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Stacked dielectric elastomer actuator for tensile force transmission
Sensors and Actuators A 155 (2009) 299–307
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Sensors and Actuators A: Physical journal homepage: www.elsevier.com/locate/sna
Stacked dielectric elastomer actuator for tensile force transmission G. Kovacs ∗ , L. Düring, S. Michel, G. Terrasi Empa, Swiss Federal Laboratories for Materials Testing and Research, Laboratory for Mechanical System Engineering, Uberlandstrasse 129, 8600 Dubendorf, Switzerland
a r t i c l e
i n f o
Article history: Received 26 February 2009 Received in revised form 25 May 2009 Accepted 30 August 2009 Available online 8 September 2009 Keywords: Active structures Electro-active polymers (EAPs) Soft dielectric EAPs Contractive tension force actuator
a b s t r a c t This paper presents a novel approach for active structures driven by soft dielectric electro-active polymers (EAPs), which can perform contractive displacements at external tensile load. The active structure is composed of an array of equal segments, where the dielectric films are arranged in a pile-up configuration. The proposed active structure has the capability of exhibiting uniaxial contractive deformations, while being exposed to external tensile forces. The serial arrangement of active segments has one contracting degree of freedom in the thickness direction of the dielectric EAP film layers. Due to the envisaged tension force transmission capability, special attention is paid to the electrode design which is of paramount importance with regard to functionality of the actuator. A compliant electrode system with anisotropic deformation properties is presented based on nano scale carbon powder. In experiments, the free deformation as well as the contractive motion under external tensile loading of several actuator configurations with different setups is characterized. These involve the study of various sizes and numbers of stacked film layers as well as different electrode designs. © 2009 Elsevier B.V. All rights reserved.
1. Introduction In the last decade, the interest in “smart materials”, which respond to external stimuli by changing their shape or size, has essentially increased. In particular soft dielectric EAPs as musclelike actuators, a subgroup of the electro-active polymers (EAPs), have attracted much interest in recent years due to their outstanding active deformation potential [1–4]. Soft dielectric EAPs consist of a thin elastomer film, which is coated on both sides with compliant electrodes (Fig. 1, left). When applying a DC high voltage U (in the range of several kV) to this compliant capacitor, the electrodes squeeze the elastomeric dielectric in the thickness direction (electrode pressure, Pequivalent [5]), and thus the nearby incompressible film expands in the planar direction (Fig. 1, right). Pequivalent = ε0 · εr ·
U 2 d
(1)
Thereby ε0 is the free-space dielectric permittivity (ε0 = 8.85 × 10−12 F/m), εr is the relative permittivity of the dielectric material and d represents the thickness of the dielectric film. As soon as the voltage is switched off and the electrodes are short-circuited the film deforms back to its initial state. So far, a variety of different types of dielectric elastomer (DE) actuators have demonstrated the versatile capabilities of this actuator technology (e.g. [6,7]). Due to their intrinsic compliance and
∗ Corresponding author. Tel.: +41 0 44 823 4063; fax: +41 0 44 823 4011. E-mail address: [email protected] (G. Kovacs). 0924-4247/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.sna.2009.08.027
unique deformation potential, soft dielectric EAPs are promising for compliant, lightweight structures in the macro-scale, which can perform continuous displacements. The actuation of the dielectric elastomer films can be used in two different ways. Based on the principle of operation, two directions are possible to perform work against external loads (Fig. 2): • Work in the planar directions (expanding actuator): Under electrical activation of a DE basic unit the film expands in the x, y plane (Fig. 1) and can thus work against external pressure loads in both planar directions x and y. • Work in the thickness direction (contractile actuator): Under electrical activation the electrodes squeeze the DE film in the thickness direction (z) (Fig. 1). Thus, the actuator can work against external tensile loads acting in the direction of the electric field lines of the compliant capacitor. With regard to the elastomers used as the dielectric in DE actuators, the best overall performances have been shown by mainly silicone and acrylic films, such as commercially available VHB 4910 from 3 M [8,9]. The dielectric film, made from acrylic VHB 4910 especially for DE actuators, is strongly pre-strained in planar directions in order to reduce the thickness and therefore the required activation voltage level. To maintain the DE film in this biaxially pre-strained state a support structure is needed. To enable the required displacements this support structure must offer the corresponding mechanical degrees of freedom (DOF). Obviously, in pre-strained DE actuators the design of the support structure is one of the key issues and causes many design constraints.
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Fig. 1. Structure and working principle of soft dielectric EAP (Q = electrical charge).
To date most scientific research work has been focused on the in-plane expanding actuation mode. This is due to the fact that the commercially available acrylic material VHB 4910 (3 M) can easily be processed to planar actuators and has demonstrated a very high actuation performance, in particular with high breakdown fields of up to 200 V/m, when pre-strained [10–12]. Many different actuator designs have been studied for experimental purposes and represent the present state-of-the-art. They include the extender (planar), unimorph-, bi-morph-, spring roll-, push-pull, bow-tie-, diamond-, diaphragm-, spider-, inchworm segmentand universal muscle actuators [6,7]. They all have the same basic electro-mechanical characteristic. The actuator expands in-plane when voltage is applied and shrinks back as soon as the applied charges are removed from the electrodes (following the principles depicted in Fig. 1). For example, the development of the Empa arm wrestling robot in 2005 has been a major contribution to the production technology of expanding actuators in rolled configuration [13]. Due to the expanding actuation characteristic an agonistantagonistic arrangement of the actuator bundles was necessary to produce a bidirectional motion. In this configuration all actuators were fixed in a pre-strained state in order to execute the required contracting motion under external tensile force in the deactivated mode. Without exception all highly pre-strained actuators of the Empa robot have poor durability due to the required high pre-stretching ratio in combination with the necessary rigid, often heavy and bulky supporting structure. When contraction at activation is required the discussed “expanding planar EAP actuator” does not represent the appropriate solution for many applications. The basic DE working principle can be exploited directly (Fig. 2, lower picture left) in order to build linear contractive actuators. A few approaches have been carried out so far to provide actuators with contractive properties when activated. Based on the folded and helical design with non-pre-
strained silicone film contractive motion of the actuator has been demonstrated [14]. The so-called folded actuator consists of a single strip of an elastomer that is first successively coated with continuous compliant electrodes and then folded up, so as to form a monolithic compact body. The compliant electrodes are made of a silicone/carbon-black mixture. In the case of the silicone elastomer, the folded structure is finally ‘sealed’ with a thin coating made of the same elastomer used for the strip. When using acrylic elastomers the intrinsic adhesiveness of the material does not require any further treatment for good structural integrity. A high voltage applied between the electrodes causes a thickness squeezing of the entire elastomeric layer under the influence of the induced electro-static field. The resulting axial contraction is accompanied by associated related lateral expansion. Furthermore the basic design of the stack configuration has been developed and demonstrated for electrostatic tactile displays with high structural compliance [15]. When a voltage is applied between neighbouring electrode layers of this device, the actuator stack will contract and the stimulator tip will disappear below the surface of the device. Due to the fact that the fingertip is basically pressing on the stimulator tip no significant tensile force has to be produced by this actuator structure for the pull-back motion. By reducing the voltage the electrodes are discharged beyond the voltage source causing a relaxation of the actuator stack. The stimulator tip is thereby pressed against the skin on top of the device because of the stored elastic energy. In contrast to this last actuator design, a new actuator in pileup design is proposed here which exhibits contractive deformation when electrically activated and can work against external tensile loads acting in the stack’s axial direction. The working principle of the actuator design is based on the contraction effect of the dielectric film when an electrostatic field is applied. Due to the fact that the actuation direction is parallel to the electrostatic field lines, the electrostatic force can be directly turned into mechanical force (the compressed DE film produces the contraction motion of the actuator). The electrode design plays a central role in enabling transmission of the tensile force from one DE layer to the next. In contrast to the expanding membrane actuator, the proposed contractive actuator demonstrates the feasibility and convenience of the pile-up configuration for many applications where contraction at activation and production of tensile forces are of central interest. 2. Conceptual approach
Fig. 2. Activation modes: in-plane expanding–out-of plane contractive actuation.
