Antagonistically actuated compliant joint: Torque and stiffness control

The current research effort in the design of lightweight and safe robots is resulting in increased interest for the development of variable stiffness actuators. Antagonistic pneumatic muscle actuators (pMAs) have been proposed for this purpose, due to their inherent nonlinear spring behavior resulting from both air compressibility and their nonlinear force-length relation. This paper addresses the simultaneous torque and stiffness control of an antagonistically actuated joint with pneumatic muscles driven by compact, fast-switching solenoid valves. This strategy allows compensation of unmodeled joint dynamics while adjusting the joint stiffness depending on the task requirements. The proposed controller is based on a sliding mode force control applied to an average model of the valve-pneumatic muscle system. This was necessary to cope with both the well known model uncertainties of the pMA and the discontinuous on-off behavior of the solenoid valves. Preliminary experimental results verified the effectiveness of the proposed implementation

Robust Estimation of Variable Stiffness in Flexible Joints

We consider the problem of estimating on line the nonlinear stiffness of flexible transmissions in robot joints, with special reference to actuation devices with adjustable stiffness in serial configuration. These joints are characterized by a principal motor for controlling the link motion and secondary motor for adjusting the stiffness. In this actuation configuration, the flexible transmission undergoes relatively small deformations and the stiffness estimation problem is more challenging due to poor excitation conditions. We improve our previous general approach, combining a residual-based flexibility torque estimator that uses also a kinematic Kalman filter to handle discretization and quantization errors with an enhanced recursive least squares algorithm that does not suffer from lack of persistent excitation. As a result, stiffness is estimated in a more robust way using only position measurements on the motor sides and motor dynamic parameters. The performance of the proposed estimation method is illustrated through simulations and experiments on the AwAS joint developed at IIT

A decoupled Impedance observer for a Variable Stiffness Robot

This paper focuses on the estimation of the impedance for a Variable Impedance Actuator (VIA) through torque and position measurements. Despite the recent development of several VIA, impedance control is not yet implemented in closed loop because of the difficulty of obtaining in real-time measurements of time-varying impedance. The estimation algorithm is proposed as an alternative approach to the standard procedures of impedance identification, to robustly tolerate the variability of the mechanical stiffness due, for example, to model uncertainties. The impedance estimator is therefore implemented on the Actuator with Adjustable Stiffness (AwAS). The effectiveness of the proposed estimator is proved through simulation and experimental results

On-line estimation of variable stiffness in flexible robot joints

Variable stiffness actuators (VSAs) are currently explored as a new actuation approach to increase safety in physical human-robot interaction (pHRI) and improve dynamic performance of robots. For control purposes, accurate knowledge is needed of the varying stiffness at the robot joints, which is not directly measurable, nonlinearly depending on transmission deformation, and uncertain to be modeled. We address the online estimation of transmission stiffness in robots driven by VSAs in antagonistic or serial configuration, without the need for joint torque sensing. The two-stage approach combines (i) a residual-based estimator of the torque at the flexible transmission, and (ii) a recursive least squares stiffness estimator based on a parametric model. Further design refinements guarantee a robust behavior in the lack of velocity measures and in poor excitation conditions. The proposed stiffness estimation can be easily extended to multi-degree-of-freedom (multi-DOF) robots in a decentralized way, using only local motor and link position measurements. The method is tested through extensive simulations on the VSA-II device of the University of Pisa and on the Actuator with Adjustable Stiffness (AwAS) of IIT. Experiments on the AwAS platform validate the approach