Torsional Actuator Research Articles

Humidity-responsive fiber actuators assembled from cellulose nanofibrils

Fiber actuators, particularly valuable in soft robotics and environmental sensing, are at the forefront of “smart” materials and materials innovation. In this realm, torsional and tensile biofiber actuators, notable for their cost-effectiveness and biodegradability, mark a critical gap in the development of next-generation functional systems and devices. To address this gap, this study showcased moisture-responsive fiber actuators made from cellulose nanofibrils (CNFs). The initial focus of this contribution was on an innovative torsional actuator, which capitalized on the hydrophilic nature of the CNFs filaments produced through wet-spinning processes. These robust filaments, with a mechanical strength of (237.0 ± 10.7) MPa, were twisted to form the torsional actuator. This actuator demonstrated a rapid rotational response, achieving up to 1180 rpm within merely 10 s of exposure to moisture, and maintained high durability over multiple cycles. Building upon this platform, the study continued and aimed to build up a tensile actuator, which ingeniously integrated a supercoiled nylon fiber core within a moisture-sensitive CNFs sheath. This design enhances the structural support and functionality of the actuator. The pursued transition from torsional to tensile actuator demonstrates an iterative and innovative approach in actuator technology, underscoring the versatility and potential of CNFs in the realm of “smart” actuation materials.

The Design, Simulation, and Parametric Optimization of an RF MEMS Variable Capacitor with an S-Shaped Beam

This study presents the design and simulation of an RF MEMS variable capacitor with a high tuning ratio and high linearity factor of capacitance–voltage response. An electrostatic torsion actuator with planar and non-planar structures is presented to obtain the high tuning ratio by avoiding the occurrence of pull-in point. In the proposed design, the capacitor plate is connected to the electrostatic actuators by using the s-shaped beam. The proposed design shows a 138% tuning ratio with the planar structure of the actuator and 167% tuning ratio by implementing the non-planar structure. A linearity factor of 99% is attained by adjusting the rates at which the capacitor plate rises as the actuation voltage increases and the rate at which the capacitance decreases as the plate rises. Parametric optimization of the design is performed by utilizing the finite element method (FEM) analysis and high-frequency structural simulator (HFSS) analysis to obtain an optimized high-tuning ratio RF MEMS varactor at low actuation voltage. S-parameters of the design are presented on HFSS, with a 50 ohm coplanar waveguide (CPW) serving as the transmission line. The proposed RF MEMS varactor can be utilized in tunable RF devices.

Пьезоэлектрический дисковый актюатор кручения с дискретно-спиральными электродами

A mathematical model of a disk (ring) torsion actuator with curvilinear interdigital electrodes – «villi» of a spiral form, which are characterized by a constant value of the orientation angle of an arbitrary section of the villus to the circumferential direction of the disk, has been developed. Bases of electrode-villi interacting with each other through local areas of piezoelectric layer are connected with periodic alternation to central or peripheral concentric circular base electrodes, to outputs of which control voltages are connected. As a result of numerical modeling of the «twisting bimorph» graphs of the dependence of the twist angle and the blocking torque on the ratio of the inner and outer radii and the values of the control voltage at the outputs of the base electrodes of the annular two-layer actuator with symmetrical (relative to the radial line) directions of the spiral orientations of the electrode-villi and, as a result, the spiral polarization lines of the piezoelectric layers working for tension/compression.

A Novel Design of a Torsional Shape Memory Alloy Actuator for Active Rudder.

SMA actuators are a group of lightweight actuators that offer advantages over conventional technology and allow for simple and compact solutions to the increasing demand for electrical actuation. In particular, an increasing number of SMA torsional actuator applications have been published recently due to their ability to supply rotational motion under load, resulting in advantages such as module simplification and the reduction of overall product weight. This paper presents the conceptual design, operating principle, experimental characterization and working performance of torsional actuators applicable in active rudder in aeronautics. The proposed application comprises a pair of SMA torsion springs, which bi-directionally actuate the actuator by Joule heating and natural cooling. The experimental results confirm the functionality of the torsion springs actuated device and show the rotation angle of the developed active rudder was about 30° at a heating current of 5 A. After the design and experiment, one of their chief drawbacks is their relatively slow operating speed in rudder positioning, but this can be improved by control strategy and small modifications to the actuator mechanism described in this work.

