Dielectric Actuators Research Articles

A novel dielectric elastomer membrane actuator concept for high-force applications

Dielectric elastomer actuators (DEAs) are known to be lightweight, energy efficient, and scalable in performance. Two well-studied DEA concepts are represented by stacked and membrane DEAs, respectively. The first configuration is known for providing high forces, while the second one results in high strokes. This work proposes a novel design solution, which combines the advantages of both concepts, i.e., high force and stroke, based on a stack of high stroke (3 mm) membrane DEAs. While a single silicone-based DE membrane provides an actuation force on the order of hundreds of mN only, the proposed stacking results into a parallel mechanical connection, which increases the overall force to the double-digit newton range. The biasing mechanism, which is necessary to operate the dielectric elastomer membrane as an actuator, is placed within the passive frame of the DEAs, leading to an overall compact design. To demonstrate the potential of the concept, two prototypes capable of generating record high forces are assembled and tested. The first one allows to lift a weight of 10 kg up to 3 mm, while the second one allows to generate a compression force of 87 N, while working compressing a linear spring with a stiffness of 30.7 N mm−1 up to 2.8mm. These prototypes allow to establish a boosting factor of about 200 for the force output of membrane DEAs.

Parameters identification method for viscoelastic dielectric elastomer actuator materials using fractional derivatives

Since dielectric elastomer actuators are commonly used as artificial muscles, different approaches in material parameters identification have been used. Most dielectric actuators are parametrized with the help of continuum mechanics. Alternatively, rheological models can be used. Unfortunately, in basic rheological models, frequency dependence of viscoelastic materials cannot be obtained with a single equation. In order to obtain frequency dependences of viscoelastic material, fractional Kelvin-Voigt model is used. Fractional Kelvin-Voigt model can be applied for parameters identification in case of dynamical or cyclical excitation with different frequencies and for creep and stress relaxation analysis. Basic Kelvin-Voigt is limited on specific frequency and on creep. It cannot provide stress relaxation. Fractional Kelvin-Voigt model can further be used for fractional control. All previous work dealing with this subject is focused on analysis of material properties. Our contribution and novelty is in preparing governing equations for development control algorithms for dielectric elastomer actuators.

A novel application of dielectric stack actuators: a pumping micromixer

The fabrication of pumping micromixers as a novel application of dielectric stack actuators is proposed in this work. DEA micromixers can be valuable for medical and pharmaceutical applications, due to: firstly, the biocompatibility of the used materials (PDMS and graphite); secondly, the pumping is done with peristaltic movements, allowing only the walls of the channel to be in contact with the liquid, avoiding possible contamination from external parts; and thirdly, the low flow velocity in the micromixers required in many applications. The micromixer based on peristasltic movements will not only mix, but also pump the fluids in and out the device. The developed device is a hybrid micromixer: active, because it needs a voltage source to enhance the quality and speed of the mixing; and passive, with a similar shape to the well-known Y-type micromixers. The proposed micromixer is based on twelve stack actuators distributed in: two pumping chambers, consisting of four stack actuators in series; and a mixing chamber, made of four consecutive stack actuators with 30 layers per stack. The DEA micromixer is able to mix two solutions with a flow rate of 21.5 μl min–1 at the outlet, applying 1500 V at 10 Hz and actuating two actuators at a time.

Silicone based dielectric elastomer strip actuators coupled with nonlinear biasing elements for large actuation strains

