Several forces are always exerted on locomotion systems, such as inertial forces, gravitational forces and ground reaction forces. Among these, we focus on the ground reaction forces in order to consider the balance control of a simple two-link legged robot model in the sagittal plane, which contains one actuator at the ankle and two force sensors on the sole. To maintain the balance, it is necessary that the ground reaction forces are kept positive at both ends of the foot, i.e. the heel and the toe. To achieve this, we design two control laws for each ground reaction force. By alternatively switching them, the standing posture can be maintained. Examining the behavior in the phase plane, the stability of the control laws are considered. Furthermore, we also study posture changes caused by stationary external forces in the horizontal direction. Introducing a torque input that changes on a large time scale, we realize a postural adjustment whereby the body is adaptively inclined to the opposite direction of the external force. By computer simulations, we confirm the convergence of such a posture adjustment.
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