Active elastomer components based on dielectric elastomers

Transcription

1 Gummi Fasern Kunststoffe, 68, No. 6, 2015, pp Active elastomer components based on dielectric elastomers W. Kaal and S. Herold Fraunhofer Institute for Structural Durability and System Reliability LBF, Darmstadt, Germany Selected from International Polymer Science and Technology, 42, No. 8, 2015, reference GK 15/06/412; transl. serial no Translated by M. Grange A new design concept developed at the Fraunhofer LBF allows dielectric elastomers to be integrated efficiently as load-bearing components, serving as both actuator and sensor at the same time. Since the concept is based on perforated metal electrodes with high electrical conductivities, it can also be used for highly dynamic and acoustic applications to which dielectric elastomers would not usually lend themselves. This article focuses on the adaptive properties of these elastomer components. By applying a static voltage, the effective complex elastic modulus and the loss angle of the composite material can be adjusted within certain limits and varied as required. Experimental results are provided to illustrate this approach and the potential of this new technology for innovative solutions is discussed. INTRODUCTION Passive elastomer components have proved their ability to influence the dynamic behaviour of machine and vehicle structures in specific ways. Individual components can be tailored to particular applications thanks to the wide range of available materials with their different suspension and damping characteristics. In addition, electroactive polymers (EAPs), and particularly dielectric elastomers (DEs), allow active elastomer components to be produced which are capable of integrating adaptive and active functionalities. For example, the elastomer components mechanical properties can be modified by electrical stimulation during operation and, if necessary, active forces can also be applied. New concepts are therefore available for optimising vibration behaviour and acoustics, which can enhance performance, increase comfort or extend service life depending on the application. A concept that has been developed at the Fraunhofer LBF allows dielectric elastomers to be integrated particularly efficiently as load-bearing components which can, at the same time, serve as actuators and sensors. Experimental results are provided to illustrate this approach and the potential of this new technology for innovative solutions is discussed. DE ACTUATORS Dielectric elastomer stack actuators The term electroactive polymers refers to a variety of flexible functional materials with electromechanical coupling characteristics. The dielectric elastomers (DEs) are the most well-established and most widely researched of these. Their high deformability and rapid response times make them suitable for use as actuators in a variety of applications. A DE transducer generally consists of a thin polymer film provided on both sides with a compliant electrode, thus forming a flexible capacitor (Figure 1). The deformation that occurs when an electrical voltage is applied is based on electrostatic attraction between opposite charge carriers, causing a mechanical pressure s M inside the capacitor which is known as Maxwell stress. Depending on the construction, the elongation in the plane of the film or its change in thickness can be utilised for actuation purposes. This is achieved 2015 Smithers Information Ltd. T/1

2 by stacking multiple individual layers with alternately arranged electrodes electrically connected in parallel to form a stack actuator. From an electrical point of view, a stack transducer of this type essentially represents a capacitance which is calculated using the formula for a plate capacitor from the geometric values A (plate area), h (plate spacing), n (number of layers) and the permittivity e of the dielectric as: DE transducers are also characterised by a parallel resistance R p, which is obtained from the resistivity of the elastomer, and by a series resistance R s, which is determined by the conductivity of the electrodes and supply lines as well as the contact resistances. This gives the simplified equivalent model of a DE actuator shown in Figure 2. The series connection of serial resistance (1) and capacitance leads to low-pass behaviour by the actuator, since at high frequencies only part of the voltage U o applied to the terminals is effectively applied to the capacitor. Some of the properties of conventional DE stack actuators would be disadvantageous if used in structural dynamics applications. On the one hand, rigid connections as used in mechanical engineering applications restrict elongation at the edges (Figure 3a), which can entail significant overall reductions in performance. This effect is more pronounced in flatter actuators with larger surface areas, which are needed in load-bearing applications for reasons of stability. On the other hand, the relatively poor conductivity of the compliant electrodes leads to a high series resistance. For the system as a whole, this means a low upper limit to the frequencies at which the actuator can reasonably be operated (Figure 3b). DE stack actuators with perforated electrodes Figure 1. Operating principle of dielectric elastomers To facilitate the use of DE stack actuators in structural dynamics applications, an actuator design based on nonelongating metal electrodes with a microscopically fine perforated structure (Figure 4) has been developed at the Fraunhofer LBF. This design enables the incompressible elastomer to expand locally into the hollow spaces, giving a macroscopically compressible composite stack. The actuator area therefore remains constant under both mechanically and electrically induced pressure, and even very flat versions of the actuator can be attached to rigid structures with low losses. The extremely high conductivity of the metal electrodes also favours operation at high frequencies and ensures that electrical losses are low. The precise design of the pattern of holes can be optimised and fine-tuned by numerical methods, particularly using finite element models [1]. Figure 2. Simplified equivalent electrical circuit diagram of a DE stack actuator Figure 3. Disadvantages of conventional DE stack actuators for dynamic applications: a) mechanical restriction of elongation with rigid connection, b) low-pass electrical characteristics Functional model of a DE stack actuator with perforated electrodes Based on the design concept described above, various functional models of DE stack actuators with perforated metal electrodes have been constructed at the LBF. The stack actuator illustrated in Figure 5 consists of 44 active layers of natural latex in a polymer housing, which was made from polyamide in a rapid prototyping process. The 45 electroformed electrodes consist of nickel and they each T/2 International Polymer Science and Technology, Vol. 42, No. 9, 2015

