Electroactive Polymers in Space: Design Considerations and Possible Applications

Maarja Kruusmaa(1), Paolo Fiorini(2)

(1)Intelligent Materials and Systems Laboratory Tartu University Institute of Technology

Nooruse 1, 50411 Tartu, Estonia Email: maarja.kruusmaa@ut.ee

(2)Department of Computer Science, University of Verona Ca' Vignal 2 - Strada Le Grazie 15, 37134 Verona, Italy

Email: paolo.fiorini@univr.it ABSTRACT This paper gives an overview of the technology of Electroactive Polymer (EAP) materials. We focus specifically on ionic conductive polymer materials (IPMC) as a rapidly maturing technology with first commercial applications available, which have also been considered for space applications for almost a decade. We briefly describe their properties and their working principle. Next, we describe IPMC materials working as sensors and actuators and their potential of use in biomimetic devices. In the following we briefly discuss the challenges of IPMC sensor and actuator control. Finally, we envision some possible applications of these materials to space systems. INTRODUCTION

The robotic applications developed and exploited so far use almost exclusively electromechanical actuators. The technology of electromechanical devices is very well established, and has thorough theoretical background, control methods and reliable applications demonstrated during several decades. This technology has obviously reached its maturity and therefore its limits have also become visible. Devices using this technology need rigid links to connect the rotating joints, gears and bearings and they are therefore unavoidably complex, rigid and noisy. At the current state of development it is hard to reduce the size and energy consumption of these devices.

This paper gives an insight to another, alternative method, of actuating robotic devices, by means of shape-changing materials. Electroactive polymers (EAP) change their shape and size in response to an electric stimulus (see [1] for an overview). Compared to electromechanical devices, EAPs have many complimentary advantages. They are lightweight, soft and flexible, easy to miniaturize, and permit distributed actuation and sensing. The behaviour of the EAP materials in the electric field somewhat resembles the performance of biological muscles, therefore EAP materials are considered to be good candidates for building biomimetic devices [2]. However, compared to the technology of electromechanical devices, EAPs have many drawbacks typical to developing technologies such as low output force or small strain (depending on the material used), high energy consumption and lack of well-established control methods.

In the following, we give a short overview of EAP materials, their properties and the estimate of the maturity of their technology and then concentrate on ionic conductive electroactive polymers (IPMC) as a relatively mature technology in the EAP domain, which has been considered for space applications for more than a decade.

The technology of EAPs is often proposed as a promising alternative to overcome the drawbacks of bulky, noisy, rigid electromechanical devices. The idea we envision in this paper is, instead of opposing these two alternatives, to combine their complementary advantages into a new device. As an example, we propose a bio-inspired design concept of a flexible, compliant manipulator with distributed actuation and sensing. The manipulator has a DC-motor driven semi-rigid plastic backbone surrounded by the layer of IPMC artificial muscle bundles. The DC-motor driven backbone permits precise positioning of the manipulator, holds the manipulator in the steady position and guarantees a sufficient amount of output force and torque. At the same time the IPMC materials in a form of artificial “skin” and “muscles” provide softness and flexibility and distributed sensing and actuation. We surmise that in contact with an object this kind of manipulator is capable of sensing the object, and the distributed actuation would permit fine manipulation and grasping.

ELECTROACTIVE POLYMER MATERIALS

There are many types of electroactive polymers with various properties and with various types of reaction to electrical stimuli. By and large, EAP materials can be divided into few major groups.

Electronic polymers are driven by electric field or Coulomb forces. Materials like dielectric EAPs, electrostictive elastomers and ferroelectric polymers belong into this group [3]. They shrink and expand when electric stimulation is applied. Electronic polymers can operate at room conditions, have a low response time, can hold strain under activation

In Proceedings of the 9th ESA Workshop on Advanced Space Technologies for Robotics and Automation'ASTRA 2006' ESTEC, Noordwijk, The Netherlands, November 28-30, 2006

and they are relatively strong. These polymers require high voltage (several kV) at very low current for operation. Electronic polymers have been used in robotic applications such as a biologically inspired hexapod robot [4] and a micro-robot mimicking annelid animals [5].

