Alexander York

Experimental characterization of the hysteretic and rate-dependent electromechanical behavior of dielectric electro-active polymer actuators

Dielectric electro-active polymers (DEAPs) can achieve substantial deformation (>300% strain) while sustaining, compared to their ionic counterparts, large forces. This makes them attractive for various actuation and sensing applications such as in light weight and energy efficient valve and pumping systems. Many applications operate DEAP actuators at higher frequencies where rate-dependent effects influence their performance. This motivates the seeking of dynamic characterization of these actuators beyond the quasi-static regime. This paper provides a systematic experimental investigation of the quasi-static and dynamic electromechanical properties of a DEAP actuator. In order to completely characterize the fully coupled behavior, force versus displacement measurements at various constant voltages and force versus voltage measurements at various fixed displacements are conducted. The experiments are conducted with a particular focus on the hysteretic and rate-dependent material behavior. These experiments provide insight into the electrical dynamics and viscoelastic relaxation inherent in DEAP actuators. This study is intended to provide information, including high frequency performance analysis, useful to anyone designing dynamic actuator systems using DEAPs.

Experimental comparison of bias elements for out-of-plane DEAP actuator system

This paper presents an experimental comparison of three different biasing elements utilized to produce out-of-plane actuation for a diaphragm dielectric electro-active polymer (DEAP). A?hanging mass, a linear coil spring, and a nonlinear (bistable) mechanism are individually paired with an unloaded DEAP actuator. High voltage (2.5?kV) is applied to the DEAP and the out-of-plane stroke of the DEAP is measured. The actuator stroke is notably different for each bias element. Results show that as the bias element stiffness increases, the actuator stroke decreases. However, the bistable element, when coupled with the DEAP, demonstrated improved actuation in a specific range of DEAP pre-deflection. Not only was the stroke larger for this case, the stroke also did not attenuate at higher voltage frequencies as much as the linear coil spring bias elements. This study demonstrates a promising method for obtaining high performance DEAP actuators in the future.

Closed loop control of dielectric elastomer actuators based on self-sensing displacement feedback

This paper describes a sensorless control algorithm for a positioning system based on a dielectric elastomer actuator (DEA). The voltage applied to the membrane and the resulting current can be measured during the actuation and used to estimate its displacement, i.e., to perform self-sensing. The estimated displacement can be then used as a feedback signal for a position control algorithm, which results in a compact device capable of operating in closed loop control without the need for additional electromechanical or optical transducers. In this work, a circular DEA preloaded with a bi-stable spring is used as a case of study to validate the proposed control architecture. A comparison of the closed loop performance achieved using an accurate laser displacement sensor for feedback is also provided to better assess the performance limitations of the overall sensorless scheme.

Modeling, Identification, and Control of a Dielectric Electro-Active Polymer Positioning System

This paper deals with a positioning system based on a dielectric electro-active polymer membrane. The motion is generated by the deformation of the membrane caused by the electrostatic compressive force between two compliant electrodes applied on the surface of the polymer. This paper proposes a detailed electro-mechanical nonlinear model of the system, which is subsequently used to develop (in both time and frequency domains) various model-based feedback control laws. Accurate modeling is useful to compensate the nonlinear behavior of the actuator (caused by the material characteristics and geometry) and obtain PID controllers providing precise tracking of steps or sinusoidal reference signals. The various design strategies are compared on various experimental tests.

Modeling and experimental validation of a bi-stable out-of-plane DEAP actuator system

This paper presents modeling and experimental validation of a small profile, scalable DEAP actuator system. The actuator system consists of a bi-stable mechanism (a negative-rate bias spring, or NBS) coupled with an out-of-plane dielectric electro-active polymer (DEAP). The NBS biases the DEAP allowing actuation when the voltage is cycled and is shown to have a major impact on the overall system performance. Particularly in comparison with conventional linear springs, the NBS-biased actuator exhibits a considerably larger displacement stroke. A first order model of the NBS–DEAP coupled system is developed based on minimization of the systems potential energy. This approach allows for the determination of quasi-static force equilibria in the presence of multiple stable positions. The model is validated with experimental data and provides insight into system trends and related parameter optimization.

