Various Robots Made from Piezoelectric Materials and Electroactive Polymers: A Review <s> Property overview <s> Electroelastomers (also called dielectric elastomer artificial muscles) have been shown to exhibit up to 380% electrically-driven strain. Their elastic energy density and power output compare favorably with natural muscles. A variety of actuator configurations have been developed for a variety of important applications including linear actuation, fluidic control, and flat panel speakers. In particular, spring rolls are compact, have a potentially high electroelastomer-to-structure weight ratio, and can be configured to actuate in several ways, including axial extension, bending, and as multiple degree-of-freedom actuators. They combine load bearing, actuation, and sensing functions. Simulation and robot fabrication have produced six-legged robots, using a spring roll as each of the robots' legs, that can run and clear obstacle. <s> BIB001 </s> Various Robots Made from Piezoelectric Materials and Electroactive Polymers: A Review <s> Property overview <s> Electroelastomers (electroactive elastomers) such as the 3M VHB 4910 acrylic adhesive films have exhibited up to 380% strain in area expansion at 5?6?kV when they are highly prestrained. By rolling highly prestrained electroelastomer films around a compression spring, we have demonstrated multifunctional electroelastomer rolls (MERs, or spring rolls) that combine load bearing, actuation, and sensing functions. We extended the design to two-degree-of-freedom (2-DOF) and 3-DOF spring rolls by patterning the electrodes to align radially on two and four circumferential spans of the rolls, respectively. Multiple-DOF spring rolls retain the linear actuation of 1-DOF spring rolls with additional bending actuation. Mathematical equations are derived to correlate the bending angle and lateral force of the rolls with the actuated stroke in one of the electroded spans. Two-DOF spring rolls with a 1.4?cm outside diameter, 6.8?cm axial length, and 11?g weight have been fabricated; these rolls have a 90? maximum actuation bending angle, 0.7?N maximum lateral force, and up to 15?N blocked axial force. Three-DOF spring rolls with a 2.3?cm outside diameter, 9.0?cm axial length, and 29?g weight exhibit a 35? maximum bending angle and 1.0?N maximum lateral force. These specifications can be modified by variations in roll parameters according to the equations. Multi-DOF spring rolls are easy to fabricate, compact, multifunctional, and mechanically robust. They represent a radically new actuation technology and may enable a number of unique applications. We have demonstrated a small walking robot, MERbot, with one 2-DOF spring roll as each of its six legs. The robot's speed is as high as 13.6?cm?s?1 or two-thirds of its length per second. 'Sushi rolls' have also been fabricated: these consist of six 2-DOF springs connected in series and monolithic in structure. The sushi rolls can be driven so as to generate wavelike or serpentine motion. <s> BIB002 </s> Various Robots Made from Piezoelectric Materials and Electroactive Polymers: A Review <s> Property overview <s> Face robots capable of expressing their emotional status, can be adopted as an efficient tool for friendly communication between the human and the machine In this paper, we present a face robot actuated with artificial muscle based on dielectric elastomer By exploiting the properties of dielectric elastomer, it is possible to actuate the covering skin, eyes as well as provide human-like expressivity without employing complicated mechanisms The robot is driven by seven actuator modules such eye, eyebrow, eyelid, brow, cheek, jaw and neck module corresponding to movements of facial muscles Although they are only part of the whole set of facial motions, our approach is sufficient to generate six fundamental facial expressions such as surprise, fear, angry, disgust, sadness, and happiness In the robot, each module communicates with the others via CAN communication protocol and according to the desired emotional expressions, the facial motions are generated by combining the motions of each actuator module A prototype of the robot has been developed and several experiments have been conducted to validate its feasibility <s> BIB003 </s> Various Robots Made from Piezoelectric Materials and Electroactive Polymers: A Review <s> Property overview <s> The first arm wrestling match between a human arm and a robotic arm driven by electroactive polymers (EAP) was held at the EAPAD conference in 2005. The primary objective was to demonstrate the potential of the EAP actuator technology for applications in the field of robotics and bioengineering. The Swiss Federal Laboratories for Materials Testing and Research (Empa) was one of the three organizations participating in this competition. The robot presented by Empa was driven by a system of