design and characterization of an active hinge segment

Available online at www.sciencedirect.com

Sensors and Actuators A 141 (2008) 577587

Design and characterization of an active hinge segment based on soft dielectric EAPsP. Lochmatter a,b , G. Kovacs a,a

Empa, Swiss Federal Laboratories for Materials Testing and Research, Laboratory for Mechanical Systems Engineering, Uberlandstrasse 129, 8600 Dubendorf, Switzerland b ETH, Swiss Federal Institute of Technology Zurich, Centre of Structures and Technologies, Zurich, Switzerland Received 28 March 2007; received in revised form 14 August 2007; accepted 5 October 2007 Available online 18 October 2007

Abstract This paper presents an active hinge conguration driven by soft dielectric EAPs, which is able to perform specic rotary motions back and forth. The active structure is composed of a hinged support structure to which from both sides pre-strained dielectric elastomer (DE) lms are attached. The resulting structure is similar to a biological agonistantagonist conguration. Based on a hyperelastic model for the dielectric lm the design of the active hinge segment was optimized to achieve the best overall performance in terms of the angle of displacement of the free segment and the blocking force of the constrained segment, respectively. In experimental tests the blocking force and the free angle of deection of active hinge segments with different setups were characterized. The effects from asymmetrical and symmetrical pre-straining of the DE lms as well as from stacking of several DE lm layers were investigated. Fairly good agreement between the modelling and the experiments was found. In addition, the durability of the active hinge samples under cyclical activation was experimentally determined. Typically, rather few activation cycles were endured before a dielectric breakdown occurred. 2007 Elsevier B.V. All rights reserved.Keywords: Active structures; Electroactive polymers (EAPs); Soft dielectric EAPs; Active hinge segment

1. Introduction Among the active materials, in particular soft dielectric EAPs as a subgroup of the electroactive polymers (EAPs) have attracted much interest in recent years due to their outstanding active deformation potential [14]. Soft dielectric EAPs consist of a thin elastomer lm, which is coated on both sides with compliant electrodes (Fig. 1, left). When applying a DC high voltage of several kilovolts to this compliant capacitor the electrodes squeeze the elastomeric dielectric in its thickness direction (equivalent electrode pressure, pequivalent [5]) and thus, the incompressible lm expands in the plane (Fig. 1, right). Due to their unique deformation potential [1], soft dielectric EAPs are promising for lightweight structures in the macroscale, which can undergo large shape changes. So far, different

Corresponding author. Tel.: +41 44 823 40 63; fax: +41 44 823 40 11. E-mail address: [email protected] (G. Kovacs).

approaches to mostly linear dielectric elastomer (DE) actuators demonstrated the versatile capabilities of this actuator technology (e.g. [6,7]). The unique properties of soft dielectric EAPs as a compliant, actively expanding/contracting skin, however, have not been exploited so far. Following this fundamental lack, we present an active hinge segment driven by soft dielectric EAPs. Therein, the active material covers the mechanical backbone structure of the hinge and thus serves both as driving and as structural element. Given the advantages of its continuous surface and linkage-free drive, the proposed conguration is promising for instance for aerodynamic applications (e.g. wing aps). In this study, the conceptual approach of the active hinge segment is addressed (Section 2). Following the modelling of the so-called agonistantagonist conguration (Section 3) corresponding samples of the active hinge segment with optimized design were prepared and experimentally characterized in terms of their active performance (Section 4).

0924-4247/$ see front matter 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.sna.2007.10.029

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P. Lochmatter, G. Kovacs / Sensors and Actuators A 141 (2008) 577587

Fig. 1. Structure and principle of operation of soft dielectric EAPs.

2. Conceptual approach Regarding the elastomers used as dielectrics in DE actuators mainly silicone and acrylic lms (e.g. VHB 4910 supplied by 3M) have shown the best overall performances [8]. Especially for DE actuators from acrylic VHB 4910, the dielectric lm is strongly pre-strained in its planar directions x and y (Fig. 1) to thus reduce the required activation voltage level. To maintain the DE lm in this biaxially pre-strained state a support structure is needed. This support structure requires specic mechanical degrees of freedom (DOF) to allow the resulting actuator for the desired active deformation. Inspired by the structure of biological agonistantagonist systems, where two opposing muscles are xed at jointed bones (e.g. human arm, Fig. 2, left), a novel conguration where two pre-strained DE lms are mounted to a hinged support structure is proposed (Fig. 2, centre). Under active expansion of one of the pre-strained DE lms at a time, the support structure executes a rotational motion in the one or the other direction (Fig. 2, right). Note that the directions of motion of the arm and the DE conguration are opposite to each other since a biological muscle contracts under activation, while a DE actuator expands. With the proposed conguration the two DE lms pre-strain each other via the hinged support structure. This arrangement has some benets compared for instance to the approach where a single DE lm (stack) is pre-strained by an elastic support structure: Since the two muscles can be independently activated, rotary motions back and forth become possible, which results in a wide range of angular displacement. The active forces can be simply scaled up by applying stacks of DE lm layers to both sides of the support structure.

