Scaling-associated aspects of liquid–gas phase change actuators in robots and wearable technologies

A novel linear-type piezoelectric ultrasonic actuator for application in a Smart Fuze Safety System (SFSS) is investigated in this paper. Considering the requirements of SFSS, the structural parameters of the piezoelectric ultrasonic actuator are initially determined. Furthermore, sensitivity analysis of the structural parameters to the frequency consistency is conducted using finite element (FEM) software. To validate the results of FEM, the frequency sweep tests of the piezoelectric ultrasonic actuator are performed to determine the stator's actual working mode frequencies with PSV-300-B Doppler laser vibrometer system. Additionally, the results of frequency sweep test are compared with that of the FEM analysis, and further verified by impedance analyzer. To investigate the overall performance of the piezoelectric ultrasonic actuator, vibration modes of actuator's stator, output speed and force of the piezoelectric ultrasonic actuator are tested. The experimental results show that the output speed and force of the actuator can reach 88.2 mm/s and 2.3 N respectively, which can meet the demands of the SFSS.

Untethered soft actuators that operate without physical connections to external power sources or control system cables are an active area of research for their potential applications in human-robot interaction, haptic devices, and 4D printing due to their wide range of motion and high degrees of freedom. Among these actuators, liquid-to-gas phase change actuators have a simple structure and can generate higher output force using photothermal conversion. In this study, we propose a novel control method for untethered soft actuators by dyeing the low-boiling-point liquids used in liquid-to-gas phase change actuators. The dyed low-boiling-point liquids exhibit different light absorption spectra matching their color, enabling wavelength-selective actuation using light sources with corresponding wavelengths. We evaluated and discussed this control method in terms of the relationship between irradiance and output, actuation speed, and continuous operation. Additionally, we prototyped a robotic arm to demonstrate selective motion according to the light color.

Soft actuators, as the core components of soft robots, play a crucial role in the entire robot system. However, due to the limited output force and response speed of soft actuators based on smart materials, soft robots have difficulty achieving effective movement in amphibious scenarios. Inspired by the hydraulic drive mechanism of leaping spiders, a flexible and controllable amphibious robot module is developed by designing a new type of joint actuator and introducing an efficient underwater encapsulation method. This new type of joint electro-hydraulic actuator can achieve a large swinging angle and a fast dynamic response speed. In addition, the crawling robot developed by using this joint actuator can not only realize fast crawling speed (<tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$3.3 \text{BL} / \mathrm{s}$</tex>) but also perform climbing motion at an angle of 30°. Moreover, it can be applied underwater and achieve rapid propulsion by splashing water, demonstrating excellent amphibious motion performance. This work not only contributes to the development of high-performance amphibious soft actuators but also provides essential references for the design of soft robots.

&lt;p&gt;Soft robots have become important members of the robot community with many potential applications owing to their unique flexibility and security embedded at the material level. An increasing number of researchers are interested in their designing, manufacturing, modeling, and control. However, the dynamic simulation of soft robots is difficult owing to their infinite degrees of freedom and nonlinear characteristics that are associated with soft materials and flexible geometric structures. In this study, a novel multi-flexible body dynamic modeling and simulation technique is introduced for soft robots. Various actuators for soft robots are modeled in a virtual environment, including soft cable-driven, spring actuation, and pneumatic driving. A pneumatic driving simulation was demonstrated by the bending modules with different materials. A cable-driven soft robot arm prototype and a cylindrical soft module actuated by shape memory alley springs inspired by an octopus were manufactured and used to validate the simulation model, and the experimental results demonstrated adequate accuracy. The proposed technique can be widely applied for the modeling and dynamic simulation of other soft robots, including hybrid actuated robots and rigid-flexible coupling robots. This study also provides a fundamental framework for simulating soft mobile robots and soft manipulators in contact with the environment.&lt;/p&gt; &lt;p&gt; &lt;/p&gt;

