Marc Matysek

Fabrication and Application of Miniaturized Dielectric Elastomer Stack Actuators

In dielectric elastomer stack actuators (DEAs), the driving voltage is reduced using thin dielectric layers. They are efficiently fabricated in an automated process. To produce thin dielectric films, uncured polydimethylsiloxane (PDMS) is spun to a thickness below 100 μm. After curing graphite, electrodes are sprayed on the PDMS surface and the next dielectric film can be applied on top. The main process steps are explained in detail. We show that the thickness variation within one layer is smaller than 4%. The electrodes are sprayed to have a sheet resistance of 10 kΩ/sq. The technology presented here is able to produce DEAs with layer thicknesses down to 5 μm. Hence, it is possible to design and fabricate actuators, which are driven only at 150 V. Arrays of small actuators with a resolution of 1 mm can be produced as well as single actuator elements with a diameter up to 40 mm. Finally, the potential of this technology is demonstrated by two examples varying highly in their application. The first application is a vibrotactile display. It generates a perceptive vibration of 125 Hz at a driving voltage of 600 V. The second application is a peristaltic pump with a maximum flow rate of 12 μL/min.

Standards for dielectric elastomer transducers

Dielectric elastomer transducers consist of thin electrically insulating elastomeric membranes coated on both sides with compliant electrodes. They are a promising electromechanically active polymer technology that may be used for actuators, strain sensors, and electrical generators that harvest mechanical energy. The rapid development of this field calls for the first standards, collecting guidelines on how to assess and compare the performance of materials and devices. This paper addresses this need, presenting standardized methods for material characterisation, device testing and performance measurement. These proposed standards are intended to have a general scope and a broad applicability to different material types and device configurations. Nevertheless, they also intentionally exclude some aspects where knowledge and/or consensus in the literature were deemed to be insufficient. This is a sign of a young and vital field, whose research development is expected to benefit from this effort towards standardisation.

Novel multilayer electrostatic solid state actuators with elastic dielectric

Solid state actuators provide deformation and actuation forces mainly excited by electric fields. Piezoelectric actuators are well established providing high forces at low strain due to their material characteristic. Electrostatic solid-state actuators consist of elastic dielectric layers between compliant electrodes. Applying electric fields of up to 100 V/μm at the electrodes the dielectric contracts due to electrostatic forces and expands in orthogonal direction. We use high elastic silicone elastomers with thin graphite powder electrodes. In order to increase the absolute strain values at limited voltage, we have developed a novel multilayer process technology to fabricate elastomer stack actuators with up to 100 layers. The electromechanical properties of the actuators have been evaluated theoretically and characterised experimentally. Maximum strain values up to 20% for prestressed multilayer films have been achieved. The novel multilayer fabrication technology provides multilayer stack actuators with various electrode patterns like universal linear actuators or matrix arrays for a wide range of applications as tactile displays for telemanipulation or Braille displays. The strain in vertical direction versus driving voltage shows a hysteresis due to viscous friction in the elastomer layers. These measurements correspond to a viscoelastic theoretical model. The mechanical stress versus strain characteristic shows a strong nonlinearity for strains > 30%. The dynamic characteristic has been evaluated by measuring the mechanical impedance in the frequency range of 2 to 1000 Hz.

The dielectric breakdown limit of silicone dielectric elastomer actuators

Soft silicone elastomers are used in a generation of dielectric elastomer actuators (DEAs) with improved actuation speed and durability compared to the commonly used, highly viscoelastic polyacrylate 3M VHB™ films. The maximum voltage-induced stretch of DEAs is ultimately limited by their dielectric breakdown field strength. We measure the dependence of dielectric breakdown field strength on thickness and stretch for a silicone elastomer, when voltage-induced deformation is prevented. The experimental results are combined with an analytic model of equi-biaxial actuation to show that accounting for variable dielectric field strength results in different values of optimal pre-stretch and thickness that maximize the DEA actuation.

Combined driving and sensing circuitry for dielectric elastomer actuators in mobile applications

Dielectric elastomer stack actuators (DESA) promise breakthrough functionality in user interfaces by enabling freely programmable surfaces with various shapes. Besides the fundamental advantages of this technology, like comparatively low energy consumption, it is well known that these actuators can be used as sensors simultaneously. The work we present in this paper is focused on the implementation of a DEA-based tactile display into a mobile device. The generation of the driving voltage of up to 1.1 kV out of a common rechargeable battery and the implementation of the sensor functionality are the most challenging tasks. To realize a large range of tactile experiences, both static and dynamic driving voltages are required. We present a structure combining different step-up topologies to realize the driving unit. The final circuitry complies with typical requirements for mobile devices, like small size, low weight, high efficiency and low costs. The sensing functionality has to be realized for different actuator elements regardless of their actual state. An additional sensing layer on top or within the actuators would cause a higher fabrication effort and additional interconnections. Therefore, we developed a high voltage compatible sensing system. The circuitry allows sensing of user input at every actuator element. Both circuits are implemented into a handheld-like device.

