## 3 Novel Rotating Motion Mechanism
### Rotary Actuator
#### 3.6.5 Rotary Piezoelectric Stick-Slip Actuators  
Referring to Zhong BW [141], the current status of rotary piezoelectric stick-slip actuators was summarised, as shown in Table 17. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Types** & **Advantages** & **Disadvantages** & **Applications** \\ \hline Based on the conjugate & Reduce motor inertia & Depending on the cam & Artificial knee joint, a low \\ cylindrical cam [137] & energy loss, compact & design, high cost, and & power wearable \\ structure & limited adaptability & assistive device \\ \multirow{3}{*}{Bistable rotating & \multirow{3}{*}{\begin{tabular}{c} Love energy consumption, \\ repeatability, high \\ precision, and large load \\ capacity \\ \end{tabular} } & \multirow{3}{*}{\
\begin{tabular}{c} Relying on mechanical \\ locks, the range of action \\ is limited \\ \end{tabular} } & \multirow{3}{*}{
\begin{tabular}{c} Biomimetic robots or soft \\ robots, medical, and \\ surgical instruments \\ \end{tabular} } \\ \cline{3-3}  & & Wide adaptability, & \\ \cline{3-3}  & & lightweight, improved & Complex structure, relying & Human-robot collaborative \\ nonlinear stiffness actuator & safety through & on cam profile & robots, highly dynamic \\ \multirow{3}{*}{[139]} & biomechanical properties, & optimisation, limited & environment \\  & and low cost & real-time performance & walking robots \\ \cline{3-3}  & Flexible configuration of & Multiple motors need to be & Humanoid/quadruped \\ \multirow{3}{*}{Disc spring variable } & equivalent stiffness, high & coordinated for control, & robots, \\  & torque density, fast and & and fatigue of the disc & exoskeleton/rehabilitation \\ \cline{3-3}  & wide range stiffness & spring may affect long & robots, robot arm joint \\ \cline{3-3}  & adjustment & term reliability & \\ \hline \hline \end{tabular}
\end{table} 
Table 16: Comparison of rotary actuators. Figure 80: Structure of SDS-VSA: 1. Positioning motor; 2. Harmonic reducer; 3. Output shaft; 4. Base: 5. Cam disc; 6. Roller retaining ring; 7. Disc-spring combination; 8. Harmonic reducer; 9. Rigid motor. The rotary actuator discussed in this section demonstrates diverse innovative designs for different application requirements. The nonlinear series elastic actuator based on a conjugate cylindrical cam transforms the joint nonlinear load into uniform motor motion by customising the cam profile. Combined with a unidirectional driving strategy, it significantly reduces inertia and Joule heat loss. Its high integration is particularly suitable for wearable devices such as prosthetic knee joints, but the dependence on cam design limits its universality. The bistable rotating mechanism utilises fibre-constrained dielectric elastomer actuators and mechanical locks with symmetrical adversarial layout to achieve high repeatability angle positioning (90\({}^{\circ}\)/150\({}^{\circ}\)) and zero power consumption after switching. The negative stiffness design extends the actuation range (45\({}^{\circ}\)), but mechanical locks and finite angles are its main constraints. The torsion spring-connected nonlinear stiffness actuator adopts a symmetrical torsion spring sleeve cam mechanism, which converts the radial deformation of the torsion spring into axial compression through rollers, achieving biomimetic passive stiffness adjustment. Its lightweight design improves the safety of human-machine cooperation, but the structural complexity and real-time performance need to be optimised. The compact and reconfigurable disc spring variable stiffness actuator innovatively utilises symmetrical disc spring groups and dual cam angle adjustment to achieve flexible configuration of equivalent stiffness and high torque density (30% higher than single-sided schemes), significantly expanding the range and speed of stiffness adjustment. However, multi-motor collaborative control and disc spring fatigue are reliability challenges.