The hinged backbone structure controls the admissible displacement states of the conguration. Thus, the support structure may be frozen by blocking the hinge bearing. In this state, any external loads acting upon the structure would be taken by the rigid support structure and the shape of the segment would be maintained despite the intrinsic softness of the DE lms. 3. Design optimization 3.1. Goal and parameters The electromechanical behaviour of the agonistantagonist conguration (Fig. 3, left) was modelled in order to nd a setup for the segment with largest angle of displacement, , of the free segment (Fig. 3, centre) as well as largest, blocking moment, m, per unit width of the constrained segment (Fig. 3, right) achieved under activation of only one DE lm stack (e.g. upper stack (a), Ua > 0, Ub = 0). For the optimization the following parameters were taken intro account: Core structure: The position of the hinge bearing (LP /L), and the aspect ratio of the segment (H/L), were varied. DE actuators: As pre-strain setups for the DE lms we selected a symmetrical pre-straining with equal stretch ratios in both planar directions ((i) (i) = 4 4) and two x y converse asymmetrical pre-strain states ((i) (i) = 3 x y 5 and 5 3). Moreover, the impact of the number of DE lm layers, N, was examined.

Fig. 2. Active hinge segment driven by soft dielectric EAPs (right) corresponding to a biological agonistantagonist conguration (e.g. human arm, left).


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Fig. 3. Design parameters of the active hinge segment (left). Angle of displacement, , of the activated, free segment (centre). Blocking moment, m, per unit width needed to hold the activated segment in its neutral position (right).

3.2. Constitutive model equations The quasi-static mechanical behaviour of the stacks from acrylic lm VHB 4910 (supplied by 3M) was modelled with an isotropic hyperelastic lm model [9]. The corresponding Eq. (1) relates the stresses, j , and the stretch ratios, j , in the lms principal directions j = x,y,z j = Kj (j 1) p, j = x, y, z, x y z = 1 (1)

where the stretch ratios, j , are dened as the ratio of the actual lm dimension, Lj , compared to the corresponding undeformed (0) (0) length, Lj (j = (Lj /Lj ) for j = x, y, z), K is the stiffness parameter of the lm and p is the hydrostatic pressure. The electromechanical coupling was accomplished by introducing the compressive equivalent electrode pressure, pequivalent , according to Ref. [5] in the thickness direction of the lm z = pequivalent = 0 r U Lz2

Fig. 4. Main dimensions of the active hinge segment with large width, W, compared to its length, L ((W/L) 1).

(2)

where 0 is the free-space permittivity, r the relative permittivity of the dielectric lm, U the applied voltage and Lz is the resulting thickness of the lm. We assume that the two equal DE lm stacks (a) and (b) are identically pre-strained with given planar pre-stretch ratios (i) (i) to the pre-strain state (i). Thus, after mountx y

ing the pre-strained DE lm stacks to the support structure the agonistantagonist segment remains in neutral position ((i) = 0) since the DE lm stacks introduce the same forces per unit width (i) (i) in the x direction (fa,x = fb,x in Fig. 3, left). As illustrated in Fig. 4, we selected a segment geometry with large width, W, compared to its length, L, for the modelling ((W/L) 1). Hence, the pre-stretch ratio of the DE lm stacks in the y direction remains approximately constant across the lm (y = (i) = const.). The stretch ratios for the stacks (a) and (b) y in the x and y direction for a general angular displacement state

Fig. 5. Theoretical prediction of the angular displacement for varied position of the hinge bearing (left, for (H/L) = 1) and for varied geometry aspect ratio of the (i) (i) (i) (i) (i) (i) segment (right, for (LP /L) = 0). The given curves correspond to the pre-strain setups , x y = 3 5; , x y = 4 4; , x y = 5 3.

