This paper presents a novel wearable tactile haptic display for rendering soft body sensations to multiple fingertips with electroactive smart elastomers. The system uses newly developed multi-layered hydrostatically coupled dielectric elastomer actuators (DEAs), which have been designed to apply a localised tunable force to a user’s fingertip via a soft electrically-deformable interface. The system is comprised of DEAs which are fingertip mounted and are driven individually by a wired connection to a control unit. The force applied to the user’s fingertip is based on the user’s fingertip position which is monitored by an optical three dimensional finger tracking system. This novel tactile display system is conceived to convey soft body interactions within virtual environments. To demonstrate this, a simulator capable of demonstrating virtual objects of varying tactile haptic properties has been developed. This paper presents preliminary results of ongoing testing, as well as data pertaining to the characterization of the device in terms of force response. The paper also outlines the current limitations of the proposed technology and challenges to be addressed for further developments.
KEYWORDS: Actuators, Dielectric elastomer actuators, Electrical breakdown, System integration, Dielectrics, Polymers, Safety, Combustion, Electrodes, Microcontrollers, Signal detection, Analog electronics, Control systems, Detection and tracking algorithms
Electrical breakdown of dielectric elastomer actuators (DEAs) is an issue that has to be carefully addressed when designing systems based on this novel technology. Indeed, in some systems electrical breakdown might have serious consequences, not only in terms of interruption of the desired function but also in terms of safety of the overall system (e.g. overheating and even burning). The risk for electrical breakdown often cannot be completely avoided by simply reducing the driving voltages, either because completely safe voltages might not generate sufficient actuation or because internal or external factors might change some properties of the actuator whilst in operation (for example the aging or fatigue of the material, or an externally imposed deformation decreasing the distance between the compliant electrodes). So, there is the clear need for reliable, simple and cost-effective detection systems that are able to acknowledge the occurrence of a breakdown event, making DEA-based devices able to monitor their status and become safer and “selfaware”. Here a simple solution for a portable detection system is reported that is based on a voltage-divider configuration that detects the voltage drop at the DEA terminals and assesses the occurrence of breakdown via a microcontroller (Beaglebone Black single-board computer) combined with a real-time, ultra-low-latency processing unit (Bela cape an open-source embedded platform developed at Queen Mary University of London). The system was used to both generate the control signal that drives the actuator and constantly monitor the functionality of the actuator, detecting any breakdown event and discontinuing the supplied voltage accordingly, so as to obtain a safer controlled actuation. This paper presents preliminary tests of the detection system in different scenarios in order to assess its reliability.
Dielectric elastomers are widely investigated for use as actuators, stretch/force sensors and mechanical energy harvesters. As performance of such devices is limited by the elastomer’s dielectric strength, it is important to investigate the factors that mostly affect the electrical breakdown of those materials. In this paper, we present a preliminary study on the breakdown strength of a widely used poly-acrylic elastomer film, VHB 4905 by 3M with an equi-biaxial pre-strain of 300%. The breakdown was measured with two metal electrodes, one hemispherical and the other one planar, and was characterized under different conditions to investigate the effects of the hemispherical electrode’s curvature, the force applied by the two electrodes and the environmental humidity. With a given radius of curvature, the breakdown field increased by about 50% for a nearly ten-fold increase of the applied mechanical force, while, for a given mechanical force, the field decreased by about 20% for a two-fold increase of the radius of curvature. Furthermore, for a given radius of curvature, an increase of the environmental relative humidity from 0% to 80% caused a reduction of the breakdown field of about 20%. This study shows that the breakdown field of the studied dielectric elastomer is highly dependent on the boundary conditions of the breakdown test, as well as the environmental/storage conditions of the material. Therefore, such conditions must be reported carefully to allow for critical evaluations/comparisons of experimental results. As suggested by our data, variations of the compression, electrode’s curvature and environmental humidity are likely to cause a diversity of possible interplaying effects, some of which are preliminary proposed in this paper and are referred to as topics requiring deeper future investigations.
