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Spray deposited multilayered dielectric elastomer actuators
Sensors and Actuators A 167 (2011) 459–467
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Sensors and Actuators A: Physical journal homepage: www.elsevier.com/locate/sna
Spray deposited multilayered dielectric elastomer actuators O.A. Araromi a,∗ , A.T. Conn b , C.S. Ling a , J.M. Rossiter b , R. Vaidyanathan a , S.C. Burgess a a b
Department of Mechanical Engineering, University of Bristol, Queen’s Building, University Walk, Bristol BS8 1TR, UK Department of Engineering Mathematics, University of Bristol, Merchant Venturers Building, Woodland Road, Bristol BS8 1UB, UK
a r t i c l e
i n f o
Article history: Received 2 November 2010 Received in revised form 28 February 2011 Accepted 1 March 2011 Available online 9 March 2011 Keywords: Dielectric elastomer Electroactive polymers (EAPs) Unimorph actuators Spray deposition DEA fabrication Figure of merit
a b s t r a c t Dielectric elastomer electroactive polymers are an emerging class of actuation technology which is inherently compliant and capable of large actuation stresses and strains. Despite promising performance characteristics, their fabrication has been inhibited by two significant factors: (i) the requirement for consistently thin dielectric layers, to minimise activation voltages; (ii) automated production of multilayered configurations, to increase the actuation power. This paper presents a robust, low-cost fabrication technique that overcomes these issues by utilising optimised spray deposition. Spray deposition of silicone dielectric elastomer actuators (DEAs) offers numerous benefits including scalability, flexibility for different DEA configurations and multilayered assembly with a high degree of automation. A predictive model based on the Gaussian distribution is used to characterise the profile of deposited elastomer layers for principal fabrication parameters. This model enables individual dielectric layers to be composed from multiple parallel depositions, which greatly increases scalability as demonstrated by fabricated DEA films with planar dimensions from 25 mm2 to over 10,000 mm2 . Using the predictive model, a new figure of merit is introduced for analyzing DEA film profiles by considering the estimated mean Maxwell stress that is feasible for a specific dielectric breakdown strength. The analysis suggests that compared to a single deposition, a film composed of four parallel depositions will increase the maximum characteristic DEA dimension by an order of magnitude, while producing a comparable mean Maxwell stress. A significant advantage of the presented spray deposition technique is the semi-automated layering process, creating stratified solid-state actuators. By eliminating the stacking of layers from the fabrication process, inherent electrical isolation, good layer-to-layer bonding and capacity for more complex 3D geometries is achieved. A proof-of-concept multilayer unimorph and stack DEA is presented to validate the fabrication technique through static and dynamic displacement tests. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Dielectric elastomer actuators (DEAs) are a type of electroactive polymer (EAP) that attracts significant research interest because of their inherent compliance, light weight, high energy densities and actuation strains up to 380% as demonstrated with 3M VHB 4910 tape [1,2]. DEAs are made up of dielectric elastomer (DE) films sandwiched between two compliant electrodes, essentially forming an ‘elastic capacitor’. When a large voltage field (typically around 50–200 MV/m [1]) is applied across the electrodes, coulomb forces cause compression of the film in the thickness direction and expansion in planar direction. The effective induced pressure in the thickness direction is often referred to as the Maxwell stress, p, given by Eq. (1). p = er e0
V 2 y
∗ Corresponding author. Tel.: +44 1173315936. E-mail address: [email protected] (O.A. Araromi). 0924-4247/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.sna.2011.03.004
(1)
where er is the relative dielectric constant of the elastomer film, e0 is the permittivity of free space (equal to 8.85 × 10−12 F/m), y is the initial film thickness and V is the voltage across the elastomer film. From Eq. (1), it is clear that actuation stress, p, is inversely proportional to the square of the thickness, y, of the DE film for a fixed voltage. It is therefore more beneficial, for a given voltage, to have a large number of thin elastomer layers than to have a smaller number of thick layers. However, fabricating functional multilayer DEAs is non-trivial, especially if stacking is performed manually [3,4] which can result in inconsistent actuator performance. Layerto-layer bonding also has a significant impact on the structural integrity of multilayer DEA, especially if the actuator is required to hold a load in both its passive and active states [3]. Interlayer bonding is also important for preventing arcing around the sides of the DE films and for electrode adhesion if the silicone elastomer being used is not naturally adhesive [3]. Some fabrication methods attempt to provide electrical isolation by coating actuators with a passive soft insulating layer [5], however, this coating can inhibit actuator performance.
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Currently, acrylic and silicone based dielectric elastomers have been shown to have the best mechanical and actuation properties [6]. Silicone elastomers are easy to work with, inexpensive and can be easily fabricated into thin films with various shapes, in contrast to the commonly used acrylic type elastomers (e.g. 3M VHB 4910), allowing for more flexibility in actuator design. Advanced methods for fabricating silicone based DEAs have been developed by various research groups [7–9], all of which are effective in producing useable actuators. Commercial silicone based DEA products exist with highly conductive metal electrodes [9], however, the fabrication process used to produce them is complex and optimised for the large volume production of single film actuators. The equipment is also large, expensive and therefore not ideal for small laboratories. The fabrication processes outlined in Lotz et al. [7] and Schlaak et al. [8] are similar to the work presented here. However, the spin coating equipment used is expensive and requires accurate speed and acceleration control, and is limited in its flexibility for producing films of different shapes and dimensions. Other fabrication methods, such as mould casting [5] or K-bar/Mayer bar coating [10] are more simple and inexpensive. However, such methods are only suited to fabricating single layer DE films, multilayer DEAs can only be obtained by folding, rolling or manually stacking individual films, and therefore are subject to issues of interlayer bonding and electrical insulation. A potential fabrication process that has not received significant research study is that of spray deposition. Spray deposition offers a number of benefits over existing methods including: (i) versatility in the fabrication of actuators of different sizes, thicknesses and profiles; (ii) the potential to automate the process to create a full 3D actuator prototyping system; (iii) high quality control due to the well established parameter optimisation of spray material for the painting, printing and coating industries; (iv) the ready adaptation of spray methods to deposit both dielectric and compliant electrode materials. This paper presents a flexible fabrication technique for producing silicone DEAs based on spray deposition which can consistently produce multilayer DEAs. The general fabrication technique, applicable for any silicone dielectric elastomer, is described in Section 2. Section 3 presents the results of the fabrication characterisation and optimisation study based on deposition distance and silicone mass and introduces a new figure of merit for variable thickness DEA actuators. In Section 4 proof of concept unimorph and stack actuators are used to demonstrate the fabrication technique’s ability to produce functional DEAs. Finally, the main conclusions are drawn in Section 5. 2. Generalised fabrication method We present a new DEA fabrication method where the dielectric layers, and optionally the electrodes layers, are spray deposited. The fabrication method described here requires only basic components; a DC motor, a wheel and an airbrush system. Since the principle components can be procured and assembled quickly and cheaply the whole fabrication system is inexpensive. The rotating wheel-based spray deposition concept has the inherent advantage of giving a consistent layer thickness without requiring a change in direction of the motor or precise control over its velocity profile. Fig. 1 shows the proposed DE fabrication technique. The fabrication process can be summarised as follows: (1) Prepare a flexible substrate and affix around the circumference of a motorised wheel. (2) Rotate the wheel at constant speed and spray deposit dielectric elastomer using constant air pressure (Fig. 1a). (3) Apply electrode mask to the cured elastomer layer.
(4) Apply compliant electrode material, e.g. by brush or spray deposition (Fig. 1b). (5) Repeat steps 2–4 for the required number of DEA layers. (6) Remove complete actuator including substrate from wheel. (7) Electrically connect layers to give a parallel circuit. (8) Connect to high voltage supply and test (Fig. 1d). 2.1. Dielectric elastomer deposition Since spray deposition is only suitable for low viscosity fluids most dielectric materials must be thinned or dissolved in a suitable solvent before spraying. In this study silicone is used as the dielectric material so we must consider how liquid silicone can be prepared at the appropriate low viscosity. Uncured silicone can be dissolved in solvent to form a relatively low viscosity liquid. With condensation cure silicone this is typically achieved by thinning the base part with an organic solvent and subsequently adding the curing agent. For other silicones, such as addition cure silicones, the individual constituents are typically dissolved in solvent separately before being mixed. The prepared low viscosity solvent–silicone mixture is poured into the airbrush container through a strainer to remove any undissolved elastomer clumps. The mixture is then airbrushed at a constant pressure onto the circumference of a rotating wheel which has steel substrate sections mounted to it as shown in Fig. 1a. The circumference of the wheel is lined with magnetic tape to ensure the substrate remains flat against the surface of the wheel circumference during spraying. The wheel is mounted onto a DC motor rotating at constant velocity. The airbrush spray is aligned with the centreline of the substrate. The solvent–silicone mixture is spray deposited in the form of discrete droplets, which build up on the substrate surface with each revolution of the wheel, creating a continuous deposition with a smooth surface topography. This is important as large variations in the surface topography could create local regions with a thickness much smaller than the rest of the deposition and, therefore, more susceptible to failure by dielectric breakdown. The surface topography was investigated by performing tapping mode atomic force microscopy (AFM) on a 75.9 m thick sample deposition (Fig. 2) using a Veeco Multimode V Scanning Probe Microscope. The color axis in the right of Fig. 2 maps the colors in the scan to relative height values. Fig. 2 clearly shows that at the micro-scale the variation in surface feature height is less than 70 nm, and the variation in surface height relative to the mean thickness of the sample deposition is less than 0.09%. Hence the risk of premature dielectric breakdown due to localised droplet deposition topography can assumed to be negligible. The airbrush and rotating wheel are housed within an extraction hood in order to minimise the likelihood of foreign airborne particles embedding in the deposited silicone film before it has cured. It is important to fabricate films in as clean an environment as possible to avoid small airborne particles adversely affecting the quality of layers and layer bonding as shown in Fig. 3a. This figure shows how a particle causes the layers to warp and distort around it, resulting in a region of reduced thickness and hence a point of weakness in terms of dielectric breakdown. 2.2. Electrode deposition After the dielectric elastomer has been spray deposited the solvent evaporates leaving the silicone layer. Once this sprayed layer has fully cured, electrodes can now be applied to the surface of the silicone. Electrodes can be applied using a variety of methods including airbrushing, hand brushing, stamping or smearing. Suitable electrode materials for this fabrication process are dry graphite powder, conductive carbon grease and graphite powder suspended
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Fig. 1. Fabrication process of dielectric elastomer multilayer DEA.
