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Variable stiffness mechanisms have become popular for improving the performance of meso/microscale robots due to their ability to enable safe, effective, and versatile operation without additional actuators. In minimally invasive surgery (MIS), they can provide low stiffness for adaptability and an extended workspace and high stiffness for stable manipulation. This study aims to optimize the pattern design that maximizes the stiffness ratio while preventing buckling. Longitudinal slits were identified as the ideal shape for maximizing the ratio using topology optimization. The effects of the design parameters were investigated by analytical modeling and finite element analysis (FEA). Finally, we established a design optimization process to maximize the variable stiffness ratio while ensuring safety. The load and buckling tests of 19 cases were verified in both the experiment and FEA. Buckling occurred in eight cases, consistent with the expected results from the proposed map. Among the tested cases, the tube with the optimal pattern parameter set exhibits the highest stiffness ratio of 2.67 while satisfying the given stiffness constraint (greater than 0.3 times the flexural stiffness of the initial tube) and avoiding buckling. The proposed optimization method potentially enables an extended workspace and more versatile functionality for MIS instruments.

Tendon-driven soft wearable robots should be simple, compact, and safe because they are worn on the human body and interacts directly with the human. Here, unlike rigid robots, wire pretension should be removed at the end-effector due to the inherent characteristics of the soft robot. In this paper, for the stable and compact actuation system without pretension, a linear actuator named Slider-Tendon Linear Actuator is proposed. The proposed actuator is designed to make a linear motion using a tendon rather than other linear transmissions such as ball screw or lead screw transmission. By using the proposed design method that uses the tendon, the actuator size was reduced to 23.6% of the size of an actuator using a ball screw because the tendon can endure high tensile force with a small cross-section area. Furthermore, this paper proposes a robot design methodology to locate the robot mechanism in the actuator, rather than in the end-effector. The proposed actuator is designed to contain fast-connection and stroke amplified under-actuation mechanism. With this method, not only is the complexity and size of the worn part reduced but also its performance is improved. Finally, by applying this actuator to a specific wearable robot, this paper verifies that the proposed actuator sufficiently satisfies the wearable robot requirements.

Origami provides a unique tool for the design of robotic frames owing to its simple shaping principle by “folding.” However, achieving the fast and reversible activeness of a highly reconfigurable structure remains challenging owing to the limitations of accessible actuators. In particular, it is difficult to find an actuator that can realize a simultaneously large, rapid, reversible, and stable movement while leading to a favorable form factor for the origami. To overcome this, in this article, we propose a 3-D shape-shifting system consisting of a morphing origami block that complements the stability problem of shape memory alloy wire actuators by tuning its structural characteristics. This cooperative scheme improves the reversibility and stability of the shape-shifting system, which enables the rapid transformation with high degrees of freedom unlike in existing programmable origami. As a stand-alone unit of transformation, morphing block equipped with deployable mechanism and actuators weighs 6 g and has a volume change factor of ten. Furthermore, the transformation time in both directions is less than 5 s, and the block can carry more than 120 g of payload in the deployed state. The proposed system composed of multiple origami blocks can reconfigure itself into diverse 3-D target shapes.

Layer-by-layer manufacturing of composite mechanisms allows fast and cost-effective fabrication of customized robots in millimeter and centimeter scales which is promising for research fields that rely on frequent and numerous physical iterations. Due to the limited number of components that can be directly integrated in composite structures, however, often an assembly step is necessary which diminishes the benefits of this manufacturing method. Inspired by the Japanese craft of cutting (kiri-) paper (-gami), Kirigami, we introduce quasi-2D and highly stretchable functional Kirigami layers for direct integration into the composite mechanisms. Depending on the material and geometrical design; functional Kirigami layers can perform as flat springs, stretchable electronics, sensors or actuators. These components will facilitate the design and manufacturing of composite robots for different applications. To illustrate the effectiveness of these components, we designed and realized a foldable composite inchworm robot with three Kirigami layers serving as actuator, sensor and contact pad with directional friction. We elaborate on the working principle of each layer and report on their combined performance in the robot.

Soft wearable hand robots with tendon-sheath mechanisms are being actively developed to assist people with lost hand mobility. For these robots, accurately estimating fingertip forces leads to successful object grasping. An approach can utilize information from actuators assuming quasi-static environments. However, non-linearity and hysteresis with regards to the dynamic changes of the tendon-sheath mechanism hinder accurate fingertip force estimation. This paper proposes a learning-based method to estimate fingertip forces by integrating dynamic information of motor encoders, wire tension, and sheath bending angles. The model is modified from Long Short-Term Memory by incorporating a residual term that governs the dynamic changes in sheath bending angles. Using a tendon-driven soft wearable hand robot, the proposed model obtained RMSE less than 0.44 N. It was further evaluated under criteria ranging from different object sizes, bending angle ranges, and forces. Finally, a repeatability test (0.46 N in RMSE), real-time applicability (125 Hz), and force control (12.7% in MAPE) were performed to verify the feasibility of the proposed method.