The proposed contractive actuator consists of stacked small DE film pieces coated with compliant electrodes (Fig. 3). The electrostatic field induced actuation force and displacement is perpendicular to the plane of the electrode and dielectric film. The obtained contraction of the actuator can be used to drive the device under a specific service load. The design of the pile-up actuator is based on a series of capacitors electrically connected in parallel with alternating polarities of each layer. When transmission of an external tension force is required, the internal force (stress) dis-
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Fig. 3. Layerwise composite structure of the stacked DE actuator consisting of alternating electrode and DE film layers.
tribution of the actuator is of central interest. The actuator has to be capable, therefore, of robustly transmitting the external tension force from one load introduction point placed at one end of the actuator to the second one positioned at the opposite end when activated and contraction motion is supposed to be executed. With respect to the composed structure of the proposed actuator the tensile stress transfer occurs as follows. When a voltage is applied between two electrodes, external tension force can be transmitted by the electrostatic field induced Maxwell force. As a result the DE is squeezed by both neighbouring electrodes, and contractive motion of the entire actuator occurs. Although considerable external tension load can be transmitted by the electrostatic force, solely compression stress is triggered within the dielectric film which acts as a “spacer” and does not take any tensile stress (in either planar or normal directions). This is beneficial when good fatigue strength under cyclic tension load and long term stability of the actuator are of central interest. Obviously the electrostatic field vanishes inside the electrode due to its high electrical conductivity and the capability of freely moving charges. In order to allow the tensile force transmission within the electrode layer, its electro-mechanical properties are of paramount importance and will be discussed from a constitutive perspective below. The external tension force has to be transmitted over the electrode layer by inducing tensile stresses inside the electrode material (as sketched in Fig. 4). This can easily be accomplished when the dielectric medium is made of compressible elastic material. In such a case no planar expansion occurs when the dielectric is compressed by the Maxwell force and therefore the electrode can be made of a solid conductive material layer (stiff conductive plate). When using incompressible material like hyper-elastic elastomer films as dielectric, the electrode has to be compliant in the planar direction in order to allow unrestricted planar expansion of the dielectric film without reducing the electric conductivity needed to provide the electrostatic field, even in the deformed state. On the other hand, the electrode has to be designed in such a way
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that the tension force transmission capability in the normal direction of the film is assured. For this reason a certain stiffness and tensile strength of the electrode material in the thickness direction (z) (parallel to the electrostatic field) is required to robustly transmit mechanical tensile stress. The following explanation of the working principle is based on an idealized and macroscopic layered system with homogeneous properties. In general an electro-static field occurs between two electrodes when a voltage is applied to the capacitor. Thereby the electric charges will be arranged on the surface of each electrode and thus located near to the dielectric film. Additionally it can be assumed that a layer of oppositely poled electric charges on the surface of the dielectric film is formed, which is thus positioned close to the electrode. As a result attraction forces of the counter-poled charges between both joining surface layers of the dielectric and the electrode occur. When external tension force is applied to the actuator these interlayer attraction forces create mechanical tensile stress within the electrode material (Fig. 4). Therefore the external force can be transmitted from one side to the opposite side of the electrode by the resulting tension stress state of the electrode material, which needs high stiffness and tensile strength in the thickness direction (z) (Fig. 4). Any adhesive bonding effects between the dielectric and electrode layers might be helpful for mechanical stability in the passive state but are not necessary for the force transmission at activation. Consequently an anisotropic material system has to be used as the electrode with direction dependent stiffness in order to fulfill the in-plane compliancy and the out-of-plane stiffness and strength demand. 3. Investigations of carbon powder electrode layers The proposed pile-up configuration with an appropriate electrode allows the realization of novel dielectric elastomer actuators which can exhibit contractive motion under an applied external tension load. To date, carbon powder Ketjenblack 600 from Akzo Nobel (or a similar material) is widely used as the most compliant electrode material [16]. Due to its low stiffness (loose powder) it can easily follow the hyper-elastic planar deformation of the film, still depicting satisfactory electric conductivity. Any other material consisting of electrically conductive particles doped in a bonding matrix is not recommended in order to profit from the very low in-plane stiffness. Strong cohesion of the electrode material is necessary, however, to fulfill the force transmission requirement in the transverse direction (parallel to the electro-static field lines). The above discussed load transfer mechanism can be accomplished by applying the carbon powder in a very thin layer form (in the range of several hundred nm). In this configuration (called here Single Layer Electrode, SLE) it can be assumed that the compact clusters consisting of primary electrode particles can take the attracting electro-static forces of the counter-pole type charges arranged on the surface of each electrode and dielectric. Thus the
Fig. 4. Mechanical tensile stress transmission within the electrode.
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Fig. 5. Morphology of the electrode surface on a relaxed dielectric elastomer film. General overview (left) and detail of the particle cluster.
transmission of the applied external force can be provided by the arising electrode material tensile stress. To confirm this hypothesis we have manufactured some coated film samples for appropriate morphological material characterization. Acrylic film (VHB 4910, IPN post-processed [17]), was the used dielectric film and its sticky surface was coated with powder by the smearing method. After application of the electrode material, the surplus amount of powder could easily be removed by suction cleaning which is possible due to the high adhesiveness of the acrylic film. Thereby a minimum required amount of electrode material remains on the film, exhibiting the required transversal isotropic mechanical property. The obtained stack actuator, designed in a multilayer configuration for practical reasons, has shown a high tensile strength under external tension force and when voltage is applied. The same actuator could easily be separated at each layer when actuation voltage was absent. This approach can be followed for any other compliant and conductive material with similar morphological layout, like spray-coated SWCNT, metallic particles prepared by sputtering and (atmospheric plasma) chemical vapour deposition. For the latter, a biaxial corrugated film surface has to be considered in order to provide unrestricted planar strain of the film [18]. Due to the very low thickness of the applied carbon powder electrode with its planar compliancy, the in-plane stiffening effect of the film is negligible. In the case of non-sticky dielectric elastomer materials the same result can be obtained by removing the surplus electrode powder material when voltage is applied. In this situation all those particles, which are not charged and therefore not pressed onto the surface of the dielectric film by the electrostatic force, will be mechanically removed. 3.1. Morphological investigation of the electrode In order to simulate two different manufacturing methods and different physical versions of the actuator (passive and activated mode) the electrode coating as well as the microscopic investigation has been carried out under diverse pre-strain states of the
dielectric film. In the classical handcrafted assembly process of a multilayer actuator the electrode is applied in the pre-strained state of the dielectric film. However, the basic design of the stack actuator presented in this study requires dielectric films applied in a stress free state. For this reason the pre-strained film has to be relaxed after the coating process, whereby the electrode’s surface becomes crumpled. After relaxing the film a SEM investigation of the surface of the electrode was performed by a Hitachi S-4800 machine at 5 kV acceleration voltage and between 4 and 7.8 mm working distance. The picture in Fig. 5, left, depicts the electrode’s morphology with its strongly corrugated surface after relaxation of the film. In this arrangement the electrode is easily capable of following the planar deformation of the dielectric film positioned beneath it (not visible on the picture). In Fig. 5, right, the agglomerated primary electrode particles in the size range of <50 nm are arranged in compact agglomerates, forming “mountains” and “valleys”. As a result these “mountains” with stochastic alignment can freely crimp or stretch in the plane but are capable of transferring tensile stress in the thickness direction (z) at small strain. As a very attractive option for an automated manufacturing process the coating of an already relaxed film is important. For comparison, a similar SEM investigation was performed at 2 kV acceleration voltage and between 7.4 and 11.7 mm working distance. In Fig. 6 the morphology of the electrode surface in the strained state is displayed, whereas the electrode was applied in the relaxed state of the film. As soon as the film is strained (200% in area) the clusters are still noticeable but show essentially smaller corrugations, as may be expected. In this expanded state, which corresponds to the deformation state after activation, the electrode is still capable of taking tensile stresses and therefore provides tension force transmission. Due to the pile-up design of the actuator it has to be pointed out that the actuator does not have any coherence in the deactivated state. Any bonding effects, by reason of stickiness of the dielectric film, or of possibly appearing Van-der-Waals forces, can be helpful to control the system’s shape and to prevent it from breaking into pieces. Otherwise some sort of design solution
Fig. 6. Morphology of the electrode surface in the strained state. General overview (left) and detail of the particle cluster (right).
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Fig. 7. Stacked configuration of dielectric elastomer layers.
or packaging has to be considered in order to avoid total collapse of the actuator. However strong attracting forces are produced by the electrostatic field when voltage is applied which enables high structural stability of the actuator. As an essential conclusion the actuator tends to break apart by tension creep when no voltage is applied and a tension load is applied. Accordingly the dielectric elastomer stack actuator represents a new kind of system where the structural integrity and shape is mainly governed by the electrostatic field strength. As a matter of fact, stack actuators made with “thick” particle electrodes (>1 m) are not capable of taking external tension force. Due to the loose electrode powder layer they simply break apart when loaded and may only be used as the actuator where external pressure force is applied. Tests have shown that SLE is strictly necessary to provide tension force transmission. Actuators equipped with essentially thicker powder electrodes (>1 m) were not able to transmit tensile force and broke apart in one of the electrode layers when actuated with attached external load. 4. Design optimization 4.1. Basic design The proposed actuator is based on a series of elastic capacitors consisting of many small pieces of dielectric elastomer films coated with compliant electrically conductive material as the electrode. They are layered in serial configuration but electrically connected in parallel with alternating polarities of each layer (Fig. 7). Therefore each electrode has an extension at the border of the corresponding layer. Alternating polarities can be achieved by arranging the electrodes in alternating alignment mode of the extensions when stacked up. The extensions are connected with
a compliant conductive band placed on two opposite side faces of the actuator addressing the associated conductive layers. This electrode attachment configuration is essential for the low electrical resistance of the entire system. For activation the main feeding (supply) line of the voltage source has to be attached to the actuator, electrically connecting each electrode layer. In this configuration, where the actuation direction is parallel to the electrostatic field lines and therefore normal to the plane of the electrode and dielectric film, the electrostatic force can be directly turned into a specific mechanical service force to drive the device. The shape of the applied electrodes is of central importance for a reliable operation and represents one of the major design parameters. In this context a small passive zone as a circular cut-out section has to be adopted near to the brink of each layer in order to ensure the electrical separation of each electrode. Furthermore the pull-in effect [19] (with a high risk of electrical breakdown) has to be avoided by alternate offsetting of the electrode edges due to the electro-static field density concentration at the edges of each electrode. The actuation potential and the appearance of the stack actuator are shown by two different samples depicted in Fig. 8, each in the passive and in the activated state. The left actuator has a length of 70 mm (fixing parts not included) and a diameter of 14 mm. The maximum achieved non-loaded contraction of the displayed actuator is about 30%. In comparison to this slim shaped device an actuator with an essentially lower length to diameter ratio (length = 21 mm and diameter = 20 mm) is presented in Fig. 8, right. In this picture the ability to produce considerable tension force (lifting a weight from a support) is demonstrated. By lifting 1 kg of mass, 10% of contraction has been achieved at activation.
