Series Elastic Actuators-Driven Parallel Robot With Wide-Range Impedance Realization for Balance Assessment and Training

In this article, a novel robotic device is proposed that can realize a large range of compliance and a force sufficient to achieve human-balance training. The proposed device is called a virtual ground robot (VGR) and consists of a 3-degree-of-freedom parallel mechanism, which is driven by three series elastic actuators (SEAs). The proposed VGR can produce a large force and impedance overcoming the limitation of SEA using properly designed parallel legs, 3-PRRS, which can also reduce the size of the VGR. As a result, the proposed VGR can achieve 2.38-times stiffness ellipsoid as a conventional 3-RPS parallel robot. It can also achieve significantly large compliance by exploiting the advantages of SEAs, which have not been realized using conventional motion platforms. The dynamics and kinematics of the VGR are investigated from the SEA space to the entire task space, based upon which, the spatial impedance control of the VGR is designed by utilizing the robust force control of the SEA. The VGR is utilized as a device for measuring and training the balance of a human; a person can stand on the VGR, and balance-related values such as force, position, and center of the pressure are observed and controlled by the robot. The features and performance of the developed robot were theoretically examined and validated experimentally.