A new model of thermo-mechanical actuation for twisted and coiled polymer muscles with initial curvature

The twisted and coiled polymer (TCP) artificial muscle is one type of novel soft actuator for mimicking natural skeletal muscle that can provide large linear and torsional actuation and energy density. Twisting and coiling are the pivotal steps in fabricating TCP muscles. The influence of twisting on the actuation response of TCP muscles has been extensively investigated recently. However, the influence of coiling remains unclear. Based on the finite strain theory, we establish a new thermo-mechanical actuation model for TCP muscles with initial curvature. The theoretical predictions based on the model align well with the finite element simulations, accurately capturing the actuation response of thermally-activated TCP muscles. It is revealed that twisting contributes positively to the actuation, while coiling has a passive effect. Geometrical parameters, such as the helix radius and helix angle, can effectively regulate the actuation performance of TCP muscles. Furthermore, an optimal bias angle is identified that maximizes both the recovery torque and the linear actuation. This study sheds light on the structural optimization design of TCP muscles.

Torsional actuation and vari-stiffness characteristics of SMA/basalt hybrid braided composite tubes

Shape memory alloy hybrid composite (SMAHC) tubes have the potential to actively control the torsional stiffness of structures. In this work, shape memory alloy (SMA) wires orthogonally braided with basalt fiber were heated by electricity to generate the torsional torque of the SMAHC tube, and then the torsional stiffness of the tube was measured. The macroscopic finite element model of the SMAHC tube was established to assist in the analysis of the torsional stiffness variation before and after heating. According to the experimental results, the actuating torsional ability was affected by the phase transformation recovery force of SMA. The active control of torsional stiffness depends on the change of SMA modulus, the magnitude and direction of SMA wires’ recovery force, and the thermodynamic properties of matrix at different temperatures. Then an evaluation model for the torsional stiffness control effect was proposed. These research results can be used for the adaptive control of the stiffness and vibration characteristics of rotating composite structures, and provide a novel and effective method for the control of structural torsional characteristics with lightweight effects.

Periodic oscillations in electrostatic actuators under time delayed feedback controller

In this paper, we prove the existence of two positive T-periodic solutions of an electrostatic actuator modeled by the time-delayed Duffing equation ẍ(t)+fD(x(t),ẋ(t))+x(t)=1−eV2(t,x(t),xd(t),ẋ(t),ẋd(t))x2(t),x(t)∈]0,∞[where xd(t)=x(t−d) and ẋd(t)=ẋ(t−d), denote position and velocity feedback respectively, and V(t,x(t),xd(t),ẋ(t),ẋd(t))=V(t)+g1(x(t)−xd(t))+g2(ẋ(t)−ẋd(t)),is the feedback voltage with positive input voltage V(t)∈C(R/TZ) for e∈R+,g1,g2∈R, d∈0,T. The damping force fD(x,ẋ) can be linear, i.e., fD(x,ẋ)=cẋ, c∈R+ or squeeze film type, i.e., fD(x,ẋ)=γẋ/x3, γ∈R+. The fundamental tool to prove our result is a local continuation method of periodic solutions from the non-delayed case (d=0). Our approach provides new insights into the delay phenomenon on microelectromechanical systems and can be used to study the dynamics of a large class of delayed Liénard equations that govern the motion of several actuators, including the comb-drive finger actuator and the torsional actuator. Some numerical examples are provided to illustrate our results.

Investigation of mechanical and electrical characteristics of self-sensing pneumatic torsional actuators

Investigation of mechanical and electrical characteristics of self-sensing pneumatic torsional actuators

High-Performance One-Body Electrochemical Torsional Artificial Muscles Built Using Carbon Nanotubes and Ion-Exchange Polymers.