Fig. 8. Two stack actuators in action: left half in passive mode–right half in activated mode.
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resents an absolute value and does not depend on the size of the actuator’s cross-sectional area.
4.2. Manufacturing process Single dielectric elastomer film discs, which are coated on one side, are used as the basic material for the pile-up process. High precision and accuracy of the coating, cutting and stacking process steps is a prerequisite for a successful manufacturing procedure, leading to the full utilization of the theoretical potential (up to 40% contraction and 0.16 N/mm2 actuation pressure) of the present stacked actuator. Furthermore special attention has to be paid to the design of the load introduction parts at both ends of the actuator and includes the determination of the film shear deformation. The pile-up actuator configuration represents a fault tolerant system because every eventually broken layer can be localized and decoupled from the voltage source. Reactivation tests on failed devices have shown that malfunctioning actuators can be repaired by replacing or just removing the broken layers. This can be achieved by easily dismounting the actuator stack close to the place of the failed layer and re-assembling it again after having completed the repair. 4.3. Design parameters
5. Experimental characterization The performance of the actuator was experimentally determined in terms of the static contraction performance when activated under different conditions, which are: • Free contraction of the entire actuator (no external constraints and loads) • Contraction of the unloaded actuator equipped with fasteners at both terminal points • Contraction of the unloaded entire actuator with different passive/active area ratios • Contraction of the actuator under different external tension forces (equipped with fasteners at both terminal points). 5.1. Characterization setups The setup for the characterization of the EAP stack actuator prototypes consists of three major components.
In addition to the electro-mechanical properties of the dielectric film and the electrode material, the performance of the actuator is essentially determined by the following 3 main design parameters: • The number of applied layers affects the length of the actuator and therefore the absolute stroke when activated. Based on the produced electrostatic pressure the absolute exhibited force is determined by the cross-section area of one dielectric elastomer film layer. By adding a large number of equal elastomer and electrode layers a macro-scale actuator can be produced. • The envisaged service force has to be introduced into both ends of the actuator which requires appropriate end fixing parts consisting of stiff material. The interface between the soft actuator material and the stiff end fixing part leads to a local multi-axial stress state of the DE material. As a consequence, actuator performance is reduced within this transition zone with limited deformation capability. • To avoid electrical breakdown between the electrodes close to the outside surface, an uncoated passive border area is necessary (Fig. 7). This leads to a loss of the electro active area, which reduces the effectiveness of the actuator. Therefore the width of the passive border has to be designed to be as small as possible in order to maximize performance. The passive, uncoated material area generates a mechanical resistance due to its elastic property and reduces the actuation performance. For this reason the ratio of the uncoated and coated areas essentially determines the effectiveness of the actuation. In terms of optimization it is of central interest to decrease the size of the passive area to a minimum required width. However, the required minimum width of the passive area is determined by the applied voltage amplitude and the electrical breakdown strength of the dielectric film. Consequently the size of the passive strip rep-
• High voltage source: The used high voltage source can generate high voltage from 0 to 12.5 kV. • Contact-free displacement sensor: The deformation of the actuator is measured by a laser displacement sensor. Its absolute measuring accuracy is 0.01 mm. • Tension load: For the characterization of loaded actuators weights from 0 to 24.5 N were used. The supply/control of the actuator system was managed via a graphical user interface (GUI) based on LabView. The control signals from the GUI were transmitted via a PCI card to the high voltage amplifier (model 5/80 from TREK Inc.). 5.2. Characterization procedures For all tests the DE actuators were activated with increasing activation voltage levels from 0 up to 4.2 kV. Preliminary tests showed that the actuators with dielectric film thickness of approx. 80 m became electromechanically unstable over an activation voltage of 4.5 kV. To avoid electrical breakdown the activation voltage was therefore limited to 4.2 kV. The tests were carried out gradually with 1 kV steps up to 3 kV and subsequently with 100 V steps up to 4.2 kV. The corresponding displacement was measured after a short rest of >5 s, waiting for a stable situation in each case. 5.3. Characterization results In order to demonstrate the design parameter dependency of the actuation performance and to characterize the deformation potential, five actuators with circular punched layers and different electrode design and size were tested respectively. The key specifications of the actuators are described in the following Table 1.
Table 1 Key data of the characterized actuators. Actuator
Total diameter (mm)
Active diameter (mm)
Height (mm)
Number of DE layers
1 2 3 4 5
18 20 20 20 20
16 16 16 16 16
18.3 21.2 14.0 22.9 25.8
Approx. 280 Approx. 330 Approx. 210 Approx. 350 Approx. 400
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Fig. 9. Comparison of the measurement data for unloaded free end actuators (left) and unloaded fixed end actuators (right).
To determine the influence of the stiffening effect at both terminal points, each actuator was once actuated without fasteners (Fig. 9, left) and once actuated with fasteners (Fig. 9, right). As an additional design parameter variation, actuator #1 was manufactured with a passive border width of only 1 mm, which has revealed the difficulties of the handcrafted processing for actuators with slim passive borders. All other actuators were produced with 2 mm width of uncoated area. Obviously all actuators depict an essential contraction offset when actuated at different edge conditions. For all tested actuators the stiffening effect of the fittings positioned at both terminal points is not negligible (Fig. 9). However it has to be pointed out that all actuators were relatively short in comparison to the diameter. Therefore the transition zone between the stiff part and the freely deformable region is relatively large compared to the entire length of the actuator. This effect can be visually observed on the actuator depicted in Fig. 11, which is very different to the “slim” actuator in Fig. 8, left, where the length to width ratio is essentially larger. As expected the curves in Fig. 10 reveal the performance dependency of the passive/active area ratio as the most relevant design parameter. The largest measured contraction for “free end” actuators was 46% at an activation voltage of 4.1 kV. This was achieved with actuator #1 which has the smallest amount of passive area (21%). Actuator #2 has reached a lower contraction of up to 35% at an even higher activation voltage of 4.2 kV. The same test was carried out when the actuators were equipped with fittings. The largest measured contraction for actuator #1 was reduced to 30% at an activation voltage of 4.2 kV whereas actuator #2 reached a contraction of only 20% at the same activation voltage. Without fasteners at both terminal points the deformation of the actuator can take place without any external constraints: the passive (uncoated) circular border strip does not participate in the contraction motion and provides a resistance to deformation. However, the inner active part contracts when activated and simultaneously expands in the
Fig. 10. Comparison of the measurement data for unloaded free end actuators and unloaded fixed end actuators.
planar direction. As a result, the surface of the actuator’s top face displays a circular bathtub like shape, which can be easily observed in Fig. 11. In this configuration the actuator has shown a considerable contractive deformation, which can never be achieved with mounted fasteners. The performance limitation by passive border size as well as by the stiff end fixing parts is remarkable and therefore represents the main design criterion. Assuming that the most used actuators in practice will be loaded with external forces, the actuators have to be equipped with appropriate fittings, operating as load introduction parts. For this reason the significant difference in maximum contraction between “free end” and “fixed end” actuators has to be accounted for in the design process. This factor is relevant for actuators with large form factors (width-to-length ratio). On the other hand this stiffening effect is negligible for long and slender actuators. For DE stack actuators with a larger length to
Fig. 11. DE stack actuator with free end: deactivated (left)–activated (right).
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Fig. 12. DE stack actuator equipped with fixed end parts: deactivated (left)–activated (right).
Fig. 13. Contraction of a DE stack actuator at different actuation voltages and external loads (left diagram). By connecting the end points of each curve the force-contraction curve is obtained (right diagram).
width ratio and a smaller passive border area with respect to the total cross-sectional area, a smaller difference is expected. For practical applications the contraction of the actuator under varying external tension loads is of central interest. For this reason we have carried out tensile activation tests with actuator #5 in order to study the electro-mechanical performance of the contractive stack actuator subjected to external tensile force. The tested actuator was provided with appropriate fasteners at both terminal sections for the tension force introduction (Fig. 12). In the vertical position the upper end of the actuator was fixed to a stiff frame whereas the load, in this case a lead weight, has been attached to the lower end of the actuator. It is important to emphasize that the actuator was not loaded in the passive state before starting the actuation tests. Accordingly the weight was positioned on top of a fixed supporting plate in the passive state. At activation, the weight was lifted-off by the contraction motion of the actuator. By applying different voltages in the static mode, the particular vertical displacement of the weight was measured, representing the deformation of the entire actuator. In Fig. 13, left, the typical “lift off” effect can be observed, which is characterized by the offset of the contraction motion at increased voltage and given external weight. This effect is clearly noticeable for the case with the highest load (2614 g): The curve forms a sharp bend at 3.2 kV actuation voltage. At this point the appropriate electro-static field is created to produce the minimum “lift off” force in order to lift the weight. With further increase in the voltage the contraction motion continues in a similar manner to all other curves. Connecting all end points of the set of curves, the characteristic force-deformation curve of the actuator can be defined (Fig. 13, right), which is mostly relevant for the design process drawn to a specific application. Based on this characteristic, which is valid for a specific actuator’s shape and material, the whole area below the curve represents the field of possible actuations. With reference to these curves, the net actuation energy can be evaluated, when performing mechanical work in a closed loop mode. For a load Fext = 20.74 N e.g. a contraction of 10% was reached with the actuator, corresponding to scontr = 2.5 mm absolute displacement. The active part of the actuator weighs mact approx.
