# Design and Experiment of a Passive Vibration Isolator for Small Unmanned Aerial Vehicles
## 1 Introduction  
### Research Background  
Uncrewed aerial vehicles (UAVs) refer to uncrewed flying crafts manufactured in the form of fixed-wing or rotary-wing aircraft. In terms of nomenclature, the international civil aviation organization uses the term UAV, whereas the federal aviation administration refers to them as uncrewed aircraft, commonly known as drones. The field of application of UAVs is gradually expanding owing to the sophistication of related sensor-fusion technologies, the development of ultra-high-efficiency motors, and the improved reliability of navigation systems [1]. In the military market, the increasing size and enhanced performance of uncrewed systems are replacing certain mission scopes of manned aircraft. In the civil market, the small-to-medium-sized drone market is rapidly growing owing to the expansion of commercial business model applications [2]. Drones employed in the precision-detection field have demonstrated innovative application in various domains, such as surveillance, reconnaissance, and target coordination [3]. Recently, small to medium-sized drones with enhanced performance have been analyzed based on the development of onboard hyper-spectral camera technology, mapping techniques, and optimal flight technologies [4]. However, while the high efficiency and output of the motor have enabled the attachmentof heavier payloads relative to the size of the drone, this also makes it more susceptible to disturbances, significantly affecting the precision of the optical equipment [5; 6]. Also, the use of vibration isolation devices to maintain the precision of optical equipment is considered critical for improving the quality of optical devices [7; 8]. Therefore, to mitigate the effects of vibrations on optical equipment, the installation of vibration isolators is crucial.  
According to a 2022 report by the United States Congressional Research Service [9], optical surveillance equipment such as the electro-optical targeting system (EOTS), electro-optical (EO), and infra-red (IR), which are used in military UAVs, are trending towards weight reduction and high precision owing to the high integration of sensors and actuators.  
Figure 1 presents a graph depicting the correlation between the weight of the optical equipment and its precision, based on the catalog information from View-pro (Established in 2005, Shenzhen, China), GSTiR (established in 2013, Wuhan, China), and SPI Corp (established in 1997, Las Vegas, NV, USA). Since optical equipment, which requires high precision, is vulnerable to external vibrations, a vibration isolator must be equipped to enhance their robustness against vibrations. Also, tests must be conducted to select an appropriate vibration isolator for optical equipment.  
This study aims to develop a new custom vibration isolator tailored for optical equipment and propose a testing methodology. It achieves this by using a fixed-fixed beam as an equivalent model to replicate the drone's first natural frequency. Along with the design of the vibration isolator, rubber stiffness tests and finite element method (FEM) analysis were conducted and comparatively analyzed. This led to the classification of rubber based on stiffness, which aided in evaluating the damping characteristics of the vibration isolator. To verify the damping characteristic of the vibration isolator, a testing method simulating the first natural frequency of a drone as a fixed-fixed beam was proposed. Also, the damping characteristic was confirmed based on the results of the random vibration test.