# Optimal Attitude Determination for the CR200 Underwater Walking Robot
## 1 Introduction  
Biomimetics, the practice of drawing inspiration from nature's designs and mechanisms to address engineering challenges, is being increasingly applied in the development of real-world engineering products. In this study, we present the results of a fluid dynamics analysis on the application of biomimetic concepts in underwater robot development. The Crabster CR200 [1] is a six-legged underwater walking robot inspired by sea creatures like crabs and lobsters, designed for precise seabed inspection and manipulation. One of the CR200's standout features is its ability to maintain stability and position on the sea floor, even in strong currents. As an application example of the Crabster CR200 underwater robot, various underwater exploration tasks can be highlighted, including recent preliminary experiments for underwater cultural heritage excavation. Key experiments involved using the robotic arm to lift pottery and testing the robot's surrounding exploration capabilities withan ultrasonic camera and scanning sonar. These efforts led to the successful development of robotic arm operation techniques applicable to underwater cultural heritage surveys. In carrying out these missions, it can accurately hold its underwater position and maintain a stable posture in moving water. The CR200 walks on the seabed and can adjust its body posture using its six legs, each with four degrees of freedom [2]. This stability allows the CR200 to capture clear acoustic images in murky water and perform precise manipulations in currents. The main external disturbance for the CR200 in tidal areas is the hydrodynamic load acting on its body. The robot maintains its position and posture by maintaining mechanical contact with the seabed, with more than three legs always touching the ground during underwater operations. The stability in currents is further enhanced by controlling its heading and pitch angles, aligning its heading with the current direction, and adjusting the pitch angle to minimize drag and increase downforce on the body. A multi-jointed underwater walking robot like the CR200 can adjust its posture according to operating conditions to respond to various external environmental loads encountered during underwater walking and swimming. In harsh underwater environments, such as those in the Yellow Sea, identifying the optimal posture that allows the robot to maintain its position without being swept away is crucial for operating walking robots. This study aims to use Computational Fluid Dynamics (CFD) analysis to determine the optimal posture that enables the underwater robot, while resting on the seabed, to resist uniform currents and maintain its position without drifting. CFD simulation has become a widely adopted technique for the development and operation of underwater robots. Its primary application is to predict hydrodynamic forces to better understand the motion and trajectory of these robots. [PERSON] et al. [3] conducted a simulation of an Autonomous Underwater Vehicle (AUV) to evaluate its docking capability, focusing on the hydrodynamic interactions between the AUV and the docking structure under varying current conditions. [PERSON] et al. [4] used CFD simulations to calculate fluid forces on a small Intervention-AUV during cruising and analyzed its hydrodynamic performance in various postures, replicating realistic operational conditions by assuming an infinite fluid field. [PERSON] et al. [5] extended the capabilities of an existing vertically maneuvering soft underwater robot, enabling horizontal movement and path tracking through CFD analysis. Additionally, CFD analysis was used to estimate the hydrodynamic coefficients of the 6-DOF dynamic equation, which were then applied to construct a dynamic model for path tracking and attitude control of the robot [6; 7]. [PERSON] et al. [8] developed a control algorithm based on CFD-derived drag force data to enable stable path tracking and attitude control in underwater environments.