Development of a Fatigue Testing Setup for Dielectric Elastomer Membrane Actuators

Transcription

1 Development of a Fatigue Testing Setup for Dielectric Elastomer Membrane Actuators M. Hill* a,b, G. Rizzello a, S. Seelecke a a Department of Systems Engineering, Department of Materials Science and Engineering, Saarland University, Saarbrücken, Germany, Saarland University, Saarbrücken, D 66123; b Zentrum für Mechatronik & Automatisierungstechnik ggmbh, Eschberger Weg 46, Saarbrücken, D ABSTRACT Dielectric elastomers (DE s) represent a transduction technology with high potential in many fields, including industries, due to their low weight, flexibility, and small energy consumption. For industrial applications, it is of fundamental importance to quantify the lifetime of DE technology, in terms of electrical and mechanical fatigue, when operating in realistic environmental conditions. This work contributes toward this direction, by presenting the development of an experimental setup which permits systematic fatigue testing of DE membranes. The setup permits to apply both mechanical and electrical stimuli to several membranes simultaneously, while measuring at the same time their mechanical (force, deformation) and electrical response (capacitance, resistance). In its final state, the setup will allow to test up to 15 DE membranes at the same time for several thousands of cycles. Control of the modules, monitoring of the actuators, and data acquisition are realized on a crio FPGA-system running with LabVIEW. The setup is located in a climate chamber, in order to investigate the fatigue mechanisms at different environmental conditions, i.e., in terms of temperature and humidity. The setup consists of two main parts, namely a fatigue group and a measurement group. The fatigue group stays permanently in the climate chamber, while the measurement group is assembled to the fatigue group and allows to perform measurements at 20 C. Keywords: Dielectric Elastomers, Dielectric ElectroActive Polymers, DE, DEA, Fatigue Testing 1. INTRODUCTION Dielectric elastomer (DE) transducers are attractive for industrial application due to their multifunctionality, i.e., the possibility of working either as sensors, actuators, or generators 1. Some advantages of DE s over alternative transduction technologies are high power density, low weight, large deformation, high energy efficiency, silent operation, and fast response time. Additionally, DE s can be used as sensors and actuators at the same time, by exploiting the so called selfsensing feature 3-5. Possible applications of DE s include pumps 6-8 and valves Even if DE s appear as promising in a number of industrial applications, up to now only little effort has been dedicated to systematic investigation of their lifetime and fatigue when operating for many cycles. Clearly, this investigation represents a fundamental step in order to ensure the wide spread of DE technology. A number of works reported the mechanical fatigue analysis of pure elastomer materials, and some relevant examples are reported in the following. Cadwell et al. 13 investigated the influence of dynamic load, applied at constant mechanical frequency, to rubber. They reported a dependence of maximal strain on the number of cycles until failure. Also Lake et al. 14 investigated the fatigue mechanisms of elastomers. In their work, they observed the dynamic growth of damages caused by razor cuts. It was found that the elastomer chains are destroyed if the stress overcomes a certain limit value, while keeping the stress below this limit allows to increase the lifetime of the elastomer. In 15, Mars et al. investigated the factors which influence the fatigue behavior of elastomers. It was observed that, additionally to mechanical load and temperature, the composition of the atmosphere and especially electrical charge have a major impact. All of these works focused on mechanical behavior of elastomers, thus neglecting the electric behavior. Some works have also reported actual fatigue investigation of DE s. In 16, Zhang et al. investigated the aging of several silicone based DE membranes. It was shown that the Young s modulus and the breakdown strength of the silicone increase with the number of cycles, while permittivity decreases. Matysek et al. 17 investigated the fatigue of DE stack actuators under electric load. It was found that these actuators can perform up to several million cycles at a specific electric field of 20 V/µm. Moving towards the same direction of the aforementioned works, this paper presents design and assembly of a

