Process Time Constant Research Articles

Dynamics during holographic exposure in photopolymers for single and multiplexed gratings

Abstract Under the assumption that at low recording intensity the grating formation process is much slower than the free monomer diffusion, we present a model that describes the dynamics of the holographic grating formation in photopolymers. The model indicates how the beam intensity modulation influences the saturation value of the grating modulation, while the average recording intensity is related in a nonlinear fashion to the process time constant. An extension of this model describes and predicts the recording of a single grating in the presence of an additional mutually incoherent light beam, and the simultaneous recording of angularly mutliplexed gratings. The model is validated by comparing with experimental results obtained using DuPont HRF-150-38 photopolymer.

Dynamics during holographic exposure in photopolymers for single and multiplexed gratings

Under the assumption that at low recording intensity the grating formation process is much slower than the free monomer diffusion, we present a model that describes the dynamics of the holographic grating formation in photopolymers. The model indicates how the beam intensity modulation influences the saturation value of the grating modulation, while the average recording intensity is related in a nonlinear fashion to the process time constant. An extension of this model describes and predicts the recording of a single grating in the presence of an additional mutually incoherent light beam, and the simultaneous recording of angularly mutliplexed gratings. The model is validated by comparing with experimental results obtained using DuPont HRF-150-38 photopolymer.

Inherent Dynamic Problems with On-Demand Control Structures

There are two basic control structures for chemical plants: fixed feeds and fixed products. In the former, liquid levels are controlled by manipulating flows leaving individual units down through a series of reactors, tanks, distillation columns, etc. In the latter, which is called “on-demand” control, the product flow rate leaving the last unit is fixed (by a downstream user), and liquid levels are controlled by manipulating streams fed to each individual unit sequentially from its upstream unit. Buckley called these structures “material balance in the direction of flow” and “material balance in the reverse direction of flow”. This paper points out that the on-demand structure has several inherent dynamic disadvantages compared to the more conventional approach of setting the feed streams to a process. The most important disadvantage is that level control loops typically have more dynamic lags with this structure, which makes level controller tuning more difficult and degrades dynamic performance. A second problem is that this structure can produce interaction between level and composition loops that is not present in the conventional structure. Another disadvantage is that the on-demand structure tends to produce larger changes in process time constants as throughput changes, making the plant more nonlinear. In addition, the propagation of disturbances in the on-demand control structure is more complex than that in the conventional control structure because flow disturbances move upstream while composition disturbances move both upstream and downstream. Several of these features are shown mathematically using a simple stripper process. Then they are illustrated on two processes of increasing complexity. The first is a binary system with the reaction A → B and a plant topology of one reactor, one stripping column, and one recycle stream. The second is a ternary system with the reaction, A + B → C and a flowsheet containing one reactor, two distillation columns, and two recycle streams. Dynamic simulations demonstrate that the on-demand structure introduces larger disturbances into the system, which results in more variability in product quality.

Dynamics of the photo-induced orientation and relaxation of new polymethacrylates containing carbazolyl and azobenzene pendant groups

Polarized-light-induced birefringence has been investigated in copolymers of [ω-(N-carbazolyl)alkyl methacrylate] and [2,5-dimethylphenyl-[(4-nitrophenyl)azo]phenoxyalkyl methacrylate]. Optical properties of the side-chain copolymers have been compared with materials in which the azo-dye was merely dispersed in a poly[ω-(N-carbazolyl)alkyl methacrylate] matrix. Increasing the temperature of the film has a dramatic depressive effect on the maximum of photo-induced orientation near and above the glass transition temperature (Tg) region, while its effect was restricted under this temperature. Kinetics of the photo-orientation and the thermal relaxation as a function of temperature have been analysed by a biexponential model based on two rate constants. The universal Williams–Landel–Ferry (WLF) theory has been used to correlate the writing process and relaxation relative time constants with the relative temperature, i.e. T−Tg . The validity of this model for the studied materials leads to the assumption that photo-orientation and relaxation processes are mainly dependent on the free volume in the polymer matrix. This hypothesis has been assessed in the case of [11-(N-carbazolyl)undecyl methacrylate]-based materials. © 1999 Society of Chemical Industry

Miscible porous media displacements in the quarter five-spot configuration. Part 1. The homogeneous case

