Bipedal Robot: Leg Kinematics for Stable Walking

Legged robots have high mobility for application with uneven terrain. Bipedal robots require the lower number of actuators with less complication of control system, but their walking stability is the main challenge. This paper presents a design of the bipedal robot having two actuated joints for the hip and the knee and one passive joint for the ankle of each leg. To minimize the inertia of the moving legs, the four brushless motors with integrated controller and planetary gearbox are concentrically located at the hip axis while the knee joints are driven through the parallel linkages. The zero-moment point (ZMP) along the foot support contacting the floor was derived based on the table-cart model. During the stance phase of stable walking, the desired ZMP must be located within the foot boundary. The ZMP generator for shifting the center of mass (COM) forward according to the stride was simulated along with kinematics of the legs. The hip and the knee joint trajectories were implemented on the bipedal robot prototype. The desired leg kinematics was validated by the experiment. As the load supported by the feet, the effect of gravity on the leg joint positions and torques was also studied. Additional sensing and the control of joint stiffnesses will be applied for achieving the dynamic walking of the bipedal robot.