Actuation Frequency Research Articles

Flow induced by a pair of plasma actuators on a circular cylinder in still air under duty-cycle actuation

A pair of surface mounted dielectric-barrier-discharge (DBD) plasma actuators is placed diametrically on a circular cylinder model. The flow field generated by the actuators in steady and duty-cycle actuation modes is measured by particle image velocimetry. Salient features of the flow and vortices are discovered for the duty-cycle actuation modes with frequency ranging from 5 to 1000 Hz and duty-cycle ratio from 0.01 to 0.99. The results are compared with those for continuous steady actuation. For a given duty-cycle ratio, a discrete vortex is generated by each of the DBD actuator in an actuation cycle for low frequencies. Such vortices are created alternately from each side of the cylinder. They interact and move laterally in response to the on-off signals of the actuators. As the frequency increases, the distance between the vortices generated from successive duty-cycles decreases and the lateral motion of the vortices becomes smaller so that the vortices are packed together as a vortex train along a narrow path on each side of the cylinder. As the frequency is further increased, the vortices in the vortex trains lose their individual characteristics and the vortex trains become steady jets. Regardless of the actuation frequency, the direction of the average total momentum of the air is found to be continuously dependent on the duty-cycle ratio. The magnitude of the average total momentum depends on the frequency. An optimum frequency exists for maximum average momentum, which may exceed that in the continuous steady actuation mode.

Comparative Analysis of a Locally Resampling MR Elastography Reconstruction Algorithm in Liver Fibrosis.

In magnetic resonance elastography (MRE), the precision of the observed mechanical depends on the ratio between mechanical wavelength and spatial resolution. Since the mechanical wavelength may vary with actuation frequency, between patients and depending on position, a unique spatial resolution may not always generate an optimal ratio for multifrequency acquisitions, in patients with varying degrees of disease or in mechanically heterogeneous organs. To describe an MRE reconstruction algorithm that adjusts the ratio between shear wavelength and pixel size, by locally resampling the matrix of shear displacement, and to assess its performance relative to existing reconstructions in different use cases. Prospective. Four phantoms, 20 healthy volunteers (5 men, median age 34, range 20-56) and 46 patients with nonalcoholic fatty liver disease (37 men, median age 63, range 33-83). A 3 T; gradient-echo elastography sequence with 40 Hz, 60 Hz, and 80 Hz frequencies. For each algorithm, phantoms stiffness were compared against their nominal values, repeatability was calculated in healthy volunteers, and diagnostic performance in detecting advanced liver fibrosis was assessed in 46 patients. Linear regression was used to evaluate the agreement between stiffness values and phantoms stiffnesses. Bland-Altman method was used to evaluate repeatability in volunteers. The ability to diagnose advanced fibrosis was assessed by receiver operating curve analysis (with Youden index thresholds). Significance was considered at P value of 0.05. From the linear regression, the slope closest to 1 is provided by MARS (40 Hz) and k-MDEV (60H, 80 Hz). Repeatability index was best with MDEV (23%) and lowest with k-MDEV (53%). The best performance in detecting advanced fibrosis was provided by MARS at 40 Hz (area under the operating curve, AUC=0.88), k-MDEV and MARS at 60 Hz (AUC=0.91), and multimodel direct inversion (MMDI) and MARS at 80 Hz (AUC=0.90). MARS shows the best diagnostic performance to detect advanced fibrosis and the second-best results in phantoms after k-MDEV. 1. Stage 2.

Frictional Anisotropic Locomotion and Adaptive Neural Control for a Soft Crawling Robot.

Crawling animals with bendable soft bodies use the friction anisotropy of their asymmetric body structures to traverse various substrates efficiently. Although the effect of friction anisotropy has been investigated and applied to robot locomotion, the dynamic interactions between soft body bending at different frequencies (low and high), soft asymmetric surface structures at various aspect ratios (low, medium, and high), and different substrates (rough and smooth) have not been studied comprehensively. To address this lack, we developed a simple soft robot model with a bioinspired asymmetric structure (sawtooth) facing the ground. The robot uses only a single source of pressure for its pneumatic actuation. The frequency, teeth aspect ratio, and substrate parameters and the corresponding dynamic interactions were systematically investigated and analyzed. The study findings indicate that the anterior and posterior parts of the structure deform differently during the interaction, generating different frictional forces. In addition, these parts switched their roles dynamically from push to pull and vice versa in various states, resulting in the robot's emergent locomotion. Finally, autonomous adaptive crawling behavior of the robot was demonstrated using sensor-driven neural control with a miniature laser sensor installed in the anterior part of the robot. The robot successfully adapted its actuation frequency to reduce body bending and crawl through a narrow space, such as a tunnel. The study serves as a stepping stone for developing simple soft crawling robots capable of navigating cluttered and confined spaces autonomously.

