## 4 Common Driving Methods for Rotating Motion Mechanisms
### Electric Drive
#### 4.1.2 Piezoelectric Drive
Utilising the resonance characteristics of vibrators to enhance energy transfer efficiency and support high power output. The output speed and torque can be flexibly regulated by changing the transmission ratio (TR), excitation frequency, and voltage amplitude. The motor integrates gear transmission and piezoelectric drive, which can achieve miniaturisation and high power density. [PERSON] et al. [176] proposed a rotary piezoelectric actuator (RPA) based on a spatial spiral compliance mechanism (SSCM). The proposed RPA structure [176] is shown in Figure 91. A single PZT (lead zirconate titanate) stack generates linear displacement through the inverse piezoelectric effect. SSCM converts the linear displacement of the P stack into pure rotational motion. SSCM consists of spiral rods and parallel platforms. The symmetrical layout of the spiral rod counteracts lateral forces and avoids axis deviation. By constraining the design of thrust bearings and bolt connections, the longitudinal degrees of freedom are limited to ensure that the output is a single rotational motion. This actuator can achieve high precision and fast response. Table 20 summarizes and compares the driving performance of the different piezoelectric motors mentioned above. Figure 91: Proposed Structure of RPA. Figure 90: Three-dimensional model (**a**) Proposed assembly diagram of the new piezoelectric motor, (**b**) Structure of the vibrator, (**c**) Decomposition diagram of the rotor component. The current research on motor drive in precision rotary motion mechanisms presents a pattern of diversified technological routes and parallel development, focusing on three dimensions: efficiency improvement, structural optimisation, and precision control. Applying high temperature superconducting (HTS) materials has significantly improved system energy efficiency and power density in superconducting motors. Synchronous HTS motors reduce thermal management complexity through a non-rotating cooling coupling design. However, the physical limits of critical current characteristics and eddy current losses of superconducting materials still limit their magnetic field modulation stability and cooling system integration under dynamic conditions. The highly efficient topology structure represented by the fully superconducting motor is theoretically close to energy-lossless transmission. Still, the dynamic mismatch problem caused by the magnetic flux pinning effect between the dual superconducting circuits leads to torque fluctuations and transient response hysteresis in practical applications. The spherical motor breaks through the traditional single-axis rotation limitation through a non-integrated rotor permanent magnet topology and multi-degree of freedom decoupling control. Its modular electromagnetic field design and ABS lightweight rotor effectively reduce motion inertia. However, the suppression of magnetic field distortion and three-dimensional attitude accuracy feedback under multi-physical field coupling still needs to be further optimised based on magnetic, mechanical, and electrical collaborative simulation. The hollow cup motor eliminates the cogging effect with a coreless structure, and its dual rotor magnetic circuit configuration achieves magnetic flux path refocusing through a hybrid topology of injection moulded magnets and sintered neodymium magnets. However, the uniformity of the air gap magnetic field and the balance of axial magnetic pulling force still rely on precision mechanical tolerance control. The piezoelectric drive technology expands the boundaries of traditional friction transmission through non-contact ultrasonic excitation and a spatial spiral compliant mechanism (SSCM). The gear meshing piezoelectric motor uses the resonance enhancement effect to improve energy conversion efficiency to 68%. However, the nonlinearity of hysteresis and high-frequency vibration noise caused by the relaxation of the preload force of the piezoelectric stack are still key bottlenecks restricting its industrial application.