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of the segment become thus Stack (a) : Stack (b) : a,x = b,x = La,x(0) La,x

= =

L(i) a,x(0) La,x (i) Lb,x (0) Lb,x

La,x(i) La,x

= (i) x = (i) x

La,x L Lb,x L

a,y = (i) y (3) b,y = (i) . y

Lb,x Lb,x(0)

Lb,x Lb,x(i)

Assuming that the DE lm stack (a) is activated, while stack (b) remains deactivated (Ua > 0, Ub = 0), the stresses, a,x and b,x , acting in the x direction of the DE lm layers are derived from the hyperelastic lm model (1) when introducing the electromechanical boundary conditions (2) and (3): a,x = K a,x (a,x 1) 1 a,x y 1(i) b,x y (i)

has to be introduced according to m= a,x (i) y Lz(0)

H (i) (i) H x y 2 (fb,x fa,x ) = N 0 r Ua 2 2 L(0) z 2

(7)

1 a,x y 1 b,x y(i) (i)

1

0 r K

Ua 2 (4)

b,x = K b,x (b,x 1)

1

The overall forces per width, W, of the DE lm stacks in the x direction, fa,x and fb,x , are obtained when introducing the stresses (4) into the relationships fa,x = N a,x Aa,x /W and fb,x = N b,x Ab,x /W (0) L 1 1 o r fa,x = NK z (i) a,x (a,x 1) 1 (i) (i) K a,x y a,x y a,x y fb,x = NK L(0) z(i) b,x y

a,x (i) y(0) Lz

2

Ua 2 (5)

b,x (b,x 1)

1(i) b,x y

1 b,x y(i)

1 where the forces of the DE lm stacks from Eq. (5) were introduced. 3.3. Modelling results The widely used acrylic lm VHB 4910 was employed for the investigation. The corresponding lm stiffness parameter K 0.035 N/mm2 was adapted from Ref. [9]. The relative permittivity r 3.35 of the pre-strained lm was estimated from Ref. [10]. For simplication the relative permittivity was assumed to remain constant under active deformation of the DE lm. For the evaluation a typical activation voltage level of Ua = 4 kV was selected. 3.3.1. Angle of displacement The evaluation results for the free segment show a symmetrical distribution of the angles of displacement with maximum value when the hinge bearing is positioned in the centre of the segment at (LP /L) = 0.5 (Fig. 5, left). Unfortunately, the central position of the hinge causes a mechanically unstable behaviour. Under active rotation, the activated DE lm stack approaches the hinge. As soon as the lm stack collides with the hinge the self-aligning torque becomes zero and the displaced position is maintained even when the segment is deactivated. Thus, the hinge bearing positions at (LP /L) = 0 and 1,

where Aa,x = WLa,z = WL(0) /(a,x (i) ) and Ab,x = WLb,z = z y WL(0) /(b,x (i) ) are the cross-sectional areas of one lm layer z y of the stack (a) or (b) in the x direction and N is the number lm layers of each stack. 3.2.1. Angle of displacement The angle of displacement, , of the free segment under activation of the DE lm stack (a) (Fig. 3, centre) can be numerically evaluated when introducing the forces per unit width fa,x = fa,x () and fb,x = fb,x () from Eq. (5) into the equilibrium of moments given by 2 H L1

1

LP L

=

fa,x cos() fb,x cos() fa,x sin() + fb,x sin()

(6)

The sines and cosines of the angles = () and = () were derived from the scalar products including the position vectors of the corners A, B, C and D of the segment. The resulting angle of displacement is obtained by numerically solving Eq. (6). 3.2.2. Blocking moment When the rotational displacement of the segment under activation of one of the DE lm stacks, e.g. stack (a), shall be prevented ( = 0 a,x = b,x =: (i) in Fig. 3, right) an x external blocking moment, m, per unit width of the segment


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respectively, were preferred, where the segment shows stable behaviour. Referring to the aspect ratio of the conguration the model predicts strongly increasing angles of displacement for segments with (H/L) 1 (Fig. 5, right). In accordance to experimental observations [1] large elongation and thus large angles of displacement are expected for asymmetrical pre-straining with dominant stretch ratio perpendicular to the elongation direction ((i) (i) = 3 5) in x y Fig. 5). As expected, the model (Eq. (6) including Eq. (5)) proposes that the number of stacked DE lm layers, N, has no impact on the achieved angle of displacement. 3.3.2. Blocking moment According to Eq. (7) the blocking moment is not affected by the position of the hinge bearing. Large blocking moments are achieved when the two parallel DE lm stacks are distant ((H/L) 1). Considering the setup of the DE actuators, the blocking moment grows directly proportionally to the product of the pre-stretch ratios, (i) (i) , and the number of DE lm x y layers, N. 3.3.3. Optimal segment design In order to achieve the best overall performance, combining large angle of displacement and large blocking moment, the following design is proposed by the model: Core structure: The lateral position of the hinge bearing ((LP /L) = 0) is preferred, despite better displacement performance with central hinge, for the absence of any instability. Since the largest angles of displacement and largest blocking moments do not arise for the same geometry aspect ratio, the product of these two performance parameters was considered. Thus, good overall performance is obtained when the aspect ratio is in the range of about (H/L) 0.51. DE actuators: The DE lms have to be pre-strained predominantly perpendicular to the direction of elongation ((i) y (i) ) to reach large angles of displacement. In addition, strong x pre-straining (large product (i) (i) ) is desired to achieve large y x blocking moments. An increasing number of DE lms, N,

