Dual-mode systems

The dual-mode system gives the best set-point response attainable. 

The dual-mode system needs six adjustments, which fall into two... 

It has been demonstrated that a loop whose gain varies inversely with amplitude is prone to limit-cycle. Any controller with similar characteristics can promote limit cycling in an otherwise linear loop. On-off controllers are in this categoiy. So any nonlinear device that is purpose inserted into a loop for the sake of engendering stability must have the opposite characteristic gain increasing with amplitude. The only stabilizing nonlinear devices discussed up to this point have this property -it was manifested as a dead zone in the three-state controller and as the linear mode in the dual mode system. 

It is possible to create a controller with a continuous nonlinear function whose gain increases with amplitude. In contrast to the three-state controller, its gain in the region of zero error would be greater than zero, with integrating action to avoid offset. But its change in gain with amplitude should be less severe than that of a dual-mode system. Thus it would be more tolerant of inaccuracy in the control parameters. 

A batch chemical reactor is to be brought up to operating temperature with a dual-mode system. Full controller output supplies heat through a hot-water valve, while zero output opens a cold-mater valve fully at 50 percent output, both valves are closed. While full heating is applied, the temperature of the batch rises at, IT/min the time constant of the jacket is estimated at 3 min, and the total dead time of the system is 2 min. The normal load is equivalent to 30 percent, of controller output. Estimate the required values for the three adjustments in the optimal switching program. 

FIG 10.19. The dual-mode system requires different values of preload for reaction and cure... 

Many IC techniques are now available using single column or dual-column systems with various detection modes. Detection methods in IC are subdivided as follows [838] (i) electrochemical (conductometry, amper-ometry or potentiometry) (ii) spectroscopic (tJV/VIS, RI, AAS, AES, ICP) (iii) mass spectrometric and (iv) postcolumn reaction detection (AFS, CL). The mainstay of routine IC is still the nonspecific conductometric detector. A significant disadvantage of suppressed conductivity detection is the fact that weak to very weak acid anions (e.g. silicate, cyanide) yield poor sensitivity. IC combined with potentiometric detection techniques using ISEs allows quantification of selected analytes even in complex matrices. The main drawback... 

It has been recently demonstrated that squaraine dye 3, first synthesized by Treibs and Jacob [40], acts as a dual mode recognition system for serum albumin... 

An additional feature of CCC is its ability to be used in either normal or reversed-phase elution with the same two-phase partition solvent system (dual mode). Both polar and nonpolar compounds are certain to be retrieved in a single chromatographic run. These features prompted us to use CCC as the initial fractionation step for active microbial extracts. 

Membrane-penetrant systems, whose sorption and diffusion properties can be described by Eqs. (5)-(7) with N = 2 ( dual mode sorption and diffusion models ) have attracted much interest. The most important examples of such systems are considered in the next two sections. 

Earlier work on the application of the concept of dual mode sorption and diffusion to glassy polymer-gas systems has been reviewed in detail 6) and important aspects of more recent work have been dealt with in more recent reviews 7 10). Eq. (5) was first applied by Michaels et al U). Sorption in the polymer matrix and in the specific sorption sites was represented by linear (Henry s law) and Langmuir isotherms respectively so that Sj in Eq. (5) is given by... 

Application of the dual mode sorption and diffusion models to homogeneous polymer blend-gas systems 26,65) and filled polymers 66) has also been reported. 

It is particularly interesting and instructive to note that application of Henry + Langmuir dual-mode sorption and diffusion models is not confined to glassy polymer-gas systems. Sorption and transport of high affinity ionic species, exemplified by anionic dyes, in charged polymers, exemplified by polyamides at low pH, has been treated in the same way. These systems are of considerable importance both from the bio-mimetic and from the textile processing point of view, but have received limited atten-... 

Each heliostat is designed to illuminate a specific area on the parabola and is equipped with a dual optical control system, which maintains the proper orientation for each hcliostat by means of a dual hydraulic system. This dual system permits each heliostat lo be operated in either a search or track mode. In both cases, the optical guidance system uses an optical tube, which contains four photodiodes that control the heliostat motion in east-west and up-down direction. 

As pointed out above, IC is a well-established method for the analysis of inorganic anions and has become the method of choice in many application areas. Many techniques are available using singlecolumn [46] or dual-column systems with various detection modes. IC can be used both for analytical and preparative purposes. Large sample volumes, up to 1300 pul, can be injected to determine trace anions and cations and to attain detection limits of 10-400 ng/1. For determinations at a pig/1 to mg/1 level, a sample size of 10-50 xl is sufficient. Preconcentration is necessary for lower concentrations (an additional column, a sample pump, an extra valve and an extra time are the disadvantages of this approach [47]). With an IEC column and isocratic... 

Electro-photoswitching devices possess a dual mode operation resulting from different electrochemical properties in the two photo-interconvertible states. They may display both photochromism and electrochromism. They allow the photomodulation of electrochemical properties as was shown to occur in photochromic dihydroazulene derivatives [8.247]. Separate photochemical and electrochemical interconversion has been realized in systems containing a photoactive azo [8.248, 8.249] or dihydroazulene [8.250] unit appended with an anthraquinone moiety. 

The pressure dependence of the concentration of sorbed gas was consistent with the dual mode model while the relaxation data addressed itself to the validity of the assumptions made by the model. The assumption of rapid interchange was found to be valid for this system while the assumption of an immobile adsorbed phase could introduce a small error in the analysis It should be possible to reduce this error by more exact measurements of the concentration of sorbed gas as classical pressure experiments could... 

Section IIA summarizes the physical assumptions and the resulting mathematical descriptions of the "concentration-dependent (5) and "dual-mode" ( 13) sorption and transport models which describe the behavior of "non-ideal" penetrant-polymer systems, systems which exhibit nonlinear, pressure-dependent sorption and transport. In Section IIB we elucidate the mechanism of the "non-ideal" diffusion in glassy polymers by correlating the phenomenological diffusion coefficient of CO2 in PVC with the cooperative main-chain motions of the polymer in the presence of the penetrant. We report carbon-13 relaxation measurements which demonstrate that CO2 alters the cooperative main-chain motions of PVC. These changes correlate with changes in the diffusion coefficient of CO2 in the polymer, thus providing experimental evidence that the diffusion coefficient is concentration dependent. 

A number of attempts have been made to explain the nonlinear, pressure-dependent sorption and transport in polymers. These explanations may be classified as "concentration-dependent (5) and "dual-mode (13) sorption and transport models. These models differ in their physical assumptions and in their mathematical descriptions of the sorption and transport in penetrant-polymer systems. 

In the dual-mode sorption and transport model the pressure-dependence of a (= C/p), P and 0 in gas-glassy polymer systems arises from the pressure-dependent distribution of the sorbed gas molecules between Langmuir sites and Henry s law dissolution. Although k, Dg and are assumed to be constant, the average or effective solubility and diffusion coefficients of the entire ensemble of gas molecules change with pressure as the ratio of Henry s to Langmuir s population, C /C, changes continuously with pressure [eq. (14)]. 

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