Tripedal walking robot with fixed coxa driven by radially stretchable legs

In this paper, we propose a new kind of tripedal locomotion-namely, robotic walking with three legs. We develop a prototype robot, which we call Martian III, having three stretchable legs fixed to each other at the center of the body. Therefore the coxa (the hip joint) is completely fixed in this robot, and the only control we have are the linear actuators to stretch the three legs in the radial directions. The robot is developed as a minimal model to examine our basic idea, to make the robot swing by actuating the legs and then generate locomotion utilizing geometric effect of rigid body rolling on a flat floor. The experiments with robot succeeds in forwarding and rotary locomotion with appropriate choices of oscillatory along each leg. Further analysis of the movement of Martian III indicates that the direction of the swing leg depends on the deformation of each leg. Moreover, due to the result of the measurement experiment and movement analysis of rotation locomotion, we found out two typical type of gaits.