Academic literature on the topic 'Adaptive Dielectric'

Dielectric electroactive polymer actuators are new important transducers in control system applications. The design of a high performance controller is a challenging task for these devices. In this work, a PI controller was studied for a dielectric electroactive polymer actuator. The pole placement problem for a closed-loop system with the PI controller was analyzed. The limitations of a PI controller in the pole placement problem are discussed. In this work, the analytic PI controller gain rules were obtained, and therefore extension to adaptive control is possible. To minimize the influence of unmodeled dynamics, the robust adaptive control law is applied. Furthermore, analysis of robust adaptive control was performed in a number of simulations and experiments.

Differential diagnosis of stress adaptive mechanisms is one of the areas of modern biomedical engineering and the most complex part of the pre-nosological diagnosis of cardiac pathology. One of the manifestations of disadaptation in heart failure is a violation of the mechanisms of realization of the intracellular chain "signal-function". The concept of imbalance in the functioning of the adenylate cyclase system and β-adrenergic receptors of the erythrocytes membrane and cardiomyocytes is considered in the pathogenesis of the heart. The study of the dielectric constant (ε՜) of erythrocytes of patients with heart failure was performed using an instrument-recording complex based on microwave dielectrometry of the γ-dispersion region of free water dielectric permittivity. Testing of the β-adrenergic complex of the erythrocyte membrane by specific stimulators, blockers, and modulators was implemented at a fixed frequency of microwave radiofrequency generation (f = 37,7 GHz). Interpretation of the obtained experimental data was that the process of interaction of bioregulators with the biological system is accompanied by an increase or decrease in the relative amount of free water, which leads to a change in the real part of ε՜ complex dielectric constant. This allowed us to visualize the violation of the signal cell system at the molecular level, which manifested itself in the change of integral hydration by ε՜ parameter. It was shown that the change in the dielectric constant of the erythrocyte suspension at risk (patients with hereditary predisposition to dilated and ischemic cardiomyopathy) was significant relative to the dielectric parameters of erythrocyte samples from healthy donors; there was a tendency to block β-adrenergic receptors, with the combined action of adrenaline, PGE2 and cordanum, with Δε՜ = 0,009 ± 0,008 х 10-12 F/m. It should be noted the formation of preconditions for changes in the functioning of the adenylate cyclase system and the development of heart failure in the group at risk, is accompanied by dilated and ischemic cardiomyopathy. The effectiveness of the microwave dielectrometry method for the assessment of violations of adaptation mechanisms through the adenylate cyclase system of the erythrocyte membrane in dilated and ischemic forms of cardiomyopathies is shown. The results of the study are the basis for the introduction of the dielectric constant criterion in the general algorithm of pre-nosological diagnosis of heart failure.

A new domain decomposition method for Maxwell’s equations in conductive media is presented. Using this method, reconstruction algorithms are developed for the determination of the dielectric permittivity function using time-dependent scattered data of an electric field. All reconstruction algorithms are based on an optimization approach to find the stationary point of the Lagrangian. Adaptive reconstruction algorithms and space-mesh refinement indicators are also presented. Our computational tests show the qualitative reconstruction of the dielectric permittivity function using an anatomically realistic breast phantom.

