I. Introduction
In human–robot interaction area, compliance has an important role [1]–[4]. Physical passive compliance decouples the inertia of the actuator, which normally has a large value, from inertia of the link, and it decreases the stiffness of the joint [5]–[12]. Adaptable compliance indeed helps to improve the performance in different circumstances. For instance, energy efficiency can be improved by properly tuning the stiffness [13] [14]. In the literature, different design approaches for actuation systems that allow the stiffness regulation are proposed. These variable compliance actuation systems typically employ two actuator units in combination with passive elastic elements to control, independently, the compliance and the equilibrium position of the actuated joint. These systems can be implemented by both antagonistic and series configurations.