Dielectric Elastomers (DE) seem to be a promising technology for the implementation of light and compact Variable Stiffness Actuators (VSAs), thanks to their large power densities, low costs and shock-insensitivity. Nonetheless, the development of DE-based VSA is not trivial owing to the relevant dissipative phenomena that affect the DE when subjected to rapidly changing deformations. In this context, the purpose of the present paper is to investigate the practical feasibility of DE-based VSA. As a case study, two conically-shaped actuators, in agonistic-antagonistic configuration, are modeled accounting for the visco-hyperelastic nature of the DE films. The model is then linearized and employed for the design of a stiffness controller. The control algorithm requires the knowledge of the actuator configuration (via a position sensor) and of the force exchanged with the environment (via a force sensor). An optimum full-state observer is then implemented, which enables both accurate estimation of the DE time-dependent behavior and adequate suppression of sensor measurement noise. At last, experimental results are provided together with the description of an effective electronic driver that allows an independent activation of the agonistic-antagonistic DE membranes.

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