Electrochemical torsional artificial muscles have the potential to replace electric motors in the field of miniaturization. In particular, carbon nanotubes (CNTs) are some of the best materials for electrochemical torsional artificial muscles due to their remarkable mechanical strength and high electrical conductivity. However, previous studies on CNT torsional muscle utilize only half of the whole potential range for torsional actuation because the actuations in the positive and negative voltage ranges offset each other. Here, we used an ion-exchange polymer, poly(sodium 4-styrenesulfonate) (PSS), which leads to the participation of only positive ions in the actuation of CNT muscles so that the whole potential range can be used for torsional actuation. As a result, PSS-coated CNT muscle can provide 1.9 times higher torsional actuation compared to neat CNT torsional muscle. This PSS-coated CNT muscle not only provides high performance but also facilitates a one-body system for electrochemical torsional actuation. From these advantages, we implement a one-body torsional muscle for the realization of the forward motion of a model boat. This high performance and one-body structure for electrochemical torsional muscles can be used for further applications, such as soft robotics and implantable devices.

Torsional Pneumatic Actuator Based on Pre-Twisted Pneumatic Tubes for Soft Robotic Manipulators

The human forearm produces torsional motions that are essential in daily activities such as opening doors or containers. It has a large range of motion (ROM), can produce large torques, can move bidirectionally, and can stiffen at will. Implementing such motions in a rigid robotic manipulator is easy due to the inherently rotational motions of electric motors, but pneumatic actuators do not inherently produce rotational motions. Indeed, most torsional pneumatic actuators are capable of unidirectional or bidirectional motions but do not have the antagonistic actuation capabilities necessary to have variable stiffness. Furthermore, their actuation characteristics are very different from those of the human forearm. This article proposes a torsional pneumatic actuator based on pretwisted pneumatic tubes that uses a hybrid hard-soft structure. These pneumatic tubes can be installed at an offset from the rotation axis such that antagonistic tubes can be installed on a single actuator. The actuator is capable of bidirectional torsional motions, can change its torsional stiffness, and has an actuation performance comparable to that of the human forearm. It was implemented in a soft robotic manipulator with seven degrees of freedom with a wide ROM and is capable of rotating objects while manipulating them.

Fabrication and Actuation of Magnetic Shape-Memory Materials.

Soft actuators are deformable materials that change their dimensions or shape in response to external stimuli. Among the various stimuli, remote magnetic fields are one of the most attractive forms of actuation, due to their ease of use, fast response, and safety in biological systems. Composites of magnetic particles with polymer matrices are the most common materials for magnetic soft actuators. In this paper, we demonstrate the fabrication and actuation of magnetic shape-memory materials based on hydrogels containing field-structured magnetic particles. These actuators are formed by placing the pregel dispersion into a mold of the desired on-field shape and exposing it to a homogeneous magnetic field until the gel point is reached. At this point, the material may be removed from the mold and fully gelled in the desired off-field shape. The resultant magnetic shape-memory material then transitions between these two shapes when it is subjected to successive cycles of a homogeneous magnetic field, acting as a large deformation actuator. For actuators that are planar in the off-field state, this can result in significant bending to return to the on-field state. In addition, it is possible to make shape-memory materials that twist under the application of a magnetic field. For these torsional actuators, both experimental and theoretical results are given.

Influence of optical property contrast on the critical distribution of electrostatic torques in double-beam torsional Casimir actuators: Non-linear actuation toward chaotic motion.

Here, we discuss how to achieve the stable actuation of a double beam torsional micro-actuator over the largest possible displacement of the moving component under the influence of Casimir and electrostatic torques, when the rotating component is constructed from different materials. The main part of this study is devoted to finding the optimal distribution of the electrostatic torque between the left and right sides of the micro-actuator to reach the maximum stable operation of the device. The latter is manifested by switching from homoclinic to heteroclinic orbits in the phase portraits. Indeed, the bifurcation curves and the phase portraits have been employed to show the sensitivity of the critical distribution of the electrostatic torque, beyond which the device does show stable performance, on the contrast of the optical properties of the moving component and the applied voltage in a conservative autonomous system. Moreover, for driven systems, the Melnikov function approach and the Poincaré portraits are used to study the presence of chaotic motion, which eventually leads to stiction. It is shown that the application of the optimal distribution of the electrostatic torque can significantly decrease the possibility of chaotic motion, and at this optimal level, the threshold curves reveal less difference between systems with different optical contrast.

