Lag Phenomena Research Articles

Implementation of Fuzzy Self-Tuning PID and Feed-Forward Design for High-Performance Motion Control System

The existing conventional motion controller does not perform well in the presence of nonlinear properties, uncertain factors, and servo lag phenomena of industrial actuators. Hence, a feasible and effective fuzzy self-tuning proportional integral derivative (PID) and feed-forward control scheme is introduced to overcome these problems. In this design, a fuzzy tuner is used to tune the PID parameters resulting in the rejection of the disturbance, which achieves better performance. Then, both velocity and acceleration feed-forward units are added to considerably reduce the tracking error due to servo lag. To verify the capability and effectiveness of the proposed control scheme, the hardware configuration includes digital signal processing (DSP) which plays the main role, dual-port RAM (DPRAM) to guarantee rapid and reliable communication with the host, field-programmable gate array (FPGA) to handle the task of the address decoder and receive the feed-back encoder signal, and several peripheral logic circuits. The results from the experiments show that the proposed motion controller has a smooth profile, with high tracking precision and real-time performance, which are applicable in various manufacturing fields.

SU-E-I-79: Correction of Dark Current and Image Lag in Multi-Source Carbon Nanotube Imaging Systems.

To correct for dark current and detector image lag in multi-source carbon nanotube (CNT) based imaging systems. CNTs can be used as a novel electron source for X-ray generation. Here a small (∼2 kV) gate voltage induces a large electric field at the CNT tip that causes field emission as opposed to thermionic emission. This enables faster switching times and allows miniaturization which aids in the development of stationary multi-source systems. These systems may encounter two key problems: dark current in non-active sources and detector lag. In damaged and perhaps end-of-life devices, sources may develop significant electron output without gate voltage, interfering with images from non-active sources. Although the output is small, it is constant, and continuous integration of detectors compared with small X-ray exposure time magnifies its contribution to the signal. A method based on binary multiplexing is developed to mitigate the dark current contributions possibly extending device lifetime. Having discrete sources, these multi-source systems have coarser angular sampling than rotating systems. This coarser sampling means that the detector signal changes more in adjacent projections than rotating systems exacerbating the effect of detector lag. Since flat panel detectors have a capacitive behavior, readout only removes some charge from pixels. As a result, a device dependent amount (6.6% in our device) of the signal from the current image contributes to the subsequent image. A fast, stable inverse filter separating detector lag has been developed. Projection data were simulated based on parameters measured from a multi-source CNT system. Incorporating dark current and image lag phenomena into reconstruction improves contrast to noise ratio. These correction methods can be easily implemented on clinical multi-source systems to improve image quality and extend the device's usable lifetime. Supported in part by Siemens Medical.

Ancient and recent alien species in temperate forests: steady state and time lags

Temperate forests are relatively little affected by invasions globally. However, it remains unclear if these low invasion levels can be attributed to high invasion resistance, or if particularly long time lags in forests have lead to wide-spread invasion debt at the landscape and habitat scales, thus masking the invasibility of forests. Using linear and generalized linear mixed-effects models, we tested which factors determine the level of invasion of old (archaeophytes, pre-1500 alien species) and new (neophytes, post-1500 alien species) invaders across 30 study sites (5 ha size) and 16 transects in lowland forests in eastern Austria, Central Europe. We analysed the importance of different forest types, adjacent habitats, introduction pathways, species’ local residence time and proximity to human settlements as proxy for propagule pressure. We recorded 109 neophyte and 50 archaeophyte species, with significant differences between both groups with respect to region of origin, pathways, and life form. The number of neophyte species per study site varied by more than an order of magnitude and was dependent on both proximity to settlements and residence time in a non-linear manner, with proximity to settlements showing a particularly strong effect for recently introduced neophytes. In contrast, archaeophyte species numbers were only marginally affected by distance to the nearest settlement. Within habitats, transect plots close to the forest edge and adjacent to settlements showed significantly higher numbers and cover of neophyte species, whereas for archaeophytes and native species no such effect could be detected. Our results suggest that time lag phenomena stemming from dispersal limitation (e.g. aggravated by isolation of forest patches), heterogeneity in species’ local residence time as well as increases in local levels of propagule pressure (e.g. created by expanding settlements), may result in wide-spread invasion debt at the landscape and habitat scales, hence masking invasibility of forests. Spread of new invaders may ultimately, although possibly only over long time periods, result in increased frequency and impact of neophyte species in temperate forests.