scales up the blocking moment, while the angle of displacement is maintained. 4. Experimental characterization The performance of the agonistantagonist conguration was experimentally determined in terms of the (i) angle of displacement of the free segment, the (ii) blocking force/blocking moment needed to constrain the segment, as well as the (iii) durability of the free segment under cyclical activation until a dielectric breakdown occurs. In order to verify the modelling predictions the actuator setup was varied in terms of the pre-straining setup of the lms and the number of stacked lm layers: Pre-straining: Samples with pre-stretch ratios (i) (i) = x y 3 5, 4 4 and 5 3 were tested. DE lm layers: Samples with N = 1 and 3 lm layers were tested. 4.1. Sample preparation Following the modelling propositions, a wide support structure ((W/L) 1) with aspect ratio (H/L) 0.5 was manufactured (Fig. 6, left). The structure was CNC-milled from POM (polyoxymethylene or polyacetal) polymer, which is a good isolator (electrical safety) and combines good mechanical properties with a moderate mass density. As illustrated in Fig. 6, left, the support structure in lightweight design consists of a stationary front part (external anchorage via the xing extensions) at which the rotatable end part is pivot-mounted via arms in two hinge bearings. Smoothly running hinge bearings were integrated for taking into account the fairly small activation forces of the DE actuators. The angle of displacement, , of the active hinge segment was measured with a Hall angle sensor (type QP-2HC-SW4 by PEWATRON AG), which was implemented in the support structure (Fig. 6, right). Benecially, these small and lightweight sensors have very low friction (transducer axis

Fig. 6. Design of the implemented support structure for the active hinge segment in neutral state (left) and in angularly displaced states (right).

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Table 1 Summary of the design parameters of the active hinge samples Design parameter Dimensions Main dimensions (length width height over all) Active zones (length width) Materials Support structure Dielectric lms Compliant electrodes Characteristic/value 92 mm 248 mm 41 mm 50 mm 190 mm POM Acrylic VHB 4910 (by 3M) Mixture of silicone (RTV23/A7 by Swiss-Composite) and carbon black powder (Ketjenblack EC300J by Akzo Nobel) 312 g (100%) 246 g (78.8%) 48 g (15.4%) 18 g (5.8%) 44 3 5 and 5 3 1 and 3

Weight Overall segment composed of the Support structure (including angle sensor) Wiring (supply and measurement cables) Active material (three-layered DE actuators) Pre-straining of the DE lms (x y ) Symmetrical Asymmetrical Number of DE lm layers (N)(i) (i)

supported by ball bearing) and provide an analog voltage output, which is proportional over the full range of angles of = 45 . The preparation of the active hinge samples (detailed design parameters in Table 1) included the following steps:

(i) Pre-straining of the dielectric lms: The required number of dielectric lm layers (VHB 4910 supplied by 3M) were biaxially pre-strained by an automatic pre-straining facility (Fig. 7, top left). With this facility the dielectric lm is unwrapped from the roll and rstly guided over two rollers, which rotate at various speeds and thus pre-strain the material in its longitudinal direction. Subsequently, the lm is afxed to two conveyor belts, which continuously diverge and thus pre-strain the lm in its transversal direction. Finally, the pre-strained lms were manually mounted to rigid frames (inner size 300 mm 300 mm). (ii) Coating of the dielectric lms: Before coating the electrodes (size 50 mm 190 mm), a polymer stencil was applied to both sides of the tentered dielectric lms. With the sample with three stacked lm layers only the two outer lms were coated on both sides with electrodes (Fig. 7, top right). The uncoated intermediary lm shared the electrodes of the outer lm layers. Based on the approach of Kofod [11], who spray-coated silicone rubber electrodes on silicone dielectric lms, we applied such electrodes to the acrylic lm VHB 4910, as well. Thus, a mixture of air-curing, twocomponent silicone (RTV23/A7 by Swiss-Composite) and electrically conductive carbon black powder (Ketjenblack EC300J by Akzo Nobel) was manually spray-coated on the dielectric lm with airbrush (starter class set by Revell). The optimal electrode composition and the number of spray strokes were determined in preliminary tests based on circular DE actuators. The largest active strains were achieved for electrodes.

