Control-loop response, repeatability

For noninteracting control loops with zero dead time, the integral setting (minutes per repeat) is about 50% and the derivative, about 18% of the period of oscillation (P). As dead time rises, these percentages drop. If the dead time reaches 50% of the time constant, I = 40%, D = 16%, and if dead time equals the time constant, I = 33% and D = 13%. When tuning the feedforward control loops, one has to separately consider the steady-state portion of the heat transfer process (flow times temperature difference) and its dynamic compensation. The dynamic compensation of the steady-state model by a lead/lag element is necessary, because the response is not instantaneous but affected by both the dead time and the time constant of the process. 

The sample system is responsible for collecting a representative sample of the process and delivering it to the analyzer for analysis. Obviously, the reliability of the sample system directly affects the reliability of the overall composition analysis system. The transport delay associated with the sample system contributes directly to the overall deadtime associated with an on-line composition measurement. This difference in sampling deadtime can have a drastic effect on the performance of a control loop. Table 15.2 summarizes the dynamic characteristics and repeatability of typical control valve systems and several different types of sensors. 

Peiformance. Depending on the application, accuracy, repeatability, or perhaps some other measure of performance is appropriate. Where closed loop control is contemplated, speed of response must be included. 

Figure 13.15 shows the operational scheme of this automatic tltrator. The heart of the unit Is an INTEL 8080 microprocessor mounted on the central processing unit (CPU) board. The rotary reaction cell assembly can accommodate up to three different sensors for multiple measurements on the same processed sample. Each stepper burette board controls up to two burette dispensing assemblies. Function boards such as the colorimeter board, air burette board, E/I output board and RS-232 printer Interface boards are available optionally. The optional D/A and E/I board is used for closed-loop applications where the tltrator controls the final element such as a control valve. The RS-232 printer Interface board Is useful for troubleshooting the equipment and editing user-defined programs. The Instrument accuracy, repeatability and response time vary widely and depend on the particular type of measurement concerned. The system requires a.c. power, a 75-psl air supply and a dilution water supply for proper operation. The air flow-rate required is of about 50 cm3/mln... 

The Kc and 71 are used to adjust the PI controller gain to give the loop a desired response. Suppose 71 = oo, which would result in = 0, regardless of the value of Kc- In effect, the response would be that of a P-only controller with a period equal to and a sustained error. Although 71 = oo is not realizable, it can be set to a very large number in min/repeat to minimize the integral action. 

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