Actuation Speed Research Articles

Bioinspired MXene-Based Soft Actuators Exhibiting Angle-Independent Structural Color

HighlightsDesign and fabrication of MXene-based soft actuators with angle-independent structural color.The nanostructured MXene can not only facilitate the formation of short-range ordered 3D amorphous photonic crystals, but also help significantly improve structural color saturation.The soft actuators exhibit brilliant angle-independent structural color, ultrafast actuation and recovery speeds in response to vapor.

Speeding up soft pneumatic actuators through pressure and flow dynamics modeling and optimization

Soft pneumatic actuators (SPAs) possess the potential to be used in various scenarios due to the softness of their compositional materials to generate various modes of motion. The actuation frequency of SPAs is commonly in the range of 0-4 Hz, limiting their applications only to normal-speed tasks. To reveal the key affecting factors to the actuation speed and further optimize an SPA’s response speed for more dynamic applications, a predictive model of the whole pneumatic supply system (PSS) that takes the pressure and flow dynamics into consideration is needed. In this work, we introduce a comprehensive pressure and flow dynamic model of high-speed PSSs and SPAs. Here, the PSS is regarded as a compressed air source and a volume-fixed reservoir that are connected by a tube. The pressure and flow dynamic is modeled by correcting the adiabatic model to account for real heat exchange, and considering the assumptions of steady state of the gas flow through an orifice and the one dimensional unsteady gas flow in the tube. After calibration and validation with experiment results, we investigated the key affecting factors of PSSs on the attainable frequency of the SPA, which provides the design guidelines for the high-speed PSSs. Results show that the PSS with pure helium is dramatically faster than that with air. Finally, we present a demonstration of a helium-PSS actuated fiber-reinforced, bending SPA that achieves high frequency of up to 39 Hz in its blocked force.

Critical Damping in Travel Curves of Piezoelectrically Actuated Fast Mechanical Switches for Hybrid Circuit Breakers

Piezoelectrically actuated fast mechanical switches provide a low-loss conduction path in hybrid circuit breakers for protecting medium-voltage direct-current systems. As the desired actuation speed approaches the driving limit of a piezoelectric actuator, excessive vibration starts to dominate its underdamped travel curves, resulting in insulation failures and delayed switching in the fast mechanical switch. To improve the underdamped travel curves into critically damped travel curves, several switching motion control schemes with active damping filters are proposed in this paper. The switching motion controllers are based on an electromechanical model of a fast mechanical switch with experimentally identified parameters. According to hardware implementation tests, the proposed closed-loop control schemes optimized the switch travel curves with a 56% reduction in the undershoot and a 75% reduction in the settling time. Equipped with the critically damped travel curves, the piezoelectrically actuated fast mechanical switch is better suited for sequential submodule operations to reduce the interruption time, the peak current and the fault energy to be absorbed in hybrid circuit breakers.

Programmable soft valves for digital and analog control

In soft devices, complex actuation sequences and precise force control typically require hard electronic valves and microcontrollers. Existing designs for entirely soft pneumatic control systems are capable of either digital or analog operation, but not both, and are limited by speed of actuation, range of pressure, time required for fabrication, or loss of power through pull-down resistors. Using the nonlinear mechanics intrinsic to structures composed of soft materials-in this case, by leveraging membrane inversion and tube kinking-two modular soft components are developed: a piston actuator and a bistable pneumatic switch. These two components combine to create valves capable of analog pressure regulation, simplified digital logic, controlled oscillation, nonvolatile memory storage, linear actuation, and interfacing with human users in both digital and analog formats. Three demonstrations showcase the capabilities of systems constructed from these valves: 1) a wearable glove capable of analog control of a soft artificial robotic hand based on input from a human user's fingers, 2) a human-controlled cushion matrix designed for use in medical care, and 3) an untethered robot which travels a distance dynamically programmed at the time of operation to retrieve an object. This work illustrates pathways for complementary digital and analog control of soft robots using a unified valve design.

The Shape Memory Properties and Actuation Performances of 4D Printing Poly (Ether-Ether-Ketone).

Shape-memory polymers (SMPs) have gradually emerged in the mechanism and biomedical fields and facilitate the upgrading of industrial mechanisms and the breakthrough of technical bottlenecks. However, most of the SMPs are infeasible in harsh environments, such as aerospace, due to the low glass transition temperature. There are still some works that remain in creating truly portable or non-contacting actuators that can match the performances and functions of traditional metal structures. Polyether-ether-ketone (PEEK) with a high glass transition temperature of 143 °C is endowed with outstanding high-temperature resistance and radiation-resistant properties and shape memory behavior. Thus, we explore the shape-memory properties and actuation performances of high-temperature PEEK in bending behaviors. The shape-recovery ratio, actuation speed and force under different programming conditions and structure parameters are summarized to complete the actuation capacities. Meanwhile, a metallic ball transported by shape-memory PEEK and deployed drag sail with thermo-responsive composite joints were shown to verify the potential in aerospace.