Dielectric elastomer actuators (DEAs) have advantageous characteristics and, therefore, their application is widespread in the field of soft robotics. Their properties can be specifically adapted by both the selection of materials and the manufacturing process. Previous research has shown that fiber reinforcement of the structure can significantly enhance the unidirectional motion. In the presented work an actuator consisting of a silicone film as the dielectric, a textile carbon-fiber-reinforced electrode and a carbon black electrode are used. The electrode based on the carbon fibers additionally serves as unidirectional stiffener. Due to this highly anisotropic textile electrode, the DEA barely contracts in fiber direction. However, the active force of the DEA during actuation can be further increased through an initial pre-stretching. The aim of this work is to investigate the influence of the pre-stretching in fiber direction on the actuator performance and the long-term stability of the pre-stretch. The active force of different actuators is recorded with uniaxial tensile tests over several deformation cycles. This enables the investigation of effects deriving both from the manufacturing process and the layer structure of the textile DEA. The acquired data are evaluated and compared to results of an analytical model. To explore the ability of the fibers to maintain the initial pre-stretching of the DEA during activation, digital image correlation as an in-situ imaging technique is applied. It could be shown shown that there is no change in width due to the anisotropy. The results of the investigations are used to control and improve the manufacturing process of the textile DEA.

Optically actuated shape recovery materials receive much interest because of their great ability to control the creation of mechanical motion remotely and precisely. An infrared (IR) triggered actuator based on shape recovery was fabricated using polyurethane (TPU) incorporated by sulfonated reduced graphene oxide (SRGO)/sulfonated carbon nanotube (SCNT) hybrid nanofillers. Interconnected SRGO/SCNT hybrid nanofillers at a low weight loading of 1% dispersed in TPU showed good IR absorption and improved the crystallization of soft segments for a large shape deformation. The output force, energy density and recovery time of IR-triggered actuators were dependent on weight ratios of SRGO to SCNT (SRGO:SCNT). TPU nanocomposites filled by a hybrid nanofiller with SRGO:SCNT of 3:1 showed the maximum IR-actuated stress recovery of lifting a 107.6 g weight up 4.7 cm in 18 s. The stress recovery delivered a high energy density of 0.63 J/g and shape recovery force up to 1.2 MPa due to high thermal conductivity (1.473 W/mK) and Young's modulus of 23.4 MPa. Results indicate that a trade-off between the stiffness and efficient heat transfer controlled by synergistic effect between SRGO and SCNT is critical for high mechanical power output of IR-triggered actuators. IR-actuated shape recovery of SRGO/SCNT/TPU nanocomposites combining high energy density and output forces can be further developed for advanced optomechanical systems.

Emerging classes of soft robotics with flexible actuation, intelligent sensibility, and biomimetic functionality are driving significant advances in academic researches and commercial applications. Such new generation robotics relies heavily on important breakthroughs in soft matter engineering and flexible actuation systems. Unlike conventional rigid robots, soft robots generally have unique sporting styles and manufacturing strategies. Recently, a series of newly developed soft materials—gallium‐based liquid metals (LMs) are found to display dramatical roles in making soft machines and robotics with their unusual properties in soft robotic actuation and self‐driven field. With superior merits of both high stretchability and electroconductivity over conventional soft materials, LM is increasingly innovated as flexible sensors and actuators in constructing intelligent soft robots. Typical advances in LM enabled soft machines and robotics are summarized and interpreted, herein, with special focus on the driving principles of the LM objects, the preparation methods of LM flexible electronics for making soft robots, and the applications of LM in biomedicines and intelligent robots, together with the consideration of key challenges facing LM‐based soft robotics and its future prospects.

