Large Time Constants Research Articles

Coordinated DC Voltage Control of Wind Turbine With Embedded Energy Storage System

This paper investigates dc voltage control strategies for output power smoothing of a fully rated converter-based wind turbine with energy storage device connected to the common dc link via a bidirectional dc/dc converter. Since the dc link voltage ripple reflects power oscillation, coordinated dc voltage control schemes are used for the ac network side converter and energy storage system to ensure that generated high-frequency power fluctuation is absorbed by the energy storage system, whereas the low-frequency components are transmitted to the connected ac network. Two methods have been proposed: one is based on proportional-integral (PI) voltage controller and low-pass filter with large time constant, and the other is based on PI with low natural frequency and droop controller with large droop gain. The detailed controller designs are described and system stability is assessed and shown to be stable. Both the MATLAB/Simulink simulations and experimental results are presented to demonstrate the effectiveness of the proposed methods for power smoothing.

A Sub-10 nA DC-Balanced Adaptive Stimulator IC With Multi-Modal Sensor for Compact Electro-Acupuncture Stimulation

A compact electro-acupuncture (EA) system is proposed for a multi-modal feedback EA treatment. It is composed of a needle, a compact EA patch, and an interconnecting conductive thread. The 3 cm diameter compact EA patch is implemented with an adaptive stimulator IC and a small coin battery on the planar-fashionable circuit board (P-FCB) technology. The adaptive stimulator IC can form a closed current loop for even a single needle, and measure the electromyography (EMG) and the skin temperature to analyze the stimulation status as well as supply programmable stimulation current (40 μA-1 mA) with 5 different modes. The large time constant (LTC) sample and hold (S/H) current matching technique achieves the high-precision charge balancing ( <;10 nA) for the patient's safety. The measured data can be wirelessly transmitted to the external EA analyzer through the body channel communication (BCC) transceiver for the low power consumption. The external EA analyzer can show the patient's status, such as the muscle fatigue and the change of the skin temperature. Based on these analyses, the practitioner can adaptively change the stimulation parameters for the optimal treatment value. A 12.5 mm(2) 0.13 μm RF CMOS stimulator chip consumes 6.8 mW at 1.2 V supporting 32 different current levels. The proposed compact EA system is fully implemented and tested on the human body.

Accurate measurement of the rise and decay times of fast scintillators with solid state photon counters

In this work we present a measurement setup for the determination of scintillation pulse shapes of fast scintillators. It is based on a time-correlated single photon counting approach that utilizes the correlation between 511 keV annihilation photons to produce start and stop signals in two separate crystals. The measurement is potentially cost-effective and simple to set up while maintaining an excellent system timing resolution of 125 ps. As a proof-of-concept the scintillation photon arrival time histograms were recorded for two well-known, fast scintillators: LYSO:Ce and LaBr3:5%Ce. The scintillation pulse shapes were modeled as a linear combination of exponentially distributed charge transfer and photon emission processes. Correcting for the system timing resolution, the exponential time constants were extracted from the recorded histograms. A decay time of 43 ns and a rise time of 72 ps were determined for LYSO:Ce thus demonstrating the capability of the system to accurately measure very fast rise times. In the case of LaBr3:5%Ce two processes were observed to contribute to the rising edge of the scintillation pulse. The faster component (270 ps) contributes with 72% to the rising edge of the scintillation pulse while the second, slower component (2.0 ns) contributes with 27%. The decay of the LaBr3:5%Ce scintillation pulse was measured to be 15.4 ns with a small contribution (2%) of a component with a larger time constant (130 ns).

Photoinduced molecular desorption from graphene films

Photoinduced molecular desorption from solid surfaces will modify the electrical transporting properties of matter and result in significant change in resistance. The UV induced resistive response of graphene films prepared by chemical vapor deposition was investigated. The resistance of graphene increased exponentially upon UV irradiation and decreased also in an exponential form when the UV light was turned off. Both processes exhibited large time constants of hundreds of seconds. UV-induced molecular desorption mechanisms for graphene, carbon nanotubes, and reduced graphene oxides were compared and discussed.

