Conventional robots are mainly made of rigid materials, such as steel and aluminum. Recently there has been a surge in the popularity of soft robots owing to their inherent compliance, strong ...adaptability and capability to work effectively in unstructured environments. Of the multitude of soft actuation technologies, dielectric elastomer actuators (DEAs), also nicknamed 'artificial muscles', exhibit fast response, large deformation and high energy density, and can simply be actuated with electric voltage. In this paper, we will discuss applications of DEAs to soft robots, including robotic grippers, terrestrial robots, underwater robots, aerial robots and humanoid robots. We will survey the state of the art regarding these interesting applications and outline the challenges and perspectives. As we know, there have been extensive studies on dielectric elastomer technology in the aspects of materials, mechanics, design, fabrication and controls. To enable practical applications, efforts are underway to decrease operational voltages, improve reliability, and impart new functionalities. Key challenges include the development of freestanding actuators, untethered operation, smart/electronics free actuators, solid and stretchable electrodes, miniaturization, combination of synergistic actuation technologies to impart novel functionalities, development of effective control strategies, etc. We hope that this review can facilitate and enhance applications of dielectric elastomer technology to soft robots.
Grasping of complicated objects is an active research area which is developing fast throughout the years. Soft grippers can be an effective solution, since they are capable of holding workpieces of ...various shapes and interacting with unstructured environments effectively. Soft grippers generally consist of soft, flexible and compliant materials, which are able to conform to the shape of the object so that the gripper will not deform or bruise the soft object. Fast grasping of objects with various sizes and shapes remains a challenging task for soft grippers. In the present article, a soft gripper based on bi-stable dielectric elastomer actuator (DEA) inspired by the insect-catching ability of the Venus flytrap, is designed. This soft gripper can achieve good performances in grasping various objects by a simple actuation system. The gripper can switch from one stable state to another when subject to an impulse voltage of 0.04 s. The time duration for each grasping action is 0.17 s, and no continuous voltage is required for holding the gripped object. Thus, energy consumption can be achieved as low as 0.1386 J per grasping action. The mechanism of achieving bi-stable states is related to the duration of impulse voltage applied and the resonant frequency of the structure. The present study demonstrates that bi-stable dielectric elastomer actuators are capable of achieving fast speed for grasping with very low energy consumption, which is significant in the applications to soft grippers and biomimetic robots.
•Devices that interact with the human body should consider the mechanical properties of the soft tissues.•The maximum principal strain and compressive mean principal strain are observed over the ...patella tendon and popliteal region respectively.•The use of LoNE could inform prosthetic design strategies.•The method to generate the LoNE could include the use of simple, low-cost video equipment and 2D skin marker application.
This study aimed to develop a new technique to map the strain field for persons with lower-limb amputations to use for the design of comfortable prostheses.
Using a DSLR camera with stenciled 2D markers, we demonstrated a technique to measure skin strain around the residual limb of persons with lower limb amputations. We used open-source software programs to reconstruct a series of cloud points derived from the pictures of the marked residual limb into 3D models, then calculated the minimum, maximum, and non-extension lines from directional strain fields.
A DSLR camera was successful in capturing 2D markers. The maximum mean principal strain was 68% ± 14%, observed around the patella. The minimum compressive mean principal strain of −31% ± 4% was observed posteriorly in the popliteal region of the knee. Although lines of non-extension (LoNE) appear separate in different participants, they are anatomically located in regions that could be generalized for the design of prostheses.
Marker locations extracted from the video of different poses can be compared to calculate strains from which the position of LoNE can be generated. The use of LoNE could be valuable in reducing discomfort at the socket interface and informing future socket design.
