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

Workshop 6

Topics in Advanced Control

Theory without experience is sterile, practice without theory is blind.

—George Jay Anyon

Introduction

Prior to attempting this workshop, you should review Chapters 6 and 7 in the book.

This workshop will show how the response of feedback control (FBC) loops can be improved through the use of other control methods. These other methods include measuring common disturbances and taking action before they affect the controlled variable (feedforward control) and using a faster responding loop to decrease the response time of a system with a large time constant (cascade control). You will determine what conditions are necessary for feedforward or cascade control to be useful and identify which parameters reduce the effectiveness of these control methods.

Key Learning Objectives

Feedforward Control

1. Feedforward controllers can respond faster than feedback controllers since they react to process disturbances immediately without waiting for them to affect the process.

2. Feedforward control can only compensate for disturbances that are measured. Its effectiveness is reduced if unmeasured disturbances are significant.

3. Feedforward control is less effective for non-linear processes, where non-linearities exist between the disturbance measurement and controlled variable.

Cascade Control

4. Cascade control can significantly improve the control performance if a secondary variable can be found in the system that directly affects the primary loop and ...