Modeling and Position Control of an Electromechanical Actuator Based on a Mass-Spring-Biased EAP System Gianluca Rizzello 1 , David Naso 1 , Alexander York 2 , and Stefan Seelecke 2 Politecnico di Bari, Department of Electrical Engineering and Electronics, Italy Saarland University, Department of Mechatronics, Saarbrücken, Germany Contact author: seelecke@mx.uni-saarland.de Abstract This paper deals with the modeling and position control of an ElectroActive Polymer (EAP) actuator system consisting in a combination of an EAP membrane and a mass-spring biasing mechanism. First, a description of the biasing system is provided, and a physical-based dynamic model is derived for the whole system. Then, the model is used for the identification of an experimental actuator, and for the subsequent design of a position control. A simple but effective method is presented in order to compensate the model nonlinearities and achieve fast positioning. Simulation stage is used for testing several control strategies, in order to choose the most suitable one to be integrated in the experimental system. 1. Introduction Dielectric ElectroActive Polymers (EAPs) have the capacity to respond to an electrical excitation with a significant shape elongation. Their features include also high mechanical flexibility, ability to be driven by DC voltages, high energy density, structural simplicity, low response time, no acoustic noise and, in most cases, low costs [1]. These characteristics make EAPs the perfect candidates for the realization of mechatronic actuators. As smart materials are versatile and exhibit very high performance, advanced CAD and rapid prototyping softwares are required in order to optimize mechanical design and electronic hardware. Simulation softwares play a fundamental role both in design and control of the system. Physical models can be used for simulating the system behavior, both for control purposes and for the optimization of the system parameters in order to achieve a desirable performance. A biasing mechanism is usually coupled with the smart material [2]. By using masses, springs or nonlinear springs, the actuator system can be tuned in order to obtain a larger displacement, a larger force or a different dynamic behavior. This work presents a model and a position control for a Dielectric Elastomer Actuator (DEA) biased with a mass-spring system. The model takes into account of the physical phenomena occurring during the dynamic actuation process, and considers also the complexity due to the actuator geometry. The model is identified and validated with many experiments, by using a two-steps identification procedure which aims at characterizing both static and dynamic behavior of the actuator. Significant works on modeling of EAP materials can be found in [4-6]. Other works also investigated the dynamic response of EAP-based actuator systems. Particular examples are given in [7-9]. Most of the literature on EAP, however, focus on describing only actuators with elementary geometry, while a large contribution of the nonlinearities may arise as a consequence of the actuator configuration [3]. The effects of a biasing spring applied to an annular DEA are investigated in [2],[10]. Both these works do not consider the dynamic effects introduced by the spring, which will be considered during the development of the current work, as an extension of the results in [3]. As a final step of this work, a robust position control system is designed and implemented. A simple compensation strategy against the model nonlinearities is proposed. Several experiments are performed to show how the proposed compensation improves significantly the closed loop response. Some literature on EAP position control can be found in [11-13]. The control strategy proposed here uses a different approach from previous literature, based on a robust performance against nonlinearities formulation and a H ∞ synthesis. 2. Dielectric elastomer actuator The DEA analyzed in this work is a circular diaphragm actuator. The outer frame and the inner circular plate are made of rigid plastic, while the intermediate annular ring is the DE silicone membrane (Figure 1.a). The polymeric film is mechanically pre- stretched in the radial direction. Compliant carbon electrodes are printed all over the active area, allowing the polymer to be electrically actuated. The maximum applicable voltage is 2.5 kV. 2.1. Biasing mechanism In order to improve the actuator stroke and force, a biasing system is usually attached on the central plate. In the current work the biasing mechanism is obtained with a combination of a mass and a linear spring. A section sketch of the EAP before and after the biasing is shown