3 contain over 280,000 holes with a 90 µm diameter. This and similar functional models have already been used successfully to demonstrate active vibration reduction in structural dynamic systems [2]. Potential semi-active applications for dielectric elastomers As well as active applications, dielectric elastomers are also suitable for semi-active uses since their mechanical properties can be varied by electrical stimulation. Because passive elastomer components are already used in a variety of ways for vibration reduction in structural dynamics, the approach of varying their stiffness and damping properties in operation represents an obvious extension of their field of use [3]. In some cases, their mechanical properties vary substantially when voltage is applied because of the extent of their deformation. The advantage of the semi-active application of dielectric elastomers lies in the low amount of electrical energy that has to be supplied. Since the element s capacitance only has to be charged statically and leakage currents have to be compensated, high currents are not needed. The high voltage can therefore be generated by compact, quasi-static voltage amplifiers. Figure 4. Operating principle of DE stack actuator with perforated electrodes Figure 5. Functional model of a DE stack actuator with perforated electrodes, CAD model and photo (without cover) EXPERIMENTAL CHARACTERISATION OF THE FUNCTIONAL MODEL When voltage is applied in the design approach using perforated electrodes, the mechanical pre-stressing of the stack changes and, because of the nonlinear characteristics, even with small deformations the external stiffness also changes. In order to quantify this effect, the functional model described above was measured in a test rig for the dynamic characterisation of elastomer components. The set-up is illustrated in Figure 6. The test involves subjecting the elastomer sample or stack transducer to cyclic loads from an electrodynamic shaker, using closed-loop control to establish a constant force amplitude. The acting force is measured at the bottom of the stack transducer to minimise the dynamic effect of the upper vibrating mass. Since the entire test rig is set up on a low-frequency vibration isolation table, the lower mounting of the sample is almost completely at rest above the fundamental resonance of the isolation table at approx. 10 Hz. The sample deformation is determined from the difference between two acceleration signals which record acceleration in the vertical direction at the upper and lower points of force application. Figure 7 first shows an example of measurement data over the time range and illustrates the way in which adaptive stiffness works. The elastomer stack Figure 6. Test rig for the experimental characterisation of elastomer components was excited with a constant force amplitude of 5 N (top) and the high voltage was activated at t = 0. This voltage was generated by a miniature high voltage amplifier from Emco (Q30 model, dimensions 38 x 38 x 16 mm 3 ), supplying a maximum of 0.5 ma with a maximum of 3 kv and requiring an input current of less than 300 ma. The electrically activated increase in stiffness is associated with a corresponding reduction in amplitude of the differential acceleration (bottom); the amplitude drops after a certain time delay mainly because of the limited charging current that can be generated by the amplifier. In the present case, it was clearly possible to reduce the differential acceleration 2015 Smithers Information Ltd. T/3

