Multi-scale mechanical modelling of a tubular actuator with compliant metal electrodes

Rolled tubular dielectric elastomer-based actuators provide larger forces by making multiple thin layers of the dielectric elastomer apply their actuation forces in parallel. Such a rolled actuator is currently being marketed by Danfoss PolyPower A/S. This actuator is core-free has no pre-strain and is free-standing The dielectric elastomer (DE) material used to construct the actuator combines a silicon elastomer with compliant metal electrode technology that provides unidirectional motion. In this contribution the focus is on the three dimensional (3-D) modelling of this core-free tubular actuator. 3-D modelling is achieved by representing the actuator by two two-dimensional (2-D) models thus ensuring that the resulting 2-D models can be handled numerically by finite element simulation and analysis. Initially the Voltagestrain and Strain-blocking force characteristics of the actuators are investigated using only the modelled 'active' area, the electrode covered part of the actuator. The 'passive' area, which reduces the total force provided by the 'active' area, is modelled by approximating the steady-state characteristics of the passive area as a spring with a specified stiffness. The spring stiffness is related directly to the geometry and elastic modulus of the elastomer material in the 'passive' area. Using this simple model in conjunction with the force provided by the 3-D 'active' area model the 'effective force' of the actuator can be found. The use of this 'effective force' expression provides good agreement with the experimental forcestrain data obtained from a Danfoss PolyPower tubular actuator.