Circular Dielectric Elastomer Actuators Research Articles

Influence of Active-to-Passive Ratio on the Deformation in Circular Dielectric Elastomer Actuators.

To further improve the performance of dielectric elastomer actuaotrs (DEAs), the development of novel elastomers with enhanced electro-mechanical properties is focal for the advancement of the technology. Hence, reliable techniques to assess their electro-mechanical performance are necessary. Characterization of the actuator materials is often achieved by fabricating circular DEAs with the pre-stretch of the membrane fixed by a stiff frame. Because of this set-up, the electrode size relative to the carrier frame's dimension has an impact on actuator strain and displacement. To allow for comparable results across different studies, the influence of this effect needs to be quantified and taken into account. This paper presents an in-depth study of the active-to-passive ratio by proposing two simplified analytical models for circular DEA and comparing them. The first model is taking the hyperelastic material properties of the dielectric film into account while the second model is a linear elastic lumped parameter model based on the electro-mechanical analogy. Both models lie in good agreement and show a significant linear impact of the radial active-to-passive ratio on the electro-active strain and a resulting maximum of displacement around 50% radial coverage ratio. These findings are validated by experiments with actuators fabricated using silicone membranes. It is shown that the electrode size is not only an important parameter in the experimental design, but in some cases of higher significance for the accuracy of analytical models than the hyperelastic properties of the material. Furthermore, it could be shown that a radial coverage ratio of around 50% is desirable when measuring displacement as it maximizes the displacement and lowers the impact of deviations in electrode sizes due to fabrication errors.

Material characterization of dielectric elastomers for soft actuators

Because the demand for soft actuators and soft robots is increasing in space engineering, medical care, and others, the research on dielectric elastomers has been active in recent years. In this study, to develop better performance dielectric elastomer actuators (DEAs), viscoelastic and tensile properties of elastomers were measured using a dynamic viscoelasticity tester and a tabletop tensile tester. It was confirmed that the strain-removed film of commercially available acrylic elastomer has excellent viscoelastic and mechanical properties compared to commercially available materials. Furthermore, circular DEA was fabricated, and their electrodynamic properties and electric field response were evaluated. It was found that the material with the largest deformation as a circular DEA is acrylic DE with strain removed, and it is also superior in terms of both electric field strength and deformation.

A detailed solution framework for the out‐of‐plane displacement of circular dielectric elastomer actuators

AbstractThe out‐of‐plane displacement (OPD) of a circular dielectric elastomer actuator (DEA) membrane has been explored in recent years for a variety of important applications. Circular DEAs consist of an elastomer membrane that is prestretched to a rigid frame and coated with compliant electrodes. Such a simple configuration has found many interesting applications such as in pumps, pulse tracking, hopping search and rescue robots, dielectric elastomer generators for renewable energy harvesting, linear actuators, and many others. Here, we present an easy‐to‐implement solution framework of the OPD for circular DEAs via a detailed numerical solution using the shooting method (SM) on a system of differential algebraic equations (DAE). The material model incorporated follows the robust Gent hyperelastic representation, and a comparison (of the OPD of a single‐cone DEA) against a neo‐Hookean based solution is included. The SM‐based numerical solution to the resulting boundary value DAE problem is applied to a practical range of values based on the reported literature. Analysis of the results and comparison against other studies are provided. The current work provides a go‐to framework for implementation in further research and development that can be useful to a broader audience including nonexperts in the field of soft robotics.

Identification of a Nonlinear Dielectric Elastomer Actuator Based on the Harmonic Balance Method

This article presents a control-oriented modeling of a circular dielectric elastomer actuator loaded with a mass. Precise dynamic position control of these actuators is a challenge, because of the high level of nonlinearities. Our model takes into account nonlinear mechanical phenomena such as hyperelasticity and viscoelasticity. The behavior of dielectric elastomer actuators is analyzed by a series of experiments on three actuators with different parameters. Furthermore, the model parameters are found using optimization procedures. To improve the performance of the optimization, the steady-state solution is found using the harmonic balance method (HBM). Compared to a forward integration method, the time gain of the HBM is significant, and exceeds two orders of magnitude when six or less harmonics are considered. The application of steady-state solver enables taking the frequency response into account for the parameter identification procedure. The results obtained from the model are compared with experiments and show an excellent agreement.

