CASPlus

Function

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.


CASPlus Example

       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


Inputs and Outputs

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



Parameter List

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

%