The cascade controller acts as a PI controller with a master and cascade controller. It provides both a master Proportional-Integral (PI) controller and a secondary PI controller to handle difficult control systems.
The master PI controller provides the setpoint for the secondary (cascade) PI. The secondary PI provides the setpoint reset which, because of the PI function, can be nonlinear
Fig. 127. CasPlus function
The user can enable the integral parts of the two PI controllers setting the input “In Integral En.” manually in the property sheet or via a digital input in the wire-sheet. Therefore he has to set the “Summary” flag of this input in the Slot sheet.
This additional input “In Integral En.” can be used to enable CASPlus to act as a P controller during plant start‑up and then switch on an Integral component after start‑up is complete.
The following diagram shows how the Integral enable of the CASPlus operates in AX wire-sheet editor.
Fig. 128. CASPlus example schematics
The picture above shows an automatic switchover from P control during a 15 minutes start‑up phase to PI control during normal operation.
The “StartupFlag” statement is triggered by a controller or a station start new start.
The “ControlDelayOnOff” provides a 15 minutes start-up delay. Its outputs switches to true 15 minutes after the application new start. And enables the integral part of the “CASPlus” controller.
The Sequence splits the control output of the “CASPlus” in different heating and cooling sequences. The direct and reverse actions of the output sequence can be set via parameters in the “Sequence” statement – see the description of the “Sequence” statement.
Fig. 129. Wire-sheet Icon
Fig. 130. Property Sheet
Fig. 131. Slot Sheet
Table 91. Inputs
Inputs |
Type |
Functional description |
In X |
numeric |
Master-controlled variable |
In XH |
numeric |
Cascade- or auxiliary-controlled variable |
In W |
numeric |
Reference variable, also known as setpoint |
In Integral En. |
boolean |
Enables and disables integral control action. When “In Integral En.” is zero, integral action in the master and cascade controllers is disabled and the integral sum is reset |
In Enabled |
boolean |
If the input “In Enabled” is set to false disabled in the slot sheet, the output “Out” will be disabled and is set to the value “0”. |
By default the input “In Integral En.” is accessible on the Property sheet where it can be set manually to true or false.
If a user wants to dynamically change the integral part, he can reconfigure this input in the Slot Sheet checking the “Summary flag.
One digital input “In Integral En.” that enables and disables integral control action. When “In Integral En.” is zero, integral action in the master and cascade controllers is disabled and the associated integral sum is reset.
If the input “In Enabled” is set to false disabled in the slot sheet, the output “Out” will be disabled and is set to the value “0”.
Table 92. Outputs
Outputs |
Type |
Functional description |
Out |
numeric |
Control output |
Table 93. Parameter List
Description |
Value Range |
Default Value |
Unit |
P1 Proportional band Xp master |
0...∞ |
4 K |
K |
P2 Integral action time master |
0…7200 |
0 s |
S |
P3 Maximum output master |
0…100 |
35 |
% |
P4 Minimum output master |
0…100 |
0 |
% |
P5 Proportional band Xp cascade |
0...∞ |
20K |
K |
P6 Integral action time cascade |
0…7200 |
0 s |
S |
P7 Maximum output cascade |
0…100 |
100 |
% |
P8 Minimum output cascade |
0…100 |
0 |
% |