3D dynamics of bipedal running: Effects of step width on an amputee-inspired robot

The goal of reproducing human-like dynamic running remains a challenge for roboticists. Some robots, such as the bipedal RHex, have achieved basic features of dynamic running with hip-driven springy-legs that are similar to above-knee amputee running with prostheses. However, little is currently understood about the 3D dynamics governing such robots, and current robots still remain far from achieving human-like dynamics and stability. One apparent difference between the bipedal RHex robot and human runners is their step width: human runners, including above-knee amputees, run with footfalls close to the body centerline. Here we study the effects of varying step width on the 3D running stability of a bipedal amputee-inspired robot. The robot has a PD controller for pitch and no control for yaw or roll, based on that used for the bipedal RHex robot. We begin with a step width close to shoulder width, and then study the effects of decreasing the step width. We discovered that as the step width is decreased, yaw stability decreases and roll stability is lost corresponding with a qualitative change in a dimensionless roll inertia number. Due to this, the robot is unable to achieve human-like step widths. This implies that additional roll and yaw control may be required for stable locomotion with such narrow step widths. This discovery provides an early understanding of bipedal running dynamics and some early design guidelines for bipedal robots.