  Although the application of Series elastic actuators (SEAs) in the
biomechatronic field has proved its appropriation in many aspects so far, the
problems of maintaining the stability for the SEAs still remains. This paper
proposes a robust controller so that to overcome the drawbacks of the previous
researches. Firstly, a mathematical model considering both the SEAs and the hip
joint of humanoid UXA-90 is obtained. Secondly, a reference input of the
proposed controller that is achieved from desired hip joint's angle in a
walking cycle is utilized. Then, a backstepping based sliding mode force
control approach is employed to ensure the precise movement of robot's link as
well as meeting the requirement of robustness for the whole system, which is
significant for the task of walking of a humanoid. Finally, some simulations
are carried out to verify the quality and effectiveness of the proposed
controller.
