Open AccessEditorial
Acknowledgement to Reviewers of Actuators in 2017
Actuators 2018, 7(1), 4; doi:10.3390/act7010004 -
Abstract
Peer review is an essential part in the publication process, ensuring that Actuators maintains high quality standards for its published papers [...]
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Open AccessArticle
Comprehensive Analytical Approximations of the Pull-In Characteristics of an Electrostatically Actuated Nanobeam under the Influences of Intermolecular Forces
Actuators 2018, 7(1), 3; doi:10.3390/act7010003 -
Abstract
In this paper, analytical closed-form expressions to accurately estimate the pull-in characteristics of an electrostatically-actuated doubly-clamped nanobeam are derived and examined. In this regard, a coupled electro-mechanical problem for the nano-actuator is first presented assuming a single mode approximation while taking into account
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In this paper, analytical closed-form expressions to accurately estimate the pull-in characteristics of an electrostatically-actuated doubly-clamped nanobeam are derived and examined. In this regard, a coupled electro-mechanical problem for the nano-actuator is first presented assuming a single mode approximation while taking into account all the possible structural, electrical and nanoscale effects: the fringing of the electrical actuating force, the geometric mid-plane stretching and intermolecular (van der Walls and Casimir) forces. The complicated nonlinear resultant equations are numerically approximated in order to derive the closed-form expressions for the important nano-actuator pull-in characteristics: i.e., the detachment length, the minimum reachable gap size before the collapse and the respective pull-in voltage. The resulting closed-form expressions are first quantitatively validated with other previously published results, and comparisons showed an acceptable agreement. Unlike the reported expressions in the literature, the proposed closed-form expressions in this work are proper approximations, fairly accurate and, more importantly, provide a quick estimate of the critical design pull-in parameters of the nano-actuator. In addition, the analysis of these expressions demonstrated that the consideration of the intermolecular forces together with the fringe effect tends to significantly reduce the threshold pull-in voltage, whereas the mid-plane stretching parameter tends to the contrary to increase the voltage at the pull-in collapse. The derived expressions of these analytical/approximate solutions could hopefully be appropriately used by NEMS engineers as simple/quick procedures for successful design and fabrication of electrostatically-actuated nano-devices. Full article
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Open AccessArticle
Piezoelectric Motor Using In-Plane Orthogonal Resonance Modes of an Octagonal Plate
Actuators 2018, 7(1), 2; doi:10.3390/act7010002 -
Abstract
Piezoelectric motors use the inverse piezoelectric effect, where microscopically small periodical displacements are transferred to continuous or stepping rotary or linear movements through frictional coupling between a displacement generator (stator) and a moving (slider) element. Although many piezoelectric motor designs have various drive
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Piezoelectric motors use the inverse piezoelectric effect, where microscopically small periodical displacements are transferred to continuous or stepping rotary or linear movements through frictional coupling between a displacement generator (stator) and a moving (slider) element. Although many piezoelectric motor designs have various drive and operating principles, microscopic displacements at the interface of a stator and a slider can have two components: tangential and normal. The displacement in the tangential direction has a corresponding force working against the friction force. The function of the displacement in the normal direction is to increase or decrease friction force between a stator and a slider. Simply, the generated force alters the friction force due to a displacement in the normal direction, and the force creates movement due to a displacement in the tangential direction. In this paper, we first describe how the two types of microscopic tangential and normal displacements at the interface are combined in the structures of different piezoelectric motors. We then present a new resonance-drive type piezoelectric motor, where an octagonal plate, with two eyelets in the middle of the two main surfaces, is used as the stator. Metallization electrodes divide top and bottom surfaces into two equal regions orthogonally, and the two driving signals are applied between the surfaces of the top and the bottom electrodes. By controlling the magnitude, frequency and phase shift of the driving signals, microscopic tangential and normal displacements in almost any form can be generated. Independently controlled microscopic tangential and normal displacements at the interface of the stator and the slider make the motor have lower speed–control input (driving voltage) nonlinearity. A test linear motor was built by using an octagonal piezoelectric plate. It has a length of 25.0 mm (the distance between any of two parallel side surfaces) and a thickness of 3.0 mm, which can produce an output force of 20 N. Full article
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Open AccessArticle