With the rapid of growth of wireless connectivity more demand is placed on the need for innovative technologies capable of satisfying increasing user demand and network capacity. Adaptive antennas systems or most commonly known as Smart Antennas are expected to be implemented in the next generation of wireless systems. Their implementation avails in dynamic adaptation to spatial and temporal conditions affecting the quality of communication, while offering tremendous flexibility to wireless providers. However one of the major challenges facing Smart Antenna technology is the inherent complexity of the antenna structure, associated control algorithm and implemented RF components possibly contributing to the delay of commercial interest. This thesis will present various adaptive antenna configurations that utilize an embedded dielectric in order to achieve significant size reduction and mechanical rigidity while maintaining favorable electromagnetic performance. In order to constrict the lateral ground plane dimension, a cylindrical shaped hollow ground skirt was attached to the antenna structures effectively compromising between effective beam forming in the azimuth plane and physical size. The complexity of these antenna structures requires a more contemporary design approach which involved computer modeling using a commercial available Finite Element software package and optimization using a developed generic Genetic Algorithm based optimization program. A dielectric embedded 7-element monopole array antenna featuring switched parasitic elements is presented and optimized for maximum vertically polarized gain in the horizontal plane, producing an antenna structure with a radial length of less then 0.25λ and total height of 0.4&alamba which was shown to radiate a main lobe beamwidth of 80 degrees with an absolute gain of 4.8dBi at 2.45GHz. Further on a dielectric embedded 7-element monopole array antenna featuring parasitic elements terminated with finite set of terminating reactive loads is presented with a radial length of less then 0.25&alambda and total height of 0.4&alambda. The antenna structure and reactive load combination were optimized for maximum horizontal gain producing a principal main lobe with a measured gain of 5.1dBi and beamwidth of 110 degrees at 2.48GHz. Finally it was shown single and dual radiation lobes maybe produced when active monopoles elements are placed eccentric in a circular shaped dielectric material. A circular array of elements embedded in a dielectric material was realized with measured gains of single and dual beam radiation at 2.45GHz was shown to be 5.18dBi and 3.65Bi respectively with corresponding beamwidths of 78.5 degrees and 53 degrees.

With the rapid of growth of wireless connectivity more demand is placed on the need for innovative technologies capable of satisfying increasing user demand and network capacity. Adaptive antennas systems or most commonly known as Smart Antennas are expected to be implemented in the next generation of wireless systems. Their implementation avails in dynamic adaptation to spatial and temporal conditions affecting the quality of communication, while offering tremendous flexibility to wireless providers. However one of the major challenges facing Smart Antenna technology is the inherent complexity of the antenna structure, associated control algorithm and implemented RF components possibly contributing to the delay of commercial interest. This thesis will present various adaptive antenna configurations that utilize an embedded dielectric in order to achieve significant size reduction and mechanical rigidity while maintaining favorable electromagnetic performance. In order to constrict the lateral ground plane dimension, a cylindrical shaped hollow ground skirt was attached to the antenna structures effectively compromising between effective beam forming in the azimuth plane and physical size. The complexity of these antenna structures requires a more contemporary design approach which involved computer modeling using a commercial available Finite Element software package and optimization using a developed generic Genetic Algorithm based optimization program. A dielectric embedded 7-element monopole array antenna featuring switched parasitic elements is presented and optimized for maximum vertically polarized gain in the horizontal plane, producing an antenna structure with a radial length of less then 0.25λ and total height of 0.4&alamba which was shown to radiate a main lobe beamwidth of 80 degrees with an absolute gain of 4.8dBi at 2.45GHz. Further on a dielectric embedded 7-element monopole array antenna featuring parasitic elements terminated with finite set of terminating reactive loads is presented with a radial length of less then 0.25&alambda and total height of 0.4&alambda. The antenna structure and reactive load combination were optimized for maximum horizontal gain producing a principal main lobe with a measured gain of 5.1dBi and beamwidth of 110 degrees at 2.48GHz. Finally it was shown single and dual radiation lobes maybe produced when active monopoles elements are placed eccentric in a circular shaped dielectric material. A circular array of elements embedded in a dielectric material was realized with measured gains of single and dual beam radiation at 2.45GHz was shown to be 5.18dBi and 3.65Bi respectively with corresponding beamwidths of 78.5 degrees and 53 degrees.
Thesis (Masters)
Master of Philosophy (MPhil)
School of Microelectronic Engineering
Full Text

The growth in the number of wireless devices and applications underscores the need for characterizing and mitigating interference induced problems such as distortion and blocking. A typical interference scenario involves the detection of a small amplitude signal of interest (SOI) in the presence of a large amplitude interfering signal; it is desirable to attenuate the interfering signal while preserving the integrity of SOI and an appropriate dynamic range. If the frequency of the interfering signal varies or is unknown, an adaptive notch function must be applied in order to maintain adequate attenuation. This work explores the performance space of a phase cancellation technique used in implementing the desired notch function for communication systems in the 1-3 GHz frequency range. A system level model constructed with MATLAB and related simulation results assist in building the theoretical foundation for setting performance bounds on the implemented solution and deriving hardware specifications for the RF notch subsystem devices. Simulations and measurements are presented for a Low Noise Amplifer (LNA), voltage variable attenuators, bandpass filters and phase shifters. Ultimately, full system tests provide a measure of merit for this work as well as invaluable lessons learned. The emphasis of this project is the on-wafer LNA measurements, dependence of IC system performance on mismatches and overall system performance tests. Where possible, predictions are plotted alongside measured data. The reasonable match between the two validates system and component models and more than compensates for the painstaking modeling efforts. Most importantly, using the signal to interferer ratio (SIR) as a figure of merit, experimental results demonstrate up to 58 dB of SIR improvement. This number represents a remarkable advancement in interference rejection at RF or microwave frequencies.