The electroactive properties of ionic polymers are caused by mobility or diffusion of ions. EAP materials of this group include carbon nanotubes, conductive polymers, ionic polymer gels and ionic polymer metal composites (IPMC). Ionic polymers bend when electric stimulation is applied. Opposite to electronic polymers they produce large displacement when stimulated and operate at low voltages. At the same time their response is relatively slow, they produce small actuation force and usually need a wet environment for operation. Because of the dynamic processes inside the materials they can not keep the strain but relax after a while to the initial configuration. Therefore the applications of ion conducting polymers are usually inspired by aquatic animals. For example, they mimic motion of the caudal fin [6], pectoral fins [7], a mollusk [8] or a tadpole [9].

Conductive polymers are able to displace large loads (around an order or magnitude larger than ionic polymers) with moderate displacements (around 2%) [10]. They can hold the strain without consuming energy. Like other types of EAPs, conductive polymers are considered to be possible implementations the areas of bionic devices, robotics and tactile interfaces [11]. The problems to overcome are mainly focused around increasing their displacement and long term stability.

This paper focuses on potential applications and challenges of ionic conducting polymer metal composites (IPMC), a configuration of ionic conducting polymer materials. These materials have been investigated and developed for space applications for almost a decade by NASA JPL and applications such as a dust wiper for a planetary rover have been proposed [12].

IONIC CONDUCTIVE POLYMER METAL COMPOSITES

Fig. 1 shows a stripe of IPMC material clamped between the contacts with the lower part moving freely. The photo in the middle shows the IPMC sheet in its initial configuration, with no electric stimulus applied [13]. When the electric field is applied between the contacts, then it causes bending of the material to the left or right, depending on the polarity of the electric field. These materials cannot hold the strain but after a while (usually few seconds) relax back to the initial configuration. As an example of material performance, an IPMC developed using the latest techniques exhibits a huge bending deformation (up to 180º) when activated by a low voltage signal (ca. 2 V). Current consumption depends on the required force output (typically 100-200 mA). The material is flexible and bends without fatigue up to 107 cycles when adequate measures have been taken to control the solvent inside the membrane, e.g., by coating. Forces can be generated to allow lifting of 10 times their own weight [14]. A sheet of IPMC material requires some care in manufacturing since it will require the integration of stripes of EAP and conductive materials, to ensure proper activation and control.

Fig. 1. An IMPC sheet in a bent configuration with the opposite driving voltage polarity (A and C) and an initial configuration with no electric stimulus applied (B).

Ionic polymer materials are made of a highly porous ion fluorinated polymer, like Nafion®, Flemion®, Teflon®

and their modifications, filled with ionic conductive liquid. During material fabrication the proton connected to the terminal group (the chemical unit in the end of a polymer chain), is replaced with a metal ionic cat-ion (Na+, Li+). These cat-ions will dissociate in water, so that terminal groups will have a negative charge and at the same time there will be an excess of free cat-ions in the material (see Fig. 2, to the left). A sheet from this kind of material is then covered with a metal coating, usually platinum or gold. The metal covered polymer is called ion-polymer metal composite (IPMC).

Since water molecules are dipoles, they orient themselves in electromagnetic field and get attached to the free metal cat-ions. An applied electric field causes an electric current and the cat-ions start to move to one side of the material causing expansion of the material on that side and contraction on the other side (Fig.2, in the middle). The bent conformation is an imbalanced situation. Water starts to diffuse in the opposite direction and the polymer sheet relaxes after some time (Fig. 2, to the right). These materials do not keep their position under direct current. At the same time, their action length is remarkable and they operate at low voltage (1.2 – 7V). At the same time they are not as strong as electric EAPs and require from dozens up to several hundreds of mA of current. The actuator performance of

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IPMCs depends on their morphology, as well as on other parameters such as membrane thickness, electrodes surface conductivity, solvent type and anion doping. These parameters can be tuned during the manufacturing process [15, 16]. IPMC is therefore an engineering material that can be customized to application requirements.

