1 Selecting the Feedback Controller in .NET framework

Draw QR Code ISO/IEC18004 in .NET framework 1 Selecting the Feedback Controller

110 1 Selecting the Feedback Controller
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FIG 4.17. Three lags in the reset circuit can improve performance on processes.
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but it can be made from a conventional controller. The most effective use will be made of the three lags if they are noninteracting. This can be done by selecting t,hc capacity of RI to be 100 times that of R3, and Rz 10 times that of R3, while setting their time constants equal by appropriate adjustment of their resistances. The degree of improvement will vary with the difficulty of the process. On processes that, are fairly easy to control, improvement over two-mode control may be marginal. In fact, derivative would normally be of more value. But where dead time is dominant, or where derivative cannot be used because of noise level, delayed reset may be of considerable worth.
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INTERRUPTING THE CONTROL LOOP
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In some control loops, feedback of information from the process is only available on an intermittent basis. The on-stream chromatograph is perhaps the most common transmitter of intermittent information, although many less familiar analyzers also have this characteristic. In a loop such as this, only one piece of information is transmitted within a cert)ain space of time, known as t,he sampling interval. The control loop is open, except at the first instant of each sampling interval. This dynamic property differs from anything discussed thus far. Som&imes the conkol loop is opened int,entionally. A single continuous analyzer may be used to sample two streams, being alternately a member of each loop. Recently single controllers have been shared among a number of loops, an operation called time-shared control. But all of these situations have one common property-a periodically open control loop. In order to provide the most effective control under these circumstances, an understanding of the influence of the sampling clement is necessary.
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Open-loop Response
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The first distinction to be made is whether the sampling element is dominant or not. Figure 4.18 compares the open-loop step response of two processes which are affected differently by sampling. The abscissa is the number of samples n, taken at intervals of At, from the initiation of the step. The actual track of the controlled variable c is shown as a broken line, while its sampled value c* is indicated by the solid line.
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process
FIG 4.18. The dynamic characteristics of the slow process are less affected by sampling.
fi I 5[r -4 0 1 2 3 4 = t/Al 5 6 7 0
If a process can respond fast enough, its dynamic characteristics will be masked by the sampling system. An extremely slow process, on the other hand, would contain dynamic elements whose values far exceed the sampling interval. In the latter case, the loop is, for all practical purposes, continuous, and can be considered in that light. But as the time constants of the process approach the sampling interval, the effect of sampling increases. From this point of view, if means are found to control the loop dominated by the sampling element, less severe situations can be readily accommodated. A flow blending process, such as the one pictured in Fig. 4.19, can be dominated by the sampling interval of a discontinuous analyzer. The residence time of the fluids in the piping is normally very short, and the delay in sample piping can be made less than the cycle time of the analyzer. Typically, fluid could be transported from the control valve to the analyzer in less than 35 min, while the analyzer might produce results at intervals of 5 min. In a process of this kind, the blend analyzed has gone downstream long before any corrective action can begin-a truly difficult control loop.
Closing the Control Loop
The sampling element that will be discussed here is generally referred to as a sample and hold element, or more specifically as sample and zero-order hold. Sampling, by itself, produces an instantaneous signal only at the start of each sampling interval. This kind of signal is not especially useful for control, unless it is held or memorized until the start of the next interval. A series of steps (Fig. 4.18) is thereby generated as the process changes state.
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