Transparency in electronics can provide extra functionality and esthetic impression. Transparency plays an important role in accurate soft robot control because one can directly observe target surface condition that is usually blocked by a robot's body. Nowadays, demand for soft actuators has been rapidly increasing because soft robots have attracted much attention recently. However, conventional soft actuators are usually nontransparent with simple isotropic bending, limited performance, and limited functionality. To overcome such limitations of current soft robots, we developed a novel soft shape morphing thin film actuator with new functionalities such as high transparency and unique directional responses to allow complex behavior by integrating a transparent metal nanowire heater. A figure of merit was developed to evaluate the performance and derive an optimum design configuration for the transparent actuator with enhanced performance. As a proof of concept, various transparent soft robots such as transparent gripper, Venus flytrap, and transparent walking robot were demonstrated. Such transparent directional shape morphing actuator is expected to open new application fields and functionalities overcoming limitations of current soft robots.

Soft robots have been extensively researched due to their flexible, deformable, and adaptive characteristics. However, compared to rigid robots, soft robots have issues in modeling, calibration, and control in that the innate characteristics of the soft materials can cause complex behaviors due to non-linearity and hysteresis. To overcome these limitations, recent studies have applied various approaches based on machine learning. This paper presents existing machine learning techniques in the soft robotic fields and categorizes the implementation of machine learning approaches in different soft robotic applications, which include soft sensors, soft actuators, and applications such as soft wearable robots. An analysis of the trends of different machine learning approaches with respect to different types of soft robot applications is presented; in addition to the current limitations in the research field, followed by a summary of the existing machine learning methods for soft robots.

Soft actuators have been widely studied in recent years because of their ability to adapt to diverse environments and safely interact with humans. Their softness broadens their potential range of medical applications since they can provide inherent safety. Among the various motions a soft robot can perform, "torsion" can maximize the efficiency of motion in confined spaces like the human abdominal cavity. This paper presents a fully soft actuator with a double-helix tendon routing path for large-angle torsional motions. The double-helix tendon routing enables the actuator to generate large twisting deformations, while also avoiding buckling generally associated with the torque imbalance in small diameter soft cylinder structures. A sequential casting method was developed for cylindrical structures with internal double-helix pathing. A parametric study of the actuator’s twisting angle and the axial contraction with respect to different design parameters was conducted, including the wire tension and path pitch. From the results, when the tendon was pulled with 40 N after the pitch was decreased, the axial contraction of the soft actuator was reduced by half and the torsional angle was doubled up to 600° without buckling.

The size of a device and its adaptability to human properties are important factors in developing a wearable device. In wearable robot research, therefore, soft materials and tendon transmissions have been utilized to make robots compact and adaptable to the human body. However, when used for wearable robots, these methods sometimes cause uncertainties that originate from elongation of the soft material or from undefined human properties. In this research, to consider these uncertainties, we propose a data-driven method that identifies both kinematic and stiffness parameters using tension and wire stroke of the actuators. Through kinematic identification, a method is proposed to find the exact joint position as a function of the joint angle. Through stiffness identification, the relationship between the actuation force and the joint angle is obtained using Gaussian Process Regression (GPR). As a result, by applying the proposed method to a specific robot, the research outlined in this paper verifies how the proposed method can be used in wearable robot applications. This work examines a novel wearable robot named Exo-Index, which assists a human&rsquo, s index finger through the use of three actuators. The proposed identification methods enable control of the wearable robot to result in appropriate postures for grasping objects of different shapes and sizes.

Customization is an important issue for assistive gloves because it affects glove performance. In this letter, we propose an assembly based customizable soft pneumatic assistive glove, named Exo-Glove PM. Pneumatic soft robots generally consist of a single structure with embedded actuators. However, when assembled, the region where the assemblies are connected easily undergoes large stress concentration that leads to failure. To overcome this issue, we have developed a hybrid actuator module that combines a soft actuation structure and rigid joining methods. In general, rigid joining methods require a bulky design to enable easy assembly, but Exo-Glove PM is designed to create pathways for assembly tools to access the bolt heads by bending certain parts. This novel way of joining assemblies allows soft robots to be built of multiple parts while minimizing the volume. It ensures small module size and enables modules to cover a wide range of hand sizes; the distance between each module is matched to the length of the user's phalanges using spacers. This approach allows users to customize the glove by assembly of standardized modules and maximizes the comfort and the performance of the glove without custom manufacturing and, thus, reduces costs.

Tendon drives are widely used in robotics. The compliance of the tendon in such drives suits them for soft robots, including soft wearable robots, but several issues impede their use. Generally, the tendon should always maintain tension to prevent derailment from the spool. However, in soft robots, tendon tension induces high friction forces owing to the absence of ball bearings. Because the kinematics of the soft robot is basically nonlinear and changed by the deformation of the structure, the kinematic difference between the soft structure and the spool causes derailment of the tendon. Moreover, continuously maintained tension in soft wearable robots causes safety issues. The linear actuator can be an option. However, the need to increase the length of the linear actuator to accommodate the excursion length of its tendon is a barrier to its use in small-scale applications. To preclude this issue, a slack-enabling actuator that employs a spool is proposed. The space efficiency of the spool enables the mechanism to be small, and a one-way clutch applies unidirectional friction force to the tendon to tighten the tendon around the spool. This paper describes the design concept for the slack-enabling mechanism, its design optimization, and system identification for force control.

Muscular hydrostats have long been a source of inspiration for soft robotic designs. With their inherent compliance, they excel in unpredictable environments and can gently manipulate objects with ease. However, their performance lacks where high force or a fast-dynamic response is needed. In this study, we propose a novel spring reinforced actuator (SRA) that explores the intermediate state between muscular hydrostats and endoskeletal mechanisms. The result is that we dramatically enhance the robot dynamic performance, which is unprecedented in similar kinds of soft robots, while retaining compliant omnidirectional bending. Analytical modeling of the flexible backbone was built and experimentally validated. This is also the first attempt to perform detailed finite element analysis to investigate the strain-stress behavior of the constraining braided bellow tube. The braided interweaving threads are modeled, in which complex thread-to-thread contacts occur. Experimental evaluation of SRAs was performed for actuation force, stiffness, and dynamic response. We showcase the enhanced actuator's performance in several applications such as locomotion and heavy object manipulation.

