Regulatory Control Engineering

In the past these functions were performed by panel boards consisting of indicators, alarms, strip-chart recorders, and single-loop controllers. Today, distributed control systems (DCS) and data historians perform these functions (see Section 12.4.6). 

When rearranged this gives the exponential smoothing filter in Eq. (36), where the filter factor, 0 a 1, is defined by Eq. (37). 

This filter can be implemented in a digital computer. Other filters, such as a moving-average filter, are also possible [7]. 

The signals leaving the controller are used to affect temperatures, pressures, and flow rates by means of actuators. Actuators are usually valves whose flow area changes with the signal coming from the controller. Metering machines, or positive displacement pumps, can also be used as actuators. 

Expenditure for measurement, control, and actuator systems in the process industries is substantial. The proportional cost is higher the smaller the process is, because the absolute expenditure depends only on the number of measurement points and control loops, not on the size of the installation. 

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The disturbance variables are those over which the control engineer has no control. Disturbances may be stochastic (random) or deterministic. Stochastic disturbances arise from the natural variability of the process. Examples are short-term variations in flow rates caused by mechanical inaccuracies. Deterministic disturbances arise from known causes, and they usually occur at longer intervals. Examples are lot-to-lot variations in feedstock quality and changes in production rates mandated by the operation of some upstream or downstream process. Although the cause of such disturbances may be known, the disturbances themselves cannot be eliminated because of constraints external to the system. Some disturbances, stochastic and deterministic, may be measurable, but by definition they cannot be eliminated. However, the effect of such disturbances on the final product can be eliminated by compensating for them by adjusting the manipulated variables. This is the function of regulatory control. 

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