Parallel Compliance Design for Increasing Robustness and Efficiency in Legged Locomotion—Proof of Concept

Benefiting from serial compliance in series elastic actuators can be considered as a breakthrough in robotics. Recently, applying the parallel compliance in robot designs is growing based on its advantages such as reduction in consumed torques. In this paper, we aim at employing parallel compliance to increase walking robustness of bipedal robots against model uncertainties. Utilizing passive compliant elements instead of adapting the controller in order to cope with uncertainties makes the system more efficient and less sensitive to measurement issues such as delays and noise. We introduce a methodology for designing both parallel compliance and controller using hybrid zero dynamics concept. This study includes simulation results representing the design approach and preliminary experiments on parallel compliance effects on efficiency of a robot joint position control. The simulations comprise a compass gait (2-link) model and a 5-link model. The ground slope and robot segment lengths are considered as uncertain parameters in the first and second models, respectively. The control target is met by the insertion of compliant structures parallel to the actuators. In order to employ the proposed method on a real robot, we suggest using pneumatic air muscles as parallel compliant elements. Pilot experiments on the knee joint of BioBiped3 robot support the feasibility of suggested method.