CascadeExtractSupplyAirX

CascadeExtractSupplyAirX (FB)

FUNCTION_BLOCK CascadeExtractSupplyAirX

Short description

Generation of a supply air setpoint temperature using a cascade controller (PI controller) Additional functions: Limitation of the supply air setpoint temperature Typical application: Supply air temperature control in a ventilation system\

Representation

Function description

General

This function block is used to form the supply air setpoint temperature of a ventilation unit from \

1. Current setpoint temperature (exhaust air or room temperature) in °C\

2) Current actual temperature (exhaust air or room temperature) in °C taking into account the relative (*rMinSUPTemp* and *rMaxSUPTemp*) and absolute (**rMinSUP** and **rMaxSUP**) limits. In addition, the special operating modes ‘Rapid heating’ and ‘Free night cooling’ are taken into account when forming the supply air setpoint temperature. The formation of the supply air setpoint temperature is time-delayed (**tDelayControl**) after the enabling / disabling of the controller (*xEn* = TRUE). The cascade controller is enabled if the following conditions described below are simultaneously met: 1) The special operating mode *Rapid heating* is disabled (*xFastHeatEn* = FALSE). 2) The special operating mode *Free night cooling* is disabled (*xFNCEn* = FALSE). 3) The current actual temperature *rActTemp* does not violate its absolute limits (+5.0 - +50.0 °C). 4) The control is enabled (*xEn* = TRUE) and the start delay (duration = **tDelayControl**) has ended. In all other situations the cascade controller is disabled.\

Control

The current actual temperature rActTemp is, when the cascade controller is enabled (PI controller, xEn = TRUE, expiry of the time duration tDelayControl, no limit violation at the input rActTemp ) continuously compared with the current setpoint temperature rSetPtTemp and depending on the magnitude / sign of the difference or depending on the controller settings ( rPI_Kp or rPI_Tn ) a offset temperature is generated in the cascade controller. In the heating case ( rActTemp < rSetPtTemp ) a positive offset temperature thus arises in the range 0.0 - rMaxRelativeSUP K. In the cooling case ( rActTemp > rSetPtTemp ) a negative offset temperature thus arises in the range 0.0 - rMinRelativeSUP K. In the fully regulated state ( rActTemp = rSetPtTemp ) the offset temperature does not change. This offset temperature is then added to the current setpoint temperature and, if necessary, limited to the calculated limits &#xNAN;rMinSUPTemp or rMaxSUPTemp limited. At the output rMinSUPTemp the calculated minimum supply air setpoint temperature is continuously displayed ( rActTemp + rMinRelativeSUP, absolutely limited to rMinSUP or rMaxSUP ) . At the output rMaxSUPTemp the calculated maximum supply air setpoint temperature is continuously displayed ( rActTemp + rMaxRelativeSUP, absolutely limited to rMinSUP or rMaxSUP ). The current actual temperature rActTemp is monitored for compliance with limits. On every limit violation ( rActTemp < +5.0 °C or rActTemp > +50.0 °C ) the cascade controller is deactivated.\

Selection of the supply air setpoint temperature

The selection of the supply air setpoint temperature is made according to the following function table.\

xFastHeatEn	xFNCEn	rActTemp within limit	xEn	Supply air setpoint temperature	Notes
TRUE	X	X	X	rMaxSUP	Special operating mode ‘Rapid heating’
FALSE	TRUE	X	X	rMinSUP	Special operating mode ‘Free night cooling’
FALSE	FALSE	FALSE	X	rSetPtTemp	Limit violation actual temperature
FALSE	FALSE	X	FALSE	rSetPtTemp	Locking of the control
FALSE	FALSE	TRUE	TRUE	rSetPtTemp + Offset temperature	Normal operation

Legend: X = any\

Formation of the current supply air setpoint temperature rSetPtSUPTemp

The current supply air setpoint temperature rSetPtSUPTemp is formed from the supply air setpoint temperature, extended by a manual override module.\

Supply air setpoint temperature	eManModeN	rSetPtSUPTemp	Notes
X	eMANNUM.Auto	Supply air setpoint temperature	Manual override module in automatic
X	eMANNUM.Man	rManValue	Manual override module in manual operation

Legend: X = don't care

CODESYS

InOut:

Scope	Name	Type	Initial
Input	xEn	BOOL
Input	rActX	REAL	9
Input	rSetPtHum	REAL	55
Input	rSetPtTemp	REAL	21
Input	rSUPTemp	REAL	20
Output	rSetPtSUPX	REAL
Output	rMinSUPX	REAL
Output	rMaxSUPX	REAL
Output	rSetPtX	REAL
Input Const	rPI_Tn	REAL	400
Input Const	rPI_Kp	REAL	1.4
Input Const	rMaxHumSUP	REAL	80
Input Const	rMinHumSUP	REAL	30
Input Const	tDelayControl	TIME	TIME#5m0s0ms
Input Const	rP	REAL	980.0
Input Const	rManValue	REAL
Input Const	eManModeN	eManNum	eManNum.Auto