Advanced Process Control Techniques

Advantages of Feedback Control Loop

• Corrects as soon as the controlled variable deviates from the set point.

• Feedback control requires minimal knowledge about the process to be controlled

• The PID controller is versatile and robust.

In PID control, what can be done if process conditions change?

• the controller can be “retuned” to produce satisfactory control.

Disadvantages of Feedback Control

• Needs an error in the PV to create a corrective action

• Feedback control does not provide predictive control (not referring to derivative action)

• It may not be satisfactory for processes with large time constants and/or long time delays.

• In some situations, the controlled variable cannot be measured on-line

  • consequently, feedback control is not feasible

Needs an error in the PV to create a corrective action

It relies on what has happened, which has limitations in more complex systems.

Feedback control does not provide predictive control (not referring to derivative action)

No action to compensate for the effects of known or measurable disturbances.

It may not be satisfactory for processes with large time constants and/or long time delays.

If large and frequent disturbances occur, the process may operate continuously in a transient state and never attain the desired steady state

Split Range Control

• a wider range of control can be achieved by using two FCE to control the Manipulated Variable (MV)

• Energy or material is applied in two directions to force the process variable back to the setpoint.

What happens when using two FCEs?

By having two final control elements we can adjust the

process with two different MV

Drawbacks of Single manipulated variable

May not respond:

• fast enough

• Accurate enough

• Compensate for disturbances correctly, or for process safety.