A New and Versatile Adjustable Rigidity Actuator with Add-on Locking Mechanism (ARES-XL)
Actuators 2018, 7(1), 1; doi:10.3390/act7010001 -
Abstract
Adjustable compliant actuators are being designed and implemented in robotic devices because of their ability to minimize large forces due to impacts, to safely interact with the user, and to store and release energy in passive elastic elements. Conceived as a new force-controlled
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Adjustable compliant actuators are being designed and implemented in robotic devices because of their ability to minimize large forces due to impacts, to safely interact with the user, and to store and release energy in passive elastic elements. Conceived as a new force-controlled compliant actuator, an adjustable rigidity with embedded sensor and locking mechanism actuator (ARES-XL) is presented in this paper. This compliant system is intended to be implemented in a gait exoskeleton for children with neuro muscular diseases (NMDs) to exploit the intrinsic dynamics during locomotion. This paper describes the mechanics and initial evaluation of the ARES-XL, a novel variable impedance actuator (VIA) that allows the implementation of an add-on locking mechanism to this system, and in combination with its zero stiffness capability and large deflection range, provides this novel joint with improved properties when compared to previous prototypes developed by the authors and other state-of-the-art (SoA) devices. The evaluation of the system proves how this design exceeds the main capabilities of a previous prototype as well as providing versatile actuation that could lead to its implementation in multiple joints. Full article
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Open AccessArticle
Modeling the Static Force of a Festo Pneumatic Muscle Actuator: A New Approach and a Comparison to Existing Models
Actuators 2017, 6(4), 33; doi:10.3390/act6040033 -
Abstract
In this paper, a new approach for modeling the static force characteristic of Festo pneumatic muscle actuators (PMAs) will be presented. The model is physically motivated and therefore gives a deeper understanding of the Festo PMA. After introducing the new model, it will
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In this paper, a new approach for modeling the static force characteristic of Festo pneumatic muscle actuators (PMAs) will be presented. The model is physically motivated and therefore gives a deeper understanding of the Festo PMA. After introducing the new model, it will be validated through a comparison to a measured force map of a Festo DMSP-10-250 and a DMSP-20-300, respectively. It will be shown that the error between the new model and the measured data is below 4.4% for the DMSP-10-250 and below 2.35% for the DMSP-20-300. In addition, the quality of the presented model will be compared to the quality of existing models by comparing the maximum error. It can be seen that the newly introduced model is closer to the measured force characteristic of a Festo PMA than any existing model. Full article
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Open AccessEditorial
Actuators for Active Magnetic Bearings
Actuators 2017, 6(4), 31; doi:10.3390/act6040031 -
Abstract
The literature of active magnetic bearing (AMB) technology dates back to at least 1937 when the earliest work that clearly describes an active magnetic bearing system was published by Jesse Beams [...]
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Open AccessArticle
Efficient Structure-Based Models for the McKibben Contraction Pneumatic Muscle Actuator: The Full Description of the Behaviour of the Contraction PMA
Actuators 2017, 6(4), 32; doi:10.3390/act6040032 -
Abstract
To clarify the advantages of using soft robots in all aspects of life, the effective behaviour of the pneumatic muscle actuator (PMA) must be known. In this work, the performances of the PMA are explained and modelled with three formulas. The first formula
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To clarify the advantages of using soft robots in all aspects of life, the effective behaviour of the pneumatic muscle actuator (PMA) must be known. In this work, the performances of the PMA are explained and modelled with three formulas. The first formula describes the pulling force of the actuator based on the structure parameters; furthermore, the formula presented is the generalised contraction force for wholly-pneumatic muscle actuators. The second important model is the length formula, which is modified to our previous work to fit different actuator structures. Based on these two models, the stiffness of the actuator is formulated to illustrate its variability at different air pressure amounts. In addition, these formulas will make the selection of proper actuators for any robot arm structure easier using the knowledge gained from their performance. On the other hand, the desired behaviour of this type of actuator will be predefined and controlled. Full article
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Open AccessArticle
Electric-Pneumatic Actuator: A New Muscle for Locomotion
Actuators 2017, 6(4), 30; doi:10.3390/act6040030 -
Abstract
A better understanding of how actuator design supports locomotor function may help develop novel and more functional powered assistive devices or robotic legged systems. Legged robots comprise passive parts (e.g., segments, joints and connections) which are moved in a coordinated manner by actuators.
