Passively deformable flipper legs for an amphibious quadruped

Amphibious robots can play important roles in tasks like field exploration and disaster rescue, where robots may need to cope with complex environments including both lands and water. However, the development of the propelling mechanism for amphibious robots can be challenging due to the distinct working conditions, respectively. In this paper, we report an amphibious quadruped that possesses four newly designed passively deformable flipper legs made of spring steel. These strip legs can act as paddles to propel the robot in water and be bent to c shape under the robot's weight on land to thrust the robot forward upon release. Specifically, we focused on the effect of stiffness distribution of these legs on the performance of the amphibious robot. Inspired by pectoral fins of mudskippers, we carefully designed five different tapering profiles for these legs such that they can have variable stiffness along the length direction. Experiments were then conducted under three different environments, grass, sand and water, to find the optimal stiffness distribution of the leg. Based on the evaluation of average forward velocity and Cost of Transport (COT) of the robot, we find that the flipper leg with decreasing stiffness from proximal to distal end yielded faster and more efficient performance, both on land and in water. This points out future direction for further development of the platform.