Theta entrainment of gamma modules: effects of heterogeneity and non-stationarity

Transient synchronization of neurons in the gamma range (30-100 Hz) is believed to play an important role in attention, memory tasks[1] and other cognitive functions[2]. In many brain regions, gamma oscillations are modulated by the lower frequency theta rhythm[3]. There are at least two open questions with respect to gamma frequency synchronization: how this synchronization might occur between distal regions without direct connections and whether inputs from slow theta oscillations can synchronize populations of neurons firing at gamma frequency. It is well known that a low frequency input is capable of phase-locking a higher frequency oscillator, thus a population of gamma neurons with similar frequencies may synchronize due to common theta input. Here we exploit maps based upon the phase resetting curves [4] (PRCs) to examine minimal model of a feedforward network of oscillating neurons and to derive existence and stability criteria for phase-locking of high frequency oscillators by a common theta input. Then we extend these results for heterogeneous network with Gaussian-distributed periods and show good agreement among direct simulation of 200,000 Morris-Lecar model neurons, results obtained by PRC mapping of the same population, and theoretical predictions. These results may be extended to a special case when biological noise is approximated by treating the period of each gamma oscillator as non-stationary, specifically as an OrnsteinUhlenbeck process (OUP) with a fixed time constant and variance. Good agreement between theory and the PRC mapping results were obtained for small variance and a sufficiently large time constant of the OUP such that the time scale is slower than the rate of convergence of gamma neurons to their preferred phase. However, the limitation of the method is that the predicted and actual distributions diverge with increasing OUP variance as well as with decreasing time constant. For example, if the PRC has two stable branches and the period varies rapidly compared to the speed of attraction of the stable locking point, phases corresponding to the unstable branches will have an inflated probability density compared to our theoretical predictions due to transient crossing of these phases as the fixed point switches between branches. Finally we adopt this theory for the case in which there are multiple theta inputs to the gamma population and show that regions of existence and stability can thereby be significantly extended. Moreover, this result suggests a possible control scheme for switching the activity of a gamma population between synchrony, a cluster solution or a completely desynchronized pattern by varying the phase between theta inputs, which could theoretically lead to theta – gamma nesting, which is widely observed experimentally.

Effect of semiconductive nanoparticles on insulating performances of transformer oil

In this paper, TiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> semiconductive nanoparticles with a large relaxation time constant is added into transformer oil to form semiconductive nanofluids (SNFs), with the aim of enhancing insulating characteristics. ac, dc and lightning impulse breakdown voltage and partial discharge (PD) characteristics of oil samples before and after modification were measured according to ASTM standard methods. It was found that SNFs have ac, dc and lightning impulse breakdown voltage up to 1.2 times compared with pure oil. Meanwhile, the partial discharge resistance of SNFs was also dramatically improved. Charge trap and transportation characteristics of both samples have been measured by thermally stimulated current method (TSC) and pulse electroacoustic technique (PEA). It was found that electron shallow trap density and charge decay rate are greatly increased in semiconductive nanoparticles modified transformer oil. It is proposed that electron trapping and de-trapping processes in the shallow traps could be one of the main charge transport processes in dielectric liquids.