Largely deformable torsional soft morphing actuator created by twisted shape memory alloy wire and its application to a soft morphing wing

We present a new type of torsional soft morphing actuator designed and fabricated by twisted shape memory alloy (SMA) wires embedded in polydimethylsiloxane matrix. The design and fabrication process of the proposed soft morphing actuator are presented with investigations of its working mechanism. Actuation performance was evaluated with respect to the temporal response, the maximum torsional deformation under an applied electric current, and various design parameters including the twist direction, wire diameter, helical pitch of the SMA wire, and the actuator’s thickness and length. We demonstrate potential applications of the proposed soft morphing actuator as a soft morphing wing and airfoil. The proposed actuator will aid in the development of soft actuators, soft robotics, and other relevant scientific and engineering applications.

Design and Characteristics of Shape Memory Alloy Bidirectional Torsional Actuator

To overcome the problems of the low power/weight ratio of traditional servo motors, complex transmission systems, and large space occupied by hydraulic drive systems, this paper designs a bidirectional torsional actuator using the shape memory effect of Ti-Ni shape memory alloy (SMA) wire, which aims to realize the folding and unfolding of the wings of flying cars. The model of the designed actuator was established and the theoretical calculation was analyzed, based on which the experimental verification was carried out. It is found that when the heating current is 1.6 A, the mechanism can take into account the economy under the premise of meeting a certain folding speed. When the pre-stretching amount of SMA wire is 3%, the residual strain corresponding to the wire is small, about 0.577%, and it can provide driving torque and torsion angle to meet the design requirements of the actuator. This verifies the feasibility of the actuator and obtains the driving characteristics of the actuator. Finally, the shortcomings and improvement measures of the actuator are analyzed briefly.

Design and Performance Analysis of a Torsional Soft Actuator Based on Hyperelastic Materials

Conducting research on soft robots is crucial as there are still many problems that need to be resolved in the areas of material selection, structure design and manufacture, and drive control. Soft manipulators, a subset of soft robots, are now a popular area of study for many researchers. In comparison to typical manipulators, soft manipulators feature a high degree of gripping flexibility and a basic morphological structure. They are composed of flexible materials. They have a wide range of potential applications in healthcare, rehabilitation, bionics, and detection, and they can compensate for the drawbacks of rigid manipulators in some use scenarios. A modular soft-body torsional gripping system is developed after a torsional and gripping actuator is conceived and constructed, and its performance examined. The torsion actuator and the grasping actuator can be combined in the system in a modular fashion. With the help of RGB-D vision algorithms, this multi-modular setup makes it possible to combine soft actuators with various twisting degrees and achieve exact gripping. Through pneumatic control, the target object is precisely grasped and rotated at various angles, enabling the rotation of the target object in three dimensions.

Twistocaloric Modeling of Elastomer Fibers and Experimental Validation.

The twistocaloric effect is attributed to the change in entropy of the material driven by torsional stress. It is responsible for the torsional refrigeration of fiber materials that has been widely exploited as one of the solid-state cooling techniques with high efficiency and low volume change rate. The lack of theories and mathematical models of twistocaloric effect, however, limits broad applications of torsional refrigeration. In this work, a twistocaloric model is established to capture the relationship between twist density and temperature variation of natural rubber fibers and thermoplastic elastomer yarns. An experimental setup consisting torsion actuator and torque sensor coupled with a temperature measurement system is built to validate the model. Using the Maxwell relationship, twistocaloric coefficient is measured by quantifying the thermal effect induced by torsion under shear strain. The experimental characterization of the twistocaloric effect in natural rubber fiber and thermoplastic elastomer yarn are consistent with the theoretical predictions.

Reconfigurable Soft Pneumatic Actuators Using Extensible Fabric-Based Skins.