Polymer Cation

Water Platinum

Water Cation

Polymer

Cation Water

Platinum

Fig. 2. The IPMC sheet in the initial configuration without and electric stimulus applied (to the left), bent configuration caused by ion migration and water swelling in an electric field (in the middle) and in the relaxed state caused by water diffusion (to the right). In addition to actuation properties, IPMC materials can also work as sensors. If the IPMC material is mechanically bent, then a voltage is generated between the surface electrodes due to the non-uniform concentration of ions in the membrane. The amplitude of the output signal is around 1mV to 2 mV and the effect is observed when the sheet is in motion, so the sensor works as an accelerometer. For that reason, IPMC sensors are investigated as vibration sensors for active noise damping [17-20]. An alternative way, reported very recently, is to use the change of the surface resistance to measure the bending of the actuator. [21]. It can be shown that resistance of the metal surface electrodes of the IPMC sheet changes during bending and the change of the resistance is highly correlated to the bending curvature. The change of the surface resistance is apparently not caused by the electroactive properties of the IPMC sheet (like in the case of vibration sensors) but to the properties of the metal surface electrode. The resistance of a thin metal coating increases or decreases if the metal layer is compressed or stretched out. This effect can be used to determine the position of the IPMC sheet and a design has been proposed that permits the IPMC sheet to be used as a self-sensing actuator [22]. It can be demonstrated that the output signal of such a sensor is at least an order of magnitude stronger (10mV-20mV) than of the conventional vibration sensor. with a very good signal to noise ratio. Unlike the conventional vibration sensor this self-sensing actuator gives accurate information about the configuration of the sheet also when the sheet is not in motion. Therefore it can be used as a position sensor but also as an accelerometer if the sensor data is sampled over time [23]. MODELING AND CONTROL OF IPMC SENSORS AND ACTUATORS Several models have been proposed to study the behavior of IPMC actuators. The best known of them describe the IPMC actuator with discrete electrical models [24, 25]. Fig. 3 represents an improvement of such a model that also takes into account the surface resistance change of the IPMC material during bending. It consists of a series of connected resistors Ra and Rb, indicating the surface electrodes along the IPMC. Between the resistors representing the two surfaces there are single-unit cells consisting of resistors Rx representing the resistance of the polymer gel layer as an electric conductor and a capacitor C in conjunction with resistor Rc representing the characteristics of the exponential step response curve of the current. This combination forms a two-dimensional linear approximate model of the IPMC.

Fig. 3. The equivalent circuit of an IPMC actuator The electromechanical models are addressed in several studies, where the EAP actuator is modeled in a cantilever beam configuration. So far the models are usually verified at small bending angles by measuring the tip force against a load cell [26] or a constant curvature is presumed [27]. Usually, at large angles the constant curvature presumption does not apply since the bending moment caused by current varies along the surface of the beam, decreasing towards the tip of the beam [28].

