Modeling guided design of dielectric elastomer generators and actuators

Giant voltage-induced deformation in dielectric elastomers near the verge of snap-through instability

ABSTRACTOperation of a dielectric elastomer (DE) transducer is limited by the way it fails, and any other limits imposed by the user. For example, material rupture and dielectric breakdown are two examples of transducer failure, while tension loss is a user-imposed limit. Electromechanical instability may or may not result in transducer failure, but is often considered as a boundary to denote the onset of state transition. For a DE undergoing homogeneous deformation, constructing operational limits on work-conjugate state diagrams are rather straight-forward. However, for DE subjected to non-homogeneous deformation, the process is more complex. We present a method to plot the operational boundaries of a DE subject to non-homogeneous deformation, known commonly as the Universal Muscle Actuator or the loudspeaker configuration. Our analysis show that tension loss need not be imposed as a hard operational limit, and that there is further operational space for the transducer, post tension loss. By plotting the operational limits on work-conjugate planes, we determine an optimal inner-to-outer ring ratio and the required pre-stretches that maximize energy conversion.

Modeling of static reliability assessment in dielectric elastomer transducers subject to electric loads

As a widely used configuration for dielectric elastomer (DE) actuators, DE tube actuators (or cylindrical actuators) are also found to be susceptible to electromechanical instability (EMI), which may lead to a premature electrical breakdown (EB), and inhibit the potential actuation of DE actuators. This work investigates the electromechanical response of a DE tube actuator with and without boundary constraints to demonstrate an alternative to avoid EMI while achieving large actuation. Our simulation results based on the Gent strain energy model show that the EMI of a DE tube actuator can be eliminated, and larger actuation deformation can be achieved by applying boundary constraints. As a result of these constraints, consideration is also given to the possible mechanical buckling failure that may occur. Mechanisms of possible failure modes of constrained and unconstrained DE tube actuators, such as electromechanical instability, electrical breakdown and mechanical buckling, are elucidated. This paper should provide better theoretical guidance on how to improve the actuation performance of DE actuators, thus leading to the optimal design of DE-based devices.

Electromechanical instability (EMI) is one of most common failure modes for dielectric elastomers (DEs). It has been reported that pre-stretching a DE sample can suppress EMI due to strain stiffening taking place for larger strains and a higher elastic modulus are achieved at high stretch ratios when a voltage is applied to the material. In this work, the influence of equi-biaxial stretch on DE secant modulus was studied using VHB 4910 and silicone rubber (SR) composites containing barium titanate (BaTiO3, BT) particles and also dopamine coated BT (DP-BT) particles. The investigation of equi-biaxial deformation and EMI failure for VHB 4910 was undertaken by introducing a voltage-stretch function. The results showed that EMI was suppressed by equi-biaxial pre-stretch for all the DEs fabricated and tested. The stiffening properties of the DE materials were also studied with respect to the secant modulus. Furthermore, a voltage-induced strain of above 200% was achieved for the polyacrylate film by applying a pre-stretch ratio of 2.0 without EMI occurring. However, a maximum voltage-induced strain in the polyacrylate film of 78% was obtained by the SR/20 wt% DP-BT composite for a lower applied pre-stretch ratio of 1.6 and again EMI was eliminated.

Dielectric elastomer is a kind of typical electro-active polymer material. Under external electric field it can produce large electrostriction deformation and possesses the advantages of high elastic energy density, super short response time, high efficiency, and so on. It is widely used in the artificial muscles, facial expressions, actuators, energy harvesters, sensors, robots and Braille display devices, and also shows huge application potential in the aerospace and intelligent bionic areas. We built the free energy of the dielectric elastomer electrical-mechanical coupling system and investigated its constitutive relation and stability behavior. Then we calculated the elastomer’s critical deformation suffering from the voltage. If electrical breakdown, electromechanical instability and snap-through instability can be avoided, the large electrostriction deformation can induce adiabatic temperature change and isothermal entropy change of the dielectric elastomer. We used the entropy-temperature or electric displacement-electric field plane to describe the temperature change and entropy change of dielectric elastomer undergoing large electrostriction deformation. With the influence of temperature, we developed a temperature and deformation coupling thermodynamical free energy model to calculate the electric field induced variation of temperature and entropy in dielectric elastomers. The results should offer great help in guiding the design and fabrication of excellent actuators featuring soft dielectric elastomers.

