## 3 Mechanism Design Based on Multi-Body Dynamics
### Control Strategy  
In traditional dock mooring operations, cables and fenders provide elastic restoring forces and damping forces for the entire ship-shore mooring system to restrict the ship's movement within a certain range, while traditional cables and fenders provide a mooring stiffness for the ship. When the ship is subjected to hydrodynamic forces such as waves and currents, the entire mooring system can resist the ship's movement in the form of the elastic restoring force--just like the role of force vector F in the analysis of the ship motion time-domain equation mentioned above. If the intelligent automatic mooring system is used to operate the ship instead of the traditional cable and fender device, the intelligent mooring system will provide the ability to resist changes in terminal position and pose when the ship is subjected to external forces such as waves, and also play the role of force vector \(F\). Through the analysis of the motion and force characteristics of the ship mooring system mentioned above, it can be found that the existence of both the restoring force and damping force can restrain the slow drift of the mooring ship and are more conducive to reducing the low-frequency amplitude of the mooring ship. Based on this, the active  
Figure 3: Equivalent model schematic diagram of a double parallel butt mechanism.  
control of an intelligent automatic mooring system can be realized through the following three methods:  
1. Friction damping is provided by auxiliary branches in the parallel system, which is a passive force and cannot be controlled by the intelligent mooring system;
2. The aerodynamic damping provided by the driving branch in the parallel system is the active force, which can be interfered with by adjusting the relevant parameters of the intelligent mooring system. The damping force can inhibit the amplitude of the ship at various frequencies;
3. The constant pressure at both ends of the driving branch is maintained to provide the restoring force, which is the active force, and can be intervened by adjusting relevant parameters of the intelligent mooring system. The restoring force can change the slow drift motion of the ship into a low frequency oscillation.  
Figure 4 shows the control strategy diagram of the intelligent mooring system. According to the requirements of the mooring control task, it is necessary for the intelligent mooring system to maintain the ship's attitude within the range of motion expected in safe mooring standards. Therefore, by means of the mooring force--composed of the damping force and restoring force--the position and posture of the adsorbed ship can be adjusted actively. Among these, the damping force can suppress the ship's amplitude at various frequencies, and the restoring force can change the ship's slow drift motion into a low frequency oscillation. What the intelligent mooring system can control is the damping \(F_{2i}\) provided by the drive branch and the restoring force \(F_{1i}\) provided by the constant pressure at both ends of the drive branch. Therefore, in order to make the control strategy of the above system feasible, the damping coefficient and restoring force coefficient of the control drive branch are largely regulated, so as to realize the active control of the mooring power output equipment--the drive branch.  
Considering that intelligent mooring systems are highly nonlinear, strongly coupled complex control objects, in order to obtain good control accuracy and robustness--and to meet the requirements of ship-to-shore environment engineering availability under wave and current conditions--it is necessary to adopt an intelligent control algorithm with low computational complexity, good control performance and relative maturity; this will be studied and analyzed in the next chapter.  
Figure 4: Control strategy diagram of an intelligent mooring system.