MUSCLE-LIKE ACTUATORS? A COMPARISON BETWEEN THREE ELECTROACTIVE POLYMERS.

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1 MUSCLE-LIKE ACTUATORS? A COMPARISON BETWEEN THREE ELECTROACTIVE POLYMERS. Kenneth Meijer*, Marcus Rosenthal and Robert J Full Dept. Integrative Biology, University of California at Berkeley. ABSTRACT Muscles fulfill several functions within an animal s body. During locomotion they propel and control the limbs in unstructured environments. Therefore, the functional workspace of muscle needs to be represented by variables describing energy management (i.e. power output, efficiency) as well as control aspects (i.e. stiffness, damping). Muscles in the animal kingdom vary greatly with respect to those variables. To study if ElectroActive Polymer s (EAP) can be considered as artificial muscles we are making a direct comparison between the contractile properties of EAP s and biological muscle. We have measured the functional workspace of EAP actuators using the same setup and techniques that we use to test biological muscle. We evaluated the properties of three different EAP materials; the acrylic and silicone dielectric elastomers developed at SRI International and the high-energy electron-irradiated co-polymers (P(VDF-TrFE)) developed at the MRL laboratory at Penn State University. Initial results indicate that the EAP materials partly capture the functional workspace of natural muscle and sometimes even exceed the capabilities of muscle. Based on the data we have collected it seems that both EAP technologies have characteristics that could qualify them as artificial muscles. 1. INTRODUCTION Biological muscle is a magnificent actuator with the capacity to perform many functions. During activities as diverse as running, flying and swimming, muscles operate as motors, brakes, springs and struts 1. Some muscles, such as insect flight muscle, appear to be designed for more specific functions, whereas others are capable of changing their function in response to the requirements of the task at hand. For example, the calf muscles of turkeys function as struts during level running but they transform into motors when a turkey runs uphill 2. Muscles have good energy density values and they integrate actuation, support and fuel systems. Compared to human-made actuators, muscles are considered to be compliant actuators 3. The compliance of muscle may be one of the reasons why animals are capable of agile locomotion in unstructured environments where legged robots fail. Considering the performance capacities of biological muscle, it is no wonder that there has been considerable effort to develop human-made actuator technologies that can mimic muscle performance 3,4. Recently, several promising new technologies, based on polymer science, emerged that are expected to accomplish this feat. These technologies are classified as ElectroActive Polymers and details of their actuation mechanisms are described elsewhere 5,6,7. If we are to call a human-made actuator an artificial muscle, we must detail precisely the tasks that uniquely define what muscles do. The development of the appropriate tests is an ongoing challenge, because we are still discovering how muscles work in animals. In previous papers 8,9 we described a framework that serves as the basis for comparison and as a guideline for the evaluation of muscle-like properties in human-made actuators. This framework emphasizes the importance of modifying, enhancing or abstracting the functionality of nature instead of copying it directly. When comparing actuators, it is important to distinguish between potential and realized performance. Most studies on biological as well as human-made actuators focus on a single variable, e.g. force production or strain, which is tested under idealized conditions. Such experiments may reveal important information about the mechanism of actuation or material properties, however they do not take into account the complex interaction between variables when the actuator operates within a system. At best those experiments give information about the performance limitations. To determine the realized performance, the actuators should be tested under conditions that mimic the real life situation within the animal or application. Such tests will take into account explicitly the trade offs between different parameters and the influence of external factors such as the support and control system. On biological muscles such tests are performed by using the workloop technique. 10 This technique has revealed the wonderful and rich behavior of muscle that could never have been estimated from more basic tests. * kenneth@socrates.berkeley.edu; phone ; fax ; University of California at Berkeley, Department of Integrative Biology VLSB 3060, Berkeley CA Smart Structures and Materials 2001: Electroactive Polymer Actuators and Devices, Yoseph Bar-Cohen, Editor, Proceedings of SPIE Vol (2001) 2001 SPIE X/01/$

