Influence of loads and design parameters on the closed-loop performance of Series Elastic Actuators

摘要

Many of the potential robotic applications for Series Elastic Actuators (SEAs), such as legged walking or exoskeletons, are characterized by the interaction with an unknown environment. Especially in the case of exoskeletons, the SEA is directly acting on the user's limbs. Thus, the rendering of the actuator's mechanical output impedance is a major concern. A second commonality is the fact that the loads at the output ports are changing drastically [1]. In these scenarios, a SEA should be considered as a two-port system floating between two - not necessarily consecutive - links. Hence, this paper is focusing on the investigation of the influence of the actuator dynamics and the varying loads on the control performance. The theoretical analysis is based on an electric circuit model of a SEA that clearly and intuitively describes the role of each mechanical element [2]. This model is gradually extended by a set of cascaded control structures - a PD force control, a disturbance observer and the impedance control. The impedance interpretations of the various control structures are introduced. For each cascade, the influential elements on the closed-loop performance are highlighted. It becomes apparent that a low reflected inertia is very beneficial for the closed-loop performance. Two of the findings are validated in experiments using the RRLAB SEA in a two-port setup.