Twisted string actuators (TSAs) convert rotational motion from twisting into linear motion. They are known for high energy efficiency, and large linear strain and stress outputs. Although they have been successfully applied as the moving mechanism for different robot applications, their potential in soft robotics is mainly challenged by two aspects: First, the conventional strings of TSAs are stiff and strong but not compliant. Second, precise control of TSAs predominantly relies on external position or force sensors. Because of these, TSA-driven robots are often rigid and bulky. In this study, we propose the design, modeling, and robotic application of TSAs that are compliant, can produce large strain, and are capable of self-sensing during twisting-induced actuation. The design is realized by replacing conventional stiff strings with compliant, thermally activated, and conductive supercoiled polymer strings. Experiments show that the developed TSAs have normalized stiffness of 30%, and position self-sensing capability during twisting. The quasi-static actuation and self-sensing properties are accurately captured by the Preisach hysteresis operators. In particular, both the twisting-induced actuation and thermally induced actuation are considered. Finally, the proposed TSAs are successfully demonstrated in a low-cost three-dimensionally printed compliant robotic gripper.

Compliant, Large-Strain, and Self-Sensing Twisted String Actuators.

Fiber-Shaped Soft Actuators: Fabrication, Actuation Mechanism and Application

Realizing high-performance soft robotic grippers is challenging because of the inherent limitations of the soft actuators and artificial muscles that drive them, including low force generation, small actuation range, and poor compactness to name a few. Despite advances in this area, realizing compact soft grippers, which exhibit high dexterity and force output, is still challenging. This article explores using twisted string actuators (TSAs) to drive a soft robotic gripper. TSAs have been widely used in numerous robotic applications, but their inclusion in soft robots has been limited. The proposed design of the gripper was inspired by the human hand, with four fingers and a thumb. Tunable stiffness was implemented in the fingers by using antagonistic TSAs. The fingers' bending angles, actuation speed, blocked force output, and stiffness tuning are experimentally characterized. The gripper achieves a score of 6 on the Kapandji test and recreate 31 of the 33 grasps of the Feix GRASP taxonomy. It exhibits a maximum grasping force of 72 N, which is almost 13 times its own weight. A comparison study reveals that the proposed gripper exhibits equivalent or superior performance compared to other similar soft grippers. 

Anthropomorphic Twisted String-Actuated Soft Robotic Gripper With Tendon-Based Stiffening

In recent years, the field of soft robotics has gained much attention by virtue of its aptness to work in certain environments unsuitable for traditional rigid robotics. Along with the uprising field of soft robotics is the increased attention to soft actuators which provide soft machines the ability to move, manipulate, and deform actively. This article provides a focused review of various high-performance and novel electrically driven soft actuators due to their fast response, controllability, softness, and compactness. Furthermore, this review aims to act as a reference guide for building electrically driven soft machines. The focus of this paper lies on the actuation principle of each type of actuator, comprehensive performance comparison across different actuators, and up-to-date applications of each actuator. The range of actuators includes electro-static soft actuators, electro-thermal soft actuators, and electrically driven soft pumps.

/pdf/a-review-of-electrically-driven-soft-actuators-for-soft-k4jgjzm1.pdf

A Review of Electrically Driven Soft Actuators for Soft Robotics

Over the past decade, soft robot research has expanded to diverse fields, including biomedicine, bionics, service robots, human–robot interaction, and artificial intelligence. Much work has been done in modeling the kinematics and dynamics of soft robots, but closed‐loop control is still in its early stages due to limited sensory feedback. Thanks to the advancement in functional materials, structures, and manufacturing techniques for flexible electronics, flexible and stretchable sensors are developing rapidly. These sensors provide feedback for closed‐loop control tasks and enable soft robots to effectively explore the unknown and safely interact with humans and the environment. Herein, recent advances in perceptive soft robots that utilize flexible/stretchable sensors and functional materials are outlined. The perceptive functions of soft robots from two different aspects, that is, proprioception and exteroception, are summarized. Furthermore, the constructions of autonomous soft robots by integrating both proprioceptive and exteroceptive capabilities for closed‐loop control tasks and other challenging tasks in the real world are discussed.