2 The goal of the study presented in this paper was to test both the potential and realized performance of three different actuators based on EAP technology. We evaluated the muscle like properties the silicone and acrylic dielectric elastomers developed and fabricated at SRI International and the high-energy electron-irradiated co-polymers (P(VDF-TrFE)) developed and fabricated at the MRL laboratory at Penn State University. The principle of actuation is different in these particular EAP materials. In the dielectric elastomers strain is induced through Maxwell stresses caused by the application of an electric field 5, whereas the P(VDF-TrFE)) actuators use electrostriction for actuation 6. The tests were performed on integrated actuators that included a support system. All actuators were tested using the same experimental setup. To keep the tests tractable, we focused on three important parameters, strain, maximal force production and maximal power output. The results from the tests were compared to the performance space of biological muscles. 2. METHODS To study if ElectroActive Polymer s (EAP) can be considered artificial muscles, we have made a direct comparison between the contractile properties of EAP s and biological muscle. We have measured the functional workspace of EAP actuators, using the same setup and techniques (i.e. workloop technique 10 ) that is used to test biological muscle Actuator designs The actuators were very similar in design. They basically consisted of thin films of EAP material that were coated on both sides with compliant electrode material (Fig. 1). The thin films were clamped on two sides by a lightweight support system that was used to mount the actuators in the experimental apparatus. W Figure 1. Schematic design of the actuators we tested. The EAP film (transparent area) was coated on both sides with a compliant electrode (black area). The film was clamped on both sides by a support system (gray beams) to which Kevlar threads (Edmund Scientific) were connected for mounting in the experimental device. L Acrylic dielectric elastomer The acrylic actuators we tested had a double layer of acrylic dielectric elastomer film (3 M s VHB 4910 acrylic), coated on both sides with compliant carbon electrode material. The acrylic film was glued to a lightweight wooden support system. The mass of the entire actuator (EAP film, support system plus electrodes) was 2630 mg; whereas the mass of the active part (EAP film plus carbon electrodes) was 26.2 mg. At a 1.5 N pre-tension, the dimensions of the active part of the actuator (l x w x h) were 21 x 18 x 0.07 mm. The acrylic actuators were fabricated at SRI international and had previously served in the autonomous robot hexapod FLEX 11. Silicone dielectric elastomer The silicon actuators were made out of a single layer of silicone dielectric elastomer film (NuSil Technology s CF ). The film was coated on both sides with compliant electrode material (conductive carbon grease; Chemtronics Circuit works CW7200). The silicone film was glued to a lightweight wooden support system. The mass of the entire actuator (EAP film, support system plus electrodes) was 780 mg; the mass of the active part (EAP film plus carbon electrodes) was 159 mg. At a 1 N pre-tension, the dimensions of the active part of the actuator (l x w x h) were 22.4 x 54 x 0.07 mm. The silicone actuators were fabricated at SRI international Proc. SPIE Vol. 4329

3 High-energy electron-irradiated co-polymers (P(VDF-TrFE)) We tested P(VDF-TrFE) actuators that were fabricated at the MRL laboratory at Penn State University. The actuators were made from a single layer of stretched P(VDF-TrFE) 68/32 that was irradiated with 70 Mrad at 100 C, using 1,2 MeV. The film was coated on both sides with golden electrodes. The details of the treatment and fabrication process are described elsewhere 13. The ends of the films were glued (5 minute Epoxy) to a lightweight plastic support system. It is known that the stretched P(VDF-TrFE) material has anisotropic strain behavior 13 (i.e. when a voltage is applied to the material it lengthens in the direction in which it was stretched during the fabrication process and it shortens in the perpendicular direction). For this study, tests were only performed in the direction in which the actuator shortens in response to stimulation. The mass of the entire actuator (EAP film, support system plus electrodes) was 140 mg; the mass of the active part (EAP film plus carbon electrodes) was 16.1 mg. At a 0.25 N pre-tension, the dimensions of the active part of the actuator (l x w x h) were 12 x 10 x mm Description of experimental apparatus The experimental setup consisted of a muscle lever system that can simultaneously record position and force (Aurora Scientific, model 305B: motion range: 0-20 mm; length resolution 1 µm; force range: g; force resolution 1mN;). One side of the EAP actuator was connected to the muscle lever, whereas the other side was rigidly fixed with a clamp (Fig. 2). The EAP material was activated using an electrical circuit that generated square wave high voltage (0-5 kv) pulses of varying duration ( ms). A personal computer controlled the length and timing of the stimulation to the EAP actuator. All data was stored on a computer using a data acquisition board (National Instruments, NB-MIO-16) and software (LabVIEW, version 2.2). With this setup, we determined some of the parameters that define the functional workspace of the actuator including maximum stress and strain and maximum work and power production. Force length Figure 2. Picture of the acrylic dielectric elastomer actuator. The black arrow indicates the length changes that were imposed on the actuator by the muscle lever. Force was recorded at the left tip of the actuator Experimental tests To characterize the mechanical performance of the actuators, we performed three different experimental tests. First, the force and strain capabilities of the actuators were determined. Subsequently, we examined how the actuators used their capacity when they performed work on the environment. The maximal free strain was determined by allowing