Soft robots have shown great potential for surveillance applications due to their interesting attributes including inherent flexibility, extreme adaptability, and excellent ability to move in ...confined spaces. High mobility combined with the sensing systems that can detect obstacles plays a significant role in performing surveillance tasks. Extensive studies have been conducted on movement mechanisms of traditional hard-bodied robots to increase their mobility. However, there are limited efforts in the literature to explore the mobility of soft robots. In addition, little attempt has been made to study the obstacle-detection capability of a soft mobile robot. In this paper, we develop a soft mobile robot capable of high mobility and self-sensing for obstacle detection and avoidance. This robot, consisting of a dielectric elastomer actuator as the robot body and four electroadhesion actuators as the robot feet, can generate 2D mobility, i.e. translations and turning in a 2D plane, by programming the actuation sequence of the robot body and feet. Furthermore, we develop a self-sensing method which models the robot body as a deformable capacitor. By measuring the real-time capacitance of the robot body, the robot can detect an obstacle when the peak capacitance drops suddenly. This sensing method utilizes the robot body itself instead of external sensors to achieve detection of obstacles, which greatly reduces the weight and complexity of the robot system. The 2D mobility and self-sensing capability ensure the success of obstacle detection and avoidance, which paves the way for the development of lightweight and intelligent soft mobile robots.
Transparency is a surprisingly effective method to achieve camouflage and has been widely adapted by natural animals. However, it is challenging to replicate in synthetic systems. Herein, a ...transparent soft robot is developed, which can achieve effective camouflage. Specifically, this robot is driven by transparent dielectric elastomer actuators (DEAs). Transparent and stretchable conductive polymers, based on blends of poly(3,4‐ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) and water‐borne polyurethane (WPU), are employed as compliant solid‐state electrodes in the DEAs. The electrode exhibits large stretchability, low stiffness, excellent conductivity at large strain, and high transmittance. Consequently, the DEA can achieve a large voltage‐induced area strain of 200% and a high transmittance of 85.5%. Driven by these soft actuators, the robot can realize translations using its asymmetric vibration mode, which can be explained by dynamics analysis and is consistent with finite element modeling. This soft robot can achieve effective camouflage, due to its high transparency as well as thin structure. Furthermore, the robot can become completely flat for even better camouflage by converting its 3D structure to 2D. The transparent soft robot is potentially useful in many applications such as robots for battlefield, reconnaissance, and security surveillance, where effective camouflage is required in dynamic and/or unstructured environments.
A transparent soft camouflaging robot, driven by transparent dielectric elastomer actuators, is developed. The solid compliant electrodes in the actuators are conductive polymers of poly(3,4‐ethylenedioxythiophene):poly(styrenesulfonate)/water‐borne polyurethane with large stretchability, excellent conductivity, and high transmittance. The robot can achieve effective camouflage during its translational and rotational motions, due to its transparent, thin, and smooth structure.
Artificial muscles based on dielectric elastomer actuators (DEAs) have been used to mimic the motion of the human jaw. However, DEAs show strong nonlinear behavior coupled with rate dependent ...viscoelastic phenomena. Under cyclic actuation, the viscoelastic creep coupled with hysteresis further makes control of these viscoelastic membranes difficult. In this paper, we develop a nonlinear dynamic model based on the principles of nonequilibrium thermodynamics to account for viscoelasticity. Furthermore, the damping and inertial effects of the jaw and membrane are taken into account, relating the deformation of the membrane to the voltage applied. Experimental results are found to be consistent with theoretical predictions. The feedforward controller is then developed based on a viscoelastic nonlinear dynamic model. The controller can be used to track the sinusoidal, triangular, and staircase trajectories accurately. Finally, a video demonstrating the jaw movement in the speech, "I Have a Dream," by Dr. Martin Luther King Jr. has been shown to illustrate the effectiveness of the controller. This is one of the first efforts to control of soft robots driven by viscoelastic DEAs.
Abstract
Computer numerical control (CNC) knitting technology offers great potential in the creation of thermoformable textiles that can be shaped and stiffened in response to heat. Our research ...explores how CNC knitting can be used to design and fabricate textiles with precisely allocated material and microstructure layouts. These layouts pre-program specific deformation mechanism(s) into the textile that bias it to form an intended geometry, forgoing the need for a mould during the thermoforming process. We fabricate these smart textiles by knitting two thermal-reactive yarns with different extents of shrinkage, in a double layered structure akin to a bilayer strip. We develop a computational design-to-fabrication pipeline that translates raster images into machine-knittable instructions. Referencing multi-material additive manufacturing principles and self-actuating textiles, this paper proposes several design strategies of dithered gradients, tessellated patches and origami creases, to convert input pixel data into a material distribution layout. When paired with our assisted thermoforming process, this layout induces specific deformations of the textile, such as uni-/multi-axial curling, periodic buckling and sharp folding. Our prototypes implement these strategies on the micro-, meso- and macroscale, leading to the design and fabrication of an architectural cladding panel (700 × 535 × 110 mm) and a patterned clutch bag (200 × 420 × 65 mm).