Soft robotics technology can aid in achieving United Nations’ Sustainable Development Goals (SDGs) and the Paris Climate Agreement through development of autonomous, environmentally responsible machines powered by renewable energy. By utilizing soft robotics, we can mitigate the detrimental effects of climate change on human society and the natural world through fostering adaptation, restoration, and remediation. Moreover, the implementation of soft robotics can lead to groundbreaking discoveries in material science, biology, control systems, energy efficiency, and sustainable manufacturing processes. However, to achieve these goals, we need further improvements in understanding biological principles at the basis of embodied and physical intelligence, environment-friendly materials, and energy-saving strategies to design and manufacture self-piloting and field-ready soft robots. This paper provides insights on how soft robotics can address the pressing issue of environmental sustainability. Sustainable manufacturing of soft robots at a large scale, exploring the potential of biodegradable and bioinspired materials, and integrating onboard renewable energy sources to promote autonomy and intelligence are some of the urgent challenges of this field that we discuss in this paper. Specifically, we will present field-ready soft robots that address targeted productive applications in urban farming, healthcare, land and ocean preservation, disaster remediation, and clean and affordable energy, thus supporting some of the SDGs. By embracing soft robotics as a solution, we can concretely support economic growth and sustainable industry, drive solutions for environment protection and clean energy, and improve overall health and well-being.

More than 130,000 people have strokes each year in the United States [1]. Of these victims, 76% are left with disabilities that cost the nation over $54 billion in lost work and medical fees. One prominent disability is upper extremity hemiplegia, which occurs among approximately 50% of stroke sufferers [2]. Robotic technology has the potential to provide an automated platform for controlled rehabilitation and assisted, task-oriented therapy. Several systems have been designed to assist in patient articulation of an impaired hand using rigid robotic components. While these products have been successful in articulating the pure bending motions of the four fingers, the limited capabilities of rigid technologies fail to reproduce the complicated motion path [5] of the thumb during opposition grasp (see Figure 1). This is the most important articulation for normal hand function and specifically for picking up everyday objects. To date, robotic systems for thumb rehabilitation have not been widely investigated [3] apart from some recent work [4] that used a multi-joint rigid robot.

Soft robots are known for their ability to perform tasks with great adaptability, enabled by their distributed, non-uniform stiffness and actuation. Bending is the most fundamental motion for soft robot design, but creating robust, and easy-to-fabricate soft bending joint with tunable properties remains an active problem of research. In this work, we demonstrate an inflatable actuation module for soft robots with a defined bending plane enabled by forced partial wrinkling. This lowers the structural stiffness in the bending direction, with the final stiffness easily designed by the ratio of wrinkled and unwrinkled regions. We show the stiffness properties of the actuation module through a first-principle model validated by experimental characterization, and demonstrate the module's ability to maintain the kinematic constraint over a large range of loading conditions. We illustrate how these properties give the potential for complex actuation in a soft continuum robot and for decoupling actuation force and efficiency from load capacity. The module provides a novel method for embedding intelligent actuation into soft pneumatic robots.

Stacked balloon actuators, constructed from heat- and pressure-bonded, stacked layers of sheet films have demonstrated their utility in soft robotics for performing intricate tasks and adapting to space-constrained environments. However, these actuators face limitations such as reduced force output at low displacement heights due to buckling and small retractive forces at high displacement heights due to radial collapse under vacuum. To mitigate these issues, we introduce a method for integrating fiber reinforcements: a nylon mesh fabric that increases the tensile strength of the actuator's structural layers without constraining the maximum inflated height, and a heat-sealable nylon taffeta fabric that provides radial support to the actuator's multi-chamber design. We demonstrate the advantages of this approach in a quadruped soft robot, showcasing improved force output, actuation speed, and the advantageous use of actuator's collapsibility for shape morphing.

This chapter contributes to the modeling, analysis and control of terrestrial artificial locomotion systems. Inspired by previous models, we set up an unconventional model for a snake-like locomotion systems in form of a chain of visco-elastically interconnected mass points in a plane with passive joints, but – in contrast to literature – active links (time-varying link-length) and rotatable skids to change the movement direction and to avoid obstacles. We investigate this model in a dynamical way and focus on controlling these link lengths to achieve a global movement, steered by the skids. From dynamics, the actuator forces have to adjust the prescribed link length for the locomotion. Since it is impossible to determine the necessary actuator forces a-priori, we apply an adaptive lambda-tracking controller to enable the system to adjust these force outputs on-line on its own. Prescribed motion patterns, i.e. specific gaits, are required to guarantee a controlled movement that differ in the number of resting mass points, the load of actuators and spikes, and the lateral forces of the skids. In contrast to literature, the investigated system of n = 10 mass points exhibit a large variety of possible gaits. To determine the most advantageous gaits, numerical investigations are performed and a weighting function offers a decision of best possible gaits. Using these gaits, a gait transition algorithm, which autonomously changes velocity and number of resting mass points depending on the spike, actuator and lateral skid force load, is presented and tested in numerical simulations.

