11,781 research outputs found
Modeling and time delay control of shape memory alloy actuators
No full textShape memory alloy (SMA) actuators possess hard nonlinearities including backlash-like hysteresis and saturation. These nonlinearities result in steady-state error and limit-cycle problems when conventional controllers such as the proportional integral derivative (PID) are used for trajectory control. In this study, a dynamics for an SMA actuator was newly derived using the modified Liang's model. The derived dynamics showed continuity at the change of the phase transformation process, but the original model could not. SMA actuator characteristics could be well described using this dynamics. The derived dynamics could be also used effectively for the prediction of control performance and gain tuning of the time delay control (TDC). The dynamics consisted of first-order linear and second-order nonlinear equations. Accordingly, a control strategy was established for the TDC to regulate only the second-order nonlinear part for simplicity and for the internal closed loop to regulate the rest. The control strategy was examined from the point of view of influence of an anti-windup scheme and high gain tuning on control performance. An anti-windup scheme was essential to protect windup phenomenon and high gain tuning was effective when a temperature disturbance existed. In the robustness test, the TDC with high gains showed robustness to inertia variation and temperature disturbance in comparison with the TDC with low gains
Time delay control of a shape memory alloy actuator
No full textShape memory alloy (SMA) actuators have attractive advantages such as high power to volume ratio. However, they also have hard nonlinearities including backlash-like hysteresis and saturation. These nonlinearities result in steady-state errors and limit cycle problems when conventional controllers are used for trajectory control. In this paper, time delay control (TDC) is applied to SMA actuators. The TDC is a well-known robust nonlinear controller and does not require a precise mathematical plant model. The dynamics of an SMA actuator was derived based on Liang's model. This dynamics does not only describe the characteristics of SMA actuators very effectively but also helps tune the TDC gains. A control strategy for the SMA actuator was established and examined from the point of view of the influence of an anti-windup scheme and high gain tuning on the control performance. We also conducted experiments on the position control of the SMA actuator and show the results of the TDC in comparison with other control schemes
Josephson coupling realized in graphite-based vertical junction
No full textWe confirmed the Josephson effect in vertically proximity-coupled junctions prepared by sandwiching a 43-nm-thick exfoliated graphite layer between two superconducting electrodes. Josephson coupling with well-controlled contact characteristics was established by thermal deposition of electrodes on both sides of freshly cleaved graphite surfaces. The genuine Josephson coupling through the c-axis graphite was confirmed by the critical current modulation in in-plane magnetic fields (Fraunhofer pattern) and the response to the microwave irradiation (Shapiro steps). This scheme can be potentially utilized to fabricate atomically thin vertically coupled nanodevices based on assorted cleavable layered materials. (C) 2013 The Japan Society of Applied PhysicsX1144sciescopu
Switching dynamics in a short and a long natural Josephson junction of Bi2Sr2CaCu2O8+delta single crystals
No full textWe studied thermally activated and macroscopically quantum-tunneled switching from the zero-voltage state to a first resistive state in stacks of short and long Josephson junctions (JJs) naturally formed in Bi2Sr2CaCu2O8+delta single crystals. In the short (long) JJs the switching corresponds to an escape of a phase particle (a flexible phase line) from a potential well. The effective barrier height in the long JJs is reduced in proportion to the length of the junctions. (C) 2010 Elsevier B.V. All rights reserved.X110sciescopu
Heat transfer correlation for boiling flows in small rectangular horizontal channels with low aspect ratios