There are two major categories of dielectric elastomer actuators (DEAs), which differ from the way in which the actuation is exploited: stack DEAs, using the thickness compression, and membrane DEAs, which exploit the expansion in area. In this work we focus on a specific type of membrane DEAs, i.e., silicone-based strip-in-plane (SIP) DEAs with screen printed electrodes. The performance of such actuators strongly depends on their geometry and on the adopted mechanical biasing system. Typically, the biasing is based on elastomer pre-stretch or on dead loads, which results in relatively low actuation strain. Biasing systems characterized by a negative rate spring have proven to significantly increase the performance of circular out-of-plane DEAs. However, this kind of biasing has never been systematically applied to silicone SIP DEAs. In this work, the biasing design based on negative rate springs is extended to strip DEAs as well, allowing to improve speed, strain, and force of the resulting actuator. At first, the DEAs are characterized under electrical and mechanical loading. Afterwards, two actuator systems are studied and compared in terms of actuation strain, force output, and actuation speed. In a first design stage, the DEA is coupled with a linear spring. Subsequently, the membrane is loaded with a combination of linear and nonlinear spring (working in a negative stiffness region). The resulting stroke output of the second systems is more than 9 times higher in comparison to the first one. An actuation strain of up to 45% (11.2 millimeter) and a force output of 0.38 Newton are measured. A maximum speed of 0.29 m s−1 is achieved, which is about 60 times faster than the one typically measured for similar systems based on VHB.

A novel flying robot system driven by dielectric elastomer balloon actuators

Soft unmanned flying objects have been developed to improve communication in areas where natural disasters occur. This article investigates the design, modeling, and control of an unmanned flying robot, different from classic flying robots which are based on electric motors, robots driven by soft actuators that have advantages of low noise, deformable property, and fast response. Untethered system based on dielectric elastomer actuators can be controlled to move, theory and experiments are presented to show the movement accompanied by large voltage-triggered deformation. We connect a dielectric elastomer balloon-shaped shell to an inelastic chamber which has larger volume and apply voltage to trigger one-layer or two-layer VHB membrane without causing electrical breakdown. The results show that buoyancy force in the air for helium balloon system is inversely proportional to the altitude when the flight system goes up. Compared with single-layer balloon shell, we also generalize the concept of multi-layer soft actuators that offer larger deformation. The larger the original volume of the untethered system is, the more mass can be controlled at the actuated state. Voltage-triggered controllable motion is appreciating with our designed structure.

Investigation of the Effect of Geometric Parameters on EWOD Actuation in Rectangular Microchannels

Efficient actuation of liquid slugs in microfluidic circuits is a matter of interest in droplet-based microfluidic (DMF) applications. In this paper, the electrowetting on dielectric (EWOD) actuation of a liquid slug fully confined in a microchannel is studied. A set of experiments are conducted in which the mean transport velocity of a liquid slug enclosed in a microchannel of rectangular cross section and actuated by EWOD method is measured. A printed circuit board-based (PCB-based) microfluidic chip is used as the platform, and the transport velocity of the slug is measured by processing the images recorded by a high-speed camera while the slug moves in the channel. To investigate the effect of microchannel geometry on the mean transport velocity of the slugs, different channel heights and widths (ranging between 250−440μm and 1–2 mm, respectively) as well as different liquid volumes (ranging between 2.94and5.15μL) are tested and slug velocities up to 14.9 mm/s are achieved. A theoretical model is also developed to analyze the effect of involved parameters on the transport velocity. The results show that, within the range of design parameters considered in this study, for a constant slug volume and channel width, increasing the channel height enhances the velocity. Moreover, keeping the slug volume and channel height fixed, the transport velocity is increased by enlarging the channel width. An inverse proportionality between the slug length and velocity is also observed. These results are also shown to agree with the theoretical model developed.

Dynamic response of dielectric elastomer under electrical loading condition

The present paper is concerned with the analytical modeling for a dielectric elastomeric actuator under the electrical loading condition. A common usage of a dielectric elastomeric actuator is to realize simultaneous displacements when an electrical loading is applied. These simultaneous displacements are advantageous in the field of robotics and smart manufacturing industries. A fundamental deformation approach is presented to model the dynamic response of a homogeneously deformed dielectric elastomer with the application of electrical loading condition. The dynamic equation of motion is formulated through the standard Euler-Lagrange method. The numerical results obtained from the developed equation of motion are discussed in comparison with the similar results presented in the literature to show the vibration and oscillation behaviour of the dielectric elastomer.