in a silicone matrix. Hand brushed electrodes should be applied sparingly to prevent agglomeration. Agglomeration has the effect of increasing the stiffness of the elastomer layer [3] which subsequently reduces the performance of the actuator. Care should be taken when using conductive carbon grease because it was found to accumulate at the edges of the electrode mask, as shown in Fig. 3b, with consequential local increase in susceptibility to dielectric breakdown. Airbrushed graphite powder, dissolved in solvent at a ratio of 1:4, was used in this work. The electrodes produced have high conductivity and the airbrushing method allows for the fabrication process to be fully automated. Agglomeration of spray deposited graphite powder was not found to be a problem. The electrode masks are made from acetate transparency films coated on the underside with a thin layer of tacky silicone (e.g. BJB TC-5005) to provide good adhesion to the dielectric elastomer layer while the electrode is being applied. Datum marks are drawn onto the mask to ensure the electrodes line up through the layers.
2.3. Multilayered fabrication Fig. 2. Tapping mode atomic force microscopy of a typical region of the spray deposited dielectric elastomer surface (colors represent relative heights of surface features). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Although single layer dielectric elastomer actuators have application as, for example, active membranes, more practical applications demand more substantial multilayer actuators. To achieve this subsequent elastomer layers can be airbrushed directly on top of the electrode of the previous layer film. The direct deposition of one elastomer layer on top of previous layers produces a multilayered stack with good electrical insulation and interlayer bonding, without the use of an addition passive insulation layer.
Fig. 3. Regions with increased risk of failure due to electric breakdown: (a) effect of imperfection on multilayer actuator; (i) region of weakness, (ii) airborne particle. (b) Effects of accumulation of carbon grease electrodes; (i) region of weakness, (ii) accumulation of carbon grease electrode (both images are a cross-section through actuator width).
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Fig. 5. Examples of thickness measurements from three elastomer deposition samples with fitted Gaussian profile curve.
and curing process, and electrical connections were made between the layers by piercing with a conductive pin or wire (Fig. 4b); (iii) the offset regions were cut through after all the elastomer layers had been deposited and the edges were coated with a conductive medium, such as carbon grease or carbon powder (Fig. 4c). In general, the offset region exposure technique (method (i)) provides the most reliable electrode connection. One explanation for this is that methods (ii) and (iii) rely on a relatively small area for electrical connection. With through pinning in particular, the piercing of the elastomer layers can cause distortion of the electrodes, further preventing a reliable connection. However, through pinning and through cutting have the advantage of reducing the number of steps in the fabrication procedure, resulting in a reduction in overall fabrication time and complexity. Fig. 4. Through layer electrode connection techniques: (a) exposed offset region, (b) through pinning, (c) through cutting.
It is important to note that directly after fabrication each actuator layer is by default electrically isolated. To prevent arcing around the edges of adjacent layers a passive isolating border which surrounds the electrode can be used and need only be as wide as the thickness of the dielectric elastomer films [3]. This contributes to the effectiveness of the actuator. 2.4. Supply voltage terminals Once sufficient actuator layers have been deposited the actuator and substrate are removed from the wheel and strips of aluminium foil are attached to the offset regions to provide the electrical connection for the input driving voltage. Depending on the desired actuator configuration, the elastomer stack can be left on the substrate (e.g. unimorph, bimorph DEAs) or removed and used separately (e.g. stack, roll DEAs). In terms of scalability, many small stacks (<10 layers) can be combined to form larger stacks (>100 layers) as a way of reducing overall manufacturing time and increasing the versatility and standardisation of the process. For multilayered actuators, connection of electrodes through the elastomer layers is required to form a parallel circuit of electrodes. One way of achieving this is to offset the electrode pattern slightly during the electrode deposition step. The electrodes are thus staggered alternately so that they can easily be connected electrically, as shown in Fig. 4. Three methods of electrical connection were evaluated: (i) the offset regions were left exposed between layers so that when subsequent electrodes were applied electrical connections between layers were naturally made at the sides (Fig. 4a); (ii) the offset regions were left unshielded during the elastomer deposition
3. Fabrication characterisation and optimisation Unlike most other DE actuators, such as those using pre-strained VHB tapes, the thicknesses of DE films produced by the proposed fabrication method are inhomogeneous due to the profile of the spray jet leaving the airbrush system. Clearly this will have an impact on the efficacy of the final actuator. To this end we derive a predictive deposition model based on empirical analysis of spray densities. Previous experimental analysis has shown that the thickness profile of spray deposited matter fits a Gaussian distribution [11] of the form: 2
y(x) = a e−(x−b)
/c 2
(2)
where x is the lateral position along the substrate in mm, a is the peak deposition thickness in m, b is the lateral offset distance from the centre of the substrate√in mm and c is a shape factor approximating the variance term 2 2 . Gaussian distribution is proven to apply to the thickness profile of elastomer depositions specifically. Fig. 5 shows examples of thickness measurements taken from deposition samples made using the fabrication technique, a good fit of the Gaussian distribution to the measured data can be seen. 3.1. Materials and methods The deposition profile for a single layer deposition is a function of: (i) the deposition distance, D, i.e. the distance between the airbrush system and the surface of the substrate; (ii) the mass of silicone in the spray mixture, m; (iii) the spray mixture viscosity (related to the solvent–silicone ratio), v; (iv) the spray wheel diameter, w. The effect of deposition distance, D, and silicone mass, m, is
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Fig. 6. Experimental set-up for (a) spray deposition, (b) deposition profile thickness measurement.
investigated here. Single layer films were produced using the fabrication technique described in Section 2 using a Revell Starter Class airbrush system onto a wheel of diameter, w = 51 cm (Fig. 6a). A Parvalux 24 V DC motor was used to rotate the wheel at a constant speed of approximately 41 rpm. The silicone used was Dow Corning Silastic 3481. The air pressure supply to the airbrush was regulated at 2.0 bar and the mixture ratio of solvent (dichloromethane) to silicone was kept constant at 5:1 for each experiment. The thickness measurements were made using a Zeiss Jenavert incident light research microscope and Olympus Colorview 3 camera system (Fig. 6b). Images were taken at 2 mm increments along the film width and the thickness measured using ImageJ (National Institutes of Health) image processing software.
mechanism of DEAs is dielectric breakdown, which for a uniform supply voltage typically occurs where thickness is at a minimum (a theoretical analysis is used to demonstrate this in Section 3.4). Fig. 7b shows the results of the Gaussian fit for varying silicone masses at the optimal deposition distance (i.e. 150 mm). The Gaussian and the 95% confidence bounds are very close, indicating a good fit. From Fig. 7b it can be seen that there is an approximately linear relationship between silicone mass, m, and peak thickness, a. The results also demonstrate the ability of the technique to produce films with very low peak thickness (<40 m) simply by adjusting silicone mass m.
3.2. Single deposition
Clearly actuators with Gaussian-like thickness profiles are nonoptimal and so we now consider how multiple offset spray passes can yield a flatter, more ideal surface profile. Multiple depositions can be made on a single substrate if a film of larger width dimensions is required by offsetting each spray deposition laterally (i.e. across the width of the film) by a certain distance. The Gaussian distribution deposition model (Eq. (2)) can be adapted to predict the film profile produced by multiple depositions using Eq. (3):
A series of 3 single layer single pass actuators were fabricated at various spray distances and silicone masses. Layer profiles were measured and Gaussian distributions were fitted to these profiles, as shown in Fig. 7. Dotted lines indicate 95% confidence intervals. The Gaussian expression parameters, a and c for each fitted curve are given in Table 1. Fig. 7a shows the effects of varying the deposition distance with the silicone mass, m = 10 g. The D = 150 mm deposition distance was found to be optimal as it produced the flattest profile while maintaining a small 95% confidence bound region. A flatter profile is preferable since the predominant failure
3.3. Multiple depositions
2
2
2
y(x) = a e−(x/c) + a e−((x−b)/c) + a e−((x−2b)/c) + · · · 2
+a e−((x−(n−1)b)/c)
(3)
where b is the offset distance in mm i.e. the separation distance between one deposition and the next, and n is the number of depositions. Fig. 9 shows the results of multiple deposition experiments where four offset depositions were made on each substrate, two staggered on either side of the substrate centreline. For this experiment a single airbrush system was used with multiple passes but it is also possible to use multiple parallel airbrush systems, positioned such that the airbrush nozzles are separated by a fixed offset distance, b (Fig. 8). A deposition distance of D = 150 mm and silicone mass m = 10 g was used for each deposition pass. Experiments were
Fig. 7. Characterised Gaussian spray deposition profiles: (a) fixed silicone mass (m = 10 g), varying deposition distance. (b) Fixed deposition distance (D = 150 mm), varying silicone mass (solid lines are the fitted Gaussian curves, dashed lines are the 95% confidence bounds).
