Scott Stanford

Dielectric elastomers: generator mode fundamentals and applications

Dielectric elastomers have shown great promise as actuator materials. Their advantages in converting mechanical to electrical energy in a generator mode are less well known. If a low voltage charge is placed on a stretched elastomer prior to contraction, the contraction works against the electrostatic field pressure and raises the voltage of the charge, thus generating electrical energy. This paper discusses the fundamentals of dielectric elastomer generators, experimental verification of the phenomenon, practical issues, and potential applications. Acrylic elastomers have demonstrated an estimated 0.4 J/g specific energy density, greater than that of piezoelectric materials. Much higher energy densities, over 1 J/g, are predicted. Conversion efficiency can also be high, theoretically up to 80-90%; the paper discusses the operating conditions and materials required for high efficiency. Practical considerations may limit the specific outputs and efficiencies of dielectric elastomeric generators, tradeoffs between electronics and generator material performance are discussed. Lastly, the paper describes work on potential applications such as an ongoing effort to develop a boot generator based on dielectric elastomers, as well as other applications such as conventional power generators, backpack generators, and wave power applications.

Electroelastomers: applications of dielectric elastomer transducers for actuation, generation, and smart structures

Electroactive polymers (EAPs) can overcome many limitations of traditional smart material and transducer technologies. A particularly promising class of EAP is dielectric elastomer, also known as electroelastomer. Dielectric elastomer transducers are rubbery polymer materials with compliant electrodes that have a large electromechanical response to an applied electric field. The technology has been developed to the point where exceptional performance has already been demonstrated: for example, actuated strains of over 300 percent. These strains and the corresponding energy densities are beyond those of other field-activated materials including piezoelectrics. Because of their unique characteristics and expected low cost, dielectric elastomer transducers are under development in a wide range of applications including multifunctional (combined actuation, structure, and sensing) muscle-like actuators for biomimetic robots; microelectromechanical systems (MEMS); smart skins; conformal loudspeakers; haptic displays; and replacements for electromagnetic and pneumatic actuators for industrial and commercial applications. Dielectric elastomers have shown unique performance in each of these applications; however, some further development is required before they can be integrated into products and smart-materials systems. Among the many issues that may ultimately determine the success or failure of the technology for specific applications are durability, operating voltage and power requirements, and the size, cost, and complexity of the required electronic driving circuitry.

Electroadhesive robots—wall climbing robots enabled by a novel, robust, and electrically controllable adhesion technology

This paper describes a novel clamping technology called compliant electroadhesion, as well as the first application of this technology to wall climbing robots. As the name implies, electroadhesion is an electrically controllable adhesion technology. It involves inducing electrostatic charges on a wall substrate using a power supply connected to compliant pads situated on the moving robot. High clamping forces (0.2-1.4 Newton supported by 1 square centimeter of clamp area, depending on substrate) have been demonstrated on a wide variety of common building substrates, both rough and smooth as well as both electrically conductive and insulating. Unlike conventional adhesives or dry adhesives, the electroadhesion can be modulated or turned off for mobility or cleaning. The technology uses a very small amount of power (on the order of 20 microwatts/Newton weight held) and shows the ability to repeatably clamp to wall substrates that are heavily covered in dust or other debris. Using this technology, SRI International has demonstrated a variety of wall climbing robots including tracked and legged robots.

Development and Testing of the Mentor Flapping-Wing Micro Air Vehicle

In 1997 the Defense Advanced Research Projects Agency initiated a program to explore the possibility of micro air vehicles for the purpose of individually portable surveillance systems for close-range operations. The various contractors approached the problem in several ways, such as developing tiny fixed-wing airplanes, rotary-wing aircraft, and ornithopters mimicking animal flight This paper describes one such flapping-wing aircraft, which drew upon the clap-fling phenomenon that is exploited by many flying animals and insects for lift generation. Essentially this aircraft was a mechanical simulation of hummingbird flight, though with two sets of wings to eliminate the unbalanced side-to-side flapping forces. Two flying demonstration models were built, one with an internal-combustion engine and another with an electric motor. In both cases, these incorporated a drive train to reduce the high rpm rotary shaft motion to lower-frequency oscillation for flapping. Also required was a programmable logic board for stabilization. Successful hovering flight was achieved with both models, and initial studies of transition to horizontal flight were also explored.

Dielectric elastomer artificial muscle actuators: Toward biomimetic motion

To achieve desirable biomimetic motion, actuators must be able to reproduce the important features of natural muscle such as power, stress, strain, speed of response, efficiency, and controllability. It is a mistake, however, to consider muscle as only an energy output device. Muscle is multifunctional. In locomotion, muscle often acts as an energy absorber, variable-stiffness suspension element, or position sensor, for example. Electroactive polymer technologies based on the electric-field-induced deformation of polymer dielectrics with compliant electrodes are particularly promising because they have demonstrated high strains and energy densities. Testing with experimental biological techniques and apparatus has confirmed that these dielectric elastomer artificial muscles can indeed reproduce several of the important characteristics of natural muscle. Several different artificial muscle actuator configurations have been tested, including flat actuators and tubular rolls. Rolls have been shown to act as structural elements and to incorporate position sensing. Biomimetic robot applications have been explored that exploit the muscle-like capabilities of the dielectric elastomer actuators, including serpentine manipulators, insect-like flapping-wing mechanisms, and insect-like walking robots.

