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Actuators, Volume 8, Issue 1 (March 2019)

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Open AccessArticle
Simultaneous Enhancement of Bending and Blocking Force of an Ionic Polymer-Metal Composite (IPMC) by the Active Use of Its Material Characteristics Change
Actuators 2019, 8(1), 29; https://doi.org/10.3390/act8010029
Received: 24 December 2018 / Revised: 14 March 2019 / Accepted: 15 March 2019 / Published: 25 March 2019
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Abstract
The exhibition of significantly large bending is a remarkable characteristic of an ionic polymer-metal composite (IPMC). However, its inability to generate a high enough force is a major problem in achieving a practical IPMC actuator. The simultaneous enhancement of bending and force generation [...] Read more.
The exhibition of significantly large bending is a remarkable characteristic of an ionic polymer-metal composite (IPMC). However, its inability to generate a high enough force is a major problem in achieving a practical IPMC actuator. The simultaneous enhancement of bending and force generation is needed for broadening the potential of the IPMC actuator as a practical engineering device. Corrosive materials as a flexible electrode of the IPMC is usually not preferred, whereas a noncorrosive material such as platinum is broadly used. Here, we used silver, a corrosive metal, as an IPMC electrode intentionally. The silver electrode exhibits a reversible redox reaction upon an external electric stimulation. That silver redox reaction resulted in the material characteristics change of the IPMC, and it consequently resulted in the simultaneous enhancement of the IPMC bending curvature and blocking force generation. It was further found that the thicker silver coating anchored into the far inside of the IPMC led to the occurrence of a significant silver redox reaction and it altered the material characteristics of the IPMC, consequently turning the IPMC into a greatly deformable and high force generative one. Full article
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Open AccessArticle
Modified U-shaped Microactuator with Compliant Mechanism Applied to a Microgripper
Actuators 2019, 8(1), 28; https://doi.org/10.3390/act8010028
Received: 17 January 2019 / Revised: 8 March 2019 / Accepted: 15 March 2019 / Published: 19 March 2019
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Abstract
In this paper, a modified U-shaped micro-actuator with a compliant mechanism is proposed. It was analyzed with a uniform and modified thin arm, as well as a similar variation in the corresponding flexure, in order to observe the impact of the compliant lumped [...] Read more.
In this paper, a modified U-shaped micro-actuator with a compliant mechanism is proposed. It was analyzed with a uniform and modified thin arm, as well as a similar variation in the corresponding flexure, in order to observe the impact of the compliant lumped mechanism. The use of these compliant mechanisms implies an increment in the deformation and a reduction in the equivalent stress of 25% and 52.25%, respectively. This characterization was developed using the Finite Element Method (FEM) in ANSYS Workbench. The design, analysis and simulation were developed with Polysilicon. In this study, the following performance parameters were also analyzed: force and temperature distribution. This device is supplied with voltage from 0 V up to 3 V, at room temperature. The modified U-shaped actuator was applied in both arms of a microgripper, and to evaluate its electrothermal performance, a static structural analysis has been carried out in Ansys Workbench. The microgripper has an increment in deformation of 22.33%, an equivalent stress reduction of 50%, and a decrease in operation frequency of 10.8%. The force between its jaws is of 367 µN. This low level of force could be useful when sensitive particles are manipulated. Full article
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Open AccessArticle
Measurement of a Temperature Field Generated by a Synthetic Jet Actuator using Digital Holographic Interferometry
Actuators 2019, 8(1), 27; https://doi.org/10.3390/act8010027
Received: 9 January 2019 / Revised: 10 March 2019 / Accepted: 11 March 2019 / Published: 18 March 2019
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Abstract
This paper shows the possibility of the measurement of a temperature field generated by heated fluid from a synthetic jet (SJ) actuator. Digital holographic interferometry (DHI) was the main measuring method used for the experiments. A single-projection DHI was used for the visualization [...] Read more.
This paper shows the possibility of the measurement of a temperature field generated by heated fluid from a synthetic jet (SJ) actuator. Digital holographic interferometry (DHI) was the main measuring method used for the experiments. A single-projection DHI was used for the visualization of the temperature field as an average temperature along the optical axis. The DHI results are compared with data obtained from constant current anemometry (CCA) experiments for the validation of the method. Principle of 3D temperature distribution using a tomographic approach is also described in this paper. A single SJ actuator, multiple continual nozzle, and the SJ actuator with two output orifices are used as a testing device for the presented experiments. The experimental configuration can measure high-frequency synthetic jets with the use of a single slow-frame-rate camera. Due to the periodic character of the SJ flow, synchronization between the digital camera, and the external trigger driving the phenomenon is performed. This approach can also distinguish between periodic and random parts of the flow. Full article
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Open AccessArticle
Improving the Performance of Pump-Controlled Circuits for Single-Rod Actuators
Actuators 2019, 8(1), 26; https://doi.org/10.3390/act8010026
Received: 18 January 2019 / Revised: 1 March 2019 / Accepted: 5 March 2019 / Published: 11 March 2019
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Abstract
Pump-controlled hydraulic circuits offer an energy-efficient solution for many applications. They combine the high power to weight ratio of hydraulic technology with the ease of control of electric technology. Pump-controlled circuits for double-rod cylinders are well developed as compared to those of single-rod [...] Read more.
