Coil Actuator Research Articles

Bioinspired self-coiling Janus microfiber actuators for micro-lifter and humidity sensing

Many soft coiled actuators are mimic the desirable structures of plant tendril. Here, we present a facile microfluidic spinning method to fabricate self-coiling Janus microfiber actuator with tunable morphology, spontaneous coiling performance and reversible traction property. By adjusting the component of hydrogel Janus fiber in a microfluidic spinning device with theta-shaped channels, tens of hundreds of homogeneous coils can be spontaneously and rapidly generated due to asymmetric contraction upon dehydration process, and three different forms of coiled fibers with various screw diameters are presented. Furthermore, the coiled Janus microfiber with hygroscopic properties possesses a reversible coiling/uncoiling performance under humidity gradients, which can serve as a micro-actuator to lift a certain weight with enough tensile strain and long-range tractive displacement. In addition, local reversible coiling/uncoiling morphing is achieved under water contact and near infrared light optothermal stimulations. As a verification of this coiled microfiber actuator, a smart switch is designed to control different colored LED lights for sensing an ambient moisture content.

Improved performance in temperature and speed of TCP artificial muscles for soft wearables robots by length modification

Artificial muscles provide a unique solution for wearable rehabilitation robots (WRRs) because they are compliant, compact, and lightweight. Twisted and coiled polymer actuators (TCPs) are artificial muscles from thermally activated polymer fibres. They present high power density, linearity, stress and strain compared to other artificial muscles. Nevertheless, as TCPs require heat to start, their main barrier for widespread use in WRRs are their slow reaction times and the high temperatures they reach. Previous studies have analysed different parameters, like fibre material, fibre diameter, and various cooling systems, to improve TCP frequency response and working temperature. Nevertheless, the length of the actuator has not been explored as a possible parameter to enhance the actuation performance in this regard. This work focuses on studying the behaviour of TCPs with different lengths and how the performance in frequency response and temperature can be improved using the length as a primary parameter, as they are critical for wearable robots. First, a characterisation of the TCPs was performed. Then, a method to improve frequency response, based on offsets on long actuators was implemented and validated using a chirp signal. The experimental results show that the mechanical characteristics are similar regardless of the actuator’s length. They reached a strain of 10% with a power of 0.16 W cm−1. However, the electrothermal properties changed as the power needed to increase temperature was higher when the actuator was enlarged. Therefore, an improvement in the required temperature was found, able to reduce the temperature with the same frequency response. Regarding the technique to enhance the speed of the actuator, it was possible to increase the frequency by 0.0006 Hz for each mm applied as an offset. Hence, the frequency response for the same displacement was increased linearly as the actuator was elongated.

Self-Sensing of a Magnetically Actuated Prism

We demonstrate a method for self-sensing of a magnetically actuated prism that can be used, e.g., in a feedback-loop without the need of additional sensors. In order to use the impedance of the actuation coils as a measurement parameter, we first obtained the optimal measurement frequency that is well separated from the actuation frequencies and at the same time provides the best compromise between sensitivity to the position and robustness. We then developed a combined actuation and measurement driver, and correlated its output signal to the mechanical state of the prism using a defined calibration sequence. We demonstrate that we can reliably measure the state of each actuator and determine the tilt angle of the prism with an accuracy of ±0.1∘ in the polar angle over a range of ±4∘ and ±20 mrad in the azimuthal angle.

Effects of Stimulus Frequency and Location on Vibrotactile Discrimination Performance Using Voice Coil Actuators on the Forearm

What are the effects of frequency variation of vibrotactile stimuli on localization acuity? The precise localization of vibrotactile stimuli is crucial for applications that are aimed at conveying vibrotactile information. In order to evaluate the ability to distinguish between vibrotactile stimuli based on their frequency and location on the forearm, we used a relative point localization method. Participants were presented with pairs of sequential vibrotactile stimuli at three possible locations on the forearm and asked to determine whether the second stimulation occurred at the same location as the first one in the pair or not. The stimulation frequency varied between 100 Hz, 150 Hz, 200 Hz and 250 Hz, which covers the range of frequencies that human observers are most sensitive to. The amplitude was kept constant. Our results revealed that the ability to discriminate between actuators remained unaffected by variations in the frequency of vibrotactile stimulation within the tested frequency range. The accuracy of the tactile discrimination task was heavily dependent on the location of the stimulation on the forearm, with the highest accuracy close to the wrist and elbow, locations that may serve as tactile anchor points. Our results highlight the critical role of stimulation location in precise vibrotactile localization and the importance of careful consideration of location in the design of forearm-mounted vibrotactile devices.

