Chemical Process Control

What is Process Control?

The manipulation of an object (actuation device) to maintain a parameter within an acceptable deviation from an ideally required condition.

There are two basic process control philosophies, feedback and feedforward control.

Feedback Control

In feedback control, the controlled variable is measured and compared with a set-point. The deviation between the controlled variable and the set-point is the error signal. The error signal is then used to reduce the deviation of controlled variable from set-point.

Direct Acting Control

If the controlled variable increases as the manipulated variable increases, then direct acting control is used.

Reverse Acting Control

Feedforward Control

Advanced Control

Mathematical Modeling

Conservation Laws

Process Reaction Curve

Statistical Models

Glossary

Actuator 

The mechanical device that cause the activation or movement of a final control element.

Direct Synthesis 

Final Control Element 

A physical device whose activation or movement causes a change in a dynamic process. In process control, the most common final control elements are control valves.

Frequency Domain 

Internal Model Control 

IMC-PID Tuning 

A method for PID tuning that selects tuning parameters to approximate an IMC-derived controller.

Ladder Logic 

A semi-graphical programming language used to represent control algorithms. The language is expressed using symbols for logic devices. The arrangement of the device symbols and their connections has the appearance of a ladder.

Laplace Transform 

An integral transformation from time domain to Laplace domain. Given a function of time $f(t)$, the Laplace transform is given by the following

$F(s) = \int_0^\infty \! f(t)\; e^{-s t} dt$

The use of $F(s)$ to represent the Laplace transform of $f(t)$ is a common convention; however, in dynamics and control it is common to use $f(t)$ and $f(s)$ to represent a time-domain function and its Laplace transform, respectively.

PID Controller 

PLC 

Programmable Logic Controller, a microprocessor-based electronic device for implementing control algorithms.

Time Domain 

Ziegler-Nichols Tuning