FB_HVACPIDMixedAir

PID mixed air chamber controller

One of the special features of the mixed air controller is the reversal of its control direction depending on the outside temperature and the exhaust air temperature. The enthalpies of the outside air and the exhaust air can be used instead of their temperatures.

If the outside temperature is lower than the exhaust air temperature, the air can be used for cooling in the case of high thermal loads inside the building. Hence, the outside air rate increases as the control variable or the supply air temperature set value decreases (curve falls from left to right).

If the outside temperature is higher than the exhaust air temperature, the outside air proportion can be increased in the event of the supply air temperature set value being increased (curve rises from left to right). The outside air temperature is applied to the input variable rOutsideTemp. The variable rReturnAirTemp is for the exhaust air or room temperature. The Night Cooling operating mode (eHVACSequenceCtrlMode_NightCooling) is transmitted to the function block by means of the Enum eModeSeqCtrl. During night cooling the control is deactivated and 100% control value is output at rY. Further information on the Summer Night Cooling function can be found under FB_HVACSummerNightCooling.

The maximum and minimum outside air rates can be set with the parameters rYMax and rYMin.

VAR_INPUT

eDataSecurityType  : E_HVACDataSecurityType;
bSetDefault        : BOOL;
eModeSeqCtrl       : E_HVACSequenceCtrlMode;
iNumberOfSequences : INT;
iMyNumberInSequence: INT;
rW                 : REAL;
rX                 : REAL;
tTaskCycleTime     : TIME;
tCtrlCycleTime     : TIME;
rOutsideTemp       : REAL;
rReturnAirTemp     : REAL;
eCtrlMode          : E_HVACCtrlMode;
rYManual           : REAL;
rYMin              : REAL;
bResetController   : BOOL;
bReset             : BOOL;

eDataSecurityType:if eDataSecurityType:= eHVACDataSecurityType_Persistent, the persistent VAR_IN_OUT variables of the function block are stored in the flash of the computer if a value changes. For this to work, the function block FB_HVACPersistentDataHandling must be instanced once in the main program, which is called cyclically. Otherwise the instanced FB is not released internally.

A change of value can be initiated by the building management system, a local operating device or via a write access from TwinCAT. When the computer is restarted, the saved data are automatically read back from the flash into the RAM.

Application example: Example_PERSISTENT.zip

If eDataSecurityType:= eHVACDataSecurityType_Idle the persistently declared variables are not saved in a fail-safe manner.

bSetDefault: If the variable is TRUE, the default values of the VAR_IN_OUT variables are adopted.

eModeSeqCtrl: among other things, control is enabled via this enum. In addition, the operation mode of the VAC system is transmitted to the controller blocks in the sequence. See also Table operation modes.

iNumberOfSequences: number of sequence controllers in the system.

iMyNumberInSequence: the own number from the controller from the sequence.

rW: the setpoint is transferred to the controller with the variable rW.

rX: actual value of the control loop.

tTaskCycleTime: cycle time with which the function block is called. If the function block is called in every cycle this corresponds to the task cycle time of the calling task.

tCtrlCycleTime: the variable tCtrlCycleTime specifies the cycle time with which the PID controller is processed. The shortest possible cycle time is that of the controller. Since the controlled systems in building automation are predominantly slow, the cycle time of the controller can be several times that of the control cycle time.

rOutsideTemp: outside temperature

rReturnAirTemp: exhaust air temperature

eCtrlMode: the operation mode is selected via this enum. Manual or automatic operation mode.

rYManual: manual value that is set at the output rY if eCtrlMode = eHVACCtrlMode_Manual.

rYMin: lower limit of the working range of the controller.

bResetController: a positive edge on the input bResetController resets the PID controller.

bReset: acknowledge input in the event of a fault.

VAR_OUTPUT

rY               : REAL; 
rXW              : REAL;
bMaxLimit        : BOOL;
bMinLimit        : BOOL;
bActive          : BOOL;
bARWactive       : BOOL;
eState           : E_HVACState;
bError           : BOOL;
eErrorCode       : E_HVACErrorCodes;
bInvalidParameter: BOOL;

rY: control signal output of the PID controller.

rXW: control deviation

bMaxLimit: the output bMaxLimit is TRUE, if the output rY has reached the value rYMax.

bMinLimit: the output bMinLimit is TRUE, if the output rY has reached the value rYMin.

bActive: bActive is TRUE, if the controller is active and enabled.

bARWactive: bARWactiveis TRUE, if the integral component of the controller has reached the lower or upper control value limit.

eState: state of the controller. See E_HVACState.

bError: this output indicates with a TRUE that there is an error.

eErrorCode: contains the command-specific error code. See E_HVACErrorCodes.

bInvalidParameter: TRUE if an error occurs during the plausibility check. The message must be acknowledged with bReset.

VAR_IN_OUT

rDeadRange      : REAL;
rInitialValue   : REAL;
rKp             : REAL;
tTi             : TIME;
tTv             : TIME;
tTd             : TIME;
rDeadBand       : REAL;
rYMax           : REAL;
iCurrentSequence: INT;

rDeadRange: in order to avoid unnecessary driving and hence premature wear of the valves or damper drives, a dead range can be set for the controller output signal rY. This means that a control signal change is only active if the change of value is greater than the dead range. A constant change of the control signal rY is converted to a pulsating drive of the actuator if a dead range is specified. The larger the dead range the larger the pauses and the control signal jumps will be. The variable is saved persistently. Preset to 0.

rInitialValue: the restart behavior of the controller is influenced by rInitialValue. The variable is saved persistently. Preset to 0.

rKp: proportional factor gain. The variable is saved persistently. Preset to 0.

tTi: integral action time. The I-part corrects the residual control deviation following correction of the P-part. The smaller the tTi time is set, the faster the controller corrects. The control loop becomes unstable if the time is too short. Larger tTi-times must be entered in order to reduce the integration component. The integral action time should be selected to be longer than the stroke time of the valve or damper drive. The variable is saved persistently. Preset to 30 s.

tTv: rate time. The larger tTv is, the stronger the controller corrects. The control loop becomes unstable if the time is too long. Often in normal building automation applications only a PI controller is used. In this case zero must be entered for tTv. The variable is saved persistently. Preset to 0 s.

tTd : damping time. The variable is saved persistently. Preset to 0 s.

rDeadBand: if the control value is at the lower or upper limit of a controller and the actual value of the controlled system oscillates around the setpoint with a small amplitude, frequent switching back and forth between two sequence controllers can be damped by an additional parameter for the switchover. To this end, the difference between the actual values and the setpoints of the controlled system is integrated after the sequence controller has reached its lower or upper limit. Switching to the next sequence only takes place if the sum of this integration is greater than the value of rDeadband (see sample). The variable is saved persistently. Preset to 0.

rYMax: upper limit of the working range of the controller. The variable is saved persistently. Preset to 100.

iCurrentSequence: number of the active controller in the sequence.