Lymphedema is a non-curative chronic swelling caused by impairment of the lymphatic system, affecting up to 250 million patients worldwide. The patients suffer from low quality of life because of discomfort and reduced range of motion due to the swelling. Severe swellings can be immediately mediated with special massaging technique known as the Manual Lymphatic Drainage (MLD). Limitations of MLD involves long travel distances, the cost of regular treatment sessions, and the lack of lymphedema specialists. Since MLD is performed very gently, described as caressing a baby's head, soft wearable robotics with its inherent compliance and safety is the perfect solution to creating a light and safe wearable lymphedema massaging device. In this paper, origami-inspired soft fabric pneumatic actuator is developed that creates not only normal force, but also shear force which is essential in the performance of MLD. The shear is created by the unfolding of the Z-shaped fold-lines as the actuator is inflated. One Z-folded actuator module of 30 x 60 mm dimension with a single fold of 15 mm fold height creates maximum shear force of about 1.5 N and stroke displacement of about 30 mm when subjected to compression loading of 5 N. The range of forces exerted can be tuned by varying the tension of the compressive clothing covering the actuators, and the stroke displacement can be varied by changing the parameter of the actuator module itself, such as the fold height and the number of the folds. The modules can also be repeatedly actuated under compressive clothing, and therefore, the developed actuator modules have high potential as a wearable massaging device.

Inchworm, a kind of caterpillar, traverses various terrains by moving its center of mass (COM) with simple two-anchor crawling locomotion. Because of this advantage, many soft locomotion robots were developed to mimic this inchworm-like two-anchor crawling. However, most of these developed inchworm robots have difficulties in moving its COM in the vertical height direction, so these robots face difficulties in overcoming discrete high terrain, like stairs. In this paper, we propose a flexible sheet-based inchworm robot (Loco-sheet) that employs a novel locomotion mode made by morphing to overcome discrete high terrains, like stairs. In this novel mode, called the S-shape locomotion mode, the robot changes its body shape into an ‘ $\mathbf{S}$ ’ to lift the COM up, by exploiting bending stiffness of the flexible sheet to climb stairs. The bending stiffness of the sheet was designed to be sufficient to lift up the COM. The robot also does two-anchor crawling, called the omega-shape locomotion mode, to pass through low gaps. Loco-sheet is designed with anisotropic friction wheels and a mass distribution advantageous for omega-shaped locomotion without slippage. Morphing between two modes is reversible. Motor-tendon driven actuation system allows the robot to pull the far end of the sheet from the actuator to fold the body, and allows the robot to operate untethered from an external power supply with small volume actuators. The robot climbs a 90 mm stair, passes through 72 mm low gaps, and travels flat surfaces. Therefore, Loco-sheet is suitable for traveling a variety of undefined environments.

This paper introduces soft LEGO for bottom-up design platform of soft robotics that can be used for various purposes, ranging from research and fast prototyping of soft robots to toys and entertainment. We integrated the interlocking mechanism of LEGO into a modular soft robot. With this design, soft robots could be built by a simple and play-like assembling process. Three kinds of components were proposed to make soft robotics compatible with LEGO: pneumatically inflatable soft brick, flexible bending brick, and channel brick. The soft brick has an air chamber and can generate motions when inflated. The bending brick has flexure and is bendable for generating motion when the assembled soft bricks are pneumatically actuated. The air channel brick has an air channel inside and works as an interface between air hoses and soft LEGO bricks. Detailed design parameters of the soft brick were optimized based on the Taguchi method with finite-element analysis to improve robustness. Design of the bending brick was selected based on experimental results to enhance the robustness of the flexure. Thanks to the multi-material 3-dimensional printer, the soft LEGO bricks could be fabricated with a single printing process. To see the feasibility of soft LEGO as a bottom-up design platform, a simple toy robot for children and a gripper that had a hybrid mechanism of hard and soft materials were built and tested. We hope this soft LEGO could lower the hurdle of soft robotics for children, researchers from other fields, and the public interest in robotics.

Microrobots that are light and agile yet require no artificial power input can be widely used in medical, military, and industrial applications. As an actuation system to drive such robots, here we report a biologically inspired bilayer structure that harnesses the environmental humidity energy, with ratchets to rectify the motion. We named this actuator-ratchet system the hygrobot. The actuator uses a hygroscopically responsive film consisting of aligned nanofibers produced by directional electrospinning, which quickly swells and shrinks in lengthwise direction in response to the change of humidity. The ratchets based on asymmetric friction coefficients rectify oscillatory bending motion in a directional locomotion. We mathematically analyzed the mechanical response of the hygrobot, which allowed not only prediction of its performance but also the optimal design to maximize the locomotion speed given geometric and environmental constraints. The hygrobot sterilized a trail across an agar plate without any artificial energy supply.