An Ultra-Low Power OTA-C Bandpass Filter Design for ECG Application

An ultra-low power OTA-C bandpass filter is presented in this paper. An 1-150 Hz 4-th order OTC-C bandpass filter is design which is suitable for ECG application. A subthreshold region OTA is designed with current cancellation technology to achieve a very small transconductance. The OTA-C filter is design with TSMC 0.18μm CMOS process. The MIM capacitor is used in this design. The supply of the proposed circuit is 1V. The total power consumption is only 245nW. I. I NTRODUCTION The development of portable heart monitoring system is not only useful in long-term healthcare, but also demanded in an increasing number of researches in the biomedical file (1). Filters is continually a critical block of biopotential acquisition systems for enhancing the signal quality (2). However, for low frequency biomedical applications, realizing filter circuits with large time constant under an acceptable capacitor's value isn't an easy task (3). Switching-capacitor circuit is a popular technique used in implementing filter circuit. Due to the leakage problem of the advanced process, the sample-hold circuits of the switch-based topologies are not suitable for applications requiring large time constant. Compared with the switching-capacitor filters, continuous-time OTA-C filter has their benefits of lower power consumption and freeing the system from switching noise. The cut-off frequency is proportional to the transconductance (g m ) of the OTA; this allows a low-power implementation for low frequency biopotential signals. In OTA-based circuits, OTA will dominate the performance of the filter circuit, while the ratio of the capacitor to the small transconductance determines the time constant of OTA-C integrators. Thus, an OTA in which the transistors are operated in the subthreshold region is needed to save more power and realize a very low transconductance. A 4-order Butterworth bandpass is presented in this paper. The bandwidth of this filter is 1-150Hz. A subthreshold OTA is used to achieve the 1Hz cut-off frequency. The transconductance is below 1nA/V. The performance of the OTA is based on the behavior of MOS devices operating in the subthreshold region. The channel current of subthreshold region is:(3)

Applying the Difference Term Approach for Low Frequency Biomedical Filter

Low frequency continuous time filters are essential analog blocks for biomedical applications. Integrating such filters having large time constants is difficult as it requires large component values. A novel approach to scale down the pole frequency is presented. A 5-bit reduction in the cut off frequency is achieved. This is made possible through adding a passive resistor in the forward path of the op-amp based integrator introducing a difference term of the pole frequency. Also, the filter topology is modified to avoid changing the quality factor. As an example, a 2nd order low pass filter is designed and simulated. Simulation results show that the pole frequency is scaled down from 1.43 MHz to 4.97 kHz while maintaining tuning of 30% around the nominal value by controlling only one resistor.

THETRIS: A micro-scale temperature and gas release model for TRISO fuel

The dominating mechanism in the passive safety of gas-cooled, graphite-moderated, high-temperature reactors (HTRs) is the Doppler feedback effect. These reactor designs are fueled with submillimeter-sized kernels formed into tristructural-isotropic (TRISO) particles that are imbedded in a graphite matrix. The best spatial and temporal representation of the feedback effect is obtained from an accurate approximation of the fuel temperature. Micro-scale models of TRISO particles are necessary in order to obtain accurate predictions during fast transients or when parameters internal to the TRISO are needed. Most accident scenarios in HTRs are characterized by large time constants and slow changes in the fuel and moderator temperature fields. In these situations, a meso-scale, or pebble- and compact-scale, solution provides a good approximation of the fuel temperature as the fission thermal energy transports out of the kernel and into the surrounding matrix with a much shorter time constant. Therefore, in most cases, the matrix can be assumed to be in quasi-static equilibrium with the kernels. These models, however, fail to provide accurate information on the state of the various components of the TRISO during the early stages of transients. Since the coated particles constitute one of the fundamental design barriers for the release of fission products, it becomes important to understand the transient behavior inside this containment system. An explicit TRISO fuel temperature model named THETRIS has been developed and incorporated into the CYNOD–THERMIX-KONVEK suite of coupled codes. The code includes gas-release models that provide a simple predictive capability of the internal pressure during transients. The new model yields similar results to those obtained with other micro-scale fuel models of TRISO particles, but with the added capability to analyze gas release, internal pressure buildup, and effects of a gap in the TRISO. Analysis of bounding benchmark transients yield good agreement with other codes in which the TRISO particles are modeled explicitly. In addition, a sensitivity study of the potential effects on the transient behavior of high-temperature reactors due to the presence of an inter-layer gap is included. Although the formation of a gap occurs under special conditions, its consequences on the dynamic behavior of the reactor can yield responses during fast transients that depart significantly from those in which no gap is present in the model. The new model was applied to an extreme (beyond design basis) scenario in order to observe the behavior of the fuel during a large prompt critical reactivity insertion. Although a large amount of fission energy was deposited rapidly into the fuel, the kernel temperature is shown to stay well below the melting point and the silicon carbide layer remained well below the temperature above which failure is expected to occur. The explicit treatment of the TRISO particle geometry leads to much lower estimations of power peaking during the transient and a greater degree of negative Doppler feedback.