4 g (without end fixations and passive border) and is able to lift a weight of over 2000 g. This corresponds to approx. 500 times the actuator’s mass. Based on this test result the net actuation energy Etotal can be calculated: Etotal = Fext · scontr = 0.052J
(2)
For a given actuator’s mass, the specific net actuation energy density Erel can be calculated: Erel =
Etotal = 12.9 J/kg mact
(3)
6. Conclusion and outlook A novel dielectric elastomer actuator design is presented with contraction capability under external tensile force when voltage is applied. Due to the pile-up configuration of stress free dielectric films and the anisotropic property of the applied electrodes, the structural integrity is mainly governed by the electro-static field induced Maxwell stresses. As presented, a set of well functioning EAP stack actuators has been successfully built and the working principle demonstrated. A first series of basic characterizations has shown significant actuation performance of the stack actuator under attached external tensile force. Based on practical experience, the relevant design parameters have been evaluated and partly experimentally quantified. As the most relevant finding, the characterization shows that a small change of length to width ratio and the amount of passive to total area results in significant performance change. Although external tensile force can be applied to the actuator, each dielectric film is exposed to pressure stress due to the Maxwell forces when activated. This is beneficial when good fatigue strength under cyclic tension force loading and long term stability of the actuator is most important. After 1 year in operation and approx. 500 repetitive activations no failure due to fatigue has been reported to date. Specific fatigue tests are planned in the near future. A remarkable robustness against exterior influences, such as humidity and mechanical influences has been observed.
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Consequently, at the present time stacked dielectric elastomer actuators offer the unique potential of enabling really innovative actuation systems where contraction at activation is of paramount interest. The specific properties of these devices make them particularly suitable for developing light, compliant and flexible systems at noise free actuation. This new type of actuator with high reliability represents the most appropriate design when linear contractions of a soft actuator are required, in order to mimic the functional performance of human muscles, i.e. in applications where noise-free contractive actuation is needed when working in close proximity to humans. In particular this is an area of immediate interest for driving prosthetic limbs or robot arms and fingers as well as for powering human machine interfaces. It is logical to postulate that such applications can belong to very different disciplines, such as bio-medical engineering, humanoriented service robotics, robot assisted rehabilitation, wearable and/or portable orthotics and prosthetics, smart automotive and mechanisms engineering, and so on. The basic key feature of the electrode design has an important initial effect on potential applications in many different industrial fields where the specific contractive property becomes significantly important. Up to now a few basic experiments have been carried out in order to gain first findings of the electro-mechanical performance and to estimate the future application potential. As soon as one efficient and automated manufacturing process is well established a large number of actuators with varying design parameters can be built for further extensive investigations. Furthermore modelling of the electro-static behaviour of the actuator has to be performed to optimize the design and future manufacturing process. References [1] S. Ashley, Artificial muscles, Sci. Am. (2003) 52–59. [2] R. Pelrine, R. Kornbluh, Q. Pei, S. Stanford, S. Oh, J. Eckerle, Dielectric elastomer artificial muscle actuators: toward biomimetic motion, in: Proc. SPIE in Smart Struct. and Mat.: Electroactive Polymer Actuators and Devices, vol. 4695, San Diego (USA), 2002, pp. 126–137. [3] Y. Bar-Cohen, in: Y. Bar-Cohen (Ed.), Electroactive Polymer (EAP) Actuators as Artificial Muscles—Reality, Potential and Challenges, SPIE, Bellingham, 2001. [4] R. Kornbluh, Dielectric elastomer artificial muscle for actuation, sensing, generation, and intelligent structures, Mat. Techn. 19 (4) (2004) 216–224. [5] R.E. Pelrine, R.D. Kornbluh, J.P. Joseph, Electrostriction of polymer dielectrics with compliant electrodes as a means of actuation, Sens. Actuators A: Phys. 64 (1) (1998) 77–85. [6] Q. Pei, R. Pelrine, S. Stanford, R. Kornbluh, M. Rosenthal, Electroelastomer rolls and their application for biomimetic walking robots, Synthetic Metals 135 (1) (2003) 129–131. [7] F. Carpi, et al., Dielectric Elastomers as Electromechanical Transducers, Elsevier, 2008. [8] R. Pelrine, R. Kornbluh, G. Kofod, High-strain actuator materials based on dielectric elastomer, Adv. Mater. 12 (2000) 1223–1225. [9] R. Perine, R. Kornbluh, Q. Pei, J. Joseph, Hight-speed electrically actuated elastomers with strain greater than 100%, Science 287 (2000) 836–839. [10] K. Meijer, M. Rosenthal, R.J. Full, Muscle-like actuators—a comparison between three electroactive polymer, Proc. SPIE Int. Soc. Opt. Eng. 4329 (2001) 7–15.
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[11] G. Kofod, R. Kornbluh, R. Pelrine, P. Sommer-Larsen, Actuation response of polyacrylate dielectric elastomers, J. Intell. Mater. Syst. Struct. 14 (2003). [12] P. Sommer-Larsen, G. Kofod, S. MH, M. Benslimane, P. Gravesen, Performance of dielectric elastomer actuators and materials, Proc. SPIE Int. Soc. Opt. Eng. 4695 (2002) 158–166. [13] G. Kovacs, P. Lochmatter, M. Wissler, An arm wrestling robot driven by dielectric elastomer actuators, Smart Mater. Struct. 16 (2007) 306–317. [14] F. Carpi, C. Salaris, D. De Rossi, Folded dielectric elastomer actuators, Smart Mater. Struct. 16 (2007) 300–305. [15] M. Jungmann, H.F. Schlaak, Electrostatic actuators with elastic dielectric for use on tactile displays, in: 8th International Conference on New Actuators, Bremen, Germany, 2002. [16] G. Kofod, Dielectric elastomer actuators. Ph.D. Thesis, Technical University of Denmark, Lyngby (Denmark), 2001. [17] S.M. Ha, W. Yuan, Q. Pei, R. Pelrine, S. Stanford, Interpenetrating polymer networks for high-performance electroelastomer artificial muscles, Adv. Mater. 18 (2006) 887–891. [18] M. Benslimane, P. Gravesen, P. Sommer-Larsen, Mechanical properties of dielectric elastomer actuators with smart metallic compliant electrodes, in: In Proc. of SPIE Smart Struct. and Mat.: Electroactive Polymer Actuators and Devices (EAPAD), vol. 4695, San Diego (USA), 2002, pp. 150–157. [19] J. Plante, Dielectric elastomer actuators for binary robotics and mechatronics. Ph.D. Thesis, Massachusetts Institute of Technology, Cambridge (USA), 2006.
Biographies Gabor Kovacs was born in 1958 in Maennedorf, Switzerland. He studied Mechanical Engineering at ETH Zurich and received his Dr. sc. techn. degree at the ETH. Following his PhD he worked at the Institute of Lightweight Structures and Ropeway Technology at the ETH Zurich as senior scientist. From 1996 to 2001 he was the Head of the competence centre “Aerial Cableway Technology” at Empa in Dubendorf (Zurich). Between 2001 and 2003 he was group leader of electroactive polymers (EAP) actuator technology for adaptive material systems. Between January 2003 and the end of 2005 he was Head of the laboratory for “Materials and Engineering” at Empa. Since 2006 he has been senior scientist for novel actuator technologies in the field of artificial muscles based on EAP. Lukas Düring was born in Niederbüren, Switzerland in 1982. He graduated in Mechanical Engineering at the Swiss Federal Institute of Technology (ETH) in 2008. He is an engineer in material engineering at the Swiss Federal Laboratories for Materials Testing and Research (Empa). He developed the contractive tension force electroactive polymer actuator. Silvain Michel graduated in Mechanical Engineering at the Swiss Federal Institute of Technology (ETH) in 1989. After 10 years as a fatigue and damage tolerant specialist in the aerospace industry he joined the Swiss Federal Laboratories for Materials Testing and Research (Empa) in 2000. Between 2003 and 2008 he was Head of the electro active research group within the lab “Mechanical Systems Engineering”. Since 2009 he has been a senior scientist, responsible for various projects on electro-active polymer actuators and devices. Giovanni Terrasi was educated as a material science engineer and received his PhD (Civil Engineering Department) from ETH Zürich in 1997. Between 1998 and 2005 he led the R&D/Engineering Department at SACAC Ltd in Lenzburg, Switzerland with a particular interest in the behaviour of high performance concrete reinforced or prestressed with fibre reinforced polymer tendons. Since December 2005 he has been in charge of the Laboratory for Mechanical Systems Engineering at the Swiss Federal Laboratories for Materials Testing and Research, Empa. He has focused his research activities on the in-service durability of mechanical components, on polymer matrix composites, active structures (based on DE actuators and piezoelectric sensors for SHM) and biomechanical engineering.