Recently, there have been many researches about applications of origami to mechanical engineering, which realizes a 3D sturcture by folding a 2D plane material. With this simple manufacturing process, origami was even adopted by some roboticists as a way to build an entirely new robot with benefits in terms of cost, weight, and structural simplicity. In this paper, we propose a new type of a walking robot based on origami structure. Because all the components of the robot that generate gait motion are mechanically connected, it can actually walk forward with only a single actuator. We also showed the similarity of gait trajectories between a kinematic analysis and the actual gait motion measured by video tracking. This result proved the possibility of designing an origami-based robot with the identical gait trajectory as we plan.

Pneumatic artificial muscles (PAMs) are frequently used actuators in soft robotics due to their structural flexibility. They are generally characterized by the tensile force due to the axial contraction and the radial force with volume expansion. To date, most applications of P AMs have utilized axial contractions. In contrast, we propose a novel way to control radial expansions of particular P AMs using anisotropic behaviors. P AMs generally consist of a cylindrical rubber bladder that expands with injection of air and multiple flexible but inextensible strings or mesh that surround the bladder to generate axial contraction force. We propose methods of generating radial expansion force in two ways. One is to control the spatial density of the strings that hold the bladder, and the other is to give asymmetric patterns directly to the bladder for geometrical anisotropy. To evaluate the performance of the actuators, soft sensors made of a hyperelastic material and a liquid conductor were attached to the P AMs for measuring local strains and pressures of the PAMs. We also suggest use of the proposed PAMs to a wearable therapeutic device for treating text neck symptoms as an application. The P AMs were used to exert a pressure to the back of the neck to recover the original spinal alignment from the deformed shape.

Field of soft robotics has been widely researched. Modularization of soft robots is one of the effort to expand the field. In this paper, we introduce a magnet connection for modularized soft units which were introduced in our previous research. The magnet connector was designed with off the shelf magnets. Thanks to the magnet connection, it was simpler and more intuitive than the connection method that we used in previous research. Connecting strength of the magnet connection and bending performance of a soft bending actuator assembled with the units were tested. Connecting strength and air leakage prevention of the connector was affordable in a range of actuating pneumatic pressure. We hope that this magnet connector enables modularized soft units being used as a daily item in the future.

We report on the design, optimization, and performance evaluation of a new wheel-leg hybrid robot. This robot utilizes a novel transformable wheel that combines the advantages of both circular and legged wheels. To minimize the complexity of the design, the transformation process of the wheel is passive, which eliminates the need for additional actuators. A new triggering mechanism is also employed to increase the transformation success rate. To maximize the climbing ability in legged-wheel mode, the design parameters for the transformable wheel and robot are tuned based on behavioral analyses. The performance of our new development is evaluated in terms of stability, energy efficiency, and the maximum height of an obstacle that the robot can climb over. With the new transformable wheel, the robot can climb over an obstacle 3.25 times as tall as its wheel radius, without compromising its driving ability at a speed of 2.4 body lengths/s with a specific resistance of 0.7 on a flat surface.

This paper presents the complete development and analysis of a soft robotic platform that exhibits peristaltic locomotion. The design principle is based on the antagonistic arrangement of circular and longitudinal muscle groups of Oligochaetes. Sequential antagonistic motion is achieved in a flexible braided mesh-tube structure using a nickel titanium (NiTi) coil actuators wrapped in a spiral pattern around the circumference. An enhanced theoretical model of the NiTi coil spring describes the combination of martensite deformation and spring elasticity as a function of geometry. A numerical model of the mesh structures reveals how peristaltic actuation induces robust locomotion and details the deformation by the contraction of circumferential NiTi actuators. Several peristaltic locomotion modes are modeled, tested, and compared on the basis of speed. Utilizing additional NiTi coils placed longitudinally, steering capabilities are incorporated. Proprioceptive potentiometers sense segment contraction, which enables the development of closed-loop controllers. Several appropriate control algorithms are designed and experimentally compared based on locomotion speed and energy consumption. The entire mechanical structure is made of flexible mesh materials and can withstand significant external impact during operation. This approach allows a completely soft robotic platform by employing a flexible control unit and energy sources.

Soft robotics, a concept contrary to conventional “hard” robotics, is a robot design methodology that uses soft materials inspired by nature. In contrast to a hard robot, a soft robot is composed of soft and flexible materials that blur the distinction between an actuator and a structure, which leads to unique characteristics that cannot be found in a conventional hard robot. This paper presents our approach to the issues that arise when the concept of soft robotics is applied to a wheeled robot. The compliance of the wheel diversifies its potential movement and allows for a high degree of adaptability to the environment. Although the wheel radius of the robot is 50 mm, it can pass through a 30 mm gap and climb a 45 mm step. While soft robotics displays properties whose performance can be challenging to implement, it also enables us to create complex forms of movement in a cheaper and simpler way. We expect that this kind of approach can provide a new design method for a deformable wheel.

This paper proposes three design concepts for developing a crawling robot inspired by an inchworm, called the Omegabot. First, for locomotion, the robot strides by bending its body into an omega shape; anisotropic friction pads enable the robot to move forward using this simple motion. Second, the robot body is made of a single part but has two four-bar mechanisms and one spherical six-bar mechanism; the mechanisms are 2-D patterned into a single piece of composite and folded to become a robot body that weighs less than 1 g and that can crawl and steer. This design does not require the assembly of various mechanisms of the body structure, thereby simplifying the fabrication process. Third, a new concept for using a shape-memory alloy (SMA) coil-spring actuator is proposed; the coil spring is designed to have a large spring index and to work over a large pitch-angle range. This large-index-and-pitch SMA spring actuator cools faster and requires less energy, without compromising the amount of force and displacement that it can produce. Therefore, the frequency and the efficiency of the actuator are improved. A prototype was used to demonstrate that the inchworm-inspired, novel, small-scale, lightweight robot manufactured on a single piece of composite can crawl and steer.

