Propulsive Element Normal Force Based on Acceleration Measurements Experienced by a Subcarangiform Robotic Fish

The normal force exerted on a propulsive element is estimated based on acceleration measurements of an articulate-flexible propulsion mechanism in a subcarangiform swimming robotic fish. The propulsion mechanism is an articulating torso followed by a flexible caudal fin to provide thrust. The trunk is an assemblage of five ABS-plastic vertebrae driven by an actuator through a pair of wires, whereas the caudal fin is a silicone-rubber lunate-shaped tail coupled to the last vertebra. MEMS 3-axis sensors measured the linear acceleration experienced by the rigid head, articulated trunk, and compliant caudal fin at different undulation frequencies with the robotic fish prototype suspended in still water. The transverse acceleration measured was approximated as the reaction force exerted by the water on a propulsive element that accelerates the surrounding water. Subsequently, the caudal fin midline motion was analyzed by video processing to compare with the subcarangiform swimming kinematics model and to depict the normal force vectors in an undulation excursion. This study provides a feasible alternative to quantify the normal force generated by propulsive elements in bio-inspired propulsion mechanisms by using low-cost MEMS sensors to complement other well-suited techniques.