Jumping Tensegrity Robot Based on Torsionally Prestrained SMA Springs

A Multimodal Self-Propelling Tensegrity Structure

Tensegrity structure is composed of tensile cables and compressive rods, offering high stiffness-to-mass ratio, deploy ability, and excellent energy damping capability. The active and dynamic tensegrity designs demonstrate great potential for soft robots. In previous designs, the movement has relied on carefully controlled input power or manually controlled light irradiation, limiting their potential applications. Here, a hybrid tensegrity structure (HTS) is constructed by integrating thermally responsive cables, nonresponsive cables, and stiff rods. The HTS can self-propel continuously on a hot surface due to its unique geometry. The HTS allows for the easy achievement of multimodal self-propelled locomotive modes, which has been challenging for previously demonstrated self-propelling structures. Additionally, using Velcro tapes to adhere the rods and cables together, a modulable and reassemblable HTS is created. The HTS introduced in this study presents a new strategy and offers a large design space for constructing self-propelling and modulable robots.

Spikebot: A Multigait Tensegrity Robot with Linearly Extending Struts

Numerous recent research efforts have leveraged networks of rigid struts and flexible cables, called tensegrity structures, to create highly resilient and packable mobile robots. However, the locomotion of existing tensegrity robots is limited in terms of both speed and number of distinct locomotion modes, restricting the environments that a robot is capable of exploring. In this study, we present a tensegrity robot inspired by the volumetric expansion of Tetraodontidae. The robot, referred to herein as Spikebot, employs pneumatically actuated rigid struts to expand its global structure and produce diverse gaits. Spikebot is composed of linear actuators that dually serve as rigid struts linked by elastic cables for stability. The linearly actuating struts can selectively protrude to initiate thrust- and instability-driven locomotion primitives. Such motion primitives allow Spikebot to reliably locomote, achieving rolling, lifting, and jumping. To highlight Spikebot's potential for robotic exploration, we demonstrate how it achieves multi-dimensional locomotion over varied terrestrial conditions.

Design and locomotion characteristic analysis of two kinds of tensegrity hopping robots

This study proposed two kinds of tensegrity hopping robots, which were actuated by push-pull electromagnets and servo motors, respectively. Both tensegrity robots are able to conduct stable and consecutive hopping actions. This paper covers the robots' structural designs, theoretical modeling of the hopping actuators, and detailed analysis of the robot's self-righting properties, all of which are validated by corresponding experimental and simulational results. The first hopping robot could hop forward at an average speed of 0.641 body length/s. Although the second robot has a lower moving speed of 0.237 body length/s, its average jumping height of 0.301 m is nearly 2.5 times higher than that of the first robot. Then compared with other tensegrity rolling robots, the proposed two robots show obvious advantages in locomotion performance over their counterparts. Therefore, the proposed robots can have large potential in many fields such as space exploration, urban search, military surveillance, etc.

Shape Memory Alloy-Based Reactive Tubular (SMART) Brake for Compact and Energy-Efficient Wearable Robot Design

Soft wearable robots have been gaining increasing popularity for enhancing human physical abilities and assisting people who have physical limitations. These robots typically use tendon-driven mechanisms (TDMs) to enable remote actuation to provide better usability with compact design. TDMs comprise an actuator, an end-effector, and a transmission system by using cables or tendons to transfer forces from the actuator to the end-effector. Tendons are typically routed by frictionless guiding tubes to minimize force losses, variations in the force direction, and the volume. To make soft wearable robots even smaller, brakes need to be compacted because brakes are irreplaceable to ensure safety and energy efficiency. This study presents a shape memory alloy-based reactive tubular (SMART) brake for designing a compact and portable TDM-based device. The SMART brake actively adjusts the friction force between the brake and tendon, making it easy to achieve the desired friction state, ranging from low-friction states for free movement to high-friction states for effective braking. The brake is designed in a tubular shape, serving multifunctions as both a brake and a guiding tube. The brake's performance and theoretical model were validated through experiments and demonstrated by two wearable devices. The brake could hold a significant brake force of 19.37 N/11 mm while weighing only 0.3 g. These findings have major implications for the future development of TDM-based devices and soft wearable robots, paving the way for enhanced system portability, safety, and energy efficiency.