Pump-controlled hydraulic circuits offer an energy-efficient solution for many applications. They combine the high power to weight ratio of hydraulic technology with the ease of control of electric technology. Pump-controlled circuits for double-rod cylinders are well developed as compared to those of single-rod cylinders. In spite of many initiatives, certain common pump-controlled single-rod cylinder solutions present stability issues during specific modes of operation. Common examples of the solutions are circuits that utilize pilot-operated check valves and circuits that use shuttle valves. In these circuits, velocity oscillations have been reported during actuator retraction at low assistive loads. In this paper, we study the area on the load-velocity graph of the available circuits where oscillatory behavior is experienced. We then propose a solution that shifts this critical zone towards lower loading values. This in turn reduces system response oscillations. Shifting the critical zone is accomplished by utilizing two charge pressures and asymmetric flow compensating valves. The concept is evaluated via simulations and experiments. Our results clearly show the enhanced performance of the circuits incorporating the proposed solution. Full article
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Open AccessArticle
Piezoelectric Vibration-Based Energy Harvesting Enhancement Exploiting Nonsmoothness
Actuators 2019, 8(1), 25; https://doi.org/10.3390/act8010025
Received: 7 February 2019 / Revised: 1 March 2019 / Accepted: 6 March 2019 / Published: 10 March 2019
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Abstract
Piezoelectric vibration-based energy harvesting systems have been used as an interesting alternative power source for actuators and portable devices. These systems have an inherent disadvantage when operating in linear conditions, presenting a maximum power output by matching their resonance frequencies with the ambient [...] Read more.
Piezoelectric vibration-based energy harvesting systems have been used as an interesting alternative power source for actuators and portable devices. These systems have an inherent disadvantage when operating in linear conditions, presenting a maximum power output by matching their resonance frequencies with the ambient source frequencies. Based on that, there is a significant reduction of the output power due to small frequency deviations, resulting in a narrowband harvester system. Nonlinearities have been shown to play an important role in enhancing the harvesting capacity. This work deals with the use of nonsmooth nonlinearities to obtain a broadband harvesting system. A numerical investigation is undertaken considering a single-degree-of-freedom model with a mechanical end-stop. The results show that impacts can strongly modify the system dynamics, resulting in an increased broadband output power harvesting performance and introducing nonlinear effects as dynamical jumps. Nonsmoothness can increase the bandwidth of the harvesting system but, on the other hand, limits the energy capacity due to displacement constraints. A parametric analysis is carried out monitoring the energy capacity, and two main end-stop characteristics are explored: end-stop stiffness and gap. Dynamical analysis using proper nonlinear tools such as Poincaré maps, bifurcation diagrams, and phase spaces is performed together with the analysis of the device output power and efficiency. This offers a deep comprehension of the energy harvesting system, evaluating different possibilities related to complex behaviors such as dynamical jumps, bifurcations, and chaos. Full article
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Open AccessArticle
Stabilization of Magnetic Suspension System by Using Only a First-Order Reset Element without a Derivative Element
Actuators 2019, 8(1), 24; https://doi.org/10.3390/act8010024
Received: 31 January 2019 / Revised: 28 February 2019 / Accepted: 6 March 2019 / Published: 9 March 2019
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Abstract
The stabilization of a magnetic suspension system is achieved by using a low-pass filter (LPF) with a nonlinear integrator without any other element. A proportional-derivative (PD) control is commonly used as the simplest method of stabilizing a magnetic suspension system. Meanwhile, a first-order [...] Read more.
The stabilization of a magnetic suspension system is achieved by using a low-pass filter (LPF) with a nonlinear integrator without any other element. A proportional-derivative (PD) control is commonly used as the simplest method of stabilizing a magnetic suspension system. Meanwhile, a first-order reset element (FORE) was applied to improve transient characteristics. The original FORE was a first-order LPF with a nonlinear reset integrator element. A magnetic suspension system cannot be stabilized by a linear LPF, nor the original FORE. In this work, the reset conditions of the FORE were modified for magnetic suspension. This modified FORE succeeded in stabilizing a magnetic suspension system. The efficacy of the modified FORE was demonstrated by simulations and experiments. A single degree of freedom magnetic suspension system was used in the experiment. Full article
(This article belongs to the Special Issue Magnetic Bearing Actuators)
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Open AccessArticle
The Trade-Off between the Controller Effort and Control Quality on Example of an Electro-Pneumatic Final Control Element
Actuators 2019, 8(1), 23; https://doi.org/10.3390/act8010023
Received: 2 January 2019 / Revised: 21 February 2019 / Accepted: 28 February 2019 / Published: 5 March 2019
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Abstract
For many years, the programmable positioners have been widely applied in structures of modern electro-pneumatic final control elements. The positioner consists of an electro-pneumatic transducer, embedded controller, and measuring instrumentation. Electro-pneumatic transducers that are used in positioners are characterized by a relatively short [...] Read more.