Recent Advances in Perceptive Intelligence for Soft Robotics

Twisted string actuators (TSAs) have exhibited great promise in robotic applications by generating high translational force with low input torque Despite great success, it remains a challenge to reliably estimate the strain of TSAs using compact solutions while maintaining actuator compliance The inclusion of position sensors not only increases system complexity but also decreases system compliance, a property often crucial in soft robots We recently constructed a compliant TSA with self-sensing capability by adopting conductive and stretchable super-coiled polymer (SCP) strings; however, only quasi-static measurements of the strain-resistance correlation were obtained This study proposes a strategy to experimentally characterize and model the transient self-sensing property in compliant TSAs The correlation between resistance and strain is characterized under different motor twisting sequences and step durations, and exhibited transient decay, hysteresis, and creep A self-sensing model that consists of a log-based nonlinear term, a rate-dependent Prandtl-Ishlinskii hysteresis term, and a creep term is proposed for the compliant TSAs Experimental results confirm the high effectiveness of the proposed self-sensing approach, with the average model validation error less than 0036 cm

Experimental Characterization and Modeling of the Self-Sensing Property in Compliant Twisted String Actuators

Actively tunable optical filters based on chalcogenide phase-change materials (PCMs) are an emerging technology with applications across chemical spectroscopy and thermal imaging. The refractive index of an embedded PCM thin film is modulated through an amorphous-to-crystalline phase transition induced through thermal stimulus. Performance metrics include transmittance, passband center wavelength (CWL), and bandwidth; ideally monitored during operation (in situ) or after a set number of tuning cycles to validate real-time operation. Measuring these aforementioned metrics in real-time is challenging. Fourier-transform infrared spectroscopy (FTIR) provides the gold-standard for performance characterization, yet is expensive and inflexible -- incorporating the PCM tuning mechanism is not straightforward, hence in situ electro-optical measurements are challenging. In this work, we implement an open-source MATLAB-controlled real-time performance characterization system consisting of an inexpensive linear variable filter (LVF) and mid-wave infrared camera, capable of switching the PCM-based filters while simultaneously recording in situ filter performance metrics and spectral filtering profile. These metrics are calculated through pixel intensity measurements and displayed on a custom-developed graphical user interface in real-time. The CWL is determined through spatial position of intensity maxima along the LVF's longitudinal axis. Furthermore, plans are detailed for a future experimental system that further reduces cost, is compact, and utilizes a near-infrared camera.

Automated real-time spectral characterization of phase-change tunable optical filters using a linear variable filter and infrared camera

Twisted string actuators (TSAs) can produce linear motions by converting the electric motor’s rotary motion. TSAs exhibit high power density, energy efficiency, and translational force. However, a common challenge in utilizing TSAs as soft and compliant actuators is to simultaneously increase their compliance and maximum strain. Previous studies predominantly utilized strings with high stiffness. Recent studies attempted to use strings with low stiffness with a sacrifice of maximum strain, especially under large loading conditions. In this paper, a novel strategy is proposed to simultaneously increase the compliance and maximum strain of TSAs. By replacing stiff strings with stretchable and coiled nylon strings, called supercoiled polymer (SCP) strings, the compliance of the TSA is increased. Furthermore, by heating the TSA with SCP strings through Joule heating, additional linear contraction is obtained. The fabrication procedure of the proposed TSA is provided. Experimental characterizations of stiffness and actuation properties are conducted with different actuator configurations and loading conditions. Experiments confirm the enhanced performance of the proposed TSA using SCPs.

Compliant and Large-Strain Twisted String Actuators using Supercoiled Polymers


 The twisted string actuator (TSA), as a recently discovered artificial muscle, has attracted a lot of attention as a compliant and powerful actuation mechanism. A TSA consists of two strings attached to a motor on one end and a load on the other end. The motor’s rotation twists the strings and generates linear actuation. A common challenge is to obtain TSAs’ strains using compact approaches. Previous studies exclusively utilized external position sensors that not only increased system cost, size and complexity, but also lowered actuator compliance. In this paper, self-sensing strategies are presented to estimate TSAs’ strains without external sensors. By incorporating conductive and stretchable nylon strings, called super-coiled polymer (SCP) strings, into TSAs, their strains can be estimated from the resistance values of SCP strings. Two self-sensing configurations are realized: (1) TSA with one regular string and one SCP string, and (2) TSA with two SCP strings. Experiments are conducted to show the correlation between the length and resistance of TSA under different conditions. Polynomial and Preisach hysteresis models were successfully employed to capture the Length – Resistance correlation and to estimate TSA’s length using the resistance.

Self-Sensing for Twisted String Actuators Using Conductive Supercoiled Polymers

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