Design of Synthetic Optimizing Neuro Fuzzy Temperature Controller for Dual Screw Profile Plastic Extruder Using Labview

Problem statement: The temperature control in plastic extrusion machine is an important factor to produce high quality plastic products. The first order temperature control system in plastic extrusion comprises of coupling effects, long delay time and large time constants. Controlling temperature is very difficult as the process is multistage process and the system coupled with each other. In order to conquer this problem the system is premeditated with neuro fuzzy controller using LabVIEW. Approach: The existing technique involved is conventional PID controller, Neural controller, mamdani type Fuzzy Logic Controller and the proposed method is neuro fuzzy controller. Results: Manifest feature of the proposed method is smoothing of undesired control signal of conventional PID, neural controller and mamdani type FLC controller. The software incorporated the LabVIEW graphical programming language and MATLAB toolbox were used to design temperature control in plastic extrusion system. Hence neuro fuzzy controller is most powerful approach to retrieve the adaptiveness in the case of nonlinear system. Conclusion: The tuning of the controller was synchronized with the controlled variable and allowing the process at its desired operating condition. The results indicated that the use of proposed controller improve the process in terms of time domain specification, set point tracking and also reject disturbances with optimum stability.

Design of Intelligent Self‐Tuning GA ANFIS Temperature Controller for Plastic Extrusion System

This paper develops a GA ANFIS controller design method for temperature control in plastic extrusion system. Temperature control of plastic extrusion system suffers problems related to longer settling time, couple effects, large time constants, and undesirable overshoot. The system is generally nonlinear and the temperature of the plastic extrusion system may vary over a wide range of disturbances. The system is designed with three controllers. The proposed GA ANFIS controller is the most powerful approach to retrieve the adaptiveness in the case of nonlinear system. In this research the control methods are simulated using simulink. Relatively the methodology and efficiency of the proposed method are compared with those of the traditional methods and the results obtained from GA ANFIS controller give improved performance in terms of time domain specification, set point tracking, and disturbance rejection with optimum stability.

Final Comments by the Authors

E-mail: fazekas@scl.sztaki.hu Figure 2 contains a simulation of the nuclear reactor with input output linearisation, where τ is set to 20min. The response to a step change of the set-pointN is shown. One can see from the top diagram that N has the desired linear behaviour and reaches the new steady state after about 2 hours. However, the internal states are far from stationary at that time, as is illustrated by the lowest diagram (note the different time scales in the diagrams). The zero dynamics is stable but slow. It needs a very long time to settle after a step change. This may be explained by the large time constants 1/λI and 1/λX of the iodine and the xenon concentration, which are between 10 h and 13 h.

Interval Timing by Long-Range Temporal Integration

Interval Timing by Long-Range Temporal Integration

MIMO CCS model identification based on GA for closed-loop field data

The MIMO model identification methods based on experiment closed-loop field data for power plant boiler-turbine coordinated control system (CCS) is developed in this paper. The CCS fast and slow dynamic processes, which correspond to large and small time constants with time delay, are modelled by combining the genetic algorithm (GA) with step response analysis. The step response is obtained from the closed-loop system response tested by normal operation signals. The step modelling formula is derived, and the identified parameters are used to initialise and to form the ranges for GA chromosomes. This multivariable system identification is suitable for control system parameter tuning of modern power plant equipped with distributed control system (DCS). Some simulations and considerations for the stable and margin stable system application of the technology are also reported.