General Strategy for Performing Temperature Programming in High Performance Liquid Chromatography: Prediction of Linear Temperature Gradients

This paper describes how an empirical retention model is transferred from temperature-programmed gas chromatography (GC) to high temperature liquid chromatography (HT-HPLC). In order to evaluate the retention prediction, a temperature range from 50 to 180 °C was investigated using two test mixtures consisting of steroids and polycyclic aromatic hydrocarbons. In this temperature range, heating rates from 1.5 °C min(-1) up to 30 °C min(-1) were applied using four different high temperature stable HPLC columns with inner diameters of 1.0, 2.1, 3.0, and 4.6 mm. Temperature lag phenomena in the HPLC column as well as in the column oven are discussed, and it is shown that the linear elution strength (LES) model can be applied without any mathematical extension in order to take a temperature-dependent delay time into account. On the basis of this approximation, it is possible to perform a systematic method development using linear temperature gradients in liquid chromatography. Furthermore, it is shown that only two initial temperature gradient runs are necessary to predict the retention times of the analytes with a maximal relative error of less than 2%.

Effects of field plate on buffer trapping in AlGaN/GaN HEMTs

AbstractTwo‐dimensional transient analysis of field‐plate AlGaN/GaN HEMTs is performed in which a deep donor and a deep acceptor are considered in a semi‐insulating buffer layer. It is shown that drain lag is reduced by introducing a field plate, because the buffer‐trapping effects are reduced. It is also shown that buffer‐related lag phenomena and current collapse are also reduced in the field‐plate structure. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Analysis of field-plate effects on buffer-related lag phenomena and current collapse in GaN MESFETs and AlGaN/GaN HEMTs

A two-dimensional transient analysis of field-plate GaN MESFETs and AlGaN/GaN HEMTs is performed in which a deep donor and a deep acceptor are considered in a semi-insulating buffer layer, and quasi-pulsed current–voltage curves are derived from them. How the existence of a field plate affects buffer-related drain lag, gate lag and current collapse is studied. It is shown that in both MESFET and HEMT, the drain lag is reduced by introducing a field plate because electron injection into the buffer layer is weakened by it, and the buffer-trapping effects are reduced. It is also shown that the field plate could reduce buffer-related current collapse and gate lag in the FETs. The dependence of lag phenomena and current collapse on the field-plate length and on the SiN passivation layer thickness is also studied. The work suggests that in the field-plate structures, there is an optimum thickness of the SiN layer to minimize the buffer-related current collapse and drain lag in GaN MESFETs and AlGaN/GaN HEMTs.

Melting behaviors of Co N ( N = 13, 14, 38, 55, 56) clusters

Constant-temperature molecular dynamics simulations and the Gupta-type many-body interatomic potential are used to study the melting behaviors of Co N ( N = 13, 14, 38, 55, 56) clusters. The atomic equivalence indexes are used to describe the structural changes of clusters at different temperatures. Other quantities like the Lindemann index, the heat capacity and caloric curve are also employed. The lag phenomena (the rapid change of Lindemann index is not consistent with that of the heat capacity curve during the cluster melting) are discussed in detail for the Co 13 and Co 14 clusters. Our results show that the Lindemann index fails to capture the melting transition while the heat capacity does for the Co 13 or Co 14 clusters. All these clusters show premelting behaviors and the surface melting are found for the Co 14 and Co 56 clusters. The Co 38 cluster undergoes distinct isomerization transitions prior to its full melting.

