Feedforward Control in .NET

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t has been shown that the nature of the process largely determines how well it can be controlled: the proportional band, reset and derivative times, and the period of cycling are all functions of the process. Processes which cannot be controlled well because of their difficult nature are very susceptible to disturbances from load or set-point changes. W h e n a difficult process is expected to respond well to either of these disturbances, feedback control may no longer be satisfactory for these reasons: 1. The nature of feedback implies that there must be a measurable error to generate a restoring force, hence perfect control is unobtainable. In the steady state, the controller output will be proportional to the load. When the load changes, the controller output must change. In going from one output to another, a controller must reset, because in each steady state, proportional and derivative offer no contribution. Consequently the net change in output has been shown to be a function of the integrated error: Am = loo e dt PR s
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Any combination of wide band and long reset time (characteristic of difficult processes) results in a severe integrated error per unit load change : Se
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This explains why difficult processes are sensitive to disturbances. 2. The feedback controller does not know what its output should be for any given set of conditions, so it changes its output until measurement and set point are in agreement-it solves the control problem by trial and error, which is characteristic of the oscillatory response of a feedback loop. This is the most primitive method of problem solving. 3. Any feedback loop has a characteristic natural period. - Should disturbances occur at intervals less than about three periods, it is evident that no steady state will ever be reached. There is a way of solving the control problem directly, and this is called feedforward control. The principal factors affecting the process are measured and, along with the set point, are used in computing the correct output to meet current conditions. Whenever a disturbance occurs, corrective action starts immediately, to cancel the disturbance before it affects the controlled variable. Feedforward is theoretically capable of perfect control, notwithstanding the difficulty of the process, its performance only being limited by the accuracy of the measurements and computations. Figure 8.1 is a simplified diagram illustrating the arrangement of the feedforward control system as it has been described. Its essential feature is the forward flow of information. The controlled variable is not used by the system, because this would constitute feedback; this point is important because it shows how it is possible to control a variable without having a continuous measurement of it available. A set point is essential, however, because any control system needs a command to give it direction.
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FIG 8.1. The control system embodies a forward flow of information.
PO6 1 Multiple-loop Systems
Although a single controlled variable is indicated in the figure, any number may be accommodated in one feedforward system. Three forward loops are shown, to suggest that all the components of load which significantly affect a controlled variable may be used in solving for the manipulated variable. Although their configuration differs from the commonly recognized feedback loop, these loops are truly closed. Feedforward control should not, therefore, be construed as merely an elaborate form of programmed or open-loop control.
THE CONTROL SYSTEM AS A MODEL OF THE PROCESS
In practice, the feedforward control system continually balances the material or energy delivered to the process against the demands of the load. Consequently the computations made by the control system are material and energy balances on the process, and the manipulated variables must therefore be accurately regulated flow rates. An example is the balancing of firing rate vs. thermal power that is being withdrawn as steam from a boiler. Some material and energy are inevitably stored within the process, the content of which will change in passing from one state to another. This change in storage means a momentary release or absorption,of energy or material, which can produce a transient in the controlled variable, unless it is accounted for in the calculations. To be complete, then, the control computer should be programmed to maintain the process balance in the steady state and also in transient intervals between steady states. It must consist of both steady-state and dynamic components, like the process: it is, in effect, a model of the process. If the steady-state calculations are correct, the controlled variable will be at the set point as long as the load is steady, whatever its current value. If the calculations are in error, an offset wil1 result, which may change with load. If no dynamic calculations are made, or if they are incorrect, the measurement will deviate from the set point while the load is changing, and for some time thereafter, while new energy levels are being established in the process. If both the steady-state and dynamic calculations are perfect, the process will be continually in balance, and no deviation will be measurable at any time. This is the ultimate goal. The same procedure is followed in the design of a feedforward system as was used for decoupling, i.e., the process model is reversed. The manipulated variables are solved for in terms of load components and controlled variables. In a decoupling system, controller outputs were inserted where the controlled variables appeared in the equations. But for feedforward control, set points are used instead. It is the intent of a
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