Mode of control

Classical Feedback Control. The majority of controllers ia a continuous process plant is of the linear feedback controller type. These controllers utilize one or more of three basic modes of control proportional (P), iategral (I), and derivative (D) action (1,2,6,7). In the days of pneumatic or electrical analogue controllers, these modes were implemented ia the controller by hardware devices. These controllers implemented all or parts of the foUowiag control algorithm ... 

The objective of crusher control is usually to maximize crusher throughput at some specified product size, without overloading the crusher. Usually onlv three variables can be adjusted feed rate, crusher opening, and feed size in the case of a secondary crusher. Four modes of control for a crusher are (1) Setting overload control,... 

This is a mode of control that causes the output of a controller to change in a linear fashion to the error signal. 

This is a mode of control that anticipates when a process variable will reach its desired control point by sensing its rate of change. This allows a control change to take place before the process variable overshoots the desired control point. You might say that derivative control gives you a little kick ahead. 

The airflow rate to each room is controlled by a damper operated as a function of the room temperature. Sensors that measure the COt concentration in the room air provide an extra mode of control, in addition to that of air temperature. 

The choice of the mode of control of heating plant can have a considerable effect on the efficiency of the process. The process itself may dictate the controls. For example. 

The flow controllers are often used to set desired flows for the fresh feed, stripping steam, and dispersion steam. Each flow controller usually has three modes of control manual, auto, and cascade. In manual mode, the operator manually opens or closes a valve to the desired percent opening. In auto mode, the operator enters the desired flow rate as a set-point. In cascade mode, the controller set-point is an input from another controller. 

Cells in the developing imaginal discs, which form much of the adult body during metamorphosis, exhibit a cell cycle that incorporates both modes of control described above (Fig. 2C). Each imaginal disc has 10-50 cells in the newly... 

The mode of control is the manner in which a control system makes corrections relative to an error that exists between the desired value (setpoint) of a controlled variable and its actual value. The mode of control used for a specific application depends on the characteristics of the process being controlled. For example, some processes can be operated over a wide band, while others must be maintained very close to the setpoint. Also, some processes change relatively slowly, while others change almost immediately. 

Deviation is the difference between the setpoint of a process variable and its actual value. This is a key term used when discussing various modes of control. 

Four modes of control commonly used for most applications are ... 

Each mode of control has characteristic advantages and limitations. The modes of control are discussed in this and the next several sections of this module. 

Three terms commonly used to describe the proportional mode of control are proportional band, gain, and offset. 

Offset, also called droop, is deviation that remains after a process has stabilized. Offset is an inherent characteristic of the proportional mode of control. In other words, the proportional mode of control will not necessarily return a controlled variable to its setpoint. 

Bumpless transfer is the smooth transfer of a controller from one operating mode to another. The balance (BAL) position provides this smooth transfer when transferring the controller from the automatic to manual mode. In the BAL position, the controller is still in the automatic mode of operation, but the deviation meter now indicates the difference between outputs of manual and automatic modes of control. The manual output is adjusted until the deviation meter shows no deflection. Now, the controller can be transferred smoothly from automatic to manual. 

Mode selection buttons. Located directly under each mode indicating light, each button will select its respective mode of control. If the button below the "M" mode light is depressed, the controller will be in the manual mode of operation if the button below the "A" mode light is depressed, the controller will be in the automatic mode of operation. 

Figure 4. Schematic representations of possible modes of controlling permeability through an oriented membrane by host-guest complexation. (a) Control of permeability through intermolecular voids, (b) Control of permeability through intramolecular channels (reprinted with permission from Anal. Chem. 1993, 65,928. Copyright 1993 American Chemical Society).

Table 1 Comparison Between the Manual and the Automatic Mode of Controlling the Moist Agglomeration Process...

Of course, we may also control the wet-gas drum pressure very nicely by the spillback PC valve, shown in Fig. 28.5. But this mode of control causes N, the number of moles, in Eq. (28.2), to rapidly increase. And the motor amps will increase just as fast as the number of moles. 

When processes are subject only to slow and small perturbations, conventional feedback PID controllers usually are adequate with set points and instrument characteristics fine-tuned in the field. As an example, two modes of control of a heat exchange process are shown in Figure 3.8 where the objective is to maintain constant outlet temperature by exchanging process heat with a heat transfer medium. Part (a) has a feedback controller which goes into action when a deviation from the preset temperature occurs and attempts to restore the set point. Inevitably some oscillation of the outlet temperature will be generated that will persist for some time and may never die down if perturbations of the inlet condition occur often enough. In the operation of the feedforward control of part (b), the flow rate and temperature of the process input are continually signalled to a computer which then finds the flow rate of heat transfer medium required to maintain constant process outlet temperature and adjusts the flow control valve appropriately. Temperature oscillation amplitude and duration will be much less in this mode. 

