intelligent driving, and structural innovation. The current research focuses on improving joints' flexibility, dynamic response, and environmental adaptability, such as the series-parallel hybrid finger mechanism proposed by [PERSON] et al., [120] which achieves multi-degree of freedom rotation and redundant translation through TCP drive. Combining rigid and flexible coupling design balances response speed and grasping stability but is limited by material properties and structural complexity. The hydraulic spherical motion mechanism developed by [PERSON] et al. [121] adopts a compact hydraulic drive to achieve continuous dual-degree-of-freedom spherical motion, significantly reducing the inertia of traditional multi-degree-of-freedom mechanisms, but \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Types** & **Advantages** & **Disadvantages** & **Applications** \\ \hline Series-parallel hybrid & High response speed, balanced stiffness and flexibility, and adaptability to various grasping modes & Complex structure, limited material properties & Robot hands that require quick response and flexible grasping \\ Hydraulic spherical & Compact structure, high stiffness, 2-degree-of-freedom spherical motion & High energy consumption & In robot joints and spherical motion scenes \\ Stewart Platform Hook & They adapt to complex movements, have a simple structure, and are easy to integrate & Limited carrying capacity, long term wear and tear & High precision positioning platforms \\ Three degrees of freedom & Combining rigidity and flexibility with strong impact & Control complexity: Cable tension balance requires precise adjustment & Lightweight robotic arms \\ Offset slider crank & Multi-joint single-drive & Low load capacity, dependent on linear & Functional pseudo-bionic robot fingers \\ connecting rod & synchronous drive, lightweight structure & actuators, limited travel & Knee exoskeleton, human-machine motion \\ New RCM mechanism [125] & Lightweight, shock-absorbing, and highly coordinated in motion & Restricted range of motion & Axis alignment scene \\ Noncircular gear five-bar & Simplify the structure; have a high degree of freedom & Complex processing and gear meshing accuracy & Biomimetic joint with variable transmission ratio \\ Mechanism [126] & constraint and strong motion & affect transmission efficiency & characteristics \\ \hline \hline \end{tabular}
\end{table} 
Table 13: Comparison of joint transmission mechanisms. Figure 65: Three-dimensional model of a noncircular gear five-bar mechanism. (1. Sun gear; 2. Planetary Wheel I; 3. Rod 1; 4. Rod 2; 5. Actuator; 6. Rod 3; 7. Planetary Wheel II). high energy consumption remains a bottleneck. [PERSON] et al.