Some Conventional Schemes
Dr Mark J Willis
Department of Chemical and Process Engineering
University of Newcastle upon Tyne
email: mark.willis@ncl.ac.uk
Written: October, 1999
Aims and Objectives
To revise / revisit basic control schemes covered in the second year control
course (Process Control I). Schemes to be discussed are:
• Feedback control
• Feedforward Control
• Cascade Control
• Ratio Control
For each scheme a typical process example is used to highlight the
application of the methodology.
On completion of this section of the course you should have an awareness of
a common process control schemes and a basic knowledge of their
advantages and disadvantages.
Background
Normally a chemical process has numerous inputs and many outputs.
Consider the diagram below:
Manipulated Input
Disturbance Inputs
PROCESS
Reboiler
Bottom Product
B, xB
Boilup
V
Condenser
Accumulator
Reflux
L
Distillate
D, xD
Feed
P
F, zF
VT Output (s)
The inputs can generally be classified as: