Position and stiffness control based on a mechanical-equilibrium model for antagonistic joints using hyper-extension actuators

In this study, we develop an antagonistic joint using a hyper-extension actuator for the development of robots that can perform human-like movements that are smooth and flexible. Previous antagonistic joints using pneumatic artificial muscles could not maintain joint rigidity when no air pressure was applied. The hyper-extension actuator greatly elongates when air pressure is applied and has a small radial-expansion rate. By applying this actuator to the antagonistic joint, joint stiffness can be maintained even without air-pressure application. The developed antagonistic joint can achieve a target joint angle range of -90–90 deg and joint stiffness of 0.1, 0.2, and 0.3 Nm/rad by controlling the applied pressure to the actuator, as defined by a model. The pressure-control performance of the joint angles and stiffness is verified using an actual device. The results demonstrate that the expected and measured values are in agreement.