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The behaviour of a cantilever beam IPMC actuator subject to axial forces under large displacement is a challenging task to be modeled and understood. An accurate model would considerably facilitate the control of the IPMC sheet. Beside the accurate models for control, there are several other aspects to take into account that make feedback control of IPMC actuators a challenging task. First of all, there is no functional relationship between the applied voltage and output force. The force depends on the previously applied voltages and the material has a significant hysteresis. Due to the dynamic processes inside the polymer sheet, the IPMC material is able to hold the strain only for a few seconds. After that, the hydrostatic forces inside the porous material filled with ionic liquid cause the material to relax and turn back to an initial equilibrium state. Also, an IPMC actuator exhibits shape hysteresis. This means that its relaxed position changes during its operation. Under constant voltage, the output force increases at first but after a while it starts to decrease exponentially. The voltage would have to be increased continually to keep the force constant. When the voltage is decreased, the direction of the output force is reversed. There are several other factors that make the control of IPMC materials complicated and they are mostly caused by the immaturity of this new technology [28]. Generally, the properties of the materials are not uniform and change over time. Most of IPMC materials use water as a solvent of the ionic liquid. Therefore the actuator works only if it is humid. A dried actuator has to be especially processed in order to make it work again. While working in air, the hydration level of the material can change and this in turn induces changes in its behaviour. However, several solutions are considered to overcome the problem of muscle fatigue due to dehydration. For example, water can be encapsulated within the membrane by a covering material [29]. However the covering material will increase the stiffness and thereby decreases efficiency. Applying more than 1.23V to the strip causes water electrolysis which can lead to problems. First of all, electrolysis consumes energy. Also, if the strip operates in air, the area of electrolysis dries very quickly. If the strip operates in a closed environment, the emission of oxygen and hydrogen will pressurize the environment. The alternative solution would be to use other ionic solvent instead of water [30]. The metal coating of an IPMC strip has relatively high resistance because it is very thin and cracked. For that reason, the strip bends more at the contacts and less towards the tip. There is a trade-off between conductivity and elasticity. If the metal layer is too thick the strip will no longer bend. Good results have been obtained when controlling the IPMC tip force against the load cell at small tip displacements [31]. Also, attempts are made to control the system of IPMC actuators with a feedback camera image [32]. Since the IPMC materials have both sensor and actuator properties, an obvious approach would be to use the combination of them. Some encouraging results in this direction consider the control of an integrated IPMC sensor/actuator using H∞ control and the construction of IPMC sensor system [33]. A position feedback control system with the developed sensor system shows that good responses can be obtained for a step or sinusoidal reference signals. The main difficulties with developing the integrated sensor/actuator is that the IPMC sheet is essentially an accelerometer. It generates voltage on the surfaces when it is bent. Also, the signal of the sensor is very week (1mV-2mV)while the actuator is at the same time driven with 2V – 4V input signals. The equivalent circuit in Fig. 3 shows that the IPMC material is essentially an infinite lossy transmission line. Therefore, the signals, traveling back and forth along the material are considerably distorted and delayed. It is difficult to distinguish the sensor signal from the distorted and delayed driving signal. An alternative approach would be to use the surface resistance change of the IPMC material as a feedback signal as it is proposed in [23]. In this way the IPMC sensor also works as a position sensor, the sensor signal is at least an order of magnitude stronger and is not distorted by the dynamics of the transmission line. USING IPMC TECHNOLOGY FOR SPACE APPLICATIONS From the overview given above it is obvious that the technology of the IPMC materials is a fascinating alternative to the conventional approaches in robotics but at the same time faces several difficult basic problems that must be solved before reliable applications can emerge. These problems are very typical of emerging technologies, and they are mainly concerned with the poor long-term stability, lack of accurate models and control methods of the basic materials. Compared to the electromechanical devices, the devices using IPMC materials have almost orthogonal advantages and disadvantages. Moving to macro scale applications is one of the priorities if the IPMC research, however, due to the energy consumption of these materials [34] and small output force it means overcoming several fundamental problems. On the other hand, these materials are much more suitable for miniaturization due to their mechanical simplicity than conventional electromechanical components. Another fascinating property of these materials is their low elastic modulus. This aspect makes them very different form conventional electromechanical devices with rigid links and rotational joints. Theoretically, a flexible sheet of an PMC material has infinitely many degrees of freedom and the real number of possible configurations depends on the material properties, configuration, and, most of all, the topology of the electric contacts on the surface. The precise feedback control of the materials is still an unsolved problem. However, the fact that the materials are soft and flexible compensates the inaccuracy of their control. Thus, the applications of the IPMC materials would offer a new approach to the design of compliant manipulators. IPMC materials as well as other types of EAP materials have been considered for space applications for a while, mainly as artificial muscles working in biomimetic devices [12]. The main problems to overcome, to be used in this very specific domain is, as mentioned earlier, the reliability and long-term stability of these materials as well as their behavior is harsh environments of space applications. For example, an active research focuses on using various types of ionic solvents and testing their suitability at very low temperatures [35]. Moreover, the sensor properties of IPMC materials would theoretically permit building IPMC devices with continuous sensing and actuation without additional components for feedback signals. Even if the control of these devices is inaccurate, they are soft and flexible, and capable of perceiving insurgence at any point of its surface.

