The Dual-mode Concept

What would you like a controller to do, forgetting for the moment what it is reasonable to expect The most severe demand would certainly be to follow a step change in set point perfectly. This could be demanded of the controller, but not of the process, because it requires infinite process gain. The speed at which a variable may change is limited by the maximum rate at which energy may be delivered to the process. A valve may only open fully, not infinitely. Therefore it can only be asked that the controller not interfere with the maximum speed of the process. To duplicate the remainder of the step input, the control loop must be stable to the point that no overshoot or oscillation is observable. Nor should there be any offset. Finally, the controller ought to be insensitive to input noise, which is usually present in some form. T0 

With regard to difficult processes, control functions which approach the demands of the four points of performance listed above need to be set forth  

Maximum speed implies that the controller be saturated for any measurable deviation. Such a demand limits the selection to an on-off controller. But if the tolerance may be widened somewhat, then the controller need only saturate in response to a large signal. (How large the signal must be will Vary with the difficulty of the process.) 

Critical damping can be achieved by both low gain and derivative action, but the latter amplifies noise. Critical damping implies an asymptotic approach to the set point. To accommodate maximum speed, the zone of critical damping must be restricted to a narrow band about the set point. Therefore this criterion and its solution apply specifically to small-signal response. 

Zero offset requires a controller with infinite gain, in the steady state. An integrator supplying reset action is sufficient to satisfy this criterion. 

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At the empirical level, alternative ways of representing the data (which may or may not utilize the dual mode concept) are possible 33,34). Accordingly, interest in the model embodied in Eqs. (5), (8) and (9) centers primarily around the question of its physical meaningfulness and significance, as well as its predictive potential. The results of recent research will be discussed below mainly from this point of view. 

Application of the dual mode concept to gas diffusion in glassy polymers was originally subject to the limitation that DT2 = OinEq. (6) ( total immobilization model )6-Later this simplifying assumption was shown to be unnecessary, provided that suitable methods of data analysis were used 52). Physically, the assumption DX2 = 0 is unrealistic, although it is expected that DT2 < DX1 52). Hence, this more general approach is often referred to as the partial immobilization model . 

NR and — CO groups, measured after prior washing of the resin with water 85), was well represented by a Langmuir + Nernst dual mode sorption model at salt concentrations not exceeding 0.2 mol dm 3. A detailed physical interpretation of the relevant parameters was not given, however, neither was the dual mode concept utilized in a corresponding diffusion study 86). 

Studies of purified humic substances are also supportive of the dual-mode concept. Sorption of a PAH apparently depends not only on its hydrophobicity but also on its steric ability to fit into cavities in NOM (13). The abundance and microviscosity of the cavities (23,24) seems to increase with ionic strength, and divalent metal ion concentration. Schulten (25,26) has performed molecular mechanics calculations for a hypothetical humic macromolecule in the presence or absence of water. He found that small molecules—atrazine, pentachlorophenol, a peptide, and a trisaccharide— bind in nanometer-size voids in the humic structure. With continued energy minimization, the voids shrink and become more hindered. 

FIGURE 20.4-6 Schematic representation of the dual-mode concept. The sorption <4>setved during the first exposure to the penetrant is often considerably different from that observed after the isotherm has settled at the highest pressure of measurement. The conditioning process associated with the first exposure to the penetrant is believed to be associated with redistribution of unrelaxed volume elements in the polymer. 

Figure 3. Schematic representation of the dual mode sorption concept.

The great majority of CEC—MS applications is run in the reversed-phase mode using alkyl-bonded silica stationary phases [12,14,24,26,38,45,81,82,98-104], The dual functionality concept is represented in these stationary phases by the alkyl chains, most frequently octadecyl chains, that constitute the top retentive layer, and residual silanol groups on the surface, that dissociate at pH values higher than 3-4 and... 

