# A Review of Autonomous Tugboat Operations for Efficient and Safe Ship Berthing
## 4 Autonomous Tugboat Element Technologies
### Control Technology  
Control technology can be said to be a core technology, to the extent that control technology is mentioned the most in research on ship berthing. Since tugboats and large ships perform cooperative work in a relatively close distance environment, accurate position and attitude control performance is required. In the study in [52], an empirical experiment was conducted to maintain a fixed position while offsetting the influence of tidal current or wind by implementing a dynamic position control algorithm for an autonomous tugboat. In order to berth a large-scale merchant ship, it is necessary to control a group of at least two or more tugboats instead of one tugboat, so research on control technology using a group of tugboats is being actively conducted. In the study in [18], a distributed control approach was employed to independently control each tugboat instead of using a centralized control method. The study focused on investigating the tracking control of the towed vessel using a robust control strategy. However, the study only addressed the issue of controlling the vessel's simple direction without taking into account the effects of hydrodynamic forces or environmental disturbances. The study by [PERSON] [19] assumed that tugboats are strategically placed close enough so that information transmission between them is not necessary. An adaptive control technique was used to compensate for the force on the contact point between the main ship and the tugboat when it cannot be accurately known. Other studies [2; 21] have used adaptive control techniques to compensate for changes in fluid dynamic parameters, such as drag force and mass coefficient when passing through narrow channels or open sea. Another study [23] dealt with maneuvering a target vessel using an autonomous surface vessel with an overactuated system equipped with six propellers. The study designed a tracking controller using an adaptive control technique to match the desired trajectory. This tracking controller can be applied to a scenario in which the motion of a mothership is manipulated using an autonomous tugboat (Figure 19). In Figure 19 a traceable desired trajectory (\(p_{d}\)) is generated that reflects the actuator configuration and constraints, model uncertainties, and controller structure and generates the desired forces and moments(\(\tau_{net}\)) through the tracking controller. Given \(\tau_{net}\), we are given the commands for individual actuators \(\tau_{1}\), \(\tau_{2}\), etc., as well as the barge's position and yaw angle in the inertial frame (p), the barge's velocity in the body frame(v), heading(\(\psi\)), and rr of the ship. In a subsequent study [24], the authors considered modeling disturbances such as wind, current, and waves, which are types of environmental disturbances. They applied the Sliding Mode Control (SMC) technique, a robust control method, instead of the adaptive control method. SMC can handle not only uncertainties in quickly changing parameters but also disturbances or dynamic components not included in the model. SMC is more resistant to disturbances compared with the adaptive control technique. The effectiveness of these control methods was verified using a 1:70 scale model ship, also known as a \"cybership\". In another study [3], the authors proposed a new robust controller methodology through matrix decomposition. They arranged the tugboats in a specific configuration, which converted the overall problem into a second-order system with an asymmetric input gain matrix. The studies mentioned above primarily focused on developing control methods for tugboats, but did not address the control of the mothership's attitude, which involves the positioning and orientation of the ship during the berthing process. Moreover, these studies only modeled each tugboat as an azimuth propulsion system and did not consider the actual physical interaction between the tugboats and the mothership, where the tugboats push and pull the mothership.