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A hybrid chaotic controller integrating hip stiffness modulation and reinforcement learning-based torque control to stabilize passive dynamic walking

Implementation of bipedal stable walking has attracted a lot of interest. Passive dynamic walking (PDW) is one promising manner to generate natural bipedal walking with high energy efficiency. However, how to improve stability and versatility of PDW-based robot against disturbance and time varying environments is still a big challenge in robotics. Chaos and bifurcations are intrinsic features of biped dynamic walking, which are important reasons for the failure of PDW. Thus a hybrid chaotic controller to stabilize chaos and bifurcations of PDW was proposed in this paper. At first, the dynamics model of compliant biped robot was set up, and routine to bipedal chaotic motions was found through parameter study. Then one hybrid chaotic controller integrating hip stiffness modulation and hip impulse torque control was developed, where hip impulse torque control based on reinforcement learning was trained to get state variables close to fixed point of PDW, and then hip stiffness modulation was conducted based on Ott-Grebogi-Yorke method to stabilize unstable motions of fixed point. Simulation results showed that period-1 stable walking could be gained for biped robots from chaotic motions, against original value disturbance, force disturbance and in time varying environments. The proposed hybrid chaotic controller could be used to stabilize bipedal chaotic motions and make the passivity-based robot become robust and versatile to disturbed and time changing environments.