modelCascadeAntiWindup

Example of cascade control with external lock

Extends from Modelica.Icons.Example (Icon for runnable examples).

Information

Description

In this examples are compared two cascade control schemes: with and without external lock.

When two controllers are connected together in a cascade control scheme, the inner controller typically regulates the actuator, while the outer one provides the Set Point reference for the inner one.
Since the inner controller acts on the plants, its Control Signal have to be limited, using AntiWindup. Unfortunately it is not possible for the outer controller, to know the values for which the inner regulator saturates.
Such a problem can be voided by using the PID in its incremental form, using the Increment/Decrement lock feature, and creating an external loop between the controllers.
If the inner regulator saturates, its satHi signal becomes true. Connecting this signal to the forbidIncrement input of the outer controller, avoid an useless and potentially dangerous increase of its Control Signal ( that is the Set point of the inner controller that saturated). With such a scheme a windup-like effect can be avoided.
In the following figure, the green line is the CS of the outer controller with Increment/Decrement lock, while the black one is the output of the outer controller whitout Increment/Decrement lock.
The black line shows a windup like effect that turns in a slower reaction when the Set Point changes at time t = 30.

Set Point, Process Variables (with and without external lock) and outer control signals (with and without external lock)

inner regulators Control Signal (with and without external lock)

Components

Type	Name	Description
Controllers.Digital.PID_2dof	PID_outer_extLock
Controllers.Digital.PID_2dof	PID_inner_extLock
LinearSystems.Continuous.TransferFunction	P1	inner process trasfer function
LinearSystems.Continuous.TransferFunction	P2	outer process trasfer function
Modelica.Blocks.Sources.TimeTable	SetPoint
Controllers.Digital.PID_2dof	PID_outer
Controllers.Digital.PID_2dof	PID_inner
LinearSystems.Continuous.TransferFunction	P3	inner process trasfer function
LinearSystems.Continuous.TransferFunction	P4	outer process trasfer function

Revisions

Industrial Control Systems (v 1.0.0) : April-May 2012