Contents lists available at ScienceDirect
Sensors and Actuators A: Physical journal homepage: www.elsevier.com/locate/sna
Stacked dielectric elastomer actuator for tensile force transmission G. Kovacs ∗ , L. Düring, S. Michel, G. Terrasi Empa, Swiss Federal Laboratories for Materials Testing and Research, Laboratory for Mechanical System Engineering, Uberlandstrasse 129, 8600 Dubendorf, Switzerland
a r t i c l e
i n f o
Article history: Received 26 February 2009 Received in revised form 25 May 2009 Accepted 30 August 2009 Available online 8 September 2009 Keywords: Active structures Electro-active polymers (EAPs) Soft dielectric EAPs Contractive tension force actuator
a b s t r a c t This paper presents a novel approach for active structures driven by soft dielectric electro-active polymers (EAPs), which can perform contractive displacements at external tensile load. The active structure is composed of an array of equal segments, where the dielectric films are arranged in a pile-up configuration. The proposed active structure has the capability of exhibiting uniaxial contractive deformations, while being exposed to external tensile forces. The serial arrangement of active segments has one contracting degree of freedom in the thickness direction of the dielectric EAP film layers. Due to the envisaged tension force transmission capability, special attention is paid to the electrode design which is of paramount importance with regard to functionality of the actuator. A compliant electrode system with anisotropic deformation properties is presented based on nano scale carbon powder. In experiments, the free deformation as well as the contractive motion under external tensile loading of several actuator configurations with different setups is characterized. These involve the study of various sizes and numbers of stacked film layers as well as different electrode designs. © 2009 Elsevier B.V. All rights reserved.
1. Introduction In the last decade, the interest in “smart materials”, which respond to external stimuli by changing their shape or size, has essentially increased. In particular soft dielectric EAPs as musclelike actuators, a subgroup of the electro-active polymers (EAPs), have attracted much interest in recent years due to their outstanding active deformation potential [1–4]. Soft dielectric EAPs consist of a thin elastomer film, which is coated on both sides with compliant electrodes (Fig. 1, left). When applying a DC high voltage U (in the range of several kV) to this compliant capacitor, the electrodes squeeze the elastomeric dielectric in the thickness direction (electrode pressure, Pequivalent [5]), and thus the nearby incompressible film expands in the planar direction (Fig. 1, right). Pequivalent = ε0 · εr ·
U 2 d
(1)
Thereby ε0 is the free-space dielectric permittivity (ε0 = 8.85 × 10−12 F/m), εr is the relative permittivity of the dielectric material and d represents the thickness of the dielectric film. As soon as the voltage is switched off and the electrodes are short-circuited the film deforms back to its initial state. So far, a variety of different types of dielectric elastomer (DE) actuators have demonstrated the versatile capabilities of this actuator technology (e.g. [6,7]). Due to their intrinsic compliance and
∗ Corresponding author. Tel.: +41 0 44 823 4063; fax: +41 0 44 823 4011. E-mail address: [email protected] (G. Kovacs). 0924-4247/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.sna.2009.08.027
unique deformation potential, soft dielectric EAPs are promising for compliant, lightweight structures in the macro-scale, which can perform continuous displacements. The actuation of the dielectric elastomer films can be used in two different ways. Based on the principle of operation, two directions are possible to perform work against external loads (Fig. 2): • Work in the planar directions (expanding actuator): Under electrical activation of a DE basic unit the film expands in the x, y plane (Fig. 1) and can thus work against external pressure loads in both planar directions x and y. • Work in the thickness direction (contractile actuator): Under electrical activation the electrodes squeeze the DE film in the thickness direction (z) (Fig. 1). Thus, the actuator can work against external tensile loads acting in the direction of the electric field lines of the compliant capacitor. With regard to the elastomers used as the dielectric in DE actuators, the best overall performances have been shown by mainly silicone and acrylic films, such as commercially available VHB 4910 from 3 M [8,9]. The dielectric film, made from acrylic VHB 4910 especially for DE actuators, is strongly pre-strained in planar directions in order to reduce the thickness and therefore the required activation voltage level. To maintain the DE film in this biaxially pre-strained state a support structure is needed. To enable the required displacements this support structure must offer the corresponding mechanical degrees of freedom (DOF). Obviously, in pre-strained DE actuators the design of the support structure is one of the key issues and causes many design constraints.
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Fig. 1. Structure and working principle of soft dielectric EAP (Q = electrical charge).
To date most scientific research work has been focused on the in-plane expanding actuation mode. This is due to the fact that the commercially available acrylic material VHB 4910 (3 M) can easily be processed to planar actuators and has demonstrated a very high actuation performance, in particular with high breakdown fields of up to 200 V/m, when pre-strained [10–12]. Many different actuator designs have been studied for experimental purposes and represent the present state-of-the-art. They include the extender (planar), unimorph-, bi-morph-, spring roll-, push-pull, bow-tie-, diamond-, diaphragm-, spider-, inchworm segmentand universal muscle actuators [6,7]. They all have the same basic electro-mechanical characteristic. The actuator expands in-plane when voltage is applied and shrinks back as soon as the applied charges are removed from the electrodes (following the principles depicted in Fig. 1). For example, the development of the Empa arm wrestling robot in 2005 has been a major contribution to the production technology of expanding actuators in rolled configuration [13]. Due to the expanding actuation characteristic an agonistantagonistic arrangement of the actuator bundles was necessary to produce a bidirectional motion. In this configuration all actuators were fixed in a pre-strained state in order to execute the required contracting motion under external tensile force in the deactivated mode. Without exception all highly pre-strained actuators of the Empa robot have poor durability due to the required high pre-stretching ratio in combination with the necessary rigid, often heavy and bulky supporting structure. When contraction at activation is required the discussed “expanding planar EAP actuator” does not represent the appropriate solution for many applications. The basic DE working principle can be exploited directly (Fig. 2, lower picture left) in order to build linear contractive actuators. A few approaches have been carried out so far to provide actuators with contractive properties when activated. Based on the folded and helical design with non-pre-
strained silicone film contractive motion of the actuator has been demonstrated [14]. The so-called folded actuator consists of a single strip of an elastomer that is first successively coated with continuous compliant electrodes and then folded up, so as to form a monolithic compact body. The compliant electrodes are made of a silicone/carbon-black mixture. In the case of the silicone elastomer, the folded structure is finally ‘sealed’ with a thin coating made of the same elastomer used for the strip. When using acrylic elastomers the intrinsic adhesiveness of the material does not require any further treatment for good structural integrity. A high voltage applied between the electrodes causes a thickness squeezing of the entire elastomeric layer under the influence of the induced electro-static field. The resulting axial contraction is accompanied by associated related lateral expansion. Furthermore the basic design of the stack configuration has been developed and demonstrated for electrostatic tactile displays with high structural compliance [15]. When a voltage is applied between neighbouring electrode layers of this device, the actuator stack will contract and the stimulator tip will disappear below the surface of the device. Due to the fact that the fingertip is basically pressing on the stimulator tip no significant tensile force has to be produced by this actuator structure for the pull-back motion. By reducing the voltage the electrodes are discharged beyond the voltage source causing a relaxation of the actuator stack. The stimulator tip is thereby pressed against the skin on top of the device because of the stored elastic energy. In contrast to this last actuator design, a new actuator in pileup design is proposed here which exhibits contractive deformation when electrically activated and can work against external tensile loads acting in the stack’s axial direction. The working principle of the actuator design is based on the contraction effect of the dielectric film when an electrostatic field is applied. Due to the fact that the actuation direction is parallel to the electrostatic field lines, the electrostatic force can be directly turned into mechanical force (the compressed DE film produces the contraction motion of the actuator). The electrode design plays a central role in enabling transmission of the tensile force from one DE layer to the next. In contrast to the expanding membrane actuator, the proposed contractive actuator demonstrates the feasibility and convenience of the pile-up configuration for many applications where contraction at activation and production of tensile forces are of central interest. 2. Conceptual approach
Fig. 2. Activation modes: in-plane expanding–out-of plane contractive actuation.