The development of the field of soft robotics has led to the exploration of novel techniques to manufacture soft actuators, which provide distinct advantages for wearable assistive robotics. One subset of these soft pneumatic actuators is conventionally developed from silicone, fabrics, and thermoplastic polyurethane (TPU). Each of these materials in isolation possesses limitations of low-stress capacity, low-design complexity, and high-input pressure requirements, respectively. Combining these materials can overcome some limitations and maintain their desirable properties. In this article, we explore one such composite design scheme using a combination of silicone polymer-based bladder and reconfigurable fabric skin made from an anisotropic extensible fabric. The silicone polymer bladder acts as the hermetic seal, while this skin acts as the constraint. Bending and torsional actuators were designed utilizing the anisotropy of these fabrics. The torsional actuator designs can achieve over 540° of twist, significantly larger than previously reported in the literature, owing to the lower mechanical impedance of the extensible fabrics. Actuators with 360° of bending were also fabricated using this method. In addition, the lack of TPU-backed or inextensible fabrics reduces the actuator's stiffness, leading to lower actuation pressures. Skin-based designs also confer the advantage of modularity, reconfigurability, and the ability to achieve complex motions by tuning the properties of the bladder and the skin. For applications with high-force requirements, such as wearable exoskeletons, we demonstrate the utility of multilayer design schemes. A multilayer bending actuator generated 190 N of force at 100 kPa and was shown to be a candidate for wearable assistive devices. In addition, torsional designs were shown to have utility in practical scenarios such as screwing on a bottle cap and turning knobs. Thus, we present a novel fabric-skin-based design concept that is highly versatile and customizable for various application requirements.

Electromechanical analysis of a self-sensing torsional micro-actuator based on CNTs reinforced piezoelectric composite with damage

Electromechanical microstructures are increasingly attractive for various applications of smart devices. However, electrostatic actuator is sensitive to pull-in instability caused by microscopic dispersion forces, taking risks of stiction, adhesion, and even failure. Making micro-actuators self-sensing is a promising strategy for active modulation of device design. Although some progress has been made for respective sensing and actuating, it is still challenging to build integrating sensing-actuating device. In this work, we proposed one self-sensing actuating composite structure for constructing an electrostatic torsional actuator. We modelled composite nanobeams capable of self-adjusting pull-in titling angle of micro-actuator based on strain gradient theory with size dependency and damage. Theoretical and numerical results showed good agreements and verified the accuracy of proposed model in this paper. The variation of CNTs volume fraction can tune torsional stiffness of torsional nanobeam, and thus increase the stable working range of micromirror. By introducing piezoelectric layers into laminated torsional structures, the working state and the damage of device can be sensed and driven by piezoelectric effect, and the stability can be adjusted and predicted to avoid pull-in instability. This work can contribute to structural optimization of electromechanical devices.

High-fidelity modeling of dynamic origami folding using Absolute Nodal Coordinate Formulation (ANCF)

Origami has evolved into a framework for creating engineering systems at vastly different scales: from large deployable airframes to architected materials to small DNA machines. These emerging applications require us to develop high-fidelity models that can simulate and examine folding-induced mechanical responses, especially those involving significant facet rotation, non-uniform deformation, and complex dynamics. To this end, this study formulates and experimentally validates a new origami mechanics model based on Absolute Nodal Coordinate Formulation (ANCF), which has unique advantages for predicting the nonlinear dynamics of multibody systems with large rotation and deformation. This new model treats origami facets as ANCF thin plate elements rotating around compliant creases. Moreover, Torsional Spring Damper Actuator (TSDA) connectors are developed to represent crease folding. After careful calibration with experimentally measured constitutive material properties, this study provides the first reported quantitative agreement between simulation predictions and experiment results involving complex and non-uniform facet deformation and transient dynamic responses. Therefore, this model can help deepen our knowledge of folding-induced mechanics and dynamics, fostering future applications for origami.

Enhanced Hydro-Actuation and Capacitance of Electrochemically Inner-Bundle-Activated Carbon Nanotube Yarns.

Recently, several attempts have been made to activate or functionalize macroscopic carbon nanotube (CNT) yarns to enhance their innate abilities. However, a more homogeneous and holistic activation approach that reflects the individual nanotubes constituting the yarns is crucial. Herein, a facile strategy is reported to maximize the intrinsic properties of CNTs assembled in yarns through an electrochemical inner-bundle activation (EIBA) process. The as-prepared neat CNT yarns are two-end tethered and subjected to an electrochemical voltage (vs Ag/AgCl) in aqueous electrolyte systems. Massive electrolyte infiltration during the EIBA causes swelling of the CNT interlayers owing to the tethering and subsequent yarn shrinkage after drying, suggesting activation of the entire yarn. The EIBA-treated CNT yarns functionalized with oxygen-containing groups exhibit enhanced wettability without significant loss of their physical properties. The EIBA effect of the CNTs is experimentally demonstrated by hydration-driven torsional actuation (∼986 revolutions/m) and a drastic capacitance improvement (approximately 25-fold).