Toward Accurate and Robust Unloading Force Rendering via a Series Elastic Actuator-Driven Body Weight Support System

Body weight support (BWS) systems are extensively utilized in rehabilitation robots to deliver sophisticated unloading force (i.e., vertical interaction force) for gravity and impedance compensation. The robust and accurate unloading force rendering is critical for gait training. In this letter, a compliant BWS system powered by a series elastic actuator (SEA) is developed, and a concise unloading force estimation and control framework is proposed. This framework comprises an unloading force observer and a time-delay-based disturbance rejection controller (DRC). The proposed observer eliminates the influence of extensively overlooked post-SEA dynamics, thereby accurately estimating the unloading force. Instead of regulating the spring force of SEAs, this framework utilizes the estimated unloading force as feedback and achieves direct control of interaction force. This concise method is highly deployable and easy to tune, fully harnessing the potential of SEAs in safe and accurate force rendering while addressing their limitations such as post-SEA disturbances and intrinsic oscillatory modes. Interaction experiments demonstrate that the proposed control framework is robust to human disturbances up to 5 Hz, despite the SEA exhibiting resonance. Additionally, the control error of the proposed framework is reduced by up to 60% compared to the velocity-sourced SEA controller.