M.Sc Thesis

M.Sc StudentVirozub Evgania
SubjectDynamics and gait optimization of multi-link swimming robot
using "perfect fluid" model
DepartmentDepartment of Mechanical Engineering
Supervisors ASSOCIATE PROF. Yizhar Or
PROF. Alon Wolf
Full Thesis text - in Hebrew Full thesis text - Hebrew Version


Mobile autonomous robots are crucially needed for various marine applications such as protection of marine infrastructures, detection of underwater threats, search and rescue operations and maintenance of geometrically complicated marine systems.  For these potential maritime applications, robotic swimmers are currently the subject of extensive research and development. Simple theoretical models that describes the dynamics and the interaction between the robotic swimmer and the fluid are crucially needed.

In this work we use a simple model of ''perfect fluid" for the investigation of the dynamics of a planar multi-link swimmer. The swimmer is a chain of rigid ellipse-shaped links that are connected by revolute joints whose angles are controlled by motors. The model accounts for the inertial effects only and neglects the viscous drag effects. Therefore it is possible to reduce the swimmer's motion equation into a first order system due to symmetries and conservation of generalized momentum variables. Using numerical analysis of the first order system the performance of the system under harmonic inputs was examined. Using simulations it was possible to optimize the swimmer's structure and control input to find the values for maximal displacement and predict the movement of the swimmer. The optimizations showed that optimal values the result in maximal displacement for three-link swimmer can be found for amplitude of the harmonic input, phase shift between the links and combined values of amplitude and phase shift. Geometrical optimization of the length of the links under certain restrictions resulted in an optimal link's length ratio. For the five-link swimmer the model enabled us to make an evaluation of optimal phase shift, optimal amplitude of the links and combined values of phase shift and amplitude. It was shown that harmonic inputs oscillating about zero result in the robot's movement in a straight line. Additional simulation showed that it is possible to maneuver the swimmer in a moderate turning by changing the control inputs into oscillations about a constant angle.

The theoretical model was tested with experiments and motion measurements of swimming three-link and five link robots, resulting in a good agreement. The measurements of the movement were collected and processed using motion capturing system. Collected data was compared with theoretical model.

In this work the "perfect fluid" model was investigated. The model accounts for the inertial effects only, with the assumptions of an ideal inviscid fluid. The viscous drag effects, the effect of vortex shedding and hydrodynamic influence the links have on each other are neglected. The result is a very simple model that gives a very simple formulation of the motion equations for a swimming multi-link robot. Therefore the model demonstrates the importance of inertial forces and moments in the movement in an ideal fluid. The unmodelled effects resulted in some differences between the experimental results and the theoretical model