FB_HVACPIDCtrl

FB_HVACPIDCtrl 1:

Application

The PID controller is a standard controller for heating and air conditioning applications. It can be used to control temperatures, pressures, volumetric flows or humidity. The values of Ti and Tv can be set to zero. This allows a P, PI, or PD controller characteristic to be set. The controller has an ARW function (Anti-Reset-Windup). This prevents continuous integration in the event of a perpetual pending control deviation.

VAR_INPUT 

eDataSecurityType: E_HVACDataSecurityType;
bSetDefault      : BOOL;
bEnable          : BOOL;
rW               : REAL;
rX               : REAL;
tTaskCycleTime   : TIME;
tCtrlCycleTime   : TIME;
eCtrlMode        : E_HVACCtrlMode;
rYManual         : REAL;
rInitialValue    : 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: FB_HVACPIDCtrl 2:Example_PERSISTENT.zip

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

Notice

A cyclically changing variable must never be linked with the IN_OUT variable of a function block, if eDataSecurityType:= eHVACDataSecurityType_Persistent. It would lead to early wear of the flash memory.

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

bEnable: input variable for enabling the controller. The controller is active if bEnable = TRUE.

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

rX: rX acquires the 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.

eCtrlMode: the operation mode is selected via this enum.

rYManual: the controller output rY can be overridden for test purposes. The set value of rYManual will be found at the controller output in manual operation mode.

rInitialValue: the restart behavior of the controller is influenced by rInitialValue. The initial values are adopted at a rising edge of the controller enable bEnable.
0 = start with the value zero at the output rY
<>0 = start with the value of rInitialValue

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;
bDirection      : BOOL;
rKp             : REAL;
tTi             : TIME;
tTv             : TIME;
tTd             : TIME;
rYMin           : REAL;
rYMax           : REAL;

rDeadRange: in order to avoid unnecessary driving and hence premature wear of the valves or damper drives, a dead range (0..32767) 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.

bDirection: the control direction of the controller can be changed with the parameter bDirection. If bDirection is TRUE, then the direct control direction is active for cooling operation of the controller. If bDirection is FALSE, then the indirect control direction of the controller is activated for heating operation. The variable is saved persistently. Preset to 0.

rKp: proportional factor (0.01..100) gain. The variable is saved persistently. Preset to 1.0.

tTi: integral action time [s]. 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 [s]. 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 [s]. The variable is saved persistently. Preset to 0 s.

rYMin / rYMax: limiting the working range of the controller (0..32767). Several other function blocks, e.g. sequencers, require a symmetrical control range (-100 to +100). In the case of a cascade structure, the working range of the master controller determines the setpoint of the slave controller. For example, 15° to 25° as the limitation of the supply air set value for an exhaust/supply air cascade control. The variables are saved persistently. rYMin preset to 0. rYMax preset to 100.

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