The dual-mode model has also been applied with some success to the sorption and transport behavior of small- and intermediate-size organic molecules in glasses (75). Above some size, however, the penetrant may be too large to be accommodated solely within individual holes, and two truly distinct environments may not actually exist for these penetrants. Even in this case, however, the concept of unrelaxed free volume remains valid, and observations of sorption and dilation behavior that apply to gases also apply qualitatively to larger penetrants up to the point of extreme plasticization where an actual glass transition occurs. 

Another interesting approach to obtaining potent, broad-spectmm activity has been reported (127). The "dual-action" antibacterial concept involves incorporation of two moieties having complimentary antibacterial modes of action into the same molecule, and uses the mode of action of one part to release the second antibacterial at the site of action. This approach is exemplified in Ro 23—9424 (47) (127), which uses the mode of action and reactivity of the cephalosporin moiety (Fig. 2) to release the quinolone portion. 

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... 

Detection of glass transition temperatures in several NOMs has further supported the concept of dual mode sorption, a phenomenon occurring in synthetic... 

The concept of dual-mode stimulation can be expanded further. It is possible to devise systems capable of existing in several forms (multistate) that can be intercon-verted by different external stimuli (multifunctional). Such systems can give rise to intricate networks of reactions that, when examined from the viewpoint of molecular-level devices"1311 reveal very interesting properties.1321... 

The concept of two or mote modes of sorption of penetrants in polymers is very familiar to cellulose and protein chemists for the case of water vapor. In fact combined Lan uir and Henry s law sorption was proposed and correctly formulated by Matthes in 1944 for water in cellulose The discovery of dual mode sorption of gases in assy polymers and the aibsequent realization that diffusion constants determined by the time lag method did not have the same ple fundamental significance a ociated whh these parameters for mbbery polymers was of profound importance. Not only were the many carefully determined diffusion coefficients in the literature of questionable value for polymers below their ass transition but a good deal of the careful peculation about solution and diffusion and the effect of... 

The concept of dual mode sorption was first dearly demonstrated and quantified by Michaels, Vieth and Barrie in 1963 The same authors also discussed its effect on the diffusion process itself. Vieth and his co-workers aibsequently extended these findings to a number of polymer-gas systems and elaborated the theoretical aspects of the problem In particular, a model for diffusion in glassy polymers, which has come to be known as the totd inunobilization model, was developed by Vieth and Sladek ... 

Systems Analysis Figure 1 shows a concept identified by NETL for a integrated fuel processor/ fuel cell system targeted for diesel APUs. There are several favorable attributes of this system. For example, startup occurs by firing an internal combustor in the dual reactor reformer. This provides heat to the ATR reformer (via conduction) as well as supplying heat to the fuel cell cathode via direct exhaust from the combustor or preheated air from the heat exchanger (optional). If necessary, the ATR is fired in a partial oxidation mode to aid in heatup and to provide heat to the anode side of the... 

Walter, B. and Gatellier B. (2002) Development of the high power NADI concept using dual mode diesel combustion to achieve zero NOx and particulate emissions. SAE Paper 2002-01-1744. 

Unless identical display modes are adopted in both T- and R-modes, otherwise there are always some discrepancies between their voltage dependent transmittance and reflectance curves. This is the reason that none of the abovementioned transflective mixed-mode LCDs has perfectly matched voltage dependent transmittance and reflectance curves. Different from the mixed display modes employed between transmission and reflection regions as described above. Sharp Corp. also introduced the dual-cell-gap concept for transflective LCDs [57]. 

The concept of the dual-layer LNT-SCR system is qualitatively similar to the dual-bed LNT-SCR system. In this case, NH3 is produced in the bottom layer and before diffusing back to the gas phase, it is being stored in the top SCR layer. The stored NH3 is subsequently used to reduce NO during lean mode. The dual-layer catalyst simulation predicts lower conversion efficiencies compared to the dualbed suggesting that the SCR functionality of the top layer is not as efficient as in the case of the separate downstream brick. 

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