Using branch and basal trunk sap flow measurements to estimate whole-plant water capacitance: comment on Burgess and Dawson (2008)

Sap flow sensors are uniquely able to continuously monitor whole tree physiology. Recently, Burgess and Dawson (Burgess SSO, Dawson TE, Plant Soil 305:5–13, 2008) urged caution in using sap flow probes to estimate water storage use in trees. Here we respond to three criticisms raised there: (1) Sampling: that tree water storage, estimated from branch-bole sap flow lags, was compromised by unaccounted variation in branch position and orientation; (2) Instrumentation: that sap flow sensor response times may be sensor artefacts rather than manifestations of tree water storage; and (3) Theory: that tree water storage estimates are based on a faulty concept of lag phenomena in sap flow that persists in the literature. We agree with the need for caution in sap flow-based estimates of plant water storage, but here correct flaws in arguments and representations of studies presented in Burgess and Dawson (Burgess SSO, Dawson TE, Plant Soil 305:5–13, 2008).

Analysis of buffer‐trapping effects on gate lag, drain lag and current collapse in AlGaN/GaN HEMTs

AbstractTransient simulations of AlGaN/GaN HEMTs are performed in which a deep donor and a deep acceptor are considered in a semi‐insulating buffer layer. Quasi‐pulsed I‐V curves are derived from the transient characteristics, and are compared with steady‐state I‐V curves. It is shown that the lag phenomena and current collapse could be reproduced. Particularly, the gate lag is correlated with relatively high source access resistance of the FETs. The current collapse is shown to bemore pronounced when the deep‐acceptor density in the buffer layer is higher and when an off‐state drain voltage is higher, because trapping effects become more significant. It is suggested that to minimize current collapse in AlGaN/GaN HEMTs, an acceptor density in the buffer layer should be made low, although the current cutoff behavior may be degraded. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Stable response of low-gravity liquid non-linear sloshing in a circle cylindrical tank

Under pitch excitation, the sloshing of liquid in circular cylindrical tank includes planar motion, rotary motion and rotary motion inside planar motion. The boundaries between stable motion and unstable motion depend on the radius of the tank, the liquid height, the gravitational intension, the surface tensor and the sloshing damping. In this article, the differential equations of nonlinear sloshing are built first. And by variational principle, the Lagrange function of liquid pressure is constructed in volume intergration form. Then the velocity potential function is expanded in series by wave height function at the free surface. The nonlinear equations with kinematics and dynamics free surface boundary conditions through variation are derived. At last, these equations are solved by multiple-scales method. The influence of Bond number on the global stable response of nonlinear liquid sloshing in circular cylinder tank is analyzed in detail. The result indicates that variation of amplitude frequency response characteristics of the system with Bond, jump, lag and other nonlinear phenomena of liquid sloshing are investigated.

Analysis of buffer‐related lag phenomena and current collapse in GaN FETs

Drain-current responses of GaN MESFETs with a semi-insulating buffer layer are calculated when the drain voltage and/or the gate voltage are changed abruptly, and pulsed I-V curves are derived from them. It is shown that so-called current collapse or current slump is not so dependent on the gate length LG (0.3 μm – 1 μm), and this is essentially determined by a deep-acceptor density NDA in the buffer layer. LG and NDA dependence of gate lag is also studied, indicating that the gate lag is weaker for longer LG and the lag rate becomes high with NDA, although it may show a saturation behaviour. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Low-gravity liquid nonlinear sloshing analysis in a tank under pitching excitation

Under pitch excitation, the sloshing of liquid in circular cylindrical tank includes planar motion, rotary motion and rotary motion inside planar motion. The boundaries between stable motion and unstable motion depend on the radius of the tank, the liquid height, the gravitational intension, the surface tensor and the sloshing damping. In this article, the differential equations of nonlinear sloshing are built first. And by variational principle, the Lagrange function of liquid pressure is constructed in volume integration form. Then the velocity potential function is expanded in series by wave height function at the free surface. The nonlinear equations with kinematics and dynamics free surface boundary conditions through variation are derived. At last, these equations are solved by multiple-scales method. The influence of Bond number on the global stable response of nonlinear liquid sloshing in circular cylinder tank is analyzed in detail. The result indicates that the system's amplitude–frequency response changes from a ‘soft-spring’ to a ‘hard-spring’ in the planar motion with the decreasing of the Bond number, while it changes from a ‘hard-spring’ to a ‘soft-spring’ in the rotary motion. At the same time, jump, lag and other nonlinear phenomena of liquid sloshing are discovered.