A detailed two-part computational investigation is conducted into the dynamical evolution of two-dimensional miscible porous media flows in the quarter five-spot arrangement of injection and production wells. High-accuracy direct numerical simulations are performed that reproduce all dynamically relevant length scales in solving the vorticity–streamfunction formulation of Darcy's law. The accuracy of the method is demonstrated by a comparison of simulation data with linear stability results for radial source flow.Within this part, Part 1 of the present investigation, a series of simulations is discussed that demonstrate how the mobility ratio and the dimensionless flow rate denoted by the Péclet number Pe affect both local and integral features of homogeneous displacement processes. Mobility ratios up to 150 and Pe-values up to 2000 are investigated. For sufficiently large Pe-values, the flow near the injection well gives rise to a vigorous viscous fingering instability. As the unstable concentration front approaches the central region of the domain, nonlinear interactions between the fingers similar to those known from unidirectional flows are observed, such as merging, partial merging, and shielding, along with secondary tip-splitting and side-branching instabilities. At large Pe-values, several of these fingers compete for long times, before one of them accelerates ahead of the others and leads to the breakthrough of the front.In contrast to unidirectional flows, the quarter five-spot geometry imposes both an external length scale and a time scale on the flow. The resulting spatial non-uniformity of the potential base flow is observed to lead to a clear separation in space and time of large and small scales in the flow. Small scales occur predominantly during the early stages near the injection well, and at late times near the production well. The central domain is dominated by larger scales.Taken together, the results of the simulations demonstrate that both the mobility ratio and Pe strongly affect the dynamics of the flow. While some integral measures, such as the recovery at breakthrough, may show only a weak dependence on Pe for large Pe-values, the local fingering dynamics continue to change with Pe.The increased susceptibility of the flow to perturbations during the early stages provides the motivation to formulate an optimization problem that attempts to maximize recovery, for a constant overall process time, by employing a time-dependent flow rate. Within the present framework, which accounts for molecular diffusion but not for velocity-dependent dispersion, simulation results indeed indicate the potential to increase recovery by reducing the flow rate at early times, and then increasing it during the later stages.

Design of robust SISO PI controllers for a MSF desalination plant

The method of inequalities is applied to the of design a robust PI controller for a multi-stage flash (MSF) desalination plant. The method takes into account the given variations in process gain, time delay and time constant ( K p , τ d and τ) of the transfer function models. The performance of the closed loop system is evaluated for different operating points (i.e., on different transfer function models). PI controllers are also designed using an internal model controller (IMC) and by Ziegler-Nichols (Z-N) methods based on the nominal transfer function model. The performances of these controllers are compared. The controller designed by the method of inequalities gives the best results.

Force control in turning based on robust PI controller design

Real-time cutting force control is a very straightforward way to improve safety, performance and quality in rough turning operations. The main problem in controller design for force control is to achieve robustness. Several time-variable process parameters, such as the depth of cut, the spindle rotational speed and the specific cutting force, directly affect the closed-loop system gain and time constant. In this paper, two approaches to robust PI (proportional integral) controller design based on the internal model control (IMC) method are presented. The first approach treats the cutting process as a first-order system with a time- variable gain. By designing for highest process gain, the controller becomes robust, but perhaps a bit sluggish for the lower range of process gain. In the second approach, the cutting process is linearized by introducing non-linear transformations. In this way, the design may be based on a constant-gain system and the time-variable parameters may be considered as additive disturbances. This controller is somewhat faster than the first approach and shows almost identical behaviour over the entire operating range. Both controllers incorporate, in addition, simple parameter adaptation by gain scheduling with respect to spindle speed variations, in order to handle variations in the process time constant. The work presented in this paper shows that it is possible to achieve robust force control in turning using the common PI controller.

Calculating process control parameters from steady state operating data

The tutorial paper will show that chemical process control parameters can be obtained from steady state operating data in combination with simple algebraic equations. The parameters include steady state process gain, feedforward control factor and approximate process time constant. Here, we treat ‘self-regulating’ processes first. Integrating processes and unstable processes are treated separately, unlike the standard method of setting up linear differential equations followed by Laplace transformations. With the standard method, a steady state portion of dynamic response, if there is any, appears as a part of equations of a complex variable. Many control engineers in the chemical industries who might have studied the method, do not use it because of mathematical difficulties. Instead, they may resort to plant tests that are often time consuming and costly. The parameters obtained by this paper's method will aid improvement of control systems. We will address the relation of this method with that of commercial software, which implement advanced process controls based on plant tests.

