Documentation

PI-Controller - Heating (B35)

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
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 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)).