Closed Loop Depth Control of Biomimetic Robotic Fish Cooperatively Driven by Pectoral Fin and Caudal Fin

To improve the dynamic and steady-state performance of robotic fish in depth control, the depth control process is divided into two stages named the approaching stage and the cruising stage according to the depth error, and then a closed-loop motion control method is presented by combining central pattern generator (CPG) and fuzzy control. Firstly, a motion control model based on CPG is established, in which the feedback inputs are the signals of pressure sensor and the control parameters are adjusted by fuzzy controller. On this basis, a cooperative driving mode of the pectoral fin and the caudal fin is employed for the approaching stage with large depth errors, and an approaching fuzzy controller is used to control the offset and amplitude of the feather joint such that the robotic fish can move to the desired depth rapidly. For the cruising stage with small depth error, a cruising fuzzy controller is employed to control the attack angle of the pectoral fin such that the robotic fish can cruise at the desired depth. The switch between those two stages is achieved by initiating and inhibiting the CPG of the pectoral fin. In the design of fuzzy controllers, the approximate relations between the control parameters and the changing rate of pitch angle obtained through fitting the experiment results by the least squares method, is utilized. So, the speed of the robotic fish moving to the desired depth is increased and the steady-state error when cruising at the desired depth is decreased. The validity of the proposed method is verified by the simulation and experiment results.