the actuator to contract against a small finite load (isotonic experiment). Due to the specifications of our experimental setup, these experiments can only be performed on actuators that generate a contraction force in response to a stimulus (i.e. biological muscle). Therefore, this experiment was only performed on the P(VDF-TrFE) actuator. During these experiments the voltage applied to the actuator was systematically increased from 0 to 3kV. The maximal isometric stress (also referred to as blocked stress 7 ) of the actuators was determined in an experiment in which the actuator was kept at a fixed length and was activated with a high voltage stimulus. Isometric force was defined as the maximal change (increase or decrease) in force that could be attained with an applied stimulus. Isometric stress was calculated by normalizing the force to the area perpendicular to the force direction. During the experiments, voltage was systematically increased until the force of the actuator peaked or the limits of the electronic circuit were reached. Proc. SPIE Vol

4 Additionally, the bandwidth of isometric force production was studied by repeatedly stimulating the actuators at frequencies ranging from 2 to 60 Hz. The work and power output of the actuators was determined with the work loop technique 10. In these experiments, sinusoidal length oscillations were imposed on the actuator and the force response was recorded. The actuator was stimulated at a phase of the oscillatory cycle such that the work output of the actuator was maximized. The duty cycle for stimulation was fixed at 0.5, which means that the actuator was activated for 50% of the cycle time. For each actuator the imposed strain was systematically varied to determine the strain that yielded maximum work output. The optimal stimulation phase and strain amplitude was determined for workloops performed at a cycle frequency of 2 Hz. Once the optimal parameter values were found, the experiments were repeated at higher cycle frequencies (2-25 Hz) to determine the maximum power output. Each trial consisted of three sequential workloops; the second workloop was used to calculate work and power output. Work and power were normalized to the mass of the electro active film and electrodes (excluding the mass of the support system). 3. RESULTS An important distinction between the results presented here and data previously presented on the EAP materials 5,6,13 is that our results reflect the performance of the integrated system (e.g. EAP material + support system + electronic driver circuit). As a consequence the potential of the material is realized to varying degrees. Nevertheless, the test results give important information for improvements in devices to be made in the future Actuation: contract or extend? The dielectric elastomers responded to a high voltage stimulus with an increase in their length. In contrast, the P(VDF-TrFE) actuator shortened when it was activated. Actuation in the dielectric elastomers is caused by Maxwell stress that results from the applied electrical field 5. For the P(VDF-TrFE) actuators, actuation is the result of a high electrostrictive response 6. In contrast to natural muscle, the dielectric elastomers relax when they are stimulated. Hence, to get muscle-like contractions, the control of those actuators has to be opposite to that of natural muscle. In other words, to generate force, the stimulation to the actuator has to be turned off Strain For the P(VDF-TrFE) actuator in the unstretched direction we found that unloaded strain increased linearly with the applied voltage. A maximal strain of 1.2 % was attained at a voltage of 3 kv (table 1), which amounts to approximately one third of the strain values that have been measured in the stretched direction 6. The strain values in the unstretched direction found in this study correspond to previously measured values 13. The strains measured for the P(VDF-TrFE) actuator are at the low end of the spectrum of biological muscles, whereas the strain values reported for dielectric elastomers 5 are at the high end of the biological spectrum. Table 1. Peak values found in experimental tests, numbers for free strain are obtained from literature 5,6,13. Data for biological muscle is obtained from a previous review 9,14. Actuator Cross sectional area (mm 2 ) Biological muscle (9) VHB 4910 acrylic CF silicone P(VDF-TrFE) unstretched Preload (g) Action upon stimulation contraction extension extension contraction Strain (%) (5) 63 (5) 1.2 Isometric stress (MPa) Max Work output (J/kg) Max Power output (W/kg) Frequency (Hz) at max Power Proc. SPIE Vol. 4329

5 3.3. Isometric stress Maximal isometric stress of the dielectric elastomers and the P(VDF-TrFE) material in the stretched direction were all within the range found for biological muscle (Table 1). The maximal isometric stress of the P(VDF-TrFE) material in the unstretched direction is almost two times larger compared to values found in biological muscle (Table 1). For all actuators, the isometric stress increased linearly when the applied voltage was increased from zero to several kilovolts. This gives the actuators good control capability, because the force of the actuators can be controlled over a wide range of input voltages. All actuators responded to the applied voltage with a rapid change (decrease or increase) in force (Fig. 3). When the stimulus was taken away the force returned to the value set by the preload. The rate of return was fairly rapid in the dielectric elastomers, but in the