Comparison of PCB Based Actuator Coils for Untethered Actuation

Recently magnetic actuation has become a versatile tool for manipulation on both the micro and macro scale. Due to the problems arising from scale, conventional electromagnet production methods are insufficient to produce the desired electromagnets to be used for this purpose. To solve the problems arising from this situation, researchers have been forced to work in alternative electromagnet production and application areas and paved the way for PCB-based electromagnetic actuators. For this reason, this article will explain the design principles of PCB planar coils for electromagnetic actuator applications and will provide the basis for later studies to create a coil matrix using this coil design and an actuator that can move ferromagnetic particles or magnets in the plane with this 2D coil matrix. The presented project design steps can be listed as follows; First of all, to be able to design a coil that consists of copper paths on the PCB and can meet expectations (homogeneous magnetic field, high magnetic force/current ratio). PCB coils consisting of square, circular and hexagonal coils are manufactured and various measurements are made. The measured results are consistent with the simulated results proving the accuracy and applicability of PCB coil actuators, and they are highly suitable for a new actuator version. In the tests carried out, coils energized with 1A can provide a maximum field strength of 1400 (A/m) and an average of 900 (A/m) and due to the lack of magnetic core, the created magnetic field has the shape of a bell. In order to overcome this situation, it is foreseen that making studies to increase the working current and, as a more important development, switching from the planar single-layer structure to the structure of two or more layers can increase the magnetic field produced by this type of coils, but this layer increase should take into account the cost and PCB production and application capabilities. should be made available.

A machine-learning-based tool for last closed-flux surface reconstruction on tokamaks

Tokamaks allow to confine fusion plasma with magnetic fields. The prediction/reconstruction of the last closed-flux surface (LCFS) is one of the primary challenges in the control of the magnetic configuration. The evolution in time of the LCFS is determined by the interaction between the actuator coils and the internal tokamak plasma. This task requires real-time capable tools to deal with high-dimensional data and high resolution at same time, where the interaction between a wide range of input actuator coils with internal plasma state responses adds an additional layer of complexity. In this work, we present the application of a novel state-of-the-art machine learning model to LCFS reconstruction in an experimental advanced superconducting tokamak (EAST) that learns automatically from the experimental data of EAST. This architecture allows not only offline simulation and testing of a particular control strategy but can also be embedded in a real-time control system for online magnetic equilibrium reconstruction and prediction. In real-time modeling tests, our approach achieves very high accuracies, with an average similarity of over 99% in the LCFS reconstruction of the entire discharge process.

Sixty-Nine-Element Voice Coil Deformable Mirror for Visible Light Communication

To overcome the atmospheric turbulence aberration, and improve the quality of light beam in visible light communication (VLC), a compact 69-element deformable mirror (DM) using micro voice coil actuators was designed based on systematic theoretical analysis. The structural parameters of the micro voice coil actuator were optimized by electromagnetic theory and the finite element method. The DM was optimized from the aspects of thermal deformation, response time, coupling coefficient, and other parameters. Finally, wavefront fitting and residual calculation were completed according to the influence function. The optimized voice coil deformable mirror (VCDM) has a large phase stroke, good thermal stability, a short response time of less than 0.7 ms, and a large first resonance of 2045 Hz. The fitting residuals of the VCDM for the first 10 Zernike modes with a PV value of 8 μm are all below 10 nm (RMS). Compared with a similar DM, the obtained results from our compact VCDM indicate that it has a higher wavefront fitting precision. VCDM corrected complex random aberrations in the VLC scenario and improved the coupling efficiency of the signal beam, proving that the compact VCDM with high performance and low cost has a good application prospect in VLC systems.

Design and Implementation of an Active Vibration Control Algorithm Using Servo Actuator Control Installed in Series with a Spring-Damper