Multifunctional electroelastomer roll actuators and their application for biomimetic walking robots

Electroelastomers (also called dielectric elastomer artificial muscles) have been shown to exhibit excellent performance in a variety of actuator configurations, but making a compact, free-standing, muscle-like actuator capable of obtaining good performance has been a challenge. By rolling highly prestrained electroelastomer films around a compression spring, we have demonstrated Multifunctional Electroelastomer Rolls (MERs) that combine load bearing, actuation, and sensing functions. The MER spring rolls are compact, have a potentially high electroelastomer-to-structure weight ratio, and can be configured to actuate in several ways, including axial extension, bending, and as multiple degree-of-freedom actuators that combine both extension and bending. One degree-of-freedom (1-DOF), 2-DOF, and 3-DOF MERs have all been demonstrated through suitable electrode patterning on a single monolithic substrate. A 1-DOF MER with 9.6 g weight, 12 mm diameter, and 65 mm total length can deliver up to 15 N force and 12 mm stroke. Its capacitance is around 13 nF and changes linearly with strain during axial tension or compression. The MERs are useful in a number of applications where compact and high-stroke actuation is required. The applications as artificial muscles are particularly appealing, as multifunctionality prevails in natural muscles.

Interpenetrating networks of elastomers exhibiting 300% electrically-induced area strain

Mechanical prestrain is generally required for most electroelastomers to obtain high electromechanical strain and high elastic energy density. However, prestrain can cause several serious problems, including a large performance gap between the active materials and packaged actuators, instability at interfaces between the elastomer and prestrain-supporting structure, and stress relaxation. Difunctional and trifunctional liquid additives were introduced into 400% biaxially prestrained acrylic films and subsequently cured to form the second elastomeric network. The goal of this research was to determine the effect of different functional additives and concentrations on the microstructure, the mechanical properties, and the actuation of composite films. In the as-obtained interpenetrating polymer networks (IPNs), the additive network can effectively support the prestrain of the acrylic films and as a result, eliminate the external prestrain-supporting structure. However, the large amount of additive used to completely preserve prestrain was found to make the films too stiff, causing damage to IPN composite films. Furthermore, the interpenetrating network formed from a trifunctional monomer is more effective than that formed from a difunctional monomer in supporting the high tension of the VHB network. This high efficiency trifunctional additive leads to the enhancement of the breakdown field, due to less damage on the microstructure. The IPN composite films without external prestrain exhibit electrically-induced strains up to 300% in area, comparable to those of VHB 4910 films under high prestrain conditions.

Biologically inspired hexapedal robot using field-effect electroactive elastomer artificial muscles

Small, autonomous mobile robots are needed for applications such as reconnaissance over difficult terrain or internal inspection of large industrial systems. Previous work in experimental biology and with legged robots has revealed the advantages of using leg actuators with inherent compliance for robust, autonomous locomotion over uneven terrain. Recently developed field-effect electroactive elastomer artificial muscle actuators offer such compliance as well as attractive performance parameters such as force/weight and efficiency, so we developed a small (670 g) six-legged robot, FLEX, using AM actuators. Electrically, AM actuators are a capacitive, high-impedance load similar to piezoelectrics, which makes them difficult to rive optimally with conventional circuitry. Still, we were able to devise a modular, microprocessor-based control system capable of driving 12 muscles with up to 5,000 V, operating form an on- board battery. The artificial muscle actuators had excellent compliance and peak performance, but suffered from poor uniformity and degradation over time. FLEX is the first robot of its kind. While there is room for improvement in some of the robot systems such as actuators and their drivers, this work has validated the idea of using artificial muscle actuators in biologically inspired walking robots.

High electromechanical performance of electroelastomers based on interpenetrating polymer networks

The electromechanical performance of interpenetrating polymer networks (IPN) in which one elastomer network is under high tension balanced by compression of the second network, were investigated. Uniaxial stress relaxation analysis confirmed significant decrease in viscoelasticity in comparison with 3M VHB films, the primary component network in the IPN films. In dynamic mechanical analysis, the IPN composite showed a higher mechanical efficiency, suggesting delayed relaxation of the acrylic chains in the presence of IPN formation. This improvement was found to be dependant on the contents of poly(TMPTMA). Actuation performance without mechanical prestrain showed that these IPN electroelastomers had demonstrated high elastic strain energy density (3.5 MJ/m3) and a high electromechanical coupling factor (93.7%). These enhanced electromechanical performances indicate that IPN electroelastomer should be suitable for diverse applications.

Characterization of electroelastomers based on interpenetrating polymer networks

Interpenetrating polymer networks (IPN) in which one elastomer network is under high tension balanced by compression of the second network have been shown to exhibit electrically-induced strain up to 300% and promise a number of polymer actuators with substantially enhanced performance and stability. This paper describes the mechanical and thermal properties of the IPN electroelastomer films. The quasi-linear viscoelastic model and Yeoh strain energy potential are used to characterize the viscoelastic response and stress-strain behavior of the IPN films in comparison with 3M VHB films, primary component network in the IPN films. Material parameters were determined from uniaxial stress relaxation experiments. An analysis of the results confirms that the IPN composites have reduced viscoelasticity and fast stress-strain response due to preserved prestrain. Differential scanning calorimetry showed two glass transition temperatures that are slightly shifted from the two component networks, respectively. The two networks in the IPN are considered to be independent of each other. The thermal property is also studied with termogravimetric analysis (TG).