Risk reduction of operating procedures and process flowsheets

A unified risk reduction strategy that uses fault tree analysis, proposes modifications in chemical operations that include the process flowsheet, control system, and operating procedures. The modifications commonly involve the establishment of stationary states. These states allow for longer process time constants and support intermediate-state verification. Relative importance is used to identify quantitatively the dominant causes of risk, which are modified to produce safer and more reliable processes. The strategy is tested on the design of a pump system startup, and risk reduction of several orders of magnitude is achieved

Comparing two neural networks for pattern based adaptive process control

AbstractAn adaptation strategy based on an analysis of patterns exhibited in the recent controller error history is presented. The strategy is a two parameter adaptation, where the gain and time constant of the controller's internal model are adjusted to make the closed loop error response match a target pattern. Both a back‐propagation network and a vector quantizer network (VQN) are compared as pattern analysis tools. This strategy is established for a number of model based controllers and is demonstrated here using the Generalized Predictive Control algorithm. Details of this set point tracking strategy are presented along with demonstrations on both simulated and real single loop processes that experience significant changes in process gain and time constant. Results show both networks to be equally capable at pattern recognition with the VQN's ease of training and implicit ability to assess the accuracy of the pattern match as deciding factors in network selection.

Programmed Rate Processing to Increase Throughput in LPCVD

The simultaneous one‐dimensional Knudsen diffusion and chemical reaction description of low‐pressure chemical vapor deposition in features on patterned surfaces is used to show that a substantial decrease in processing time can be achieved for a given step coverage by varying the deposition rate in a prescribed manner during the process. For constant‐rate CVD (CRCVD) processes, the rate is dictated by the rate required to obtain a good step coverage at the end of the process when the instantaneous aspect ratio becomes very large. The use of a programmed‐rate CVD (PRCVD) process allows a larger average deposition rate, since the initial rate is much higher than the rate required to maintain good step coverage close to feature closure. The PRCVD concept is demonstrated for the thermally activated deposition of silicon dioxide from tetraethylorthosilicate (TEOS) by decreasing the temperature during deposition. The two‐step process path chosen for our calculations is not optimal; however, it can be easily implemented and modified. The time saved depends on the reaction kinetics, but approaches 50% of the constant rate process time for step coverages above 95%.

Experimental techniques, data treatment for studying the dynamics of ethanol production/consumption in baker's yeast

The dynamics of ethanol production/consumption in baker's yeast were studied under feed- rate controlled conditions. The yeast was grown on molasses in an 8-l fed-batch reactor and experiments were done at cell concentrations between 5 and 65 g l −1. Small changes in the feed rate were made around a feed rate corresponding to the critical growth rate, at which the yeast cell metabolism switches between ethanol consumption and production. A membrane gas sensor was used for on-line measurement of the ethanol concentration in the broth. The measured ethanol signal was used for control and the system was excited through changes in the regulator set-point. The closed-loop experiments ensured that feed variations were within the critical range, and thus facilitated reproducible experiments. Data were fitted to a second-order difference equation by statistical methods. Results were compared with a theoretically derived model. The process gain could be understood in terms of the underlying stoichiometry by using the “bottleneck” view of yeast glucose metabolism. The process time constant was found to be longer than is implied by a simple Monod relation between glucose uptake rate and concentration.

Multiple actions of substance P that regulate the functional properties of acetylcholine receptors of clonal rat PC12 cells.

1. The effects of substance P (SP) on each of the kinetic components of reversible desensitization (measured at 4 degrees C) and also on irreversible deactivation (measured at 22 degrees C) of the nicotinic acetylcholine receptor on PC12 cells were examined by 22Na+ influx measurements of the functional state of the receptor. 2. In the absence of agonists, SP converts the acetylcholine receptors in a time- and concentration-dependent manner, to a state that is not responsive to agonist. Upon removal of the peptide, this effect was reversible and the kinetics of the recovery of the permeability response were analysed to provide further characterization of the non-responsive state. Following exposure of cells to SP (10 microM) for 3 or more min, recovery was by a first-order process (time constant, t1/2 = 2.1 min), the same value, within experimental error, as that observed for recovery measured after the initial rapid phase of agonist-mediated desensitization. 3. In the presence of agonist, SP caused a strong enhancement of both the rate and extent of agonist-mediated desensitization. This effect was observed even at concentrations of peptide which produced only a small extent of desensitization when incubated alone. For 500 microM-carbamylcholine, the equilibrium level of desensitization (approximately 85% loss of the permeability response) was achieved at 4 degrees C in about 20 min by a biphasic process, while in the presence of 1.0 microM-SP, complete (100%) desensitization occurred by a single rapid exponential phase characterized by a t1/2 of 20 s. 4. The concentration of carbamylcholine required to produce half-maximal desensitization at equilibrium, Kdes, was 94 microM and was reduced by 6-fold in the presence of 0.3 microM-SP. 5. A mechanistic model is presented in which the receptor is viewed as existing in a dynamic conformational equilibrium between an activatable state Rc and the initial desensitized state Rd. It is proposed that SP binds preferentially to the Rd state and thus can allosterically (1) stabilize the receptor in the absence of agonist in that state, and (2) enhance, in an even lower concentration range, both the rate and extent of agonist-mediated stabilization of the receptor in the Rd state. 6. The second, slower component of agonist-mediated desensitization is, in contrast, inhibited by SP. This desensitization step appears to involve a covalent modification of the initial desensitized state (Rd) and is dependent on Ca2+. SP may exert this inhibitory effect by limiting the access of Ca2+ to an intracellular site of action.(ABSTRACT TRUNCATED AT 400 WORDS)

