Effects of Lags

The systems explored above illustrate a very important point about the control of openloop unstable systems the control of these systems becomes more difficult as the order of the system is increased and as the magnitudes of the first-order lags increase. 

Our examples above demonstrated this quantitatively. For this reason, it is vital to design a reactor control system with very fast measurement dynamics and very fast heat-removal dynamics. If the thermal lags in the temperature sensor and in the cooling jacket are not small, it may not be possible to stabilize the reactor with feedback control. 

Bare-bulb thermocouples and oversized cooling-water valves arc often used to improve controllability. 

Up to this point we have looked at using proportional controllers on openloop unstable systems. The controllability can often be improved by using derivative action in the controller. An example will illustrate the point. 

E3umple 11.S. Let us take the same third-order process analyzed in Example 11.4. For a t = 5 minutes and a proportional controller, the ultimate gain was 6.17 and the ultimate frequency was 1.18 radians per minute. 

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Biomedical sample analysis relies on appropriate sample collection. Although any result is unlikely to guide the response phase of the incident, the emergency response should include consideration of the collection of these samples. The best results are expected to be generated from samples taken as early as possible, and from patients considered to have had the greatest exposure. There is however little clear data available on the urgency of analysis in relation to the stability of markers, and on the effects of lag-times for bringing samples to the laboratory. 

The meaning of s, g, and c is in the 123 text as well as that of the various rates. Model 4 includes the effects of lag between genera-tions of stars. 

Akande, O.F. Eord, J.E. Rowe, P.H. Rubinstein, M.H. The effects of lag-time and dwell-time on the compaction properties of 1 1 paracetamol/microcrystalline cellulose mixtures prepared by precompression and main compression. J. Pharm. Pharmacol. 1998, 50, 19-28. 

With pulsed or transient illumination, lag also results in a nonlinear detector response to incident light intensity. The effects of lag on the linearity of detector response can be reduced by using multiple readout scans, long dwell times, both resulting in longer readout times, or other schemes (13,14, ID ... 

Figure 3. Relative integrated signal showing the effect of lag as a function of dwell time controlled by channel time. Each channel consisted of500 pixels (delta Y = 500). Signal is due to a single pulse of a Xe strobe lamp (General Radio, 1539-A Stroboslave). Light was detected in zero order with a 20-p.m entrance slit. Dwell time o, 40 ns , 80 ns A, 160 ns ...

Readout scans are then used to prepare the target for exposure. Alternatively, the electron beam potential can be shifted between the preparation and reading potential levels to reduce the effects of lag (13,14,15). 

Zeger SL, Thomas D, Dominici F et al (2000) Exposure measurement citot in time-series studies of air pollution concepts and consequences. Environ Health Perspect 108 419-426 Zeka A, Zanobetti A, Schwartz J (2005) Short term effects of particulate matter on cause specific mortality effects of lags and modification by city characteristics. Occup Environ Med 62 718-725... 

Another convention is to use a dummy variable (often u) instead of x in our functions g(u), h u). Then we use X (instead of p) for the lag or separation. After integration over u we are left with only the variable X to show clearly the effect of lag or separation on our convolution or correlation. 

The death rate coefficient is usually relatively small unless inhibitoiy substances accumulate, so Eq. (24-10) shows an exponential rise until S becomes depleted to reduce [L. This explains the usual growth curve (Fig. 24-21) with its lag phase, logarithmic phase, resting phase, and declining phase as the effect of takes over. 

Results of studies to determine the effect of stator vane control on acceleration are shown in Figures 4-67 and 4-68. The first control step shown in Figure 4-67 is to have the stator vanes normally on flow control with a 2 sec lag in the flow control. On reaching trip, the stator vanes drive to the wide open position after a 1 sec lag. The vanes move at design speed. 

Although the experiments reported by Maurer et al. (1977) were performed for a completely different reason, namely, to study effects of vapor cloud explosions (see Section 6.4), fireballs were nevertheless generated. These experiments involved vessles of various sizes (0.226-1000 1) and containing propylene at 40 to 60 bar gauge pressure. The vessels were ruptured, and the released propylene was ignited after a preselected time lag. One of these tests, involving 452 kg of propylene, produced a fireball 45 m in diameter. 

The rate of heat conduction is further complicated by the effect of sunshine onto the outside. Solar radiation reaches the earth s surface at a maximum intensity of about 0.9 kW/ m. The amount of this absorbed by a plane surface will depend on the absorption coefficient and the angle at which the radiation strikes. The angle of the sun s rays to a surface (see Figure 26.1) is always changing, so this must be estimated on an hour-to-hour basis. Various methods of reaching an estimate of heat flow are used, and the sol-air temperature (see CIBSE Guide, A5) provides a simplification of the factors involved. This, also, is subject to time lag as the heat passes through the surface. 

Fig. 7.1.6 Influence of pH and temperature on the luminescence of Cbaetopterus photoprotein elicited by old dioxane and Fe2+ in 20 mM phosphate buffer. Left panel the effect of pH in phosphate buffer solutions of various pH values, at 22°C. Right panel the effect of temperature at pH 7.2. Luminescence was initiated by the injection of Fe2+. The time lag of the light emission after the Fe2+ injection was also shown in the right panel. From Shimomura and Johnson, 1966.

Another type of NR crosstalk, which has only recently been recognized, is the so-called nongenomic actions of several receptors that induce very rapid cellular effects. Effectively, evidence has accumulated over several decades that steroid receptors may have a role that does not require their transcriptional activation, such as modifying the activity of enzymes and ion channels. While the effects of steroids that are mediated by the modulation of gene expression do occur with a time lag of hours, steroids can induce an increase in several second messengers such as inositol triphosphate, cAMP, Ca2+, and the activation of MARK and PI3 kinase within seconds or minutes. Many mechanistic details of these nongenomic phenomena remain poorly understood. Notably, controversy still exists as to the identity of the receptors that initiate the non-genomic steroid actions. However, it now appears that at least some of the reported effects can be attributed to the same steroid receptors that are known as NRs. 

It is possible to distinguish between direct and indirect nOes from their kinetic behavior. The direct nOes grow immediately upon irradiation of the neighboring nucleus, with a first-order rate constant, and their kinetics depend initially only on the intemuclear distance r" indirect nOes are observable only after a certain time lag. We can thus suppress or enhance the indirect nOe s (e.g., at He) by short or long irradiations, respectively, of Ha- a long irradiation time of Ha allows the buildup of indirect negative nOe at He, while a short irradiation time of Ha allows only the direct positive nOe effects of Ha on He to be recorded. 

A valnable comparison was made of the relative effectiveness of (a) augmentation with a cnltnre containing Dehalococcoides ethenogenes, (b) stimnlation of the natural dechlo-rinated population by addition of lactate and mineral nntrients, and (c) a recircnlation control withont amendment (Lendvay et al. 2003). The hrst procedure rapidly affected complete dechlorination, whereas the second was effective only after a lag period, and in both the hrst and second treatments the population of Dehalococcoides increased. 

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