Event‐triggering‐based robust optimal control for post‐stall pitching maneuver of aircraft

AbstractIn this paper, the problem of the post‐stall pitching maneuver of aircraft with lower deflection frequency of control actuator is studied by considering the unsteady aerodynamic disturbances (UADs). A nonlinear disturbance observer is employed to estimate the UADs in order to suppress their adverse effects. And the nonzero equilibrium adaptive dynamic programming (NEADP) control method derived from the reinforcement learning is adopted to achieve the optimal control of aircraft. Simultaneously, a critic neural network is constructed to approximate the value function in the Hamilton‐Jacobi‐Bellman equation so as to realize adaptivity. Furthermore, the event‐triggering (ET) scheme with an improved triggering condition is applied in the control method in order to decrease the deflection frequency of control actuator. In addition, the Lyapunov stability analysis method is used to prove that all signals in the closed‐loop control system are uniformly ultimately bounded. And the proof by contradiction is applied to confirm that the Zeno behavior is excluded with the analysis of the minimum dwell time. Finally, simulation results verify the effectiveness of the improved ET‐based NEADP control method.

A 50 µm acoustic resonator microchannel enables focusing 100 nm polystyrene beads and sub-micron bioparticles

The manipulation and focusing of submicron/nanometre-scale objects are becoming increasingly important in the fields of biology, chemistry, and materials science. Acoustofluidics provides for the high throughput and precise manipulation of cells and particles with simple equipment. However, application to submicron/nanoparticles is challenging due to the particle volume dependence of the acoustic radiation force and the relatively larger effect of the viscous drag force on small objects. Here, we present an acoustofluidic device that can focus submicron/nanoparticles, bacterial cells, and extracellular vesicles (EVs) using a high-frequency two-dimensional acoustic standing wave. We fabricate 50 µm × 50 µm square cross-section channels as acoustic resonators and generate a two-dimensional acoustic standing wave at an actuation frequency of 14.9 MHz. This configuration enables a strong acoustic radiation force towards the centre of the microchannel and a cross-section-centred single-vortex stream that does not counteract the acoustic radiation force. Using this device, polystyrene beads ranging in diameter from 50 to 500 nm were focused toward the channel centre, where a focused bead stream width of less than 5 µm was achieved at a flow rate of 3 µL min−1 for 100 nm beads and 9 µL min−1 for 200 nm beads. Furthermore, bacterial cells were successfully focused at a flow rate of 40 µL min−1, and the focusing of EVs was also demonstrated. Our acoustofluidic device can become a promising tool for a wide range of biological analyses targeting submicron cells, viruses, and EVs.

Heat transfer enhancement in turbulent boundary layers with a pulsed slot jet in crossflow

The convective heat transfer enhancement in a turbulent boundary layer (TBL) employing a pulsed, slot jet in crossflow is investigated experimentally. A parametric study on actuation frequencies and duty cycles is performed. The actuator is a flush-mounted slot jet that injects fluid into a well-behaved zero-pressure-gradient TBL over a flat plate. A heated-thin-foil sensor measures the time-averaged convective heat transfer coefficient downstream of the actuator location and the flow field is characterised by means of Particle Image Velocimetry. The results show that both the jet penetration in the streamwise direction and the overall Nusselt number increase with increasing duty cycle. The frequency at which the Nusselt number is maximised is independent of the duty cycle. The flow topology is considerably altered by the jet pulsation. A wall-attached jet rises from the slot accompanied by a pair of counter-rotating vortices that promote flow entrainment and mixing. Eventually, a simplified model is proposed which decouples the effect of pulsation frequency and duty cycle in the overall heat transfer enhancement, with a good agreement with experimental data. The cost of actuation is also quantified in terms of the amount of injected fluid during the actuation, leading to conclude that the lowest duty cycle is the most efficient for heat transfer enhancement.

