An Enhanced Fractional Order Model of Ionic Polymer-Metal Composites Actuator

Ionic polymer-metal composites (IPMCs) are electroactive polymers which transform the mechanical forces into electric signals and vice versa. The paper proposes an enhanced fractional order transfer function (FOTF) model for IPMC membrane working as actuator. In particular the IPMC model has been characterized through experimentation, and a more detailed structure of its FOTF has been determined via optimization routines. The minimization error was attained comparing the simple genetic algorithms with the simplex method and considering the error between the experimental and model derived frequency responses as cost functions. 1. Introduction In the last decade a new breed of polymers, known as electroactive polymers or more commonly EAPs, has emerged thanks to their electroactive capabilities [1]. Ionic polymer-metal composites [2] belong to this material class. They bend if they are solicited by an external electric field and they act as motion sensor if an external deformation is applied. They are characterized by several interesting properties such as high compliance, lightness, and softness. IPMCs exploit ionic polymers for electrochemical and mechanical transduction and noble metals as electrodes, and they represent a valid alternative to the classic actuators and/or sensors. They can be cut in any shape and size, they are characterized by large deformations applying very low level of voltage, and they can work both in a humid or in a wet environment. These properties make them particularly attractive for possible applications in very different fields such as robotics, aerospace, and biomedicine [3, 4]. An intense research has been currently carried out to improve the IPMC performances in terms of power consumption, developed force, and deformation [5]. Moreover the research efforts have been spent in finding improved models able to predict the IPMCs behavior both as actuators and sensors [6]. In the literature several models describing the IPMC as actuator can be found. In detail, they can be divided into three categories: white box, black box, and grey box. The first category, called white box, or physical models, is based on the underlying physical mechanisms of the IPMC to develop a system of equations that fully describes the device response [7]. Numerical implementation via methods as finite element analysis can be found in the literature [8]. Some difficulties are presented with physical modeling of IPMC transducers, such as a complete knowledge of the chemical and/or physical mechanisms involved in the electromechanical transduction and the