Experimental analysis of the dynamic inflow effect due to coherent gusts

Abstract. The dynamic inflow effect describes the unsteady aerodynamic response to fast changes in rotor loading due to the inertia of the wake. Fast changes in turbine loading due to pitch actuation or rotor speed transients lead to load overshoots. The phenomenon is suspected to be also relevant for gust situations; however, this was never shown, and thus the actual load response is also unknown. The paper's objectives are to prove and explain the dynamic inflow effect due to gusts, and compare and subsequently improve a typical dynamic inflow engineering model to the measurements. An active grid is used to impress a 1.8 m diameter model turbine with rotor uniform gusts of the wind tunnel flow. The influence attributed to the dynamic inflow effect is isolated from the comparison of two experimental cases. Firstly, dynamic measurements of loads and radially resolved axial velocities in the rotor plane during a gust situation are performed. Secondly, corresponding quantities are linearly interpolated for the gust wind speed from lookup tables with steady operational points. Furthermore, simulations with a typical blade element momentum code and a higher-fidelity free-vortex wake model are performed. Both the experiment and higher-fidelity model show a dynamic inflow effect due to gusts in the loads and axial velocities. An amplification of induced velocities causes reduced load amplitudes. Consequently, fatigue loading would be lower. This amplification originates from wake inertia. It is influenced by the coherent gust pushed through the rotor like a turbulent box. The wake is superimposed on that coherent gust box, and thus the inertia of the wake and consequently also the flow in the rotor plane is affected. Contemporary dynamic inflow models inherently assume a constant wind velocity. They filter the induced velocity and thus cannot predict the observed amplification of the induced velocity. The commonly used Øye engineering model predicts increased gust load amplitudes and thus higher fatigue loads. With an extra filter term on the quasi-steady wind velocity, the qualitative behaviour observed experimentally and numerically can be caught. In conclusion, these new experimental findings on dynamic inflow due to gusts and improvements to the Øye model enable improvements in wind turbine design by less conservative fatigue loads.

Magnetically actuated gearbox for the wireless control of millimeter-scale robots.

The limited force or torque outputs of miniature magnetic actuators constrain the locomotion performances and functionalities of magnetic millimeter-scale robots. Here, we present a magnetically actuated gearbox with a maximum size of 3 millimeters for driving wireless millirobots. The gearbox is assembled using microgears that have reference diameters down to 270 micrometers and are made of aluminum-filled epoxy resins through casting. With a magnetic disk attached to the input shaft, the gearbox can be driven by a rotating external magnetic field, which is not more than 6.8 millitesla, to produce torque of up to 0.182 millinewton meters at 40 hertz. The corresponding torque and power densities are 12.15 micronewton meters per cubic millimeter and 8.93 microwatt per cubic millimeter, respectively. The transmission efficiency of the gearbox in the air is between 25.1 and 29.2% at actuation frequencies ranging from 1 to 40 hertz, and it lowers when the gearbox is actuated in viscous liquids. This miniature gearbox can be accessed wirelessly and integrated with various functional modules to repeatedly generate large actuation forces, strains, and speeds; store energy in elastic components; and lock up mechanical linkages. These characteristics enable us to achieve a peristaltic robot that can crawl on a flat substrate or inside a tube, a jumping robot with a tunable jumping height, a clamping robot that can sample solid objects by grasping, a needle-puncture robot that can take samples from the inside of the target, and a syringe robot that can collect or release liquids.

Ionic coordination strengthening of temperature-driven gradient hydrogel actuators with rapid responsiveness

Emerging applications of stimuli-responsive hydrogels as soft actuators and robotics have attracted great attention, but they face challenges in practical applications due to their poor mechanical properties, low driving force, and low actuation speed. Herein, we presented tough hydrogel actuators composed of a copolymer of N-isopropylacrylamide (NIPAM) and 3-sulfopropyl methacrylate potassium salt (SPA) monomers, and tunicate cellulose nanocrystals (TCNCs). The negatively charged TCNCs as nanofillers were gradient arranged by a direct current electric field (DC-EF) via electrophoresis. After in situ polymerization, TCNCs gradient distributed across the thickness of hydrogel, resulting in the formation of gradient crosslinking density of networks. Hydrogel actuators with high strength were obtained via Zr4+/−SO3- ionic coordination, whose tensile strength was about 200 times of hydrogel without ionic coordination. These hydrogel actuators exhibited rapid bending velocity, excellent stability, and good cycling performance in response to temperature. Importantly, these hydrogel actuators could be designed as soft robots to lift and transport objects by changing environment temperature. This work provided a facile yet efficient strategy to fabricate soft actuators with high strength and rapid responsiveness.

