Joseph S. Eckerle

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.

Electrostrictive polymer artificial muscle actuators

Many new robotic and teleoperated applications require a high degree of mobility or dexterity that is difficult to achieve with current actuator technology. Natural muscle is an actuator that has many features, including high energy density, fast speed of response, and large stroke, that are desirable for such applications. The electrostriction of polymer dielectrics with compliant electrodes can be used in electrically controllable, muscle-like actuators. These electrostrictive polymer artificial muscle (EPAM) actuators can produce strains of up to 30% and pressures of up to 1.9 MPa. The measured specific energy achieved with polyurethane and silicone polymers exceeds that of electromagnetic, electrostatic, piezoelectric, and magnetostrictive actuators. A simple model using linear elastic theory can predict EPAM actuator performance from mechanical and electrical material properties and load conditions. A spherical joint for a highly articulated (snake-like) manipulator using EPAM actuator elements has been demonstrated. A rotary motor using EPAM actuator elements has been shown to produce a specific torque of 19 mNm/g and a specific power of 0.1 W/g. An improved EPAM motor could produce greater specific power and specific torque than could electric motors.

Acoustical performance of an electrostrictive polymer film loudspeaker

A new type of loudspeaker that generates sound by means of the electrostrictive response of a thin polymer film is described. Electrostrictive polymer film (EPF) loudspeakers are constructed with inexpensive, lightweight materials and have a very low profile. The films are typically silicone and are coated with compliant electrodes to allow large film deformations. Acoustical frequency response measurements from 5 x 5 cm (planar dimensions) prototype EPF loudspeakers are presented. Measurements of harmonic distortion are also shown, along with results demonstrating reduced harmonic distortion achieved with square-root wave shaping. Applications of EPF loudspeakers include active noise control and general-purpose flat-panel loudspeakers.

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.

Sound radiation properties of dielectric elastomer electroactive polymer loudspeakers

Dielectric elastomer electroactive polymer (EAP) loudspeakers have been built and demonstrated at SRI International. Dielectric elastomer loudspeakers have the advantages of being very lightweight and able to conform to any shape or surface, making them attractive as low-profile, surface-mounted speakers in rooms or vehicle interiors, and for applications in active noise control. Loudspeaker performance depends on a number of mechanical factors, such as speaker shape and mechanical bias, as well as on electrical driving characteristics. This paper discusses important aspects of loudspeaker performance, including sound pressure level and directivity.

Programmable surface deformation: thickness-mode electroactive polymer actuators and their applications

Many different actuator configurations based on SRI International’s dielectric elastomer (DE) type of electroactive polymer (EAP) have been developed for a variety of applications. These actuators have shown excellent actuation properties including maximum actuation strains of up to 380% and energy densities of up to 3.4 J/g, using the planar mode of actuation. Recently, SRI has investigated different configurations of DE actuators that allow complex changes in surface shape and thus the creation of active surface texture. In this configuration, the “active” polymer film is bonded or coated with a thicker passive layer, such that changes in the polymer thickness during actuation of the DE device are at least partially transferred to (and often amplified by) the passive layer. Although the device gives out-of-plane motion, it can nonetheless be fabricated using two-dimensional patterning. The result is a rugged, flexible, and conformal skin that can be spatially actuated by subjecting patterned electrodes on a polymer substrate to an electric field. Using thickness-mode DE, we have demonstrated thickness changes of the order of 0.5 - 2 mm by laminating a passive elastomeric layer to a DE polymer that is only 60 μm in thickness. Such thickness changes would otherwise require a very large number of stacked layers of the DE film to produce comparable surface deformations. Preliminary pressures of 4.2 kPa (0.6 psi) in a direction normal to the plane of the DE film have been measured. However, theoretical calculations indicate that pressures of the order of 100 kPa are feasible using a single layer of DE film. Stacking multiple layers of DE film can lead to a further increase in achievable actuation pressures. Even with current levels of thickness change and actuation pressures, potential applications of such surface texture change are numerous. A thin, compliant pad made from these actuators can have a massaging or sensory augmentation function, and can be incorporated into garments if desired. The bumps and troughs could act as valves or pumping elements in a fluidic or microfluidic system. Such a device could also be the basis of a smart skin that controls boundary-layer flow properties in a boat or airplane so as to reduce overall drag. The DE elements of the pad can also be used as sensors to make a touch-sensitive skin for recording human interaction with the environment. By driving a thin, compliant vibrating layer at resonant frequencies, one can also configure these devices as solid or fluidic conveyors that transport material on a macroscopic or microscopic scale.

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.

Silicon pressure transducer arrays for blood-pressure measurement

Abstract Arterial tonometry is a technique for blood-pressure measurement that uses an array of pressure transducers pressed against the skin overlying an artery. A new construction for tonometer transducer arrays in which several transducers share a single diaphragm is described. The transducer array is fabricated from silicon using anisotropic etching. An epitaxial layer is used to stop the etch and thereby define the thickness of the diaphragm. Theoretical analyses are employed to predict transducer sensitivity and crosstalk. The impact of crosstalk on blood-pressure measurement accuracy is analyzed. This impact is acceptably small if the width of the transducer diaphragm is small compared to the artery being measured. Transducer arrays employing the new, shared-diaphragm construction exhibited sensitivities of about 25 to 50 μ V V −1 mmHg −1 and acceptable crosstalk.

Stretching the Capabilities of Energy Harvesting: Electroactive Polymers Based on Dielectric Elastomers

Dielectric elastomer actuators are “stretchable capacitors” that can offer muscle-like strain and force response to an applied voltage. As generators, dielectric elastomers offer the promise of energy harvesting with few moving parts. Power can be produced simply by stretching and contracting a relatively low-cost rubbery material. This simplicity, combined with demonstrated high energy density and high efficiency, suggests that dielectric elastomers are promising for a wide range of energy-harvesting applications. Indeed, dielectric elastomers have been demonstrated to harvest energy from human walking, ocean waves, flowing water, blowing wind, pushing buttons, and heat engines. While the technology is promising and advances are being made, there are challenges that must be addressed if dielectric elastomers are to be a successful and economically viable energy-harvesting technology. These challenges include developing materials and packaging that sustain a long lifetime over a range of environmental conditions, designing the devices that stretch the elastomer material uniformly, and system issues such as practical and efficient energy-harvesting circuits.