Klaus Flittner

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.

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.

Development of an active isolation mat based on dielectric elastomer stack actuators for mechanical vibration cancellation

This paper describes the development of an active isolation mat for cancelation of vibrations on sensitive devices with a mass of up to 500 gram. Vertical disturbing vibrations are attenuated actively while horizontal vibrations are damped passively. The dimensions of the investigated mat are 140 × 140 × 20 mm. The mat contains 5 dielectric elastomer stack actuators (DESA). The design and the optimization of active isolation mat are realized by ANSYS FEM software. The best performance shows a DESA with air cushion mounted on its circumference. Within the mounting encased air increases static and reduces dynamic stiffness. Experimental results show that vibrations with amplitudes up to 200 μm can be actively eliminated.

Dielectric elastomer actuators with enhanced permittivity and strain

The impact of the modification of silicone rubber with barium titanate particles on the permittivity and hence on the performance of dielectric elastomer actuators has been investigated. Barium titanate powders with different particle sizes in the micrometer and nanometer range were used in this study. The mechanical properties of the composite materials in terms of the Youngs modulus in tension and compression load as well as the viscoelastic behavior in shear load were experimentally determined. Additionally, the electric properties like permittivity, specific conductivity and electric breakdown field strength were evaluated. Model film actuators with the modified silicone material were prepared and their actuation strain was measured. With a concentration of 20 vol.% barium titanate particles, an enhancement of the permittivity of 140 % and an increase of the actuation strain of about 100 % with respect to the unmodified material could be achieved. Furthermore, first multilayer actuators were manufactured with an automatic spin coating process and their permittivity and strain were measured. The results of these investigations are in good agreement with the data of the experiments with single layer dielectric elastomer films.

Dielectric elastomer stack actuators for integrated gas valves

Dielectric elastomer actuators are suited for the fabrication of gas valves in micro systems. Based on the application of a micro burner unit a complete valve, consisting of the seat, the actuator and a spring structure to produce the closing force, is designed and evaluated. The required flow rate and the allowed pressure drop are derived and used to define the design of the valve seat, actuator and spring structure. This includes dimensions and shape of the valve seat with the outlet and channels for the gas flow. One valve in an array has the size of 15 x 15 mm2. The actuator thickness and the shape of its active region are determined to achieve a deflection of up to 50 μm by the use of a finite element simulation. To generate the closing force a spring structure made of nickel with intrinsic layer stresses is fabricated using an electroplating process. For the fabrication a Top-Down process was chosen. The dielectric elastomer actuator is directly fabricated onto a sacrificial substrate containing the spring structure and finally assembled with the valve seat by an plasma bonding process. The fabricated valves are characterized in respect of achieved deflection and flow rates.

Electrical modeling of dielectric elastomer stack transducers

Performance of dielectric elastomer transducers (DEST) depends on mechanical and electrical parameters. For designing DEST it is therefore necessary to know the influences of these parameters on the overall performance. We show an electrical equivalent circuit valid for a transducer consisting of multiple layers and derive the electrical parameters of the circuit depending on transducers geometry and surface resistivity of the electrodes. This allows describing the DESTs dynamic behavior as a function of fabrication (layout, sheet and interconnection resistance), material (breakdown strength, permittivity) and driving (voltage) parameters. Using this electrical model transfer function and cut-off frequency are calculated, describing the influence of transducer capacitance, resistance and driving frequency on the achievable actuation deflection. Furthermore non ideal boundary effects influencing the capacitance value of the transducer are investigated by an electrostatic simulation and limits for presuming a simple plate capacitor model for calculating the transducer capacitance are derived. Results provide the plate capacitor model is a valid assumption for typical transducer configurations but for certain aspect ratios of electrode dimensions to dielectric thickness -- arising e.g. in the application of tactile interfaces -- the influence of boundary effects is to be considered.

Integration of dielectric elastomer stack actuators into micro systems

Using dielectric elastomer stack actuators the electrical contact to each conducting layer is a major concern. In order to integrate these actuators inside micro systems e.g. microfluidic systems compatibility to micro fabrication processes is required. The contact resistance and number of connected layers influence the overall actuator performance directly. Lower number of active electrodes decreases the generated deformation of the actuator. High contact resistance has a negative impact on the dynamic actuator behavior. For conventional interconnection processes with copper wires, the contact ratio is in the range of 60% to 80%, depending on the film thickness of the dielectric layer. Furthermore, this process is not compatible to standard micro fabrication technologies. In this paper we evaluate a process based on electroplating for connecting dielectric elastomer stack actuators and present a measurement system to characterize the number of connected layers. The performance of an electroplated contact is defined by the number of connected layers and the contact resistance between electroplated copper studs and graphite electrodes. It depends on different parameters like the cross sectional area of the electrode layers for connection and therefore on the layer thickness. Using multiple contacts instead of a single one the performance of the contact can also be positively influenced.

Modeling of non ideal dielectric elastomer stack actuators

In this paper we discuss the electrical and mechanical modeling of dielectric elastomer stack actuators. The model is used to extract internal electrical parameters out of measurable values of the whole actuator system. Further parameters like the uniaxial Youngs modulus, the assembly of the stack and the number of connected layers have to be considered. Using this data the static actuator performance for several driving conditions can be predicted. The final comparison of model based predicted deformation and real stack deformation show good accordance. Hence, the proven correlation of actuator layout parameters, fabrication parameters and driving conditions allow application dependent actuator design.

Dielektrische elektroaktive Polymere für strukturintegrierte Aktorik und Sensorik

Zusammenfassung Dielektrische elektroaktive Polymere (DEAP) sind eine Unterart von elektroaktiven Polymeren, die sich insbesondere zur Realisierung strukturintegrierter Aktor- und Sensorelemente eignen. Im aktorischen und sensorischen Betrieb verhalten sich dielektrische Elastomeraktoren (DEA) wie elektrostatische Aktoren bzw. Sensoren. Der Artikel stellt die Funktionsweise dar und erläutert die aktorischen und sensorischen Eigenschaften dieses aktiven Materials. Die Anwendungsmöglichkeiten werden anhand von drei unterschiedlichen Demonstratoren aufgezeigt. Abstract Dielectric electro-active polymers (DEAP) are a subclass of electro-active polymers. These materials are very well suited for the realisation of structural integrated actuators and sensors. In actuator and sensor mode dielectric elastomer actuators (DEA) show the behaviour of electrostatic actuators and sensors respectively. In this paper we are explaining the functional principle as well as the characteristics of actuation and sensing. The variety of applications of this technology is exemplarily shown by three different demonstrations.

Integrated gas valve array using dielectric elastomer actuators

In this paper we are presenting a concept of a dielectric elastomer actuator (DEA) driven gas valve array for use in a micro burner unit. Such a unit consists of a spatial array of gas nozzles. Every valve controls the gas flow through a single nozzle. With individual control of each valve the burner can be activated partially in controlled spatial patterns. Therefore, the heat dissipation can be controlled and adjusted according to the current needs. For the individual valves a simple control valve rather than a proportional valve can be used. Using dielectric elastomer actuators to control the gas flow an additional demand for thermal decoupling between firing chamber and dielectric elastomer actuator must be met. Therefore, the valve seat is made of a heat-resistant material. With a polymer based ceramic the thermal decoupling can be achieved. Additionally, this material permits the fabrication of arbitrary three dimensional structures. To control the gas flow different configurations of actuator and valve seat are possible. They are compared according to the complexity of the assembly and the possibility of a monolithic fabrication. The different configurations contain different actuation modes (thickness variation versus lateral deformation), direct or indirect control of gas flow and different valve movements relative to gas flow.