rolled dielectric elastomer (DE) actuators. Based on the calculated stress condition in the rolled actuator, a low number of pre-strained DE film wrappings were found to be preferential for achieving the best actuator performance. Because of the limited space inside the robot body, more than 250 rolled actuators with small diameters were arranged in two groups according to the human agonist–antagonist muscle configuration in order to achieve an arm-like bidirectional rotation movement. The robot was powered by a computer-controlled high voltage amplifier. The rotary motion of the arm was activated and deactivated electrically by corresponding actuator groups. The entire development process of the robot is presented in this paper where the design of the DE actuators is of primary interest. Although the robot lost the arm wrestling contest against the human opponent, the DE actuators have demonstrated very promising performance as artificial muscles. The scientific knowledge gained during the development process of the robot has pointed out the challenges to be addressed for future improvement in the performance of rolled dielectric elastomer actuators. <s> BIB004 </s> Various Robots Made from Piezoelectric Materials and Electroactive Polymers: A Review <s> Property overview <s> Polymer-based linear actuators with contractile ability are currently demanded for several types of applications. Within the class of dielectric elastomer actuators, two basic configurations are available today for such a purpose: the multi-layer stack and the helical structure. The first consists of several layers of elementary planar actuators stacked in series mechanically and parallel electrically. The second configuration relies on a couple of helical compliant electrodes alternated with a couple of helical dielectrics. The fabrication of both these configurations presents some specific drawbacks today, arising from the peculiarity of each structure. Accordingly, the availability of simpler solutions may boost the short-term use of contractile actuators in practical applications. For this purpose, a new configuration is here described. It consists of a monolithic structure made of an electroded sheet, which is folded up and compacted. The resulting device is functionally equivalent to a multi-layer stack with interdigitated electrodes. However, with respect to a stack the new configuration is advantageously not discontinuous and can be manufactured in one single phase, avoiding layer-by-layer multi-step procedures. The development and preliminary testing of prototype samples of this new actuator made of a silicone elastomer are presented here. <s> BIB005 </s> Various Robots Made from Piezoelectric Materials and Electroactive Polymers: A Review <s> Property overview <s> A number of materials have been explored for their use as artificial muscles. Among these, dielectric elastomers (DEs) appear to provide the best combination of properties for true muscle-like actuation. DEs behave as compliant capacitors, expanding in area and shrinking in thickness when a voltage is applied. Materials combining very high energy densities, strains, and efficiencies have been known for some time. To date, however, the widespread adoption of DEs has been hindered by premature breakdown and the requirement for high voltages and bulky support frames. Recent advances seem poised to remove these restrictions and allow for the production of highly reliable, high-performance transducers for artificial muscle applications. <s> BIB006 </s> Various Robots Made from Piezoelectric Materials and Electroactive Polymers: A Review <s> Property overview <s> A kind of dielectric elastomer (DE) material, called ‘synthetic elastomer’, has been developed based on acrylonitrile butadiene rubber (NBR) to be used as a dielectric elastomer actuator (DEA). By stacking single layers of synthetic elastomer, a linear actuator, called a multistacked actuator, is produced, and used by mechatronic and robotic systems to generate linear motion. In this paper, we demonstrate the application of the multistacked dielectric elastomer actuator in a biomimetic legged robot. A miniature robot driven by a biomimetic actuation system with four 2-DOF (two-degree-of-freedom) legged mechanisms is realized. Based on the experimental results, we evaluate the performance of the proposed robot and validate the feasibility of the multistacked actuator in a locomotion system as a replacement for conventional actuators. <s> BIB007
