1 Multiple-loop Systems in .NET framework

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224 1 Multiple-loop Systems
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Tz - TI. This is another argument, in favor of using feedback to t,rim the feedforward set point. This mutual adaptation is a further indication how perfectly feedforward and feedback complement one anot her. Peedforward is fast, intelligent , and responsive, but also inaccurate; feedback is slow but accurate, and is capable of regulation in t,he face of unknown load condit,ions.
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ECONOMIC CONSIDERATIONS
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Feedback control can be enforced on the heat exchanger using only three elements: transmitter, controller, and valve. Adding feedforward control requires another temperature transmitter, two flow transmitters, two square-root extractors, a steam-flow controller, a summer, a multiplier, and a lead-lag unit-nine items. Such an expense must be justified. Although the heat exchanger serves as a convenient demonstration of feedforward control, only in rare instances would such refined control be justified for this type of application. Jlanagement likes to see investments such as advanced control systems pay for themselves in less than two years. If improved temperature control had no value, there would be no payout. But if it prolonged the life of the exchanger, or saved steam, or reduced maintenance, it would have a measurable worth. Economic incentive for improved control is most likely to be found where consistent quality of a valuable product is important. i\lany unit operations are conducted at reduced capacity or low recovery, or use excessive amounts of utilities to ensure that quality will surpass specifications even with poor control. Naturally, the rate of payout from these sources will vary directly with the production rate of the plant. Large plants therefore encounter reduced risks. But the payout of a control system is as much a factor of cost as of savings. A control system can be too perfect-that is, its cost can be out of proportion to the job to be done. In the control system for the heat exchanger, for example, both flow and inlet temperature were accepted as load changes. If inlet temperature were nearly constant, or subject only to slow variations, it could be eliminated from the system as long as feedback trim were available. This would reduce cost by saving a transmitter and a summer. There are three principal areas where feedforward control can produce results unobtainable with any other technique: 1. A difficult process subject to frequent disturbances may never settle out under feedback control. Load changes occurring at intervals of less than three cycles are sufficient to develop this situation. This is not
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Feedforward
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uncommon on some towers, furnaces, and coupled processing units which may cycle at periods exceeding an hour. 2. In many plants, the variable of interest cannot be measured continuously, accurately, or quickly enough for adequate closed-loop control. Often a secondary variable is used as an inferential measure of the first, simply because it is the best available. In a case like this, however, product quality may suffer both from poor control of the inferential variable and from its indifferent relationship to the primary variable. 3. More interest is developing in the control of economic variables: cost, debits, yield, etc. These variables are not directly measured, and often not computed, to allow the closing of the economic loop. But even if they were available, conventional feedback control could not be used, because the intent is to maximize or minimize their value rather than t,o control at a given set point.
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Optimizing Programs4
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Feedforward control systems are not limited solely to regulatory duties. In fact, the controlled variable may be easily programmed with respect to any measured term, simply by making the appropriate substitution in the process equation. If, for some reason, it were desired to vary the temperature of the liquid leaving the heat exchanger relative to its flow, this could readily be accommodated. To do so, the computing system would become somewhat different but no more difficult to implement than for a constant set point. If feedback trim were used, however, the set point to the feedback controller would have to be similarly programmed, which might increase the complexity of the system significantly. An important observation is that any control program may be followed, even one that would cause the process gain dc/dm to change sign. Hence feedforward is the logical means to achieve optimizing (steady-state adaptive) control. This has already been demonstrated in Chap. 6. The first step in optimizing is the definition of the sources of loss within the plant. The process will generate valuable product somewhat proportional to the rate of material pumped into it, with the result that the profit can be subject to wide variation. If the losses are maintained at a minimum, however, the highest profit will always result, and values and rates of raw materials can be ignored. Losses, or debits, are not difficult to define. They consist principally of utility costs, streams of unrecoverable materials, and lowered market value brought about by contaminants in the product. Debits peculiar to a process may vary with feed composition, catalyst activity, atmospheric conditions, and demands elsewhere within the plant, but feed rate generally exercises the greatest influence upon them. The purpose of
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