Modeling, simulation, and control of a polypyrrole-based conducting polymer actuator

Abstract

A detailed model was developed for an ionic electro-active polymer (EAP) actuator. The electrical and chemical domains of the system were modeled using a simple electrical circuit. Ionic charge storage within the polymer was described using a linear reticulated model. This model improves upon the continuum diffusive model introduced in prior work by providing a low order model of diffusion that can be analyzed in the context of modern and classical control methods. Additionally, the reticulated diffusion model describes the dynamics of ionic charge distribution within the polymer, which enables a more precise calculation of electromechanical coupling. An interesting observation of ionic electro-active polymers is that they exhibit enormous asymmetry in coupling from electrical to mechanical domains. While electrical potentials produce large linear displacements (5% strain or greater), uniaxially-applied mechanical loads result in a negligible electrical back effect. This is surprising, suggesting that there are huge entropic losses when applying mechanical loads. After examining the mechanics of the system it was theorized that the apparent lack of coupling is actually the result of the Poisson Effect, which causes changes in the volume of an object when uniaxial loads are applied. A derivation of the stored electrical energy and strain energy led directly to a set of constitutive equations that are able to account for the asymmetric coupling observed in EAP. The solution to the uniaxial loading boundary condition was developed fully and compared to prior work. Experimental results from an EAP actuator composed of polypyrrole, a widely-used conducting polymer, validate the electro-mechanical coupling model. MATLAB was used to simulate the

(cont.) response of the actuator and the results compared to the experimental data. Results verify that the model accurately describes the electrical, mechanical, and coupled behavior of the system. The correlation between the model and experimental data is very good for electrically-induced strains up to 3% and applied potentials up to 1 Volt above the potential of zero charge (PZC); these are within the typical operating range of polypyrrole. The model is sufficiently simple to allow real-time control while also exceeding prior models in its ability to predict polymer behavior in normal operating ranges.