consisting of 1 g of carbon black, which was pre-mixed with 30 ml toluene, and 9 g of manually pre-mixed twocomponent silicone. This mixture was stirred (stirrer of type Eurostar Power-b by IKA Labortechnik) at about 500800 rpm for about half an hour until the initially liquid mixture changed into more viscous. coated by 16 spray strokes since thus sufcient electrical conductivity was provided, while the tensile stiffness of the electrodes still remained sufciently low. The curing of the electrodes took about 12 h at room temperature. The resulting electrodes adhered strongly to the acrylic lm, were completely dry-to-touch, homogenous, thin and showed compared to conventional electrodes from graphite and silicone oil much higher active strains at lower activation voltages already. (iii) Stacking of the dielectric lms: For the multilayer actuator three tentered DE lm layers were successively stuck on each other. In order to prevent trapping of air between the lm layers the frames were angled and the lms manually brought together with a foam roller. (iv) Assembly of the support structure and the DE lm stacks: To prevent the support structure from turning out of the neutral position during the assembly, its hinged parts were rmly connected by a locking rod (Fig. 7, bottom left). The locked support structure was then sandwiched in between the two tentered DE lm stacks. Subsequently, the actuator was carefully cut out of the frames (Fig. 7, bottom right). This process step was very critical since the pre-strained DE lms easily crackespecially at their freestanding edges. We thus temporarily hardened the DE lm by local application of cooling spray (type KAELTE 75 by Kontakt Chemie) shortly before cutting the lm. Finally, xing rods were applied (Fig. 6, left) to press the DE lm layers to the support structure and thus prevent them from unwanted detachment.


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Fig. 7. Manufacturing steps of the active hinge samples: pre-straining of the acrylic VHB 4910 lm with an automatic facility (top left); mounting of the pre-strained lm on a frame and double-side spraying of the rubber electrodes (top right); sandwiching of the locked support structure between the tentered and coated DE lms (bottom left); complete hinge segment after cutting the sample out of the frames (bottom right).

Fig. 8. Schematic top view (left) and picture (right) of the measurement setup for the experimental characterization of the angle of displacement of the free hinge segment.

Fig. 9. Schematic top view (left) and picture (right) of the measurement setup for the experimental characterization of the blocking force of the constrained hinge segment.

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4.2. Characterization setups The supply/control system of the agonistantagonist segment 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 amplier (model 5/80 by TREK Inc.). The high voltage provided by the amplier was subsequently switched by a computer-controlled reed relay (type HE24-1B83-BV026 by Meder Electronic Inc.) to one of the DE actuators of the agonistantagonist segment at a time. The analog signal from the angle sensor (type QP-2HC-SW4 by PEWATRON AG) was acquired via the PCI card, digitalized and saved for post-processing. 4.2.1. Angle of displacement For quantifying the active potential in angular displacement, , the free segment was oriented vertically in order to prevent the inuence of gravity (Fig. 8). At different orientation the hinge segment may exhibit passive angular displacement due to the dominating own weight of the support structure (Table 1) and the softness of the DE lms. Moreover, reduced angular displacement is expected when the active hinge segment has to actively lift its rotational part against gravity. In order to reduce the inertia of the rotational part of the support structure, arising during active rotation, the segment was xed at the end distant from the rotational axis of the hinge bearing. 4.2.2. Blocking force The active force generation potential was determined by measuring the blocking force, Fblocking , needed to maintain the activated segment in its neutral position ( = 0). Based on the blocking force, Fblocking , the corresponding blocking moment, m, per width, W, of the segment was calculated with m = Fblocking (L/W), where L = 62 mm and W = 190 mm. The segment was xed in an upright orientation at the end closer to the hinge bearing (Fig. 9). The angular displacement was constrained in both directions by two lateral load cells (type HBM S2 by HBM) calibrated to 1 N. The load cells were preloaded for preventing any active displacement of the segment (F0 0.02 N). 4.2.3. Durability The setup according to the measurement of the free angle of displacement was selected for the durability characterization. Compared to the blocked conguration this setup is more critical since during free displacement of the segment the DE lms actively expand which involves an increase in electrical eld across the DE lms. 4.3. Characterization procedures 4.3.1. Angle of displacement and blocking force For the characterization of the angle of displacement and the blocking force the DE actuators on sides (a) and (b) were alternately activated with increasing activation voltage levelsFig. 10. Supply voltage characteristics for the experimental characterization of the angle of displacement and the blocking force of the hinge segment.