Fig. 8. Multiple deposition fabrication.
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Table 1 Fitted Gaussian distribution model parameters (see Eq. (2), the 95% confidence value are given in brackets). Spray distance, D (m = 10 g) 60 mm 90 mm 120 mm 150 mm
a
c
Elastomer mass, m (D = 150 mm)
a
c
181.0 (±7.8) 123.3 (±7.0) 79.2 (±3.1) 52.6 (±3.8)
10.43 (±0.53) 12.46 (±0.86) 14.02 (±0.72) 18.72 (±2.13)
6g 12 g 18 g 24 g
40.3 (±1.5) 78.7 (±1.9) 112.1 (±1.4) 155.2 (±2.0)
16.75 (±0.91) 17.24 (±0.62) 16.58 (±0.30) 17.62 (±0.34)
conducted for offset distances of 20 mm, 25 mm and 30 mm. Three experiments were conducted for each offset distance to account for experimental scatter. The substrate width was 100 mm. The solid lines in Fig. 9 show the predicted deposition profile determined as the sum of the Gaussian fits of the individual depositions, shown as dashed lines, over the entire spray region. The results show there is a good match between the measured data and the predicted profile. The 25 mm offset, in particular, produces a good profile over a large width. Fig. 9b suggests that the 25 mm offset with four separate depositions produces a DE film around 80 mm wide. The multiple deposition samples here had a maximum area of around 10,000 mm2 . The length of the film is limited only by the length of the substrate, which can be as long as the wheel circumference (1.6 m for the set-up presented here). Hence, in principle, 4 depositions could have produced a single film with an area of up to 128,000 mm2 . Therefore the multiple deposition technique is clearly well suited to producing large DE films, which are not easily achievable using the primary competing fabrication method of spin coating. 3.4. Deposition profile analysis and figure of merit The predominant failure mechanism that limits the performance of DEAs is dielectric breakdown, which in theory should occur where the voltage field is greatest (assuming the film is properly insulated and free from impurities and external mechanical disturbances). A DEA film of non-uniform planar thickness, y(x),
such as the spray deposition profiles presented in this work, will therefore typically fail where y(x) is at minimum. This effect is accentuated by the fact that the transverse thickness strain, εy , is proportional to applied voltage field squared, (V/y)2 . Consequently, the uniformity of an elastomer film’s thickness can be used to assess its likely performance as a DEA. Since the films produced via spray deposition are not uniform, a figure of merit is introduced here to provide a quantified measure of predicted DEA performance for a specified electrode width. Assuming dielectric breakdown occurs where y(x) is minimum, ymin , and that the voltage field is equal to elastomer breakdown strength, kv , then Eq. (1) can be rewritten as follows: p = er e0 so ¯ r e0 p∼e
k y 2 v min
(4)
y
k y 2 v min
(5)
y¯
Neglecting constants, Eq. (5) suggests that a useful figure of 2 ¯ , since it yields an approximate measure of the merit is (ymin /y) ¯ produced by a non-uniform DEA. Note that mean Maxwell stress, p, the principal assumption of this method is that the maximum supply voltage is not a design constraint, whereas in practice it may be. For the single and multiple deposition profiles presented in Sections 3.2 and 3.3 the key design parameter is electrode width, L, since it defines the active DEA region. The electrode is assumed to be aligned centrally with x = 0 as shown in Fig. 10a and hence, for single pass deposition, ymin is located at x = ±L/2. The mean film ¯ for the single spray deposition film profiles in Section thickness, y, 3.2 is calculated using the integral of Eq. (2) over the electroded region:
y¯ = L−1
L/2
a e−x
2 /c 2
dx
(6)
−L/2
Fig. 9. Results of multiple deposition experiments with predicted profile: (a) 20 mm offset, (b) 25 mm offset, (c) 30 mm offset (dashed lines represent the Gaussian curves of the individual depostions).
Similarly, y¯ is found for the multiple spray deposition film profiles in Section 3.3 using the integral of Eq. (3). For these profiles, ymin may not necessarily be located at x = ±L/2. The effect of varying the electrode width, L, on the maximum voltage that may be applied to a single spray deposition film is shown in Fig. 10b, using Eq. (2) and kv = 50 MV/m (conservative estimate for Dow Corning Silastic 3481 based on [1]). Since the peak profile thickness, defined by the Gaussian fit parameter a, decreases as D increases, the maximum voltage correspondingly decreases over the range 0 < L < 25 mm. For L > 25 mm the flattest profile produced by D = 150 mm appears optimal. The figure of 2 ¯ , elucidates a more useful evaluation of the single merit, (ymin /y) deposition film profiles. Fig. 11a clearly demonstrates the intuitive result that the flattest profile (D = 150 mm) is optimal as it 2 ¯ , and hence Maxwell stress, for all has the highest value of (ymin /y) electrode widths. The results from applying the figure of merit to the multiple deposition spray profiles are more interesting. Fig. 9 suggests that, qualitatively, an offset distance of 25 mm appears optimal since it offers a relatively uniform thickness profile and a large maximum electrode width. The figure of merit, as plotted in Fig. 11b does indeed confirm this to a certain extent, as 2 ¯ > 0.9 for 0 < L < 75 mm. However, it is also noticeable that (ymin /y)
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Fig. 10. (a) Schematic diagram of deposition profile defining electrode width, L, and the corresponding active region; (b) estimated curves of maximum voltage against electrode width for the m = 10 g single deposition profiles with spray distance, D = 60, 90, 120, 150 mm (thickness profiles shown in Fig. 7a).
a larger offset of b = 30 mm may be preferable if a wider DEA and 2 ¯ > 0.84 hence a larger electrode width is desired, since (ymin /y) for 0 < L < 100 mm. An overall conclusion that can be drawn from Fig. 11 is that there is a clear advantage in using multiple depositions compared to single depositions. 4. A unimorph actuator using multilayer spray deposition In order to validate the proposed multi-layer fabrication method a number of stack and unimorph actuators were fabricated (Fig. 12). These actuators act as demonstrators and have not been optimised for maximum actuator output. The unimorph actuator has 6 active layers with an additional passive layer on top to encapsulate the electrode and protect it from external disturbances. The deposition distance and silicone mass for each layer was 75 mm and 10 g, respectively. The stacked actuator has 4 active layers with an additional passive layer on the bottom for encapsulation of the electrode. The deposition distance and silicone mass for each layer was 90 mm and 10 g, respectively. The silicone used was Dow Corning Silastic 3481 with the 10% 81-F curing agent for the unimorph actuator and 5% 81-VF curing agent for the stack actuator. The active electrode region for both actuators is 50 mm long and 10 mm wide. The total actuator width and length for both actuators is 20 mm and 80 mm, respectively, and airbrushed graphite powder was used for the electrodes of both actuators. The offset exposure technique, discussed in Section 2.3, was used to connect the electrodes of the unimorph actuator and the through cutting technique was used for stack actuator. The total actuator fabrication time was approximately 15 h for the unimorph actuator and 10 h for the stack actuator (note that these times are
2
for a batch of 16 identical actuators). However, this was primarily due to the time necessary for the silicone to cure (approximately 2 h per layer). In terms of actual man-hours, each DEA layer took a maximum of 30 min to fabricate. Faster curing silicones may be used to reduce fabrication time. DEA driving signals were generated in Mathworks Matlab and output using a National Instruments NI-PCI 6229 DAQ module. The signal was then amplified using a HA-151A potentiostat (Hokuto Denko, Japan) and stepped up to high voltages using an EMCO F121 converter (EMCO High Voltage Corporation), which is capable of supplying up to 12 kV. The actuator displacements were measured using a Keyence LK-G152 laser displacement sensor. The laser was positioned 70 mm from the fixed end of the unimorph actuator. The stack actuator was placed on a plastic substrate coated in silicone oil to reduce friction and the laser displacement sensor was positioned directly on top of the stack, perpendicular to the electrodes. The active strain of the stack was determined by dividing the initial stack thickness by the active deformation measurement from the laser displacement sensor. The output voltage and current from the potentiostat and the laser displacement output were measured using the DAQ module at a sample rate of 100 Hz for the unimorph actuator and 500 Hz for the stack actuator. Fig. 13 shows the results of static and dynamic tests for the unimorph and stack actuators. The tip displacement of a piezoelectric unimorph actuator is linearly proportional to the actuation stress, which is also proportional to the input voltage, assuming a constant elastic modulus [12]. The DE unimorph shares a similar actuation principle with the piezoelectric unimorph but in this case tip deflection is proportional to the square of the input voltage. Fig. 13a shows an approximately squared relationship between the unimorph tip deflection
2
¯ , for the single deposition profiles with variable spray distance, D = 60, 90, 120, 150 mm (m = 10 g); (b) figure of merit, (ymin /y) ¯ , for the Fig. 11. (a) Figure of merit, (ymin /y) multiple deposition profiles with variable offset distance, b = 20, 25, 30 mm (m = 10 g, D = 150 mm).