2 setup which allows to characterize both mechanical and electrical fatigue of DE s. The setup enables to investigate the influence of several external inputs to fatigue mechanisms and lifetime of DE s. The setup is capable of applying mechanical and electrical load to DE membrane actuators, with a maximum electric field up to 100 V/µm and a mechanical strain up to 100%. The setup allows also to perform tests at several environmental conditions, in terms of temperature and humidity, by means of a climate chamber. During each test, mechanical (force, deformation) and electrical (resistance, capacitance) measurements will be acquired periodically, and used to estimate the effects of fatigue on material performance. 2. DE BASIC DESCRIPTION The basic configuration of a DE Actuator (DEA) consists of a deformable capacitor made of a thin dielectric elastomer membrane surrounded by compliant electrodes. In this work, the electrodes are made of graphite compound, manufactured by screen printing, while the DE membrane is a silicone-based elastomer. The transduction principle of a DE is based on an electromechanical pressure, known as Maxwell stress 1. A voltage U applied between the electrodes generates an electric field which results into attractive electrostatic forces that squeeze the membrane, resulting in a lateral expansion motion that can be used for actuation (Figure 1). The Maxwell stress compressing the membrane can be calculated as follows 2 U σ Max = ε0ε r z where ε 0 and ε r are the void and DE relative permittivity, respectively, U is the applied voltage, and z the DE membrane thickness. (1) Figure 1: Working principle of a DEA Due to their nature as compliant capacitors, DE s can be naturally used as sensors. In fact, the application of a mechanical load results in a change in capacitance, which can be properly measured and used to reconstruct the membrane deformation or the external force 2. If we assume that a DE membrane can be regarded as a parallel-plate capacitor, by using standard parallel-plate capacitance formula we obtain A C = ε0εr, (2) z where A is the surface of the electrodes. When the membrane is deformed, A increases while z is decreases, since the volume V = Az remains constant due to material incompressibility. Therefore, the overall capacitance also increases. The setup is designed in order to test DE membranes with different kind of geometries. The geometry investigated at first is the circular membrane DE shown in Figure 2. The electrodes are made of graphite compound, manufactured by screen printing, while the DE-membrane is a silicone-based elastomer. The circular membrane is also enclosed in a passive rigid frame made of epoxy, which constraints the motion out-of-plane as shown in Figure 2. The diameter of the inner and outer frames equal 12.5 mm and 22 mm, respectively. Figure 2: Circular DE element, at rest (left-hand side) and deflected out-of-plane (right-hand side)

3 Mechanical Excitation 3. DE-ACTUATOR FATIGUE TESTING Investigation of mechanical fatigue is interesting for applications of DE s as sensors. In particular, it is important to understand the fatigue mechanisms of both membrane and electrodes printed on. Information on mechanical fatigue is provided by membrane force and deformation, resistance of the electrodes, and capacitance of the DE membrane. These indicators are measured by means of a laser displacement sensor, a load cell, and a RC-identification system (e.g., the method proposed in 5 ), respectively. The fatigue is emerged by stretching the membrane with an external mechanical load. This load can cause cracks in the electrodes as well as attrition in the membrane. The DEA s are fatigued by applying an out-of-plane deformation for a number of cycles that will typically range from several thousand to a few million cycles, as shown in Figure 3. The external inputs for this kind of experiments are the maximal displacement, the membranes predeflection, and the frequency of the mechanical load. The maximal displacement can be varied from 1 to 5 mm. The predeflections can be varied from 0 to 4 mm with increments of 0.2 mm. The frequency of the displacement can be selected up to 10 Hz. Furthermore it is possible to apply various, constant electric voltages during the mechanically excited test. Figure 3: Fatigue of DE as sensors Electrical Excitation Investigation of electrical fatigue is essential for applications of DE s as actuators. Several signals can be selected as electrical inputs, such as square- or sinewaves, up to a maximum amplitude of 3 kv. The voltage is supplied by high voltage amplifiers. The indicators for this fatigue test are stroke, current needed to generate that stroke, resistance and capacitance. The fatigue occurs as a consequence of electric field, which can result in failure by an electrical breakdown. In this testing device, electric fields up to 100V/µm can be reached. To convert an applied voltage into a mechanical stroke, the DE s must be pre-deflected out-of-plane, for example by a linear spring, as shown in Figure 4. The resulting stroke can be properly tuned by changing stiffness and pre-compression of the spring. The pre-compression of the spring, which influences the actuator stroke, represents a further control parameter for the fatigue testing. Figure 4: Fatigue of DE as actuators Fatigue implementation The implementation of the overall fatigue testing is separated in a fatigue interval and a measurement interval, which are repeated cyclically as is shown in Figure 5. During the fatigue interval, the membranes are solicited for a number of cycles (e.g., several thousands), while system variables are monitored at low sampling rate (e.g., 1 khz) and saved temporarily to detect eventual issues in the fatigue setup. During the measurement interval, data are acquired for a few number of cycles. This operation is executed with a high sampling rate (e.g., 25 khz). Measured data are stored in a computer for subsequent post-processing. Figure 5: Process of fatigue testing