The membrane-type air spring can be used to suppress lateral vibration of a vibration isolation table. However, compared to voice coil actuators, pneumatic actuators are difficult to use for precise vibration control, because servo valves have nonlinear dynamic characteristics. Therefore, actuators, such as voice coil actuators, can be placed in parallel with air springs, allowing force-type actuators to provide additional force to the system. These actuators generate force. In the case of a ball-screw mechanism device or a linear servomotor, it is an actuator that generates displacement. These actuators are represented as serial active systems. Serial active systems are structurally simpler than parallel active systems. However, there are very few studies on vibration isolation systems using serial active systems compared to parallel active systems. As the two are different types of systems, a new control algorithm suitable for the serial active system is needed. This study proposes a system in which an actuator capable of accurately controlling displacement is connected in series with a support spring-damper. A new active vibration control algorithm for the proposed control system is also developed, which is termed the position input and position output. The proposed control algorithm uses the displacement of the system as an input and outputs the desired displacement of the actuator installed in series with the damper and spring. The proposed control algorithm increases the damping at the target frequency and reduces the response of the system. Numerical studies and experiments were conducted on the single-degree-of-freedom and multi-degree-of-freedom systems. The results show the efficacy of the proposed control system and the novel control algorithm for the vibration suppression of the lateral vibration of a vibration isolation table.

Fast Actuating Multi-Helically Heated Twisted and Coiled Polymer Actuator

The twisted and coiled polymer actuator (TCPA), promisingly used in wearable robots, soft exoskeletons and prosthesis, retains the advantages of convenience, high energy density, scalable stroke and hysteresis-free. However, as a thermal actuator, its dynamic response is still rather low, not only because of its convection improvable long cooling time, but also due to its long heating time. PID control is used to shorten TCPAs' rising time. However, this improvement is restricted by the upper limit of the heater temperature and has not substantially improve the poor heat conduction between nylon and heater. In addition, the high working current is also an important defect limiting TCPAs' application. In this work, we have revealed that the practical thermoelectric model of a TCPA obeys the law of heat transfer with Dirichlet condition. Under the consideration that TCPAs with symmetrical Dirichlet condition can effectively improve their heating performance, we have proposed a multi-helically heated twisted and coiled polymer actuator (MHTCPA). Through physical and simulated experiments, we verify that the MHTCPA has better actuating performance, requires less current, and that the symmetry of boundary conditions is the principal factor to improve the heating performance. Given the same power limit, the MHTCPA has a 20% smaller time constant, adopting a 24% smaller current to reach a 15% higher temperature and takes 50% less time to finish actuation. With the help of closed-loop control method, the actuation of the MHTCPA takes 83% less time compared previous TCPA. For the mechanical properties, the MHTCPA is still hysteresis-free but 25.8% stronger in lifting a 100-g weight.

Frequency-Domain Analysis and Design of Thomson-Coil Actuators

Thomson coil actuators consist of an electric coil repelling a metallic disk. They are considered the fastest electromagnetic actuators for short displacements and for this reason are favored for DC circuit breakers. Although the basic physics are understood, the actuator's minimal components are very interdependent, making analysis difficult. Actuator design so far has thus been limited to trial and error based on finite-element models. In this paper, a novel approach is presented based on the electromagnetic frequency response of the actuator. This method is shown to be an intriguing analysis tool and various actuator design directions are presented. Experimental results provide a confirmation of the analysis.

System modeling and sliding mode control of dual-axis voice coil actuator fast steering mirror

System modeling and sliding mode control of dual-axis voice coil actuator fast steering mirror

On force generation in electro-fluidic linear actuators with ferrofluid

In ferrofluid actuation systems, forces are generated by actively controlling pressure and flow within the fluid using an applied magnetic field. There are multiple contributing factors in force generation involving complex non-linear couplings between electromagnetic and fluid pressure fields. This brings significant challenges in theory-based design and optimization. In this paper, a theoretical model of pressure transmission between a ferrofluid and solid is derived starting from Maxwell’s stress tensor and accounting for magnetic saturation within the fluid. This model shows that linear actuator designs based on orthogonal mode operation, where the field direction through the fluid is perpendicular to the motion direction, can provide the highest force capacity for a given field strength from the actuator coil. This is verified by theoretical analysis of some basic linear actuator topologies. The results are applied in the design and analysis of a novel piston-type linear actuator with sealed chamber and two internal electrical coils for bidirectional operation. Experimental measurements of both static and dynamic behaviour are shown to validate the described principles. The actuator produces smooth and precise flow-regulated motion, has zero intrinsic stiffness, and exhibits very low friction due to the suspension effect from ferrofluid layers within the actuator.

