Page 26 - Practical Control Engineering a Guide for Engineers, Managers, and Practitioners

CHAPTER 1

Qualitative

Concepts in Control

Engineering and

Process Analysis

his will be the easiest chapter in the book. There will be no math-
ematics but several qualitative concepts will be introduced. Filst,
Tthe cornerstone of control engineering, the feedback controller is
discussed. Its infrequent partner, the feedforward controller is presented.
The significant but often misunderstood differences between feedback
and feedforward control are examined. The disconcerting truth about the
difficulty of implementing error-free feedback control is illustrated with
an indusbial example. Both kinds of controllers are designed to respond
to disturbances, which are discussed briefly. Finally, we spend a few
moments on the question of what a control engineer is.

1-1 What Is a Feedback Controller?
Consider the simple process shown in Fig. 1-1. The level in the tank is to
be maintained "near'' a target value by manipulating the valve on the
inlet stream. Now, place the "~yet-undefined" controller in Fig. 1-2.
The controller must sense the level and decide how to adjust the
valve. Notice that for the controller to work properly
1. There must be a way of measuring the tank level (the "level
sensor") and a way of transmitting the measured signal to the
controller.
2. Equally important there must be a way of transmitting the
controller decision or controller output to the valve.
3. At the valve there must be a way of converting the controller
output signal into a mechanical movement to either close or
open the valve (the "actuator").

1