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Fig. 12. Multilayer fabrication using the developed fabrication technique: (a) example multilayer deposition (electrode smearing caused during specimen preparation), (b) DE multilayer unimorph actuator.
Fig. 13. DEA activation response: (a) multilayer unimorph (static), (b) multilayer unimorph (dynamic), (c) stack actuator (static), (d) stack actuator (dynamic).
and input voltage. A squared relationship can also be seen between the strain in the stack actuator and the input voltage (Fig. 13c). The maximum contractile strain of 3.4% at 4.0 kV input voltage agrees with previous studies on the silicone material used [13], hence validating the quality of the fabrication method. Fig. 13b shows the response of the unimorph actuator to a 5.0 Hz square wave input signal of 2.1 kV amplitude and 50% duty cycle. The unimorph actuator stroke amplitude reaches a value of 5.86 mm, which is 10 times greater than the static tip deflection (equal to 0.54 mm from Fig. 13a) for the same input voltage. Fig. 13d shows the dynamic response of the stack actuator to a 1 s step input voltage of 3.3 kV,
the response has been low pass filtered with cut-off at 50 Hz to eliminate noise. The stack actuator reaches a steady-state contractile strain of 1.9% in approximately 0.5 s, which is in agreement with previous studies [6]. 5. Conclusion Multilayer DEAs can provide actuators with improved actuation capabilities, including increased actuation stress and actuation strain. However, fabricating functional multilayer DEAs is nontrivial. Existing fabrication techniques require large or expensive
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equipment and have limited flexibility in producing actuators of different dimension or shapes. A flexible fabrication technique has been developed for the production of silicone based DEAs which is simple and well suited to building prototype actuators. The technique is also straightforward to operate and does not require complex acceleration or speed control unlike the primary competing fabrication method of spin coating. Experimental data shows that the thickness profile of the DE films can be controlled through the deposition distance, D, silicone mass, m, the solvent to silicone ratio and wheel diameter. The effects of varying the principle parameters, D and m, have been investigated. DE films of varying sizes have been produced by performing multiple spray depositions onto a single substrate. A figure of merit, representing the estimated mean Maxwell stress feasible from a DEA film of non-uniform thickness has been presented and used to analyse the various spray deposition film profiles. Out of the profiles analysed, it has been shown that the largest deposition distance of 150 mm will produce the optimal DEA actuation output. The analysis suggests that multiple deposition film profiles (composed of four separate depositions) will significantly increase the maximum electrode width by an order of magnitude, while producing a comparable mean Maxwell stress to that of a single deposition. The technique is also capable of producing multilayer DEAs which have inherent electrical isolation and good layer-to-layer bonding. Proof-of-concept multilayer DE unimorph and stack actuators have been fabricated using the developed fabrication technique to validate its ability to produce functional DEAs. The 4 layer stack actuator produced a contractile strain of 3.4% under a 4.0 kV activation voltage and the 6 layer, 80 mm long unimorph actuator produced a static tip deflection of 1.34 mm under an activation voltage of 3.8 kV. The unimorph actuator also operated at resonance and produced a tip amplitude of 5.86 mm at 2.1 kV activation voltage. Acknowledgements The authors would like to gratefully acknowledge Dr. Julie Etches and Dr. Maria D.M. Salinas-Ruiz from the Advanced Composites Centre for Innovation and Science (ACCIS) at the University of Bristol for their support and Dr. Andrew M. Collins from the Department of Chemistry, University of Bristol for the atomic force microscopy measurements. We would also like to thank the ACCIS Laboratory and the Bristol Robotics Laboratory (BRL) for the use of their facilities. The work presented is supported by the EPSRC (Grant Number: EP/F022824/1), the Higher Education Funding Council for England (HEFCE), the University of Bristol and the Department of Mechanical Engineering, University of Bristol. References [1] R. Pelrine, R. Kornbluh, J. Joseph, R. Heydt, Q. Pei, S. Chiba, High-field deformation of elastomeric dielectrics for actuators, Mater. Sci. Eng. C: Biomim. Mater. Sens. Syst. 11 (2000) 89–100. [2] P. Brochu, Q. Pei, Advances in dielectric elastomers for actuators and artificial muscles, Macromol. Rapid Commun. 31 (1) (2010) 10–36.
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[3] G. Kovacs, L. Düring, S. Michel, G. Terrasi, Stacked dielectric elastomer actuator for tensile force transmission, Sens. Actuators A: Phys. 155 (2009) 299–307. [4] S.C. Burgess, C.S. Ling, A. Conn, S. Araromi, J. Wang, R. Vaidyanathan, Development of a novel electro active polymer (EAP) actuator for driving the wings of flapping micro air vehicle, in: S. Hernandez, C.A. Brebbia (Eds.), Computer Aided Optimum Design in Engineering, vol. XI, WIT Press, Southampton, 2009, pp. 207–217. [5] F. Carpi, C. Salaris, D. De Rossi, Folded dielectric elastomer actuators, Smart Mater. Struct. 16 (2007) 300–305. [6] S. Michel, X.Q. Zhang, M. Wissler, C. Lowe, G. Kovacs, A comparison between silicone and acrylic elastomers as dielectric materials in electroactive polymer actuators, Polym. Int. 59 (2010) 391–399. [7] P. Lotz, M. Matysek, P. Lechner, M. Hamann, H.F. Schlaak, Dielectric elastomer actuators using improved thin film processing and nanosized particles, Proc. SPIE 6927 (2008) 23–30. [8] H.F. Schlaak, M. Jungmann, M. Matysek, P. Lotz, Novel multilayer electrostatic solid state actuators with elastic dielectric, Proc. SPIE 5759 (2005) 121–133. [9] Polypower, 2010, www.polypower.com. [10] A.T. Conn, Development of novel flapping mechanism technologies for insectinspired micro air vehicles, PhD Thesis, University of Bristol, 2008. [11] P. Mathur, D. Apelian, A. Lawley, Analysis of the spray deposition process, Acta Metall. 37 (1989) 429–443. [12] Q.M. Wang, L.E. Cross, Performance analysis of piezoelectric cantilever bending actuators, Ferroelectrics 215 (1) (1998) 187–213. [13] X. Zhang, C. Löwe, M. Wissler, B. Jähne, G. Kovacs, Dielectric elastomers in actuator technology, Adv. Mater. 7 (5) (2005) 361–367.
Biographies Oluwaseun Araromi received his undergraduate education in Mechanical Engineering at the University of Bristol and the University of Illinois at Urbana-Champaign, and received his MEng in degree 2008 from the Department of Mechanical Engineering, University of Bristol. He is currently conducting his PhD studies in electroactive polymer actuators under Professor Stuart Burgess and Dr. Ravi Vaidyanathan at the Department of Mechanical Engineering, University of Bristol. He is also the recipient of a University of Bristol Scholarship. Dr. Andrew Conn received his MEng and PhD degrees from the Department of Mechanical Engineering, University of Bristol in 2004 and 2008, respectively. He is currently a Research Assistant in the Soft Robotics research group at the Department of Engineering Mathematics, University of Bristol. His research interests include electroactive polymer actuators, bio-inspired design, soft robotics and novel mechanisms. Dr. Chung Seng Ling received his BEng degree in Mechatronics, and PhD degree in Control Engineering from the University of Leeds in 1997 and 2001, respectively. He is currently with the Bristol Robotics Laboratory developing a commercial medical device. His research interests include nonlinear control, fault detection and isolation, biomimetics and robotics, novel sensor and actuator technologies. Prof. Stuart Burgess is professor of engineering design in the Department of Mechanical Engineering at the University of Bristol. He has published over 100 technical papers on the design of mechanisms and structures and has registered 7 patents. He has industrial experience of designing precision mechanisms for the European Space Agency including the solar array deployment mechanism for the ENVISAT satellite. He is a recipient of a Turners Gold Medal for spacecraft design. Dr. Jonathan Rossiter is a Senior Lecturer at the University of Bristol and head of the Soft Robotics research group at Bristol Robotics Laboratory. His research interests include biomimetic soft robotics, soft actuator technologies, electroactive polymers, human-like tactile sensing and synthetic biology. Dr. Ravi Vaidyanathan is a Senior Lecturer at the University of Bristol and Head of Rehabilitation Robotics at the Bristol Robotics Laboratory. He earned his PhD in biologically inspired systems at Case Western Reserve University in 2001, and also holds honorary professorships at Case Western Reserve University (Cleveland, OH, USA). His research has been recognized internationally with awards from the Institute of Electrical and Electronics Engineers (IEEE), American Institute of Aeronautics and Astronautics (AIAA), and the Robotics Society of Japan (RSJ).