ranging from 2.0 up to 4.2 kV (Fig. 10). The activation voltage was maintained for 5 s at each activation step. In order to prevent early breakdowns in the DE actuators during the charging/discharging phase, the activation voltage was not applied stepwise but was established and reduced along a ramp function 200 ms before/after each activation step. A quiescent period after each activation step enabled the visco-hyperelastic DE lms to relax to their initial state. Since these viscous effects become more inuential for larger active strains the quiescent period was gradually extended for higher activation levels (15 s at beginning, increased by 2 s for each next higher voltage level). 4.3.2. Durability During the durability tests the number of stepwise activation cycles were determined until a dielectric breakdown occurred. The DE actuators on sides (a) and (b) were alternately activated for 5 s with a moderate activation voltage level of 4.0 kV followed by a remaining quiescent period of 5 s. 4.4. Characterization results 4.4.1. Angle of displacement Regarding the typical angular displacement response, (t), the inuence of the visco-hyperelastic properties of the DE actuators from VHB 4910 become clearly visible (Fig. 11). After activation the segment initially rotates fairly fast but then the motion gradually slows down. After deactivation the viscous behaviour of the DE lms delays the return of the segment to its neutral position. Especially after large active straining accomplished by large activation voltage levels and/or long activation durations considerably more time is required for the segment to return to its neutral position. Considering the rates of displacement (d/dt), one nds that the main portion of the displacement arises shortly after activation/deactivation. In addition, the maximum displacement rate increases with the activation voltage level (temporarily up to 60 /s at 4 kV). The maximum angles of displacement reached by the segments after 5 s of activation as a function of the initial electrical eld, (U/L(i) ), are presented in Fig. 12. As expected from z


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Fig. 12. Maximum angle of displacement of the tested hinge segments with various setups reached after 5 s of activation as a function of the initial electrical eld.

Fig. 11. Temporal development of the angle of displacement of a single-layered (i) (i) hinge segment with pre-strain setup x y = 3 5 at selected activation voltage levels of 2, 3 and 4 kV.

the equivalent electrode pressure (Eq. (2)), the displacement amplitude grows disproportionately with increasing level of the initial electrical eld. In accordance with the predictions from the modelling, the segment, where the lms are predominantly pre-strained perpendicular to the elongation direction ((i) (i) = 3 5), proved to exhibit the largest angles of disx y placement of more than 30 . The angular displacements of the two other pre-strain setups ((i) (i) = 4 4 and 5 3) were x y similar to each other. Only at higher electrical elds the symmetrical pre-straining ((i) (i) = 4 4) escalated to values x y in between the results for the asymmetrical pre-strain setups ((i) (i) = 3 5 and 5 3), which then better corresponds x y to the modelling predictions. According to the modelling, one would expect that with multilayer congurations the number of applied DE lm layers does not inuence the angular displacement of the segment. The tested sample with three stacked DE lm layers, pre-strained with the preferred pre-stretch ratios (i) (i) = 3 5, howx y ever, showed lower angular displacement compared to the corresponding single-layered actuator. Supposedly, this reduction was induced by a discontinuous physical contact of the intermediary DE lm layer to the electrodes of the neighbouring lms by either trapping of air during stacking of the DE lms or due to the roughness of the outer surface of the sprayed electrodes. If so, all DE lm layers would need to be double side coated. 4.4.2. Blocking force In Fig. 13, exemplarily the temporal development of the blocking force, Fblocking (t), and the corresponding blocking moment per unit width, m(t), respectively of the single-layered segment with a pre-straining of (i) (i) = 3 5 is given for x y different activation voltage levels. As can be seen, the blocking force establishes quickly after activation, maintains its level during activation and declines to zero as soon as the segment

is deactivated. As shown, the hyper-viscoelastic behaviour of the DE lms from VHB 4910 has no impact on the blocked activation since no active expansion of the actuator is allowed. The equivalent electrode pressure from the electrodes (Eq. (2)), which is transmitted via the hydrostatic pressure in the planar lm directions, establishes as soon as the DE capacitor is electrically charged. Due to the low electrode resistance and the small capacitance of the present DE actuators the characteristic charging time is short. Fig. 14 gives the collected measurement results of the blocking forces, Fblocking , and the corresponding blocking moments per unit width, m, respectively for all experimentally investigated setups as a function of the initial electrical eld, (U/L(i) ). z In addition, the corresponding modelling predictions are included in Fig. 14. The blocking moment per unit width is described according to the modelling by Ua H m = N L(i) 0 r z (i) 2 Lz2

(8)

when introducing the initial electrical eld, (Ua /L(i) ), into z Eq. (7). As seen from relationship (8), the blocking moment

Fig. 13. Temporal development of the blocking force and the moment per unit (i) width, respectively of a single-layered hinge segment with pre-strain setup x (i) y = 3 5 at selected activation voltage levels of 2, 3 and 4 kV.