For many years, the programmable positioners have been widely applied in structures of modern electro-pneumatic final control elements. The positioner consists of an electro-pneumatic transducer, embedded controller, and measuring instrumentation. Electro-pneumatic transducers that are used in positioners are characterized by a relatively short mean time-to-failure. The practical and economical method of a reasonable prolongation of this time is proposed in this paper. It is principally based on assessment and minimizing the effort of the embedded controller. For this purpose, some measures were introduced: The control value variability, mean-time, and the cumulative controller’s effort. The diminishing of controller effort has significant practical repercussions because it reduces the intensity of mechanical wear of the final control element components. On the other hand, the reduction of the cumulative effort is important in the context of process economy due to limitation of the consumption of energy of compressed air supplying the final control element. Therefore, the minimization of control effort indicators has an impact on the increase of the functional safety and economics of the controlled process. The simulations were performed in the Matlab-Simulink environment with the use of the liquid level control system in which a phenomenological model of a final control element was deployed. As a result of the performed simulations, the recommendations regarding the selection of the structure of positioner controller were elaborated. Full article
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Open AccessArticle
Time-Stepping FEM-Based Multi-Level Coupling of Magnetic Field–Electric Circuit and Mechanical Structural Deformation Models Dedicated to the Analysis of Electromagnetic Actuators
Actuators 2019, 8(1), 22; https://doi.org/10.3390/act8010022
Received: 26 December 2018 / Revised: 25 February 2019 / Accepted: 25 February 2019 / Published: 1 March 2019
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Abstract
The present paper introduced a framework for multi-level coupling transient electromagnetic fields (EMF) and mechanical structural dynamics based on the finite element method (FEM). This framework was dedicated to predicting, with better accuracy, the wave magnetic force density for obtaining the mechanical deformation [...] Read more.
The present paper introduced a framework for multi-level coupling transient electromagnetic fields (EMF) and mechanical structural dynamics based on the finite element method (FEM). This framework was dedicated to predicting, with better accuracy, the wave magnetic force density for obtaining the mechanical deformation occurring in electromagnetic actuators (EMAs). The first-level EMF transient model coupling is related to the magnetic field equations that are strongly coupled with the electric circuit input voltage equations. This is done by considering the magnetic saturation through the Newton–Raphson (N–R) method. The time-stepping solution of the EMF model resulted in the magnetic force densities being computed from the Lorentz force (LZ) expressions, based on the space–time variation of the induced eddy current. For the second coupling level, the EMF model was sequentially coupled with the mechanical structural deformation equations (MDef) through the local magnetic force density to achieve minimal material dynamic displacement and deformation. The developed multi-physics EMF–MDef time-stepping (FEM) model tools were implemented using the Matlab software. Full article
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Open AccessArticle
An Electro-Thermal Actuation Method for Resonance Vibration of a Miniaturized Optical-Fiber Scanner for Future Scanning Fiber Endoscope Design
Actuators 2019, 8(1), 21; https://doi.org/10.3390/act8010021
Received: 29 January 2019 / Revised: 23 February 2019 / Accepted: 25 February 2019 / Published: 1 March 2019
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Abstract
Medical professionals increasingly rely on endoscopes to carry out many minimally invasive procedures on patients to safely examine, diagnose, and treat a large variety of conditions. However, their insertion tube diameter dictates which passages of the body they can be inserted into and, [...] Read more.
Medical professionals increasingly rely on endoscopes to carry out many minimally invasive procedures on patients to safely examine, diagnose, and treat a large variety of conditions. However, their insertion tube diameter dictates which passages of the body they can be inserted into and, consequently, what organs they can access. For inaccessible areas and organs, patients often undergo invasive and risky procedures—diagnostic confirmation of peripheral lung nodules via transthoracic needle biopsy is one example from oncology. Hence, this work sets out to present an optical-fiber scanner for a scanning fiber endoscope design that has an insertion tube diameter of about 0.5 mm, small enough to be inserted into the smallest airways of the lung. To attain this goal, a novel approach based on resonance thermal excitation of a single-mode 0.01-mm-diameter fiber-optic cantilever oscillating at 2–4 kHz is proposed. The small size of the electro-thermal actuator enables miniaturization of the insertion tube. Lateral free-end deflection of the cantilever is used as a benchmark for evaluating performance. Experimental results show that the cantilever can achieve over 0.2 mm of displacement at its free end. The experimental results also support finite element simulation models which can be used for future design iterations of the endoscope. Full article
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Open AccessArticle
Asynchronous Control of a Prototype Inchworm Actuator: Control Design and Test Results
Actuators 2019, 8(1), 20; https://doi.org/10.3390/act8010020
Received: 17 January 2019 / Revised: 15 February 2019 / Accepted: 19 February 2019 / Published: 27 February 2019
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Abstract
Inchworm actuators are innovative mechanisms that offer nanometer-level positioning coupled with extreme dynamic range. Because of this, they have found applications in optical instruments of various types including interferometers, segmented reflectors, and coronagraphs. In this paper, we present two prototypes of flight-qualifiable inchworm [...] Read more.