P(VDF-TrFE) actuators it took almost 200 (ms) to return to the preload force (Fig. 3). As a consequence the bandwidth for isometric force production was much smaller in the P(VDF-TrFE) actuators compared to the dielectric elastomers. The P(VDF-TrFE) actuators were able to generate at least 90% of the maximal isometric force up to a frequency of 8 Hz, beyond that frequency the isometric force declined steeply. In the dielectric elastomers the frequency giving 90% force equaled 15 Hz for the acrylic material and 20 Hz for the silicone material. These low bandwidths are probably caused by limits in the electronic system and electrode conductivity that increases the time to charge and discharge the actuators. Figure 3. Maximal isometric contractions of the tested actuators. Note that the scale of the force axis is different for the individual actuators Work and Power Output To generate work and power, the actuators need to overcome visco-elastic losses in the actuators. Large energy losses could seriously limit the performance of any actuator. Figure 4 shows the results of workloops performed on the actuators when they were not stimulated. The passive workloops were moved in a clockwise direction, which means energy in the actuators is lost. The visco-elastic losses in the acrylic dielectric elastomers were substantial. Work absorbed by the acrylic actuator increased from 12 to 23 J/kg when the oscillation frequency was increased from 2 to 10 Hz. In comparison, the silicone dielectric elastomers and the P(VDF-TrFE) actuator acted more spring-like, having negligible losses during the passive workloop. Proc. SPIE Vol

6 Figure 4. Passive workloops of the tested actuators at an oscillatory frequency of 2 Hz. All three actuators show clockwise workloops, i.e. energy is absorbed by the actuators. The amount of energy absorbed is represented by the area within the loop. Note that different scales are used for different actuators. To have the dielectric elastomers generate positive work, the actuators needed to be stimulated during the lengthening phase of the workloop. This resulted in a small force during lengthening and a larger force during shortening and hence the generation of work. Figure 5 shows the active workloops that maximized work output. For all actuators, work output was largest at 2 Hz and declined with increasing frequency of oscillation. For the dielectric elastomers, maximum work output was attained at a strain of 5 %. Optimal strain for the P(VDF-TrFE) actuator was much smaller (0.4 %). Maximum massspecific work output ranged from 0.25 J/kg for the P(VDF-TrFE) actuator to 13 J /kg for the acrylic dielectric elastomer actuator (Table 1). These values are within the range of values found for biological muscle (Table 1). Despite the large visco-elastic losses the acrylic actuator still generated the largest work output of all three actuators. The small work output of the P(VDF-TrFE) actuator was caused by the small strains this actuator generates. The bandwidth for work production was limited. At oscillation frequencies larger than 8 Hz the P(VDF-TrFE) actuator and the acrylic dielectric elastomer were unable to generate positive work. For the silicone actuator, this limit was obtained at an oscillation frequency of 15 Hz. Mass-specific power output of our actuators was at the lower end of the spectrum of biological muscle (Table 1). Massspecific power output ranged from 0.5 W/kg for the P(VDF-TrFE) actuator to 35 W/kg for the acrylic dielectric elastomer. Maximum power output was limited to low oscillation frequencies. The silicone dielectric elastomer generated maximum mass-specific power of 20 W/kg at an oscillation frequency of 10 Hz, whereas maximum power output of the P(VDF-TrFE) actuator was obtained at an oscillation frequency of only 2 Hz. 12 Proc. SPIE Vol. 4329

7 Figure 5. Active workloops for the tested actuators at an oscillatory frequency of 2 Hz. All three actuators show counterclockwise workloops, i.e. energy is generated by the actuators. The amount of work produced is represented by the area within the loop. Note that different scales are used for different actuators. 4. DISCUSSION The aim of the present study was to determine whether the performance of three different EAPs fall within the functional space of natural muscle, and thereby meet the criteria for being termed an artificial muscle. We attained this goal by making a direct comparison of the EAP actuators with biological muscle. Biological muscles come in a wide range of varieties and they occupy a broad functional space. Our results show that EAP actuators fit the performance space of natural muscle to varying degrees. Specifically, the dielectric elastomer generated forces that were well within the range reported for muscle and their ability to produce work was at the lower end of muscle s capacity. The capacity for force generation in the P(VDF- TrFE) actuators was also within the range observed for muscle and sometimes even exceeded that range. However, their ability to produce work was seriously limited by their small strain capacity. Due to limitations in our experimental setup and a less than optimal actuator designs we where not able to explore the entire workspace of the actuators. Therefore, the results presented in this paper should be considered as preliminary. The bandwidth for force and work production of the actuators was limited to low frequencies (<20 Hz). In contrast, strain tests performed on the materials under ideal conditions revealed much larger bandwidths 5,13. For