Continuous Robust Control for Series Elastic Actuator With Unknown Payload Parameters and External Disturbances

In this paper, the torque tracking control problem for a class of series elastic actuators(SEAs) in the presence of unknown payload parameters and external disturbances is investigated. The uncertainties/disturbances rejection problem for SEAs is addressed from the view of a continuous nonlinear robust control development. Specifically, based on the analysis of a nonlinear SEA, the generic dynamics of SEA systems is described and a novel nonlinear control framework for SEAs is constructed. Then a RISE(robust integral of the sign of the error)-based second-order filter is introduced to synthesize the control law. Moreover, the control performance is theoretically ensured by Lyapunov analysis. Finally, some experimental results are included to demonstrate the superior performance of the proposed control method, in terms of transient response and robustness.

Augmented Virtual Stiffness Rendering of a Cable-driven SEA for Human-Robot Interaction

Human-robot interaction(HRI) is fundamental for human-centered robotics, and has been attracting intensive research for more than a decade. The series elastic actuator(SEA) provides inherent compliance, safety and further benefits for HRI, but the introduced elastic element also brings control difficulties. In this paper, we address the stiffness rendering problem for a cable-driven SEA system, to achieve either low stiffness for good transparency or high stiffness bigger than the physical spring constant, and to assess the rendering accuracy with quantified metrics. By taking a velocity-sourced model of the motor, a cascaded velocity-torque-impedance control structure is established. To achieve high fidelity torque control, the 2-DOF(degree of freedom) stabilizing control method together with a compensator has been used to handle the competing requirements on tracking performance, noise and disturbance rejection,and energy optimization in the cable-driven SEA system. The conventional passivity requirement for HRI usually leads to a conservative design of the impedance controller, and the rendered stiffness cannot go higher than the physical spring constant. By adding a phase-lead compensator into the impedance controller,the stiffness rendering capability was augmented with guaranteed relaxed passivity. Extensive simulations and experiments have been performed, and the virtual stiffness has been rendered in the extended range of 0.1 to 2.0 times of the physical spring constant with guaranteed relaxed passivity for physical humanrobot interaction below 5 Hz. Quantified metrics also verified good rendering accuracy.

Redundancy in Biology and Robotics:Potential of Kinematic Redundancy and its Interplay with Elasticity

Redundancy facilitates some of the most remarkable capabilities of humans,and is therefore omni-present in our physiology.The relationship between redundancy in robotics and biology is investigated in detail on the Series Elastic Dual-Motor Actuator(SEDMA),an actuator inspired by the kinematic redundancy exhibited by myofibrils.The actuator consists of two motors coupled to a single spring at the output.Such a system has a redundant degree of freedom,which can be exploited to optimize aspects such as accuracy,impedance,fault-tolerance and energy efficiency.To test its potential for human-like motions,the SEDMA actuator is implemented in a hopping robot.Experiments on a physical demonstrator show that the robot's movement patterns resemble human squat jumps.We conclude that robots with bio-inspired actuator designs facilitate human-like movement,although current technical limitations may prevent them from reaching the same dynamic and energetic performance.