Inchworm actuators are innovative mechanisms that offer nanometer-level positioning coupled with extreme dynamic range. Because of this, they have found applications in optical instruments of various types including interferometers, segmented reflectors, and coronagraphs. In this paper, we present two prototypes of flight-qualifiable inchworm actuators developed at the Jet Propulsion Laboratory. These actuators have two sets of brake piezoceramic (PZT) stacks and an extension PZT stack used for mobility. By proper phasing of the signals to these PZTs, a walking gait can be achieved that moves a runner attached via a flexure to the optic to be moved. A model of these devices, based on first principles, is developed as well as an estimation and control scheme for precise positioning. The estimator estimates physical parameters of the device as well as a self-induced motion disturbance caused by the brakes. Simulations and test data are presented that demonstrate nanometer-level positioning precision as well as the cause of variations in the brake-induced disturbance. Full article
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Open AccessArticle
Finite Element Model of Vibration Control for an Exponential Functionally Graded Timoshenko Beam with Distributed Piezoelectric Sensor/Actuator
Actuators 2019, 8(1), 19; https://doi.org/10.3390/act8010019
Received: 26 December 2018 / Revised: 18 February 2019 / Accepted: 21 February 2019 / Published: 26 February 2019
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Abstract
This paper presents a dynamic study of sandwich functionally graded beam with piezoelectric layers that are used as sensors and actuators. This study is exploited later in the formulation of the active control laws, while using the optimal control Linear Quadratic Gaussian (LQG), [...] Read more.
This paper presents a dynamic study of sandwich functionally graded beam with piezoelectric layers that are used as sensors and actuators. This study is exploited later in the formulation of the active control laws, while using the optimal control Linear Quadratic Gaussian (LQG), accompanied by the Kalman filter. The mathematical formulation is based on Timoshenko’s assumptions and the finite element method, which is applied to a flexible beam divided into a finite number of elements. By applying the Hamilton principle, the equations of motion are obtained. The vibration frequencies are found by solving the eigenvalue problem. The structure is analytically then numerically modeled and the results of the simulations are presented in order to visualize the states of their dynamics without and with active control. Full article
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Open AccessArticle
Optimal Magnetic Spring for Compliant Actuation—Validated Torque Density Benchmark
Actuators 2019, 8(1), 18; https://doi.org/10.3390/act8010018
Received: 18 January 2019 / Revised: 14 February 2019 / Accepted: 18 February 2019 / Published: 22 February 2019
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Abstract
Magnetic springs are a fatigue-free alternative to mechanical springs that could enable compliant actuation concepts in highly dynamic industrial applications. The goals of this article are: (1) to develop and validate a methodology for the optimal design of a magnetic spring and (2) [...] Read more.
Magnetic springs are a fatigue-free alternative to mechanical springs that could enable compliant actuation concepts in highly dynamic industrial applications. The goals of this article are: (1) to develop and validate a methodology for the optimal design of a magnetic spring and (2) to benchmark the magnetic springs at the component level against conventional solutions, namely, mechanical springs and highly dynamic servo motors. We present an extensive exploration of the magnetic spring design space both with respect to topology and geometry sizing, using a 2D finite element magnetostatics software combined with a multi-objective genetic algorithm, as a part of a MagOpt design environment. The resulting Pareto-optima are used for benchmarking rotational magnetic springs back-to-back with classical industrial solutions. The design methodology has been extensively validated using a combination of one physical prototype and multiple virtual designs. The findings show that magnetic springs possess an energy density 50% higher than that of state-of-the-art reported mechanical springs for the gigacycle regime and accordingly a torque density significantly higher than that of state-of-the-practice permanently magnetic synchronous motors. Full article
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Open AccessArticle
Condition Monitoring of Active Magnetic Bearings on the Internet of Things
Actuators 2019, 8(1), 17; https://doi.org/10.3390/act8010017
Received: 20 January 2019 / Revised: 8 February 2019 / Accepted: 14 February 2019 / Published: 20 February 2019
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Abstract
A magnetic bearing is an industrial device that supports a rotating shaft with a magnetic field. Magnetic bearings have advantages such as high efficiency, low maintenance, and no lubrication. Active magnetic bearings (AMBs) use electromagnets with actively controlled coil currents based on rotor [...] Read more.
A magnetic bearing is an industrial device that supports a rotating shaft with a magnetic field. Magnetic bearings have advantages such as high efficiency, low maintenance, and no lubrication. Active magnetic bearings (AMBs) use electromagnets with actively controlled coil currents based on rotor position monitored by sensors integral to the AMB. AMBs are apt to the Internet of Things (IoT) due to their inherent sensors and actuators. The IoT is the interconnection of physical devices that enables them to send and receive data over the Internet. IoT technology has recently rapidly increased and is being applied to industrial devices. This study developed a method for the condition monitoring of AMB systems online using off-the-shelf IoT technology. Because off-the-shelf IoT solutions were utilized, the developed method is cost-effective and can be implemented on existing AMB systems. In this study, a MBC500 AMB test rig was outfitted with a Raspberry Pi single board computer. The Raspberry Pi monitors the AMB’s position sensors and current sensors via an analog-to-digital converter. Several loading cases were imposed on the experimental test rig and diagnosed remotely using virtual network computing. It was found that remote AMB condition monitoring is feasible for less than USD 100. Full article
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Open AccessCorrection
Correction: Persoons, T.; Cressall, R.; Alimohammadi, S. Validating a Reduced-Order Model for Synthetic Jet Actuators Using CFD and Experimental Data. Actuators 2018, 7, 67
Actuators 2019, 8(1), 16; https://doi.org/10.3390/act8010016
Received: 14 February 2019 / Accepted: 14 February 2019 / Published: 19 February 2019
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Abstract
The authors wish to make the following corrections to this paper [...] Full article
Open AccessArticle
Instability and Drift Phenomena in Switching RF-MEMS Microsystems
Actuators 2019, 8(1), 15; https://doi.org/10.3390/act8010015
Received: 11 January 2019 / Revised: 8 February 2019 / Accepted: 14 February 2019 / Published: 18 February 2019
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Abstract
MEMS switches include mobile beams in their mechanical structure and these suspended parts are essential for the device functioning. This paper illustrates the most important instability phenomena related to MEMS switches. Starting from the most important instability exploited in these devices—the electrical actuation—the [...] Read more.