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The technological progress is often viewed as a competition between alternative technologies, where one approach is overtaking another. However, the trends of technological development rather confirm that this is seldom the case. On the contrary, in the developmental phase new technologies often mature in synergy with the other, established ones. Having this observation in mind, we envision a design concept where the traditional technology of electromechanical devices is combined with electroactive materials to develop a soft and compliant multi-degrees of freedom manipulator. This device would hypothetically be as strong, reliable and easy to control as conventional robotic manipulators. At the same time it would have the softness and flexibility of the IPMC materials and as well the capability of perceiving contact with objects and surfaces. The kind of design we have in mind is a biomimetic robot arm combining a semi-rigid backbone driven my DC motors with IPMC artificial muscles and skin-like surface around it. The DC-motor driven backbone would permit accurate positioning of the manipulator and guarantee required output force. At the same time IPMC material layer around it would equip the device with sensorial capabilities, at the same time permitting flexible and continuous change of the manipulator surface and compliance. In a longer perspective, the PMC skin could be used as a haptic device to perceive the shape and texture of the object. We suggest that this kind of manipulator would be used in applications where delicate manipulation of objects is required (e.g. in probes and collecting samples). BIOMIMETIC ARM CONFIGURATIONS Considering the current state of the art of the IPMC materials, a design must take into consideration the limited force generation capabilities of the material before, which currently is of the order of 100mN. However, the microgravity environment of space helps to reduce this limitation. Initially, space applications of EAP materials should follow two separate paths, the first should be to demonstrate the survivability of the material in the space environment, and the second should focus on the design of applications that can take advantage of the specific combination of low gravity and surface actuation and sensing. Applications that could satisfy the above constraints could include the development of protective covering of robotic arms and astronaut suites as well, active gloves, transportation mechanisms, shape changing supports, and small probes for planetary rovers. The proposed systems take advantage of EAP materials of forming sheets that can deflect and form surface waves, on the application of an electric field. For example, protective coverage of astronauts and robot arms, would form a protective cushion to prevent damages to space suites. On Earth, protective functions are carried out by airbags of various types, but in space airbag action would be too violent and may cause other damages and problems. A protective sheep of EAP material would be capable of changing its mechanical impedance upon sensing a contact with an external object, thus providing local stiffness and localized protection from a potential impact. Thus the conventional arm structure would provide the support and guidance to the flexible EAP actuator. Another problem, often mentioned by astronauts, is the lack of touch and grasp sensitivity. Mechanical solutions were propsed in the past, but EAP could represent a good solution. A haptic glove could be designed, which could cover the internal and the external part of the astronaut glove. The external part would act as a touch/force sensor, whereas the internal glove will mirror the external deflections for the astronaut’s hand. Haptics, in fact, could be a very good application domain for EAP materials, since they would provide distributed forces mimicking the contact with the objects. Extending this idea further, shape changing objects could provide very useful tool for space. They would consist of a “blob” of EAP material stripes, suitably connected, that could deform and assume one of a number of predetermined shapes. Such a device could provide support for tools and instruments, and would also be capable of exercising some kind of grasp to the object itself. Transportation systems would require careful design, because of the intrinsic limitation of EAP materials, however, their capability of producing sustainable surface waves, can be used to develop a system to move objects in one or two directions. This approach would greatly benefit from the microgravity environment, and would provide a very low volume and weight replacement to conventional conveyor belts. Finally, extending the ideas proposed in [12], one can consider a small octopus limb with an internal rigid structure consisting of, possibly, cable actuated links, and an external coverage of EAP material, which would provide sensing and manipulation support to the arm. The external EAP coverage would have some independent mobility from the internal rigid structure, thus performing some shape adjustment when necessary. This configuration could also include small fingers, that would be capable of entering small spaces between rock and on the ground and deploy chemical sensors and collecting samples. Clearly, the ideas proposed above face great theoretical and implementation challenges that need to be carefully addresses, such as: communication and power distribution to the distributed actuators and sensors, modelling of the interaction among EAP and conventional materials, mechanical design of attachment and support between the EAP components and the electromechanical structure, design and implementation of EAP bundles that can be actuated in a similar way to artificial muscles. However, steady progresses are made in each on these critical areas, and soon prototypes implementing the concept ideas described above will be made available.

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CONCLUSIONS In this paper we have briefly summarized the properties of electroactive materials, and focused on discussing a specific implementation, consisting of coating the polymer with a metal sheet in the ionic conductive electroactive polymers (IPMC) configuration. The challenges in using this technology are manifold, starting from the poor modeling of the actual chemical/physical process to the integration of the materials into large and more practical prototypes. However the possible benefits could include reduced weight and volume, the use of intrinsically safe materials with distributed sensing and actuation properties A possible approach to speed the development is to develop integrated devices, in which conventional electromechanical devices are coupled with EAP layers to take advantage of the material properties. Such a development will be highly relevant to space applications where safety is paramount and would allow an easier integration of robotic system in joint operations with astronauts. Similarly, planetary roves could benefit from the availability of new arms, with shapes like an elephant trunk or an octopus limb, which could allow for delicate manipulation of samples, and will require lower volume and mass budget than their electromechanical counterparts. In the paper we give a first indication of how such a device could be designed and propose some of the research steps necessary to develop such device. REFERENCES [1] Y. Bar-Cohen, Electroactive Polymer (EAP) Actuators as Artificial Muscles - Reality, Potential and Challenges

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