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A better understanding of how actuator design supports locomotor function may help develop novel and more functional powered assistive devices or robotic legged systems. Legged robots comprise passive parts (e.g., segments, joints and connections) which are moved in a coordinated manner by actuators. In this study, we propose a novel concept of a hybrid electric-pneumatic actuator (EPA) as an enhanced variable impedance actuator (VIA). EPA is consisted of a pneumatic artificial muscle (PAM) and an electric motor (EM). In contrast to other VIAs, the pneumatic artificial muscle (PAM) within the EPA provides not only adaptable compliance, but also an additional powerful actuator with muscle-like properties, which can be arranged in different combinations (e.g., in series or parallel) to the EM. The novel hybrid actuator shares the advantages of both integrated actuator types combining precise control of EM with compliant energy storage of PAM, which are required for efficient and adjustable locomotion. Experimental and simulation results based on the new dynamic model of PAM support the hypothesis that combination of the two actuators can improve efficiency (energy and peak power) and performance, while does not increase control complexity and weight, considerably. Finally, the experiments on EPA adapted bipedal robot (knee joint of the BioBiped3 robot) show improved efficiency of the actuator at different frequencies. Full article
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Open AccessArticle
Quantification of Force and Torque Applied by a High-Field Magnetic Resonance Imaging System on an Ultrasonic Motor for MRI-Guided Robot-Assisted Interventions
Actuators 2017, 6(4), 29; doi:10.3390/act6040029 -
Abstract
The risk of accidental dislodgement of robot-operated surgical mechanisms can lead to morbidity or mortality. The force and torque applied by a 3.0-tesla scanner on an ultrasonic motor are not fully known. The force and torque may displace the motor, which is not
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The risk of accidental dislodgement of robot-operated surgical mechanisms can lead to morbidity or mortality. The force and torque applied by a 3.0-tesla scanner on an ultrasonic motor are not fully known. The force and torque may displace the motor, which is not fully magnetic resonance imaging (MRI)-compatible but can be safely used in MR environments. A suspension apparatus was designed to measure the angles of deflection and rotation applied to the motor by MR magnetic fields. Three orientations and two power states of the motor were assessed inside the MR bore. The displacement force and torque were measured at eight locations with respect to the bore. The displacement force on the motor from 10 cm outside the magnet bore to 20 cm inside the bore ranged from 3 to 7 gF. The experimental measurements are in agreement with the theoretical values. Running the motor altered the force by 1 gF. The force does not significantly change when the MRI scanner is on. Considerable displacement force is applied to the motor, and no deflection torque is observed. Quantified values can be used to solve dynamic equations for robotic mechanisms intended for MRI-guided operations. Full article
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Open AccessFeature PaperArticle
Stiffness Control of Variable Serial Elastic Actuators: Energy Efficiency through Exploitation of Natural Dynamics
Actuators 2017, 6(4), 28; doi:10.3390/act6040028 -
Abstract
Variable elastic actuators are very promising for applications in physical human–robot interaction. Besides enabling human safety, such actuators can support energy efficiency, especially if the natural behavior of the system is exploited. In this paper, the power and energy consumption of variable stiffness
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Variable elastic actuators are very promising for applications in physical human–robot interaction. Besides enabling human safety, such actuators can support energy efficiency, especially if the natural behavior of the system is exploited. In this paper, the power and energy consumption of variable stiffness actuators with serial elasticity is investigated analytically and experimentally. Besides the fundamental mechanics, the influence of friction and electrical losses is discussed. A simple but effective stiffness control method is used to exploit the corresponding knowledge of natural dynamics by tuning the system to antiresonance operation. Despite nonlinear friction effects and additional electrical dynamics, the consideration of the ideal mechanical dynamics is completely sufficient for stiffness control. Simulations and experiments show that this yields a distinct reduction in power and energy consumption, which underlines the suitability of the control strategy. Full article
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Open AccessArticle
Pneumatic Multi-Pocket Elastomer Actuators for Metacarpophalangeal Joint Flexion and Abduction-Adduction
Actuators 2017, 6(3), 27; doi:10.3390/act6030027 -
Abstract
During recent years, interest has been rising towards developing fluidic fiber-reinforced elastomer actuators for wearable soft robotics used in hand rehabilitation and power-assist. However, they do not enable finger abduction-adduction, which plays an important role in activities of daily living, when grasping larger