Laminar boundary layer forcing with active surface deformations

The use of active surface deformations as an actuation strategy has been largely overlooked in the flow control literature. In this work, we investigate how active surface deformations applied over a range of frequencies and amplitudes affect a laminar boundary layer. We find that the strategy is capable of producing both high- and low-speed motions. These motions appear well-suited for flow control, especially at the lowest actuation frequencies.

Unsteady disturbances in a swept wing boundary layer due to plasma forcing

This work investigates the response of a transitional boundary layer to spanwise-invariant dielectric barrier discharge plasma actuator (PA) forcing on a 45° swept wing at a chord Reynolds number of 2.17×106. Two important parameters of the PA operation are scrutinized, namely, the forcing frequency and the streamwise location of forcing. An array of passive discrete roughness elements is installed near the leading edge to promote and condition a set of critical stationary crossflow (CF) instability modes. Numerical solutions of the boundary layer equations and linear stability theory are used in combination with the experimental pressure distribution to provide predictions of critical stationary and traveling CF instabilities. The laminar–turbulent transition front is visualized and quantified by means of infrared thermography. Measurements of velocity fields are performed using hotwire anemometry scans at specific chordwise locations. The results demonstrate the inherent introduction of unsteady velocity disturbances by the plasma forcing. It is shown that, depending on actuator frequency and location, these disturbances can evolve into typical CF instabilities. Positive traveling low-frequency type III modes are generally amplified by PA in all tested cases, while the occurrence of negative traveling high-frequency type I secondary modes is favored when PA is operating at high frequency and at relatively downstream locations, with respect to the leading edge.

Investigation of the interaction between NS-DBD plasma-induced vortexes and separated flow over a swept wing

The effect of nanosecond pulsed dielectric barrier discharge (NS-DBD) plasma flow separation control is closely related to the actuation frequency, because it involves the interaction between plasma-induced vortexes and separated flow. In order to study the mechanism of NS-DBD plasma flow separation control over a swept wing, especially the influence of the actuation frequency, at first, experimental studies of the actuation frequencies at 100 Hz are conducted to validate the numerical simulation method. Then, numerical studies of different actuation frequencies which are 50 Hz, 100 Hz, 160 Hz, 200 Hz, 500 Hz, and 1000 Hz, respectively are conducted. The interaction between the plasma-induced vortexes and the separated flow is analyzed. Results show that there is a range of the actuation frequency which includes the frequency (160 Hz) calculated by the average aerodynamic chord length to make the control effect good, but when the actuation frequencies are too low (50 Hz) or too high (1000 Hz), the control effect will get worse. The former is because plasma-induced vortexes disappear in a period within an actuation cycle; the latter is because plasma-induced vortexes cannot develop completely, resulting in a weak vortex intensity.

Electromagnetic actuator design for distributed stiffness

Origami robots characterized by rigid plates linked together with flexible joints have inherent compliance and large deformations. Their low-profile design and folding mechanical transmissions provide substantial force and torque, especially for wearables and human-interactive devices. The current major limitation in the field is the need for embeddable dynamic actuation systems that produce actuation frequencies similar to human motion. In this study, we propose a novel low-profile printable electromagnetic actuator that can be integrated into thin large robotic surfaces made of rigid panels and into the streamlined pushbutton manufacturing process. We use already existing variable stiffness joints to demonstrate the scalability and distributable properties of the proposed actuation system on a locally controllable variable-stiffness distributed surface.

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.

Compact, repetition rate locked all-PM fiber femtosecond laser system based on low noise figure-9 Er:fiber laser

We demonstrate a compact femtosecond fiber laser system based on all polarization-maintaining (PM) fiber and fiber components integrated structure. The figure-9 oscillator which incorporated a nonlinear amplifying loop mirror in the cavity features a 103.4-MHz high repetition rate with up to 93.1 dB signal-to-noise ratio of the radio frequency spectrum, 0.0056% [1 Hz, 1 MHz] integrated root-mean-square amplitude noise at the fundamental repetition rate and 63.7-fs timing jitter [100 Hz, 1 MHz]. Meanwhile, the fundamental repetition frequency was also locked to a stable radio frequency reference by using a self-designed frequency actuator and a relative frequency stability of 2.1 × 10−12 at 1-s gate time was obtained. Moreover, benefitting from the large positive group-velocity dispersion and negative third-order dispersion at 1.5-μm wavelength band, we also achieved 48.2 fs compressed pulse duration as well as an amplified average power of 199 mW via one-stage all-PM fiber amplifier and compressor. At last, as a performance proof, by directly splicing 38-cm long PM highly nonlinear fiber to the pulse compressor, a broadband coherent supercontinuum spanning from 950 nm to 2150 nm was generated. Our all-PM fiber laser system is suitable for the further buildup of a low noise PM fiber optical frequency comb.

