A Sliding Mode Force and Position Controller Synthesis for Series Elastic Actuators

This paper deals with the robust force and position control problems of Series Elastic Actuators. It is shown that a Series Elastic Actuator's force control problem can be described by a second-order dynamic model which suffers from only matched disturbances. However, the position control dynamics of a Series Elastic Actuator is of fourth-order and includes matched and mismatched disturbances. In other words, a Series Elastic Actuator's position control is more complicated than its force control, particularly when disturbances are considered. A novel robust motion controller is proposed for Series Elastic Actuators by using Disturbance Observer and Sliding Mode Control. When the proposed robust motion controller is implemented, a Series Elastic Actuator can precisely track desired trajectories and safely contact with an unknown and dynamic environment. The proposed motion controller does not require precise dynamic models of the actuator and environment. Therefore, it can be applied to many different advanced robotic systems such as compliant humanoids and exoskeletons. The validity of the motion controller is experimentally verified.Comment: Accepted by Robotica in 201

Evaluation and Comparison of SEA Torque Controllers in a Unified Framework

Series elastic actuators (SEA) with their inherent compliance offer a safe torque source for robots that are interacting with various environments, including humans. These applications have high requirements for the SEA torque controllers, both in the torque response as well as interaction behavior with its the environment. To differentiate state of the art torque controllers, this work is introducing a unifying theoretical and experimental framework that compares controllers based on their torque transfer behavior, their apparent impedance behavior, and especially the passivity of the apparent impedance, i.e. their interaction stability, as well as their sensitivity to sensor noise. We compare classical SEA control approaches such as cascaded PID controllers and full state feedback controllers with advanced controllers using disturbance observers, acceleration feedback and adaptation rules. Simulations and experiments demonstrate the trade-off between stable interactions, high bandwidths and low noise levels. Based on these tradeoffs, an application specific controller can be designed and tuned, based on desired interaction with the respective environment

A Rationale for Acceleration Feedback in Force Control of Series Elastic Actuators

Series elastic actuators (SEAs) have become fundamental components in robots that physically interact with unstructured environments and humans. Force control of SEAs is indeed an active area of research. This paper proposes a theoretical foundation for acceleration feedback (AF) in SEA force control. Even if AF already appeared in early works on SEAs, its advantages have not been properly highlighted in the literature. In particular, this paper formally motivates improved performance robustness and transparency exactly as if using a softer and lighter actuator. Taking advantage of AF, we propose a generic control architecture characterized by impressive performance robustness in spite of even high environment uncertainties. A comparison with state-of-the-art force control solutions such as disturbance observers and adaptive controllers is reported using a comprehensive set of simulations and experiments. As a result, AF methods exhibit the higher performance robustness and accuracy. Beside this outcome, AF controllers are extremely easy to implement and the rise of low-cost miniaturized accelerometers based on micro electromechanical systems (MEMS) represents an additional motivations for their use