Research On Characteristics Of Cylindrical Dielectric Electoactive Polymer (EAP) Actuator

Dielectric electroactive polymer(EAP) is a kind of high molecular polymer with viscoelasticity, which can convert mechanical energy to electrical energy. With the advantages of large deformation, zero noise, smooth motion, high energy density, low cost, faster response, good environment adaptability, etc, EAPs offer potential application in the numerous fields, such as bionic robot, rehabilitation training, etc. Supported by the National and Province Natural Science Fund Projects, the visco-hyperelastic behavior of dielectric EAP membranes(VHB4910) are studied deeply, and the theoretical and experimental researches on dielectric EAP cylindrical actuator also carried out, so the references and guidances can be provided for the design and manufacture of dielectric EAP actuator eventually.According to the continuum mechanics theory, the relationships between the stress and strain of EAP membrane are derived using typical strain energy function. A nonlinear elastic model with a few elasticity moduli is put forward. The apparatus for equi-biaxial tensile are designed to fulfill the fixed-corner-points equi-biaxial tensile experiments. The model parameters are derived through fitting the tensile data to the models, then the fitting precision compared. The equi-biaxial tensile processes analyzed with finite element software ABAQUS, the results show that the nominal stress error is small.Applying the general Maxwell viscoelastic model to deduce the Prony series of the quasi linear viscoelastic model, the coefficients are fitted out through implementing the biaxial relaxation experiments. The biaxial tensile-relaxation experiment and simulation are also carried out while the results are consistent with each other well. The nonlinear viscoelastic model based on Ogden model elasticity networks for EAP membrane is presented, the parameters fitted out from the data of the equi-biaxial tensile experiment which is performed under different stretch speed. After deducing the viscoelastic electromechanical coupling equations for EAP membrane, the working stability is analyzed when the membrane is in free boundary and unilateral constraint state. The results show that: in the free boundary state, instability will occur when the thickness decreases more than 37%; the viscous flow can improve the stability and increase the deformation; pre-stretch can also enlarge the relative critical stretch ratio, and lower the excitation voltage(or nominal electric field).Analyze the structure and stress state of the EAP film of the cylindrical actuator, as well as the influences of the factors(the radial shrinkage, the interlayer pressure after the EAP film wound, etc.) on the axial stress. Study the effects of axial elongation, the radius-thickness ratio of the innermost EAP film and the number of the winding layer on the thickness. Based on the ideal electromechanical coupling equations of an EAP film, the properties of the relationship between voltage and displacement is analyzed, and the results are verified valid through displacement experiment. Introducing the failure modes of the actuators, and the influences of structure parameters(such as pre-stretch, wound layer number and the parameters of the spring) within the valid working range of the actuator on the output displacements and forces, some guiding principles are derived for designing the actuators.The dynamic model for the cylindrical actuator is presented to analyze its dynamic responses when activated by the step, ramp and period voltage. Under the step voltage activation, increasing the voltage can generate large displacement and viscous deformation process become obvious, and the pre-stretch can realize larger instantaneous elastic deformation while the viscous deformation is small. Larger instantaneous elongation can be obtained through reducing the rise rate of the ramp excitation voltage. Changing the relative offset of periodic voltage, the different displacement outputs generate, while increasing the frequency will inhibit the output amplitude. The dynamic experiments show that the two-time-parameter viscoelastic model can describe the dynamic displacement more accurately, and the voltage frequency influnce the displacement amplitude of actuator. It can also been verified that the vibration center offset and amplitude reduced due to “excitation dead zone”.Equivalent circuit model for cylindrical actuator is constructed, and the parameters are measured. Through calculation the relative dielectric constant, εr=3.24, of VHB membrane in the cylinder actuator is nearly identical to the experiment results. The leakage current experiments are carried out and the results show that the leakage current is an important factor that affects the actuation efficiency. In quasi-static actuating cycle, the electromechanical conversion efficiency of the actuator is significantly affected due to the energy stored by capacitor and the current leakage.