This paper presents gain optimization for a controller of a 6- DOF underwater robot with tilting thrusters. PID control system with anti-windup technique is designed to stabilize the hovering motion of the robot. The controller comprises thrust vector decomposition to overcome nonlinearity of the thrust vector and also includes an algorithm to compensate for saturation of thrusters. A total of 24 control gains should be tuned in this controller, and gain optimization is performed according to four system errors using genetic algorithm. First, 18 PID control gains were optimized and then 6 gains were optimized to affect anti-windup. As a result, control gains optimized by the integral absolute error showed the best performance, and it is verified that tracking error in position and orientation of the robot were reduced by 29.38% compared with initial gains.

This paper presents control performance improvement by modifying center of gravity (COG) of an underwater robotic platform. To reduce the oscillation or to increase the positioning accuracy, it is important to accurately know the COG of an underwater robotic platform. The COG is determined by the three measured tilting angles of the platform in different postures. The tilting angle is measured while the platform is hanged by two strings. Using coordinate transformation, the plane of intersection is defined from the angle of the platform and the position of the string. The COG of the robotic platform is directly calculated by the intersected point in three defined planes. The measured COG is implemented to the control algorithm that is pre-designed in the previous research, and the empirical result on tilting gives 48.26% improved oscillation performance comparing to the oscillation result with the ideal COG position.