No full textIn the present experimental study, a correlation is proposed to represent the heat transfer coefficients of the boiling flows through horizontal rectangular channels with low aspect ratios. The gap between the upper and the lower plates of each channel ranges from 0.4 to 2 mm while the channel width being fixed to 20 mm. Refrigerant 113 was used as the test fluid. The mass flux ranges from 50 to 200 kg/m(2) s and the channel walls were uniformly heated up to 15 kW/m(2). The quality range covers from 0.15 to 0.75 and the flow pattern appeared to be annular. The modified Lockhart-Martinelli correlation for the frictional pressure drop was confirmed to be within an accuracy of 20%. The heat transfer coefficients increase with the mass flux and the local quality; however the effect of the heat flux appears to be minor. At the low mass flux condition, which is more likely to be with the smaller gap size, the heat transfer rate is primarily controlled by the liquid film thickness. A modified form of the enhancement factor F for the heat transfer coefficient in the range of Re-LF less than or equal to 200 well correlates the experimental data within the deviation of 20%. The Kandlikar's flow boiling correlation covers the higher mass flux range (Re-LF > 200) with 10.7% mean deviation. (C) 2001 Published by Elsevier Science Ltd
Pressure drop correlations for two-phase flow within horizontal rectangular channels with small heights
No full textThe present paper proposes new correlations for the two-phase pressure drop through horizontal rectangular channels with small gaps (heights) based on 305 data points. The gap between the upper and the lower plates of each channel ranges from 0.4 to 4 mm while the channel width being fixed to 20 mm. Water and air were used as the test fluids. The superficial velocity ranges of water and air were (0.03-2.39 and 0.05-18.7 m/s, respectively. The atmospheric pressure condition was maintained throughout the experiments. In the present study, the two-phase frictional multiplier was expressed using the Lockhart-Martinelli type correlation but with the modification on parameter C. Effects of the mass Hus and the gap size were considered. The correlations with the modified C successfully cover wide ranges of the Martinelli parameter (X = 0.303-79.4) and the all-liquid Reynolds number (RELo = 175-17700) based on the hydraulic diameter within the deviation of +/- 10%. (C) 2001 Elsevier Science Ltd. All rights reserved
A simple and color-free WDM-passive optical network using spectrum-sliced Fabry-Perot laser diodes
No full textWe propose and demonstrate a simple and color-free wavelength-division-multiplexing passive optical network (WDM-PON) using spectrum-sliced Fabry-Perot laser diodes (F-P LDs). Due to its low front facet reflectivity (similar to 0.1%), the F-P LD shows a broadband lasing spectrum and reduced mode partition noise. By utilizing multiple peaks that are separated by the free-spectral range of the cyclic arrayed waveguide gratings, the 16-channel WDM-PON is successfully demonstrated at 155 Mb/s
Observation of negative refraction of Dirac fermions in graphene
No full textHalf a century ago, Veselago(1) proposed 'left-handed' materials with negative permittivity and permeability, in which waves propagate with phase and group velocities in opposite directions. Significant work has been undertaken to attain this left-handed response, such as establishing a negative refractive index in so-called metamaterials, which consist of periodic sub-wavelength structures(2-4). However, an electronic counterpart has not been demonstrated owing to difficulties in creating repeated structures smaller than the electronic Fermi wavelength of the order similar to 10 nm. Here, without needing to engineer sub-wavelength structures, we demonstrate negative refractive behaviour of Dirac fermions in graphene, exploiting its unique relativistic band structures(5). Analysis of both electron focusing through an n-p-n flat lens and negative refraction across n-p junctions confirms left-handed behaviour in the electronic system. This approach to electronic optics is of particular relevance to the on-going efforts to develop novel quantum devices with emergings layered materials.116950sciescopu
DYNAMICAL PROPERTIES OF AN UNDERDAMPED SINGLE-PLAQUETTE JOSEPHSON-JUNCTION ARRAY
No full textWe have studied, both analytically and numerically, the dynamical properties of a single-plaquette square array consisting of 2x2 superconducting islands coupled by resistively and capacitively shunted Josephson junctions (RCSJ). In zero magnetic field the dynamics of the system becomes identical to that of a single Josephson junction. But in an external magnetic field it shows a transition to chaos even in a dc bias current only.X11sciescopu
- …