Figure 3.3. Response of various modes of control to step input (Eckman, Automatic Process Control, Wiley, New York, 1958).

Figure 3.6. Some modes of control of liquid level, (a) Level control by regulation of the effluent flow rate. This mode is externally adjustable, (b) Level control with built in overflow weir. The weir may be adjustable, but usually only during shutdown of the equipment, (c) Overflow weir in a horizontal kettle reboiler. The weir setting usually is permanent.

Figure 3.9. Steam heaters, (a) Flow of steam is controlled off the PF outlet temperature, and condensate is removed with a steam trap or under liquid level control. Subject to difficulties when condensation pressure is below atmospheric, (b) Temperature control on the condensate removal has the effect of varying the amount of flooding of the heat transfer surface and hence the rate of condensation. Because the flow of condensate through the valve is relatively slow, this mode of control is sluggish compared with (a). However, the liquid valve is cheaper than the vapor one. (c) Bypass of process fluid around the exchanger. The condensing pressure is maintained above atmospheric so that the trap can discharge freely, (d) Cascade control. The steam pressure responds quickly to upsets in steam supply conditions. The more sluggish PF temperature is used to adjust the pressure so as to maintain the proper rate of heat transfer.

Three modes of control of air flow are shown in Figure 3.3(e). Precautions may need to be taken against subcooling to the freezing point in winter. 

Recent studies indicate that the various phosphohydrolase and phosphotransferase activities of glucose-6-phosphatase are affected by numerous metabolites (see Table X and Sections II,C and III,D,4). The possible significance of observed activation or inhibition by a number of these compounds in vitro relative to regulation of both types of activity of the enzyme in vivo has been considered in a number of instances. Possible modes of control of net glucose release, involving the regulation by a variety of factors, of both hydrolytic and synthetic activities of the enzyme, have been discussed in considerable detail in earlier reviews by the author (9, 10). 

There are three principal types or modes of control action which are more generally employed, viz. proportional (P), integral (I), and derivative (D). In the first the controller produces an output signal J which is proportional to the error, i.e. ... 

PID action should be employed for systems which respond rather sluggishly. The presence of the derivative term allows a higher proportional gain to be used which will speed up the control action. Such controllers are frequently installed because of their versatility and not because analysis of the system has indicated the need for the presence of all three modes of control. 

One of the most controversial topics in the recent literature, with regard to partition coefficients in carbonates, has been the effect of precipitation rates on values of the partition coefficients. The fact that partition coefficients can be substantially influenced by crystal growth rates has been well established for years in the chemical literature, and interesting models have been produced to explain experimental observations (e.g., for a simple summary see Ohara and Reid, 1973). The two basic modes of control postulated involve mass transport properties and surface reaction kinetics. Without getting into detailed theory, it is perhaps sufficient to point out that kinetic influences can cause both increases and decreases in partition coefficients. At high rates of precipitation, there is even a chance for the physical process of occlusion of adsorbates to occur. In summary, there is no reason to expect that partition coefficients in calcite should not be precipitation rate dependent. Two major questions are (1) how sensitive to reaction rate are the partition coefficients of interest and (2) will this variation of partition coefficients with rate be of significance to important natural processes Unless the first question is acceptably answered, it will obviously be difficult to deal with the second question. 

Chemical reactions conducted within microemulsions occur with the same modes of control afforded by micelles. Thus, photoactivity of semiconductors [75] and metal colloids [76] formed within microemulsions are maintained, as is the capacity for initiation of photooxidative polymerization [77],... 

In a feedback configuration the controlled variable (temperature) has to be upset before correction can take place. Feedforward is a mode of control that corrects for a disturbance before it can cause an upset. Figure 2.108 illustrates feedforward control of a steam heater. The feedforward portion of the loop detects the major load variables (the flow and temperature of the entering process fluid) and calculates the required steam flow (Ws) as a function of these variables. When the process flow increases, it is matched with an equal increase in the steam flow controller set point. Because instantaneous response is not possible, dynamic correction by a lead-lag element is provided. 

Globin is synthesized in reticulocytes (see Chap. 1, Prob. 1.1). which have no nucleus and therefore cannot utilize transcriptional and other potential modes of control. Control of globin synthesis from the pool of globin-enriched mRNA is geared to the concentration of hemin (Fe(III)-protoporphyrin]. which has the ability to inactivate a translational inhibitor of protein synthesis. The inhibitor is a protein kinase that phosphorylates and inactivates one of the initiation factors involved in initiation of translation. When the concentration of hemin is high, it binds to a regulatory subunit of the kinase and. as a result, initiation of globin synthesis can proceed. 

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