On-line estimation of time delay and continuous-time process parameters†

A simple technique is presented for on-line estimation of constant or slowly-varying continuous-time process parameters and time delay. The method is shown to allow considerable flexibility for application to systems of varying complexity. A major advantage of the algorithm lies in its ability to track time-delay variations over a practically unlimited range. The technique is based on approximation of time delay in the frequency domain by a rational transfer function, construction of the derivatives of process input and output using multiple filters, and estimation using a model which is non-linear in the desired parameters. In spite of this inherent non-linearity with respect to the sought parameters, in general the estimation schemes lead to the true, unique solution. The cases when this is not true are shown not to be of serious consequence.

Action potential prolongation and induction of abnormal automaticity by low quinidine concentrations in canine Purkinje fibers. Relationship to potassium and cycle length.

Marked QT prolongation with induction of polymorphous ventricular tachycardia ("Torsades de Pointes") is a well-described phenomenon during quinidine therapy, frequently occurring at low plasma quinidine concentrations, low serum potassium, and slow heart rates. We have therefore assessed the dose-electrophysiological effects of quinidine as a function of extracellular potassium and cycle length in canine Purkinje fibers, using standard microelectrode techniques. Quinidine (1 microM) prolonged action potential at 90% repolarization, while leaving phase zero upstroke slope (Vmax) unchanged at a cycle length 300-8000 msec; at 10 microM, Vmax depression became evident. Increases in the action potential at 90% repolarization were most marked at long cycle lengths and low extracellular potassium (in contrast to Vmax depression) and were partially reversed by tetrodotoxin (1 microM). The relationship between log of cycle length and action potential at 90% repolarization was linear (for cycle length 300-8000 msec) in the absence of quinidine. Quinidine increased the slope of this relationship in a concentration-related fashion, whereas increasing extracellular potassium shifted the curve rightward (without changing slope), regardless of the presence or absence of quinidine. Action potentials were also measured following pauses of 5-60 seconds. In the absence of quinidine, the action potential depolarization returned to its baseline value in a monoexponential fashion (time constant 36.0 +/- 4.9 sec, mean +/- SE, n = 10). In the presence of 1 microM quinidine, this return was better fit as a biexponential process (time constants 4.2 +/- 1.2 and 40.7 +/- 6.2 seconds, n = 14). At slow stimulation rates (cycle length greater than 4000 msec) in low extracellular potassium (2.7 mM), quinidine produced early afterdepolarizations in 7/14 (50%) of fibers at 1 microM and 14/18 (78%) at 10 microM. Early afterdepolarizations were eliminated by increasing stimulation rates, raising the extracellular the extracellular potassium concentration to 5 mM, or adding tetrodotoxin. These data suggest that prolongation by quinidine of action potentials at 90% repolarization is multifactorial, with both a "tonic" prolonging effect and a prominent frequency-dependent action potential shortening effect. At long cycle lengths and low extracellular potassium, low quinidine concentrations consistently produced early after-depolarizations. The parallels between this form of triggered activity and clinical arrhythmias induced by quinidine suggest that early afterdepolarizations may play a role in quinidine-induced Torsades de Pointes.

Temperature dependence of enhancement and diminution of tension evoked by staircase and by tetanus in rat muscle.

