Ionic polymers are compliant, light weight materials that operate under low voltage levels as transducers. They can be used as both sensors and actuators for various applications, primarily those involving flexible structures. The electromechanical transduction properties of these materials were discovered just over a decade ago, spawning the development of ionic polymer research. While the debate continues over the dominant physical mechanisms of actuation, several model forms have been proposed. The majority of these existing models are stated to be linear approximations and some were derived with input-dependence. However, nonlinear characteristics have been observed in both the electrical and mechanical response of cantilever actuators, including harmonic distortion in the time-domain and magnitude scaling of the frequency response. Characterization results indicate that the nonlinear mechanisms are dynamic since they have dominance at low frequencies, but are essentially negligible as the excitation frequency increases. This research uses knowledge gained from the characterization results to
develop a dynamic model that can predict the observed nonlinear behavior. The empirical model is constructed from input-output data collected using a Gaussian input current signal and is validated using the measured frequency response function and single-frequency sinusoidal responses.
Ionic polymer transducers exhibit coupling between the electrical, chemical, and mechanical domains, allowing their use as both sensors and actuators. Because of their compliance, light weight, and low voltage operation, ionic polymers have spawned an area of much research, although their fundamental mechanisms are still open for debate. While most of the existing models provide linear, dynamic approximations of the response, nonlinear characteristics have been observed experimentally. Some of these include the introduction of permanent strain in the step response and distortion in the forced response to harmonic excitations. Recent experimental results have shown that the solvent plays a significant role in the dynamic response of ionic polymer actuators. Given a single-frequency input voltage, the major difference from changing solvent materials was concluded to be a nonlinear distortion with varying influence, seen in both the actuation current and tip velocity measurements. These results compared the response of a water-based sample to a sample prepared with the ionic liquid EMI-Tf, where it was found that the voltage-to-current relationship was much more nonlinear in the water sample, while it was predominantly linear with the ionic liquid sample. This research looks to further explore this nonlinear distortion by incorporating a larger set of candidate solvent materials and investigating the impact of how changing properties affect the overall response. System identification techniques using the Volterra series are employed to aid in the characterization of the harmonic distortion. The knowledge gained in this study will provide useful information about the nature of the nonlinearity and some of the factors that affect its relative influence, which will assist physical model development.
Ionic polymers are a class of electromechanically coupled materials that can be used as flexible transducers. When set up in the cantilever configuration, the actuators exhibit a large bending deflection when an electric field is applied across their thickness. Being a relatively new research topic, the governing physical and chemical mechanisms are not yet fully understood. Experimental results have demonstrated nonlinear dynamic behavior. The nonlinear dynamics can be seen in the response of current, displacement, and velocity of the actuator. This work presents results for the nonlinear identification of ionic polymer actuator systems driven at a specific frequency. Identification results using a 5th-degree Volterra expansion show that the nonlinear distortion can be accurately modeled. Using such a high power in the series expansion is necessary to capture the most dominant harmonics, as evidenced when examining the power spectral density of the response. An investigation of how nonlinearities enter into the response is also performed. By analyzing both the actuation current and tip velocity, results show that both the voltage to current and current to velocity stages influence the nonlinear response, but the voltage to current stage is more dominantly nonlinear.