Walking Pattern Filter for Dynamic Biped Robot with Sma Actuators

In this paper we present a walking pattern filter for Shape Memory Alloy (SMA) actuated biped robots. SMAs are known for their slow response. The actuation speed limitation can potentially lead to stability problems for biped robots. This filter adapts the human motion for a SMA biped in order to have a stable walking pattern. The Zero Moment Point (ZMP) is used as a main criterion of the filter to guarantee the stability of the motion. The SMA actuators are designed based on the dynamic and kinematics data 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 new trajectories are generated and checked by the ZMP criterion. The output of the filter can generate smooth trajectories for the SMA biped robots. The filter furthermore guarantees the stability while mimicking the human motion. The filter provides a practical way to create stable walking patterns using SMA actuators. INTRODUCTION Shape Memory Alloy (SMA) composites are a class of smart materials that exhibit extremely large recoverable strains. The shape memory effect occurs due to a temperature and stress dependent shift in the materials’ crystalline structure between two different phases called martensite and austenite. The use of SMAs in applications involving actuation has several advantages such as large deformation, excellent power to mass ratio, maintainability, reliability, and clean and silent actuation. The disadvantages are slow response, low energy efficiency due to conversion of heat to mechanical energy and motion control difficulties due to hysteresis, nonlinearities, parameter uncertainties, and unmodeled dynamics. When an SMA wire is at a high temperature without stress, it will be in the austenitic phase. At this phase the material is in a body centered cubic structure configuration. As the temperature of the wire decreases, the phase transformation will become martensitic. The martensitic phase is most often a face centered cubic structure. In shape memory effect, a specimen exhibits a large residual strain after loading and unloading. This strain can be fully recovered upon heating the material. In pseudoelastic effect, the SMA material provides a large strain upon loading that is fully recovered in a hysteresis loop upon unloading. [1, 2] When the stress–free austenite phase cools down below the martensite start temperature (Ms) the phase starts transforming to martensite. The material will be completely martensitic when the temperature drops below the martensite finish temperature (Mf). At this phase, the material has multiple variants and twins. As long as the temperature of the material is below the austenite start temperature (As), no phase transformation to austenite takes place. When, however, this material is loaded, it will initially start deforming elastically. If the stress increases above a certain amount, the pairs of martensite twins begin detwinning to the stress-preferred twins. During this reorientation process, stress rises very slightly at therefore the stiffness of the material is at its minimum. This single variant of the martensite is thermodynamically stable at T < As. Therefore, upon unloading there is no conversion to the multiple variant martensite and only a small elastic negative strain will take place. This will leave the detwinned material with a residual strain. The detwinned material can recover the residual strain by heating above the austenite final temperature (Af). The transformation to austenite starts at the austenite start temperature As, thus creating the shape memory effect.