On the failure modes and maximum stretch of circular dielectric elastomer actuators

Dielectric elastomer actuators have been widely studied theoretically and experimentally thanks to its superior attributes such as large deformation. The actuator failure has long been an issue placing barriers to large deformation in practical applications. In this paper, we investigate the various failure modes and the maximum stretch of the circular dielectric elastomer actuator. An analytical model depicting the actuation mechanism of the dielectric elastomer actuator is proposed to help analyze the failure onset. Several types of normally observed failure modes, i.e. electromechanical instability, loss of tension, and electrical breakdown, are analyzed theoretically based on the proposed model. A 3D figure that interprets the relationship of the corresponding factors and the possible failure modes are proposed as well, by which we could know the possible failure onset, as well as the associated maximum stretch under specific conditions. Experiments are carried out and the results are in accordance with the prediction by the proposed 3D figure.

Miniaturized Circuitry for Capacitive Self-Sensing and Closed-Loop Control of Soft Electrostatic Transducers.

Soft robotics is a field of robotic system design characterized by materials and structures that exhibit large-scale deformation, high compliance, and rich multifunctionality. The incorporation of soft and deformable structures endows soft robotic systems with the compliance and resiliency that makes them well adapted for unstructured and dynamic environments. Although actuation mechanisms for soft robots vary widely, soft electrostatic transducers such as dielectric elastomer actuators (DEAs) and hydraulically amplified self-healing electrostatic (HASEL) actuators have demonstrated promise due to their muscle-like performance and capacitive self-sensing capabilities. Despite previous efforts to implement self-sensing in electrostatic transducers by overlaying sinusoidal low-voltage signals, these designs still require sensing high-voltage signals, requiring bulky components that prevent integration with miniature untethered soft robots. We present a circuit design that eliminates the need for any high-voltage sensing components, thereby facilitating the design of simple low cost circuits using off-the-shelf components. Using this circuit, we perform simultaneous sensing and actuation for a range of electrostatic transducers including circular DEAs and HASEL actuators and demonstrate accurate estimated displacements with errors <4%. We further develop this circuit into a compact and portable system that couples high voltage actuation, sensing, and computation as a prototype toward untethered multifunctional soft robotic systems. Finally, we demonstrate the capabilities of our self-sensing design through feedback control of a robotic arm powered by Peano-HASEL actuators.

Modeling and feedforward control of a soft viscoelastic actuator with inhomogeneous deformation

Recently, dielectric elastomer actuators (DEAs) have been popularly employed for several soft robotic applications. However, achieving effective control of their highly nonlinear deformation is a key issue. Besides material nonlinearity and electromechanical coupling which accounts for their strong nonlinear behavior, response of DEAs is also affected by their viscoelasticity and inhomogeneous deformation. In this paper, we present an analytical model that can account for all these factors in a circular DEA. Both the viscoelastic behavior and the effect due to inhomogeneous deformation of passive area are well exhibited. Algorithm to obtain the numerical solution is proposed and the simulation results show good agreement with the experimental data. In addition, a feedforward controller is designed by inverse solution of the proposed analytical model which is subsequently used to test the tracking ability of the developed controller. Given a desired displacement, control voltage is first calculated by the developed controller and applied to DEA. The measured displacement of the actuator follows well with the desired trajectory, which verifies the effectiveness of the developed controller. We hope that the effective control of DEAs may pave their way to expansive future applications of soft robots.

Electrostatic field induced coupling actuation mechanism for dielectric elastomer actuators

Dielectric elastomer actuators (DEAs) exhibit many fascinating advantages, including high compliance, large actuation strain, fast response, high energy density and efficiency. These muscle-like properties make them suitable for robotic and mechatronic applications especially the actuation of soft robots. However, such actuators have experienced high rates of failure when operating at high voltage, which impedes their practical application to some extent. Here, we report an enhanced and reliable actuation mechanism which couples the electrostatic induction actuation and the electrostatic attraction or zipping mechanisms for DEAs. A circular DEA (E-DEA), a zipping DEA (EZ-DEA) and a bending DEA (E-DEMES) are redesigned and fabricated using this mechanism, which both survive the pull-in electromechanical instability and demonstrate higher operational voltages, larger attainable deformations, and longer actuation lifetime. The E-DEA achieves a large area strain up to 174% and can sustain 20-kV or even higher voltage, its equivalent dielectric strength is larger than 877 MV/m. The EZ-DEA achieves a larger out-of-plane deformation up to 8 mm compared to the traditional zipping DEAs. The E-DEMES is promoted 5 times in response speed and maximally about 87% in the transient blocking force. The improved E-DEMES also exhibits a great potential in faster actuation, larger actuation stroke and force for soft robots.