The proposed contractive actuator consists of stacked small DE film pieces coated with compliant electrodes (Fig. 3). The electrostatic field induced actuation force and displacement is perpendicular to the plane of the electrode and dielectric film. The obtained contraction of the actuator can be used to drive the device under a specific service load. The design of the pile-up actuator is based on a series of capacitors electrically connected in parallel with alternating polarities of each layer. When transmission of an external tension force is required, the internal force (stress) dis-
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Fig. 3. Layerwise composite structure of the stacked DE actuator consisting of alternating electrode and DE film layers.
tribution of the actuator is of central interest. The actuator has to be capable, therefore, of robustly transmitting the external tension force from one load introduction point placed at one end of the actuator to the second one positioned at the opposite end when activated and contraction motion is supposed to be executed. With respect to the composed structure of the proposed actuator the tensile stress transfer occurs as follows. When a voltage is applied between two electrodes, external tension force can be transmitted by the electrostatic field induced Maxwell force. As a result the DE is squeezed by both neighbouring electrodes, and contractive motion of the entire actuator occurs. Although considerable external tension load can be transmitted by the electrostatic force, solely compression stress is triggered within the dielectric film which acts as a “spacer” and does not take any tensile stress (in either planar or normal directions). This is beneficial when good fatigue strength under cyclic tension load and long term stability of the actuator are of central interest. Obviously the electrostatic field vanishes inside the electrode due to its high electrical conductivity and the capability of freely moving charges. In order to allow the tensile force transmission within the electrode layer, its electro-mechanical properties are of paramount importance and will be discussed from a constitutive perspective below. The external tension force has to be transmitted over the electrode layer by inducing tensile stresses inside the electrode material (as sketched in Fig. 4). This can easily be accomplished when the dielectric medium is made of compressible elastic material. In such a case no planar expansion occurs when the dielectric is compressed by the Maxwell force and therefore the electrode can be made of a solid conductive material layer (stiff conductive plate). When using incompressible material like hyper-elastic elastomer films as dielectric, the electrode has to be compliant in the planar direction in order to allow unrestricted planar expansion of the dielectric film without reducing the electric conductivity needed to provide the electrostatic field, even in the deformed state. On the other hand, the electrode has to be designed in such a way
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that the tension force transmission capability in the normal direction of the film is assured. For this reason a certain stiffness and tensile strength of the electrode material in the thickness direction (z) (parallel to the electrostatic field) is required to robustly transmit mechanical tensile stress. The following explanation of the working principle is based on an idealized and macroscopic layered system with homogeneous properties. In general an electro-static field occurs between two electrodes when a voltage is applied to the capacitor. Thereby the electric charges will be arranged on the surface of each electrode and thus located near to the dielectric film. Additionally it can be assumed that a layer of oppositely poled electric charges on the surface of the dielectric film is formed, which is thus positioned close to the electrode. As a result attraction forces of the counter-poled charges between both joining surface layers of the dielectric and the electrode occur. When external tension force is applied to the actuator these interlayer attraction forces create mechanical tensile stress within the electrode material (Fig. 4). Therefore the external force can be transmitted from one side to the opposite side of the electrode by the resulting tension stress state of the electrode material, which needs high stiffness and tensile strength in the thickness direction (z) (Fig. 4). Any adhesive bonding effects between the dielectric and electrode layers might be helpful for mechanical stability in the passive state but are not necessary for the force transmission at activation. Consequently an anisotropic material system has to be used as the electrode with direction dependent stiffness in order to fulfill the in-plane compliancy and the out-of-plane stiffness and strength demand. 3. Investigations of carbon powder electrode layers The proposed pile-up configuration with an appropriate electrode allows the realization of novel dielectric elastomer actuators which can exhibit contractive motion under an applied external tension load. To date, carbon powder Ketjenblack 600 from Akzo Nobel (or a similar material) is widely used as the most compliant electrode material [16]. Due to its low stiffness (loose powder) it can easily follow the hyper-elastic planar deformation of the film, still depicting satisfactory electric conductivity. Any other material consisting of electrically conductive particles doped in a bonding matrix is not recommended in order to profit from the very low in-plane stiffness. Strong cohesion of the electrode material is necessary, however, to fulfill the force transmission requirement in the transverse direction (parallel to the electro-static field lines). The above discussed load transfer mechanism can be accomplished by applying the carbon powder in a very thin layer form (in the range of several hundred nm). In this configuration (called here Single Layer Electrode, SLE) it can be assumed that the compact clusters consisting of primary electrode particles can take the attracting electro-static forces of the counter-pole type charges arranged on the surface of each electrode and dielectric. Thus the
Fig. 4. Mechanical tensile stress transmission within the electrode.
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Fig. 5. Morphology of the electrode surface on a relaxed dielectric elastomer film. General overview (left) and detail of the particle cluster.
transmission of the applied external force can be provided by the arising electrode material tensile stress. To confirm this hypothesis we have manufactured some coated film samples for appropriate morphological material characterization. Acrylic film (VHB 4910, IPN post-processed [17]), was the used dielectric film and its sticky surface was coated with powder by the smearing method. After application of the electrode material, the surplus amount of powder could easily be removed by suction cleaning which is possible due to the high adhesiveness of the acrylic film. Thereby a minimum required amount of electrode material remains on the film, exhibiting the required transversal isotropic mechanical property. The obtained stack actuator, designed in a multilayer configuration for practical reasons, has shown a high tensile strength under external tension force and when voltage is applied. The same actuator could easily be separated at each layer when actuation voltage was absent. This approach can be followed for any other compliant and conductive material with similar morphological layout, like spray-coated SWCNT, metallic particles prepared by sputtering and (atmospheric plasma) chemical vapour deposition. For the latter, a biaxial corrugated film surface has to be considered in order to provide unrestricted planar strain of the film [18]. Due to the very low thickness of the applied carbon powder electrode with its planar compliancy, the in-plane stiffening effect of the film is negligible. In the case of non-sticky dielectric elastomer materials the same result can be obtained by removing the surplus electrode powder material when voltage is applied. In this situation all those particles, which are not charged and therefore not pressed onto the surface of the dielectric film by the electrostatic force, will be mechanically removed. 3.1. Morphological investigation of the electrode In order to simulate two different manufacturing methods and different physical versions of the actuator (passive and activated mode) the electrode coating as well as the microscopic investigation has been carried out under diverse pre-strain states of the
dielectric film. In the classical handcrafted assembly process of a multilayer actuator the electrode is applied in the pre-strained state of the dielectric film. However, the basic design of the stack actuator presented in this study requires dielectric films applied in a stress free state. For this reason the pre-strained film has to be relaxed after the coating process, whereby the electrode’s surface becomes crumpled. After relaxing the film a SEM investigation of the surface of the electrode was performed by a Hitachi S-4800 machine at 5 kV acceleration voltage and between 4 and 7.8 mm working distance. The picture in Fig. 5, left, depicts the electrode’s morphology with its strongly corrugated surface after relaxation of the film. In this arrangement the electrode is easily capable of following the planar deformation of the dielectric film positioned beneath it (not visible on the picture). In Fig. 5, right, the agglomerated primary electrode particles in the size range of <50 nm are arranged in compact agglomerates, forming “mountains” and “valleys”. As a result these “mountains” with stochastic alignment can freely crimp or stretch in the plane but are capable of transferring tensile stress in the thickness direction (z) at small strain. As a very attractive option for an automated manufacturing process the coating of an already relaxed film is important. For comparison, a similar SEM investigation was performed at 2 kV acceleration voltage and between 7.4 and 11.7 mm working distance. In Fig. 6 the morphology of the electrode surface in the strained state is displayed, whereas the electrode was applied in the relaxed state of the film. As soon as the film is strained (200% in area) the clusters are still noticeable but show essentially smaller corrugations, as may be expected. In this expanded state, which corresponds to the deformation state after activation, the electrode is still capable of taking tensile stresses and therefore provides tension force transmission. Due to the pile-up design of the actuator it has to be pointed out that the actuator does not have any coherence in the deactivated state. Any bonding effects, by reason of stickiness of the dielectric film, or of possibly appearing Van-der-Waals forces, can be helpful to control the system’s shape and to prevent it from breaking into pieces. Otherwise some sort of design solution
Fig. 6. Morphology of the electrode surface in the strained state. General overview (left) and detail of the particle cluster (right).
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Fig. 7. Stacked configuration of dielectric elastomer layers.
or packaging has to be considered in order to avoid total collapse of the actuator. However strong attracting forces are produced by the electrostatic field when voltage is applied which enables high structural stability of the actuator. As an essential conclusion the actuator tends to break apart by tension creep when no voltage is applied and a tension load is applied. Accordingly the dielectric elastomer stack actuator represents a new kind of system where the structural integrity and shape is mainly governed by the electrostatic field strength. As a matter of fact, stack actuators made with “thick” particle electrodes (>1 m) are not capable of taking external tension force. Due to the loose electrode powder layer they simply break apart when loaded and may only be used as the actuator where external pressure force is applied. Tests have shown that SLE is strictly necessary to provide tension force transmission. Actuators equipped with essentially thicker powder electrodes (>1 m) were not able to transmit tensile force and broke apart in one of the electrode layers when actuated with attached external load. 4. Design optimization 4.1. Basic design The proposed actuator is based on a series of elastic capacitors consisting of many small pieces of dielectric elastomer films coated with compliant electrically conductive material as the electrode. They are layered in serial configuration but electrically connected in parallel with alternating polarities of each layer (Fig. 7). Therefore each electrode has an extension at the border of the corresponding layer. Alternating polarities can be achieved by arranging the electrodes in alternating alignment mode of the extensions when stacked up. The extensions are connected with
a compliant conductive band placed on two opposite side faces of the actuator addressing the associated conductive layers. This electrode attachment configuration is essential for the low electrical resistance of the entire system. For activation the main feeding (supply) line of the voltage source has to be attached to the actuator, electrically connecting each electrode layer. In this configuration, where the actuation direction is parallel to the electrostatic field lines and therefore normal to the plane of the electrode and dielectric film, the electrostatic force can be directly turned into a specific mechanical service force to drive the device. The shape of the applied electrodes is of central importance for a reliable operation and represents one of the major design parameters. In this context a small passive zone as a circular cut-out section has to be adopted near to the brink of each layer in order to ensure the electrical separation of each electrode. Furthermore the pull-in effect [19] (with a high risk of electrical breakdown) has to be avoided by alternate offsetting of the electrode edges due to the electro-static field density concentration at the edges of each electrode. The actuation potential and the appearance of the stack actuator are shown by two different samples depicted in Fig. 8, each in the passive and in the activated state. The left actuator has a length of 70 mm (fixing parts not included) and a diameter of 14 mm. The maximum achieved non-loaded contraction of the displayed actuator is about 30%. In comparison to this slim shaped device an actuator with an essentially lower length to diameter ratio (length = 21 mm and diameter = 20 mm) is presented in Fig. 8, right. In this picture the ability to produce considerable tension force (lifting a weight from a support) is demonstrated. By lifting 1 kg of mass, 10% of contraction has been achieved at activation.