In recent years, the field of soft robotics has received considerable attention due to its potential in increasing the safety of human-robot interaction. The design of soft robots possesses great challenges. For example, the longstanding challenge of co-design morphology and actuation makes designing them by hand a trial-and-error process. Earlier work presented by the authors proposes a computational design synthesis (CDS) method for the automated design of virtual, soft locomotion robot morphologies. This work extends the CDS method for morphologies with the automated co-design of actuation. Two methods are considered. In the first method, the actuation of designs is described by parametric actuation curves (PACs) that model feedforward actuation patterns. For every morphology in the design process, a set of PACs is optimized that assumes symmetric and cyclic gaits. The second method, soft actor-critic (SAC) reinforcement learning, removes this assumption as well as models feedback control for comparison. Adding PAC optimization to the CDS method is shown to improve the performance of the resulting designs and to achieve better results within less design iterations. SAC is, however, deemed less effective, due to the need for design specific problem tuning for each new morphology. The SAC experiments also show that the best found soft robot gaits are symmetric and cyclic, although this is not a constraint in the SAC problem formulation, thus verifying the assumptions made in the PAC formulation. To validate the search space modeled in the co-design CDS method, a state-of-the-art soft robot is replicated and compared.

Repetitive motion rehabilitation training is vital for stroke recovery and postoperative rehabilitation, and the robot-assisted approach has been increasingly adopted in recent years. Soft wearable robots have attracted research interest due to their high compliance and favorable power-to-weight ratio. However, current research mainly focuses on assisting single upper-limb joints, while the limited output force of soft pneumatic actuators remains a significant limitation. In this work, we propose a rigidsoft coupled pneumatic actuator to address this limitation, enhancing output force while preserving compliance by embedding rigid sheets between continuously arranged bladders. By adjusting key parameters, we developed three actuators for the shoulder, elbow, and wrist joints, forming an integrated soft wearable suit. The joint output angles under different pressures were experimentally obtained, achieving shoulder abduction of 44.6°, elbow flexion of 97.3°, and wrist flexion of 48.4° at relatively low pressures. Surface electromyography (sEMG) tests reveal that under no-load conditions, the elbow actuator significantly reduces biceps brachii activity. Furthermore, under a 1.5 kg load, reductions of 17.1% and 26.7% in muscle activity were observed for dynamic and static assistance scenarios, respectively. The proposed suit enables safe, flexible, and effective multi-joint rehabilitation, supporting both active and passive training needs.

The field of soft robotics is rapidly evolving, and there is a growing interest in developing soft robots with bioinspired features for use in various applications. This research presented the design and development of 3D-printed origami actuators for a soft robot with amphibious locomotion and tongue hunting capabilities. Two different types of programmable origami actuators were designed and manufactured, namely Z-shaped and twist tower actuators. In addition, two actuator variations were developed based on the Z-shaped actuator, including the pelvic fin and the coiling/uncoiling types. The Z-shaped actuators were used for the rear legs to facilitate the locomotion of the water-like frogs. Meanwhile, the twisted tower actuators were used for the rotation joints in the forelegs and for locomotion on land. The pelvic fin actuator was developed to imitate the land locomotion of the mudskipper, and the coiling/uncoiling actuator was designed for tongue hunting motion. The origami actuators and soft robot prototype were tested through a series of experiments, which showed that the robot was capable of efficiently moving in water and on land and performing tongue hunting motions. Our results demonstrate the effectiveness of these actuators in producing the desired motions and provide insights into the potential of applying 3D-printed origami actuators in the development of soft robots with bioinspired features.