Fabric muscle with a cooling acceleration structure for upper limb assistance soft exosuits

Soft exosuits used for supporting human muscle strength must be lightweight and wearable. Shape memory alloy (SMA) spring-based fabric muscles (SFM) are light and flexible, making them suitable for soft and shape-conformable exosuits. However, SFMs have a slow actuation speed owing to the slow cooling rate of the SMA spring. This paper proposes a forced air-cooling fan-integrated fabric muscle (FCFM) that improves the cooling rate by arranging a thin-diameter SMA spring bundle with a high surface-area-to-volume ratio inside a breathable fabric with integrated fans. The relaxation time of an FCFM weighing 30 g and containing a 2.6 g SMA spring bundle, which contains 200 thin springs, was reduced by over 70.2% via forced-air cooling using the integrated fans. A 4 kg weight, which is 1530 times the mass of the SMA spring bundle, was hung from the FCFM and was repeatedly actuated in ten-second cycles. An upper limb assistive soft exosuit with FCFMs was fabricated and worn on a mannequin holding a dumbbell, and the arm extension time after flexion was improved by 4.5 times. Additionally, the assistive performance of the exosuits for repetitive tasks in specific scenarios was evaluated, and the strong potential of the proposed FCFM for soft exosuits was verified.

Unsteady Effects of Blowing over Actuated Surfaces Using Phase-Locked Particle Image Velocimetry

An experimental study is conducted to investigate the effects of upper-surface trailing-edge blowing on a low-aspect-ratio rectangular wing with an actively actuated flap. Two wing configurations, the wing with a baseline flap and a dual-radius flap, are wind-tunnel tested at three spanwise locations to investigate the three-dimensional and unsteady effects of blowing during flap actuation at a freestream Reynolds number (based on the airfoil chord) of . Two-dimensional and stereoscopic phase-locked particle image velocimetry (PIV) measurements are used to collect data during flap actuation at different actuation speeds and blowing intensities. Proper orthogonal decomposition and modified Q criterion are used to analyze the PIV data. The wake dynamics reveal that flap actuation contributes to tip-vortex strengthening when active blowing is not present. The combination of flap actuation and active continuous blowing is more effective in controlling the boundary layer compared to the flap at rest for the same blowing intensities.

Design, simulation, and motion characteristics of a novel impact piezoelectric actuator using double stators

This study presents a novel impact piezoelectric motor that excites double stators through a sinusoidal signal. A sawtooth signal drives the traditional impact piezoelectric actuator, and its working frequency is limited by the resonant frequency. This study uses sine signals to drive the double stators to produce a sinusoidal vibration. The sinusoidal vibration of different frequencies and amplitudes are synthesised into a sawtooth vibration on the stage plate. The directional movement of the slider is realised using the vibration of the stage plate to drive the slider. This structure reduces the space required for the piezoelectric actuator to work. The working principle of the motor is discussed, and the structure is constructed. The dynamics model of the whole system is established on the based of the dynamics model of the actuator and the LuGre friction model. Moreover, the dynamics model was simulated and analysed through MATLAB/Simulink. The prototype is fabricated and tested. Experimental results confirm the effectiveness of using sinusoidal signals to drive the piezoelectric actuator, and the motion process of the piezoelectric motor is consistent with the theoretical analysis. The maximum speed of the piezoelectric actuator is 5.54 mm s−1, and the resolution is 0.72 μm. This study provides an effective driving method for the quasi-static piezoelectric motor to improve the working frequency.

Extending the Force Balancing Method of Adjusting Kinematic Parameter to Spatial Mechanisms

Robotic mechanisms is one that the mechanism includes no less than one variable speed actuator. As for force balancing, which refers to mechanisms with a complete cancelation of the inertia-induced force on the ground (shaking force). A few strategies are accessible for reducing shaking force, including the counterweight (CW), add-of-spring (AOS), add-of-linkage (AOL), and adjusting kinematic parameter (AKP). AKP is only applicable to planar robotic mechanisms when it was developed. The primary goal of the present paper is to extend AKP approach to a spatial mechanism. Considering the complex kinematic characteristics of spatial mechanism, a simple spherical parallel robot (SPR) is used as the research tool. The equations for force balancing of the spherical robotic mechanism using AKP were derived. Simulation verification was performed by a reliable software called SPACAR available in the Matlab environment. The results demonstrated the effectiveness of the AKP method in eliminating the unbalanced force to spatial mechanisms. The main contribution is the provision of a new force balancing method to spatial mechanisms, which will enrich the field of balancing of spatial robotic mechanisms.

