BAC_AC_ErcT_PID_01

Functional description

The subtemplate BAC_AC_ErcT_PID_01 is an supply air temperature sequence controller for energy recovery.
The set value, actual value and control output are referenced via the BACnet value objects X, W and Y.

The PID sequence controller is enabled based on the plant operation mode udiOpMod and the global temperature communication structure g_stSeqLinkT[PLT_NUM]. This data and command structure is the link between the individual sequence controllers and the corresponding control function block FB_BA_SeqLink of a plant.

The BACnet BV object En is used to display the controller enable.

The limit value monitoring of PID controller is controlled by the function block EnEvtEn as a function of the plant startup process.

In dehumidification mode bDeHum = TRUE, the PID sequence controller is disabled.

Interface

BAC_AC_ErcT_PID_01 1:

Block diagram

Direction of action

The direction of action of the PID sequence controller is selected based on a comparison of the outside temperature with the exhaust air temperature.
If the outside temperature is lower than the outlet air temperature, the direction of action of the PID controller is indirect (heating mode)
If the outside temperature is higher than the outlet air temperature, the direction of action of the PID controller is direct (cooling mode)

VAR_INPUT

rX         : REAL; 
rW         : REAL; 
bDeHum     : BOOL;
bSync      : BOOL; 
rFdb       : REAL; 
rExtAT     : REAL;
udiPltStp  : UDINT; 
udiOpMod   : UDINT;

rX: Measured value supply air temperature

rW: Set value of the supply air temperature

bDeHum: Dehumidification active, supply air temperature controller disabled. If dehumidification is active, energy recovery is disabled in heating mode. If the heat recovery unit recovers ‘coolth’ from the outlet air, it is switched to 100% in dehumidification mode.

bSync: Input for synchronisation of the controller

rFdb: Position feedback actuator

rExtAT: Measured value outlet air temperature

udiPltStp: Steps during startup of the air-conditioning plant. See also BAC_AC_StartT_01

udiOpMod: Plant operation mode. See also BAC_AC_OpMod_01

VAR_OUTPUT

rY        : REAL; 
bEn       : BOOL;
bActn     : BOOL;

rY: Control value output

bEn: Energy recovery control enabled

bActn: Direction of action of the supply air controller, required for the set value strategy; TRUE = direct = cooling; FALSE = reverse = heating

VAR CONSTANT

PLT_NUM    : BYTE := 1;

PLT_NUM: all alarms and events of all plant components within a controller are included in a global alarm and event list. The assignment of events and alarms to a plant is defined by the assignment of a PLT_NUM plant number.
The recording and processing of an alarm from an aggregate or a device takes place within the templates by means of the alarm function block FB_BA_Alarm.
The evaluation of the alarms of a plant, e.g. for the generation of a collective message or for plant shutdown in case of relevant faults, takes place within the template BAC_PltAlm_01 by means of the function block FB_BA_AlarmPlt.
The evaluation of different plant events within the templates of a plant, takes place within the template BAC_PltComnMsg_01 by means of the function block FB_BA_ComnMsg.

Important! The assignment and evaluation of the alarms and events of a plant can only be done correctly if all templates of a plant have the same plant number!

Within a ventilation system with sequence controller the plant number indicates which field from the global data structure g_stSeqLinkT[PLT_NUM] is used as link between the individual sequence controllers and the corresponding control function blockFB_BA_SeqLink.

The plant number can be assigned in the Project Builder in the parameter menu for the templates or via a column in the Excel import.