Better Production Forecasting for Unconventional-Gas Wells

This article, written by Senior Technology Editor Dennis Denney, contains highlights of paper SPE 134231, ’A Better Way To Forecast Production From Unconventional-Gas Wells,’ by Peter P. Valko, SPE, and W. John Lee, SPE, Texas A&amp;M University, prepared for the 2010 SPE Annual Technical Conference and Exhibition, Florence, Italy, 19-22 September. The paper has not been peer reviewed. The stretched-exponential-decline-curve model and data-intensive discovery provide a controlled production forecast for an individual tight gas well or shale-gas well on the basis of data gathered through parameter processing for a large group of wells. Group production for a large number of wells follows stretched-exponential-decline behavior of production rates. This approach moves production forecasts in tight and unconventional gas fields from individual and subjective curve matching to a group-data controlled-forecast methodology. Stretched-Exponential Decline A natural interpretation of the stretched-exponential decay of a quantity is that it is generated by a sum (integral) of pure-exponential decays with a “fat-tailed” probability distribution of the time constants. Therefore, the stretched-exponential-production-decline (SEPD) model can be interpreted as the acknowledgement of the heterogeneity. The actual production decline is determined by many individually contributing volumes in exponential decay (i.e., in some kind of pseudosteady state), but with a specific distribution of characteristic time constants. The family of the background distribution of characteristic time constants is known analytically. The distribution is determined by a parameter pair: the exponent parameter n and the characteristic time parameter τ. Broadly, the τ parameter is the median of the characteristic time constants. The nearer parameter n is to zero, the larger is the tail of the distribution (i.e., more-elementary volumes have very large time constants). One result of using the hyperbolic-decline approach in analytical models and reservoir simulators is that boundary-dominated flow is necessary for meaningful extrapolation. Because the traditional boundary-dominated state is questionable in tight and, especially, shale gas, little use can be made of the method. The stochastic interpretation, however, provides that the distribution of time constants is an intrinsic property of a continuous unconventional gas reservoir, and, hence, it determines the totality of the production-decline curve. Data-Intensive Discovery The desire of capturing the “individuality” of any well will remain. However, the probabilistic criterion for proved (P90) reserves does not require a good fit. Rather, it requires some kind of proof that a given estimate is met or surpassed by (future) reality in at least 90% of the cases. Strictly, such proof can be provided only if a sufficient number of wells have been abandoned, which is not the case for unconventional resources. Industry always will be interested in the performance of the newest technology in the newest plays. There are no long-duration data sets, and those of shorter duration are affected by the accelerated evolution of completion technology.

Convergence Acceleration of Time-Periodic Electromagnetic Field Analysis by the Singularity Decomposition-Explicit Error Correction Method

This paper proposes a novel method for the improvement of the convergence to a steady state in time-periodic transient nonlinear eddy-current analyses. The proposed method, which is based on the time-periodic finite element method and the singularity decomposition-explicit error correction method, can extract poorly converged error components corresponding to the large time constants of an analyzed system. The correction of the extracted error components efficiently accelerates the convergence to a steady state. Numerical results verify the effectiveness of the proposed method.

Textural controls on low-frequency electrical spectra of porous media

The results from several laboratory studies of the relationships between electrical polarization and physical properties of porous media have prompted interest in the potential use of low-frequency electrical spectra to qualitatively or quantitatively map variation in hydrogeologic properties in the field. Compiling several published and unpublished data sets, supported by new measurements, we have examined the low-frequency electrical spectra of a range of natural and artificial porous media to assess the generality of proposed relationships between electrical and physical properties. Our work confirms a significant positive correlation between the magnitude of electrical polarization (quantified as imaginary conductivity at a specific frequency) and the surface-area/pore-volume ratio [Formula: see text]. Analyzing the parameters of ageneralized Cole-Cole resistivity relaxation model fitted to many electrical spectra, we observe two apparent controls on the electrical relaxation. For samples with abundant relatively large pore throats, we observe a distinct increase in the time constant of the model with modal pore-throat size, in accordance with classical electrical relaxation models. However, for media with pore structures dominated by small pore throats, the diffusion-length scales do not appear to be controlled by modal pore-throat size. We conclude that for such media, the microstructure of the network of small pores leads to some connectivity of diffusion paths; thus, these samples exhibit relatively large time constants. There is potential value in addition to limitations when using electrical spectra to estimate physical properties of porous media, and we see the need for more appropriate generalized theories of electrical polarization in hydrogeologic media.