DE is the representative material among electronic EAPs, and has been used as actuator in robot applications because of its large deformation ability, high force density, and cost-effectiveness  . DE actuators consist of a thin elastomeric film covered on both sides with electrodes. The basic operating principle of a DE is presented in Figure  8 . By applying an electric field across the electrodes, an electrostatic attraction between opposite charges on opposite electrodes and a repulsive force between equal charges on each electrode are generated. Those forces contract the elastomeric film in the thickness direction and expand it in the in-plane direction. The typical operating voltage for DE films 10-100 μm in thickness ranges from 0.5 to 10 kV BIB006 . Because DE has an inherent electrostatic characteristic, power is consumed during thickness reduction and, theoretically, the deformed state can be maintained without additional power consumption. The first application of a dielectric elastomer actuator in robots was for artificial muscles. Kwak et al. reported about a face robot actuated with artificial muscles based on a dielectric elastomer in 2005 BIB003 . To generate six fundamental facial expressions of surprise, fear, disgust, anger, happiness, and sadness, seven actuator modules for the eye, eyebrow, eyelid, brow, cheek, jaw, and neck were incorporated in the robot. In this facial robot, each actuator module communicated with the other modules via the CAN (control area network) communication protocol, and facial motions were generated according to the desired emotional expression by combining the motions of each actuator module. A prototype robot was constructed and the feasibility was validated through several experimental results. The most representative feature of the proposed method over other face robots is its embedded and multiple DOF actuation. Kovacs et al. participated that the first arm wrestling match between a human and a robotic arm driven by EAPs was held at the Electroactive Polymer Actuators and Device (EAPAD) conference in 2005, and they reported on the entire development process of the robot in 2007 BIB004 . They proposed a spring roll actuator to achieve large actuating force with dielectric elastomers as shown in Figure 9 . The actuator consisted of a bi-directionally pre-stretched and double-side-coated dielectric film wrapped around an elastic coil spring. The actuator elongated in in the axial direction and contracted back to its original length when an electric voltage was and was not applied, respectively. With the proposed actuator, elongation and generated force were up to 26% and 15 N, respectively. Based on the calculated stress condition in the rolled actuator, a low number of pre-strained dielectric elastomer film wrappings were preferable for achieving the best actuator performance. Because of the limited space inside the robot body, 256 rolled actuators with diameter of 12 mm were arranged in two groups, according to the human agonist-antagonist muscle configuration, in order to achieve an arm-like bidirectional rotation movement as shown in Figure 10 . The rotary motion of the arm was activated and deactivated electrically by the corresponding actuator groups. The developed arm wrestling robot was made of a carbon fiber composite as shown in Figure 11 . Although the robot lost the arm wrestling contest against the human opponent, the dielectric elastomer actuators demonstrated a promising performance as artificial muscles. Various configurations of dielectric elastomer-based actuators have been proposed and designed, such as roll extended folded and stacked BIB001 BIB005  . Choi et al. proposed a new design of a multi-jointed robot finger driven by an artificial muscle actuator and evaluated its control performance through an experimental test  . They proposed a new type of multi-stacked actuator that could generate linear motion like natural muscles. To transfer the linear motion into rotational motion, a simple slider crank mechanism was adopted. The manufactured prototype of the multi-joint robot finger and the movement of the proposed finger are presented in Figure 12 . To control the motion of the proposed robot finger, a PWM-PID feedback controller was designed based on a high-voltage switching circuit and implemented to the system. Nguyen et al. also designed a small biomimetic quadruped robot driven by multi-stacked dielectric elastomer actuators BIB007 . A miniature legged robot driven by a biomimetic actuation system with four 2-DOF leg mechanisms was realized and the feasibility of the dielectric elastomer actuator for a locomotion mechanism was verified by replacing conventional actuators. The walking motion of the proposed robot is illustrated in Figure 13 . The total size of the robot was 191 mm, 100 mm, and 115 mm of length, width, and height, respectively. The total weight, excluding and including the weight of the actuators, was 243 g and 450 g, respectively. They compared the performance of the proposed leg with the leg using a 3-DOF rolled dielectric elastomer actuator through numerical simulation and an experimental test and obtained a similar performance, with maximum angle of the leg of 20°-35°, by applying a driving voltage of 0-5 kV BIB002 .