Application of multiplicative dimensional reduction method for uncertainty quantification and sensitivity analysis of MEMS electrostatic actuators

The effect of electrostatic actuation allows MEMS devices to have specific physical movements. They have several advantages including low power, low cost, and small size and are used widely in variable capacitors, micro-accelerometers, etc. In this study, we consider a MEMS actuator consisting of a moveable plate and a fixed plate in the presence of an applied electric field. The gap between the two plates can normally be changed by voltage control. It is known that as the gap reduces to two thirds of the original gap, the so-called “pull-in effect” tends to occur, causing the plates to collide (resulting in dielectric breakdown and actuator failure). It is therefore important to predict the onset of the “pull-in effect”. As it is practically impossible to obtain the model parameters precisely, this prediction should account for the presence of uncertainties. Sampling methods such as Monte Carlo and Quasi Monte Carlo are easy to use with the caveat of low accuracy and high computational cost. The other popular method is polynomial chaos. It has high accuracy and low computational cost under smoothness assumption for problems with small number of uncertain parameters. In this study, we consider a two-stage approach to quantify the parametric uncertainty of MEMS electrostatic actuators with a moderate number of causal stochastic factors. In the first stage, a multiplicative dimensional reduction method is used to approximate the variance-based global sensitivity measures in order to simplify the model for the uncertainty quantification stage. The second stage involves the use of the generalized polynomial chaos (gPC) approach to quantify uncertainty of the simplified model from the first stage.

Robust Interaction Control of a Dielectric Elastomer Actuator With Variable Stiffness

This paper presents an interaction control algorithm for a dielectric elastomer membrane actuator. The proposed method permits efficient exploitation of the controllable stiffness of the material, allowing to use the membrane as a "programmable spring" in applications such as robotic manipulation or haptic devices. To achieve this goal, we propose a design algorithm based on robust control theory and linear matrix inequalities. The resulting controller permits to arbitrarily shape the stiffness of the elastomer, while providing robust stability and performance with respect to model nonlinearities. A self-sensing displacement estimation algorithm allows implementation of the method without the need of a deformation sensor, thus reducing cost and size of the system. The approach is validated on an experimental prototype consisting of an elastomer membrane preloaded with a bistable biasing spring.

Modeling, control and experimental investigation of time-average flow rate of a DEAP actuator based diaphragm pump

Over the past three decades, micropumps have been intensively interested to handle and to carry small and precise volumes of liquid in many fields of modern industry. Especially, smart actuator based diaphragm micropumps have received considerable attention because of its simple structure and operation precisely. However, hysteresis phenomenon in the smart actuator is the barrier which prevents the system functioning correctly. This study deals with a mitigation strategy of the hysteresis effects for a typical dielectric electro-active polymer actuator based diaphragm micropump in order to improve the performance of the micropump. Firstly, the forward and inverse hysteresis models are designed and optimized to describe the rate-dependent hysteresis behavior of the actuation system. The model-based control method is then developed and validated to drive the system precisely at high frequencies. Finally, experiments have been conducted to demonstrate the effectiveness of the control strategy in a real application. As a result, the hysteresis behaviors of the smart actuator are effectively compensated at the frequencies up to 1Hz and a maximum time average flow rate of 1.443 mL/min and maximum backpressure of 833.5 Pa at room temperature are recorded.

Microplasma Actuator for EHD Induced Flow

Dielectric barrier discharge microplasma actuator was used for flow modification. The actuator was energized at a relatively low discharge voltage of about 1 kV. The movement of incense particles was tracked by the high-speed camera. A modulated ac voltage was applied to electrodes. It was observed using a high-speed camera how the flow velocity and direction was changed at different duty ratios of the applied voltage. The numerical simulation of the flow was carried out using the fluid model, which considers the electric potential to be the combination of the potential due the external electric field and the charge density of plasma. Based on these potentials, the body force was calculated. Furthermore, the body force was implemented in the Navier–Stokes equations and the simulated flow was obtained. The numerical results matched the obtained results in experiments.