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During recent years, interest has been rising towards developing fluidic fiber-reinforced elastomer actuators for wearable soft robotics used in hand rehabilitation and power-assist. However, they do not enable finger abduction-adduction, which plays an important role in activities of daily living, when grasping larger objects. Furthermore, the developed gloves often do not have separate control of joints, which is important for doing various common rehabilitation motions. The main obstacle for the development of a fully-assisting glove is moving a joint with multiple degrees of freedom. If the functions are built into the same structure, they are naturally coupled and affect each other, which makes them more difficult to design and complex to control than a simple flexion-extension actuator. In this study, we explored the key design elements and fabrication of pneumatic multi-pocket elastomer actuators for a soft rehabilitation glove. The goal was to gain more control over the metacarpophalangeal joint’s response by increasing the degree of actuation. Three main functional designs were tested for achieving both flexion and abduction-adduction. Five prototypes, with four different actuator geometries and four different reinforcement types, were designed and fabricated. They were evaluated by recording their free motion with motion capture and measuring their torque output using a dummy finger. Results showed the strengths and weaknesses of each design in separating the control of the two functions. We discuss the different improvements that are needed in order to make each design plausible for developing an actuator that meets the requirements for full assist of the hand’s motions. In conclusion, we show that it is possible to produce multi-pocket actuators for assisting MCP joint motion in both flexion and abduction-adduction, although coupling between the separate functions is still problematic and should be considered further. Full article
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Open AccessArticle
Generalization of Series Elastic Actuator Configurations and Dynamic Behavior Comparison
Actuators 2017, 6(3), 26; doi:10.3390/act6030026 -
Abstract
The Series Elastic Actuator (SEA) has recently been developed by many research groups and applied in various fields. As SEA is the combination of motor, spring, gear and load, various types and configurations of mechanism have been developed as SEAs to satisfy many
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The Series Elastic Actuator (SEA) has recently been developed by many research groups and applied in various fields. As SEA is the combination of motor, spring, gear and load, various types and configurations of mechanism have been developed as SEAs to satisfy many requirements necessary for the applications. This paper provides a theoretical framework to categorize and compare these various configurations of SEAs. The general structure and model of SEA is provided, and SEA configurations are categorized into Force-sensing Series Elastic Actuator, Reaction Force-sensing Series Elastic Actuator and Transmitted Force-sensing Series Elastic Actuator, based on the relative location of the spring. Criteria such as Force sensitivity, Compliance and Transmissibility of SEA are derived and compared using actual SEAs that have been developed previously. Full article
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Open AccessArticle
Optimization Design of Electromagnetic Actuator Applied as Fast Tool Servo
Actuators 2017, 6(3), 25; doi:10.3390/act6030025 -
Abstract
Fast tool servos (FTS) for single point diamond turning are widely employed for machine optical free-form surfaces. A FTS with a large stroke and a high bandwidth can increase the efficiency of a machine and complexity of a work-piece. In this paper, a
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Fast tool servos (FTS) for single point diamond turning are widely employed for machine optical free-form surfaces. A FTS with a large stroke and a high bandwidth can increase the efficiency of a machine and complexity of a work-piece. In this paper, a FTS driven by a Maxwell electromagnetic actuator is developed to obtain a relatively large stroke and high bandwidth. In this study, a multi-objective optimization model is proposed based on the whole system by considering electromagnetic driving principles, kinematic model, and mechanical model. The proposed optimization model can be applied to solve the parameters of electromagnetic actuators with diverse application requirements. A sequential quadratic programming (SQP) algorithm is implemented to solve the problem. The optimization results are verified through both finite element analysis and experiments. The optimized FTS can produce 49.55 μm of full stroke with a frequency response of 3.2 kHz. Full article
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Open AccessReview
Recent Progress on BaTiO3-Based Piezoelectric Ceramics for Actuator Applications
Actuators 2017, 6(3), 24; doi:10.3390/act6030024 -
Abstract
