In this paper, we study the underactuated fivelink biped walking on stochastically rough terrain. We propose a simple and powerful Sliding Mode Control scheme. By taking Poincaré sections just before the impact, we accurately represent ten dimensional system dynamics of metastable walking as a Markov process. By switching between two qualitatively different controllers, we show that the number of steps before failure can be increased by more than 10 million times compared to using either one of

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... the controllers only. To achieve this, only the current state and approximate terrain slope for a one step lookahead on geometrically rough terrain is needed. The analysis techniques in this paper are also designed for future application to a range of other simulated or experimental walkers.