Soft robots with many degrees of freedom, modelled after snakes or tentacles, can locomote through a combination of controlled friction and phased multi-segment deformation. Using different periodic motions (gaits) snakes (and snake-like robots) are able to cross open space, climb narrow passages, side-wind across granular substrates, and more. Unlike their biological counterparts, snake-like robots should be able to adapt easily to space flight by utilising controllable friction elements in the form of electro-adhesive pads to selectively attach to and detach from surfaces and objects. They could operate on the interior and exterior of satellites to perform maintenance and repair, or even explore the surfaces of small astronomical bodies which do not produce enough gravity to allow for traditional wheeled rovers.
Dielectric elastomer actuators (DEAs) are an ideal candidate for the driving system of such a robot, and have already demonstrated the ability to form multi-degree of freedom actuators of varying design. They are lightweight, use minimal current to maintain a given position, have high energy density, and are capable of self-sensing their strain, reducing the need for external monitoring. We present here the design and analysis of a lightweight DEA snake-like robot incorporating electro-adhesive elements for operating in zero-gravity environments, including different gait waveforms for enhanced performance and finite element analysis for design optimisation. We conclude with discussion of future improvements, such as the incorporation of dielectric elastomer switches for greater autonomy.
Dielectric elastomer switches have shown large potential for integrating signal processing directly into multifunctional dielectric elastomers. Previously presented continuum dielectric elastomer switches (cDES) utilize percolation effects within piezoresistive membranes to directly switch high voltages, controlling dielectric elastomer actuators. The here presented geometric dielectric elastomer switches (gDES) use geometric air gaps in encapsulated soft sensor structures to switch both high and low voltages.
gDES consist only of soft materials such as silicones and carbon-doped conductive silicones. The structured conductive electrode areas are arranged in such a way that they create small gaps within a shielded cavity. The gap can be opened or closed by an external deformation or pressure. Depending on the electrode design and mechanical characteristics, the necessary amount of deformation and pressure can be tailored exactly to the requirements of the application. Arrays of these switches can be integrated in soft robotic grippers and extend the features of those grippers by touch and shear force detection. Furthermore, gDES can act as limit switches and can be introduced in automation technology. One of the key advantages is that the switches themselves are entirely shielded and not affected by environmental influences.
gDESs have an advantage of operation at lower voltages than related cDES. This reduces the necessary amount of driving voltage and opens up the application in classic automation technologies and robotics. gDESs possess conductive silicone composites containing conductive fillers. The switching points and the general behaviour (normally-open [NO] or normally-closed [NC]) are tuned by the geometry of conductive parts associated with the shielding silicone structures. Related to percolation-based cDESs, the gDESs are produced by classic microelectronic production technologies, such as soft lithography.
We present the principle design of different gDESs and arrays of the same, the production techniques and first results of distributed touch sensing for soft robotic grippers. Methods and design parameters for adjusting the switching characteristics are presented and experimentally evaluated.
Inkjet material deposition is a promising approach to print multiple functional components for dielectric elastomer (DE) devices. The automatic fabrication process promotes reliable and repeatable results, and allows scaling to a few millimetres, which is advantageous in areas such as microfluidics and optics. We present here the printing and evaluation of novel ink formulae comprising silicone and a conductive filler. Carbon black, the conductive filler, is a popular electrode material. Although it has a relatively high resistance, it has been shown to produce compliant electrodes of good performance for dielectric elastomer actuators (DEA). Carbon black is added to liquid silicone rubber and solvents in order to obtain a solution that can be inkjet-printed. The silicone provides binding of the carbon particles into a soft matrix as well as bonding to the elastomer membrane on which it is printed. Each ink has unique electromechanical properties, e.g. sheet resistances ranging from a few kΩ/sq to MΩ/sq. We can apply different inks to provide conductive electrodes for DEA or piezoresistive components such as the dielectric elastomer switch (DES) - able to locally control charge over DEA - or simple resistor and electrode tracks. We discuss ink behaviours and printed sample components for networks of DEA and combined driving circuitry, all with soft, flexible materials.