MEMS switches include mobile beams in their mechanical structure and these suspended parts are essential for the device functioning. This paper illustrates the most important instability phenomena related to MEMS switches. Starting from the most important instability exploited in these devices—the electrical actuation—the paper also analyzes other important effects related to instability phenomena, which are very common in this type of technology. Instabilities due to dielectric charge trapping, fabrication tolerances, mechanical deformation, contact wear, and temperature variation are duly analyzed, giving a comprehensive view of the complexity encountered in the reliable functioning of these apparently simple devices. Full article
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Open AccessArticle
Condition Monitoring with Prediction Based on Diesel Engine Oil Analysis: A Case Study for Urban Buses
Actuators 2019, 8(1), 14; https://doi.org/10.3390/act8010014
Received: 14 November 2018 / Revised: 22 January 2019 / Accepted: 13 February 2019 / Published: 16 February 2019
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Abstract
This paper presents a case study and a model to predict maintenance interventions based on condition monitoring of diesel engine oil in urban buses by accompanying the evolution of its degradation. Many times, under normal functioning conditions, the properties of the lubricants, based [...] Read more.
This paper presents a case study and a model to predict maintenance interventions based on condition monitoring of diesel engine oil in urban buses by accompanying the evolution of its degradation. Many times, under normal functioning conditions, the properties of the lubricants, based on the intervals that manufacturers recommend for its change, are within normal and safety conditions. Then, if the lubricants’ oil condition is adequately accompanied, until reaching the degradation limits, the intervals of oil replacement can be enlarged, meaning that the buses’ availability increases, as well as their corresponding production time. Based on this assumption, a mathematical model to follow and to manage the oil condition is presented, in order to predict the next intervention with the maximum time between them, which means the maximum availability. Full article
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Open AccessArticle
Multi-Objective Design Optimization of a Shape Memory Alloy Flexural Actuator
Actuators 2019, 8(1), 13; https://doi.org/10.3390/act8010013
Received: 7 January 2019 / Revised: 31 January 2019 / Accepted: 10 February 2019 / Published: 16 February 2019
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Abstract
This paper presents a computational model and design optimization strategy for shape memory alloy (SMA) flexural actuators. These actuators consist of curved SMA wires embedded within elastic structures; one potential application is positioning microcatheters inside blood vessels during clinical treatments. Each SMA wire [...] Read more.
This paper presents a computational model and design optimization strategy for shape memory alloy (SMA) flexural actuators. These actuators consist of curved SMA wires embedded within elastic structures; one potential application is positioning microcatheters inside blood vessels during clinical treatments. Each SMA wire is shape-set to an initial curvature and inserted along the neutral axis of a straight elastic member (cast polydimethylsiloxane, PDMS). The elastic structure preloads the SMA, reducing the equilibrium curvature of the composite actuator. Temperature-induced phase transformations in the SMA are achieved via Joule heating, enabling strain recovery and increased bending (increased curvature) in the actuator. Actuator behavior is modeled using the homogenized energy framework, and the effects of two critical design parameters (initial SMA curvature and flexural rigidity of the elastic sleeve) on activation curvature are investigated. Finally, a multi-objective genetic algorithm is utilized to optimize actuator performance and generate a Pareto frontier, which is subsequently experimentally validated. Full article
(This article belongs to the Special Issue Actuators Based on Shape Memory Alloys)
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Open AccessArticle
Design and Analysis of Novel Actuation Mechanism with Controllable Stiffness
Actuators 2019, 8(1), 12; https://doi.org/10.3390/act8010012
Received: 3 December 2018 / Revised: 31 January 2019 / Accepted: 6 February 2019 / Published: 9 February 2019
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Abstract
Actuators intended for human–machine interaction systems are usually designed to be mechanically compliant. Conventional actuators are not suitable for this purpose due to typically high stiffness. Advanced powered prosthetic and orthotic devices can vary their stiffness during a motion cycle and are power-efficient. [...] Read more.
Actuators intended for human–machine interaction systems are usually designed to be mechanically compliant. Conventional actuators are not suitable for this purpose due to typically high stiffness. Advanced powered prosthetic and orthotic devices can vary their stiffness during a motion cycle and are power-efficient. This paper proposes a novel actuator design that modulates stiffness by means of a flexible beam. A motorized drive system varies the active length of the cantilever beam, thus achieving stiffness modulation. New large deflection formulation for cantilever beams with rolling contact constraints is used to determine the moment produced by the actuator. To validate the proposed solution method, an experiment was performed to measure large static deformations of a cantilever beam with the same boundary conditions as in the actuator design. The experiments indicate excellent agreement between measured and calculated contact forces between beam and roller, from which the actuator moment is determined. Full article
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Open AccessArticle
Stability and Performance Analysis of Electrodynamic Thrust Bearings
Actuators 2019, 8(1), 11; https://doi.org/10.3390/act8010011
Received: 30 December 2018 / Revised: 20 January 2019 / Accepted: 29 January 2019 / Published: 1 February 2019
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Abstract
Electrodynamic thrust bearings (EDTBs) provide contactless rotor axial suspension through electromagnetic forces solely leaning on passive phenomena. Lately, linear state-space equations representing their quasi-static and dynamic behaviours have been developed and validated experimentally. However, to date, the exploitation of these models has been [...] Read more.
