Characterization, Modeling, and Control of the Nonlinear Actuation Response of Ionic Polymer Transducers
Kothera, Curt S.
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Ionic polymer transducers are a class of electroactive polymer materials that exhibit coupling between the electrical, chemical, and mechanical domains. With the ability for use as both sensors and actuators, these compliant, light weight, low voltage materials have the potential to benefit diverse application areas. Since the transduction properties of these materials were recently discovered, full understanding of their dynamic characteristics has not yet been achieved. This research has the goal of better understanding the actuation response of ionic polymers. A specific emphasis has been placed on investigating the observed nonlinear behavior because the existing proposed models do not account for these characteristics. Employing the Volterra representation, harmonic ratio analysis, and multisine excitations, characterization results for cantilever samples showed that the nonlinearity is dynamic and input-dependent, dominant at low frequencies, and that its influence varies depending on the solvent. It was determined that lower viscosity solvents trigger the nonlinear mechanisms at higher frequencies. Additionally, the primary components of the harmonic distortion appear to result from quadratic and cubic nonlinearities. Using knowledge gained from the characterization study, the utility of different candidate system structures was explored to model these nonlinear response characteristics. The ideal structure for modeling the current-controlled voltage and tip velocity was shown to consist of an underlying linear system with a dynamic input nonlinearity. The input nonlinearity is composed of a parallel connection of linear and nonlinear terms, where each nonlinear element has the form of a Hammerstein system. This system structure was validated against data from measured time and frequency responses. As a potential application, and consequently further validation of the chosen model structure, a square-plate polymer actuator was considered. In this study, the plate was clamped at the four corners where a uniform input was applied, measuring the center-point displacement. Characterization and modeling were performed on this system, with results similar to the cantilever sample. Applying output feedback control, in the form of proportional-integral compensation, showed that accurate tracking performance could be achieved in the presence of nonlinear distortions. Special attention was extended here to the potential application in deformable mirror systems.
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