Virtual Model Control for Planetary Hexapod Robot Walking on Rough Terrain

In this paper, Virtual Model Control (VMC) is proposed to address the gait control problem of a space hexapod robot. In the VMC framework, a series of virtual elements, like springs and dampers, are attached to specific points on the body to generate desired joint torques. Especially, the control of the gait is divided into two phases: stance phase and swing phase. In the former, the VMC is exploited to compute the torques for the standing legs required to control the body height and attitude. The virtual elements are attached to the hips in such a way to govern each degree of freedom. On the other hand, in the latter phase, the VMC provides the control actions for the swing legs to follow a desired trajectory. In this case, the springs and dampers are attached between the foot of the leg and a point on the desired trajectory. The legs alternate these two modes cyclically and this switch is commanded by a state machine. In this work, three possible gaits are considered: tripod gait, wave gait and stick gait. The strength of the VMC and its suitability for space applications lie on its intuitiveness, robustness and computational efficiency. The effectiveness and performance of the proposed approach are assessed through numerical simulations considering different terrain roughness.