So-called 'hydrostatically coupled' dielectric elastomer actuators (HC-DEAs) have recently been shown to offer new
opportunities for actuation devices made of electrically responsive elastomeric insulators. HC-DEAs include an
incompressible fluid that mechanically couples a dielectric elastomer based active part to a passive part interfaced to the
load, so as to enable hydrostatic transmission. Drawing inspiration from that concept, this paper presents a new kind of
actuators, analogous to HC-DEAs, except for the fact that the fluid is replaced by fine powder. The related technology,
here referred to as 'granularly coupled' DEAs (GC-DEAs), relies entirely on solid-state materials. This permits to avoid
drawbacks (such as handling and leakage) inherent to usage of fluids, especially those in liquid phase. The paper presents
functionality and actuation performance of bubble-like GC-DEAs, in direct comparison with HC-DEAs. For this
purpose, prototype actuators made of two pre-stretched membranes of acrylic elastomer, coupled via talcum powder (for
GC-DEA) or silicone grease (for HC-DEA), were manufactured and comparatively tested. As compared to HC-DEAs,
GC-DEAs showed a higher maximum stress, the same maximum relative displacement, and nearly the same bandwidth.
The paper presents characterization results and discusses advantages and drawbacks of GC-DEAs, in comparison with
HC-DEAs.
As a means to improve versatility and safety of dielectric elastomer actuators (DEAs) for several fields of application,
so-called 'hydrostatically coupled' DEAs (HC-DEAs) have recently been described. HC-DEAs are based on an
incompressible fluid that mechanically couples a DE-based active part to a passive part interfaced to the load, so as to
enable hydrostatic transmission. This paper presents ongoing developments of HC-DEAs and potential applications in
the field of haptics. Three specific examples are considered. The first deals with a wearable tactile display used to
provide users with tactile feedback during electronic navigation in virtual environments. The display consists of HCDEAs
arranged in contact with finger tips. As a second example, an up-scaled prototype version of an 8-dots refreshable
cell for dynamic Braille displays is shown. Each Braille dot consists of a miniature HC-DEA, with a diameter lower than
2 mm. The third example refers to a device for finger rehabilitation, conceived to work as a sort of active version of a
rehabilitation squeezing ball. The device is designed to dynamically change its compliance according to an electric
control. The three examples of applications intend to show the potential of the new technology and the prospective
opportunities for haptic interfaces.
Hydrostatic coupling has been recently reported as a means to improve versatility and safety of dielectric elastomer (DE)
actuators. Hydrostatically coupled DE actuators rely on an incompressible fluid that mechanically couples a DE-based
active part to a passive part interfaced to the load. In this paper, we present ongoing development of bubble-like versions
of such transducers, made of silicone and oil. In particular, the paper describes millimeter-scale actuators, currently being
developed as soft, light, acoustically silent and cheap devices for two types of applications: tactile displays and
cutaneous stimulators. In both cases, the most significant advantages of the proposed technology are represented by high
versatility for design (due to the fluid based transmission mechanism), tailorable stiffness perceived by the user (obtained
by adjusting the internal fluid pressure), and suitable electrical safety (enabled by both a passive interface with the user
and the insulating internal fluid). Millimeter-scale prototypes showed a resonance frequency of about 250 Hz, which
represents the value at which Pacinian cutaneous mechanoreceptors exhibit maximum sensitivity; this provides an
optimum condition to eventually code tactile information dynamically, either in combination or as an alternative to static
driving.
So-called dielectric elastomer (DE) actuators represent today one of the best performing technologies for electroactive
polymer based actuation. This paper presents the concept for a new of class of DE actuators, with attractive potential
capabilities for specific application needs. The proposed actuators use an incompressible fluid to mechanically couple an
active elastic part with a passive elastic part. The active part works according to the DE actuation principle, while the
passive part represents the end effector, in contact with the load. The fluid is used to transfer actuation hydrostatically
from the active to the passive part and, then, to the load. This can provide specific advantages, including improved safety
and less stringent design constraints for the architecture of the actuator, especially for its soft end effector. These might
be of particular interest for many types of applications. Such a simple concept can be readily implemented according to
different structures and intended functionalities of the resulting actuators. The paper describes some examples of
actuators based on this concept and reports the preliminary performance of the first prototypes.
The rapidly growing adoption of dielectric elastomer (DE) actuators as a high performance EAP technology for many
kinds of new applications continuously opens new technical challenges, in order to take always the most from each
adopted device and actuating configuration. This paper presents a new type of DE actuators, which show attractive
potentialities for specific application needs. The concept here proposed adopts an incompressible fluid to mechanically
couple active and passive parts. The active parts work according to the DE actuation principle, while the passive parts
represent the end effector, in contact with the load. The fluid is used to transfer actuation hydrostatically from an active
to a passive part and, then, to the load. This can provide specific advantages, including improved safety and less stringent
design constraints for the architecture of the actuator, especially for soft end effectors. Such a simple concept can be
readily implemented according to different shapes and intended functionalities of the resulting actuators. The paper
describes the structure and the performance of the first prototype devices developed so far.
