Stable Walking Pattern for an SMA-Actuated Biped

In this paper, a walking pattern filter for shape-memory-alloy (SMA)-actuated biped robots is presented. SMAs are known for their high power-to-mass ratio as well as slow response. When used as actuators, the SMA speed limitation can potentially lead to stability problems for biped robots. The presented filter adapts the human motion such that an SMA biped robot maintains a stable walking pattern. The zero moment point (ZMP) is used as the main criterion of the filter to guarantee the stability of the motion. The SMA actuators are designed based on the dynamics and kinematics of the motion. The response time of each SMA actuator is modeled in order to estimate the behavior of the actuator in realizing the given trajectory. After applying the delay times to the motion, the new trajectories are generated and evaluated by the filter for the ZMP criterion. Using simulations, it is shown that the filter can generate smooth trajectories for the SMA-actuated biped robots. The filter furthermore guarantees the stability of a robot mimicking the human walking motion.