Envelope-Based Variable-Gain Control Strategy for Vibration Suppression of Solar Array Using Reaction Wheel Actuator

The orbital operation of spacecraft can excite the long-drawn and low-frequency vibration of the solar array, which is prone to affecting the task execution of the system. To address this issue, an envelope-based variable-gain control strategy is proposed to suppress vibration of the solar array using the reaction wheel (RW) actuator. The RW actuator is individually mounted on the solar array to provide reaction torque through the speed change of its rotor. The governing equation of motion of the solar array actuated by a RW actuator is deduced with the state space representation. The control relation between the measured bending moment and the rotational speed of the RW actuator with the constant-gain coefficient is firstly developed and demonstrated in numerical simulation. Changing the gain coefficient to be inversely proportional to the envelope function of vibration, a variable-gain control strategy is proposed to improve the damping effect of the RW actuator. Simulation results show that the vibration suppression performance of the RW actuator is improved compared to the constant-gain control. As the actual on-orbit natural frequency of the solar array is not always exactly known, the robustness of the control system is analyzed for the deviation between the estimated and the actual natural frequency values. The proposed variable-gain control is also experimentally verified using a simplified elastic plate model. Experimental results indicate that the vibration attenuation time is decreased to 29.1% and 50.22% compared to the uncontrolled and the constant-gain controlled states, respectively.

ADRC Control System of PMLSM Based on Novel Non-Singular Terminal Sliding Mode Observer

In an attempt to solve the problem of the many parameters of the traditional active disturbance rejection controller (ADRC) and to accurately estimate the mover position and speed required by a permanent magnet synchronous linear motor (PMLSM) system, an improved ADRC and a novel nonsingular fast terminal sliding mode observer (NFTSMO) are proposed. Firstly, the traditional first-order ADRC is simplified, the tracking differentiator (TD) module is removed, and the direct error is used to replace the nonlinear function in the extended state observer (ESO) and nonlinear state error feedback (NLSEF) module. Based on the traditional NFTSMO, the smooth back electromotive force (EMF) is obtained by adding the TD to reduce the phase delay caused by the low-pass filter in the traditional sliding mode observer (SMO), and the actuator position and speed information are modulated from the observed back EMF based on the principle of a phase-locked loop (PLL). Simulation and experiments show that this method not only simplifies the system structure of PMLSM but also optimizes many parameters in ADRC while retaining the original excellent performance. Compared with the traditional NFTSMO, the improved NFTSMO enhances the observation accuracy, reduces the chattering phenomenon of the system, and improves the robustness of the system.

Cooling‐Accelerated Nanowire‐Nitinol Hybrid Muscle for Versatile Prosthetic Hand and Biomimetic Retractable Claw (Adv. Funct. Mater. 18/2022)

Artificial Muscles In article number 2111145, Il-Kwon Oh and co-workers grow a copper nanowire forest directly on the surface of a shape memory alloy coil (SMAc) to provoke thermal dissipation and accelerate its actuation speed. Then, the cooling-accelerated Cu nanowire grown SMAc is applied as artificial muscles for a versatile prosthetic hand capable of making a series of sign languages and playing the grand piano. Moreover, a unique feature of felines, a retractable claw, is realized by using the cooling-accelerated Cu nanowire grown SMAc for successful control of traction force.

Soft fabric muscle based on thin diameter SMA springs

This study proposes fabric muscles based on shape memory alloy (SMA) springs coiled with a thin diameter SMA wire of 80 to improve heat dissipation under natural convection and forced air cooling. A new structure and its fabrication process are developed to create soft and flexible fabric muscles capable of supplying current and withstanding high external force, using a large number of thin SMA spring bundles. A total of 250 bundles of SMA springs weighing only 3.2 g constitute the fabric muscle that can lift a 1500 times-heavier mass, where each strand behaved like a muscle fiber. The fabric muscle generates a blocking force of up to 56.54 N and a power density of 2072 W kg−1. In addition, an air-cooling structure similar to a showerhead is proposed to further improve the actuation speed of the fabric muscles and maintain the flexibility of the fabric muscles. The fabric muscle with forced air cooling is cooled 40.5% faster than that with natural cooling. Finally, the results confirm that forced cooling can drive the muscle with a cycle of 5.5 s.