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Fig. 14. Experimentally determined maximum blocking forces and blocking moments per unit width, respectively reached after 5 s of activation including the corresponding modelling predictions for various segment setups as a function of the initial electrical eld.

is affected by the pre-straining level of the DE lms (m L(i) = L(0) /((i) (i) )) when maintaining the initial electrical z z x y eld. As a consequence, the theoretical predictions for the blocking moment per unit width are equal for the pre-strain setups (i) (i) = 3 5 and 5 3 but slightly smaller for x y (i) (i) = 4 4 (Fig. 14). x y Comparing the measurement results with the modelling predictions, a fairly good match was found for the samples with one DE lm layer. The segment with three DE lm layers, however, exhibited slightly lower active forces than the tripled values according to the modelling (m N). As with the results from the angular displacement, this performance loss may have originated from a discontinuous contact of the intermediary DE lm layer to the electrodes of the neighbouring lms. 4.4.3. Durability Referring to the lifetime of DE actuators, basically one has to distinguish between durability without or with activation: Without activation, DE actuators consisting of pre-strained DE lms (e.g. VHB 4910 supplied by 3M) were observed to mechanically fail when shelved long-time (shelf life). The deformation energy stored in the highly pre-strained visco-hyperelastic material drove crack growth at local imperfections in the DE lm. Whenever a crack was initiated, the crack growth usually continued until the entire lm was torn. Under activation, the electromechanical response of DE actuators may possibly decline before nally a dielectric breakdown occurs. The inuencing factors include the boundary conditions during operation of the actuator (e.g. free or blocked), the applied activation voltage characteristic (spectrum from single activation with long intermediary rest periods to continuous activation) and the corresponding activation voltage level. According to our practical experiences with for instance circular DE actuators from VHB 4910 [12] generally, long-term activations are possible for low activation voltages and initial activation elds, respectively (below about 2.5 kV, 40 V/ m), while dielectric breakdowns arise within few tens of seconds at moderate (between about 2.5 and 3.5 kV, 40 and 56 V/ m) and already within few seconds at high activation levels (above 3.5 kV, 56 V/ m). This

behaviour is also inuenced by the disproportionate increase in active expansion of DE actuators with increasing activation voltage, which likewise results in a disproportionate growth in electrical eld across the DE lm in the actively strained state. Here, the durability of the freely displacing agonist antagonist segments was characterized by life tests, i.e. the numbers of stepwise activation cycles were determined until a dielectric breakdown occurred. In general, only up to a few tens of additional activation cycles were endured by the single-layered segments. Proper statistical statements on the durability in dependence on the different pre-strain setups cannot be given since only few samples were tested. Nevertheless, the DE actuators of the segments with three lm layers (pre-strain setup (i) (i) = 3 5) surprisingly x y survived more than 200 cycles. With all durability tests the displacement characteristics were repetitive in time. No signicant deterioration in displacement response was observed until the dielectric breakdown occurred (e.g. results presented in Fig. 15). The typically observed difference in amplitudes of the angle of displacement forth and back indicates the sensitivity of the displacement performance on imperfections, e.g. from the manual manufacturing process or the hinge friction in the support structure.

Fig. 15. Temporal development of the angular amplitude of displacement of (i) (i) a three-layered hinge segment with pre-strain setup x y = 3 5 under cyclical, alternate activation of the sides (a) and (b) with 4 kV for each 5 s.


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5. Discussion and conclusion We proposed an active hinge segment driven by soft dielectric EAPs, which can actively accomplish specic rotary motions. The suggested conguration resembles a biological agonistantagonist structure. The soft dielectric EAP lms acting as an active skin are thereby mounted to a hinged backbone structure, which maintains the dielectric lms in a pre-strained state. The quasi-static electromechanical modelling of the agonistantagonist segment showed that the largest angles of displacement and blocking moments are expected for asymmetrically pre-strained DE lms, applied to a core with central position of the hinge bearing and aspect ratio in the range of (H/L) = 0.51. Since this hinge position involves a mechanically unstable behaviour the lateral hinge position was preferred, leading, however, to a reduced angular displacement performance. In order to verify the predictions from the modelling, the performance of selected agonistantagonist samples was experimentally determined. While the DE actuator setup was varied in terms of the pre-stretch ratios and the number of lm layers, the design of the support structure was adapted from the modelling ((H/L) 0.5 and ((W/L) 1)). As performance parameters for the active segment, the accomplished blocking force, the angle of displacement and the durability were taken into account. The single-layered segments exhibited blocking forces, which agreed fairly well with the model prediction. Furthermore, the scalability of the active forces with increasing number of DE lms could be veriedthe three-layered segment exhibited nearly tripled blocking forces. In correspondence to the modelling results, the largest angles of displacement were found for the asymmetrical pre-strain setup of the DE lm with strong pre-straining perpendicular to the elongation direction ((i) (i) = 3 5). The sample x y with three stacked DE lms layers, however, showed smaller displacement than the corresponding single-layered segments. This may have resulted from discontinuous contact between the electrodes from the outer lm layers to the intermediary layer. The durability of all single-layered segments under cyclical activation until a dielectric breakdown occurred was fairly poor. Only the three-layered segment could stand more than 200 activation cycles without dielectric breakdown. Benecially, all tested samples showed stable amplitude in angular displacement back and forth without any degradation until the dielectric breakdown occurred. In our future work, we will focus on improving the active hinge segment in such way that its active shape change performance becomes independent of its orientation relative to gravity. This may be achieved by drastically reducing the dominating own weight of the support structure and by increasing the number of driving DE lm layers. Moreover, we intend to build an actuator composed of several interlinked agonistantagonist segments. The resulting structure would have the ability to actively perform complex shape changes. One of the key issues related to such active structure will be the design of an appropriate electrical supply/control

