Biomimetic airship driven by dielectric elastomer actuators

Dynamic reconfiguration of antenna devices is becoming a prime need in emerging telecommunication and remote sensing systems operating in the portion of the electromagnetic (EM) spectrum spanning from millimeter-wave (MMW) to terahertz. Different techniques and materials are currently available for the implementation of a given EM reconfiguration in antenna systems at microwaves andMMW, such as semiconductors, RF-MEMS, Liquid Crystal, and ferroelectrics. However, a common feature to all these technologies is the significant increased loss and complexity with regard to the devices fixed counterpart. Both loss and complexity further increase when high-performance reconfiguration capabilities are addressed atMMWand above, constituting a major limitation to the future deployment of dynamically controllable systems. Advanced performance, low-complexity and low-cost are, therefore, the cornerstones in the development of new technologies for antenna reconfiguration. In this framework, the main objective of this thesis is to contribute to the advancement of low-cost and efficient technologies enabling antenna reconfiguration capabilities fromMMWto THz frequencies. Within this scope, it is proposed the analysis, design and implementation of mechanically reconfigurable devices using dielectric elastomer actuators (DEAs). DEAs are an emerging technology that possesses unique properties, and represents a potential alternative to conventionalmechanical reconfiguration schemes. DEAs are especially attractive for their large strain combined with low-costmaterials and fabrication, analog control and near-zero DC power consumption. These characteristics make them particularly suited to the realization of low-cost and low-loss reconfigurable antennas. The high potential of DEAs for the realization of adaptive devices is experimentally validated by the development of different prototypes operating atMMWand THz frequencies: i) a very low-loss true-time-delay phase shifter operating at Ka-band; ii) a Ka-band reconfigurable reflectarray with 1-D beam-scanning capability; iii) a stretchable and beam-scanning THz reflectarray exhibiting the uncommon potential for the implementation of conformal or reconfigurable devices based on mechanical stretching. The designs and concepts demonstrated in this thesis pave the way for the evolution of new DEA-based reconfigurable devices resulting in low-cost, low loss and compact structures.

Agriculture and horticulture depend heavily on human labor to perform tasks that are often dirty, hazardous, and highly repetitive. One reason for the lack of automation of these tasks is the absence of suitable robotic handling equipment. Rigid robotic manipulators are typically incapable of performing dexterous manipulation tasks such as harvesting apples as they lack the ability to adapt to objects of various shapes and sizes. Such robotic manipulators need a large number of sensors and actuators to overcome these challenges, making them overly complex and not very robust. Therefore, the development of robotic manipulators for dexterous manipulation tasks has begun to focus on morphological computation, in which at least some aspects of the control are outsourced to the body of the robot. Taking inspiration from grasping mechanisms in natural systems, the field of soft robotics attempts to address this problem by constructing robots from soft materials. Although soft robotics may be the key to realizing automation of dexterous manipulation tasks, the current commercially available soft robotic grippers are only capable of performing simple pick-and-place tasks with open-loop control. This limited capability is in large part due to a lack of techniques to endow these manipulators with a sense of self-movement and body position, known as proprioception. Proprioception is a simple problem for conventional robots with rigid members and discrete joints, as the body position can be easily reconstructed using the information from encoders in the robots’ joints. However, it is a highly challenging problem for soft robots with virtually infinite degrees of freedom and above all, no suitable off-the-shelf sensors…

Soft robotics, a research field wherein robots are fabricatedfromcompliant materials, has sparked widespread research interest becauseof its potential applications in a variety of scenarios. In soft robots,luminescence is an important functionality for communication and informationtransmission, and it is typically achieved through electroluminescence,which relies on synthetic substances activated by external electricsources, such as batteries. This paper focuses on the use of luciferase,a biologically derived luminescent enzyme, as a luminescent material.Bioluminescence, which is triggered by the luciferin–luciferasereaction, is highly energy-efficient, nontoxic, and eco-friendly.In this regard, a mammalian cell-derived secreted luciferase bioluminescentliquid was developed. This bioluminescent liquid is strongly bright,stable, freezable, and scalable for use as a soft robotic material.To investigate the applicability of this bioluminescent liquid tosoft robotics, it was incorporated as an electrode in electricallydriven soft actuators, sensors, and robots. Specifically, dielectricelastomer sensors (DESs) and dielectric elastomer actuators (DEAs)were fabricated and characterized using established fabrication processes.The resistivity of the bioluminescent liquid was found to be 448.1Ω·cm. When the DES was subjected to uniaxial strain, itexhibited a linear response and large deformation of up to 200% strain,with a simultaneous luminance change of 27%. The DEA displayed anareal strain of 46.0% and a luminance change of 31% at an appliedvoltage of 3.4 kV. The waterproof bending DEA generated a tip angleof 21.8° at 10 kV and was applied to a jellyfish robot that couldswim in water at a speed of 2.1 mm/s. The experimental results demonstratedthe successful operation of these devices, validating the conceptof energy-efficient, safe, and environmentally friendly bioluminescentsoft robots.