Contents lists available at ScienceDirect
Sensors and Actuators A: Physical journal homepage: www.elsevier.com/locate/sna
Spray deposited multilayered dielectric elastomer actuators O.A. Araromi a,∗ , A.T. Conn b , C.S. Ling a , J.M. Rossiter b , R. Vaidyanathan a , S.C. Burgess a a b
Department of Mechanical Engineering, University of Bristol, Queen’s Building, University Walk, Bristol BS8 1TR, UK Department of Engineering Mathematics, University of Bristol, Merchant Venturers Building, Woodland Road, Bristol BS8 1UB, UK
a r t i c l e
i n f o
Article history: Received 2 November 2010 Received in revised form 28 February 2011 Accepted 1 March 2011 Available online 9 March 2011 Keywords: Dielectric elastomer Electroactive polymers (EAPs) Unimorph actuators Spray deposition DEA fabrication Figure of merit
a b s t r a c t Dielectric elastomer electroactive polymers are an emerging class of actuation technology which is inherently compliant and capable of large actuation stresses and strains. Despite promising performance characteristics, their fabrication has been inhibited by two significant factors: (i) the requirement for consistently thin dielectric layers, to minimise activation voltages; (ii) automated production of multilayered configurations, to increase the actuation power. This paper presents a robust, low-cost fabrication technique that overcomes these issues by utilising optimised spray deposition. Spray deposition of silicone dielectric elastomer actuators (DEAs) offers numerous benefits including scalability, flexibility for different DEA configurations and multilayered assembly with a high degree of automation. A predictive model based on the Gaussian distribution is used to characterise the profile of deposited elastomer layers for principal fabrication parameters. This model enables individual dielectric layers to be composed from multiple parallel depositions, which greatly increases scalability as demonstrated by fabricated DEA films with planar dimensions from 25 mm2 to over 10,000 mm2 . Using the predictive model, a new figure of merit is introduced for analyzing DEA film profiles by considering the estimated mean Maxwell stress that is feasible for a specific dielectric breakdown strength. The analysis suggests that compared to a single deposition, a film composed of four parallel depositions will increase the maximum characteristic DEA dimension by an order of magnitude, while producing a comparable mean Maxwell stress. A significant advantage of the presented spray deposition technique is the semi-automated layering process, creating stratified solid-state actuators. By eliminating the stacking of layers from the fabrication process, inherent electrical isolation, good layer-to-layer bonding and capacity for more complex 3D geometries is achieved. A proof-of-concept multilayer unimorph and stack DEA is presented to validate the fabrication technique through static and dynamic displacement tests. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Dielectric elastomer actuators (DEAs) are a type of electroactive polymer (EAP) that attracts significant research interest because of their inherent compliance, light weight, high energy densities and actuation strains up to 380% as demonstrated with 3M VHB 4910 tape [1,2]. DEAs are made up of dielectric elastomer (DE) films sandwiched between two compliant electrodes, essentially forming an ‘elastic capacitor’. When a large voltage field (typically around 50–200 MV/m [1]) is applied across the electrodes, coulomb forces cause compression of the film in the thickness direction and expansion in planar direction. The effective induced pressure in the thickness direction is often referred to as the Maxwell stress, p, given by Eq. (1). p = er e0
V 2 y
∗ Corresponding author. Tel.: +44 1173315936. E-mail address: [email protected] (O.A. Araromi). 0924-4247/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.sna.2011.03.004
(1)
where er is the relative dielectric constant of the elastomer film, e0 is the permittivity of free space (equal to 8.85 × 10−12 F/m), y is the initial film thickness and V is the voltage across the elastomer film. From Eq. (1), it is clear that actuation stress, p, is inversely proportional to the square of the thickness, y, of the DE film for a fixed voltage. It is therefore more beneficial, for a given voltage, to have a large number of thin elastomer layers than to have a smaller number of thick layers. However, fabricating functional multilayer DEAs is non-trivial, especially if stacking is performed manually [3,4] which can result in inconsistent actuator performance. Layerto-layer bonding also has a significant impact on the structural integrity of multilayer DEA, especially if the actuator is required to hold a load in both its passive and active states [3]. Interlayer bonding is also important for preventing arcing around the sides of the DE films and for electrode adhesion if the silicone elastomer being used is not naturally adhesive [3]. Some fabrication methods attempt to provide electrical isolation by coating actuators with a passive soft insulating layer [5], however, this coating can inhibit actuator performance.
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Currently, acrylic and silicone based dielectric elastomers have been shown to have the best mechanical and actuation properties [6]. Silicone elastomers are easy to work with, inexpensive and can be easily fabricated into thin films with various shapes, in contrast to the commonly used acrylic type elastomers (e.g. 3M VHB 4910), allowing for more flexibility in actuator design. Advanced methods for fabricating silicone based DEAs have been developed by various research groups [7–9], all of which are effective in producing useable actuators. Commercial silicone based DEA products exist with highly conductive metal electrodes [9], however, the fabrication process used to produce them is complex and optimised for the large volume production of single film actuators. The equipment is also large, expensive and therefore not ideal for small laboratories. The fabrication processes outlined in Lotz et al. [7] and Schlaak et al. [8] are similar to the work presented here. However, the spin coating equipment used is expensive and requires accurate speed and acceleration control, and is limited in its flexibility for producing films of different shapes and dimensions. Other fabrication methods, such as mould casting [5] or K-bar/Mayer bar coating [10] are more simple and inexpensive. However, such methods are only suited to fabricating single layer DE films, multilayer DEAs can only be obtained by folding, rolling or manually stacking individual films, and therefore are subject to issues of interlayer bonding and electrical insulation. A potential fabrication process that has not received significant research study is that of spray deposition. Spray deposition offers a number of benefits over existing methods including: (i) versatility in the fabrication of actuators of different sizes, thicknesses and profiles; (ii) the potential to automate the process to create a full 3D actuator prototyping system; (iii) high quality control due to the well established parameter optimisation of spray material for the painting, printing and coating industries; (iv) the ready adaptation of spray methods to deposit both dielectric and compliant electrode materials. This paper presents a flexible fabrication technique for producing silicone DEAs based on spray deposition which can consistently produce multilayer DEAs. The general fabrication technique, applicable for any silicone dielectric elastomer, is described in Section 2. Section 3 presents the results of the fabrication characterisation and optimisation study based on deposition distance and silicone mass and introduces a new figure of merit for variable thickness DEA actuators. In Section 4 proof of concept unimorph and stack actuators are used to demonstrate the fabrication technique’s ability to produce functional DEAs. Finally, the main conclusions are drawn in Section 5. 2. Generalised fabrication method We present a new DEA fabrication method where the dielectric layers, and optionally the electrodes layers, are spray deposited. The fabrication method described here requires only basic components; a DC motor, a wheel and an airbrush system. Since the principle components can be procured and assembled quickly and cheaply the whole fabrication system is inexpensive. The rotating wheel-based spray deposition concept has the inherent advantage of giving a consistent layer thickness without requiring a change in direction of the motor or precise control over its velocity profile. Fig. 1 shows the proposed DE fabrication technique. The fabrication process can be summarised as follows: (1) Prepare a flexible substrate and affix around the circumference of a motorised wheel. (2) Rotate the wheel at constant speed and spray deposit dielectric elastomer using constant air pressure (Fig. 1a). (3) Apply electrode mask to the cured elastomer layer.
(4) Apply compliant electrode material, e.g. by brush or spray deposition (Fig. 1b). (5) Repeat steps 2–4 for the required number of DEA layers. (6) Remove complete actuator including substrate from wheel. (7) Electrically connect layers to give a parallel circuit. (8) Connect to high voltage supply and test (Fig. 1d). 2.1. Dielectric elastomer deposition Since spray deposition is only suitable for low viscosity fluids most dielectric materials must be thinned or dissolved in a suitable solvent before spraying. In this study silicone is used as the dielectric material so we must consider how liquid silicone can be prepared at the appropriate low viscosity. Uncured silicone can be dissolved in solvent to form a relatively low viscosity liquid. With condensation cure silicone this is typically achieved by thinning the base part with an organic solvent and subsequently adding the curing agent. For other silicones, such as addition cure silicones, the individual constituents are typically dissolved in solvent separately before being mixed. The prepared low viscosity solvent–silicone mixture is poured into the airbrush container through a strainer to remove any undissolved elastomer clumps. The mixture is then airbrushed at a constant pressure onto the circumference of a rotating wheel which has steel substrate sections mounted to it as shown in Fig. 1a. The circumference of the wheel is lined with magnetic tape to ensure the substrate remains flat against the surface of the wheel circumference during spraying. The wheel is mounted onto a DC motor rotating at constant velocity. The airbrush spray is aligned with the centreline of the substrate. The solvent–silicone mixture is spray deposited in the form of discrete droplets, which build up on the substrate surface with each revolution of the wheel, creating a continuous deposition with a smooth surface topography. This is important as large variations in the surface topography could create local regions with a thickness much smaller than the rest of the deposition and, therefore, more susceptible to failure by dielectric breakdown. The surface topography was investigated by performing tapping mode atomic force microscopy (AFM) on a 75.9 m thick sample deposition (Fig. 2) using a Veeco Multimode V Scanning Probe Microscope. The color axis in the right of Fig. 2 maps the colors in the scan to relative height values. Fig. 2 clearly shows that at the micro-scale the variation in surface feature height is less than 70 nm, and the variation in surface height relative to the mean thickness of the sample deposition is less than 0.09%. Hence the risk of premature dielectric breakdown due to localised droplet deposition topography can assumed to be negligible. The airbrush and rotating wheel are housed within an extraction hood in order to minimise the likelihood of foreign airborne particles embedding in the deposited silicone film before it has cured. It is important to fabricate films in as clean an environment as possible to avoid small airborne particles adversely affecting the quality of layers and layer bonding as shown in Fig. 3a. This figure shows how a particle causes the layers to warp and distort around it, resulting in a region of reduced thickness and hence a point of weakness in terms of dielectric breakdown. 2.2. Electrode deposition After the dielectric elastomer has been spray deposited the solvent evaporates leaving the silicone layer. Once this sprayed layer has fully cured, electrodes can now be applied to the surface of the silicone. Electrodes can be applied using a variety of methods including airbrushing, hand brushing, stamping or smearing. Suitable electrode materials for this fabrication process are dry graphite powder, conductive carbon grease and graphite powder suspended
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Fig. 1. Fabrication process of dielectric elastomer multilayer DEA.