Soft Artificial Muscle Based on Pre-Detwinned Shape Memory Alloy Spring Actuator Achieving High Passive Assistive Torque for Wearable Robot

For designing the assistive wearable rehabilitation robots, it is challenging to design the robot as energy efficient because the actuators have to be capable of overcoming human loads such as gravity of the body and spastic torque continuously during the assistance. To address these challenges, we propose a novel design of soft artificial muscle that utilizes shape memory alloy (SMA) spring actuators with pre-detwinning process. The SMA spring was fabricated through a process called pre-detwinning, which enhances the linearity of the SMA spring in martensite phase and unpowered restoring force, which is called passive force. The fabricated SMA spring can contract >60%. Finally, the soft wearable robot that can assist not only the gravitational torque exerted on the elbow by passive force, but also the elbow movements with active force was designed with a soft artificial muscle. A soft artificial muscle consists of the bundles of pre-detwinned SMA springs integrated with the stretchable coolant vessel. The stiffness of the muscle was measured as 1125 N/m in martensite phase and 1732 N/m in austenite phase. In addition, the muscle showed great actuation frequency performances, the bandwidth of which was measured as 0.5 Hz. The proposed wearable mechanism can fully compensate the gravitational torque for all the angles in passive mode. In addition, the proposed mechanism can produce high torque up to 3.5 Nm and movements in active mode.

Shape Memory Alloy (SMA) Actuators: The Role of Material, Form, and Scaling Effects

Shape memory alloys (SMAs) are smart materials that are widely used to create intelligent devices because of their high energy density, actuation strain, and biocompatibility characteristics. Given their unique properties, SMAs are found to have significant potential for implementation in many emerging applications in mobile robots, robotic hands, wearable devices, aerospace/automotive components, and biomedical devices. Here, the state-of-the-art of thermal and magnetic SMA actuators in terms of their constituent materials, form, and scaling effects are summarized, including their surface treatments and functionalities. The motion performance of various SMA architectures (wires, springs, smart soft composites, and knitted/woven actuators) is also analyzed. Based on the assessment, current challenges of SMAs that need to be addressed for their practical application are emphasized. Finally, how to advance SMAs by synergistically considering the effects of material, form, and scale is suggested.

Shape Memory Alloys in Textile Platform: Smart Textile-Composite Actuator and Its Application to Soft Grippers

In recent years, many researchers have aimed to construct robotic soft grippers that can handle fragile or unusually shaped objects without causing damage. This study proposes a smart textile-composite actuator and its application to a soft robotic gripper. An active fiber and an inactive fiber are combined together using knitting techniques to manufacture a textile actuator. The active fiber is a shape memory alloy (SMA) that is wire-wrapped with conventional fibers, and the inactive fiber is a knitting yarn. A knitted textile structure is flexible, with an excellent structure retention ability and high compliance, which is suitable for developing soft grippers. A driving source of the actuator is the SMA wire, which deforms under heating due to the shape memory effect. Through experiments, the course-to-wale ratio, the number of bundling SMA wires, and the driving current value needed to achieve the maximum deformation of the actuator were investigated. Three actuators were stitched together to make up each finger of the gripper, and layer placement research was completed to find the fingers' suitable bending angle for object grasping. Finally, the gripping performance was evaluated through a test of grasping various object shapes, which demonstrated that the gripper could successfully lift flat/spherical/uniquely shaped objects.

