asp net display barcode Multiple-loop Systems in .NET framework

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1 Multiple-loop Systems
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puting system. Bear in mind, however, that the amount of offset, dc, resulting from an error in the manipulated variable dm, varies inversely with load:
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Although feedforward systems are designed primarily for regulation, some rules regarding set-point changes are noteworthy. To avoid the usual dynamic problems associated with feedback loops, the feedback controller should be placed in manual during set-point changes. In the case where this is a frequent occurrence, set-point changes can be introduced in automatic if sent directly to the forward loop; this requires multiplication of the set point by the output of the controller as shown in Fig. 8.16. It is impossible, of course, for the process to respond instantaneously to a step in set point. Since the controlled variable will lag behind the set point, a positive error will develop before the new set point is reached. The feedback controller, being in automatic, will integrate the error, changing it s output to a new but incorrect value; it must then bring its output back to the previous state by generating a negative error, equal in area to the earlier positive error. The effect is the same as that shown in Fig. 6.14, produced by the blending control system. This situation can be remedied simply by inserting a lag in the set point to the feedback controller (but not to the multiplier), as Fig. 8.16 illustrates. The lag should be adjusted to prevent t he overshoot that would be realized without it. The feedback controller will always equalize the integrated error promoted by any disturbance entering a forward loop. For this reason, it should remain in manual while dynamic compensation is being adjusted. Another important consideration when adding feedback is the location of the dynamic compensator. Although lead-lag can be beneficial to the response of a feedback loop, it interferes with manual operation. W h e n an operator changes the output of the controller manually, he likes to see that action reproduced exactly by the manipulated variable. With a lag or lead-lag between the controller and the manipulated variable, several minutes-possibly even an hour-could elapse before the effect of the adjustment is complete. Therefore it is mandatory to arrange the system so that dynamic compensation is out of the feedback loop.
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FIG 8.16. If set-point response is important, the set point should go directly into the forward loop.
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Feedforward Control
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FIG 8.17. The complete control system for the heat exchanger includes feedback and dynamic compensation.
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In theory, each forward loop should have its own dynamic compensator, but ordinarily only flow inputs can change fast enough to warrant dynamic compensation. Observe the location of the lead-lag unit in the system with two forward loops and one feedback loop, which is shown in Fig. 8.17.
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An adaptive control system was defined as having the ability to change its parameters in accordance with the changing character of the process. A feedforward control system, by itself, can only generate an output relative to known and measurable inputs as prescribed by a fixed program. Some factors relating to the process may be unknown and variable. F o r optimum performance, the feedforward system ought to be supplied with information regarding these unknowns. A feedback controller, on the other hand, is geared to solve for unknowns. So the inclusion of a feedback signal in a forward loop actually adapts the forward loop to unmeasured changes in the process. Remarkably enough, the feedforward system also adapts the feedback loop to variations in process gain. Figure 8.14 shows the load response of the cited heat exchanger under feedback control. With increasing load, the transient is overdamped. With decreasing load, the transient is greater and underdamped, indicating that the process gain changes inversely with liquid flow, ( T h is characteristic was discussed under Variable Dynamic Gain in Chap. 2, and again under Dynamic Adaptive Systems in Chap. 6.) Since the process gain varies inversely with flow, the controller gain ought to vary directly with flow. The complete control system for the heat exchanger, Fig. 8.17, illustrates how this is brought about. The feedback loop sees Tz as its input and W, as its output. But within the loop, T1 is subtracted from, and W, multiplied by, the controller output. Subtraction is a linear operation, so gain is not changed therein; but multiplication is nonlinear, causing feedback gain to vary directly with flow W,. Correct loop-gain adaptation cannot be achieved if the feedback is introduced in an ~ other place. If the output of the feedback controller were to set K, then feedback gain would vary both with W, and with Tz - T1. But process gain does not vary with
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