in a silicone matrix. Hand brushed electrodes should be applied sparingly to prevent agglomeration. Agglomeration has the effect of increasing the stiffness of the elastomer layer [3] which subsequently reduces the performance of the actuator. Care should be taken when using conductive carbon grease because it was found to accumulate at the edges of the electrode mask, as shown in Fig. 3b, with consequential local increase in susceptibility to dielectric breakdown. Airbrushed graphite powder, dissolved in solvent at a ratio of 1:4, was used in this work. The electrodes produced have high conductivity and the airbrushing method allows for the fabrication process to be fully automated. Agglomeration of spray deposited graphite powder was not found to be a problem. The electrode masks are made from acetate transparency films coated on the underside with a thin layer of tacky silicone (e.g. BJB TC-5005) to provide good adhesion to the dielectric elastomer layer while the electrode is being applied. Datum marks are drawn onto the mask to ensure the electrodes line up through the layers.
2.3. Multilayered fabrication Fig. 2. Tapping mode atomic force microscopy of a typical region of the spray deposited dielectric elastomer surface (colors represent relative heights of surface features). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Although single layer dielectric elastomer actuators have application as, for example, active membranes, more practical applications demand more substantial multilayer actuators. To achieve this subsequent elastomer layers can be airbrushed directly on top of the electrode of the previous layer film. The direct deposition of one elastomer layer on top of previous layers produces a multilayered stack with good electrical insulation and interlayer bonding, without the use of an addition passive insulation layer.
Fig. 3. Regions with increased risk of failure due to electric breakdown: (a) effect of imperfection on multilayer actuator; (i) region of weakness, (ii) airborne particle. (b) Effects of accumulation of carbon grease electrodes; (i) region of weakness, (ii) accumulation of carbon grease electrode (both images are a cross-section through actuator width).
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Fig. 5. Examples of thickness measurements from three elastomer deposition samples with fitted Gaussian profile curve.
and curing process, and electrical connections were made between the layers by piercing with a conductive pin or wire (Fig. 4b); (iii) the offset regions were cut through after all the elastomer layers had been deposited and the edges were coated with a conductive medium, such as carbon grease or carbon powder (Fig. 4c). In general, the offset region exposure technique (method (i)) provides the most reliable electrode connection. One explanation for this is that methods (ii) and (iii) rely on a relatively small area for electrical connection. With through pinning in particular, the piercing of the elastomer layers can cause distortion of the electrodes, further preventing a reliable connection. However, through pinning and through cutting have the advantage of reducing the number of steps in the fabrication procedure, resulting in a reduction in overall fabrication time and complexity. Fig. 4. Through layer electrode connection techniques: (a) exposed offset region, (b) through pinning, (c) through cutting.
It is important to note that directly after fabrication each actuator layer is by default electrically isolated. To prevent arcing around the edges of adjacent layers a passive isolating border which surrounds the electrode can be used and need only be as wide as the thickness of the dielectric elastomer films [3]. This contributes to the effectiveness of the actuator. 2.4. Supply voltage terminals Once sufficient actuator layers have been deposited the actuator and substrate are removed from the wheel and strips of aluminium foil are attached to the offset regions to provide the electrical connection for the input driving voltage. Depending on the desired actuator configuration, the elastomer stack can be left on the substrate (e.g. unimorph, bimorph DEAs) or removed and used separately (e.g. stack, roll DEAs). In terms of scalability, many small stacks (<10 layers) can be combined to form larger stacks (>100 layers) as a way of reducing overall manufacturing time and increasing the versatility and standardisation of the process. For multilayered actuators, connection of electrodes through the elastomer layers is required to form a parallel circuit of electrodes. One way of achieving this is to offset the electrode pattern slightly during the electrode deposition step. The electrodes are thus staggered alternately so that they can easily be connected electrically, as shown in Fig. 4. Three methods of electrical connection were evaluated: (i) the offset regions were left exposed between layers so that when subsequent electrodes were applied electrical connections between layers were naturally made at the sides (Fig. 4a); (ii) the offset regions were left unshielded during the elastomer deposition
3. Fabrication characterisation and optimisation Unlike most other DE actuators, such as those using pre-strained VHB tapes, the thicknesses of DE films produced by the proposed fabrication method are inhomogeneous due to the profile of the spray jet leaving the airbrush system. Clearly this will have an impact on the efficacy of the final actuator. To this end we derive a predictive deposition model based on empirical analysis of spray densities. Previous experimental analysis has shown that the thickness profile of spray deposited matter fits a Gaussian distribution [11] of the form: 2
y(x) = a e−(x−b)
/c 2
(2)
where x is the lateral position along the substrate in mm, a is the peak deposition thickness in m, b is the lateral offset distance from the centre of the substrate√in mm and c is a shape factor approximating the variance term 2 2 . Gaussian distribution is proven to apply to the thickness profile of elastomer depositions specifically. Fig. 5 shows examples of thickness measurements taken from deposition samples made using the fabrication technique, a good fit of the Gaussian distribution to the measured data can be seen. 3.1. Materials and methods The deposition profile for a single layer deposition is a function of: (i) the deposition distance, D, i.e. the distance between the airbrush system and the surface of the substrate; (ii) the mass of silicone in the spray mixture, m; (iii) the spray mixture viscosity (related to the solvent–silicone ratio), v; (iv) the spray wheel diameter, w. The effect of deposition distance, D, and silicone mass, m, is
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Fig. 6. Experimental set-up for (a) spray deposition, (b) deposition profile thickness measurement.
investigated here. Single layer films were produced using the fabrication technique described in Section 2 using a Revell Starter Class airbrush system onto a wheel of diameter, w = 51 cm (Fig. 6a). A Parvalux 24 V DC motor was used to rotate the wheel at a constant speed of approximately 41 rpm. The silicone used was Dow Corning Silastic 3481. The air pressure supply to the airbrush was regulated at 2.0 bar and the mixture ratio of solvent (dichloromethane) to silicone was kept constant at 5:1 for each experiment. The thickness measurements were made using a Zeiss Jenavert incident light research microscope and Olympus Colorview 3 camera system (Fig. 6b). Images were taken at 2 mm increments along the film width and the thickness measured using ImageJ (National Institutes of Health) image processing software.
mechanism of DEAs is dielectric breakdown, which for a uniform supply voltage typically occurs where thickness is at a minimum (a theoretical analysis is used to demonstrate this in Section 3.4). Fig. 7b shows the results of the Gaussian fit for varying silicone masses at the optimal deposition distance (i.e. 150 mm). The Gaussian and the 95% confidence bounds are very close, indicating a good fit. From Fig. 7b it can be seen that there is an approximately linear relationship between silicone mass, m, and peak thickness, a. The results also demonstrate the ability of the technique to produce films with very low peak thickness (<40 m) simply by adjusting silicone mass m.
3.2. Single deposition
Clearly actuators with Gaussian-like thickness profiles are nonoptimal and so we now consider how multiple offset spray passes can yield a flatter, more ideal surface profile. Multiple depositions can be made on a single substrate if a film of larger width dimensions is required by offsetting each spray deposition laterally (i.e. across the width of the film) by a certain distance. The Gaussian distribution deposition model (Eq. (2)) can be adapted to predict the film profile produced by multiple depositions using Eq. (3):
A series of 3 single layer single pass actuators were fabricated at various spray distances and silicone masses. Layer profiles were measured and Gaussian distributions were fitted to these profiles, as shown in Fig. 7. Dotted lines indicate 95% confidence intervals. The Gaussian expression parameters, a and c for each fitted curve are given in Table 1. Fig. 7a shows the effects of varying the deposition distance with the silicone mass, m = 10 g. The D = 150 mm deposition distance was found to be optimal as it produced the flattest profile while maintaining a small 95% confidence bound region. A flatter profile is preferable since the predominant failure
3.3. Multiple depositions
2
2
2
y(x) = a e−(x/c) + a e−((x−b)/c) + a e−((x−2b)/c) + · · · 2
+a e−((x−(n−1)b)/c)
(3)
where b is the offset distance in mm i.e. the separation distance between one deposition and the next, and n is the number of depositions. Fig. 9 shows the results of multiple deposition experiments where four offset depositions were made on each substrate, two staggered on either side of the substrate centreline. For this experiment a single airbrush system was used with multiple passes but it is also possible to use multiple parallel airbrush systems, positioned such that the airbrush nozzles are separated by a fixed offset distance, b (Fig. 8). A deposition distance of D = 150 mm and silicone mass m = 10 g was used for each deposition pass. Experiments were
Fig. 7. Characterised Gaussian spray deposition profiles: (a) fixed silicone mass (m = 10 g), varying deposition distance. (b) Fixed deposition distance (D = 150 mm), varying silicone mass (solid lines are the fitted Gaussian curves, dashed lines are the 95% confidence bounds).