Dielectric Elastomer Transducers (DETs) integrated into inflatable structures can form the basis for soft, low mass robots. Such robots will have very high packaging efficiency and be simple to deploy. These attributes, combined with the high power density of DETs make them ideal for space robots. In this paper we present a study of different motions achieved from the actuation of three distinct simple experimental designs. Firstly, the dome actuator constructed from a sheet of silicone rubber with segmented electrodes. Secondly, an elongation of the former, capable of producing locomotory motion from phased actuation of segments. Finally, a rolled cylindrical design varying the seam geometry, and electrode position and composition to produce different resonant and non-resonant motion. This study is comprised of experimental results, and finite element modelling of each design using commercially available FEM software. The different structures are simulated undergoing inflation and actuation, and the results compared to experimental data. Modal analyses of the inflated cylindrical structures are also compared with the frequency responses of the experimental models. Extrapolation of these basic units to more complex structures, designed to complement or replace existing space equipment, is presented for discussion alongside the remaining challenges.
Conventional electronics are typically rigid, introducing unwanted stiffness to otherwise entirely soft systems. Emerging soft and stretchable electronics provide a platform for integrating driving electronics in soft robotics and structures. A stretchable electrode having strain-dependent resistance is the dielectric elastomer switch (DES). The DES enables direct control of artificial muscles, or dielectric elastomer actuators (DEA), a popular material in soft robotics. Electromechanically interacting DEA and DES together make up smart actuator networks, with the DES as piezoresistive-charge gates. The DES is a unique stretchable electrode in that it directly couples mechanical strain with a logic state change. We have previously demonstrated logic gates and memory elements using DES/DEA arrays. Performance, particularly speed and cycle life, were limited due largely to acrylic-based, viscoelastic materials and hand-made fabrication process. Here we present computing elements with enhanced performance, comprising silicone membranes and airbrushed silicone-based electrodes. We also demonstrate a new model - a dielectric elastomer digital oscillator. The oscillator provides the timing signal for sequential logic elements, which reduces number of wires and inputs needed for DE circuits. Finally, we also use the mechanosensitive DES to implement adjustable frequency of the DE oscillators.
Multifunctional Dielectric Elastomer (DE) devices are well established as actuators, sensors and energy har- vesters. Since the invention of the Dielectric Elastomer Switch (DES), a piezoresistive electrode that can directly switch charge on and off, it has become possible to expand the wide functionality of DE structures even more. We show the application of fully soft DE subcomponents in biomimetic robotic structures.
It is now possible to couple arrays of actuator/switch units together so that they switch charge between them- selves on and off. One can then build DE devices that operate as self-controlled oscillators. With an oscillator one can produce a periodic signal that controls a soft DE robot – a DE device with its own DE nervous system. DESs were fabricated using a special electrode mixture, and imprinting technology at an exact pre-strain. We have demonstrated six orders of magnitude change in conductivity within the DES over 50% strain. The control signal can either be a mechanical deformation from another DE or an electrical input to a connected dielectric elastomer actuator (DEA). We have demonstrated a variety of fully soft multifunctional subcomponents that enable the design of autonomous soft robots without conventional electronics. The combination of digital logic structures for basic signal processing, data storage in dielectric elastomer flip-flops and digital and analogue clocks with adjustable frequencies, made of dielectric elastomer oscillators (DEOs), enables fully soft, self-controlled and electronics-free robotic structures.
DE robotic structures to date include stiff frames to maintain necessary pre-strains enabling sufficient actuation of DEAs. Here we present a design and production technology for a first robotic structure consisting only of soft silicones and carbon black.
Electromechanically coupled dielectric elastomer actuators (DEAs) and dielectric elastomer switches (DESs) may form digital logic circuitry made entirely of soft and flexible materials. The expansion in planar area of a DEA exerts force across a DES, which is a soft electrode with strain-dependent resistivity. When compressed, the DES drops steeply in resistance and changes state from non-conducting to conducting. Logic operators may be achieved with different arrangements of interacting DE actuators and switches. We demonstrate combinatorial logic elements, including the fundamental Boolean logic gates, as well as sequential logic elements, including latches and flip-flops. With both data storage and signal processing abilities, the necessary calculating components of a soft computer are available. A noteworthy advantage of a soft computer with mechanosensitive DESs is the potential for responding to environmental strains while locally processing information and generating a reaction, like a muscle reflex.
