Flexible, Low-mass Robotic Arm Actuated by Electroactive Polymers and Operated Equivalently to Human Arm and Hand

Actuation devices are used for many space applications and there is increasing need to reduce their size, mass, cost and power consumption. To address this need, JPL is developing electroactive polymers (EAP) with emphasis on two EAP categories that induce large bending and longitudinal actuation strains. Comparison between EAP and the widely used transducing actuators shows that, while lagging in force delivering capability, these materials are superior in mass, power consumption and actuation strain levels. This study is concentrating on the development of effective EAPs and the enabling of mechanisms that employ their unique characteristics. Several EAP driven mechanisms that emulate human hand were developed including a gripper, manipulator arm and surface wiper. The manipulator arm is made of a composite rod that is lifted by a longitudinal rope actuator and has an end-effector gripper with bending EAP fingers allowing to grab and hold such objects as rocks. The EAP surface wiper operates like a human finger and can be used to remove dust from windows and solar cells. These EAP driven devices take advantage of the large displacement capability with less concern for their limited force actuation capability. INTRODUCTION Efficient miniature actuators that are light, compact and driven by low power are needed to support telerobotic requirements of future NASA missions. Generally, actuators are used to operate telerobotic devices that include robotic arms, rovers, etc. Other space applications include release mechanisms, antenna and instrument deployment, positioning devices, aperture opening and closing devices, and real-time compensation for thermal expansion in space structures, etc. Increasingly, there are 1 Jet Propulsion Laboratory (JPL), Caltech., Pasadena, CA, [email protected] 2 Artificial Muscles Research Institute, UNM, Albuquerque, NM 3 Composites and Polymers Branch, NASA LaRC, Hampton, VA requirements to reduce the size, mass, and power consumption of actuation devices, and lower their cost. Electroceramics (piezoelectric and electrostrictive) offer effective, compact, actuation materials and they can be incorporated into ultrasonic motors, inchworms, translators and manipulators. In contrast to electroceramics, electroactive polymers (EAP) are emerging as new actuation materials [1] with displacement capabilities that cannot be matched by the striction-limited and rigid ceramics. Table 1 shows a comparison between the capability of EAP materials, electroactive ceramics (EAC) and shape memory alloys (SMA). As shown in Table 1, EAPs are lighter and their striction capability can be as high as two orders of magnitude more than EACs. Further, their response speed is significantly higher than SMAs. The authors’ current study is directed towards taking advantage of these polymers’ resilience and the ability to engineer their properties. The mass producibility of polymers and the fact that EAP do not require poling (in contrast to piezoelectric materials) help to produce them at low cost. EAPs can be easily formed in any various shapes and can be used to build micro-electro-mechanical systems (MEMS). They can be designed to emulate the operation of biological muscles [2-4] with unique characteristics of high toughness, large actuation strain constant and inherent vibration damping. TABLE 1: Comparison of the properties of EAP, SMA and EAC Property Electroactive polymers (EAP) Shape memory alloys (SMA) Electroactive Ceramics (EAC) Actuation strain >10% <8% short fatigue life 0.1 0.3 % Force (MPa) 0.1 3 about 700 30-40 Reaction speed μsec to sec sec to min μsec to sec Density 12.5 g/cc 5 6 g/cc 6-8 g/cc Drive voltage 2-7V/10-100V/μm NA 50 800 V Consumed Power m-watts Watts watts Fracture toughness resilient, elastic Elastic fragile The development of muscle actuators is involved with an interdisciplinary effort using expertise in materials science, chemistry, electronics, and robotics. At the initial phase of the authors' study efforts were made to identify electroactive polymers that offer large actuation strains. Two categories of EAPs were identified including (a) bending actuators Perfluorinated Ion-exchange membrane platinum (IEMP) composites, and (b) longitudinal actuators employing electrostatically activated EAPs. These two EAP actuators offer the capability to bend or stretch/extend, which essentially emulate the operation of biological muscles and limbs. IONOMERS AS BENDING EAP ACTUATORS The bending EAP actuator is composed of a perfluorinated ion exchange membrane, with chemically deposited platinum electrodes on its both sides. The formed muscle actuator is 0.18-mm in thickness and formed in 1x0.125-inch strips weighing 0.1-g. To maintain the actuation capability of IEMP, it needs to be kept moist continuously. Efforts are currently being made to overcome this limitation and success was observed when using thick platinum electrodes where the voltage is limited to <2-V rather than the levels of 3-5 volts. Using such electrodes an IEMP film was demonstrated to actuate continuously for more than