In order to operate a search and rescue robot in hazardous area, the robot requires high mobility and adaptable locomotion for moving in unpredictable environments. In this paper, we propose the deformable soft wheel robot that can produce three kinds of driving modes; caterpillar driving mode, normal wheel driving mode, legged-wheel driving mode. The robot changes its driving mode as it faces the various obstacles such as a small gap, stairs etc. Soft film and composite materials are used for fabrication of deformable wheel structure and Shape Memory Alloy (SMA) coil spring actuators are attached on the structure as an artificial muscle. Film lamination and an composite manufacturing process is introduced and the robot design is required to be modified and compromised to applying the manufacturing process. The prototype is developed and tested for verifying feasibility of the deformable wheel locomotion.

Legged locomotion has been widely researched due to its effectiveness in overcoming uneven terrains. Due to previous efforts there has been much progress in achieving dynamic gait stability and as the next step, mimicking the high speed and efficiency observed in animals has become a research interest. The main barrier in developing such a robotic platform is the limitation in the power efficiency of the actuator: the use of pneumatic actuators produce sufficient power but are heavy and big; electronic motors can be compact but are disadvantageous in producing sudden impact from stall which is required for high speed legged locomotion. As a new attempt in this paper we suggest a new leg design for a mobile robot which uses the slider-crank mechanism to convert the continuous motor rotation into piston motion which is used to impact the ground. We believe this new mechanism will have advantage over conventional leg mechanism designs using electronic motors since it uses the continuous motion of the motor instead of sudden rotation movements from stall state which is not ideal to draw out maximum working condition from an electronic motor. In order to control impact timing from the periodic motion of the piston a mechanical passive clutch trigger mechanism was developed. Dynamic analysis was performed to determine the optimal position for the mechanical switch position of the clutch trigger mechanism, and the results were verified through simulation and experiment. Development of a legged locomotion with two degrees of freedom, slider-crank mechanism for impact and additional actuation for swing motion, is proposed for future work.

Small mobile robots which use round wheels are suitable for driving on a flat surface, but it cannot climb the obstacle whose height is greater than the radius of wheels. As an alternative, legged-wheels have been proposed by many researchers due to its better climbing performance. However, driving and climbing performances have a trade-off relationship so that their driving performance should be sacrificed. In this study, in order to achieve both driving and climbing performances, a new transformable wheel was developed. The developed transformable wheel can have a round shape on a flat surface and change its shape into legged-wheel when it makes a contact with an obstacle. For design of the transformable wheel, the performance of legged-wheel was analyzed with respect to the number and curvature of the leg, and then the new transformable wheel was designed based on the analysis. Contrary to the existing transformable wheels that contain additional actuators for the transformation, the developed transformable wheel can be unfolded without any additional actuator. In this study, in order to validate the transformable wheel, a simple robot platform was fabricated. Consequently, it climbed the obstacle whose height is 2.6 times greater than the wheel radius.

In this paper, we propose a self-deployable origami structure and its application to a gliding wing module for jump-gliding. Generally, origami structures are composed of flat rigid facets. By using curved compliant facets instead of flat rigid facets, we endowed the origami structure with the ability to self-deploy and self-lock. When the origami structure is folded, the curved facets gradually flatten, and potential energy is stored on the flattened facets. By using the stored energy, the structure can rapidly deploy without an actuator. The geometry of the curved facet generates a notably high joint stiffness in the deployed state, which can be used as a self-locking mechanism. In jump-gliding, the gliding wing should be foldable and lightweight for efficient jumping and capable of rapid deployment for efficient gliding. By applying the suggested origami structure as a basic frame, the deployable gliding wing module can satisfy these requirements. The wing span of the module is 44.7 cm, and it weighs 20.3 g including a battery and a servo motor. The module can be folded in 1/8.1 of the deployed area, and it deploys within 350 ms when it needs to glide.

In this paper, we propose a new shape-shifting system. Shape-shifting is achieved by the integration of arranged identical transformable units. These are self-folding origami modules that transform their shape from a cubic to a flat shape, and vice versa, similarly to a three-dimensional (3D) morphing pixel. By changing the shape of individual modules, the system can generate desired shapes. Each module is based on a modified Kresling pattern, which has bi-stability during deployment so that it can maintain its shape in both states without any actuator force. Also, it can rapidly and reversibly transform its shape via folding and unfolding motions that are generated by a low-profile torsional SMA wire actuator positioned at fold lines. This approach is a hybrid of the modular and folding approaches so that the system can generate shape-shifting without complicated communication methodology between modules and bulky infrastructure, and has fast, compact and lightweight hardware. We demonstrate performance with a 3×3 matrix form of this module (40 mm side length) that can make five different shapes and it required only 15 seconds per transformation. This new shape-shifting system could be applied to design multi-functional self-folding origami such as a morphing 3D map.