Program description

Instance	Type	Task
X	FB_BACnetAVDisplay	The AV object is referenced to the actual value input of the BACnet loop object
W	FB_BACnetAVDisplay	The AV object is referenced to the setpoint input of the BACnet loop object
OpModPrio	FB_BA_MMUX_B08	The multiplexer defines the enable conditions of the sequence controller depending on the plant operation mode.
		udiOpMod		Enable	Comment
		OPMOD_AC_MANON	Manual on	TRUE	The plant is switched on manually via the plant selector switch
		OPMOD_AC_ON	On	TRUE	The plant runs in automatic mode via the timer program
		OPMOD_AC_OVRHTGPRTC	Overheating protection	TRUE	The plant operates in overheating protection mode
		OPMOD_AC_COLDWNPRTC	Support operation, cooling protection	TRUE	The plant operates in cooling protection mode
		OPMOD_AC_FORCVENT	Forced ventilation	TRUE	The plant operates in forced ventilation mode
En	FB_BACnetBV1204	The BV object is used to display the sequence controller enable in the MCL.
En	AND	When OpModPrio is enabled in dehumidification mode ( bDeHum = TRUE) , energy recovery is disabled.
Actn	FB_BA_Swi2P	The function block FB_BA_Swi2P determines the control direction of the sequence controller. If the subtraction of the outside temperature and the exhaust air temperature is < - 0.25, the output of the function block Actn is FALSE. The control direction of the loop object is thus indirect and the energy recovery is in heating mode. If the subtraction of the outside temperature and the exhaust air temperature is > + 0.25, the output of the function block Actn is TRUE. The control direction of the loop object is thus direct and the energy recovery is in cooling mode. The hysteresis between - 0.25 and + 0.25 of the Actn function block prevents frequent switching of the control direction at approximately the same outside and exhaust air temperature.
EnEvtEn	FB_BA_LmtCtrl	The BACnet loop object PID monitors the control function by comparing the setpoint W with the actual value X. If the deviation W-X is greater than the property ErrorLimit, then the loop object sends a message to the MCL. When the plant is at a standstill, at the moment of startup and until the plant is in a controlled state, the loop object is suppressed so that no incorrect messages are sent to the MCL. Reporting of the loop object should not be activated until the air-conditioning system is fully running and the control is stable. In addition, reporting is enabled if the control has not reached the range around the setpoint defined by the property ErrorLimit after a long time. Enabling the object internal reporting is done by writing to the BACnetEventTransitionBits of the loop object. The following conditions must be met to enable reporting from the loop object: 1. The plant start program BAC_AC_StartT_01 has enabled control monitoring and sensor limit monitoring udiPltStp >= PLTSTP_AC_ENLMTMONIT and energy recovery is the active controller in the control sequence. g_stSeqLinkT[PLT_NUM].usiCurCtrl = SEQNUM_T_ERC and the supply air temperature has approached the setpoint to a point where it has settled into a range between rSp - ErrorLimit and rSp + ErrorLimit and the supply air temperature must have remained within the range of rSp - ErrorLimit and rSp + ErrorLimit for at least the duration of EnEvtEn_udiTiSttDly. 2. The timer EnEvtEn_udiTiSttAuto has expired and the control has not reached its setpoint range.
	F_BA_EvtTrans	writes to the BACnetEventTransitionBits to_offnormal, to_fault and to_normal of the loop object. The input bEnEvtEn must be TRUE
PID	FB_BACnetLoopSeq1202	Sequence controller supply air temperature energy recovery.
rtrigSync	R_TRIG	A rising edge at input bSync triggers synchronization of the loop object to the value of lrSync. If the actuator of the energy recovery was overridden by writing a higher priority to the corresponding AO object of the MCL or by activating the local priority operation, the current position of the actuator deviates from the output of the loop object. The variablesbSync and rFdbVlv can be used to restore synchronicity between the position of the actuator and the controller.
Y	FB_BACnetAVSetpoint	The AV object is referenced to the control value output of the BACnet loop object
	AND, SEL, MAX	Network which outputs the output value rY by the MAX selection. On the one hand, the value of the supply air temperature controller PID is output at the MAX selection. On the other hand, a value of 100 or 0 is output depending on the dehumidification operation bDeHum and the comparison of the outdoor temperature with the exhaust air temperature.

Version history

Version number	Comments
1.0.1	First release