Vibrotactile display for mobile applications based on dielectric elastomer stack actuators

Dielectric elastomer stack actuators (DESA) offer the possibility to build actuator arrays at very high density. The driving voltage can be defined by the film thickness, ranging from 80 μm down to 5 μm and driving field strength of 30 V/μm. In this paper we present the development of a vibrotactile display based on multilayer technology. The display is used to present several operating conditions of a machine in form of haptic information to a human finger. As an example the design of a mp3-player interface is introduced. To build up an intuitive and user friendly interface several aspects of human haptic perception have to be considered. Using the results of preliminary user tests the interface is designed and an appropriate actuator layout is derived. Controlling these actuators is important because there are many possibilities to present different information, e.g. by varying the driving parameters. A built demonstrator is used to verify the concept: a high recognition rate of more than 90% validates the concept. A characterization of mechanical and electrical parameters proofs the suitability of dielectric elastomer stack actuators for the use in mobile applications.

Dielectric elastomer actuators for tactile displays

In this paper we discuss dielectric elastomer actuators (DEA) as one of the most promising technologies in electroactive polymers (EAP). For tactile display applications a large number of actuator elements is essential. The multilayer technology presented here offers the possibility to build up independent actuators and arrays at a high density within one substrate. The functional principle of these electromechanical transducers provides a simple way to measure the actual deformation. The paper concludes with a concept to determine the active deformation as well as user applied forces.

Tactile Display with Dielectric Multilayer Elastomer Actuators.

Tactile perception is the human sensation of surface textures through the vibrations generated by stroking a finger over the surface. The skin responds to several distributed physical quantities. Perhaps the most important are high-frequency vibrations, pressure distributions (static shape) and thermal properties. The integration of tactile displays in man-machine interfaces promises a more intuitive handling. For this reason many tactile displays are developed using different technologies. We present several state-of-the-art tactile displays based on different types of dielectric elastomer actuators to clarify the advantages of our matrix display based on multilayer technology. Using this technology perpendicular and hexagonal arrays of actuator elements (tactile stimulators) can be integrated into a PDMS substrate. Element diameters down to 1 mm allow stimuli at the range of the human two-point-discrimination threshold. Driving the elements by column and row addressing enables various stimulation patterns with a reduced number of feeding lines. The transient analysis determines charging times of the capacitive actuators depending on actuator geometry and material parameters. This is very important to ensure an adequate dynamic characteristic of the actuators to stimulate the human skin by vibrations. The suitability of multilayer dielectric elastomer actuators for actuation in tactile displays has been determined. Beside the realization of a static tactile display - where multilayer DEA are integrated as drives for movable contact pins - we focus on the direct use of DEA as a vibrotactile display. Finally, we present the scenario and achieved results of a recognition threshold test. Even relative low voltages in the range of 800 V generate vibrations with 100% recognition ratio within the group of participants. Furthermore, the frequency dependent characteristic of the determined recognition threshold confirms with established literature.

Dielectric elastomer actuators using improved thin film processing and nanosized particles

Stacked dielectric elastomer actuators are fabricated by an automated process using spin coating of uncured elastomers. To improve the performance of these multilayer actuators we present two different ways. To reduce the driving voltage it is desirable to fabricate dielectric films with a thickness below 20 μm. This can be achieved by high speed spin coating of an uncured elastomer. Analyzing the automated process reveals nine principal process parameters. An adequate design of experiment reduces the number of necessary tests to an acceptable value. With these results we are able to spin thin films with a thickness of less than 5 μm and a thickness variation of about 3%. Secondly, we examine the influence of nanosized particles of metal oxide powder on the permittivity of the elastomer film. Three different materials, namely aluminiumoxide, titaniumdioxide and bariumtitanate with a bulk permittivity of about 10, 100, 1000, respectively, are used to increase the overall permittivity of the composite. To predict the resulting performance of an elastomer actuator the figure of merit Κ is introduced.

High-precision characterization of dielectric elastomer stack actuators and their material parameters

Stacked dielectric elastomer actuators (DEA) act as solid state actuators. Modeling such an electromechanical system demands the knowledge about the mechanical and electrical parameters of the used materials as well as the real static and dynamic behavior. In elastomer actuators the electrical properties of the materials might change with applied mechanical stress or applied voltage as it is known from some materials (e. g. polyacryl). Therefore, we examined the PDMS used in stacked dielectric elastomer actuators regarding such dependencies. We present results from testing the permittivity of two different silicones (Elastosil P7670, Wacker Silicones; RTV410, Bayer) versus mechanical stress, frequency of the driving voltage, film thickness and curing temperature. The resulting movement of a stacked actuator is not a single displacement of the elements but a rather complex bulk deformation. Therefore, a planar displacement measurement system is necessary. Laser displacement sensors offer the possibility of a two-sided measurement. This allows to determine the actual thickness variation even if the actuator array moves out of plane. The setup includes a prestretching device to clamp the actuators symmetrically and to simulate an uniaxial load. The realized measurement setup has an effective vertical measurement range of 10 mm, a resolution of 100 nm at a sample rate of 20 kHz. This allows the static and dynamic displacement measurement of planar actuators.