Stability of the control scheme of a design of a robotic fish

Abstract

Fishes are known to be efficient swimmer with teleost species known for their high speed and acceleration inside water. Imitating these abilities requires building an efficient controller to manage the robotic fish system. The efficiency is in terms of faster swimming ability while consuming less energy as well as stable response to input commands. The control scheme used for this work is based on PIC18F4520 microcontroller which generates 3 rigidly coupled pulse width modulated (PWM) signals that is used for controlling (in an open-loop fashion) 3 Futaba RC servo motors which in turn manages the robotic fish tail configurations dynamically. A mathematical model of this robotic fish controller was setup which incorporates the tail fin drag, the rubber joint resistance and the hydrodynamic drag which was then used in MATLAB/SIMULINK environment for stability tests. The results indicated that the system is stable for open loop design but is unstable if used in closed loop mode - that is, if there is positional feedback to the controller. Furthermore, the system was found to be very sensitive to perturbation with a settling period of 2.17 seconds. It was therefore concluded that there is need to modify the design of this controller so that feedback from sensors (for example) will not negatively affect its performance when implemented. A successful implementation of a stable controller system will contribute to the science behind autonomous underwater vehicles (AUV).