# Vibration Isolation of a Surveillance System Equipped in a Drone with Mode Decoupling
## 4 Results and Suggestions
### Vibration Characteristics of the Original System  
Frequency response functions were calculated using measured accelerations from modal testing for elucidating the vibration transmissibility between a source and a receiver for the original mobile surveillance system. Vibrations from the drone and surveillance system were measured by exciting the drone using an impact hammer. Note that an impact force was applied to the drone at an oblique direction to excite the mobile surveillance system in all directions. The transmissibility was calculated using Equation (19) (Figure 3). The operating frequency of the drone ranged from 30 to 85 Hz duringflight; therefore, this study aims to isolate the vibration in this range to minimize the transmission of vibrations to the camera in the corresponding frequency range. The black lines in Figure 3 denote the transmissibility for the original system; it shows that transmissibility in various operational frequencies exceeds unity in all directions. It means that vibrations originating from rotors were first amplified and then transmitted to the camera in the surveillance system. Specifically, a frequency range of 43-85 Hz was amplified in the fore-after direction, that of 10-38 Hz and 55-64 Hz was amplified in the side-by-side direction, and that of 10-40 Hz was amplified in the vertical direction. It implies that vibration in all operating frequencies was magnified, and the tremble of vibration was being measured. More information on the location and amplitude of natural frequencies in each direction is described in Table 1. Interestingly, several natural frequencies of each direction overlapped. The frequency range of approximately 60 Hz of the fore-after and side-by-side directions overlapped; the third natural frequency of the fore-after direction was close to the fifth natural frequency of the side-by-side direction. The natural frequency close to 36 Hz of the side-by-side and vertical directions overlapped; the fourth and fifth natural frequencies of the side-by-side and vertical directions were close to each other, suggesting that vibrations of these directions were coupled to each other. It can be inferred from this observation that a coupled model should be used for system identification to ensure the accurate prediction of vibration responses. Moreover, side-by-side and vertical directions more significantly affect the blurriness of an image. Figure 3: Measured transmissibility with modal testing: (**a**) fore-after direction, (**b**) side-by-side direction, and (**c**) vertical direction. The most straightforward approach to isolate transmissibility for different operational frequencies is to install commercial vibration isolators in the gimbal (Figure 4). Vibrations from rotors (\(\text{vib}_{\text{-source}}(\omega)\) in Figure 4) excite all components in a drone. The vibration is transmitted to the surveillance system through a gimbal that connects it to the drone causing vibration in the surveillance camera (\(\text{vib}_{\text{-target}}(\omega)\) in Figure 4), which suggests that a gimbal plays a critical role in transmitting vibration from the drone body to the surveillance system. However, vibration from rotors is transmitted to the surveillance system in an original configuration (Figure 4a) owing to the lack of a vibration isolation system in a gimbal. Therefore, this vibration influences images recorded by a surveillance camera and deteriorates monitoring performance. Hence, a vibration isolation system is necessary for the better performance of a surveillance camera. Four vibration isolators of DJI (a black isolator in a yellow box in Figure 5) were installed where the gimbal and drone body connect, and then transmissibility was measured. The red lines in Figure 3 demonstrate that this simple approach enhances the vibration isolation performance at different operational frequencies, especially in the side-by-side and vertical directions. However, significant amplification occurred at approximately 50 Hz in the fore-after direction.