Terrain edge detection for biped walking robots using active sensing with vCoP-position hybrid control

Perception of terrain is one of the most important technical impediments for biped robots to locomote in human environments. Perception error exists due to imperfect exteroceptive devices causing issues such as limited viewing angles or occlusion of visual sensors. This error can cause unexpected contact between the robot and the environment. The reaction wrenches from unexpected contact negatively affect the control and balance of the biped robot. In this study, active sensing method is proposed to estimate the edges of the contact terrain using robot link geometry information. By generating proper active motion that maintains the contact, the geometric information of the contact link is collected to estimate the edge of the contact terrain. To recognize contact and calculate its geometry, a multi-modal sensory strategy with 6 degrees of freedom (DOF) force/torque sensors at robot ankles and joint encoders is proposed. The concept of virtual Center-of-Pressure (vCoP) is utilized to generate active motion of the foot while maintaining the contact. To prevent loss of balance during active motion, the normal force condition of the vCoP is taken into account and the contact moment of the supporting foot is controlled. The operational space control scheme for a floating-base robot is adopted to apply compliant motion and vCoP-position hybrid control framework. The proposed method is verified by experiments using a 12-DOF biped robot to estimate the edge line of a block that has contact with the robot foot.