Dielectric elastomers are novel soft smart material that could deform sustainably when subjected to external electric field. That makes dielectric elastomers promising materials for actuators. In this paper, a spring-roll actuator that would bend when a high voltage is applied was fabricated based on dielectric elastomer. Using such actuators as active parts, the flexible grippers and inchworm-inspired crawling robots were manufactured, which demonstrated some examples of applications in soft robotics. To guide the parameters design of dielectric elastomer based spring-roll bending actuators, the theoretical model of such actuators was established based on thermodynamic theories. The initial deformation and electrical induced bending angle of actuators were formulated. The failure of actuators was also analyzed considering some typical failure modes like electromechanical instability, electrical breakdown, loss of tension and maximum tolerant stretch. Thus the allowable region of actuators was determined. Then the bending angle-voltage relations and failure voltages of actuators with different parameters, including stretches of the dielectric elastomer film, number of active layers, and dimensions of spring, were investigated. The influences of each parameter on the actuator performances were discussed, providing meaningful guidance to the optical design of the spring-roll bending actuators.

The type of electroactive polymer, known as dielectric elastomers, has shown considerable promise for harvesting energy from environmental sources such as ocean waves, wind, and human motions, etc. The high energy density and conversion efficiency of dielectric elastomers (DEs) can allow for very simple and robust “direct drive” generators. In such generators, a DE transducer is directly driven by the up and down motions of waves. These generators can be simple, low-cost and robust. The efficiency of a wave energy convertor using DEs was confirmed by our previous experimental work. In this study, a wave energy convertor using DEs was numerically investigated as a moored floating body. First, the numerical model is validated against existing benchmark experiments. Then the moored floating body is numerically modelled, and the simulated results are compared to physical wave tank tests. The analyzed conditions vary from single body, two bodies with different intervals, and three bodies. The fluctuation, energy absorption efficiency, as well as the influence of multiple moored floating bodies on each other are discussed. The purpose of present work is to clarify the influence of floating body intervals and number of floating bodies on the motions, the mooring tensions, as well as the power generation efficiencies for a plurality of floating bodies arranged in a series in head seas. This kind of optimal arrangement is expected to be popularized in the future, which may contribute to the increasing demand of renewable energy solutions.

Dielectric elastomers can exhibit load deformation of more than 1692% percent. The deformation is made possible when an external electric field interacts with the dielectric elastomer material, this leads to electrical polarization within the bulk media of the material and charge accumulation on a compliant electrode. This action causes a compressive force that squeezes the film thickness and expands the surface area leading to actuation of the material. The geometric configuration of the complaint electrode on the surface area of the dielectric elastomer can directly influence the electromechanical response of the film. The surface of the dielectric elastomer can be divided into two regions: the active region, which is the surface area of the film that is covered by the compliant electrode, and the passive region, which is the area of the film that remains uncovered. The purpose of this research is to determine the optimal ratio of active vs passive regions and its impact on the electromechanical performance (actuation, dielectric and electromechanical breakdown).

Electromechanical instability in anisotropic dielectric elastomers

Currently used dielectric elastomers do not have the ability to self-heal after detrimental events such as tearing or electrical breakdown, which are critical issues in relation to product reliability and lifetime. In this paper, we present a self-healing dielectric elastomer that additionally possesses high dielectric permittivity and consists of an interpenetrating polymer network of silicone elastomer and ionic silicone species that are cross-linked through proton exchange between amines and acids. The ionically cross-linked silicone provides self-healing properties after electrical breakdown or cuts made directly to the material due to the reassembly of the ionic bonds that are broken during damage. The dielectric elastomers presented in this paper pave the way to increased lifetimes and the ability of dielectric elastomers to survive millions of cycles in high-voltage conditions.

The development of accurate sensing strategies for soft robotic systems represents an ongoing challenge. In general, it is difficult to integrate a sensory system that preserves the robot's compliance while simultaneously providing accurate measurements. Dielectric elastomer (DE) transducers offer a potential solution to this problem owing to their ability to function as soft actuators and sensors at the same time, thus enabling the realization of self-sensing soft robotic systems. In this work, we propose a bi-modal sensory scheme for a DE-based soft robot, which allows simultaneous estimation of the robot's own configuration and detection of external contacts without relying on any external sensors. Self-sensing proprioception and contact detection are performed at the same time while actuating the robot, based on online processing of electrical signals measured directly at the actuators' level, i.e., DEs' voltage and current. Both estimation tasks are achieved in real time by means of artificial neural networks. After describing the system and the bi-modal self-sensing architecture, the approach is validated experimentally on a DE soft robot.

