Variable Impedance Actuator with Exponential Elasticity for Flexible-Joint-Robot and Estimation of the Joint Impedance

New generation robots, meant for physical human robot interactions, are no longer rigid; it has become soft in terms of introduction of considerable flexibility at the actuated joints, flexibility of the links and in terms of compliant coverings. Safety requirement becomes primary in situations where physical interactions occur, calling for compliance; but accuracy and control bandwidth get compromised. While it has been established in literature that flexible-link-robots are difficult to control, it is found feasible to recover some of the lost bandwidth by varying stiffness in flexible-joint-robots, maintaining safety of human during interaction. This article addresses development of such a flexible robot joint actuation system with stiffness/impedance variability. This variability of stiffness is achieved passively by the flexible transmission interposed between the prime mover and the actuated link, and in doing so, nonlinearity in the elastic transmission characteristic becomes essential. The Variable Stiffness Actuation (VSA) system of this article employs an elastic element having an exponential force-displacement characteristic, which has the property of stiffness varying linearly with transmission force. This property is favourably utilized in the estimation of stiffness, and in turn can be used in control of stiffness. The variable stiffness actuation is realized here by assembling two transmissions in agonist-anatagonistic arrangement in order to achieve simultaneous control of both joint-motion and stiffness, resembling biological musculo-skeletal system. By adding nonlinear damping elements in parallel to the elastic transmission, variability in mechanical impedance has been achieved. Joint stiffness is computed with the estimated stiffness of individual transmissions, which are obtained experimentally. Extended Kalman Filter is employed for the estimation of stiffness and other impedance components. Results are reported in support of the effectiveness of the joint actuator in achieving variability in stiffness and impedance and their estimation.