Deformation study of an in-plane oscillating dielectric elastomer actuator having complex modes

Due to the deformation ability over a wide frequency range, dielectric elastomer actuators (DEAs) under an alternating electric load have received increasing attention. In the theoretical analysis of an in-plane oscillating DEA (edge clamped membrane without a pressure load), the assumption of homogeneous deformation is applied, which conflicts with the observed non-uniform deformation under a combination of pressure and voltage load. To validate the simplification, experimental and theoretical studies of a circular DEA under alternating voltage are reported here. A laser Doppler vibrometer is used to acquire the full-field oscillating response of the DEA fabricated by commercial VHB4905 with hydrogel electrodes. The measured amplitude and phase illustrate that the assumption of homogeneous deformation only works at low frequencies, whereas the non-uniform deformation dominates the oscillation with the increase of excitation frequency. In addition, the in-plane oscillating DEA behaves as a complex mode system. Accordingly, the motion equation is proposed by considering the general viscous damping effect. The experimentally observed transition (from uniform to non-uniform deformation) is analyzed numerically. The normalized upper frequency, under which the deformation field can be treated as uniform, is also obtained based on full-wave simulations.

Experimental Investigations on the Out-of-Plane Sub-harmonic Vibration of a Circular Dielectric Elastomer Actuator

Dielectric elastomers have found interesting applications in soft loudspeakers, where vibrations subject to alternating electrical excitations are the key features. Although there are many theoretical studies on the nonlinear vibrations of dielectric elastomers subject to electromechanical coupling loads, the systematic experimental research is rare. In this work, we design a simple experimental setup to observe the out-of-plane vibrations of a circular dielectric elastomer actuator. We find that the dielectric elastomer has different response modes including the harmonic, super-harmonic and sub-harmonic responses at different excitation frequencies. We analyze the responses by using the short-time Fourier transformation. We find that the equivalent voltage and the AC/DC ratio are the main parameters affecting the occurrence of sub-harmonic responses. The phenomenon of mode shift is also observed in our experiments. These experimental observations provide a deeper understanding of the dynamic responses of dielectric elastomer subject to electromechanical loads.

Viscoelastic effects on the actuation performance of a dielectric elastomer actuator under different equal, un-equal biaxial pre-stretches

Dielectric elastomers (DEs) are belonged to a group of polymers which cause a time-dependence deformation due to the effect of viscoelastic. In recent years, viscoelasticity has been accounted in the modeling in order to understand the complete electromechanical behavior of dielectric elastomer actuators (DEAs). In this paper, we investigate the actuation performance of a circular DEA under different equal, un-equal biaxial pre-stretches, based on a nonlinear rheological model. The theoretical results are validated by experiments, which verify the electromechanical constitutive equation of the DEs. The viscoelastic mechanical characteristic is analyzed by modeling simulation analysis and experimental to describe the influence of frequency, voltage, pre-stretch, and waveform on the actuation response of the actuator. Our study indicates that: The DEA with different equal or un-equal biaxial pre-stretches undergoes different actuation performance when subject to high voltage. Under an un-equal biaxial pre-stretch, the DEA deforms unequally and shows different deformation abilities in two directions. The relative creep strain behavior of the DEA due to the effect of viscoelasticity can be reduced by increasing pre-stretch ratio. Higher equal biaxial pre-stretch obtains larger deformation strain, improves actuation response time, and reduces the drifting of the equilibrium position in the dynamic response of the DEA when activated by step and period voltage, while increasing the frequency will inhibit the output stretch amplitude. The results in this paper can provide theoretical guidance and application reference for design and control of the viscoelastic DEAs.

A method to analyze the voltage-actuation response of a pre-strained circular dielectric elastomer actuator model

Dielectric elastomers (DEs) are the polymers capable of inducing deformation under electrical stimulation. When subject to a voltage across its thickness, the material reduces in thickness and expands in area. This paper presents a new method to analyze deformation and stress distribution response of the dielectric elastomer actuator (DEA) model under different applied voltage. An equal-biaxial pre-strained circular actuator model was built. The Yeoh strain energy potential and the collocation method are used for describing the large strain actuation response and stress distribution. The study in this paper has shown that: the stress and the stretch distributions in the passive region of the DE actuator depend on the radial distance from the center at the calculation point of the passive region and the magnitude of the applied voltage; with the same excitation applied voltage, we can get a larger deformation actuation by choosing an appropriate pre-stretch ratio; the influence of the non-ideal material has seriously affected the actual deformation of the DE actuators. This analytical model has a reference potential for the design optimization of high performance DEA systems and the model-based control of the DEA robot.

