A Real-Time Approach to Process Control, 3rd Edition

Workshop 4

Feedback Control

Nothing comes from doing nothing.

—Shakespeare

Introduction

Prior to attempting this workshop, you should review Chapters 3 and 4 in the book.

The previous workshop introduced the concepts of capacitance and attenuation. These are ‘natural’ characteristics of a system, as are dead time and the process time constant. Now that we have a basic understanding of the way processes behave, we can apply this knowledge to control the process response.

Once the process personality is understood, we can manipulate process flows to maintain a desired variable at constant conditions, which are called set points. This is known as feedback control, where the value of a variable is ‘fed back’ to a controller which manipulates another variable according to the difference between the controlled variable and its set point.

Key Learning Objectives

1. Feedback control is easiest and most successful for low-capacity processes without dead time.

2. Dead time reduces the ultimate gain of a process.

3. A large time constant decreases the responsiveness of a process and reduces the achievable control performance.

4. Proportional-only control suffers from offset, which can be eliminated through integral action.

5. Derivative action can only be used where there is no significant process noise and relatively little dead time.

6. Buffer tanks and surge drums can help smooth out changes in flow and, thereby, isolate equipment from upstream disturbances. This can only result from ...