Systematic approach to development of pressure sensors using dielectric electro-active polymer membranes

Dielectric electro-active polymers (DEAPs) have become attractive materials for various actuation and sensing applications due to their high energy and power density, high efficiency, light weight, and fast response speed. However, commercial development has been hindered due to a variety of constraints such as reliability, non-linear behavior, cost of driving electronics, and form factor requirements. This paper presents the systematic development from laboratory concept to commercial readiness of a novel pressure sensing system using a DEAP membrane. The pressure sensing system was designed for in-line pressure measurements for low pressure applications such as health systems monitoring. A first generation sensor was designed, built and tested with a focus on the qualitative capabilities of EAP membranes as sensors. Experimental measurements were conducted that demonstrated the capability of the sensor to output a voltage signal proportional to a changing pressure. Several undesirable characteristics were observed during these initial tests such as strong hysteresis, non-linearity, very limited pressure range, and low fatigue life. A second generation prototype was then designed to remove or compensate for these undesirable characteristics. This prototype was then built and tested. The new design showed an almost complete removal of hysteretic non-linear effects and was capable of operating at 10 × the pressure range of the initial generation. This new design is the framework for a novel DEAP based pressure sensor ready for commercial applications.

An electro-mechanically coupled model for the dynamic behavior of a dielectric electro-active polymer actuator

Dielectric electro-active polymer (DEAP) technology holds promise for enabling lightweight, energy efficient, and scalable actuators. The circular DEAP actuator configuration (also known as cone or diaphragm actuator) in particular shows potential in applications such as pumps, valves, micro-positioners and loudspeakers. For a quantitative prediction of the actuator behavior as well as for design optimization tasks, material models which can reproduce the coupled electromechanical behavior inherent to these actuators are necessary. This paper presents a non-linear viscoelastic model based on an electro-mechanical Ogden free energy expression for the DEAP. The DEAP model is coupled with a spring/mass system to study the dynamic performance of such a representative system from static behavior to 50 Hz. The system is identified and validated by several different experiments.

Modeling of the effects of the electrical dynamics on the electromechanical response of a DEAP circular actuator with a mass–spring load

This paper presents a modeling approach of an actuator system based on a dielectric electro-active polymer (DEAP) circular membrane mechanically loaded with a mass and a linear spring. The motion is generated by the deformation of the membrane caused by the electrostatic compressive force between two compliant electrodes applied on the surface of the polymer. A mass and a linear spring are used to pre-load the membrane, allowing stroke in the out-of-plane direction. The development of mathematical models which accurately describe the nonlinear coupling between electrical and mechanical dynamics is a fundamental step in order to design model-based, high-precision position control algorithms operating in high-frequency regimes (up to 150 Hz). The knowledge of the nonlinear electrical dynamics of the actuator driving circuit can be exploited during the control system design in order to achieve desirable features, such as higher modeling accuracy for high-frequency actuation, self-sensing or control energy minimization. This work proposes a physical model of the DEAP actuator system which couples both electrical and mechanical dynamics occurring during the actuation process. By means of numerous experiments, it is shown that the model can be used to predict both actuator current and displacement, and therefore to increase the overall displacement prediction accuracy with respect to actuator models which neglect electrical behavior.

A Self-Sensing Approach for Dielectric Elastomer Actuators Based on Online Estimation Algorithms

This paper develops a position self-sensing approach for a motion actuator based on a dielectric elastomer membrane. The proposed method uses voltage and current measurements to estimate the electrical resistance and capacitance online by means of a high-frequency low-amplitude voltage component injected in the actuation signal. The actual deformation is subsequently reconstructed using a model-based estimate of the electrical parameters implemented on a field programmable gate array platform (FPGA) with a sampling frequency of 20 kHz. The main peculiarity of the approach is the use of recursive identification and filtering algorithms that avoid the need of charge measurements. The self-sensing algorithm is extensively validated on a precision linear-motion actuator, which uses a nonlinear biasing system to obtain large actuation strokes.

Self-sensing in dielectric electro-active polymer actuator using linear-in-parametes online estimation

This paper presents a self-sensing methodology for Dielectric ElectroActive Polymer actuators. The proposed approach is based on using DEAP voltage and current to estimate electrical resistance and capacitance, and using the latter to reconstruct the actuator deformation. For the estimation of the electrical parameters, the performance of two standard linear regression algorithms are compared, i.e. standard Least Mean Squares (LMS) and Recursive Least Squares (RLS). Some filtering techniques are also suggested in order to improve the quality of the estimation. The full algorithm is first illustrated in detail and then validated on an experimental actuator prototype, consisting in a DEAP membrane combined with a bi-stable biasing element which enables large actuation stroke.