An electromechanical model for characterizing sensing and actuating performance of unimorphs based on plain dielectric polymers

An energy minimization approach is used to determine the electromechanical response of unimorphs composed of active and passive layers of plain dielectric polymers which exhibit electric field induced strains only due to Maxwell stresses. The deformed shape of the unimorph, its coupling efficiency, stall force and sensitivity of capacitance of the active layer to the tip force are determined as a direct function of the applied voltage difference, external forces, elastic moduli of the layers, dielectric constant of the active layer, and geometric characteristics of the unimorph. Expressions for tip displacement and stall force derived here are shown to describe, reasonably accurately, some of the experimental results published in literature. For the unimorphs considered here, it is shown that the sensing and actuating performance characteristics are directly proportional to the dielectric constant and the elastic compliance of the layers. While the sensing characteristic (sensitivity of capacitance of active layer to the tip force) is independent of the applied voltage, the actuating characteristics (tip displacement, coupling efficiency and stall force) are quadratically proportional to the applied voltage. The electromechanical coupling in dielectric polymers, occurring from Coulomb interaction between charges on the electrodes, is fundamentally different from that in piezoelectric, ionic, and electrostrictive polymers. Therefore, the models reported in literature, characterizing the deformation behavior of beam bending actuators based on these other electroactive polymers, are not applicable for the class of unimorphs considered in this work. (C) 2011 Elsevier B.V. All rights reserved.