system to individually provide a multitude of DE actuators with specic DC high voltages. Acknowledgements The experimental work for the present study was greatly supported by Ms. M. Hoepinger, D. Vrinic and A. Schmidlin. References[1] R. Pelrine, R. Kornbluh, Q. Pei, J Joseph, High-speed electrically actuated elastomers with strain greater than 100%, Science 287 (5454) (2000) 836839. [2] F. Carpi, A. Mazzoldi, D. De Rossi, High-strain dielectric elastomer for actuation, in: Proc. SPIE in Smart Struct. and Mat.: Electroactive Polymer Actuators and Devices, vol. 5051, San Diego, USA, 2003, pp. 419422. [3] R. Kornbluh, R. Pelrine, Q. Pei, S. Oh, J Joseph, Ultrahigh strain response of eld-actuated elastomeric polymers, in: Proc. SPIE in Smart Struct. and Mat.: Electroactive Polymer Actuators and Devices, vol. 3987, Newport Beach, USA, 2000, pp. 5164. [4] G. Kofod, P. Sommer-Larsen, R. Kornbluh, R Pelrine, Actuation response of polyacrylate dielectric elastomers, J. Intell. Mat. Syst. Struct. 14 (2003) 787793. [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) 7785. [6] R. Pelrine, P. Sommer-Larsen, R. Kornbluh, R. Heydt, G. Kofod, Q. Pei, Applications of dielectric elastomer actuators, in: Proc. SPIE in Smart Struct. and Mat.: Electroactive Polymer Actuators and Devices, vol. 4329, Newport Beach, USA, 2001, pp. 335349. [7] Y. Bar-Cohen, S. Leary, M. Shahinpoor, J.O. Harrison, J. Smith, Flexible low-mass devices and mechanisms actuated by electroactive polymers, in: Proc. SPIE in Smart Struct. and Mat.: Electroactive Polymer Actuators and Devices, vol. 3669, Newport Beach, USA, 1999, pp. 5156. [8] Y. Bar-Cohen, Electroactive Polymer (EAP) Actuators as Articial Muscles: Reality, Potential, and Challenges, 2nd ed., SPIE Press, Washington, 2004, p. 765. [9] P. Lochmatter, G. Kovacs, S. Michel, Characterization of dielectric elastomer actuators based on a hyperelastic lm model, Sens. Actuators A: Phys. 135 (2) (2007) 748757. [10] M. Wissler, E. Mazza, Electromechanical coupling in dielectric elastomer actuators, Sens. Actuators A: Phys. 138 (2) (2007) 384393. [11] G. Kofod, Dielectric Elastomer Actuators, Technical University of Denmark, Lyngby, 2001, p. 130. [12] M. Wissler, E. Mazza, Modeling of a pre-strained circular actuator made of dielectric elastomers, Sens. Actuators A: Phys. 120 (1) (2005) 184192.

BiographiesPatrick Lochmatter was born in Visp, Switzerland in 1976. He graduated in mechanical engineering at the Swiss Federal Institute of Technology (ETH) in 2002. He is a PhD student in material engineering at the Swiss Federal Laboratories for Materials Testing and Research (Empa). His research goal is to develop a shell-like electroactive polymer actuator. Gabor Kovacs was born in 1958 in Maennedorf, Switzerland. He studied mechanical engineering at the Swiss Federal Institute of Technology (ETH) and received his Dr. Sc. Techn. Degree from the ETH. After his PhD he has been working at the Institute of Lightweight Structures and Ropeway Technology of the ETH Zurich as senior scientist. From 1996 to 2001 he was head of the competence centre Aerial Cableway Technology at Empa in Dubendorf (Zurich). Between 2001 and 2003 he was group leader of electroactive polymers (EAPs) actuator technology for adaptive material systems. Between January 2003 and end of 2005 he was head of the laboratory for Materials and Engineering at Empa. Since 2006 he is senior scientist for novel actuator technologies in the eld of articial muscles based on EAPs.

design and characterization of an active hinge segment

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