Double actuation architectures for rendering variable impedance in compliant robots: a review

Novel compliant actuation systems have been developed in recent years for a variety of possible advantages,such as establishing a safe human–robot interaction, increasing energy efficiency, reducing theeffects of impacts and even for the development of neuro-inspired robotic platforms to be used in humanmotor control studies.In this rapidly growing and transversal research field, systems involving more than one active element(typically motors) for each actuated degree of freedom are being investigated to allow separate positionand impedance regulations. Considering the wide range of applications and the large number of differentarrangements deriving from the combination of two active elements and passive elastic components, severalactuation architectures have been devised.This paper reviews state-of-the-art rotary variable impedance units incorporating two separate motors.Existing devices are grouped in three main categories. A critical and comparative analysis of the most relevantfeatures is carried out, also based on most representative prototypes. Recently proposed methodologiesand evaluation criteria for design optimization are illustrated and perspectives on potentialapplications of double actuation systems are presented.