Table of Contents

PI Controller - Heating

Block name

B35_REVERSE_PI_CONTROLLER

ST call

PROGRAM B35_TEST
VAR 
   XIN,WIN,COOLING:REAL;   
    FB: LIB.CORE.V1_0.B35_REVERSE_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
CONTROLLERTYPECONTROLLERTYPEController 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 TypeDescription
OUT REALOutput signal (mostly valve or damper, 0 to 100%).
PFACTORREALP-part of the output signal
IFACTORREALI-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 B33. 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