The significant electromechanical performances typically shown by dielectric elastomer actuators make this polymer
technology particularly attractive for possible active orthoses for rehabilitation. Folded contractile actuators made of
dielectric elastomers were recently described as a simple configuration, suitable to easily implement linear contractile
devices. This paper describes an application of folded actuators for so-called hand splints: they consist of orthotic
systems for hand rehabilitation. The dynamic versions of the state-of-the-art splints typically include elastic bands, which
exert a passive elastic resistance to voluntary elongations of one or more fingers. In order to provide such splints with the
possibility of electrically modulating the compliance of the resistive elements, the substitution of the passive elastic
bands with the contractile actuators is here described. The electrical activation of the actuators is used to vary the
compliance of the system; this enables modulations of the force that acts as an antagonist to voluntary finger movements,
according to programmable rehabilitation exercises. The paper reports results obtained from the first prototype
implementations of such a type of system.
The need for high driving electric fields currently limits the diffusion of dielectric elastomer actuation in some areas of
potential application, especially in the case of biomedical disciplines. A reduction of the driving fields may be achieved
with new elastomers offering intrinsically superior electromechanical properties. So far, most of attempts in this
direction have been focused on composites between elastomer matrixes and high-permittivity ceramic fillers, yielding to
limited results. In this work, the electromechanical response of a silicone rubber (poly-dimethyl-siloxane) was improved
by blending, rather than loading, the elastomer with a highly polarizable conjugated polymer (undoped poly-hexyl-thiophene).
Very low percentages (1-6 wt%) of poly-hexyl-thiophene yielded both an increase of the dielectric
permittivity and an unexpected reduction of the tensile elastic modulus. Both these factors contributed to a remarkable
increase of the electromechanical response, which reached a maximum at 1 wt% content of conjugated polymer. This
approach may lead to the development of new types of improved dielectric elastomers for actuation.
New lightweight, compliant, reliable and cheap contractile linear actuators are demanded today for many fields of
application, such as robotics, automation and biomedical disciplines. Within the family of electroactive polymers,
dielectric elastomers are rapidly emerging as high-performance transduction materials, resulting particularly attractive in
order to accomplish different kinds of tasks. The design of efficient device architectures, capable of taking the most
from the material properties with practical solutions, is not trivial. In particular, the state of the art of contractile
dielectric elastomer actuators offers device configurations resulting not always of easy fabrication. To overcome this
drawback, a new actuating configuration, referred to as 'folded dielectric elastomer actuator', has been recently
described. This paper presents prototype samples of this new type of actuator, along with different examples of
applications currently being developed.
A research project called FACE (Facial Automaton for Conveying Emotions) in course at the Research Centre "E. Piaggio" of the University of Pisa is aimed at developing an android face endowed with dynamic expressiveness and artificial vision. The bioinspired approach behind the development of this system foresees the adoption of electroactive polymers as pseudo-muscular actuators to provide motion to the silicone skin of FACE, as well as to its eyeballs. The eyes of such a human-like automaton, and in particular the achievable movements of them, play a relevant role for the "believability" of the overall system, and thus of its effectiveness, as well as for the performance of the embedded artificial vision. This work presents preliminary results related the actuation of the FACE eyeballs by means of a new type (buckling) of dielectric elastomer actuator. This kind of actuator operates with out-of-plane unidirectional displacements. It is similar to the diaphragm-type one, with the difference that the necessary pre-deformation is enabled by an underlying hemispheric support, instead of pressurised air. One silicone-based buckling actuator was connected to a plastic eyeball of FACE via a tendon-like wire, in order to enable unidirectional rotations. Relative out-of-plane displacements of the actuator larger than 50% were achieved and used to provide rotations up to 13 degrees.