Electrodynamic thrust bearings (EDTBs) provide contactless rotor axial suspension through electromagnetic forces solely leaning on passive phenomena. Lately, linear state-space equations representing their quasi-static and dynamic behaviours have been developed and validated experimentally. However, to date, the exploitation of these models has been restricted to basic investigations regarding the stiffness and the rotational losses as well as qualitative stability analyses, thus not allowing us to objectively compare the intrinsic qualities of EDTBs. In this context, the present paper introduces four performance criteria directly related to the axial stiffness, the bearing energy efficiency and the minimal amount of external damping required to stabilise the thrust bearing. In addition, the stability is thoroughly examined via analytical developments based on these dynamical models. This notably leads to static and dynamic conditions that ensure the stability at a specific rotor spin speed. The resulting stable speed ranges are studied and their dependence to the axial external stiffness as well as the external non-rotating damping are analysed. Finally, a case study comparing three topologies through these performance criteria underlines that back irons fixed to the windings are not advantageous due to the significant detent force. Full article
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Open AccessReview
A Review on the Control of the Mechanical Properties of Ankle Foot Orthosis for Gait Assistance
Actuators 2019, 8(1), 10; https://doi.org/10.3390/act8010010
Received: 20 December 2018 / Revised: 9 January 2019 / Accepted: 24 January 2019 / Published: 28 January 2019
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Abstract
In the past decade, advanced technologies in robotics have been explored to enhance the rehabilitation of post-stroke patients. Previous works have shown that gait assistance for post-stroke patients can be provided through the use of robotics technology in ancillary equipment, such as Ankle [...] Read more.
In the past decade, advanced technologies in robotics have been explored to enhance the rehabilitation of post-stroke patients. Previous works have shown that gait assistance for post-stroke patients can be provided through the use of robotics technology in ancillary equipment, such as Ankle Foot Orthosis (AFO). An AFO is usually used to assist patients with spasticity or foot drop problems. There are several types of AFOs, depending on the flexibility of the joint, such as rigid, flexible rigid, and articulated AFOs. A rigid AFO has a fixed joint, and a flexible rigid AFO has a more flexible joint, while the articulated AFO has a freely rotating ankle joint, where the mechanical properties of the AFO are more controllable compared to the other two types of AFOs. This paper reviews the control of the mechanical properties of existing AFOs for gait assistance in post-stroke patients. Several aspects that affect the control of the mechanical properties of an AFO, such as the controller input, number of gait phases, controller output reference, and controller performance evaluation are discussed and compared. Thus, this paper will be of interest to AFO researchers or developers who would like to design their own AFOs with the most suitable mechanical properties based on their application. The controller input and the number of gait phases are discussed first. Then, the discussion moves forward to the methods of estimating the controller output reference, which is the main focus of this study. Based on the estimation method, the gait control strategies can be classified into subject-oriented estimations and phase-oriented estimations. Finally, suggestions for future studies are addressed, one of which is the application of the adaptive controller output reference to maximize the benefits of the AFO to users. Full article
(This article belongs to the Special Issue Low-Profile Compliant Actuators for Wearable Devices)
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Open AccessArticle
Conductive Fabric Heaters for Heat-Activated Soft Actuators
Actuators 2019, 8(1), 9; https://doi.org/10.3390/act8010009
Received: 28 December 2018 / Revised: 11 January 2019 / Accepted: 18 January 2019 / Published: 21 January 2019
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Abstract
We examine electrically conductive fabrics as conductive heaters for heat-activated soft actuators. We have explored various fabric designs optimized for material properties, heat distribution and actuation/de-actuation characteristics of the soft actuators. We implemented this approach in the silicone/ethanol composite actuators, in which ethanol [...] Read more.
We examine electrically conductive fabrics as conductive heaters for heat-activated soft actuators. We have explored various fabric designs optimized for material properties, heat distribution and actuation/de-actuation characteristics of the soft actuators. We implemented this approach in the silicone/ethanol composite actuators, in which ethanol undergoes a thermally-induced phase change, leading to high actuation stress and strain. Various types of conductive fabrics were tested, and we developed a stretchable kirigami-based fabric design. We demonstrate a fabric heater that is capable of cyclic heating of the actuator to the required 80 °C. The fabric with the special kirigami design can withstand temperatures of up to 195 °C, can consume up to 30 W of power, and allows the actuator to reach >30% linear strain. This technology may be used in various systems involving thermally-induced actuation. Full article
(This article belongs to the Special Issue New Materials and Designs for Soft Actuators)
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Open AccessArticle
Piezoelectric Energy Harvesting with an Ultrasonic Vibration Source
Actuators 2019, 8(1), 8; https://doi.org/10.3390/act8010008
Received: 10 December 2018 / Revised: 4 January 2019 / Accepted: 8 January 2019 / Published: 11 January 2019
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Abstract
A piezoelectric energy harvester was developed in this paper. It is actuated by the vibration leakage from the nodal position of a high-power ultrasonic cutting transducer. The harvester was excited at a low displacement amplitude (0.73 µmpp). However, its operation frequency [...] Read more.