Conventional rigid actuators, such as DC servo motors, face challenges in utilizing them in artificial muscles and soft robotics. Dielectric elastomer actuators (DEAs) overcome all these limitations, as they exhibit complex and fast motions, quietness, lightness, and softness. Recently, there has been much focus on studies of the DEAs material’s non-linearity, the non-linear electromechanical coupling, and viscoelastic behavior of VHB and silicone-based conical DEAs having compliant electrodes that are based on graphite powder and carbon grease. However, the mitigation of overshoot that arises from fast response conical DEAs made with solid electrodes has not received much research focus. In this paper, we fabricated a conical configuration of multi-walled carbon nanotube/polydimethylsiloxane (MWCNT/PDMS) based DEAs with a rise time of 10 ms, and 50% peak overshoot. We developed a full feedback state-based linear-quadratic regulator (LQR) having Luenberger observer to mitigate the DEAs overshoot in both the voltage ON and OFF instances. The cone DEA’s model was identified and a stable and well-fitting transfer function with a fit of 94% was obtained. Optimal parameters Q = 70,000, R = 0.1, and Q = 7000, R = 0.01 resulted in the DEA response having a rise time value of 20 ms with zero overshoot, in both simulations and experiments. The LQR approach can be useful for the control of fast response DEAs and this would expand the potential use of the DEAs as artificial muscles in soft robotics.

For dielectric elastomer actuators (DEAs) to find more widespread adoption in commercial products, a better understanding of their durability is needed. By determining the different mechanisms contributing to ageing and breakdown in DEAs, new methods for improving their lifetime could be devised. Using a newly-developed setup for performance and lifetime evaluation of DEAs, we investigate how the choice of electrode material and application process influence lifetime. Our investigation is focused on DEAs made from a slide ring material (SRM) as the dielectric. SRM is an interesting candidate for DEAs as it offers a much higher permittivity (εr < 7) than commonly used silicone rubbers. However, the log-term durability of SRM DEAs, and how it is influenced by external factors such as the type of electrode material, has not yet been thoroughly studied. We tested SRM DEAs made with various carbon-based electrode materials, different application methods including blade casting, inkjet- and pad-printing, and ink formulations with different solvents. Average lifetime of the DEAs at 50 Hz and 40 V/μm varied from under 1 h (160,000 cycles) to over 100 h (16,000,000 cycles) between the different electrode types. Our experiments show that not only the type of material but also the thickness and topology of the electrode layer can greatly influence lifetime. We observed that some solvents such as isopropyl alcohol cause swelling and wrinkling of the SRM membrane, but that this has no negative impact on lifetime. We propose that high electrode resistance can improve lifetime by limiting current and reducing the damage from heating, arcing, or partial discharge. This effect can also be achieved by addition of an external resistor. Our results suggest that charge and discharge current of DEAs should always be limited to the very minimum needed for the intended application in order to reduce degradation.

Dielectric elastomer (DE) is widely used in various fields because of its advantages of large deformation, lightweight, and good flexibility. In this article, based on our previous research work, the actuation performance of the cone dielectric elastomer actuator (DEA) is studied first, and the influence of loading voltage, domain ratio, and additional load on the displacement and output force of DEA is analyzed and discussed. Then, a three-dimensional model of displacement and force of DEA is established. At last, the relationship between the structure and the performance of DE is discussed from the microscopic point of view. The results show that the output displacement and force of the cone DEA increase with the increase of loading voltage and additional load. The three-dimensional model graph of cone DEA can reflect the performance change of DEA well. The design with different domain ratios is the key factor that influences the final DEA actuator, which has a great influence on the cross-link density and chain length in the DE structure. By making clear the regulating function of external environment factors, we can design DEA with different configurations, which lays a good foundation for the further development of DEA and enlarges the potential application scope of DEA.

Energy in Robotics: An Interdisciplinary Challenge

Dielectric elastomer actuators (DEA) have gained increased attention in the design of electrically driven soft and lightweight robotic devices. They rely on Coulomb forces for their actuation and are able to undergo large displacements as a function of the applied electrical field. Fabricating DEAs with additive manufacturing opens the way to a personalized and cost‐efficient fabrication of soft active devices. In particular, extrusion‐based techniques provide a versatile strategy to multi‐material 3D print DEAs by using electrically conductive and soft filaments with high elongation. One key aspect of DEAs is the electrode's performance, which has to provide a high electrical conductivity and a good adhesion to the sandwiched membrane. Herein, selected 3D printing parameters were varied to optimize the electrode infill density and electrode infill direction. Mechanical and electrical properties, as well as actuation performance, of fully 3D‐printed DEA demonstrators were studied comprehensively. With the optimized parameter set, a maximum DEA displacement of 91% was achieved with respect to the free length of the actuator. This is comparable to the performance of non‐3D‐printed DEAs and demonstrates the potential of 3D printing for the production of DEAs undergoing large deformations while leveraging the design and manufacturing freedom of multi‐material 3D printing.