Characterization of land subsidence induced by groundwater withdrawals in Su-Xi-Chang area, China

Su-Xi-Chang area is one of the typical regions in China which suffers from severe land subsidence. Various tools of field monitoring were integrated to study the characteristics and mechanisms of land subsidence in this region. The occurrence and the development of the land subsidence in this region are strongly related to the groundwater pumping both in time and space. The main consolidation layers are the soft mud layers; however, the compressibility of the confined sandy layers should not be ignored. The second and third confined aquifers contributed more than 30% of total subsidence. Meanwhile, irrecoverable deformations were also observed in the sandy layers. Different sandy layers deform diversely under different stress conditions. Some have the elastic feature. But the soil strata, including both sandy layers and clayey layers, located in the center of the groundwater level depression cone exhibited obvious viscous mechanical behavior which caused the common lag phenomenon. The sand composition (mingled with small clay particles or interbeds) and sand rheology are the two main reasons for the lag phenomena in sandy layers. A series of laboratory tests for modeling the effective stress changes due to groundwater withdrawals, were conducted to investigate the mechanism of the lag phenomenon. Based on the test results, the relationship of stress–strain–time for saturated sands is obtained; and it could be expressed as power functions. The results also showed that the compression of the sandy layers was time dependent, and its deformation could be remarkable. When establishing land subsidence model, the deformation for the similar soil formation could be elastic, visco-elastic and even visco-elastic–plastic, because of the different groundwater level fluctuation experienced.

Steady-state Kinetic Mechanism of PDK1

PDK1 catalyzes phosphorylation of Thr in the conserved activation loop region of a number of its downstream AGC kinase family members. In addition to the consensus sequence at the site of phosphorylation, a number of PDK1 substrates contain a PIF sequence (PDK1-interacting fragment), which binds and activates the kinase domain of PDK1 (PDK1(deltaPH)). To gain further insight to PIF-dependent catalysis, steady-state kinetic and inhibition studies were performed for His6-PDK1(deltaPH)-catalyzed phosphorylation of PDK1-Tide (Tide), which contains an extended "PIF" sequence C-terminal to the consensus sequence for PDK1 phosphorylation. In two-substrate kinetics, a large degree of negative binding synergism was observed to occur on formation of the active ternary complex (alphaKd(ATP) = 40 microM and alphaKd(Tide) = 80 microM) from individual transitory binary complexes (Kd(ATP) = 0.6 microM and Kd(Tide) = 1 microM). On varying ATP concentrations, the ADP product and the (T/E)-PDK1-Tide product analog (p'Tide) behaved as competitive and noncompetitive inhibitors, respectively; on varying Tide concentrations, ADP and p'Tide behaved as noncompetitive and competitive inhibitors, respectively. Also, negative binding synergism was associated with formation of dead-end inhibited ternary complexes. Time progress curves in pre-steady-state studies under "saturating" or kcat conditions showed (i) no burst or lag phenomena, (ii) no change in reaction velocity when adenosine 5'-O-(thiotriphosphate) was used as a phosphate donor, and (iii) no change in reaction velocity on increasing relative microviscosity (0 < or = eta/eta0 < or = 3). Taken together, PDK1-catalyzed trans-phosphorylation of PDK1-Tide approximates a Rapid Equilibrium Random Bi Bi system, where motions in the central ternary complex are largely rate-determining.