Circular Cylinder Drag Reduction By Three-Electrode Plasma Symmetric Forcing

This study reports an efficient reduction of the drag exerted by a flow on a cylinder when the former is forced with a plasma actuator. A three-electrode plasma device (TED) disposed on the surface of the body is considered, and the effect of the actuation frequency and amplitude is studied. Particle image velocimetry (PIV) measurements provided a detailed information that was processed to obtain the time-averaged drag force and to compare the performances of TED actuator and the canonical dielectric discharge barrier actuator. For the Reynolds number considered (Re = 5500), excitations with the TED actuator were more efficient, achieving drag reductions that attained values close to 40% with high net energy savings. The reduction of coherent structures using the instantaneous vorticity fields and a clustering technique allowed us to gain insight into the physical mechanisms involved in these phenomena. This highlights that the symmetrical forcing of the wake flow at its resonant frequency with the TED promotes symmetrical vorticity patterns which favor drag reductions.

Autonomous oscillatory shape change of DEA induced by the charge–discharge process under a constant voltage

Despite the promising characteristics of Dielectric Elastomel Actuator (DEA) as a practical soft actuator, the need of high voltage for its operation prevents the successful fabrication of a practical DEA, that is, the high voltage generation takes a bulky and costly power supply. Induction of complex shape change motion of DEA such as oscillatory shape change takes even a more bulky and costly multipurpose power supply. It is a serious practical issue to be overcome. In our latest study, however, we could build a simple DEA system which exhibited a relatively complex and autonomous oscillatory shape change merely under a constant voltage, though the voltage needed was high. This successful outcome must broaden the potential usefulness of DEA as a practical soft actuator.

Relative planar strain control and minimizing deformation work in elastomeric sheets via reinforcing fiber arrays

This study investigates soft composite sheets that undergo significant deformations. The fiber reinforcement in these systems not only increases the stiffness of sheets like a traditional composite, but also controls the relationships between strains in orthogonal planar directions. Such an ability is useful in controlling the deformation of soft robots, and also enhancing the output of soft actuation techniques like electro-active polymer actuators (also called dielectric actuators). The inspiration for this work comes from squid mantle structures that couple orthogonal components of strain using helical fiber reinforcement. The resulting null space of deformations corresponding to the fiber restrictions creates a family of body deformations that optimize propulsion.The strain dynamics in the composite sheet are modeled geometrically from fiber orientations, assuming that the fibers are inextensible. After the strain dynamics have been determined, the stress/strain relationship is modeled by considering the matrix and reinforcing fibers to be two separate homogeneous systems interacting through local stresses. Both steps of this modeling technique are validated experimentally showing planar strains in a preferred direction to be as high as 16 times the resulting planar strain of an equivalent unreinforced sheet by forcing negative strains in the orthogonal planar directions. The work required for deformation is derived from the stress/strain relationship by calculating the strain energy stored in the material, and an optimal balance between increased planar strain output and increased material stiffness is analyzed. It is shown that for the specific materials used to create the soft composite sheets (thermoplastic elastomer with cotton fibers) optimal fiber angles lie between 15° and 25° to minimize work required for deformation, but this optimal range will increase with increasing ratio of fiber to matrix modulus.

Three-Electrode Sliding Nanosecond Dielectric Barrier Discharge Actuator: Modeling and Physics

We present a numerical study of plasma dynamics in a three-electrode sliding nanosecond dielectric barrier discharge flow actuator. A two-dimensional self-consistent plasma model including the effect of detail air plasma chemistry and ultrafast gas heating is used in our studies. When a third electrode is placed downstream of a classical two-electrode nanosecond dialectric barrier discharge actuator and powered by a negative direct-current voltage, a coronalike discharge is formed in its immediately vicinity, which in turn changes the dynamic of the primary streamers propagating from the pulsed electrode. The primary streamer slides and can even extend to the entire interelectrode distance. The potential difference between the third electrode, the positively charged dielectric surface, and the virtual anode formed by the streamer itself is found to be the main reason behind this elongation because of the electric field enhancement at the streamer head. Preionization and charge accumulation from the third electrode also contribute to this behavior. The numerical results also indicate that for high electric fields a negative streamer can be produced at the third electrode that merges with the positive streamer from the pulsed electrode. Consequently, the plasma channel covers the entire interelectrode space. The elongation of the primary streamer leads to an increase of the effective energy release region, which can result in a more efficient flow actuation mechanism.

Slide-Ring Materials Using Cyclodextrin.