Fig. 8. Two stack actuators in action: left half in passive mode–right half in activated mode.
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resents an absolute value and does not depend on the size of the actuator’s cross-sectional area.
4.2. Manufacturing process Single dielectric elastomer film discs, which are coated on one side, are used as the basic material for the pile-up process. High precision and accuracy of the coating, cutting and stacking process steps is a prerequisite for a successful manufacturing procedure, leading to the full utilization of the theoretical potential (up to 40% contraction and 0.16 N/mm2 actuation pressure) of the present stacked actuator. Furthermore special attention has to be paid to the design of the load introduction parts at both ends of the actuator and includes the determination of the film shear deformation. The pile-up actuator configuration represents a fault tolerant system because every eventually broken layer can be localized and decoupled from the voltage source. Reactivation tests on failed devices have shown that malfunctioning actuators can be repaired by replacing or just removing the broken layers. This can be achieved by easily dismounting the actuator stack close to the place of the failed layer and re-assembling it again after having completed the repair. 4.3. Design parameters
5. Experimental characterization The performance of the actuator was experimentally determined in terms of the static contraction performance when activated under different conditions, which are: • Free contraction of the entire actuator (no external constraints and loads) • Contraction of the unloaded actuator equipped with fasteners at both terminal points • Contraction of the unloaded entire actuator with different passive/active area ratios • Contraction of the actuator under different external tension forces (equipped with fasteners at both terminal points). 5.1. Characterization setups The setup for the characterization of the EAP stack actuator prototypes consists of three major components.
In addition to the electro-mechanical properties of the dielectric film and the electrode material, the performance of the actuator is essentially determined by the following 3 main design parameters: • The number of applied layers affects the length of the actuator and therefore the absolute stroke when activated. Based on the produced electrostatic pressure the absolute exhibited force is determined by the cross-section area of one dielectric elastomer film layer. By adding a large number of equal elastomer and electrode layers a macro-scale actuator can be produced. • The envisaged service force has to be introduced into both ends of the actuator which requires appropriate end fixing parts consisting of stiff material. The interface between the soft actuator material and the stiff end fixing part leads to a local multi-axial stress state of the DE material. As a consequence, actuator performance is reduced within this transition zone with limited deformation capability. • To avoid electrical breakdown between the electrodes close to the outside surface, an uncoated passive border area is necessary (Fig. 7). This leads to a loss of the electro active area, which reduces the effectiveness of the actuator. Therefore the width of the passive border has to be designed to be as small as possible in order to maximize performance. The passive, uncoated material area generates a mechanical resistance due to its elastic property and reduces the actuation performance. For this reason the ratio of the uncoated and coated areas essentially determines the effectiveness of the actuation. In terms of optimization it is of central interest to decrease the size of the passive area to a minimum required width. However, the required minimum width of the passive area is determined by the applied voltage amplitude and the electrical breakdown strength of the dielectric film. Consequently the size of the passive strip rep-
• High voltage source: The used high voltage source can generate high voltage from 0 to 12.5 kV. • Contact-free displacement sensor: The deformation of the actuator is measured by a laser displacement sensor. Its absolute measuring accuracy is 0.01 mm. • Tension load: For the characterization of loaded actuators weights from 0 to 24.5 N were used. The supply/control of the actuator system was managed via a graphical user interface (GUI) based on LabView. The control signals from the GUI were transmitted via a PCI card to the high voltage amplifier (model 5/80 from TREK Inc.). 5.2. Characterization procedures For all tests the DE actuators were activated with increasing activation voltage levels from 0 up to 4.2 kV. Preliminary tests showed that the actuators with dielectric film thickness of approx. 80 m became electromechanically unstable over an activation voltage of 4.5 kV. To avoid electrical breakdown the activation voltage was therefore limited to 4.2 kV. The tests were carried out gradually with 1 kV steps up to 3 kV and subsequently with 100 V steps up to 4.2 kV. The corresponding displacement was measured after a short rest of >5 s, waiting for a stable situation in each case. 5.3. Characterization results In order to demonstrate the design parameter dependency of the actuation performance and to characterize the deformation potential, five actuators with circular punched layers and different electrode design and size were tested respectively. The key specifications of the actuators are described in the following Table 1.
Table 1 Key data of the characterized actuators. Actuator
Total diameter (mm)
Active diameter (mm)
Height (mm)
Number of DE layers
1 2 3 4 5
18 20 20 20 20
16 16 16 16 16
18.3 21.2 14.0 22.9 25.8
Approx. 280 Approx. 330 Approx. 210 Approx. 350 Approx. 400
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Fig. 9. Comparison of the measurement data for unloaded free end actuators (left) and unloaded fixed end actuators (right).
To determine the influence of the stiffening effect at both terminal points, each actuator was once actuated without fasteners (Fig. 9, left) and once actuated with fasteners (Fig. 9, right). As an additional design parameter variation, actuator #1 was manufactured with a passive border width of only 1 mm, which has revealed the difficulties of the handcrafted processing for actuators with slim passive borders. All other actuators were produced with 2 mm width of uncoated area. Obviously all actuators depict an essential contraction offset when actuated at different edge conditions. For all tested actuators the stiffening effect of the fittings positioned at both terminal points is not negligible (Fig. 9). However it has to be pointed out that all actuators were relatively short in comparison to the diameter. Therefore the transition zone between the stiff part and the freely deformable region is relatively large compared to the entire length of the actuator. This effect can be visually observed on the actuator depicted in Fig. 11, which is very different to the “slim” actuator in Fig. 8, left, where the length to width ratio is essentially larger. As expected the curves in Fig. 10 reveal the performance dependency of the passive/active area ratio as the most relevant design parameter. The largest measured contraction for “free end” actuators was 46% at an activation voltage of 4.1 kV. This was achieved with actuator #1 which has the smallest amount of passive area (21%). Actuator #2 has reached a lower contraction of up to 35% at an even higher activation voltage of 4.2 kV. The same test was carried out when the actuators were equipped with fittings. The largest measured contraction for actuator #1 was reduced to 30% at an activation voltage of 4.2 kV whereas actuator #2 reached a contraction of only 20% at the same activation voltage. Without fasteners at both terminal points the deformation of the actuator can take place without any external constraints: the passive (uncoated) circular border strip does not participate in the contraction motion and provides a resistance to deformation. However, the inner active part contracts when activated and simultaneously expands in the
Fig. 10. Comparison of the measurement data for unloaded free end actuators and unloaded fixed end actuators.
planar direction. As a result, the surface of the actuator’s top face displays a circular bathtub like shape, which can be easily observed in Fig. 11. In this configuration the actuator has shown a considerable contractive deformation, which can never be achieved with mounted fasteners. The performance limitation by passive border size as well as by the stiff end fixing parts is remarkable and therefore represents the main design criterion. Assuming that the most used actuators in practice will be loaded with external forces, the actuators have to be equipped with appropriate fittings, operating as load introduction parts. For this reason the significant difference in maximum contraction between “free end” and “fixed end” actuators has to be accounted for in the design process. This factor is relevant for actuators with large form factors (width-to-length ratio). On the other hand this stiffening effect is negligible for long and slender actuators. For DE stack actuators with a larger length to
Fig. 11. DE stack actuator with free end: deactivated (left)–activated (right).
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Fig. 12. DE stack actuator equipped with fixed end parts: deactivated (left)–activated (right).
Fig. 13. Contraction of a DE stack actuator at different actuation voltages and external loads (left diagram). By connecting the end points of each curve the force-contraction curve is obtained (right diagram).
width ratio and a smaller passive border area with respect to the total cross-sectional area, a smaller difference is expected. For practical applications the contraction of the actuator under varying external tension loads is of central interest. For this reason we have carried out tensile activation tests with actuator #5 in order to study the electro-mechanical performance of the contractive stack actuator subjected to external tensile force. The tested actuator was provided with appropriate fasteners at both terminal sections for the tension force introduction (Fig. 12). In the vertical position the upper end of the actuator was fixed to a stiff frame whereas the load, in this case a lead weight, has been attached to the lower end of the actuator. It is important to emphasize that the actuator was not loaded in the passive state before starting the actuation tests. Accordingly the weight was positioned on top of a fixed supporting plate in the passive state. At activation, the weight was lifted-off by the contraction motion of the actuator. By applying different voltages in the static mode, the particular vertical displacement of the weight was measured, representing the deformation of the entire actuator. In Fig. 13, left, the typical “lift off” effect can be observed, which is characterized by the offset of the contraction motion at increased voltage and given external weight. This effect is clearly noticeable for the case with the highest load (2614 g): The curve forms a sharp bend at 3.2 kV actuation voltage. At this point the appropriate electro-static field is created to produce the minimum “lift off” force in order to lift the weight. With further increase in the voltage the contraction motion continues in a similar manner to all other curves. Connecting all end points of the set of curves, the characteristic force-deformation curve of the actuator can be defined (Fig. 13, right), which is mostly relevant for the design process drawn to a specific application. Based on this characteristic, which is valid for a specific actuator’s shape and material, the whole area below the curve represents the field of possible actuations. With reference to these curves, the net actuation energy can be evaluated, when performing mechanical work in a closed loop mode. For a load Fext = 20.74 N e.g. a contraction of 10% was reached with the actuator, corresponding to scontr = 2.5 mm absolute displacement. The active part of the actuator weighs mact approx.