1. The effect of temperature (20-37.5 degrees C) on the potentiation of twitch tension was examined during and after the staircase (250 stimuli, 5/sec) and after the tetanus (188 stimuli, 125/sec) in the extensor digitorum longus muscle of adult Lewis rats.2. During the staircase at 20 degrees C the twitch tension decreased (negative staircase) by 10-20%. At 25-30 degrees C the staircase was initially negative and later positive. At 37.5 degrees C the staircase was positive throughout the train. Both at the end of the staircase and 2 sec after the tetanus the potentiation increased linearly with increasing temperature.3. After the staircase and the tetanus at 20-30 degrees C the twitch tension increased initially rapidly and later after the staircase at a slower rate. Maximal potentiation at 20 degrees C was attained 3 min after the staircase (+ 30 +/- 3%, n = 10, s.e. of mean) and 1 min after the tetanus (+ 16 +/- 1%, n = 10, s.e. of mean). At 37.5 degrees C the potentiation decayed rapidly after the staircase and the tetanus.4. During the staircase the time course of the twitch was shortened twice as much at 20 as at 37.5 degrees C. At the end of the staircase and 2 sec after the tetanus the contraction time was the more prolonged the greater the potentiation. At maximal potentiation the contraction time was prolonged three times as much at 20 degrees C (+ 19 +/- 3%, n = 10, s.e. of mean) as at 37.5 degrees C (P < 0.005). The half-relaxation time at the end of the staircase was prolonged 10 times more at 20 than at 37.5 degrees C (P < 0.02).5. When extrapolated to time zero after the staircase and the tetanus the potentiation at 20 degrees C was still marked (20-50%). The rate of decay of potentiation (time constant, 20 degrees C, 561.2 +/- 37.4 sec, n = 20, s.e. of mean) increased with increasing temperature (Q(10) = 2.6). The event of potentiation with a fast rate of decay, present after the tetanus but not after the staircase at 37.5 degrees C, was abolished below 30 degrees C.6. The increase in twitch tension after the staircase and the tetanus at 20-30 degrees C was taken to indicate the recovery of events that diminished the twitch, occurring simultaneously with potentiation.7. (i) One process of diminution, present after the staircase but not after the tetanus, increased on cooling and was assumed to be due to fatigue. The rate of recovery of the process (time constant, 20 degrees C, 79.6 +/- 7.4 sec, n = 10, s.e. of mean) increased with increasing temperature (Q(10) = 1.9). The half-relaxation time of the last twitch in the staircase was the more prolonged the greater the process. (ii) Another process causing diminution was present after the staircase and the tetanus at 20-30 degrees C. It recovered at 20 degrees C with a time constant of 14.9 +/- 2.2 sec (n = 10, s.e. of mean). This process, possibly responsible for the initially negative staircase, was not thought to be due to fatigue. It may reflect a diminished depolarization of the transverse tubules by repetitive stimuli.

Application of Microcomputers to Automatic Weld Quality Control

The demand for increased productivity of welding operations has led to the expanded use of computer control to allow higher production rates while maintaining weld quality. The charateristics of the gas metal arc welding process and the relationship between welding parameters, the desired output of the welding process, and the automation of the process are discussed. A strategy for two-axis welding torch positioning and velocity control is developed based on preview control techniques. To evaluate the applicability of the proposed control method the motion of heat source along different welding paths is simulated on an analog computer with on-line control of process time constants by an LSI-11 microcomputer. The simulation results show that high quality seam tracking can be accomplished by controlling the torch motion using the proposed method. The method appears to be suitable for on-line control of welding processes.

Approximate Evaluation of Process Time Constants and Dominant Variables by Quasidiagonalization

Approximate Evaluation of Process Time Constants and Dominant Variables by Quasidiagonalization

The study of photoconduction of artificial lipid membranes incorporating rhodopsin. The simultaneous changes of membrane conduction and rhodopsin fluorescence

The protein fluorescence changes of rod outer segment fragments during bleaching were studied. Flash caused a fluorescence intensity drop by about 6%. The time constant of this process was approximately 30 msec and coincided with the time constant of increasing the permeability of an artificial lipid membrane containing rhodopsin and of Metarhodopsin I decay. In the presence of hydroxylamine the fluorescence intensity increases after the initial drop. The second process time constant was about 300 msec and coincided with the conduction drop time constant of the artificial membrane containing rhodopsin. A new intermediate -- Metarhodopsin II1 is proposed. It has the Metarhodopsin II absorption spectrum, lives for about 300 msec at room temperature, does not react with hydroxylamine, and increases the permeability of a disk membrane.

Microstructure of hot ? pressed molybdenum carbide

1. A study was made of the effect of temperature on the mean size and the density and type of distribution of grains in the hot pressing of molybdenum carbide. 2. Raising the hot-pressing temperature in the range 1900–2200°C at constant process pressure and time is accompanied by an appreciable growth in the mean size of Mo2C grains. At the same time, both the symmetry of grain size distribution and the grain size spectrum increase. 3. The energy of activation for the growth of Mo2C grains, which has been found to be 49 ± 3 kcal/ mole, characterizes collective recrystallization for pressing under the conditions employed.