Impact of the Soundproofing in the Cavity of the Synthetic Jet Actuator on the Generated Noise

The synthetic jet actuator (SJA) generated high noise which limits the area of its application. In this paper, the five actuators with different types of soundproofing in the cavity were tested and compared to the classic actuator. The resistance and the sound pressure level (SPL) were measured for real power P=1, 2, 4 W, and frequency in a range of 20–150 Hz. The resonant frequency of actuators was designed. Only one type of soundproofing had a significant impact on the resonant frequency. The use of soundproofing in the actuator cavity increased or did not affect the generated noise at a frequency below 120 Hz and only the mineral wool significantly decreased the noise at a frequency above 120 Hz– even 7 dBA. The direction for further investigations was set.

Study of the transient flow structures generated by a pulsed nanosecond plasma actuator on a delta wing

The transient flow structures produced by a pulsed nanosecond plasma actuator and the mechanism by which they are generated are investigated experimentally and through simulations for the case of flow control on a non-slender delta wing. Phase-averaged particle image velocimetry reveals a phenomenon in which, after each discharge pulse, two sub-vortices are generated in sequence and move along the shear layer regardless of the angle of attack, and this is confirmed by hot-wire anemometry. However, at high actuation frequencies (F+ = fc/U∞ ≥ 6.435), this phenomenon of double sub-vortices is not observed, and only one sub-vortex is generated per period. The results of pressure measurements indicate that each sub-vortex gives rise to a distinct pressure fluctuation on the wing surface. Numerical simulations reveal a number of residual heats resulting from plasma thermal effects in the shear layer, each of which turns out to induce a corresponding sub-vortex. At low actuation frequencies (F+ ≤ 4.29), there is a division of the initial residual heat into two independent residual heats and, hence, double sub-vortices per period, whereas at high actuation frequencies (F+ ≥ 6.435), residual heats from two consecutive periods merge into one, resulting in just one sub-vortex per period, which provides an explanation for the experimentally observed flow behavior.

Twisted and coiled polymer muscle actuated soft 3D printed robotic hand with Peltier cooler for drug delivery in medical management

Many robotic hands have been proposed to have unique designs and capabilities, focusing on sensing, actuation, and control. This paper presents experimental studies on a soft 3D-printed robotic hand whose fingers are actuated by twisted and coiled polymer (TCP<sub>FL</sub>) muscles, driven by resistive heating, and cooled by water and Peltier mechanism (thermoelectric cooling) for increasing the actuation frequency. The hand can be utilized for pick and place applications of drugs in clinical settings, which may be repetitive for humans. A combination of ABS plastic and thermoplastic polyurethane material is used to additively manufacture the robotic hand. The hand along with a housing tank for the muscles and Peltier coolers has a length of 380 mm and weighs 560 gm. The fabrication process of the TCP<sub>FL</sub> actuators coiled with 160 µm diameter nichrome wires is presented. The actuation frequency in the air for TCP<sub>FL </sub>is around 0.01 Hz. This study shows the effect of water and Peltier cooling on improving the actuation frequency of the muscles to 0.056 Hz. Experiments have been performed with a flex sensor integrated at the back of each finger to calculate its bend-extent while being actuated by the TCP<sub>FL</sub> muscles. All these experiments are also used to optimize the TCP<sub>FL</sub> actuation. Overall, a low-cost and lightweight 3D printed robotic hand is presented in this paper, which significantly increases the actuation performance with the help of cooling methods, that can be used in applications in medical management.

