High-frequency process disturbances

High-frequency process disturbances. The frequency of these disturbances is beyond the capability of the control system to effectively react. 

Measurements are often corrupted by high frequency, zero-mean, random variations (i.e., measurement noise). These variations can be caused by high-frequency disturbances from the process (e.g., vortices shedding in flow measurement) or electrical interference during signal transmission. Plant measurements should be filtered to reduce the impact of high-frequency variations on the quality of the estimated parameters and as a result, the predicted optimal plant operation. 

The equation derived for the transport of surfactant ions through the DL describes the adsorption kinetics as a reversible process. The qualitatively new result in the theory of ionic adsorption kinetics is the incorporation of electrostatic retardation for both the adsorption and desorption process, which is of essential importance for processes close to equilibrium. Such a situation exists at harmonically disturbed surfaces, used in investigations of adsorption dynamics like the damping of capillary waves or oscillating bubbles. At sufficiently high frequencies the diffusion layer becomes very thin and the adsorption-desorption exchange is controlled only by the ion transport through the DL, i.e. by the electrostatic retardation. At... 

The process sensitivity function for the modified reactor-separator problem is shown in Fig. 6. As seen from the figure, the recycle feedback increases the disturbance sensitivity by a factor 4,2 at low frequencies, while it in fact serves to slightly dampen the disturbance sensitivity, i.e., Sp < 1, at high frequencies. 

The Bode plots are useful, since they can be used in controller tuning, which will be discussed in chapter 32. They also are useful if we know the fi equency of the disturbance that the process is subjected to. It can easily be seen that high fi equency disturbances have less impact on the process than low frequency disturbances. Controller tuning also depends on the disturbance frequency as will be shown later. 

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