# Development of Piezoelectric Inertial Rotary Motor for Free-Space Optical Communication Systems
## 1 Introduction  
The advent of small satellites has revolutionized the sector of space exploration and communications, marking a significant shift toward more cost-effective, accessible, and versatile space missions [1, 2, 3]. These satellites are increasingly popular due to their low cost [4], shorter development cycles [5], and the ability to form constellations that can deliver comprehensive data coverage [6]. Their applications span from Earth observation and scientific research to national security and telecommunications, reflecting their growing importance in the modern space industry [7, 8, 9]. Despite the wide application range of small satellites, their most notable impact is to communication and data transfer areas. Therefore, in conjunction with the rise of small satellites, free-space optical (FSO) communications have emerged as a critical technology that offers high-speed data transmission without reliance on physical media [10, 11]. FSO communications present numerous advantages over traditional radio frequency (RF) communications, including greater bandwidth, enhanced security, and reduced risk of interference [12, 13]. However, technology also faces significant challenges, notably the need for precise pointing to maintain the alignment of communication links over vast distances [14]. This requirement underscores the importance of robust and accurate actuation systems. Currently, the majority of small satellite missions with FSO or other loads rely on brushless direct current or stepper motors, fororientation and positioning tasks. Although commonly used, these motors often fail in terms of precision, responsiveness, and power efficiency, particularly in the vacuum of space [15; 16; 17]. Furthermore, electromagnetic motion sources have relatively high mass and demand on gears or reduction systems and do not exhibit self-locking ability [18]. Furthermore, their susceptibility to magnetic interference can degrade performance in critical applications and create an additional motion error [19; 20; 21]. Therefore, it can be found that the usage of electromagnetic motion sources can be one of the limiting aspects for the further development of small satellites and their systems. These aspects create a demand to replace electromagnetic motion sources. One of the most promising motion sources that could replace electromagnetic ones are piezoelectric actuators and motors [22]. Piezoelectric actuators and motors offer numerous advantages, including high precision, electromagnetic cleanliness, low power consumption, and self-locking ability [23; 24; 25]. Inherent scalability and low power demand, low mass, the possibility of direct integration to systems, free operation of gear or reduction systems, as well as direct load driving make them particularly suitable for small satellites [26]. Furthermore, piezoelectric actuators and motors can be designed to drive loads with several degrees of freedom and provide linear and angular motions without changes in their structural design [27]. Therefore, recent developments in piezoelectric motion sources have led to the innovation of rotary piezoelectric motors, which provide precise rotational movements essential for accurate alignment required in FSO communications or at other tasks. [PERSON] et al. explored the potential of a novel ultrasonic traveling wave rotary motor, which includes a piezoelectric backup function designed to ensure operational continuity under various conditions. This motor uses the elastic properties of materials to generate rotary motion through traveling wave vibrations. The backup piezoelectric mechanism provides redundancy, crucial for applications where failure is not an option, such as in satellite orientation systems. Their study highlighted the characteristics of motor motion, including high precision in angular displacement and control of rotational speed. The traveling wave vibrations enabled for continuous and smooth motion, minimizing energy losses and maintaining stability.