Exploiting passive stability for hierarchical control

The dynamics of a Spring Loaded Inverted Pendulum (SLIP) \template " [1] approximate well the center of mass (COM) of running animals, humans, and of the robot RHex [2]. Running control can therefore be hierarchically structured as a high level SLIP control and the anchoring of SLIP in the complex morphology of the physical system. Analysis of the sagittal plane lossless SLIP model has shown that it includes parameter regions where its gait is passively stabilized, i.e. with the discrete control input | the leg touchdown angle | held constant. We present numerical evidence to suggest that an open loop \clock" excitation of a high degree of freedom hexapedal robot model can lead to asymptotically stable limit cycles that \anchor " [1] the SLIP model in its self stabilizing regime. This motivates the search for completely feedforward SLIP locomotion control strategies, which we now speculate may be successfully used to elicit a self-stabilizing running robot such as RHex.