## 4 Common Driving Methods for Rotating Motion Mechanisms
### Electric Drive
#### 4.1.2 Piezoelectric Drive  
The basic principle of piezoelectric driving is to utilise the inverse piezoelectric effect of piezoelectric ceramic materials, which means that under the action of an electric field, the piezoelectric material will deform, resulting in mechanical displacement [171]. This effect enables piezoelectric materials to convert between electrical and mechanical energy, generating rotation by controlling their mechanical deformation. Resonant piezoelectric actuators operate in a resonant state and are typically designed in the ultrasonic frequency range; hence, they are also known as ultrasonic motors. Piezoelectric ultrasonic motors (USMs) have received more attention due to their unique qualities compared to traditional magnetic coil-based motors, such as miniaturisation, high precision, speed, non-magnetic, silent operation, simple structure, and adaptability [172]. [PERSON] et al. [173] proposed a novel rotating ultrasonic motor that only uses longitudinal vibration mode, as shown in Figure 88. The motor consists of two symmetrically arranged longitudinal sensors, a rotor, a base, and an adjustable mounting base. The motor utilises the longitudinal vibration mode of piezoelectric ceramics to generate a frictional force through the longitudinal displacement of the foot driven by sensors, driving the rotor to rotate. Only a single-frequency excitation voltage is required, and directional control can be achieved by adjusting the voltage phase. The relative position between the sensor  
Figure 87: Component description of prototype motor: (**a**) stator, (**b**) outer rotor with magnet, and (**c**) completed hollow cup motor prototype. and the rotor can be adjusted, providing an additional means of changing the output characteristics. The dual-sensor synchronous drive enhances the stability of torque output. However, it requires a high-voltage drive, which may limit low power applications, and relies on friction transmission. Long term operation may cause contact surface wear and require regular maintenance. [PERSON] et al. [174] proposed a novel non-contact ultrasonic motor that can generate levitation force and acoustic drive torque, as shown in Figure 88. The motor includes a stator and a rotor. The stator structure is fixed on the mounting plate, and the entire structure is fixed on the support frame. The composition of the stator is shown in Figure 89b [174]. Figure 89c [174] shows the stator's main dimensions. The high-frequency longitudinal vibration of the motor stator compresses the air film, forming a high-pressure air film, generating axial and radial suspension forces, overcoming the gravity of the rotor, and maintaining stable suspension. The gradual deepening of the design of artificial grooves leads to uneven distribution of gas film pressure in the circumferential direction, forming a pressure gradient, driving the airflow, and generating circumferential shear stress, thereby generating driving torque. The air film completely isolates the stator and rotor, avoiding friction and wear and significantly reducing heat. [PERSON] et al. [175] proposed a resonant piezoelectric rotary motor using a parallel moving gear mechanism, as shown in Figure 90. The piezoelectric transducer generates linear displacement under a sinusoidal voltage and synthesises the planar circular translational motion of the stator through a hinge connection. The stator (internal gear) and  
Figure 88: Proposed structure of a novel rotary ultrasonic motor using only longitudinal vibration mode. Figure 89: Structure and main dimensions of non-contact ultrasonic motor. rotor (external gear) undergo periodic misalignment meshing due to the difference in tooth count, driving the rotor to decelerate and rotate. By adjusting the phase difference between the excitation voltages of the two transducers, the direction of rotor rotation can be changed. Replacing traditional friction transmission with gear meshing reduces wear and energy loss and improves efficiency.