Selection of Measured Variables

Both input and output variables may be measured variables, with on-line measurement preferred to off-line measurement. Seborg et al. (1989) discuss the importance of measurements in control, and provide three guidelines for the selection of variables to be measured and the location of the measurements. 

Guideline 10. Reliable, accurate measurements are essential for good control. Examples of poorly designed measurements include orifices with insufficient straight piping, and/or saturated liquids that flash in the orifice. 

Guideline 11. Select measurement points that are sufficiently sensitive. Consider, for example, the indirect control of the product compositions from a distillation column by the regulation of a temperature near the end of the column. In high-purity distillation columns, where the terminal temperature profiles are almost flat, it is preferable to move the temperature measurement point closer to the feed tray. 

Guideline 12. Select measurement points that minimize time delays and time constants. Large time delays and dynamic lags in the process limit the achievable closed-loop performance. These should be reduced, whenever possible, in the process design and the selection of measurements. 

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Let us summarize. The examples have shown that, according to the selection of measured variables and their admitted values... 

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