Dielectric Elastomer Generators (DEG) can capture energy from natural movement sources such as wind, the tides and human locomotion. The harvested energy can be used for low power devices such as wireless sensor nodes and wearable electronics. A challenge for low power DEG is overcoming the losses associated with charge management. A circuit which can do this exists: the Self Priming Circuit (SPC) which consists of diodes and capacitors. The SPC is connected in parallel to the DEG where it transfers charge onto/o_ the DEG based on changes in the DEG capacitance. Modelling and experimental validation of the SPC have been performed in the past, allowing design and implementation of effective SPCs which match a particular DEG. While the SPC is effective, it is still an external circuit which adds additional mass and cost to the DEG. By splitting the DEG into separate capacitors and using them to build an SPC, the Integrated SPC (I-SPC) can be realized. This reduces the components required to build a SPC/DEG and improves the performance. This paper presents a mathematical model with experimental data of a first order I-SPC. Additionally, comparisons between the SPC and I-SPC are drawn.
There are great efforts in developing effective composite structures for lightweight constructions
for nearly every field of engineering. This concerns for example aeronautics and spacecrafts, but
also automotive industry and energy harvesting applications. Modern concepts of lightweight
components try to make use of structures with properties which can be adjusted in a controllable
was. However, classic composite materials can only slightly adapt to varying environmental
conditions because most materials, like carbon or glass-fiber composites show properties which
are time-constant and not changeable.
This contribution describes the development, the potential and the limitations of novel smart,
self-controlling structures which can change their mechanical properties - e.g. their flexural
stiffness - by more then one order of magnitude. These structures use a multi-layer approach
with a 10-layer stack of 0.75 mm thick polycarbonate.
The set-up is analytically described and its mechanical behavior is predicted by finite element
analysis done with ABAQUS.
The layers are braided together by an array of shape memory alloy (SMA) wires, which can be
activated independently. Depending on the temperature applied by the electrical current flowing
through the wires and the corresponding contraction the wires can tightly clamp the layers so
that they cannot slide against each other due to friction forces. In this case the multilayer acts
as rigid beam with high stiffness. If the friction-induced shear stress is smaller than a certain
threshold, then the layers can slide over each other and the multilayer becomes compliant under
The friction forces between the layers and, hence, the stiffness of the beam is controlled by the
electrical current through the wires. The more separate parts of SMA wires the structure has
the larger is the number of steps of stiffness changes of the flexural beam.
This contribution describes the development, the potential and the limitations of planar actuators for controlling bending devices with variable stiffness. Such structures are supposed to be components of new smart, self-sensing and -controlling composite materials for lightweight constructions. To realize a proper stiffness control, it is necessary to develop reliable actuators with high actuation capabilities based on smart materials. Several actuator designs driven by electroactive polymers (EAPs) are presented and discussed regarding to their applicability in such structures. To investigate the actuators, variable-flexural stiffness devices based on the control of its area moment of inertia were developed. The devices consist of a multi-layer stack of thin, individual plates. Stiffness variation is caused by planar actuators which control the sliding behavior between the layers by form closure structures. Previous investigations have shown that actuators with high actuation potential are needed to ensure reliable connections between the layers. For that reason, two kinds of EAPs Danfoss PolyPower and VHB 4905 by 3M, have been studied as driving unit. These EAP-driven actuators will be compared based on experimental measurements and finite element analyses.
The contribution describes a new kind of multi-layer beam with a variable stiffness based on electroactive polymers
(EAP). These structures are supposed to be components of new smart, self-sensing and -controlling composite materials
for lightweight constructions. Dielectric Elastomer foils from Danfoss PolyPower are used to control the beam's
The basic idea is to change the area moment of inertia of bending beams. These beams are built up as multi-layer stacks
of thin metal or PMMA plates. Its internal structure can be changed by the use of the electroactive polymers for
controlling the area moment of inertia. So it is possible to strongly change the stiffness of bending beams up to two
orders of magnitude. Thereby, the magnitude of varying the stiffness can be scaled by the number of layers and the
number and type of electroactive polymer elements used within the bending beam.
The mechanisms for controlling the area moment of inertia are described in detail. Modeling of the mechanical structure
including the EAP uses a pseudo rigid-body model, a strain energy model as well as a finite element analysis. The
theoretical calculations are verified by experiments. The prototype described here consists of two structural layers. First
results show the feasibility of the proposed structure for mechanical components with stiffness control.