Due to issues with Pb toxicity, there is an urgent need for high performance Pb-free alternatives to Pb-based piezoelectric ceramics. Although pure BaTiO3 material exhibits fairly low piezoelectric coefficients, further designing of such a material system greatly enhances the piezoelectric response by
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Due to issues with Pb toxicity, there is an urgent need for high performance Pb-free alternatives to Pb-based piezoelectric ceramics. Although pure BaTiO3 material exhibits fairly low piezoelectric coefficients, further designing of such a material system greatly enhances the piezoelectric response by means of domain engineering, defects engineering, as well as phase boundary engineering. Especially after the discovery of a Ba(Zr0.2Ti0.8)O3x(Ba0.7Ca0.3)TiO3 system with extraordinarily high piezoelectric properties (d33 > 600 pC/N), BaTiO3-based piezoelectric ceramics are considered as one of the promising Pb-free substitutes. In the present contribution, we summarize the idea of designing high property BaTiO3 piezoceramic through domain engineering, defect-doping, as well as morphotropic phase boundary (MPB). In spite of its drawback of low Curie temperature, BaTiO3-based piezoelectric materials can be considered as an excellent model system for exploring the physics of highly piezoelectric materials. The relevant material design strategy in BaTiO3-based materials can provide guidelines for the next generation of Pb-free materials with even better piezoelectric properties that can be anticipated in the near future. Full article
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Open AccessReview
Flexible Medical Devices: Review of Controllable Stiffness Solutions
Actuators 2017, 6(3), 23; doi:10.3390/act6030023 -
Abstract
In the medical field and in soft robotics, flexible devices are required for safe human interaction, while rigid structures are required to withstand the force application and accuracy in motion. This paper aims at presenting controllable stiffness mechanisms described in the literature for
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In the medical field and in soft robotics, flexible devices are required for safe human interaction, while rigid structures are required to withstand the force application and accuracy in motion. This paper aims at presenting controllable stiffness mechanisms described in the literature for applications with or without shape-locking performances. A classification of the solutions based on their working principle is proposed. The intrinsic properties of these adaptive structures can be modified to change their mechanical characteristics from a geometrical point of view or equivalent elastic properties (with internal mechanisms or with a change in material properties). These solutions are compared quantitatively, based on selected criteria linked to the medical field as the stiffness range, the activation time and the working conditions. Depending on the application and its requirements, the most suitable solution can be selected following the quantitative comparisons. Several applications of these tunable stiffness structures are proposed and illustrated by examples of the literature. Full article
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Open AccessArticle
A Novel Mechanically Overdamped Actuator with Adjustable Stiffness (MOD-AwAS) for Safe Interaction and Accurate Positioning
Actuators 2017, 6(3), 22; doi:10.3390/act6030022 -
Abstract
This paper presents the design and development of a novel mechanically overdamped actuator with adjustable stiffness (MOD-AwAS). The novelty of MOD-AwAS compared to other variable stiffness actuators relates to its mechanical design, which prevents oscillations at the output link. Almost all variable stiffness
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This paper presents the design and development of a novel mechanically overdamped actuator with adjustable stiffness (MOD-AwAS). The novelty of MOD-AwAS compared to other variable stiffness actuators relates to its mechanical design, which prevents oscillations at the output link. Almost all variable stiffness actuators have an overshooting problem that require a sophisticated control algorithm to be able to perform accurate positioning. MOD-AwAS can regulate the stiffness from zero to its maximum (theoretically infinite) in less than 0.2 s by changing the position of the pivot point of its lever mechanisms. MOD-AwAS employs only one rotational spring with no pre-deflection, which gives it full accessibility to its energy storage capacity. Experimental results are presented to show the ability of MOD-AwAS to control its position accurately with a wide range of stiffness adjustment. Full article
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Open AccessArticle
Modeling and Testing Strategies for an Interconnected Four-Pole Magnetic Bearing
Actuators 2017, 6(3), 21; doi:10.3390/act6030021 -
Abstract
An unusual idea for the construction of active magnetic bearings has been recently discussed in the literature. Theoretical results predict a greater equivalent stiffness for it, when compared with traditional active magnetic bearings. The development of a mathematical model that allows these predictions