Correlation Between Resistance Degradation and Thermally Stimulated Depolarization Current in Acceptor (Mg)‐Doped BaTiO3 Submicrometer Fine‐Grain Ceramics

Thermally stimulated depolarization current (TSDC) of acceptor (Mg)‐doped BaTiO3 ceramics was analyzed for submicrometer fine grain specimens. For a fixed condition of polarization field (EP) and polarization time (tP), the TSDC associated with an oxygen vacancy relaxation showed two peaks with relaxation temperatures (Tm) under low polarization temperature (TP) conditions. It is inferred that the TSDC peak of the lower Tm is due to the relaxation of a distributed space charge within a grain and has a short relaxation time constant (τ0), and the higher Tm is due to the long‐range relaxation across grain boundaries with a larger time constant, τ0. The onset condition for resistance degradation can be correlated with the breaking point of TP, at which the TSDC peak changes from in‐grain relaxation to across‐grain boundary relaxation. The breaking point of TP and the time to degradation systematically decreased with the increase of acceptor concentration. Such behavior can be correlated with the decrease of TP, Tm, τ0, and little change of activation energy of relaxation, which is due to the increase of concentration.

Interface depolarization field as common denominator of fatigue and size effect in Pb(Zr0.54Ti0.46)O3 ferroelectric thin film capacitors

Dielectric, hysteresis and fatigue measurements are performed on Pb(Zr0.54Ti0.46)O3 (PZT) thin film capacitors with different thicknesses and different electrode configurations, using platinum and LaNiO3 conducting oxide. The data are compared with those collected in a previous work devoted to study of size effect by R. Bouregba et al., [J. Appl. Phys. 106, 044101 (2009)]. Deterioration of the ferroelectric properties, consecutive to fatigue cycling and thickness downscaling, presents very similar characteristics and allows drawing up a direct correlation between the two phenomena. Namely, interface depolarization field (Edep) resulting from interface chemistry is found to be the common denominator, fatigue phenomena is manifestation of strengthen of Edep in the course of time. Change in dielectric permittivity, in remnant and coercive values as well as in the shape of hysteresis loops are mediated by competition between degradation of dielectric properties of the interfaces and possible accumulation of interface space charge. It is proposed that presence in the band gap of trap energy levels with large time constant due to defects in small nonferroelectric regions at the electrode—PZT film interfaces ultimately governs the aging process. Size effect and aging process may be seen as two facets of the same underlying mechanism, the only difference lies in the observation time of the phenomena.

Internal model-based control for integrating processes

Internal model-based control (IMC) has been shown to possess many advantages over PID control, particularly in the presence of significant process deadtime. Implementation of IMC is simplified in a large class of industrial applications where the process dynamics can be adequately characterized by a simple first-order model requiring only estimates of process gain, lag time constant, and deadtime for implementing the controller design. Tuning of the controller is easily and intuitively done by adjusting the filter time constant; decreasing the time constant speeds up the closed-loop response, and increasing it yields generally a slower but more stable response. There are problems, however, in applying the IMC approach to an integrating process, i.e., a process with a pole at the origin. First, for a first-order lag filter, the steady-state error due to a process input disturbance is generally non-zero. This error can be reduced to zero with a higher-order lead lag filter with proper choice of filter parameters. This is at a cost, however, of increased design complexity, amplification of noise in the controller signals, and a potential numerical overflow issue due to an integrator within the IMC computation loop. To overcome these problems, an alternative IMC implementation is proposed where the integrator in the model is approximated by a first-order lag with a very large time constant. It is shown analytically and verified by computer simulation that this approach assures zero steady-state error for setpoint (SP) changes and process disturbance inputs. Computer simulation studies also show that the transient response can be satisfactorily tuned by proper choice of the filter time constant and that potential numerical instability issues are essentially eliminated. Further, since the choice of time constant in the lag used to approximate the integrator function in the modified IMC also affects the P V ( t ) response approach, another degree of freedom in the tuning process is introduced. This modified IMC approach has been used successfully in several real-world applications.