Soft electroactive materials can undergo large deformation subjected to either mechanical or electrical stimulus, and hence, they can be excellent candidates for designing extremely flexible and adaptive structures and devices. This paper proposes a simple one-dimensional soft phononic crystal (PC) cylinder made of dielectric elastomer (DE) to show how large deformation and electric field can be used jointly to tune the longitudinal waves propagating in the PC. A series of soft electrodes, which are mechanically negligible, are placed periodically along the DE cylinder, and hence, the material can be regarded as uniform in the undeformed state. This is also the case for the uniformly prestretched state induced by a static axial force only. The effective periodicity of the structure is then achieved through two loading paths, i.e., by maintaining the longitudinal stretch and applying an electric voltage over any two neighboring electrodes or by holding the axial force and applying the voltage. All physical field variables for both configurations can be determined exactly based on the nonlinear theory of electroelasticity. An infinitesimal wave motion is further superimposed on the predeformed configurations, and the corresponding dispersion equations are derived analytically by invoking the linearized theory for incremental motions. Numerical examples are finally considered to show the tunability of wave propagation behavior in the soft PC cylinder. The outstanding performance regarding the band gap (BG) property of the proposed soft dielectric PC is clearly demonstrated by comparing with the conventional design adopting the hard piezoelectric material. One particular point that should be emphasized is that soft dielectric PCs are susceptible to various kinds of failure (buckling, electromechanical instability (EMI), electric breakdown (EB), etc.), imposing corresponding limits on the external stimuli. This has been carefully examined for the present soft PC cylinder such that the applied electric voltage is always assumed to be less than the critical voltage except for one case, in which we illustrate that the snap-through instability of the axially free PC cylinder made of a generalized Gent material may be used to efficiently trigger a sharp transition in the BGs.

A dielectric elastomer generator (DEG) can be used for converting mechanical energy from natural motion sources, such as walking, waves, trees, etc., into electrical energy. A DEG is comprised of a soft and flexible dielectric elastomer (DE) capacitor, a priming circuit (PC), which transfers high potential charge onto/off the DE electrodes, and a power extraction circuit that harvests the generated power. To generate power, the PC must charge and discharge the DE in synchronization with the DE's capacitance change. A simple circuit to do this exists: the self-priming circuit (SPC). The SPC consists of diodes and capacitors that passively switch between charge delivery and charge receiving states in synchronization with the DE's capacitance change. Until now, there has been no understanding of how to design an SPC in order to maximize harvested energy from the DE. A new mathematical model for an SPC is presented, leading to design and optimization. An accuracy of 0.1% between model, simulation, and experiment over five cycles is obtained, once losses are taken into consideration. The behavior of the SPC is shown to be related to the maximum and minimum capacitances of the DE, but is unaffected by the exact shape of the capacitance waveform.

Interpenetrating polymer network–reinforced acrylic elastomers used for dielectric elastomer actuators are made by introducing a curable and cross-linkable liquid additive into a highly prestretched acrylic elastomer membrane. After curing the additive, force equilibrium between the polymeric networks preserves part of the prestretch, after the external forces are removed. This leads to a material system with promising properties for applications as dielectric elastomer transducers. This work presents an experimental electromechanical characterization of circular actuators made of interpenetrating polymer network–reinforced acrylic elastomers. The relative dielectric permittivity and passive mechanical response of interpenetrating polymer network membranes with different material compositions were characterized. Circular actuators were tested leading to 70 measured responses of 8 interpenetrating polymer network membranes at different voltage levels. The active deformation at a given voltage was found to increase with the amount of secondary network. This effect is mainly attributed to the decreasing membrane thickness. The actuation performance and variability of the active response can be evaluated based on these observations. Furthermore, the present data can be used to develop and validate models of interpenetrating polymer network actuators. All experiments were compared with corresponding finite element simulations using membrane-specific model equations.