Low-delay Admittance Control of Hydraulic Series Elastic Actuator for Safe Human-Robot Collaboration

Abstract

Current industrial manufacturing standards highlight safe human-robot collaboration. Therefore, compliance solutions, whether passive or active, are necessary. In this regard, series elastic actuators (SEAs) have attracted extensive interest for applications in industrial robotics owing to the low output impedance of these actuators, which can enhance interaction safety. Although considerable efforts have been expended to improve the control accuracy of SEAs, the time delay in impedance/admittance control has remained an unexplored problem. The use of filters in conventional methods can result in large time delay and considerable amplitude errors that degrade the performance and increase the risk of injury. This study resolves this problem by developing a method for low-delay admittance control of hydraulic SEAs. An external force observer (EFO)-based admittance scheme is first formulated. Thereafter, adaptive time series (ATS) compensators are introduced to improve position control and EFO. Based on the improved position controller and the ATS-based EFO (ATS-EFO), a novel low-delay admittance control scheme is developed. The proposed method only uses the load mass and spring stiffness information, thereby facilitating its application. Simulation experiments are performed to demonstrate the SEA control performance on position tracking, external force estimation, and stiffness tracking. The results confirm that the time delay and the amplitude error in these tests are significantly reduced by the proposed method.