one million cycles. Another alternative that is considered is the use of encapsulation methods to form quasi-skin to protect the ionic constituents of IEMP composite films. The structure and properties of the IEMP have been the subject of numerous investigations (see for example [5]). One of the interesting properties of this material is its ability to absorb large amounts of polar solvents, i.e. water. In order to chemically electrode IEMPs, platinum (Pt) metal ions are dispersed throughout the hydrophilic regions of the polymer, and are subsequently reduced to the corresponding zero valent metal atoms. This results in the formation of a dendritic type electrode. When equilibrated with aqueous solutions these membranes are swollen and they contain a certain amount of water. Swelling equilibrium results from the balance between the elastic forces of the polymeric matrix and the water affinity to the fixed ion-exchanging sites and the moving counter ions. The water content depends on the hydrophilic properties of the ionic species inside the membrane and also on the electrolyte concentration of the external solution. To enhance the force actuation capability of IEMPs, techniques of producing thicker films as well as modification of the ionomer processing were investigated. Success was observed in processing the material to induce more than two times the strain with a higher response consistency. To better understand the actuation mechanism in ionomers the phenomena is studied and modeled. Also, alternative ionomer actuators are being searched. Figure 1: A view of a surface wiper with a simulated window, where an ionomer is bending back and forth next to a glass plate (the slanted double image is a shadow). When an external voltage is applied on an IEMP composite film, it bends towards the anode at a level that increases with the voltage (reaching saturation at about 6 or 7-V). Under AC voltage, the film undergoes swinging movement and the displacement level depends not only on the voltage magnitude but also on the frequency. Lower frequencies (down to 0.1 or 0.01 Hz) lead to higher displacement (approaching 1 inch). The movement of the muscle is fully controllable by the applied electrical source but it is strongly affected by the water content that serves as an ion transport medium. The operation of the ionomer as a bending actuator is demonstrated in a configuration of a window surface wiper in Figure 1. As can be seen in this Figure, an ionomer film is attached to the surface of a glass plate and it is actuated left or right as desired by changing the polarity of the drive voltage. This ionomer film was driven by 2.5V and 20-mW. LONGITUDINAL ELECTROSTATIC POLYMER ACTUATORS Polymers with low elastic stiffness and high dielectric constant can be used to induce large actuation strain by subjecting the material to an electrostatic field. These characteristics of polymers allow producing longitudinal actuators that operate similar to biological muscles. The governing principle is the response of the material to Coulomb forces between charged particles. Traditional electrostatic actuators are fabricated as a capacitor with parallel electrodes with a thin air gap between them. One of the major disadvantages of this type of actuators is their relatively low breakdown voltage. The authors adopted the approach that was reported in reference [4], where a longitudinal electrostatic actuator was made of dielectric elastomer film coated with carbon electrodes. The force (stress) that is exerted normally on such a film with compliant electrodes is as follows: Where: P is the normal stress, ε0 is the permittivity of vacuum and ε is the relative permittivity (dielectric constant) of the material, E is the electric field across the thickness of the film, V is the voltage applied across the film and t is the thickness of the film. Examining the equation above, it is easy to notice that the force magnitude is twice as large as that for the case of rigid parallel electrodes. To obtain the thickness strain the force needs to be divided by the elastic modulus of the film. Use of polymers with high dielectric constants and application of high electric fields allow inducing large forces and strains. To obtain the required electric field levels there is a need for either to use high voltage and/or employ thin films. For elastomers with low elastic modulus, it is reasonable to assume a Poisson’s ratio of 0.5. This means that the volume of the polymer is kept constant while the film is deformed under the applied field. As a result, the film is squeezed in the thickness direction causing expansion in the transverse plane. For a pair of electrodes with circular shape, the diameter and thickness changes can be determined using the following relation, where the second order components are neglected. Where: D0 is the original diameter of the electrodes and ∆D is the resultant diameter change, t0 is the original thickness and the ∆t is its change under electric activation. To produce a longitudinal actuator with large actuation force, a stack of two silicone layers (Dow Corning Sylgard 186) was used with carbon electrodes on both sides of P E V t = = εε εε 0 2