Electroactive Polymer Artificial Muscles (EPAM<sup>TM</sup>) based on dielectric elastomers have the bandwidth and the energy
density required to make haptic displays that are both responsive and compact. Recent work at Artificial Muscle Inc. has
been directed toward the development of thin, high-fidelity haptic modules for mobile handsets. The modules provide
the brief tactile "click" that confirms key press, and the steady state "bass" effects that enhance gaming and music. To
design for these capabilities we developed a model of the physical system comprised of the actuator, handset, and user.
Output of the physical system was passed through a transfer function to covert vibration into an estimate of the intensity
of the user's haptic sensation. A model of fingertip impedance versus button press force is calibrated to data, as is
impedance of the palm holding a handset. An energy-based model of actuator performance is derived and calibrated, and
the actuator geometry is tuned for good haptic performance.
Dielectric elastomer actuators exert strain due to an applied electric field. With advantageous properties such as high efficiency and their light weight, these actuators are attractive for a variety of applications ranging from biomimetic robots, medical prosthetics to conventional pumps and valves. The performance and reliability however, are limited by dielectric breakdown which occurs primarily from localized defects inherently present in the polymer film during actuation. These defects lead to electric arcing, causing a short circuit that shuts down the entire actuator and can lead to actuator failure at fields significantly lower than the intrinsic strength of the material. This limitation is particularly a problem in actuators using large-area films. Our recent studies have shown that the gap between the strength of the intrinsic material and the strength of large-area actuators can be reduced by electrically isolating defects in the dielectric film. As a result, the performance and reliability of dielectric elastomers actuators can be substantially improved.
We compression molded 50 micron thick silicone films between hot platens (150°C) and room temperature platens (23°C), under high shear flow. After molding and post-curing, a voltage ramp was used to measure dielectric strength in 72 samples, each 1 cm in diameter. Breakdown strengths ranged from 16 to 183 V/μm. Overall, the two methods yielded dielectric films with comparable breakdown strengths.
Wearable dielectric elastomer actuators have the potential to enable new technologies, such as tactile feedback gloves for virtual reality, and to improve existing devices, such as automatic blood pressure cuffs. They are potentially lighter, quieter, thinner, simpler, and cheaper than pneumatic and hydraulic systems now used to make compliant, actuated interfaces with the human body. Achieving good performance without using a rigid frame to prestrain the actuator is a fundamental challenge in using these actuators on body. To answer this challenge, a new type of fiber-prestrained composite actuator was developed. Equations that facilitate design of the actuator are presented, along with FE analysis, material tests, and experimental results from prototypes. Bending stiffness of the actuator material was found to be comparable to textiles used in clothing, confirming wearability. Two roll-to-roll machines are also presented that permit manufacture of this material in bulk as a modular, compact, prestressed composite that can be cut, stacked, and staggered, in order to build up actuators for a range of desired forces and displacements. The electromechanical properties of single- layered actuators manufactured by this method were measured (<i>N</i>=5). At non-damaging voltages, blocking force ranged from 3,7-5,0 gram per centimeter of actuator width, with linear strains of 20,0-30%. Driving the actuators to breakdown produced maximum force of 8,3-10 gram/cm, and actuation strain in excess 30%. Using this actuator, a prototype tactile display was constructed and demonstrated.