My new colleague has artificial muscles: a DEA based approach for inherently compliant robotic systems

Optical imaging systems using varifocal lenses have been widely used in many applications over the past several decades, such as machine vision devices, consumer electronic products, and medical instruments. Traditional varifocal lenses often consist of multiple solid focal length refractive optical elements. The varifocal ability is obtained by dislocating these optical elements along the optical axis over specific distances using mechanical driving mechanisms. It makes the traditional optical varifocal imaging systems suffer from bulky dimensions, slow response speed, complicated configuration, and discrete magnifications. Adaptive varifocal lenses have been a better choice to address the aforementioned limitations of traditional varifocal lenses. Dielectric elastomer actuators (DEA), which can effectively respond to an electric field and result in shape deformation, have been used to develop various adaptive lenses. This paper aims to give a brief review of adaptive varifocal lenses based on DEA. First, this paper describes the basic physical mechanism of DEA. Second, this paper reviews adaptive varifocal liquid lenses based on DEA and introduces their material, structure, and fabrication process, focusing on their unique advantages, such as fast response speed and compactness. However, despite these merits, the adaptive varifocal liquid lens still has challenges in environment stability and liquid leakage. To address these challenges, adaptive varifocal soft solid lenses based on DEA have been proposed, which are also reviewed. In addition, other adaptive varifocal lenses, including metalens, Fresnel lens, microlens array, and Alvarez lens, are also presented. Finally, the prospects and challenges for the development of adaptive varifocal lenses based on DEA are discussed.

When dielectric elastomer actuators (DEAs) are actuated via high voltage, their electrical capacitance changes according to the geometry. Therefore, displacement of the actuator can be correlated to the change in capacitance, thus opening up the possibility of self-sensing DEA devices. Self-sensing can be exploited to achieve a sensorless closed loop DEA system, which is attractive from size, weight, and cost perspectives. This research work presents an embedded control system, which enables self-sensing closed loop position control of a DEA. The proposed architecture is cost effective, compact in size, easy to integrate as well as to reprogram in comparison to previous self-sensing implementations relying on FPGA systems. In the developed setup, the online self-sensing algorithm is used for estimation of displacement in a spring-biased strip DEA. For this system, understanding and mapping the correlation between estimated capacitance, applied voltage, and resulting displacement is essential for achieving an accurate DEA position reconstruction. An experimental setup is developed, and used to test a spring-biased DEA system. Self-sensing based feedback control is then used to achieve a tight regulation of the actuator displacement. To verify the effectiveness of the sensorless closed loop control system, its performance is finally compared to sensor-based feedback architectures.

Dielectric elastomer actuators (DEAs) can generate a large strain, but they are often short of strength and stress change for robotic applications. Their maximum actuation is limited by pre-mature electrical breakdown. This paper showed that dielectric oil encapsulation helps stop pre-mature breakdowns. Consequently, the oil encapsulated DEAs can sustain an ultra-high electrical breakdown field up to 835MV/m, which is 46% higher than 570MV/m in air without oil encapsulation. These oil-encapsulated DEA can produce muscle-like high stress change up to 1.05MPa, which is 75% higher than that produced by the DEA in air. Furthermore, oil encapsulation is also found effective to suppress electrical breakdown of tubular DEAs. These high-stress dielectric elastomer actuators show good potential to drive human-inspired robots.

Dielectric elastomer actuators (DEAs) stick out among explored soft actuators due to their advantages of large deformation, high energy density, fast response and low cost. However, DEAs still are far away from robotic practical applications because of the difficulty of the actuation control. This paper presents a novel control approach for control a 3D DEA control. The fabrication of the proposed soft actuator is introduced and the dynamics of the DEA is analyzed via several experiments. Furthermore, a bio-inspired cerebellum nonlinear controller for achieving accurate movements of the DEAs are designed and analyzed. Finally, the simulation and experiments have been carried out for the developed system validation.

Dielectric elastomer actuator (DEA) based flexible and stretchable electronics have attracted considerable attention over the past decades. The electrode components play an important role in the DEA performance. In this work, we studied how the incorporation of soft matrices in the electrodes affects the DEA actuation. The ultrasonic spraying was used to fabricate multiwall carbon nanotube (MWCNT) based electrodes for DEA. The results indicated that the addition of a water-soluble block polymer and silicone gel (acting as the soft matrices) could improve the actuation of the DEA with neat MWCNT electrodes by ~10% and ~24%, respectively. An inkjet printing ink, consisting of polydimethylsiloxane (PDMS), carbon black (CB) and chlorobenzene, was further developed. The stability, particle size, resistance, and morphology of 1-3 printing layers were characterized. The DEA with the inkjet-printed CB/PDMS electrodes showed 50% area stain at 2500 V, which is higher than the actuation with neat CB powder or CNT electrodes reported previously. Both results of the ultrasonic spraying and inkjet printing confirmed that the incorporation of soft matrices in the electrodes is helpful for DEA actuation.

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.