instance, the P(VDF-TrFE) material is capable of attaining 80% of its nominal strain at a stimulation frequency of 1 khz 13, whereas the half strain bandwidth of the VHB 4910 acrylic material lies between 30 and 40 Hz and the CF can reach full strain at stimulation frequencies up to 170 Hz 5. The reasons for the narrow bandwidth found in our study are twofold. First, the rate of force changes in our isometric tests (Fig. 3) indicates that our electronic circuit isn t powerful enough to quickly charge and discharge the actuators. This problem is most severe for the P(VDF-TrFE) actuators which have a capacitance that is 4 times larger than that of the silicone elastomers and almost 10 times larger than that of the acrylic elastomers. We expect this problem to be solved when our new stimulator comes into service. Second and a more fundamental problem, the actuators all suffered from a poor ratio between the masses of the active material relative to the mass of the integrated actuator. This Proc. SPIE Vol

8 problem was most severe for the acrylic actuator, which had a ratio of 1%. The silicone actuator and the P(VDF-TrFE) actuators were less affected but still suffered from unfavorable ratios of 20 % and 11% respectively. These ratios can explain the relatively poor mass-specific power output of the actuators. The main cause of the unfavorable ratios is the relatively large mass of the support systems needed to keep the EAP films in tension. Smart actuator designs making use of fiber reinforced composite techniques 15 may solve this problem by laying down stiff lightweight fibers in the EAP material. We may turn to biological muscle for inspiration on how to design such constructions. In biological muscle the contractile proteins are embedded in a connective tissue matrix that consists mainly of layers of collagen laid down in a specific pattern 16. The emerging view in muscle biology is that this connective tissue matrix is an integrated part of the actuator, not just giving support but actually playing an important role in guiding the forces generated in the actuator to the outside world and allowing the actuator to perform work on the environment 17,18. One behavior that is clearly not muscle like is the human-made actuators increase in length when they are stimulated. In fact, only the P(VDF-TrFE) actuator was capable of muscle-like contraction. The implications of the need for a pre-tension to allow actuation are important for robotic applications. To get muscle-like contractions, a single actuator needs to be stimulated continuously to keep it in a relaxed (elongated, low tension) state and then the stimulus needs to be removed if one wants to shorten the EAP actuator and exert force on the environment. Alternatively, in a skeletal system one could place EAP actuators on opposite sites of a joint and use reverse control to flex or extend the joint, i.e. stimulate the actuator that is on the opposite site of the direction in which you want to move the joint. Neither option is very attractive. The first option will not be very energy efficient because a voltage needs to be applied to the actuator at all times. The second option, which was used in the FLEX robot 11, will lead to very stiff joint system, which can be a disadvantage if the robot needs to interact with the environment. Potentially, these problems can be circumvented by actuator designs that include springs to keep the EAP film in tension or through using lever systems that transform lengthening in contraction. Biology may be able to provide inspiration for such designs. For example, animals that have a hydraulic skeleton (like sea anemones) have muscular skeletons that are reinforced by stiff helical fibers. It has been shown that depending on the angle that the stiff fibers make with the longitudinal axis of the skeleton the animal either shortens or lengthens when it is pressurized 19. It is conceivable that reinforcing the EAP materials with stiff fibers that are laid down in the appropriate configuration may help to direct the action of the EAP film from lengthening to shortening. Interestingly, this is exactly the mechanism that is employed by the McKibben artificial muscles 4. To conclude, we have shown that EAPs clearly possess some muscle-like properties. However, many questions and tests remain. How scalable are EAPs? What is the energy efficiency when operating during activities such as locomotion? What types of controls and devices are necessary to allow the most effective transmission of force and energy? We have only just begun the characterization and biological inspiration of EAPs. We must expand the exploration of their potential workspace beyond measurements of power output to include other muscle-like functions such as energy storage and return and stabilization. We need to consider the use of multiple, lightweight EAPs operating at a single joint and spanning more than one joint. ACKNOWLEDGEMENTS We thank R. Kornbluh, R. Pelrine, J Eckerle, S. Oh and S. V. Shastri from SRI for supplying the dielectric elastomers and the expertise to use them. Development of the acrylic EAP actuators was done for the U.S. Naval Explosive Ordinance Technology Division supported by the Office of Naval Research under contract N C Development of the silicone EAP actuators was done for DARPA/TTO under contract DABT63-98-C We thank ZY, Cheng and Q. Zhang from Penn State University for supplying the P(VDF-TrFE) actuators and the expertise to use them. 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