# Configuration Design and Trans-Media Control Status of the Hybrid Aerial Underwater Vehicles
## 3 Trans-Media Control  
The process and control requirements of HAUVs are special and challenging for multidomain locomotion. As a combination of AUVs and UAVs, the control design of HUAVs could benefit from the control schemes of AUVs and UAVs. However, there are some differences in the design of control schemes between UAVs and AUVs. UAVs are generally designed to be neutrally stable that their buoyancy is equal to gravity. The angle of attack (AOA) is generally too small to prevent the shell from being damaged by excessive shear force during movement, and the design of hydrodynamic configuration is symmetrical, so the effect of lift is generally negligible. UAV requires the wing to generate lift to balance gravity. So, the range of AOA is also much bigger than that of AUV's. Therefore, in the process of controller design, the dynamic model of AUVs is generally expressed in the body's coordinate system, while the UAV is generally expressed in the velocity coordinate system, so as to make the expression of the dynamic equation more convenient for controller design. Then, the velocity, angular velocity, and Euler angle are selected as virtual control variables for the AUV, and the AOA, sideslip angle, and flight path angle could also be selected for the UAV, as presented in Figure 5[25]. Combined with different control requirements, many linear and nonlinear control methods have been widely used to the control design of UAVs and AUVs, including fuzzy control [26; 27], dynamic-inverse control [28], Lyapunov-based backstepping control [29; 30; 31], adaptive neural network control [32; 33], sliding mode variable structure control [34], and so on. Benefiting from these methods, outstanding works were carried out to rectify the control issues of HAUVs. For multirotor HAUVs, [PERSON] established a dynamic model of the trans-media process, regardless of the added mass and external disturbances during the water-air transition process [19]. The structure of the dynamic models in air and underwater are similar, since air and water are both fluid. Added mass and floatage are extra-specific in water. Following this, the nonlinear dynamic models in air and underwater are transformed into linear variable parameter models, and the state feedback attitude controllers are designed. Simulation results showed that the proposed control scheme could achieve attitude stability in a preset medium [35]. This is the first attempt at air-water trans-media control. [PERSON] designed switching PID controllers for air and underwater mediums. Euler angles and altitude in air or depth in water were controlled regardless of the horizon position. Underwater fixed-depth experiments were performed using an HAUV in Figure 4a called 'Navigator'. It was equiped with a water pressure sensor which could measure its underwater depth. The depth error reached one meter. [PERSON] defined the water-air trans-media process as the transition zone. The dynamic model was divided into the flight model, underwater model, and transition zone model. Then, the gain-scheduling control scheme was developed for the trans-media process. It achieved vertical ingress and egress on 'Navigator' but could not track maneuver trajectories [36]. [PERSON] introduced the unit quaternion to describe the attitude motion to avoid this singular problem and designed the water-air switching cascade PID controller. It could track the underwater maneuver trajectories on  
Figure 5: The Lyapunov-based backstepping method for AUVs and UAVs. 'Naviator' [37]. Added mass, water surface effect and disturbance were not considered, so that the control effect of egress on the process was not considered good. Furthermore, the adaptive sliding-mode controller and the adaptive dynamic-surface controller were developed with the added mass. The parameters' uncertainty and external disturbances were considered [38]. Simulation results showed good tracking ability.