Modelling the work to be done by Escherichia coli to adapt to sudden temperature upshifts

This paper studies and models the effect of the amplitude of a sudden temperature upshift DeltaT on the adaptation period of Escherichia coli, in terms of the work to be done by the cells during the subsequent lag phase (i.e., the product of growth rate mumax and lag phase duration lambda). Experimental data are obtained from bioreactor experiments with E. coli K12 MG1655. At a predetermined time instant during the exponential growth phase, a sudden temperature upshift is applied (no other environmental changes take place). The length of the (possibly) induced lag phase and the specific growth rate after the shift are quantified with the growth model of Baranyi and Roberts (Int J Food Microbiol 23, 1994, 277). Different models to describe the evolution of the product lambda x mumax as a function of the amplitude of the temperature shift are statistically compared. The evolution of lambda x mumax is influenced by the amplitude of the temperature shift DeltaT and by the normal physiological temperature range. As some cut-off is observed, the linear model with translation is preferred to describe this evolution. This work contributes to the characterization of microbial lag phenomena, in this case for E. coli K12 MG1655, in view of accurate predictive model building.

Cell division theory and individual-based modeling of microbial lag: Part II. Modeling lag phenomena induced by temperature shifts

This paper is the second in a series of two, and studies microbial lag in cell number and/or biomass measurements caused by temperature changes with an individual-based modeling approach. For this purpose, the theory of cell division, as discussed in the first part of this series of research papers, was implemented in the individual-based modeling framework BacSim. Simulations of this model are compared with experimental data of Escherichia coli, growing in an aerated, glucose-rich medium and subjected to sudden temperature shifts. The premise of a constant cell volume under changing temperature conditions predicts no lag in cell numbers after the shift, in contrast to the experimental observations. Based on literature research, two biological mechanisms that could be responsible for the observed lag phenomena are proposed. The first assumes that the average cell volume depends on temperature while the second assumes that a lag in biomass growth occurs after the temperature shift. For a lag in cell number caused by an increased average cell volume, the cell biomass always increases at the maximal rate. Therefore, cells are evidently not stressed and do not have to adapt to the new conditions, as opposed to a lag in biomass growth. Implementation and simulation of both mechanisms are found to describe the experimental observations equally well. Therefore, further research is needed to distinguish between the two mechanisms. This can be done by observing, in addition to cell numbers, a measure for the average cell volumes. In conclusion, the individual-based modeling approach is a good methodology to investigate and test biological theories and assumptions. Also, based on the simulations, suggestions for further experimental observations can be made.

The effects of thermal annealing on the relaxation of rubbing-induced birefringence in polystyrene

We have conducted a systematic study on the effects of post rubbing annealing on the relaxation of rubbing-induced birefringence of polystyrene. It is found that annealing at T(0) only affects the relaxation up to T(0)+T(Lag), where T(Lag) is proportional to the logarithm of the annealing time t(A). A theoretical model based on the distribution of relaxation times due to the individual birefringence elements is proposed. To remove its contribution to the net birefringence each element must overcome an energy barrier E=(317+1.17xi)x10(3) J/mol, and therefore must have a characteristic relaxation time tau which depends on temperature T and a barrier height which ranges from 340.4 kJ/mol to 445.7 kJ/mol. The relaxation of birefringence is expressed by the equation NB(T, t)=[Formula: see text]N(xi)e(-t/tau(T,xi))dxi, in which both the relaxation time tau(T,xi) and the distribution function N(xi) can be extracted from experimental data. The predictions of the model agree well with all the experimental results presented in this work. The differences and similarities of the relaxation of birefringence with respect to the physical aging of quenched PS are discussed. In particular, similarities in terms of the general temperature lag phenomena are noted.