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An unusual idea for the construction of active magnetic bearings has been recently discussed in the literature. Theoretical results predict a greater equivalent stiffness for it, when compared with traditional active magnetic bearings. The development of a mathematical model that allows these predictions and the use of recently-built prototypes for testing if the expectations hold true are the main goals of this paper. Full article
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Open AccessArticle
Measuring the Temperature Increase of an Ultrasonic Motor in a 3-Tesla Magnetic Resonance Imaging System
Actuators 2017, 6(2), 20; doi:10.3390/act6020020 -
Abstract
This paper aims to evaluate the temperature increase caused by a 3.0-T magnetic resonance imaging (MRI) system on an ultrasonic motor (USM) used to actuate surgical robots in the MRI environment. Four fiber-optic temperature sensors were attached to the USM. Temperature was monitored
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This paper aims to evaluate the temperature increase caused by a 3.0-T magnetic resonance imaging (MRI) system on an ultrasonic motor (USM) used to actuate surgical robots in the MRI environment. Four fiber-optic temperature sensors were attached to the USM. Temperature was monitored outside the five-Gauss boundary and then inside the bore for 20 min while the USM was powered on. The USM temperature was tested for two states of the scanner, “off” and “on”, by employing common clinical imaging sequences and echo planar imaging sequences. The USM showed a slight temperature increase while operating in the static field of the MRI. A considerable temperature increase (~10 °C) was observed when the scanner was on. The temperature increased to 60 °C, which is beyond the acceptable safe temperature and can result in thermal burns. Most of the temperature increase (80%) was due to effects of the static field on the motion of the rotating parts of the motor, while the remainder (20%) derived from heat deposited in the conductive components of the USM due to radiofrequency pulses and gradient field changes. To solve the temperature increase, the metal components of the USM’s case can be replaced by silicon carbide. Full article
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Open AccessArticle
Design and Characterization of In-Plane Piezoelectric Microactuators
Actuators 2017, 6(2), 19; doi:10.3390/act6020019 -
Abstract
In this paper, two different piezoelectric microactuator designs are studied. The corresponding devices were designed for optimal in-plane displacements and different high flexibilities, proven by electrical and optical characterization. Both actuators presented two dominant vibrational modes in the frequency range below 1 MHz:
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In this paper, two different piezoelectric microactuator designs are studied. The corresponding devices were designed for optimal in-plane displacements and different high flexibilities, proven by electrical and optical characterization. Both actuators presented two dominant vibrational modes in the frequency range below 1 MHz: an out-of-plane bending and an in-plane extensional mode. Nevertheless, the latter mode is the only one that allows the use of the device as a modal in-plane actuator. Finite Element Method (FEM) simulations confirmed that the displacement per applied voltage was superior for the low-stiffness actuator, which was also verified through optical measurements in a quasi-static analysis, obtaining a displacement per volt of 0.22 and 0.13 nm/V for the low-stiffness and high-stiffness actuator, respectively. In addition, electrical measurements were performed using an impedance analyzer which, in combination with the optical characterization in resonance, allowed the determination of the electromechanical and stiffness coefficients. The low-stiffness actuator exhibited a stiffness coefficient of 5 × 104 N/m, thus being more suitable as a modal actuator than the high-stiffness actuator with a stiffness of 2.5 × 105 N/m. Full article
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Open AccessFeature PaperArticle
Static and Dynamic Studies of Electro-Active Polymer Actuators and Integration in a Demonstrator
Actuators 2017, 6(2), 18; doi:10.3390/act6020018 -
Abstract
Nowadays, the haptic effect is used and developed for many applications—particularly in the automotive industry, where the mechanical feedback induced by a haptic system enables the user to receive information while their attention is kept on the road and on driving. This article
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Nowadays, the haptic effect is used and developed for many applications—particularly in the automotive industry, where the mechanical feedback induced by a haptic system enables the user to receive information while their attention is kept on the road and on driving. This article presents the development of a vibrotactile button based on printed piezoelectric polymer actuation. Firstly, the characterization of the electro-active polymer used as the actuator and the development of a model able to predict the electromechanical behavior of this device are summarized. Then, the design of circular membranes and their dynamic characterization are presented. Finally, this work is concluded with the construction of a fully functional demonstrator, integrating haptic buttons leading to a clear haptic sensation for the user. Full article
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