Dynamics of conducting polymer actuators (2007)

Considerable research has been carried out to develop actuator technologies such as shape memory alloys, piezoelectric actuators, magnetostrictive actuators, contractile polymers and electrostatic actuators to use in devices such as human-like robots, micro robots and artificial organs for medical applications place of conventional actuators. Though there have been great advances, one or more of high electrical power, low efficiency or low strain limit the application of these new actuator technologies. Recently, conducting polymers have drawn considerable attention as a new class of advanced functional material for many applications based on the unique properties of electro-activity, conductivity and other physical or chemical properties. The applications being considered include batteries, photovoltaic devices, electro-chromic devices, ion selective membranes, electromagnetic interference shielding, radar absorption, electrical wires, corrosion inhibitors, electrochemical sensors and actuators. For actuator applications a comprehensive electro-chemo-mechanical model is needed to predict the mechanical output (displacement or force) from the electrical input (current and voltage) to enable control engineers to use these actuators in mechanical systems, new models which describe the dynamic response (actuator output/actuator input) as a function of time and frequency are required. The research in this thesis shows how such models can be derived by exploiting standard control theory analysis tools using Laplace transforms and State-Space techniques. For conducting polymers, such a model needs to include a description of the chemical process occurring between the conducting polymer, dopant and electrolyte. Such a model will enable the application of conducting polymer actuators in automation and robotic applications in which a predictive model is needed to design the control system and also identify the system performance to optimise the actuator characteristics. The aim of the research presented in this thesis is to create a comprehensive predictive model in order to track the output of a typical high- performance conducting polymer actuator: Polypyrrole Helix Tube Fibre Composite Actuator. The review of literature has revealed that previous models of polypyrrole actuators have meter, which has been assumed to be constant. In this work, it is shown that the strain to charge ratio is not always constant, particularly when a wide potential window is used. A master calibration curve approach has been devised to model the mechanical output when the strain to charge ratio is not constant. Secondly, the polypyrrole helix tubes were found to be viscoelastic, so the model was modified to include the viscoelastic (time-dependent) responses. Finally, the model was further modified to allow the viscoelastic parameters to vary with the applied potential. The latter two additions to the model greatly improve its predictive ability when the applied load is changing. To further investigate the effect of applied potential on the mechanical properties, a measurement method based on Quartz Crystal Microbalance technique has been developed. This method enables the thickness and shear modulus variation of polypyrrole thin films under electrochemical doping and un-doping (oxidation and reduction) to be studied. A complicated modulus shifting phenomenon in polypyrrole is revealed by these studies which depend strongly on the electrolyte. Finally, the results present a full description of the electromechanical characterisation of polypyrrole helix tubes considering the interacting effects of electrochemical and electromechanical parameters. This description may enable further optimization of the design and performance of polypyrrole helix tube actuators.