Evaluating the Optimum Distance between Voice Coil Actuators Using the Relative Point Localization Method on the Forearm

While vibrotactile stimulation shows promise for sensory substitution devices, a crucial question concerns vibrotactile spatial resolution. We examined the optimum distance between three voice coil actuators (model: lofeltL5) on the forearm. Three actuators were embedded in a fabric-based vibrotactile sleeve where the actuators were placed in enclosures 3D-printed on the fabric. We used the relative point localization method where observers must discriminate whether two successive stimulations are in the same location or not. The resolution was measured for five vibrotactile sleeves, each with different distances between the actuators on the longitudinal axis of the forearm. The various distances were tested in a random order. In experiment one, pairs of stimuli were delivered sequentially in a random order to two adjacent actuators of the tactile sleeve on the upper side of the forearm. The task was to identify the perceived direction of the second stimulation (up, down, or the same) relative to the first one. Experiment two involved the same procedure but for the underside of the forearm. Taking the restrictions of the physical dimensions of the forearm and the design considerations into account, our results suggest that 20 mm is the optimum distance between the voice coil actuators (Model: Lofelt L5) for successful discrimination with high accuracy between the two stimulus locations on the forearm. There were no significant differences between the upper and undersides of the forearm.

High Precision Hybrid Reluctance Actuator With Integrated Orientation Independent Zero Power Gravity Compensation

This article presents a novel method for enabling energy-efficient and orientation-independent gravity compensation in active vibration isolation systems by taking advantage of the inherent negative stiffness of a hybrid reluctance actuator (HRA). Counteracting the gravitational force acting on the mover of an HRA with the position dependent force of the actuator’s integrated permanent magnet, the gravitational force can be compensated without the need for an additional actuation. An HRA-system with two translational degrees of freedom (DOF) is developed, comprising a permanent magnet for generating a constant biasing flux for both system axes and two actuator coils per axis for actively controlling the position of the mover. The system prototype has an actuation range of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\pm$</tex-math></inline-formula> 0.7 mm in both DOF, while enabling energy efficient gravity compensation of payloads up to 500 g by an additional current feedback control loop. Experiments demonstrate that the current consumption for compensation of a 500 g payload is reduced from 1.58 A to 10 mA, which corresponds to a reduction of the power consumption by a factor of 25 000.

A High Thrust Density Voice Coil Actuator With a New Structure of Double Magnetic Circuits for CubeSat Deployers

The deployer uses a voice coil actuator (VCA) to control precisely the separation velocity of CubeSats with different masses. This article proposes a novel VCA with a new structure of double magnetic circuits featuring high thrust density. Unlike the conventional design, the new design achieves high thrust density by using a new structure of double magnetic circuits to concentrate the magnetic flux in the airgap and alleviate the magnetic saturation in the yoke. An improved equivalent magnetic circuit is developed to reveal the performance improvement of the novel VCA. The electromagnetic characteristics of the new VCA are investigated and compared with the conventional counterparts by finite element analysis and experimental tests. One VCA deployer prototype has been fabricated and testedexperimentally to validate the thrust density and separation characteristics.

Active cooling of twisted coiled actuators via fabric air channels.

Twisted coiled actuators (TCAs) are promising artificial muscles for wearable soft robotic devices due to their biomimetic properties, inherent compliance, and slim profile. These artificial muscles are created by super-coiling nylon thread and are thermally actuated. Unfortunately, their slow natural cooling rate limits their feasibility when used in wearable devices for upper limb rehabilitation. Thus, a novel cooling apparatus for TCAs was specifically designed for implementation in soft robotic devices. The cooling apparatus consists of a flexible fabric channel made from nylon pack cloth. The fabric channel is lightweight and could be sewn onto other garments for assembly into a soft robotic device. The TCA is placed in the channel, and a miniature air pump is used to blow air through it to enable active cooling. The impact of channel size on TCA performance was assessed by testing nine fabric channel sizes-combinations of three widths (6, 8, and 10 mm) and three heights (4, 6, and 8 mm). Overall, the performance of the TCA improved as the channel dimensions increased, with the combination of a 10 mm width and an 8 mm height resulting in the best balance between cooling time, heating time, and stroke. This channel was utilized in a follow-up experiment to determine the impact of the cooling apparatus on TCA performance. In comparison to passive cooling without a channel, the channel and miniature air pump reduced the TCA cooling time by 42% ( s to s, ). Unfortunately, there was also a 9% increase in the heating time ( s to s, ) and a 28% decrease in the stroke ( mm to mm, ). This work demonstrates that fabric cooling channels are a viable option for cooling thermally actuated artificial muscles within a soft wearable device. Future work can continue to improve the channel design by experimenting with other configurations and materials.