Fleas can jump more than 200 times their body length. They do so by employing a unique catapult mechanism: storing a large amount of elastic energy and releasing it quickly by torque reversal triggering. This paper presents a flea-inspired catapult mechanism for miniature jumping robots. A robotic design was created to realize the mechanism for the biological catapult with shape memory alloy (SMA) spring actuators and a smart composite microstructure. SMA spring actuators replace conventional actuators, transmissions, and the elastic element to reduce the size. The body uses a four-bar mechanism that simulates a flea's leg kinematics with reduced degrees of freedom. Dynamic modeling was derived, and theoretical jumping was simulated to optimize the leg design for increased takeoff speed. A robotic prototype was fabricated with 1.1-g weight and 2-cm body size that can jump a distance of up to 30 times its body size.

Tendon driven mechanism is one of the most popular mechanism for transmitting force and power from a distance. The energy efficiency of a tendon driven system can be improved if it can maintain actuation force while it is not moving without mechanical work. This could be achieved by a brake; a brake without an additional actuator is preferred for the compactness of the whole system. We present a novel passive brake mechanism, a capstan brake, which consists of a capstan and two one-way clutches. The friction between the capstan and the cable amplifies a small resisting force (originated from an inactive motor) to gain enough brake force. Because no additional actuator is involved, generation of the brake force does not consume energy. Also, the one-way clutches enable the capstan to rotate in the winding direction. Therefore, the brake force is exerted only when it is needed, and the performance of the whole device does not decrease owing to the use of the capstan brake. The performance of the proposed brake mechanism has been evaluated through several tests. The results show that the amount of the maximum brake force for the test condition is more than 55 N (and can be further increased by increasing the number of windings), and that the force loss from the brake is negligible.

In this paper, biomimetic underwater robots built using smart actuators, e.g., a shape memory alloy (SMA), an ionic polymer metal composite (IPMC), lead zirconate titanate (PZT), or a hybrid SMA and IPMC actuator, are reviewed. The effects of underwater environment were also considered because smart actuators are often affected by their external environment. The characteristics of smart actuators are described based on their actuating conditions and motion types. Underwater robots are classified based on different swimming modes. We expanded our classification to non-fish creatures based on their swimming modes. The five swimming modes are body/caudal actuation oscillatory (BCA-O), body/caudal actuation undulatory (BCA-U), median/paired actuation oscillatory (MPA-O), median/paired actuation undulatory (MPA-U), and jet propulsion (JET). The trends of biomimetic underwater robots were analyzed based on robot speed (body length per second, BL/s). For speed per body length, robots using an SMA as an actuator are faster than robots using an IPMC when considering a similar length or weight. Robots using a DC motor are longer while their speeds per body length are similar, which means that robots using smart actuators have an advantage of compactness. Finally, robots (using smart actuators or a motor) were compared with underwater animals according to their speed and different swimming modes. This review will help in setting guidelines for the development of future biomimetic underwater robots, especially those that use smart actuators.

This paper reviews various processes for manufacturing new type of robots termed “soft biomimetic robots.” Most robots are made of rigid metallic materials. But in recent years, various biomimetic robots based on soft materials and compliant parts have been developed. New manufacturing processes are required to fabricate these types of robots, and the processes include Shape Deposition Manufacturing (SDM) and Smart Composite Microstructures (SCM). Since the design of robots are limited by the available material and manufacturing processes, it is important to develop new manufacturing processes that will enable development of novel soft biomimetic robots. In this paper, various manufacturing processes which can be applied to soft robot fabrication are summarized, and features of those processes are described. Processes are divided into three categories; soft robot body fabrication, actuators for soft robots and stretchable electronics. This review provides a guideline for selecting manufacturing processes for soft robots and developing new processes that will enable new type of robots to be designed.

Small multi-legged animals that can climb vertical walls with a rough surface have inspired research on climbing locomotion. Most robots that can climb a rough vertical wall are large and heavy due to the large number of actuators required to produce the complex locomotion. This paper proposes a novel design for a small and lightweight climbing robot that uses a single actuator. To guarantee reliable wall climbing, the contact phase of two tripods should overlap. A quick return leg is designed to enable phase overlap without requiring an extra actuator. Alternating tripods are also designed, and small spines with compliance are modeled based on the pseudo-rigid-body model. Layer-based fabrication is used to reduce weight. The resulting biomimetic platform is 10cm long and 10.8g in weight and can climb up a near-vertical brick wall at a rate of 5.57mm/sec.

Tendon drive is commonly used in various kind of machines. And, the spool is commonly used to wind and unwind the tendon because of its simplicity and compactness. However, the slack of the tendon causes the derailment of the tendon out of the spool and fails the system. In order to use the tendon drive with the spool without concerning about the derailment, the slack enabling actuator is previously proposed. In this paper, the performance criteria and the design parameters affecting the performance of the slack enabling actuator are discussed.

Tendon driven mechanism has been widely used in mechanical systems because of its versatility to mechanical systems requiring complex transmission. This paper proposes a control strategy of slack enabling tendon actuator using feedback linearization. The slack enabling actuator is a newly developed tendon actuator that does not fail although the tendon slacks (non-positive tension) at the outlet of the actuator which is an essential operating phase when the tendon driven is applied to the soft wearable robots. To deal with the dynamic nonlinearities of the slack enabling actuator, input-output feedback linearization was adopted and linear disturbance observer was used to estimate the output tension of the wire and reject position following error due to the external load. The results show that the estimated tension coincide with the measured tension and a proposed method have proper characteristics to control the position of the tendon while estimating the tension of the output tendon.

