Control differences

Feedforward Control If the process exhibits slow dynamic response and disturbances are frequent, then the apphcation of feedforward control may be advantageous. Feedforward (FF) control differs from feedback (FB) control in that the primary disturbance or load (L) is measured via a sensor and the manipulated variable (m) is adjusted so that deviations in the controlled variable from the set point are minimized or eliminated (see Fig. 8-29). By taking control action based on measured disturbances rather than controlled variable error, the controller can reject disturbances before they affec t the controlled variable c. In order to determine the appropriate settings for the manipulated variable, one must develop mathematical models that relate ... 

While all these paramclcrs are extremely essential for a process line, with the R D in the field these limitations have been overcome with the use of phasor controls. To implement the.se controls different manufacturers have tidopled different control and feedback systems to monitor and control the torque and field components. They have also given these controls different trade names. The basic technological concept remains the same but process implementation may vary from one manufacturer to another. Below we attempt to identify the more common phasor controls introduced by a few leading manufacturers. 

They attend to and control different technical plants and systems, and they have to deal with all processes required. In data-based plants, controllers usually communicate with one or several local or central computers. 

Biocides are added to the wet end process to prevent slime formation. Introduction of neutral or alkaline sizing instead of acidic papermaking, the closed water circuits, and the increasing proportions of recycled paper have required changes in biocide types in order to control different microbial populations. 

Proportional gain, integral and derivative time constants to PI and PID controllers. Different settings for load and set point changes. Different settings for different definitions of the error integral. The minimum ITAE criterion provides the least oscillatory response. 

The neutralizer in the previous example might be controlled differently if the main fluctuation in the load occurs in one or two of the streams. Instead of combining all the streams together before they enter the neutralizer, those streams that vary widely might enter an additional holding tank, where they would be neutralized using traditional feedback control. They would then be added to the main neutralizer, which also has a feedback controller. Which system is best can be determined by running an economic analysis (see Chapters 10 and 11). 

As previously discussed, a particular plant will probably have controllers different from the two described here. Although most information provided can be generally applied, it is extremely important that the operator know the specific plant s controllers and their applications. 

Adsorption of nonionic and anionic polyacrylamides on kaolinite clay is studied together with various flocculation properties (settling rate, sediment volume, supernatant clarity and suspension viscosity) under controlled conditions of pH, ionic strength and agitation. Adsorption and flocculation data obtained simultaneously for selected systems were correlated to obtain information on the dependence of flocculation on the surface coverage. Interestingly, optimum polymer concentration and type vary depending upon the flocculation response that is monitored. This is discussed in terms of the different properties of the floes and the floe network that control different flocculation responses. Flocculation itself is examined as the cumulative result of many subprocesses that can depend differently on system properties. 

In the last years, the control based on differential geometry has emerged as a powerful tool to deal with a great variety of dynamic nonlinear systems [11, 15, 26], This control approach allows the transformation of a nonlinear system into a partially or totally linear one, by means of a nonlinear state transformation, which is obtained from directional derivatives of the output. It is important to remark that geometric control differs totally from the linear approximation of dynamics by calculation of the Jacobian. 

The combination of various chemical types of polymer networks in different compositions, resulting frequently in controlled, different morphologies, has produced IPNs with synergistic behavior. Thus, synergistic properties may be obtained by IPNs such as enhanced tensile and impact strength, improved adhesion and, in some cases, greater sound and shock absorption (4-7). 

Flight Control (/ig/stem section) Difference (%) Flight Control Difference (/ig/stem section) (%) ... 

Only authorized personnel should be allowed to handle the material and this handling should be done under the supervision of a responsible person. Access to stored material should be controlled. Different seed lots or cell banks should be stored in such a way to avoid confusion or cross-contamination. It is desirable to split the seed lots and cell banks and to store the parts at different locations so as to minimize the risks of total loss. 

The concentration of all automation functions within a single computer (Section 7.19.1) may be possible for a very simple plant, but this type of configuration is inefficient for more complex processes for which there could be many thousands of connections between plant and computer. Currently, small industrial processes are controlled by a hierarchical architecture consisting of a central computer (usually a minicomputer), which is used to solve central automation problems, together with a series of peripheral computers (generally microprocessors which are called front-end computers) which control different sections of the plant (Fig. 7.104a). This type of architecture is termed a decentralised computer system. 

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