We have recently synthesized slide-ring materials using cyclodextrin by cross-linking polyrotaxanes, a typical supramolecule. The slide-ring materials have polymer chains with bulky end groups topologically interlocked by figure-of-eight shaped junctions. This indicates that the cross-links can pass through the polymer chains similar to pulleys to relax the tension of the backbone polymer chains. The slide-ring materials also differ from conventional polymers in that the entropy of rings affects the elasticity. As a result, the slide-ring materials show quite small Young's modulus not proportional to the cross-linking density. This concept can be applied to a wide variety of polymeric materials as well as gels. In particular, the slide-ring materials show remarkable scratch-proof properties for coating materials for automobiles, cell phones, mobile computers, and so on. Further current applications include vibration-proof insulation materials for sound speakers, highly abrasive polishing media, dielectric actuators, and so on.

Digital Control of a High-Voltage (2.5 kV) Bidirectional DC--DC Flyback Converter for Driving a Capacitive Incremental Actuator

This paper presents a digital control technique to achieve valley switching in a bidirectional flyback converter used to drive a dielectric electroactive polymer-based capacitive incremental actuator. This paper also provides the design of a low input voltage (24 V) and variable high output voltage (0–2.5 kV) bidirectional dc–dc flyback converter for driving a capacitive incremental actuator. The incremental actuator consists of three electrically isolated mechanically connected capacitive actuators. It requires three high-voltage (HV) (2–2.5 kV) bidirectional dc–dc converters to accomplish the incremental motion by charging and discharging the capacitive actuators. The bidirectional flyback converter employs a digital controller to improve the efficiency and charge/discharge speed using the valley switching technique during both charge and discharge processes, without the need to sense signals on the output HV side. Experimental results verifying the bidirectional operation of a HV flyback converter are presented using a 3-kV polypropylene-film capacitor as the load. The energy-loss distributions of the converter are presented when 4- and 4.5-kV HV mosfet s are used on HV side. The flyback prototype with a 4 kV mosfet demonstrated 89% charge energy efficiency to charge the capacitive load from 0 V to 2.5 kV, and 84% discharge energy efficiency to discharge it from 2.5 kV to 0 V.

Interactions Between Dielectric Elastomer Actuators and Soft Bodies

Abstract Soft robots have recently attracted much interest, and dielectric elastomers have shown great promise for soft actuators because of their large voltage-induced deformation. The actuation performance is primarily characterized by the largest deformation, based on the material failure analyses. However, most previous related work usually neglected the fact that, different from their counterpart in the field of traditional rigid robots, soft actuators are deeply coupled with the environment. Interactions between soft actuators and the environment are significant and under-researched, playing an important role in evaluating their actuation capabilities and in matching them with prescribed soft robotic systems. This article investigates the interactions between a dielectric elastomer balloon actuator and an actuated soft body. We present a computational model of the coupled system and look at the effects of mechanical and material designs on the performance of the balloon actuator. Parametric studies ...

Bottlebrush Elastomers: A New Platform for Freestanding Electroactuation.

Freestanding, single-component dielectric actuators are designed based on bottlebrush elastomers that enable giant reversible strokes at relatively low electric fields and altogether avoid preactuation mechanical manipulation. This materials design platform allows for independent tuning of actuator rigidity and elasticity over broad ranges without changing chemical composition, which opens new opportunities in soft-matter robotics.

Enhancing the Electro-Mechanical Response of Stacked Dielectric Actuators

Dielectric polymer films subjected to an electric field reduce in thickness and expand in area. A pile up configuration of such films, also known as a stacked dielectric actuator, is capable of exhibiting contractive deformations while subjected to external tensile forces. This work analyzes the capabilities of the stacked actuator according to a new microscopically motivated approach which suggests that the macroscopic response is determined by four microscopic factors—the length of the polymer chains, the local behavior of the monomers, the intensity of the local dipole and the chain-density. With the aim of enhancing the actuators performance, a specific local behavior is assumed and the influence of the remaining three quantities is studied. It is shown that the actuation can be significantly improved with appropriate micro-structural changes. Interestingly, this work demonstrates that these micro-structural alterations depend on the envisaged application.