A piezoelectric energy harvester was developed in this paper. It is actuated by the vibration leakage from the nodal position of a high-power ultrasonic cutting transducer. The harvester was excited at a low displacement amplitude (0.73 µmpp). However, its operation frequency is quite high and reaches the ultrasonic range (24.4 kHz). Compared with another low frequency harvester (66 Hz), both theoretical and experimental results proved that the advantages of this high frequency harvester include (i) high current generation capability (up to 20 mApp compared to 1.3 mApp of the 66 Hz transducer) and (ii) low impedance matching resistance (500 Ω in contrast to 50 kΩ of the 66 Hz transducer). This energy harvester can be applied either in sensing, or vibration controlling, or simply energy harvesting in a high-power ultrasonic system. Full article
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Open AccessEditorial
Acknowledgement to Reviewers of Actuators in 2018
Actuators 2019, 8(1), 7; https://doi.org/10.3390/act8010007
Published: 10 January 2019
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Abstract
Rigorous peer-review is the corner-stone of high-quality academic publishing [...] Full article
Open AccessArticle
A Novel, Low-Cost and Reliable Workbench for Optimal Voltage Distribution on Piezoelectric Array Actuators
Actuators 2019, 8(1), 6; https://doi.org/10.3390/act8010006
Received: 5 December 2018 / Revised: 30 December 2018 / Accepted: 2 January 2019 / Published: 4 January 2019
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Abstract
The vibration control of structures is a valuable technique to increase their integrity and life-cycle. Among smart materials, the passive and active control systems based on piezoelectric elements have been studied in depth. Since the size and location of the piezo-elements on the [...] Read more.
The vibration control of structures is a valuable technique to increase their integrity and life-cycle. Among smart materials, the passive and active control systems based on piezoelectric elements have been studied in depth. Since the size and location of the piezo-elements on the structure are relevant matters for the damping efficiency, many works have focused on their placement optimization. Under certain circumstances, some structures may be excited by more vibration modes at the same time; hence, the signal conditioning system of the piezoelectric actuators must be capable of adjusting the driving signal in terms of voltage distribution, frequencies and associated amplitudes in order to maximize the damping efficiency. Moreover, in some applications, it could be useful to individually control each actuator therefore tailor-made power supply and signal generation systems are often necessary. This work suggests a low-cost and reliable workbench that overcomes the afore mentioned problems. The system consists of: (1) two arrays of 13 piezoceramic plates which have been glued on an aluminium beam, (2) ad hoc power supply equipment, and (3) mechanical relays arrays managed by a microcontroller for the individual handling of their operating modes. Furthermore, an Arduino board and an in-house software have been implemented in order to perform a task on each relay that is in turn wired to a single piezoelectric plate (PP). The performance of the developed system is evaluated in terms of noise and distortion of the testing signal measured at different points of the workbench. The results show that the proposed workbench may represent a good trade-off between affordability, accuracy and reliability and it can be used for several research purposes. Full article
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Open AccessArticle
Design and Fabrication of a New Dual-Arm Soft Robotic Manipulator
Actuators 2019, 8(1), 5; https://doi.org/10.3390/act8010005
Received: 28 November 2018 / Revised: 27 December 2018 / Accepted: 28 December 2018 / Published: 4 January 2019
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Abstract
This paper presents the design and implementation of a dual-arm soft robotic manipulator. It consists of two soft manipulators, which are driven by pneumatic actuators. Each soft manipulator is composed of three soft modules, and each module includes three evenly distributed cavities inside. [...] Read more.
This paper presents the design and implementation of a dual-arm soft robotic manipulator. It consists of two soft manipulators, which are driven by pneumatic actuators. Each soft manipulator is composed of three soft modules, and each module includes three evenly distributed cavities inside. The flexible bending deformation of the soft module is produced by regulating the air pressure and changing the applying sequence to the cavities. The design and fabrication of the manipulator are presented in detail. The cooperation of the dual-arm soft robotic manipulator is implemented by adopting visual servo control. Experimental testing was carried out to demonstrate the manipulator performance. Unlike a single-arm manipulator, the robotic manipulator with dual arms features high flexibility, adaptability, and safety. The feasibility of the proposed dual-arm soft robotic manipulator is demonstrated by executing assembly tasks. Full article
(This article belongs to the Special Issue Design and Control of Compliant Manipulators)
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Open AccessArticle
A Novel Actuator System Combining Mechanical Vibration and Magnetic Wheels Capable of Rotational Motion Using Shape Memory Alloy Coils
Actuators 2019, 8(1), 4; https://doi.org/10.3390/act8010004
Received: 6 December 2018 / Revised: 26 December 2018 / Accepted: 29 December 2018 / Published: 4 January 2019
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Abstract
In every country, large steel bridges, such as cable-stayed bridges, are actively being constructed, and the number of such bridges has been progressively increasing. These bridges are often inspected using drones, but inspection techniques have not been established because of strong winds and [...] Read more.