4 Environmental Conditions In addition, it is important to investigate the influence of environmental conditions on DE fatigue. To ensure that the fatigue intervals are executed under certain conditions, the fatigue mechanism is cased in a climate chamber CTS C-70/350 like the one shown in Figure 6. This climate chamber permits to control temperature in a range from -40 to 180 C and humidity between 0 and 99%. For standard testing, all the measurements will be executed at 20 C and 30% humidity. As a final remark, the fatigue tester has to be designed in order to fit into the climate chamber. Figure 6: Climate chamber CTS C-70/350 Operating mechanism 4. FATIGUE MECHANISM DESIGN The design of the fatigue device has to be customized to the special requirements of the fatigue tester. In particular, the elements used for measuring force and stroke are not compatible with a temperature range from -20 to 100 C. Therefore, the setup is separated in two main machine parts. One part is the fatigue mechanism, which remains inside the climate chamber and is exposed to controlled environmental conditions. The other part contains the sensors, which are necessary for measuring mechanical and electrical quantities. This part is the so-called measurement structure. During fatigue experiments the measurement structure remains outside the climate chamber, while it is collocated over the fatigue mechanism between two sets of experiments only for measuring force or stroke at 20 C. An illustration of this structure is shown in Figure 7. Figure 7: Schematic design of the fatigue setup DE Arrangement The DE s are arranged in three rows with five membranes each (Figure 8). This kind of distribution is chosen in such a way external inputs, like displacement or voltage, can be independently selected for every row. The number of DEA s in a row is chosen equal to 5, in order to compensate statistical deviations of DE s electro-mechanical characteristics which occurs as a consequence of the manufacturing process. Figure 8: Arrangement of DE S on baseplate

5 Mechanical design The testing system is mounted on an aluminum breadboard that allows to contain the fatigue mechanism and the measurement structure at the same time. In order to ensure that the fatigue mechanism endures millions of cycles, the mechanical design is kept as simple as possible. The displacement of the DE s is generated by a camshaft, mounted in flange bearings. There are six interchangeable cams on the shaft, made of hardened steel and mounted with a grub screw. The six cams are mounted in three couples, arranged in opposing configuration to minimize vibrations. The camshaft is driven by a stepper motor Phytron ESS 80, which allows to achieve different rotational speeds as well as to hold the camshaft to a specified position. The rotational motion of the camshaft is converted into DE s out-of-plane displacement by stroke pins, as shown in Figure 9. The stroke pins slide on the cams and are guided by linear bearings. A rake mechanism ensures that the displacement of the cams is transferred to all DE s in each rows. This design solution allows to vary the maximal displacement as well as the displacement waveform, e.g., sinewave, triangle wave. The DE s are mounted on a baseplate which contains conductor tracks, enabling the connection of the DE s with high voltage sources. A picture of the overall setup is shown in Figure 10. Figure 9: Schematic design of the fatigue mechanism Figure 10: Fatigue mechanism setup Measurement Structure The sensors used to measure force and displacement are located in the measurement structure, shown in Figure 12. It is made of aluminum profiles, mainly due to their relatively simple workability and assembly. The measurement structure is located on sliding elements and attached by toggle levers, which ensure repeatable positioning on the breadboard. The structure contains a X/Y table Jenaer Antriebstechnik XY with a travel range in x-direction of 400 mm and y-direction of 300 mm. The x-axis is firmly attached to the frame whereas the y-axis is attached to sliding elements, located in the groove of the aluminum profiles. This sliding elements allow the y-axis to move, and prevent bending of the axis. The X/Y table is controlled via a PC with LabVIEW. The measurement sensors, e.g., load cell and laser displacement sensor, are mounted to the X/Y table to perform serial measurements on each DE.

6 Figure 11: Measurement structure Control system and data processing The fatigue setup contains controllers for the stepper motor and the X/Y table. These controllers are driven by a LabVIEW program on a PC. The measurement data are acquired by a National Instruments crio FPGA board, and stored on the PC. FPGA and controllers are located in a switchbox, shown in Figure 12 (left-hand side). The high voltage components are located in a separate switchbox, shown in Figure 12 (right-hand side). This separation was made to prevent high voltage damages to the controllers and FPGA. Figure 12: Control system 5. CONCLUSION This paper has presented a setup for testing fatigue of DE membranes at several mechanical, electrical, and environmental conditions. The setup allows to apply to the DE s a mechanical displacement up to 5 mm and a voltage up to 3 kv, and permits to test the fatigue when the DE s are operating both as sensors and as actuators for millions of cycles. The setup has been separated in a fatigue mechanism and a measurement structure, to allow to run fatigue in a temperature range of -20 C to 100 C. In future research works, the setup will be used to perform actual testing of fatigue properties of DE s, in order to establish their long-term performance in typical actuators and sensors applications. ACKNOWLEDGEMENT The authors gratefully acknowledge financial support from Bürkert Fluid Control Systems

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