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.

Response time and dynamic range for a dielectric elastomer actuator

Dielectric elastomer actuators have attractive applications in many promising fields, such as artificial muscles, soft robots, micro air vehicles, etc. Quasi-static and steady-state analysis has been intensively studied in most of the existing works. However, quantitative evaluation of the response time for dielectric elastomer actuators is still unknown. Especially, with the extension of traditional operation domain to a wide frequency range, lacking the knowledge about the response time of a dielectric elastomer actuator may hinder further development of this technology. In this work, we theoretically and experimentally investigate the transient behavior of a circular dielectric elastomer actuator undergoing in-plane deformation. The response time of the actuator under a step voltage is proposed, which demonstrates that small, light and hard actuator with less damping effect reacts faster, and the response time of the commonly used VHB dielectric elastomer actuator is in the order of hundred milliseconds. Based on the transient response under harmonic voltage load, the dynamic range of a dielectric elastomer actuator and its relation to the response time are analyzed. The measured dynamic range for a circular actuator fabricated by pre-stressed commercial VHB4905 shows good consistency with theoretical prediction and verifies the influence of response time on its dynamic range. The results obtained herein are useful for optimizing the configuration and selecting proper materials of the dielectric elastomer actuator with desired operating frequency range.

高周波交流信号を用いた誘電エラストマアクチュエータのセルフセンシング

This paper reports on self-sensing of dielectric elastomer actuator (DEA). Self-sensing of DEA was demonstrated in literatures, but their responses were limited due to low frequency sensing signals. This paper describes a new method that enables the use of high frequency sensing signal, which allows faster response of self-sensing. In the experiment, two different elastomers were used to manufacture two circular DEAs to measure the difference between materials. Self-sensing circuit using 50 kHz sensing signal was developed. Experimental results show that displacement of both DEAs were successfully estimated. The difference of responses of the two materials, as well as those of two different operation speeds, are discussed.

A smart experimental technique for the optimization of dielectric elastomer actuator (DEA) systems

In order to aid in moving dielectric elastomer actuator (DEA) technology from the laboratory into a commercial product DEA prototypes should be tested against a variety of loading conditions and eventually in the end user conditions. An experimental test setup to seamlessly perform mechanical characterization and loading of the DEA would be a great asset toward this end. Therefore, this work presents the design, control and systematic validation of a benchtop testing station for miniature silicon based circular DEAs. A versatile benchtop tester is able to characterize and apply programmable loading forces to the DEA while measuring actuator performance. The tester successfully applied mechanical loads to the DEA (including positive, constant and negative stiffness loads) simulating biasing systems via an electromagnetic linear motor operating in closed loop with a force/mechanical impedance control scheme. The tester expedites mechanical testing of the DEA by eliminating the need to build intricate pre-load mechanisms or use multiple testing jigs for characterizing the DEA response. The results show that proper mechanical loading of the DEA increases the overall electromechanical sensitivity of the system and thereby the actuator output. This approach to characterize and apply variable loading forces to DEAs in a single test system will enable faster realization of higher performance actuators.

Wrinkling and pull-in instabilities of circular dielectric elastomer actuators

When subject to an electric field, a dielectric elastomer (DE) deforms, possibly straining over 100%. Due to their distinctive mechanical properties, the DEs are being developed as actuators for broad applications, such as armaria, adaptive optics, and robots. According to a method to study instabilities of DE films proposed by Tommasi et al, this paper focuses on analyzing wrinkling and pull-in instabilities of circular dielectric elastomer actuators in order to guide the design of actuators conveniently and directly. Based on simple geometrical and material assumptions, the authors consider the effects of pre-stretch, the size of activated region and material model, which are all crucial in technological applications of DEs. A valid analytical method is used to describe the occurrence of wrinkling and pull-in, which gives clear physical interpretations of these failure mechanisms. As a result, this paper gives a series of critical parameters corresponding to instabilities in different pre-stretch values and the optimal pre-stretch values for some needs. They agree with the experiments well.