Blue phase liquid crystals (BPLCs) have a delicate lattice structure existing between chiral nematic and isotropic phases, with a stable temperature range of about 2 K. But due to short coherent length, these self-assembled nano-structured BPLCs have a fast response time. In the past three decades, the application of BPLC has been rather limited because of its narrow temperature range. In 2002, Kikuchi et al. developed a polymer stabilization method to extend the blue-phase temperature range to more than 60 K. This opens a new gateway for display and photonic applications. In this dissertation, I investigate the material properties of polymer-stabilized BPLCs. According the Gerber's model, the Kerr constant of a BPLC is linearly proportional to the dielectric anisotropy of the LC host. Therefore, in the frequency domain, the relaxation of the Kerr constant follows the same trend as the dielectric relaxation of the host LC. I have carried out experiments to validate the theoretical predictions, and proposed a model called extended Cole-Cole model to describe the relaxation of the Kerr constant. On the other hand, because of the linear relationship, the Kerr constant should have the same sign as the dielectric anisotropy of the LC host; that is, a positive or negative Kerr constant results from positive or negative host LCs, respectively. BPLCs with a positive Kerr constant have been studied extensively, but there has been no study on negative polymer-stabilized BPLCs. Therefore, I have prepared a BPLC mixture using a negative dielectric anisotropy LC host and investigated its electro-optic properties. I have demonstrated that indeed the induced birefringence and Kerr constant are of negative sign. Due to the fast response time of BPLCs, color sequential display is made possible without color breakup. By removing the spatial color filters, the optical efficiency and resolution density are both tripled. With other advantages such as alignment free and wide viewing angle, polymer-stabilized BPLC is emerging as a promising candidate for next-generation displays. However, the optical efficiency of the BPLC cell is relatively low and the operating voltage is quite high using conventional in-plane-switching electrodes. I have proposed several device structures for improving the optical efficiency of transmissive BPLC cells. Significant improvement in transmittance is achieved by using enhanced protrusion electrodes, and a 100% transmittance is achievable using complementary enhanced protrusion electrode structure. For a conventional transmissive blue phase LCD, although it has superb performances indoor, when exposed to strong sunlight the displayed images could be washed out, leading to a degraded contrast ratio and readability. To overcome the sunlight readability problem, a common approach is to adaptively boost the backlight intensity, but the tradeoff is in the increased power consumption. Here, I have proposed a transflective blue phase LCD where the backlight is turned on in dark surroundings while ambient light is used to illuminate the displayed images in bright surroundings. Therefore, a good contrast ratio is preserved even for a strong ambient. I have proposed two transflective blue phase LCD structures, both of which have single cell gap, single gamma driving, reasonably wide view angle, low power consumption, and high optical efficiency. Among all the 3D technologies, integral imaging is an attractive approach due to its high efficiency and real image depth. However, the optimum observation distance should be adjusted as the displayed image depth changes. This requires a fast focal length change of an adaptive lens array. BPLC adaptive lenses are a good candidate because of their intrinsic fast response time. I have proposed several BPLC lens structures which are polarization independent and exhibit a parabolic phase profile in addition to fast response time. To meet the low power consumption requirement set by Energy Star, high optical efficiency is among the top lists of next-generation LCDs. In this dissertation, I have demonstrated some new device structures for improving the optical efficiency of a polymer-stabilized BPLC transmissive display and proposed sunlight readable transflective blue-phase LCDs by utilizing ambient light to reduce the power consumption. Moreover, we have proposed several blue-phase LC adaptive lenses for high efficiency 3D displays.
Ph.D.
Doctorate
Optics and Photonics
Optics and Photonics
Optics