The assessed high electromechanical performances of dielectric elastomer actuators are encouraging the study of possible future applications of such devices for active prosthetic or orthotic systems for humans. Although the high electric fields currently needed for their driving prevent today a short-term use in endo- prostheses, their adoption for eso-prostheses or orthoses can be considered more realistic. Exoskeletons for improving muscular performance in specific tasks or for rehabilitation are examples of possible fields of investigation. Beyond a necessary technological development towards materials and devices capable of improved performances at reduced fields, the study of such applications requires even the identification of suitable strategies of activation and control. In particular, actuators to be used for such applications may take advantage from the possibility of being activated by electrophysiological signals. This would permit advantageous body's controls of the artificial system. In this context, this work presents activities carried on towards such a goal. In particular, activations of silicone-made dielectric elastomer actuators by means of different types of electrophysiological signals, opportunely elaborated, are presented and discussed.
Linear dielectric elastomer actuators with contractile ability are demanded for several types of applications. In order to achieve such a goal, two basic actuating configurations are today available: the multilayer stack and the helical structure. The first consists of several layers of elementary planar actuators stacked in mechanical series and electrical parallel. The second relies on a couple of helical compliant electrodes alternated to a couple of helical dielectrics. The fabrication of both these configurations presents today some specific difficulties, arising from the peculiarity of each structure. Even though successful implementations have been reported and further improvements are currently in progress, the availability of simpler solutions would boost the short-term use of contractile actuators in practical applications. In order to propose a viable alternative to the present configurations, a new structure is here described. It is designed to obtain a contractile monolithic actuator, starting from a planar electroded sheet, which is then folded up. The resulting compact structure is equivalent to a multilayer stack with interdigited electrodes. With respect to the conventional multilayer stack, the new configuration is advantageously not discontinuous and can be manufactured in one single phase, avoiding layer-by-layer multistep procedures. The current developmental stage of this new actuator with a silicone elastomer is here presented.
Electromechanical devices made of dielectric elastomers represent today one of the most attractive and performing technologies within the field of electromechanical polymer actuation. We describe here a new configuration designed for monolithic dielectric elastomer actuators capable to show electrically activated linear contractions. Two helical electrodes integrated within the wall of an elastomeric hollow cylinder represent the core of the device, enabling the generation of active axial contractions and radial expansions. Detailed architecture, principle of operation and preliminary data on the performances of the new device are presented.
The human attention system is based on the capability of the eye of focusing and tracking. These actions are performed by the eyeball muscle system, as a consequence of visual stimuli. The F.A.C.E. (Facial Automaton for Conveying Emotions) project at our lab concerns the development of an android face endowed with dynamic expressiveness and artificial vision. Aimed at realising an artificial attention system for such an automaton, we present here a study for the development of pseudo-muscular polymer actuators for its eyeballs. The system is based on the mimicry of the muscular architecture of the human eye. In particular, linear actuators made of dielectric elastomers have been designed to replicate actions exerted by the main ocular muscles.
This paper presents a strategy of design, realization and control of pseudomuscular actuator controllable in position and compliance. The actuator was designed as a bundle of electromechanical actuating elements, made by dielectric elastomers. The control strategy was inspired to the Feldman's biological muscle model. Presented simulations show that opportune recruitments of the bundle active units enable a satisfying approximation of the quadratic length-force characteristic of the biological muscle.
This paper presents preliminary results on the characterisation of the actuating performances, never explored before, of an elastomeric material (Dr Scholl’s, Canada, Gelactiv tubing), to realise dielectric elastomer actuators. Strain and stress performances of this material were compared to those of the most currently used acrylic elastomer (3M, U.S.A., VHB 4910). Planar actuators were realised and tested, using films of the two materials, coated with compliant electrodes made of carbon grease. Following the application of a two-second high-voltage impulse, the isotonic transverse displacement and the isometric transverse force were separately measured along a prestrained direction. Actuators made of the new elastomer showed, respect to those made of the acrylic polymer, a lower dielectric strength, a transverse strain more than twice greater for the same applied electric field (e.g. 1.8 % against 0.7 % @ 27 V/μm) and a transverse stress less than twice smaller (e.g. 3.7 KPa against 5.3 KPa @ 24 V/μm).
A dielectric elastomer planar actuator consists of a polymeric film included between two compliant electrodes. A voltage difference applied between the electrodes generates an electrostatic pressure which compresses the flim thickness and expands its surface. The high performances and low costs of dielectric elastometers, belonging to the class of electroactive polymers, suggest their advantageous use for the actuation of small scale devices. The following figures present some possible simple applications of such actuators.
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