# Design and Analysis of the High-Speed Underwater Glider with a Bladder-Type Buoyancy Engine
## 4 Experiment  
### Gliding Experiment  
This section conducted experiments in actual waters to verify the movement speed derived from the design and modeling of the UG above. The test site was operated in deep waters of 1000 m or more near Ulleungdo, South Korea. As for the experimental method, the results were confirmed after circumnavigating the UG once at a target depth of 250 m. When the UG floated to the surface after circumnavigating, it received internal measurement data through RF communication. Figure 14 shows the UG performing a circumnavigation experiment.  
Figure 15 shows the water depth values obtained between UG circumnavigation experiments. Approximately 300 s after the start of the experiment, the UG started submerging in the direction of the sea floor due to the change in buoyancy. At about 800 s, the buoyancy engine started to turn upward, and the change in water depth became gentle  
Figure 14: Gliding experiment of UG.  
after reaching the maximum depth of approximately 260 m at 1000 s. It takes about 1900 s to circumnavigate the depth of 260 m once.  
Figure 16 shows that the UG controls the posture using the posture controller between circumnavigations. The previous speed analysis confirmed that the maximum moving speed was reached during circumnavigation at approximately 40\({}^{\circ}\). Therefore, the pitch was controlled to \(\pm\)40\({}^{\circ}\) during the circumnavigation. The target pitch was set to \(\pm\)40\({}^{\circ}\) considering the AoA at the angle derived from Figure 9. When diving, it was well controlled to the target value of \(-\)40\({}^{\circ}\), and when ascending, the pitch was not directed to +40\({}^{\circ}\) at the beginning and was controlled late. This seems to be a phenomenon that appears because the buoyancy control of the buoyancy engine takes some time.  
UG is sensitive to weight increase, and since it is a device that needs to minimize battery consumption, it cannot be equipped with a sensor such as DVL to measure speed. Therefore, the moving speed of the UG was calculated using a depth sensor and a digital compass, and the UG moving direction and sliding slope can be expressed as shown in Figure 17.  
Figure 16: Pitch result of UG while gliding.  
Figure 15: Depth result of UG while gliding.  
The calculation of the moving speed in the moving direction of the UG can be expressed as Equation (15) below:  
\[u=\frac{z_{t}-z_{t-s}}{-\sin(\theta\times\pi/180)} \tag{15}\]  
Here, \(t\) is the current time, \(s\) is the sampling time, \(z\) is the depth, and \(\theta\) is the pitch value. To simplify the movement speed calculation, only the pitch was considered in the posture drawing, and the absolute value of the pitch within 10\({}^{\circ}\) was calculated by substituting 0\({}^{\circ}\).  
Figure 18 shows the movement speed of the UG calculated using Equation (15). It accelerated from approximately 300 s and achieved the maximum moving speed of 2.11 knots at 430 s. After maintaining about 1.8 knots until the upward transition, the movement speed increased again to approximately 1.5 knots after the movement speed fluctuated during the upward shift. At 1500 s, it floated to the water, and the movement speed converged to almost 0 knots. The moving speed drops to a negative number in the 800-1000 s section because the buoyancy engine's buoyancy control was slow. Thus, the motion change could not be accelerated except for the part where the motion of the UG changes (up-down transition). The average moving speed is confirmed to be 1 knot or more. According to the experimental results, similar to the results analyzed in the design, it was confirmed that the maximum moving speed of the UG was 2 knots or more, and high-speed circumnavigation was possible.  
Figure 17: Moving direction and sliding slope of underwater glider.  
Figure 18: Velocity result of UG.