In every country, large steel bridges, such as cable-stayed bridges, are actively being constructed, and the number of such bridges has been progressively increasing. These bridges are often inspected using drones, but inspection techniques have not been established because of strong winds and thunder. Therefore, robots capable of working in difficult environments are desired. In the present study, a prototype of a rotary actuator system combining two iron disks and two electromagnetic-vibration-type actuators was fabricated. A new operation principle was developed that drives the system using the reaction force of the vibration-type actuator. Two shape memory alloy coils and two friction pads were integrated into the system to enable it to carry out turning operations, which were successfully demonstrated. The proposed actuator system can thus move in any direction. In addition, with this actuator system, both slide-on-ceiling and wall-climbing motions are possible. Full article
(This article belongs to the Special Issue Actuators Based on Shape Memory Alloys)
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Open AccessArticle
Design of Soft Origami Mechanisms with Targeted Symmetries
Actuators 2019, 8(1), 3; https://doi.org/10.3390/act8010003
Received: 16 November 2018 / Revised: 19 December 2018 / Accepted: 20 December 2018 / Published: 24 December 2018
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Abstract
The integration of soft actuating materials within origami-based mechanisms is a novel method to amplify the actuated motion and tune the compliance of systems for low stiffness applications. Origami structures provide natural flexibility given the extreme geometric difference between thickness and length, and [...] Read more.
The integration of soft actuating materials within origami-based mechanisms is a novel method to amplify the actuated motion and tune the compliance of systems for low stiffness applications. Origami structures provide natural flexibility given the extreme geometric difference between thickness and length, and the energetically preferred bending deformation mode can naturally be used as a form of actuation. However, origami fold patterns that are designed for specific actuation motions and mechanical loading scenarios are needed to expand the library of fold-based actuation strategies. In this study, a recently developed optimization framework for maximizing the performance of compliant origami mechanisms is utilized to discover optimal actuating fold patterns. Variant patterns are discovered through exploring different symmetries in the input and output conditions of the optimization problem. Patterns designed for twist (rotational symmetry) yield significantly better performance, in terms of both geometric advantage and energy requirements, than patterns exhibiting vertical reflection symmetries. The mechanical energy requirements for each design are analyzed and compared for both the small and large applied displacement regimes. Utilizing the patterns discovered through optimization, the multistability of the actuating arms is demonstrated empirically with a paper prototype, where the stable configurations are accessed through local vertex pop-through instabilities. Lastly, the coupled mechanics of fold networks in these actuators yield useful macroscopic motions and can achieve stable shape change through accessing the local vertex instabilities. This survey of origami mechanisms, energy comparison, and multistability characterization provides a new set of designs for future integration with soft actuating materials. Full article
(This article belongs to the Special Issue New Materials and Designs for Soft Actuators)
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Open AccessReview
On-Board Pneumatic Pressure Generation Methods for Soft Robotics Applications
Actuators 2019, 8(1), 2; https://doi.org/10.3390/act8010002
Received: 14 November 2018 / Revised: 8 December 2018 / Accepted: 20 December 2018 / Published: 23 December 2018
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Abstract
The design and construction of a soft robot are challenging tasks on their own. When the robot is supposed to operate without a tether, it becomes even more demanding. While a tethered operation is sufficient for a stationary use, it is impractical for [...] Read more.
The design and construction of a soft robot are challenging tasks on their own. When the robot is supposed to operate without a tether, it becomes even more demanding. While a tethered operation is sufficient for a stationary use, it is impractical for wearable robots or performing tasks that demand a high mobility. Choosing and implementing an on-board pneumatic pressure source are particularly complex tasks. There are several different pressure generation methods to choose from, each with very different properties and ways of implementation. This review paper is written with the intention of informing about all pressure generation methods available in the field of soft robotics and providing an overview of the abilities and properties of each method. Nine different methods are described regarding their working principle, pressure generation behavior, energetic considerations, safety aspects, and suitability for soft robotics applications. All presented methods are evaluated in the most important categories for soft robotics pressure sources and compared to each other qualitatively and quantitatively as far as possible. The aim of the results presented is to simplify the choice of a suitable pressure generation method when designing an on-board pressure source for a soft robot. Full article
(This article belongs to the Special Issue Actuation for Agile Robots)
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Open AccessArticle
Distributed Cooperative Avoidance Control for Multi-Unmanned Aerial Vehicles
Actuators 2019, 8(1), 1; https://doi.org/10.3390/act8010001
Received: 16 October 2018 / Revised: 21 November 2018 / Accepted: 19 December 2018 / Published: 21 December 2018
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Abstract
It is well-known that collision-free control is a crucial issue in the path planning of unmanned aerial vehicles (UAVs). In this paper, we explore the collision avoidance scheme in a multi-UAV system. The research is based on the concept of multi-UAV cooperation combined [...] Read more.
It is well-known that collision-free control is a crucial issue in the path planning of unmanned aerial vehicles (UAVs). In this paper, we explore the collision avoidance scheme in a multi-UAV system. The research is based on the concept of multi-UAV cooperation combined with information fusion. Utilizing the fused information, the velocity obstacle method is adopted to design a decentralized collision avoidance algorithm. Four case studies are presented for the demonstration of the effectiveness of the proposed method. The first two case studies are to verify if UAVs can avoid a static circular or polygonal shape obstacle. The third case is to verify if a UAV can handle a temporary communication failure. The fourth case is to verify if UAVs can avoid other moving UAVs and static obstacles. Finally, hardware-in-the-loop test is given to further illustrate the effectiveness of the proposed method. Full article
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