4 g (without end fixations and passive border) and is able to lift a weight of over 2000 g. This corresponds to approx. 500 times the actuator’s mass. Based on this test result the net actuation energy Etotal can be calculated: Etotal = Fext · scontr = 0.052J
(2)
For a given actuator’s mass, the specific net actuation energy density Erel can be calculated: Erel =
Etotal = 12.9 J/kg mact
(3)
6. Conclusion and outlook A novel dielectric elastomer actuator design is presented with contraction capability under external tensile force when voltage is applied. Due to the pile-up configuration of stress free dielectric films and the anisotropic property of the applied electrodes, the structural integrity is mainly governed by the electro-static field induced Maxwell stresses. As presented, a set of well functioning EAP stack actuators has been successfully built and the working principle demonstrated. A first series of basic characterizations has shown significant actuation performance of the stack actuator under attached external tensile force. Based on practical experience, the relevant design parameters have been evaluated and partly experimentally quantified. As the most relevant finding, the characterization shows that a small change of length to width ratio and the amount of passive to total area results in significant performance change. Although external tensile force can be applied to the actuator, each dielectric film is exposed to pressure stress due to the Maxwell forces when activated. This is beneficial when good fatigue strength under cyclic tension force loading and long term stability of the actuator is most important. After 1 year in operation and approx. 500 repetitive activations no failure due to fatigue has been reported to date. Specific fatigue tests are planned in the near future. A remarkable robustness against exterior influences, such as humidity and mechanical influences has been observed.
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Consequently, at the present time stacked dielectric elastomer actuators offer the unique potential of enabling really innovative actuation systems where contraction at activation is of paramount interest. The specific properties of these devices make them particularly suitable for developing light, compliant and flexible systems at noise free actuation. This new type of actuator with high reliability represents the most appropriate design when linear contractions of a soft actuator are required, in order to mimic the functional performance of human muscles, i.e. in applications where noise-free contractive actuation is needed when working in close proximity to humans. In particular this is an area of immediate interest for driving prosthetic limbs or robot arms and fingers as well as for powering human machine interfaces. It is logical to postulate that such applications can belong to very different disciplines, such as bio-medical engineering, humanoriented service robotics, robot assisted rehabilitation, wearable and/or portable orthotics and prosthetics, smart automotive and mechanisms engineering, and so on. The basic key feature of the electrode design has an important initial effect on potential applications in many different industrial fields where the specific contractive property becomes significantly important. Up to now a few basic experiments have been carried out in order to gain first findings of the electro-mechanical performance and to estimate the future application potential. As soon as one efficient and automated manufacturing process is well established a large number of actuators with varying design parameters can be built for further extensive investigations. Furthermore modelling of the electro-static behaviour of the actuator has to be performed to optimize the design and future manufacturing process. References [1] S. Ashley, Artificial muscles, Sci. Am. (2003) 52–59. [2] R. Pelrine, R. Kornbluh, Q. Pei, S. Stanford, S. Oh, J. Eckerle, Dielectric elastomer artificial muscle actuators: toward biomimetic motion, in: Proc. SPIE in Smart Struct. and Mat.: Electroactive Polymer Actuators and Devices, vol. 4695, San Diego (USA), 2002, pp. 126–137. [3] Y. Bar-Cohen, in: Y. Bar-Cohen (Ed.), Electroactive Polymer (EAP) Actuators as Artificial Muscles—Reality, Potential and Challenges, SPIE, Bellingham, 2001. [4] R. Kornbluh, Dielectric elastomer artificial muscle for actuation, sensing, generation, and intelligent structures, Mat. Techn. 19 (4) (2004) 216–224. [5] R.E. Pelrine, R.D. Kornbluh, J.P. Joseph, Electrostriction of polymer dielectrics with compliant electrodes as a means of actuation, Sens. Actuators A: Phys. 64 (1) (1998) 77–85. [6] Q. Pei, R. Pelrine, S. Stanford, R. Kornbluh, M. Rosenthal, Electroelastomer rolls and their application for biomimetic walking robots, Synthetic Metals 135 (1) (2003) 129–131. [7] F. Carpi, et al., Dielectric Elastomers as Electromechanical Transducers, Elsevier, 2008. [8] R. Pelrine, R. Kornbluh, G. Kofod, High-strain actuator materials based on dielectric elastomer, Adv. Mater. 12 (2000) 1223–1225. [9] R. Perine, R. Kornbluh, Q. Pei, J. Joseph, Hight-speed electrically actuated elastomers with strain greater than 100%, Science 287 (2000) 836–839. [10] K. Meijer, M. Rosenthal, R.J. Full, Muscle-like actuators—a comparison between three electroactive polymer, Proc. SPIE Int. Soc. Opt. Eng. 4329 (2001) 7–15.
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[11] G. Kofod, R. Kornbluh, R. Pelrine, P. Sommer-Larsen, Actuation response of polyacrylate dielectric elastomers, J. Intell. Mater. Syst. Struct. 14 (2003). [12] P. Sommer-Larsen, G. Kofod, S. MH, M. Benslimane, P. Gravesen, Performance of dielectric elastomer actuators and materials, Proc. SPIE Int. Soc. Opt. Eng. 4695 (2002) 158–166. [13] G. Kovacs, P. Lochmatter, M. Wissler, An arm wrestling robot driven by dielectric elastomer actuators, Smart Mater. Struct. 16 (2007) 306–317. [14] F. Carpi, C. Salaris, D. De Rossi, Folded dielectric elastomer actuators, Smart Mater. Struct. 16 (2007) 300–305. [15] M. Jungmann, H.F. Schlaak, Electrostatic actuators with elastic dielectric for use on tactile displays, in: 8th International Conference on New Actuators, Bremen, Germany, 2002. [16] G. Kofod, Dielectric elastomer actuators. Ph.D. Thesis, Technical University of Denmark, Lyngby (Denmark), 2001. [17] S.M. Ha, W. Yuan, Q. Pei, R. Pelrine, S. Stanford, Interpenetrating polymer networks for high-performance electroelastomer artificial muscles, Adv. Mater. 18 (2006) 887–891. [18] M. Benslimane, P. Gravesen, P. Sommer-Larsen, Mechanical properties of dielectric elastomer actuators with smart metallic compliant electrodes, in: In Proc. of SPIE Smart Struct. and Mat.: Electroactive Polymer Actuators and Devices (EAPAD), vol. 4695, San Diego (USA), 2002, pp. 150–157. [19] J. Plante, Dielectric elastomer actuators for binary robotics and mechatronics. Ph.D. Thesis, Massachusetts Institute of Technology, Cambridge (USA), 2006.
Biographies Gabor Kovacs was born in 1958 in Maennedorf, Switzerland. He studied Mechanical Engineering at ETH Zurich and received his Dr. sc. techn. degree at the ETH. Following his PhD he worked at the Institute of Lightweight Structures and Ropeway Technology at the ETH Zurich as senior scientist. From 1996 to 2001 he was the Head of the competence centre “Aerial Cableway Technology” at Empa in Dubendorf (Zurich). Between 2001 and 2003 he was group leader of electroactive polymers (EAP) actuator technology for adaptive material systems. Between January 2003 and the end of 2005 he was Head of the laboratory for “Materials and Engineering” at Empa. Since 2006 he has been senior scientist for novel actuator technologies in the field of artificial muscles based on EAP. Lukas Düring was born in Niederbüren, Switzerland in 1982. He graduated in Mechanical Engineering at the Swiss Federal Institute of Technology (ETH) in 2008. He is an engineer in material engineering at the Swiss Federal Laboratories for Materials Testing and Research (Empa). He developed the contractive tension force electroactive polymer actuator. Silvain Michel graduated in Mechanical Engineering at the Swiss Federal Institute of Technology (ETH) in 1989. After 10 years as a fatigue and damage tolerant specialist in the aerospace industry he joined the Swiss Federal Laboratories for Materials Testing and Research (Empa) in 2000. Between 2003 and 2008 he was Head of the electro active research group within the lab “Mechanical Systems Engineering”. Since 2009 he has been a senior scientist, responsible for various projects on electro-active polymer actuators and devices. Giovanni Terrasi was educated as a material science engineer and received his PhD (Civil Engineering Department) from ETH Zürich in 1997. Between 1998 and 2005 he led the R&D/Engineering Department at SACAC Ltd in Lenzburg, Switzerland with a particular interest in the behaviour of high performance concrete reinforced or prestressed with fibre reinforced polymer tendons. Since December 2005 he has been in charge of the Laboratory for Mechanical Systems Engineering at the Swiss Federal Laboratories for Materials Testing and Research, Empa. He has focused his research activities on the in-service durability of mechanical components, on polymer matrix composites, active structures (based on DE actuators and piezoelectric sensors for SHM) and biomechanical engineering.