In the past few decades there has been a huge growth in the wideband high-frequency communication systems. A compact size, high-efficiency antenna design with low conductor loss is essential for enabling high data rate communication in addition to other necessary requirements. In ecent years, Dielectric Resonator Antenna DRA) has evolved to establish its presence because of its incomparable low profile,light weight antenna, that provide wide bandwidth with low dissipation loss compared to microstrip antennas. The flexibility in choice of shape, relative permittivity and size enables a whole spectrum of operating frequency ranges (1GHz-40GHz), sizes, radiation patterns and bandwidths. In this project the investigation of dielectric resonator antennas are quantitatively realized by the design and evaluation of two Dielectric Resonator Antenna.The first is a stacked dielectric resonator antenna with operating frequency range 8.17GHz-12.55GHz and other is stacked dielectric resonator antenna with operating frequency range 7.81GHz-12.33GHz both in X-band applications. The hybrid mode is suitable for a broadside radiation pattern. The modes can be excited by feeding from microstrip lines and coaxial probes and the location of the excitation determines excited mode.Size, shape and permittivity of the DR element govern the resonant frequency.

Large area and flexible electronic systems are widely used in applications such as displays, image sensors, wearable electronics, and energy harvesting systems. One of the fundamental functional blocks in these systems is the thin-film transistor (TFT), which suffers from poor field-effect mobility, electrical instability, etc. due to the state localization at low-temperature fabrication process, a criterion that enables system realization on glass or flexible substrate, such as plastic. The electrostatic discharge (ESD) is a rapid transfer of static charge between two objects of dissimilar potentials, one of which is typically grounded. An electronic device could suffer ESD damage during different stages of its lifetime including manufacturing and product usage leading to a loss of billions of dollars annually to the electronics industry. Since the ESD phenomenon is unavoidable, on-chip ESD protection devices or circuits are required. An ideal ESD protection device should offer a low resistance path to the surge current during an ESD event, but a high resistance path to the signal during the normal operation to minimize the power loss. In the crystalline CMOS technology, the parasitic bipolar turn-on (snapback) is effectively used to design the ESD protection device. However, most of the TFT technologies do not exhibit any bipolar turn-on owing to the poor mobility and lack of complementary devices. Hence, the conventional protection circuit uses large aspect-ratio diode-connected TFTs that offer a low resistance path to the surge current but also does the same to signals during normal system operation resulting in power loss. Additional circuits are required to keep the protection devices turned off during normal operation, but it leads to higher routing complexity, layout area, and multi-component reliability issues. This thesis investigates the feasibility of a novel idea for ESD protection involving an adaptive-dielectric TFT (adTFT) that self-configures itself to a low resistance state during an ESD event and a high resistance state during normal operation without external control. The adTFT device is designed to differentiate between an ESD pulse, which is typically nanoseconds order, and a normal operation signal, which is either a static dc (e.g. in power line) or a pulse of width microseconds to milliseconds order (e.g. in data/address line of a switch matrix). This is achieved using a time-dependent gate field masking mechanism, which is enabled by modifying the gate dielectric of a conventional TFT to a dielectric-semiconductor-dielectric stack and attaching a charge injection/extraction terminal to the sandwiched semiconductor layer. A first-order model of the masking dynamics under a gate step-bias input is developed using the space-charge-limited-current and threshold voltage modulation. TCAD simulations are performed using poly-Si adTFT to get a detailed insight into the device operation. The HBM (human body model) ESD robustness and normal mode leakage current of the diode-connected adTFTs are evaluated and compared against that of the conventional TFTs. Next, the adTFT and an experimental control (behaving similar to the conventional TFT) are fabricated using ZnO as the semiconductor material and Al2O3 as the dielectric material. The device operation is investigated using dc I-V, C-V, and transient pulse characterizations that eventually lead to the device layout optimization for ESD protection device design. The response of the protection device during normal circuit operation is evaluated in terms of both, the constant bias and the pulsed bias. The ESD robustness is evaluated using the transmission line pulse (TLP) measurements. Finally, the performance of the adTFT is compared to that of the conventional TFT in terms of the power to thermal breakdown during ESD and the power leakage during normal operation to highlight the primary advantage of the adTFT as an ESD protection device over the conventional one. The fabricated diode-connected adTFTs result in 1000-10000 times the power savings compared to the diode-connected conventional TFTs without sacrificing the ESD robustness. Therefore, the adTFT condenses the operation of an entire circuit into a single device and shows promise as a versatile building block for ESD protection and other circuit designs.