4 Figure 7. Influence of offset voltage on deformation in the time range and thus the deflection to approx. 65% of the initial amplitude. In order to quantify this effect and present it in the frequency range more precisely, the stack transducer was excited with a harmonic signal at various frequencies from 10 to 150 Hz and the deformation, stiffness and loss angle between force and deformation were determined from the acceleration data. These measurements were performed for different offset voltages (0 V, 500 V, 750 V, 1250 V, 1500 V and 1920 V). The test results are compiled in Figure 8. The amplitude curves for force and deflection can be seen on the left-hand side and the resulting values for stiffness and loss angle on the right-hand side. It is apparent that the deflection amplitude decreases as the offset voltage rises over the whole of the frequency range observed, which translates as an increase in stiffness. This effect is slightly more pronounced at low frequencies than in the higher frequency range. The loss angle decreases over the frequency range from a maximum of about 20 at 10 Hz to about 7 at 150 Hz. In the lower frequency range in particular, the loss angle is reduced by the offset voltage, which corresponds to a reduction in system damping. As an example, Figure 9 shows the resulting stiffness at 20 Hz as a function of offset voltage. It is clear that the stiffness can be increased semi-actively by a factor of about 2. Because of the quadratic relationship between Maxwell stress and electrical field, the stiffness increases disproportionately to the voltage and so a further increase in offset voltage will make the effect even more pronounced. The elastomer s breakdown voltage, which is over 4 kv for the present material under ideal conditions, determines the maximum possible voltage. However, the present actuator was only tested up to about half of this voltage owing to imperfections in the material and because the manual production method is prone to errors. CONCLUSIONS Variable stiffness elements are of interest wherever system parameters (e.g. speed, mass etc.) vary and structural properties have to be adjusted accordingly. However, possible applications for semiactive stiffness adjustment can also be seen in the area of adaptive vibration absorbers. Many different solutions with mechanically adjustable stiffness have already been designed and implemented [4, 5]. Some of these approaches use a change in the effective length of a bending beam while others are based on changes to their cross-section or pre-stressing. All of these approaches, however, contain mechanically adjustable components which are therefore susceptible to wear. They also need a motor to activate the adjusting mechanism. To adjust stiffness and therefore resonant frequency, even when operated under load, this motor generally has to be correspondingly powerful and therefore heavy and expensive. Figure 8. Test results in the frequency range Hz Figure 9. Evaluation of measurements at 20 Hz T/4 International Polymer Science and Technology, Vol. 42, No. 9, 2015

5 In contrast, adaptive vibration absorbers based on semi-active DE stack transducers need no mechanical adjustment mechanisms and can react quickly to the need for frequency changes, while being cheap to produce and energy-efficient to operate. They take up little space and the high-voltage generator can be located separately so that it can supply multiple elastomer components with high voltage. This paper has only been able to demonstrate the effect of semi-active stiffness adjustment in DE stack actuators with perforated electrodes in principle. It will certainly be possible to increase this effect by optimising the electrode geometry, elastomer properties (particularly the surface characteristics) and design. Further work in this field will therefore enable cost-effective, energyefficient adaptive stiffness elements to be produced in the future for broad areas of application. REFERENCES 1. Kaal, W., Herold, S., Numerical investigations on dielectric stack actuators with perforated electrodes, Smart Materials and Structures 22/10 (2013), , Herold, S., Kaal, W., Melz, T., Novel dielectric stack actuators for dynamic applications, Conference on Smart Materials, Adaptive Structures and Intelligent Systems, Stone Mountain, GA, USA, Carpi, F. (Hrsg.), Dielectric Elastomers as electromechanical transducers: Fundamentals, materials, devices, models and applications of an emerging electroactive polymer technology., Elsevier, Amsterdam, Franchek, M.A., Ryan, M.W., Bernhard, R.J., Adaptive passive vibration control, Journal of Sound and Vibration 189 (1995), Brennan, M.J., Vibration control using a tunable vibration neutraliser, Journal of Mechanical Engineering Sciences 211 (1997), Smithers Information Ltd. T/5

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