Design and kinematic analysis of a rope-driven linkage frog-like swimming robot with multidirectional controllable motion

In this paper, a rope-driven linkage frog swimming robot is constructed in accordance with the delicate structure of the frog swimming mode. First, a rope-driven linkage limb propulsion mechanism is proposed. This mechanism not only enables the robot to achieve the superior motion characteristics and mechanical performance of the four limbs of a frog swimming in water but also makes the limb structures more flexible and lightweight. Using the D-H parametric method with geometric relations, the swimming kinematic model of this robotic limb propulsion mechanism is established, and the stability margin and recovery moment equations are derived. Second, based on the above mathematical model, the motion space and stability analysis of the robot limb ends are simulated by MATLAB software. Finally, an experimental prototype with a size of 18 cm in length × 7 cm in width × 7 cm in height, combined with a control system, is developed. The experimental result verifies the rationality and correctness of the robot's structural design, theoretical derivation and control system and proves that it has superior motion capability with a straight-line swimming speed of 0.54 m/s, a horizontal surface turning speed of 100°/s (turning radius of approximately 0.18 m), and an upwards and downwards submerged swimming speed of 9 cm/s during a single swim in the water.