Nanoscale Diamane Spiral Spring for High Mechanical Energy Storage

A compact, stable, sustainable, and high-energy density power supply system is crucial for the engineering deployment of mobile electromechanical devices/systems either at the small- or large-scale. This work proposes a spiral-based mechanical energy storage scheme utilizing the newly synthesized 2D diamane. Atomistic simulations show that diamane spiral can achieve a high theoretical gravimetric energy density of about 564 Wh kg^-1 , about 14 500 times the steel spring. The interlayer friction between diamane is found to cause a strong stick-slip effect that results in local stress/strain concentration. As such, the energy storage capacity of the diamane spiral can be tuned by suppressing the influence from the interlayer friction. Simulations affirm that higher gravimetric energy density can be achieved by reducing the turn number or adopting a low friction contact pair. The fundamental principles that dominate the energy storage capacity of the spiral spring are theoretically analyzed, respectively. The obtained insights suggest that the 2D vdW solids can be promising candidates to construct spiral structures with a high gravimetric energy density. This work should be beneficial for the design of reliable, stable, and sustainable nanoscale mechanical energy storage schemes that can be used as an alternative low-carbon footage energy supplier for novel micro-/nanoscale devices or systems.

Control Aspects of Shape Memory Alloys in Robotics Applications: A Review over the Last Decade

This paper mainly focuses on various types of robots driven or actuated by shape memory alloy (SMA) element in the last decade which has created the potential functionality of SMA in robotics technology, that is classified and discussed. The wide spectrum of increasing use of SMA in the development of robotic systems is due to the increase in the knowledge of handling its functional characteristics such as large actuating force, shape memory effect, and super-elasticity features. These inherent characteristics of SMA can make robotic systems small, flexible, and soft with multi-functions to exhibit different types of moving mechanisms. This article comprehensively investigates three subsections on soft and flexible robots, driving or activating mechanisms, and artificial muscles. Each section provides an insight into literature arranged in chronological order and each piece of literature will be presented with details on its configuration, control, and application.

A Review of SMA-Based Actuators for Bidirectional Rotational Motion: Application to Origami Robots

Shape memory alloys (SMAs) are a group of metallic alloys capable of sustaining large inelastic strains that can be recovered when subjected to a specific process between two distinct phases. Regarding their unique and outstanding properties, SMAs have drawn considerable attention in various domains and recently became appropriate candidates for origami robots, that require bi-directional rotational motion actuation with limited operational space. However, longitudinal motion-driven actuators are frequently investigated and commonly mentioned, whereas studies in SMA-based rotational motion actuation is still very limited in the literature. This work provides a review of different research efforts related to SMA-based actuators for bi-directional rotational motion (BRM), thus provides a survey and classification of current approaches and design tools that can be applied to origami robots in order to achieve shape-changing. For this purpose, analytical tools for description of actuator behaviour are presented, followed by characterisation and performance prediction. Afterward, the actuators’ design methods, sensing, and controlling strategies are discussed. Finally, open challenges are discussed.

A multi-motion bionic soft hexapod robot driven by self-sensing controlled twisted artificial muscles

Artificial muscles have unique advantages for driving bionic robots because their driving mode is similar to biological muscles. However, there is still a big gap between the existing artificial muscle and biological muscle in performance. The twisted artificial muscles (TAMs) from nylon 6,6 provides a low-cost, high integration, low hysteresis driving method. But as a soft actuator, the control of the TAM is so complex that the advantage of excellent embeddedness has not been brought into play. This work presents a self-sensing control method for the TAM by monitoring the real-time resistance of the heating wire which realizes the accurate controlling of the TAM temperature. The simultaneous control of 18 TAMs is realized by using the self-sensing control method. By using a new step walking method based on the principle of insect bionics, a bionic soft hexapod robot with both multi-motion and load capacity is realized. Besides, due to the excellent environmental adaptability of the TAM, the bionic robot can realize amphibious motion both on land and underwater conditions, and the corresponding maximum load capacities are 300 g and 1 kg, respectively. This work not only provides a reliable self-sensing control method of the TAMs but also promotes the development of bionic soft robots.

Shape memory materials for electrically-powered soft machines

We review the recent progress of electrically-powered artificial muscles and soft machines using shape memory alloy and liquid crystal elastomer.