Friction along the Bowden-cable transmission degenerates control performance unless it is properly compensated. Friction is produced when the bending angle of the Bowden-cable changes as the relative position of the actuator and the end-effector changes. This study proposes a method, termed loop routing, to compensate friction along the Bowden-cable. Loop routing involves making a one-round loop along the sheath that continuously maintains the sheath's bending angle at 2π regardless of the end-effector's position in 2-D space. This minimizes the bending angle change of the sheath as the end-effector translates in a 3-D workspace, which minimizes the friction change and enables feedforward friction compensation of the Bowden-cable without employing a sensor. An experiment in open-loop tension control of the Bowden-cable was conducted to evaluate the performance of the proposed method. Results show that the output tension follows the reference tension well, with an RMS error of 4.3% and a peak error of 13.3% of maximum reference.

A soft wearable robot with a tendon drive is a promising technology that enables a wearable robot to be compact and lightweight. A soft tendon routing system was previously proposed to apply a tendon drive to a soft wearable robot.

One of the major problems in utilizing origami structures is ensuring variable stiffness; a deployable structure needs to become stiff or flexible according to the requirements of its use in an application. In this study, we present a self-deploying tubular origami mechanism that switches between two distinctive states: small and flexible at its normal state and rigid and stiffened at a locked state. By embedding compact torsional SMA actuators into the mechanism in a novel way through stitching, the process of deploying from the normal to the locked state proceeds in a simple and low-profile manner. With global heating, the torsional SMA wires activate a buckling effect that draws a radical change of folding line from one diagonal to another in every unit tile of the tube, creating axial stiffness. The activated structure, which weighs only 2.9 g, can endure a load of 2.7 kg or more. Additionally, since it does not require bulky actuators, this origami structure can be highly mobile and small in size. This novel origami mechanism is expected to be useful in a wide variety of applications, such as aerospace equipment, mobile architecture, and medical devices, especially those used in minimally invasive surgery (MIS).

A soft wearable robot, which is an emerging type of wearable robot, can take advantage of tendon-driven mechanisms with a Bowden cable. These tendon-driven mechanisms benefits soft wearable robots because the actuator can be remotely placed and the transmission is very compact. However, it is difficult to compensate the friction along the Bowden cable which makes it hard to control. This study proposes the use of a position-based impedance controller, which is robust to the nonlinear dynamics of the system and provides compliant interaction between robot, human, and environment. Additionally, to eliminate disturbances from unexpected tension of the antagonistic wire arising from friction, this study proposes a new type of slack enabling tendon actuator. It can eliminate friction force along the antagonistic wire by actively pushing the wire while preventing derailment of the wire from the spool.

An active needle is proposed for the development of magnetic resonance imaging (MRI)-guided percutaneous procedures. The needle uses a low-transition-temperature shape memory alloy (LT SMA) wire actuator to produce bending in the distal section of the needle. Actuation is achieved with internal optical heating using laser light transported via optical fibers and side coupled to the LT SMA. A prototype, with a size equivalent to a standard 16-gauge biopsy needle, exhibits significant bending, with a tip deflection of more than 14 $^\circ$ in air and 5 $^\circ$ in hard tissue. A single-ended optical sensor with a gold-coated tip is developed to measure the curvature independently of temperature. The experimental results in tissue phantoms show that human tissue causes fast heat dissipation from the wire actuator; however, the active needle can compensate for typical targeting errors during prostate biopsy.

A new approach to artificial muscle actuator design is presented, and is implemented using shape memory alloys (SMA). An array of SMA actuators is segmented into many independently controlled, spatially discrete volumes, each contributing a small displacement to create a large motion. The segmented cellular architecture of SMA wires is extended to a multiaxis actuator array by arranging the segments in a two-dimensional (2-D) array. The multiaxis control is streamlined and coordinated using a 2-D segmentation method in order to activate multiple links of a robot mechanism in a coordinated manner. The basic principle of segmented binary control (SBC) is first presented, followed by multiaxis segmentation theory and a design procedure. The method is applied to a five-fingered robotic hand capable of taking a variety of postures. A 10-axis SMA actuator array is built, and SBC is implemented using Peltier-effect thermoelectric devices for selective local heating and cooling. Experiments demonstrate the feasibility and effectiveness of the new method

A new approach to the design and control of shape memory alloy (SMA) actuators is presented. SMA wires are divided into many segments and their thermal states are controlled individually as a group of finite state machines. Instead of driving a current to the entire SMA wire and controlling the wire length based on the analog strain–temperature characteristics, the new method controls the binary state (hot or cold) of individual segments and thereby the total displacement is proportional to the length of the heated segments, i.e. austenite phase. Although the thermomechanical properties of SMA are highly nonlinear and uncertain with a prominent hysteresis, segmented binary control is robust and stable, providing characteristics similar to a stepping motor. However, the heating and cooling of each segment to its bi-stable states entail longer time and larger energy for transition. In this paper, an efficient method for improving the speed of response and power consumption is developed by exploiting the inherent hysteresis of SMA. Instead of keeping the extreme temperatures continuously, the temperatures return to intermediate 'hold' temperatures closer to room temperature but sufficient to keep constant phase. Coordination of the multitude of segments having independent thermal states allows for faster response with little latency time even for thick SMA wires. Based on stress dependent thermomechanical characteristics, the hold temperature satisfying a given stress margin is obtained. The new control method is implemented using the Peltier effect thermoelectric devices for selective segment-by-segment heating and cooling. Experiments demonstrate the effectiveness of the proposed method.