Cell division theory and individual-based modeling of microbial lag: Part I. The theory of cell division

This series of two papers deals with the theory of cell division and its implementation in an individual-based modeling framework. In this first part, the theory of cell division is studied on an individual-based level in order to learn more about the mechanistic principles behind microbial lag phenomena. While some important literature on cell division theory dates from 30 to 40 years ago, until now it has hardly been introduced in the field of predictive microbiology. Yet, it provides a large amount of information on how cells likely respond to changing environmental conditions. On the basis of this theory, a general theory on microbial lag behavior caused by a combination of medium and/or temperature changes has been developed in this paper. The proposed theory then forms the basis for a critical evaluation of existing modeling concepts for microbial lag in predictive microbiology. First of all, a more thorough definition can be formulated to define the lag time λ and the previously only vaguely defined physiological state of the cells in terms of mechanistically defined parameters like cell mass, RNA or protein content, specific growth rate and time to perform DNA replication and cell division. On the other hand, existing predictive models are evaluated with respect to the newly developed theory. For the model of [Hills, B., Wright, K.A., 1994. New model for bacterial growth in heterogeneous systems. J. Theor. Biol. 168, 31–41], a certain fitting parameter can also be related to physically meaningful parameters while for the model of Augustin et al. (2000) [Augustin, J.-C., Rosso, L., Carlier, V.A. 2000. A model describing the effect of temperature history on lag time for Listeria monocytogenes. Int. J. Food Microbiol. 57, 169–181] a new, mechanistically based, model structure is proposed. A restriction of the proposed theory is that it is only valid for situations where biomass growth responds instantly to an environment change. The authors are aware of the fact that this assumption is not generally acceptable. Lag in biomass can be caused, for example, by a delayed synthesis of some essential growth factor (e.g., enzymes). In the second part of this series of papers [Dens, E.J., Bernaerts, K., Standaert, A.R., Kreft, J.-U., VanImpe, J.F., 2005. Cell division theory and individual-based modeling of microbial lag: Part II. modeling lag phenomena induced by temperature shifts. Int. J. Food Microbiol.], the theory of cell division is implemented in an individual-based simulation program and extended to account for lags in biomass growth. In conclusion, the cell division theory applied to microbial populations in dynamic medium and/or temperature conditions provides a useful framework to analyze microbial lag behavior.

Quantifying microbial lag phenomena due to a sudden rise in temperature: a systematic macroscopic study

The microbial lag phase is a complex and yet not completely understood phenomenon. Many studies on the microbial lag phase have been published but few report a systematic study; moreover, previous lag studies have involved the effect of multiple confounded factors. Here, the effect of sudden temperature rises on an exponentially growing Escherichia coli culture is systematically investigated. Experiments are performed in a computer-controlled bioreactor where E. coli K12 MG1655 is grown under aerobic conditions in Brain Heart Infusion (BHI) broth. This experimental set-up is used to characterise the effect of (i) the amplitude of the temperature shift, (ii) the pre-shift temperature level and (iii) the post-shift temperature level on the occurrence and length of a lag phase. Besides temperature, no other environmental changes take place at the moment of the temperature shift. To quantify the length of the induced lag phase, the experimental data are described with a common growth model. Depending on the three factors tested, a lag phase of more or less 1 h is induced or not. This lag/no lag behaviour can largely be explained by the existence of a normal physiological temperature range but also the amplitude of the temperature rise plays a role. It can be concluded that for the microorganism under study the lower boundary of the normal range lies approximately between 22.78 and 23.86 °C. It is shown that this boundary is no cut-off point, but rather a transition zone. Even more, repeated experiments at this boundary have yielded different results (lag or no lag). This observation points out that the mechanism triggering this lag phase is not absolute but may be subject to biological variability.

Numerical Analysis of Slow Current Transients and Power Compression in GaAs FETs

Two-dimensional transient simulation of GaAs MESFETs is performed when the gate voltage and the drain voltage are both changed abruptly. Quasi-pulsed current-voltage (I-V) curves are derived from the transient characteristics. It is discussed how the slow current transients (lag phenomena) and the pulsed I-V curves are affected by the existence of substrate traps and surface states. It is shown that the so-called power compression could occur both due to substrate traps and due to surface states. Effects of impact ionization of carriers on these phenomena are also discussed. It is shown that the lag phenomena and the power compression are weakened when impact ionization of carriers becomes important, because generated holes may help the traps to change their ionized densities quickly.