Soft dielectric elastomer actuators (DEAs) exhibit interesting muscle-like behavior for the development of soft robots. However, it is challenging to model these soft actuators due to their material ...nonlinearity, nonlinear electromechanical coupling, and time-dependent viscoelastic behavior. Most recent studies on DEAs focus on issues of mechanics, physics, and material science, while much less importance is given to quantitative characterization of DEAs. In this paper, we present a detailed experimental investigation probing the voltage-induced electromechanical response of a soft DEA that is subjected to cyclic loading and propose a general constitutive modeling approach to characterize the time-dependent response, based on the principles of nonequilibrium thermodynamics. In this paper, some of the key observations are found as follows: 1) Creep exhibits the drift phenomenon, and is dominant during the first three cycles. The creep decreases over time and becomes less dominant after the first few cycles; 2) a significant amount of hysteresis is observed during all cycles and it becomes repeatable after the first few cycles; 3) the peak of the displacement is shifted from the peak of the voltage signal and occurs after it. To account for these viscoelastic phenomena, a constitutive model is developed by employing several dissipative nonequilibrium mechanisms. The quantitative comparisons of the experimental and simulation results demonstrate the effectiveness of the developed model. This modeling approach can be useful for control of a viscoelastic DEA and paves the way to emerging applications of soft robots.
Existing stretchable capacitive sensor arrays face challenges in decoupling normal forces and stretch stimuli which restrict its applications into soft robotics. Herein, a metamaterial capacitive ...sensor array with 6 × 6 sensors is developed for the detection of normal forces on curved deforming surfaces common to both the soft universal jamming gripper and human elbow. The fabrication process involves the 3D printing of carbon black thermoplastic polyurethane (PI–ETPU) electrodes and thermoplastic polyurethane (TPU) insulation based on commercially available multimaterial fused deposition modeling (FDM). This metamaterial capacitive sensor array is unaffected by a uniaxial stretch of up to 21.6% and remains capable of detecting normal forces. The sensor array structure possesses a negative Poisson's ratio which is desirable for improving compliance and conformability to adhered expanding surfaces. Consequently, normal force‐distribution and proximity detection capabilities on curved deforming surfaces are demonstrated on a soft universal jamming gripper for measuring gripping forces and identifying object shapes. Conformal force‐sensing joint wearable on a human elbow with an individualized Poisson's ratio is explored as another application. The possibility of programming Poisson's ratio of a metamaterial capacitive sensing array opens new avenues for potential sensing applications into soft robotics and personalized wearables.
Soft stretchable sensors are becoming essential in enabling force and proximity sensing capabilities on curved deforming surfaces in unstructured environments. Mechanical metamaterials designs fabricated by affordable 3D printing are investigated as a platform to improve compliance and conformality of existing soft sensor arrays, for diverse applications in universal jamming gripper and elbow force‐sensing wearables.
A membrane of a dielectric elastomer may undergo electromechanical phase transition from the flat to wrinkled state, when the applied voltage reaches a critical value. The wrinkled region is observed ...to expand at the expense of the flat region during the phase transition. In this paper, we report on a dynamic pattern of wrinkles in a circular membrane of a dielectric elastomer. During phase transition, both the flat and wrinkled regions move interchangeably in the membrane. The radial prestretch is found to significantly affect electromechanical phase transition. For example, a membrane with a small prestretch can exhibit a dynamic pattern of wrinkles, which is essentially related to snap-through instability. However, a membrane with a large prestretch undergoes continuous phase transition, without exhibiting a dynamic pattern. An analytical model is developed to interpret these experimental phenomena. Finite element simulations are performed to predict the wrinkle morphology, especially the coexistence of flat and wrinkled regions. Both the theoretical calculations and finite element simulations are qualitatively consistent with the experiments. Additionally, we observe another type of electromechanical behavior involving a dynamic pattern of wrinkles with different wavelengths. The membrane first undergoes continuous transition from the flat to wrinkled state, followed by discontinuous transition from one wrinkled state to another. These results may inspire new applications for dielectric elastomers such as on-demand patterning of wrinkles for microfluidics and stretchable electronics.
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