An actuator with tuneable resonant frequency for active vibration damping

This paper explores implementing active resonance frequency tuning of adaptive tuned vibration absorbers (ATVA) by coupling a vibration absorber comprising a voice coil actuator (VCA) and a sprung inertial mass to a solenoid actuator (SA). In this device, the DC current flowing through the SA generates negative magnetic stiffness and varies the equilibrium position of the nonlinear spring of the VCA, thus changing the effective stiffness of the system and its resonant frequency. The paper details the analytical design steps and the parametric sizing process of designing the SA to maximize its force density within defined magnetic, dimensions and thermal constraints. The design of the SA was verified using finite element analysis (FEA), manufactured and coupled to a VCA to make the complete ATVA system. The experimental results demonstrate the tunability of the resonance frequency of the ATVA by controlling the SA’s DC current. It is shown that the system’s resonance frequency can be shifted by up to 23% of the fundamental resonance frequency, which improves the effectiveness of the ATVA at damping a broader range of excitation frequencies.

Material-based modeling of cavatappi artificial muscles

Soft actuators show much promise for use in bioinspired and biomimetic robotics as they share many similarities with actuation systems found in nature. Twisted and coiled actuators are soft actuators that have been shown to outperform many metrics of biological muscles, leading researchers to derive actuation models for future control and implementation. Although models have been proposed for twisted and coiled carbon nanotubes and polymer fibers, cavatappi artificial muscles—a novel twisted and coiled fluidic soft actuator—have not been modeled yet. This work establishes a framework for modeling cavatappi using the thick-wall pressure vessel stress analysis and the spring theory. The presented model uses the mechanical properties of the precursor drawn material used for fabrication, initial twist (internal fiber angle), muscle geometry, and internal pressure to predict the artificial muscles contraction under different external loads. The model predictions agree with the experimental results for cavatappi of different internal fiber angles and load conditions. Given their potential implementation in bioinspired applications, our model can help better design, optimize, and control the actuation response of cavatappi.

Design and construction of a lab-made calorimetric system for specific loss power (SLP) measurements in suspensions of magnetic nanoparticles

The specific loss power (SLP) is a figure of merit in nanomagnetism, which allows to classify systems of magnetic nanoparticles (MNPs) according to their capability to dissipate heating power in response to the energy absorbed from an alternating magnetic field (AMF) at radiofrequencies. SLP magnitude is relevant in biological applications of MNPs, such as nanomedicine, where the thermal energy released by these nanostructures can be used to produce apoptosis in cancer cells, becoming a promising alternative for the treatment of malignant tumors. In this work we present details of the design and construction of a lab-made electromagnetic system to perform SLP measurements in systems of MNPs. The system is composed by an actuation stage, which generates the AMF to excite the sample, a thermal insulation stage between the sample and its surroundings, and two sensing stages: the first one for measuring the magnetic field applied to the sample and the second one for measuring the changes of temperature of the sample. The system developed allows to reach AMFs around 7.7 kA m−1 in the center of the actuation coil, operating at 260 kHz and with a power consumption less than 150 W. The architecture of the system allows the automatization and control of SLP measurements from remote places through the IEEE 488 bus available in the power source and a graphic interface developed in LabVIEW software, which allows to visualize temperature versus time measurements, control the magnetic field amplitude and store the data in a public folder available in the cloud. Some results are presented, where the system is employed to measure the SLP of a colloidal suspension of MNPs of maghemite (γ-Fe2O3) stabilized with sodium polyacrylate. The results obtained show a satisfactory performance of the device to measure the SLP of samples of MNPs for different purposes.

Force Enhanced Multi-Twisted and Coiled Actuator and Its Application in Temperature Self-Adaptive Tensegrity Mechanisms

Twisted and coiled actuators (TCAs) are a type of thermal-driven actuator with high energy density and display the enormous application potential to drive soft robots. However, the existing soft robots driven by TCAs are scarce and can only realize the grabbing and creeping movement almost without load capacity. The main reason is that the TCA fabricated by traditional self-coiling method has low force and long thermal response time. Aiming to improve the output force of TCA, the fabrication and force enhancement principle of multitwisted and coiled actuators (MTCAs) are developed. Compared with the traditional single-twisted and coiled actuators, the output force of the MTCA can be increased to more than three times with the same fiber. The proposed MTCA with a mass of 0.1 g achieves deformations of 14.46% of its length under load of 1200 g. Besides, we expand the application field of the MTCA into the self-adaptive mechanisms. Two temperature self-adaptive tensegrity mechanisms with self-folded and self-upraised ability are designed and tested to further display the excellent performances of the MTCA. This article not only proposed the MTCA with significantly improved output force, but also promoted the application of the TCAs in temperature self-adaptive mechanisms.