This doctoral thesis presents the results of the three-years activities carried out during the XXXIII cycle of the Ph.D. program in Electrical and Information Engineering of the Polytechnic University of Bari, Bari, Italy. The topic of this thesis is the optimal control of complex systems using Reinforcement Learning (RL) based techniques. Optimal control theory is aimed at finding control policies that minimize a predefined closed-loop performance criterion, namely the utility function. While optimal control for linear systems is a well-established framework, several issues arise when nonlinearities come into the picture. Feedback optimal control policies for nonlinear systems are found by solving the Hamilton-Jacobi-Bellman (HJB) equation, which is in general analytically intractable. Starting from the 1980s, considerable efforts have been made by the research community to overcome such intractability. This resulted in the development of new approaches based on RL that find approximated solutions of the HJB equation using Neural Networks (NNs). RL is an important branch of the Machine Learning theory. It is inspired by the animal world where living beings improve their behaviors by interacting with an unknown environment, evaluating the effect of their actions and modifying them accordingly. The combination of RL paradigms, NNs, and optimal control results in the Adaptive Dynamic Programming (ADP) approach. ADP algorithms find optimal control laws by means of different learning strategies. Such approach demonstrates the increasing penetration of Artificial Intelligence (AI) in the field of complex control systems. The main purpose of this thesis is to show the effectiveness of ADP-based control systems in real-world scenarios. In fact, although most of the ADP theory has been developed since the second half of the 2000s, experimental tests of real-world ADP-based controllers have only been published more recently. This thesis begins by over-viewing the main ADP algorithms that solve optimal control problems for nonlinear systems, covering the two main learning strategies: the Policy Iteration (PI) algorithm with on-policy learning and the PI algorithm with off-policy learning. The mathematical details of such approaches are presented, discussing the main properties along with pros and cons. Then, the powerful features of the ADP algorithm with off-policy learning are exploited to provide novel control strategies according to two different complex systems. It will be shown how the versatility and power of ADP-based techniques allow to solve control problems with different contexts and objectives in an innovative way. As first case study, the optimal control of mechatronic devices based on dielectric elastomer membranes, namely the Dielectric Elastomer Actuators (DEAs), is considered. A DEA is typically constituted by a flexible polymeric membrane that undergoes a deformation when excited with an electrical voltage. DEAs have recently received a significant interest due to their high energy density, high deformation ranges, and low production costs. They have also showed to be quite attractive in the context of several applications, ranging from micro-positioning systems to soft-robotic structures. However, the interesting features of the DEAs are limited by their strong nonlinear behavior and sensitivity to environmental conditions, which limit their penetration in the industrial sector. The strong nonlinearities due to the underlying physical behavior encouraged the development of advanced control strategies. Nevertheless, energy-efficient controllers have never been developed for such class of actuators. In this thesis, a novel minimum energy control strategy for DEAs is developed. The objective is to minimize the electrical energy required during a positioning task. In principle, the DEA dynamics can be detailed by an energy consistent model, which also describes the losses that occur in the actuator during any positioning task. An optimal feedback control strategy can be employed to minimize those losses, by formulating the energy-minimization problem as an optimal control problem. However, due to the involved nonlinearities, an analytic solution of the HJB equation does not exist. In this thesis, an ADP algorithm with off-policy learning is employed to deal with the optimal energy control problem. In particular, the ADP approach will be used as a tool to solve offline the HJB equation, deriving energy-efficient control laws for a given set of target displacement values. Finally, experimental tests will validate for the first time an energy consistent model of the DEA as well as the energy-efficient controllers. Substantially improvements in terms of energy saving will arise when comparing the proposed approach with other traditional control methods, such as Proportional Integral or feed-forward