Boron nitride enhanced shape memory poly(aryl ether ketone) actuators with rapid self‐deformation and staged responsive behaviors

AbstractIn this paper, boron nitride (BN) sheets, as two‐dimensional thermal conductive fillers, were integrated into shape memory poly(aryl ether ketone) (PAEK) matrix to improve the response speed of actuators. The thermal conductivity of composites was found to be positively correlated with the content of BN sheets. Significantly, a highest in‐plane thermal conductivity of 2.66 W m−1 K−1 was obtained, which resulted in the increase of the recovery speed of the PAEK/BN actuators from 18 to 6 s. Besides, the integration of BN sheets improved the shape memory effect of PAEK matrix, including the higher shape recovery ratio (up to 99.9%) and fixity ratio (over 97%). In addition, through the assembling of four components with designed BN contents, we achieved the interesting staged shape recovery behaviors for PAEK/BN composite actuators, which enabled programmable and smart manipulations. These fabricated PAEK/BN composite actuators with rapid response and staged deformation behaviors possessed the wide utilization potential in smart devices.

Hydrogel-based strong and fast actuators by electroosmotic turgor pressure.

Hydrogels are promising as materials for soft actuators because of qualities such as softness, transparency, and responsiveness to stimuli. However, weak and slow actuations remain challenging as a result of low modulus and osmosis-driven slow water diffusion, respectively. We used turgor pressure and electroosmosis to realize a strong and fast hydrogel-based actuator. A turgor actuator fabricated with a gel confined by a selectively permeable membrane can retain a high osmotic pressure that drives gel swelling; thus, our actuator exerts large stress [0.73 megapascals (MPa) in 96 minutes (min)] with a 1.16 cubic centimeters of hydrogel. With the accelerated water transport caused by electroosmosis, the gel swells rapidly, enhancing the actuation speed (0.79 MPa in 9 min). Our strategies enable a soft hydrogel to break a brick and construct underwater structures within a few minutes.

Rapid Actuation of Thermo-Responsive Polymer Networks: Investigation of the Transition Kinetics.

The swelling and collapsing of thermo-responsive poly(N-isopropylacrylamide)-based polymer (pNIPAAm) networks are investigated in order to reveal the dependency on their kinetics and maximum possible actuation speed. The pNIPAAm-based network was attached as thin hydrogel film to lithographically prepared gold nanoparticle arrays to exploit their localized surface plasmon resonance (LSPR) for rapid local heating. The same substrate also served for LSPR-based monitoring of the reversible collapsing and swelling of the pNIPAAm network through its pronounced refractive index changes. The obtained data reveal signatures of multiple phases during the volume transition, which are driven by the diffusion of water molecules into and out of the network structure and by polymer chain re-arrangement. For the micrometer-thick hydrogel film in the swollen state, the layer can respond as fast as several milliseconds depending on the strength of the heating optical pulse and on the tuning of the ambient temperature with respect to the lower critical solution temperature of the polymer. Distinct differences in the time constants of swelling and collapse are observed and attributed to the dependence of the cooperative diffusion coefficient of polymer chains on polymer volume fraction. The reported results may provide guidelines for novel miniature actuator designs and micromachines that take advantages of the non-reciprocal temperature-induced volume transitions in thermo-responsive hydrogel materials.

Harnessing Cascading Dominoes for Peristaltic Wave Motion

Organisms have evolved many simple yet effective muscular movements. Of these, peristaltic wave motion is among the most prolific and can be found across organism scales and in a variety of organs that transport objects or provide locomotion. Several robotic platforms have mimicked peristaltic movement, which resembles soliton propagation; however, the complexity of integrating multiple sequential actuators and the fragility of these systems have limited their practical application. In this work, we demonstrate the potential of cascading dominoes to realize soliton-like wave movements that can transport objects and locomote with only two actuators. Unlike conventional domino series where falling dominoes produce a soliton at the collapsing frontier, we added a revolute joint on each domino to ensure all the dominoes stack continuously, which inherently changes the soliton wave shape. We show how domino geometry determines the wave shape of the soliton, both analytically and experimentally, and how actuator speed affects the wave speed. Inspired by peristaltic wave motion in living organisms, we used the cascading dominoes to move objects through an artificial esophageal conduit and build a “mantis shrimp” crawler. Cascading dominoes enable a new type of actuator mechanism and provide new opportunities for robotic motion.