Fig. 8. Multiple deposition fabrication.
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Table 1 Fitted Gaussian distribution model parameters (see Eq. (2), the 95% confidence value are given in brackets). Spray distance, D (m = 10 g) 60 mm 90 mm 120 mm 150 mm
a
c
Elastomer mass, m (D = 150 mm)
a
c
181.0 (±7.8) 123.3 (±7.0) 79.2 (±3.1) 52.6 (±3.8)
10.43 (±0.53) 12.46 (±0.86) 14.02 (±0.72) 18.72 (±2.13)
6g 12 g 18 g 24 g
40.3 (±1.5) 78.7 (±1.9) 112.1 (±1.4) 155.2 (±2.0)
16.75 (±0.91) 17.24 (±0.62) 16.58 (±0.30) 17.62 (±0.34)
conducted for offset distances of 20 mm, 25 mm and 30 mm. Three experiments were conducted for each offset distance to account for experimental scatter. The substrate width was 100 mm. The solid lines in Fig. 9 show the predicted deposition profile determined as the sum of the Gaussian fits of the individual depositions, shown as dashed lines, over the entire spray region. The results show there is a good match between the measured data and the predicted profile. The 25 mm offset, in particular, produces a good profile over a large width. Fig. 9b suggests that the 25 mm offset with four separate depositions produces a DE film around 80 mm wide. The multiple deposition samples here had a maximum area of around 10,000 mm2 . The length of the film is limited only by the length of the substrate, which can be as long as the wheel circumference (1.6 m for the set-up presented here). Hence, in principle, 4 depositions could have produced a single film with an area of up to 128,000 mm2 . Therefore the multiple deposition technique is clearly well suited to producing large DE films, which are not easily achievable using the primary competing fabrication method of spin coating. 3.4. Deposition profile analysis and figure of merit The predominant failure mechanism that limits the performance of DEAs is dielectric breakdown, which in theory should occur where the voltage field is greatest (assuming the film is properly insulated and free from impurities and external mechanical disturbances). A DEA film of non-uniform planar thickness, y(x),
such as the spray deposition profiles presented in this work, will therefore typically fail where y(x) is at minimum. This effect is accentuated by the fact that the transverse thickness strain, εy , is proportional to applied voltage field squared, (V/y)2 . Consequently, the uniformity of an elastomer film’s thickness can be used to assess its likely performance as a DEA. Since the films produced via spray deposition are not uniform, a figure of merit is introduced here to provide a quantified measure of predicted DEA performance for a specified electrode width. Assuming dielectric breakdown occurs where y(x) is minimum, ymin , and that the voltage field is equal to elastomer breakdown strength, kv , then Eq. (1) can be rewritten as follows: p = er e0 so ¯ r e0 p∼e
k y 2 v min
(4)
y
k y 2 v min
(5)
y¯
Neglecting constants, Eq. (5) suggests that a useful figure of 2 ¯ , since it yields an approximate measure of the merit is (ymin /y) ¯ produced by a non-uniform DEA. Note that mean Maxwell stress, p, the principal assumption of this method is that the maximum supply voltage is not a design constraint, whereas in practice it may be. For the single and multiple deposition profiles presented in Sections 3.2 and 3.3 the key design parameter is electrode width, L, since it defines the active DEA region. The electrode is assumed to be aligned centrally with x = 0 as shown in Fig. 10a and hence, for single pass deposition, ymin is located at x = ±L/2. The mean film ¯ for the single spray deposition film profiles in Section thickness, y, 3.2 is calculated using the integral of Eq. (2) over the electroded region:
y¯ = L−1
L/2
a e−x
2 /c 2
dx
(6)
−L/2
Fig. 9. Results of multiple deposition experiments with predicted profile: (a) 20 mm offset, (b) 25 mm offset, (c) 30 mm offset (dashed lines represent the Gaussian curves of the individual depostions).
Similarly, y¯ is found for the multiple spray deposition film profiles in Section 3.3 using the integral of Eq. (3). For these profiles, ymin may not necessarily be located at x = ±L/2. The effect of varying the electrode width, L, on the maximum voltage that may be applied to a single spray deposition film is shown in Fig. 10b, using Eq. (2) and kv = 50 MV/m (conservative estimate for Dow Corning Silastic 3481 based on [1]). Since the peak profile thickness, defined by the Gaussian fit parameter a, decreases as D increases, the maximum voltage correspondingly decreases over the range 0 < L < 25 mm. For L > 25 mm the flattest profile produced by D = 150 mm appears optimal. The figure of 2 ¯ , elucidates a more useful evaluation of the single merit, (ymin /y) deposition film profiles. Fig. 11a clearly demonstrates the intuitive result that the flattest profile (D = 150 mm) is optimal as it 2 ¯ , and hence Maxwell stress, for all has the highest value of (ymin /y) electrode widths. The results from applying the figure of merit to the multiple deposition spray profiles are more interesting. Fig. 9 suggests that, qualitatively, an offset distance of 25 mm appears optimal since it offers a relatively uniform thickness profile and a large maximum electrode width. The figure of merit, as plotted in Fig. 11b does indeed confirm this to a certain extent, as 2 ¯ > 0.9 for 0 < L < 75 mm. However, it is also noticeable that (ymin /y)
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Fig. 10. (a) Schematic diagram of deposition profile defining electrode width, L, and the corresponding active region; (b) estimated curves of maximum voltage against electrode width for the m = 10 g single deposition profiles with spray distance, D = 60, 90, 120, 150 mm (thickness profiles shown in Fig. 7a).
a larger offset of b = 30 mm may be preferable if a wider DEA and 2 ¯ > 0.84 hence a larger electrode width is desired, since (ymin /y) for 0 < L < 100 mm. An overall conclusion that can be drawn from Fig. 11 is that there is a clear advantage in using multiple depositions compared to single depositions. 4. A unimorph actuator using multilayer spray deposition In order to validate the proposed multi-layer fabrication method a number of stack and unimorph actuators were fabricated (Fig. 12). These actuators act as demonstrators and have not been optimised for maximum actuator output. The unimorph actuator has 6 active layers with an additional passive layer on top to encapsulate the electrode and protect it from external disturbances. The deposition distance and silicone mass for each layer was 75 mm and 10 g, respectively. The stacked actuator has 4 active layers with an additional passive layer on the bottom for encapsulation of the electrode. The deposition distance and silicone mass for each layer was 90 mm and 10 g, respectively. The silicone used was Dow Corning Silastic 3481 with the 10% 81-F curing agent for the unimorph actuator and 5% 81-VF curing agent for the stack actuator. The active electrode region for both actuators is 50 mm long and 10 mm wide. The total actuator width and length for both actuators is 20 mm and 80 mm, respectively, and airbrushed graphite powder was used for the electrodes of both actuators. The offset exposure technique, discussed in Section 2.3, was used to connect the electrodes of the unimorph actuator and the through cutting technique was used for stack actuator. The total actuator fabrication time was approximately 15 h for the unimorph actuator and 10 h for the stack actuator (note that these times are
2
for a batch of 16 identical actuators). However, this was primarily due to the time necessary for the silicone to cure (approximately 2 h per layer). In terms of actual man-hours, each DEA layer took a maximum of 30 min to fabricate. Faster curing silicones may be used to reduce fabrication time. DEA driving signals were generated in Mathworks Matlab and output using a National Instruments NI-PCI 6229 DAQ module. The signal was then amplified using a HA-151A potentiostat (Hokuto Denko, Japan) and stepped up to high voltages using an EMCO F121 converter (EMCO High Voltage Corporation), which is capable of supplying up to 12 kV. The actuator displacements were measured using a Keyence LK-G152 laser displacement sensor. The laser was positioned 70 mm from the fixed end of the unimorph actuator. The stack actuator was placed on a plastic substrate coated in silicone oil to reduce friction and the laser displacement sensor was positioned directly on top of the stack, perpendicular to the electrodes. The active strain of the stack was determined by dividing the initial stack thickness by the active deformation measurement from the laser displacement sensor. The output voltage and current from the potentiostat and the laser displacement output were measured using the DAQ module at a sample rate of 100 Hz for the unimorph actuator and 500 Hz for the stack actuator. Fig. 13 shows the results of static and dynamic tests for the unimorph and stack actuators. The tip displacement of a piezoelectric unimorph actuator is linearly proportional to the actuation stress, which is also proportional to the input voltage, assuming a constant elastic modulus [12]. The DE unimorph shares a similar actuation principle with the piezoelectric unimorph but in this case tip deflection is proportional to the square of the input voltage. Fig. 13a shows an approximately squared relationship between the unimorph tip deflection
2
¯ , for the single deposition profiles with variable spray distance, D = 60, 90, 120, 150 mm (m = 10 g); (b) figure of merit, (ymin /y) ¯ , for the Fig. 11. (a) Figure of merit, (ymin /y) multiple deposition profiles with variable offset distance, b = 20, 25, 30 mm (m = 10 g, D = 150 mm).