schemes. The second complex system where ADP is applied is a DC microgrid featuring power buffers. Due to the increasing penetration of DC sources and loads, such as photo-voltaic generators or electrical vehicles, DC microgrids have recently gained significant attention. DC distribution systems are more efficient and reliable than AC microgrids, where redundant conversion stages are present. Moreover, DC microgrids do not suffer of many AC-related issues, such as frequency synchronization or reactive power flows. However, due to a lack of damping inertia, DC systems can face instability issues when volatile source and loads are considered. A possible solution is represented by power buffers, which can be used as damping elements in the DC microgrid. A power buffer is a power converter with a large storage element that can be exploited to decouple the distribution grid from the final load. In fact, when abrupt load changes occur, the energy stored in the buffer compensates the transient mismatch. The input impedance seen by the network can be actively controlled by the power buffer during transients, so that the stability properties of the DC system are improved. By introducing a communication network on top of the physical grid, distributed control policies for such buffers are enabled. Their effective range of action is thus extended to the neighboring power buffers. In this way, power buffers can assist each other during abrupt load changes. This thesis investigates the cooperative distributed control of power buffers. The cooperative assistive control objective is formulated as an optimal control objective, where the single utility function is shared among all the buffers. In contrast with the existing literature, the nonlinear dynamics is considered. Thus, ADP will represent the key tool in designing such optimal policies. Clearly, when dealing with distributed control schemes, the communication topology plays a crucial role. Based on the configuration of the communication network, in this thesis two different control approaches will be presented. Firstly, the communication topology is fixed and inspired by the physical vicinity of the buffers. A set of optimal control policies able to provide assistance during abrupt load changes are learned offline, using the ADP with off-policy learning approach. Such policies are then interpolated in a real-time control scheme. The proposed approach overcome the issues of the existing distributed solutions for power buffers. For example, a feedback controller is directly provided, instead of open-loop policies that require additional control loop to be implemented. By considering the fully nonlinear dynamics, the proposed approach does not rely on small-signal approximations. Thus, performances and stability will be guaranteed also for large-signal variations. Experimental validations conducted in a Controller/Hardware-in-the-Loop (CHIL) environment will asses the effectiveness of the proposed approach. A second approach considers the communication topology a free parameter subject to optimization. In fact, there is no guarantee that a communication topology inspired by the physical vicinity is optimal with regard to the control objectives. Moreover, the energy availability of each power buffer is limited, thus the co-optimization of control performances and communication topologies is important when distributed solutions are considered. A sparsity-promoting optimal controller optimizes a closed-loop utility function, while minimizing at the same time the number of interactions between different control loops. Clearly, DC systems can benefit from sparse communication structures, minimizing computational and communication costs with a limited impact on the resulting closed-loop performances. However, the existing linear formulations for the sparsity-promoting optimal control are not practical for nonlinear systems as the DC microgrid with power buffers. This thesis presents the first attempt in solving sparsity-promoting optimal control problems for nonlinear systems. The versatility properties of the ADP algorithm with off-policy learning are exploited to provide such solution, without requiring the exact knowledge of the system dynamics. In fact, a single set of learning data is repetitively used to find optimal controllers for different communication topologies. The proposed data-driven algorithm employs Domain-of-Attraction estimation methods to check the stability of each distributed controller, while a Tabu Search procedure optimizes the combinatorial problem. The obtained sparsity-promoting controllers are then employed in the DC microgrid. The validity of the proposed approach will be assessed through exhaustive CHIL experiments. Quantitative and qualitative comparisons will show how the proposed methodology significantly outperforms existing approaches.

