Stable walking of 3D compass-like biped robot with underactuated ankles using discrete transverse linearization

In this study, a walking manner of a spatial compass-like biped robot with underactuated ankles is presented. This biped robot has 3 degrees of freedom with only one actuator. The hip joint is equipped with a constraint mechanism to lock the angle when it retracts to a desired value. With the Poincare shooting method, a limit cycle of 3D walking gait on level ground is obtained. This limit cycle is unstable by checking the Jacobian of the Poincare map. We use this limit cycle as a reference trajectory and then introduce a method based on discrete transverse linearization to transform the reference trajectory tracking problem to a stabilization problem of a discrete periodic linear system. Thus, the classical discrete-time linear quadratic regulator is recalled to solve the stabilization problem. That is, a stable walking control law is obtained for the proposed 3D robot with a constraint mechanism.