Inverse kinematics of a subsea constrained manipulator based on FABRIK-R

Although underwater manipulators are considered the most suitable tools for executing many subsea intervention operations, they are an example of robots not designed for autonomous tasks. The subsea manipulator Schilling Titan 2, for instance, is composed of a sequence of constrained joints that makes impossible finding a closed-form analytical solution for its inverse kinematics. For this reason, numerical methods have normally been used. Nevertheless, these methods typically result in high computational load and may need many iterations to find a solution. FABRIK is an inverse kinematics approach which has as main advantages a fast convergence and a low computational cost, avoiding matrix inversion and being robust to singularities. However, the constraints imposed by the joints of the Titan 2 manipulator also makes impossible solving its inverse kinematics using FABRIK. A recent extension of FABRIK, named FABRIK-R, presents a solution for this problem. Therefore, this paper proposes to overcome the mentioned issues by solving the inverse kinematics of a high constrained subsea manipulator based on FABRIK-R and presents a comparison between this solution and a classical approach based on a second-order pseudo-inverse Jacobian. The advantages of the proposed approach are demonstrated in simulation experiments, and its feasibility is demonstrated in real experiments in manipulation tasks performed in dry lab conditions.