Abstract This paper closely examines the nature of the dielectric response of piezoceramics that are used as Adaptive Piezoelectric Sensoriactuators (APSAs). Firstly, it is demonstrated that he APSA possesses real time structural health monitoring abilities, based on the capacitance measurement of the piezoceramic. Secondly, nonideal behavior including lossy, hysteretic, and field dependence is measured in the piezoceramics and a method mitigating some of this response in the Adaptive Piezoelectric Sensoriactuator is proposed.

Dielectric elastomer(DE) is a kind of promising material bearing excellent activate properties including large deformations (up to 380%), high energy densities (up to 3.4 J/g), high efficiency, high responsive speed, good reliability and durability. Thus the DE actuator, sensors and energy harvester is widely used in the field of aeronautics and smart bionics. When an electric field is applied on the compliant electrodes of the dielectric elastomers, the polymer shrinks along the electric field and expands in the transverse plane. In consequence, the electric field becomes higher. This kind of positive feedback may cause the elastomer to thin down, resulting in an electromechanical stability. An analysis on the electromechanical stability of dielectric elastomer using arbitrary free-energy function with constant dielectric constant has been presented in Suo’s papers. In many research on the electromechanical stability analysis of DE actuator, DE’s dielectric constant is assumed to be a constant. This is only the truth if the dielectric elastomer undergoing limited deformation. Actually, a typical dielectric elastomer is a kind of crosslinked polymer. The structural symmetry of the macromolecular, the crosslinking degree, along with the tensile deformation can affect the dielectric permittivity enormously. For dielectric elastomers with higher crosslinking degree, or higher degree of molecular structural symmetry, its permittivity is relatively low. In addition, stretching can guide the macromolecule to be arranged in order, this can increase the intermolecular forces and reduce the activities of polar group, as a results, the dielectric constant will decrease. However, if the crosslinking degree is low and the deformation is well below the extension limit, the molecular units in the polymers can be polarized as freely as in a polymeric liquid. In this case the corresponding permittivity is unaffected by the deformation. Recent experimental research results also proved that the dielectric permittivity of dielectric elastomer changed while undergoing large deformation. According to Pelrine, the dielectric constant of the DE film is variable and it is a function of the area increase ratio which depends on stretch ratio. In this paper, approach for the electromechanical stability of a dielectric elastomer having variable dielectric constant is developed. The critical breakdown electric field is obtained. Simulation results proved that the prestretching process can enhance remarkably the electromechanical stability of dielectric elastomer. These results agree well with the experimental data and can be used as guidances in the design and fabrication of dielectric elastomer actuators.

Dielectric electroactive polymers are materials capable of mechanically adjusting their volume in response to an electrical stimulus. However, currently these materials require multi-step manufacturing processes which are not additive. This paper presents a novel 3D printed flexible dielectric material and characterizes its use as a dielectric electroactive polymer (DEAP) actuator. The 3D printed material was characterized electrically and mechanically and its functionality as a dielectric electroactive polymer actuator was demonstrated. The flexible 3-D printed material demonstrated a high dielectric constant and ideal stress-strain performance in tensile testing making the 3-D printed material ideal for use as a DEAP actuator. The tensile stress-strain properties were measured on samples printed under three different conditions (three printing angles 0°, 45° and 90°). The results demonstrated the flexible material presents different responses depending on the printing angle. Based on these results, it was possible to determine that the active structure needs low pre-strain to perform a visible contractive displacement when voltage is applied to the electrodes. The actuator produced an area expansion of 5.48% in response to a 4.3 kV applied voltage, with an initial pre-strain of 63.21% applied to the dielectric material.

ZnO based polymer composite materials are of great interest because of their excellent electrical, optical, semiconductor and biocompatible properties. In this study, we synthesize anisotropic composites of aligned ZnO rods in polydimethylsiloxane (PDMS) elastomer and study their dielectric properties as a function of applied electric field and frequency. Submicron ZnO rods are synthesized using an inexpensive, high yield chemical route. Washed and purified ZnO rods are then aligned in uncured PDMS at different electric field and frequency. We find that under electric field, ZnO rotates with their long axis in the direction of the electric field and before coalescing form chains in the silicone elastomer. From the optical microscopy images and in situ dielectric measurements, the best alignment parameters are found at 4 kV/mm and 10 kHz. These conditions are then selected to prepare aligned ZnO-PDMS composites. Complete curing of composites is confirmed using dynamic mechanical analysis (DMA). Our results show that aligned ZnO in uncured PDMS exhibit higher dielectric permittivity compared to random dispersion with the same composition. For the cured ZnO-PDMS composites, dielectric permittivity increases by 80% compared to random composites.

Actuators based on dielectric elastomers are a promising technology in robotic and mechatronic applications. Up to now, the practical electro-mechanical response and controllability of actuators based on dielectric elastomers are limited by the inadequacy of the employed driving circuits, which are based on voltage-regulated converters. In order to circumvent the aforementioned activation issues, the design procedure of a novel activation strategy for controlling dielectric elastomer actuators is presented in this article. The proposed electronic driver derives from the flyback converter topology and it is able of delivering to the dielectric elastomer actuator middle-frequency, current-pulse trains dependent on the duty-cycle value. The driver’s transformer, switching and protection circuit components design and optimization are based on an estimation of the dielectric elastomer actuator’s electrical parameters. The design of the transformer is crucial for the actuator’s performance and energy efficiency, meanwhile the driver’s switching and protection circuit components are important for the appropriate driver functioning and safety operation. The reported experimental results show that the proposed electronic driver performances are in accordance with the driver’s design.

