Documentation
PI-Controller - Cooling (B34)
Block name
B34_PI_CONTROLLER
ST call
PROGRAM B34_TEST
VAR
XIN,WIN,COOLING:REAL;
FB: LIB.CORE.V1_0.B34_PI_CONTROLLER;
END_VAR
FB(X:=XIN, W:=WIN, OUT=>COOLING);
END_PROGRAM
Library
LIB\CORE
Version
V1.0
Description
The block is an implementation of a PI controller (with parameters: P-band, Integration constant, Offset, Min and Max values, and output value of an inactive controller).
Inputs
| Input | Type | Description | Default value |
|---|---|---|---|
| W | REAL | Setpoint (usually temperature). | 0 |
| X | REAL | Measured value (usually temperature). | 0 |
| CONTROLLERTYPE | CONTROLLERTYPE | Controller type (cooling/heating). | cooling |
| BP | BOOL | When set to TRUE, the I part is disabled (the controller acts as a P controller) | False |
| ENABLED | BOOL | Controller enable (FALSE = controller is deactivated). | True |
| XP | REAL | Proportional band | 30 |
| OFF | REAL | Offset, added to the setpoint W. | 0 |
| TI | REAL | Integration constant | 120 |
| MIN | REAL | Minimum output value OUT (usually 0). | 0 |
| MAX | REAL | Maximum output value OUT (usually 100). | 100 |
| DISVAL | REAL | Value of the OUT output when the controller is deactivated (ENABLED=FALSE). | 0 |
Outputs
| Output | Type | Description |
|---|---|---|
| OUT | REAL | Output signal (mostly valve or damper, 0 to 100%). |
| PFACTOR | REAL | P-part of the output signal |
| IFACTOR | REAL | I-part of the output signal |
Function
There is a control deviation at the controller input, E=(W+OFF)-X. The controller output OUT is given by the relation OUT=-100/Xp*(E+1/Ti*∫e)+MIN, where XP is the proportional band. The integration part (I-part) is calculated as follows: When OUT=0 at t=0 and the inputs are W+OFF=0, X=1, then the I-part rises to OUT=100 at t=Ti, see figure below. The I-part is limited to 0...100 before it is added to the P-part. The resulting value is finally limited by MIN and MAX, see figure at block B32. The I-part helps to achieve the exact control value.
The BP input sets the I-part to 0, if set to true.
The ENABLED parameter switches the controller off (ENABLED=FALSE, controller disabled, and OUT=DISVAL). If Xp<0 then Xp=0, see below. If MAX<MIN then OUT=MAX (or minimum from (MIN, MAX)).
Application example
A simple PI controller using analogue setpoint over a time program block TPG.