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Fig. 12. Multilayer fabrication using the developed fabrication technique: (a) example multilayer deposition (electrode smearing caused during specimen preparation), (b) DE multilayer unimorph actuator.
Fig. 13. DEA activation response: (a) multilayer unimorph (static), (b) multilayer unimorph (dynamic), (c) stack actuator (static), (d) stack actuator (dynamic).
and input voltage. A squared relationship can also be seen between the strain in the stack actuator and the input voltage (Fig. 13c). The maximum contractile strain of 3.4% at 4.0 kV input voltage agrees with previous studies on the silicone material used [13], hence validating the quality of the fabrication method. Fig. 13b shows the response of the unimorph actuator to a 5.0 Hz square wave input signal of 2.1 kV amplitude and 50% duty cycle. The unimorph actuator stroke amplitude reaches a value of 5.86 mm, which is 10 times greater than the static tip deflection (equal to 0.54 mm from Fig. 13a) for the same input voltage. Fig. 13d shows the dynamic response of the stack actuator to a 1 s step input voltage of 3.3 kV,
the response has been low pass filtered with cut-off at 50 Hz to eliminate noise. The stack actuator reaches a steady-state contractile strain of 1.9% in approximately 0.5 s, which is in agreement with previous studies [6]. 5. Conclusion Multilayer DEAs can provide actuators with improved actuation capabilities, including increased actuation stress and actuation strain. However, fabricating functional multilayer DEAs is nontrivial. Existing fabrication techniques require large or expensive
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equipment and have limited flexibility in producing actuators of different dimension or shapes. A flexible fabrication technique has been developed for the production of silicone based DEAs which is simple and well suited to building prototype actuators. The technique is also straightforward to operate and does not require complex acceleration or speed control unlike the primary competing fabrication method of spin coating. Experimental data shows that the thickness profile of the DE films can be controlled through the deposition distance, D, silicone mass, m, the solvent to silicone ratio and wheel diameter. The effects of varying the principle parameters, D and m, have been investigated. DE films of varying sizes have been produced by performing multiple spray depositions onto a single substrate. A figure of merit, representing the estimated mean Maxwell stress feasible from a DEA film of non-uniform thickness has been presented and used to analyse the various spray deposition film profiles. Out of the profiles analysed, it has been shown that the largest deposition distance of 150 mm will produce the optimal DEA actuation output. The analysis suggests that multiple deposition film profiles (composed of four separate depositions) will significantly increase the maximum electrode width by an order of magnitude, while producing a comparable mean Maxwell stress to that of a single deposition. The technique is also capable of producing multilayer DEAs which have inherent electrical isolation and good layer-to-layer bonding. Proof-of-concept multilayer DE unimorph and stack actuators have been fabricated using the developed fabrication technique to validate its ability to produce functional DEAs. The 4 layer stack actuator produced a contractile strain of 3.4% under a 4.0 kV activation voltage and the 6 layer, 80 mm long unimorph actuator produced a static tip deflection of 1.34 mm under an activation voltage of 3.8 kV. The unimorph actuator also operated at resonance and produced a tip amplitude of 5.86 mm at 2.1 kV activation voltage. Acknowledgements The authors would like to gratefully acknowledge Dr. Julie Etches and Dr. Maria D.M. Salinas-Ruiz from the Advanced Composites Centre for Innovation and Science (ACCIS) at the University of Bristol for their support and Dr. Andrew M. Collins from the Department of Chemistry, University of Bristol for the atomic force microscopy measurements. We would also like to thank the ACCIS Laboratory and the Bristol Robotics Laboratory (BRL) for the use of their facilities. The work presented is supported by the EPSRC (Grant Number: EP/F022824/1), the Higher Education Funding Council for England (HEFCE), the University of Bristol and the Department of Mechanical Engineering, University of Bristol. References [1] R. Pelrine, R. Kornbluh, J. Joseph, R. Heydt, Q. Pei, S. Chiba, High-field deformation of elastomeric dielectrics for actuators, Mater. Sci. Eng. C: Biomim. Mater. Sens. Syst. 11 (2000) 89–100. [2] P. Brochu, Q. Pei, Advances in dielectric elastomers for actuators and artificial muscles, Macromol. Rapid Commun. 31 (1) (2010) 10–36.
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[3] G. Kovacs, L. Düring, S. Michel, G. Terrasi, Stacked dielectric elastomer actuator for tensile force transmission, Sens. Actuators A: Phys. 155 (2009) 299–307. [4] S.C. Burgess, C.S. Ling, A. Conn, S. Araromi, J. Wang, R. Vaidyanathan, Development of a novel electro active polymer (EAP) actuator for driving the wings of flapping micro air vehicle, in: S. Hernandez, C.A. Brebbia (Eds.), Computer Aided Optimum Design in Engineering, vol. XI, WIT Press, Southampton, 2009, pp. 207–217. [5] F. Carpi, C. Salaris, D. De Rossi, Folded dielectric elastomer actuators, Smart Mater. Struct. 16 (2007) 300–305. [6] S. Michel, X.Q. Zhang, M. Wissler, C. Lowe, G. Kovacs, A comparison between silicone and acrylic elastomers as dielectric materials in electroactive polymer actuators, Polym. Int. 59 (2010) 391–399. [7] P. Lotz, M. Matysek, P. Lechner, M. Hamann, H.F. Schlaak, Dielectric elastomer actuators using improved thin film processing and nanosized particles, Proc. SPIE 6927 (2008) 23–30. [8] H.F. Schlaak, M. Jungmann, M. Matysek, P. Lotz, Novel multilayer electrostatic solid state actuators with elastic dielectric, Proc. SPIE 5759 (2005) 121–133. [9] Polypower, 2010, www.polypower.com. [10] A.T. Conn, Development of novel flapping mechanism technologies for insectinspired micro air vehicles, PhD Thesis, University of Bristol, 2008. [11] P. Mathur, D. Apelian, A. Lawley, Analysis of the spray deposition process, Acta Metall. 37 (1989) 429–443. [12] Q.M. Wang, L.E. Cross, Performance analysis of piezoelectric cantilever bending actuators, Ferroelectrics 215 (1) (1998) 187–213. [13] X. Zhang, C. Löwe, M. Wissler, B. Jähne, G. Kovacs, Dielectric elastomers in actuator technology, Adv. Mater. 7 (5) (2005) 361–367.
Biographies Oluwaseun Araromi received his undergraduate education in Mechanical Engineering at the University of Bristol and the University of Illinois at Urbana-Champaign, and received his MEng in degree 2008 from the Department of Mechanical Engineering, University of Bristol. He is currently conducting his PhD studies in electroactive polymer actuators under Professor Stuart Burgess and Dr. Ravi Vaidyanathan at the Department of Mechanical Engineering, University of Bristol. He is also the recipient of a University of Bristol Scholarship. Dr. Andrew Conn received his MEng and PhD degrees from the Department of Mechanical Engineering, University of Bristol in 2004 and 2008, respectively. He is currently a Research Assistant in the Soft Robotics research group at the Department of Engineering Mathematics, University of Bristol. His research interests include electroactive polymer actuators, bio-inspired design, soft robotics and novel mechanisms. Dr. Chung Seng Ling received his BEng degree in Mechatronics, and PhD degree in Control Engineering from the University of Leeds in 1997 and 2001, respectively. He is currently with the Bristol Robotics Laboratory developing a commercial medical device. His research interests include nonlinear control, fault detection and isolation, biomimetics and robotics, novel sensor and actuator technologies. Prof. Stuart Burgess is professor of engineering design in the Department of Mechanical Engineering at the University of Bristol. He has published over 100 technical papers on the design of mechanisms and structures and has registered 7 patents. He has industrial experience of designing precision mechanisms for the European Space Agency including the solar array deployment mechanism for the ENVISAT satellite. He is a recipient of a Turners Gold Medal for spacecraft design. Dr. Jonathan Rossiter is a Senior Lecturer at the University of Bristol and head of the Soft Robotics research group at Bristol Robotics Laboratory. His research interests include biomimetic soft robotics, soft actuator technologies, electroactive polymers, human-like tactile sensing and synthetic biology. Dr. Ravi Vaidyanathan is a Senior Lecturer at the University of Bristol and Head of Rehabilitation Robotics at the Bristol Robotics Laboratory. He earned his PhD in biologically inspired systems at Case Western Reserve University in 2001, and also holds honorary professorships at Case Western Reserve University (Cleveland, OH, USA). His research has been recognized internationally with awards from the Institute of Electrical and Electronics Engineers (IEEE), American Institute of Aeronautics and Astronautics (AIAA), and the Robotics Society of Japan (RSJ).