Abstract Dielectric Elastomers (DE) considered as smart materials have increasing popularity in the scientific community. Despite a multitude of presented applications and devises based on DE, there is no standard model to describe them as transducers. Thus, their design is in general often time-consuming. To describe the operational behavior, mostly material equations describing the materials energy state through strain and stress are utilized. The driving mechanism is expressed through the commonly known Maxwell-pressure which is the result of the energy balance. This often-used approach is sufficient for quasi-static applications. Yet when considering the dynamic regime, a description of the locally effective directional forces is necessary. We propose an expansion to the existing modelling approach. Deepening the view on force mechanisms of electrostatic actuators and incorporating the solid body properties of elastomers into consideration. Thus, we gain a network description for dielectric elastomers as reversible electromechanical transducers near an operating point. The network model, allows for a sped-up design and simulation process, especially in the development of dynamic applications. Our findings are supported findings from previous literature and FEM-simulations.

Abstract This work calculates the quasi-static and dynamic stiffnesses of dielectric elastomer isolators that are mechanically loaded by nominal (constant) compressive forces with sinusoidal fluctuations and have constant voltages applied through their thickness. Quasi-static membrane stretches due to nominal compressive loads are determined numerically for a wide range of voltages, and quasi-static compressive stiffnesses are calculated from the corresponding through-thickness displacements. Two calculations of quasi-static stiffness are presented, with each intended to be used for a different analysis relevant to isolator system design. The first determines the stiffness by finding the average slope of the force-displacement curve, where the nominal compressive load of interest is divided by the corresponding nominal compressive deflection. This is called the average slope stiffness, and it should be used for static calculations that determine nominal load sharing in vibration isolation systems. The second approach calculates the local slope of the force-displacement curve numerically at a particular nominal compressive load. This is called the local slope stiffness. It is intended for vibration isolation analyses. The two notions of stiffness introduced from quasi-static loading are then used to calculate dynamic stiffnesses from the time-dependent stretches induced by sinusoidal load fluctuations. For the average slope calculation, the stiffness is determined by dividing the nominal compressive load by the average dynamic stretch of the isolator. The local slope stiffness is calculated by dividing the fluctuating component of the compressive load by the first harmonic of the dynamic stretch. Comparisons of the stiffness calculations are presented for a wide range of excitation frequencies that include the resonances of the isolator. The resulting stiffnesses are shown to be controllable by varying the though-thickness voltage. The frequency dependence of the isolator’s stiffness is determined numerically. At excitation frequencies near resonance, the dynamic stiffness of the isolator changes substantially, and multi-valued stiffnesses are possible. The stiffnesses determined in this work could be used in component or system-level lumped-parameter models used in the design of vibration control systems. Dielectric elastomer isolators could be used as variable stiffness devices with semi-active, open-loop control of their properties.

Artificial Muscles based on Dielectric Elastomers (DE) can potentially enable the realization of bio-inspired actuation systems whose intrinsic compliance and damping can be varied according to the task requirements. Nonetheless, the control of DE-based Variable Impedance Actuators (VIA) is not trivial owing to the non-linear viscoelastic response which characterizes the acrylic dielectrics commonly employed in practical devices. In this context, the purpose of the present paper is to outline a novel strategy for the control of DE-based VIA. Although the proposed methodology is applicable to generic DE morphologies, the considered system is composed of a couple of conically-shaped DE films in agonistic-antagonistic configuration. Following previously published results, the system dynamic model is firstly recalled. Then, a DE viscoelasticity compensation technique is outlined together with a control law able to shape the DE actuator impedance as desired. The operative limits of the system are explicitly considered and managed in the controller by increasing the operating DE actuator stiffness if required. In addition, the problem of model uncertainties compensation is also addressed. Finally, as a preliminary step towards the realization of a practical DE-based VIA, the proposed control approach is validated by means of simulations.