Self-folding origami requires a low-profile actuator to be embedded in a sheet of paper-like planar material. Various actuation methods have been employed to actively fold such sheets. This paper presents a torsion shape-memory alloy (SMA) wire actuator embedded in patterned origami structures that actively folds the origami by twisting the SMA wire. A simple wire is aligned with the fold line, and each end is fixed to a facet. The twisting of the wire directly rotates the facets. This method has the advantage of using an easily available wire SMA and the advantage of a flat form factor similar to that of sheet SMA. Generally, SMA wire is used in a linear manner or as a spring. The torsion SMA wire presented in this paper is trained to generate torsional force when heated. The amount of rotation depends on the length of the wire; a 200-μm-diameter SMA wire 12 mm in length can induce 540° rotation. SMA wires are arranged in pairs side by side to rotate the facets in both directions. Maximum torque of 70 mNcm is generated in this antagonistic arrangement. The torsion SMA wire actuators enable a novel design for a programmable folding sheet that is easily manufactured and exhibits fast folding and unfolding.Copyright © 2014 by ASME

The Venus flytrap uses bistability, the structural characteristic of its leaf, to actuate the leaf's rapid closing motion for catching its prey. This paper presents a flytrap-inspired robot and novel actuation mechanism that exploits the structural characteristics of this structure and a developable surface. We focus on the concept of exploiting structural characteristics for actuation. Using shape memory alloy (SMA), the robot actuates artificial leaves made from asymmetrically laminated carbon fiber reinforced prepregs. We exploit two distinct structural characteristics of the leaves. First, the bistability acts as an implicit actuator enabling rapid morphing motion. Second, the developable surface has a kinematic constraint that constrains the curvature of the artificial leaf. Due to this constraint, the curved artificial leaf can be unbent by bending the straight edge orthogonal to the curve. The bending propagates from one edge to the entire surface and eventually generates an overall shape change. The curvature change of the artificial leaf is 18 m(-1) within 100 ms when closing. Experiments show that these actuation mechanisms facilitate the generation of a rapid and large morphing motion of the flytrap robot by one-way actuation of the SMA actuators at a local position.

The electromechanical impedance (EMI) method for NDE uses a single piezoelectric material to act as an actuator and a sensor simultaneously, and the EMI method is suitable for structures with complex surfaces. However, this technique still has wide range of problems which needs to be investigated. For one, locating damaged areas on a host structure precisely is known to be extremely difficult as this non-model based technique heavily relies on the variations in the impedance signatures. In this study, an attempt to locate the damaged areas on an ordinary concrete panel and a lightweight concrete panel using bottom ash is carried out by using different frequency ranges. Since the sensing range decreases as the excitation frequency of piezoelectric material increases, one can possibly predict the damaged areas by analyzing the impedance signatures from different frequency ranges. Statistical analysis method such as root mean square deviation (RMSD) is applied to determine the changes of the experimental structures, and the RMSD values of low frequency range and high frequency range are compared to verify the relationship between the frequency range and sensing range. Furthermore, the applicability of this method to locating the damaged areas is investigated on various materials including the lightweight concrete.

An insect size micro jumping mechanism is developed and jumps 40cm. The prototype is fabricated with the composite structures cut by precision UV laser. The robot mechanism is bio-mimetic system that is inspired by the small jumping insect, Flea. A single sheet shape memory alloy coil actuator is used for propulsion and energy storage. The compliant mechanism in the body allows to reduce the number of actuators for triggering. The robot mechanism has 36mg weight, 2 cm length and 2mm height except of wire legs.

The transmission determines the speed and force of the driven part with respect to the actuator. Because the transmission ratio is generally fixed, the velocity and force of the driven part are in trade off relationship. In this paper, to vary the transmission ratio of the tendon driven links, variable moment arm mechanism is proposed. The moment arm of the wire to the joint is changed according to the increase of the tension of the wire. By the static analysis, the motion and force characteristics of the mechanism is verified. Simulation results show that the rotational joint with the mechanism can rotates faster and generates larger force compared with conventional rotational joints while same actuator is used.

Various oscillating parameters have been used to increase the velocity of a robotic dolphin. Among them, an efficient and easy way to increase the thrust is to increase an oscillating frequency of the robotic dolphin. However, there is a trade-off. If the oscillating frequency increases, the stability of the robotic dolphin decreases. Especially, a reactive motion is occurred in a head of the robotic dolphin due to the fluctuating motion of a caudal fin. The unsteady motion in the head position can be controlled using an additional actuator. However, the best solution is to reduce an undesired motion without adding the actuator. In this paper, to reduce the fluctuating problem, we consider the flexibility of the caudal fin. An oscillating amplitude of the caudal fin and head motion is changed with varying the stiffness of the caudal fin. Therefore, if we find the appropriate stiffness of the caudal fin, the head motion can be stabilized.

In this paper, we present a deformable wheel robot using the ball-shaped waterbomb origami pattern, so-called magic-ball pattern. The magic-ball origami pattern is a well-known pattern that changes its shape from a long cylindrical tube to a flat circular tube. By using this special structure, a wheel with mechanical functionalities can be achieved without using many mechanical parts. Moreover, because of the characteristic that the structure constrains its own movement, it is possible to control the whole shape of the wheel using only few actuators. And also, from analysis of the wheel structure in kinematic model, the performance of the wheel and determine the condition for actuators can be predicted. We think that the proposed design for the deformable wheel shows the possibility of using origami structure as a functional structure with its own mechanism.Copyright © 2013 by ASME