Optimization of thermal characteristics of axisymmetric synthetic air jet impingement on flat surface

The local heat transfer of axisymmetric synthetic air jet impinging on flat surface is investigated experimentally. Acoustic speaker is used for generation of synthetic air jet with cavity of cylindrical shape.The experiments is conducted for actuator frequency ranging from 50Hz to 400Hz, orifice diameter 2mm to 10mm, orifice plate thickness 2mm to 10mm, cavity diameter 50mm to 75mm, cavity depth 30mm to 60mm, jet to plate distance 16mm to 112mm. A steady heat flow is maintained on the flat surface. Local heat transfer characteristics on flat surface is found by thermal images using IR thermal imaging and thin foil technique.The experimental results reveals that the heat transfer is highly effected by frequency, orifice diameter, orifice plate thickness, jet to plate distance. However, cavity depth and cavity diameter has small influence on thermal performance.The higher cavity volume show more influence on heat transfer characteristics. An optimization of synthetic jet performance parameters is studied for maximum thermal performance interms of heat transfer characteristics.

Trajectory Envelope of a Subsea Shuttle Tanker Hovering in Stochastic Ocean Current—Model Development and Tuning

Abstract A subsea shuttle tanker (SST) concept for liquid carbon dioxide transportation was recently proposed to support studies evaluating the ultra-efficient underwater cargo submarine concept. One important topic is the position keeping ability of SST during the offloading process. In this process, the SST hovers above the well and connects with the wellhead using a flowline. This process takes around 4 h. Ocean currents can cause tremendous drag forces on the subsea shuttle tanker during this period. The flow velocities over hydroplanes are low throughout this process, and the generated lift forces are generally insufficient to maintain the SST’s depth. The ballast tanks cannot provide such fast actuation to cope with the fluctuation of the current. It is envisioned that tunnel thrusters that can provide higher frequency actuation are required. This paper develops a maneuvering model and designs a linear quadratic regulator that facilitates the SST station-keeping problem in stochastic current. As case studies, the SST footprints at 0.5 m/s, 1.0 m/s, and 1.5 m/s mean current speeds are presented. Numerical results show that the designed hovering control system can ensure the SST’s stationary during offloading. The required thrust from thrusters and the propeller are presented. The presented model can serve as a basis for obtaining a more efficient design of the SST and provide recommendations for the SST operation.

Drug-Loaded IRONSperm clusters: modeling, wireless actuation, and ultrasound imaging

Individual biohybrid microrobots have the potential to perform biomedical in vivo tasks such as remote-controlled drug and cell delivery and minimally invasive surgery. This work demonstrates the formation of biohybrid sperm-templated clusters under the influence of an external magnetic field and essential functionalities for wireless actuation and drug delivery. Ferromagnetic nanoparticles are electrostatically assembled around dead sperm cells, and the resulting nanoparticle-coated cells are magnetically assembled into three-dimensional biohybrid clusters. The aim of this clustering is threefold: First, to enable rolling locomotion on a nearby solid boundary using a rotating magnetic field; second, to allow for noninvasive localization; third, to load the cells inside the cluster with drugs for targeted therapy. A magneto-hydrodynamic model captures the rotational response of the clusters in a viscous fluid, and predicts an upper bound for their step-out frequency, which is independent of their volume or aspect ratio. Below the step-out frequency, the rolling velocity of the clusters increases nonlinearly with their perimeter and actuation frequency. During rolling locomotion, the clusters are localized using ultrasound images at a relatively large distance, which makes these biohybrid clusters promising for deep-tissue applications. Finally, we show that the estimated drug load scales with the number of cells in the cluster and can be retained for more than 10 h. The aggregation of microrobots enables them to collectively roll in a predictable way in response to an external rotating magnetic field, and enhances ultrasound detectability and drug loading capacity compared to the individual microrobots. The favorable features of biohybrid microrobot clusters place emphasis on the importance of the investigation and development of collective microrobots and their potential for in vivo applications.

An investigation of acoustic relocation phenomenon in a microchannel under acoustic fields

This work investigates the effects of actuation frequency and fluid properties on the relocation of miscible inhomogeneous fluids inside a microchannel under acoustic standing waves. Remarkably, we demonstrate the cases in which relocation is achieved relatively faster in the case of fluids with smaller impedance difference between them (1.5%), than in the case of fluids with larger impedance difference (10%) when actuated at an optimum frequency. Subsequently, we show that, if the impedance difference between two fluids is less, actuation of the microchannel at a single frequency is sufficient for the fast and complete relocation, whereas, in the case of larger impedance difference between the fluids, sweeping at multiple frequencies results in fast and more complete relocation compared to actuation of the system by a single frequency. Furthermore, the role of gravity in the process is also analyzed in detail.

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.