ELEMENTS OF THE DESIGN OF A CONTROL SYSTEM

Let us see now what are the basic questions that we must ask while attempting to design a control system that will satisfy the control needs for a chemical process.

1. Define Control Objectives

The central element in any control configuration is the process that we

want to control. The first question that is raised by the control designeri is:

- Ensuring the stability of the process, or



- suppressing the influence of external disturbances, or


- optimizing the economic performance of a plant, or

- combination of the above

At the beginning the control objectives are 'defined qualitatively and subsequently they are quantifi'ed, usually in terms of the output variables.

2.Select Measurements

Whatever are our control objectives, we need some means to monitor the

performance of the chemical process. This is done by measuring the values of certain processing variables (temperatures, pressures, concentrations, flowrates, etc.).

It is self-evident that we would like to monitor directly the variables thatre present our control objectives, and this is what is done whenever possible. Such measurements are called primary measurements.

3.Select Manipulated Variables

Once the control objectives have been specified and the various measurements identified, the next question is how do we effect a change on the process, i.e.

Usually in a process we have a number of available input variables which can be adjusted freely. Which ones we select' to use as manipulated variables is a crucial question as the choice will affect the quality of the control actions we take.

4.Select the Control Configuration

After the control objectives, the possible measurements, and the available manipulated variables have been identified, the final problem to be solved is that of defining the control configuration.

5.Design the Controller
In every control configuration, the controller is the active element that receives the information from the measurements and takes appropriate control actions to adjust the values of the manipulated variables.