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BACnet: The Loop Object

The Loop object is defined by the standard in terms of the object properties representing the external, physical attributes of any category of control loop. The Loop object is a detailed and complex object in the standard.

The optimal flexibility of this object is attained by including three specific gain constants with no assumed values as the units:

  • Proportional Constant
  • Integral Constant
  • Derivative Constant

The developer should pay close attention to these three constants when drafting the object structure for BACnet devices. The suitable gain units are figured by a control algorithm, which is a local matter for the automation developer.

The following table will present the actual properties and datatypes of the Loop object from the BACnet standard:

PropertyDatatype
Object_IdentifierBACnet Object Identifier
Object_NameCharacter String
Object_TypeBACnet Object Type
Present_ValueReal
DescriptionCharacter String
Status_FlagsBACnet Status Flags
Event_StateBACnet Event State
ReliabilityBACnet Reliability
Out_of_ServiceBoolean
Update_IntervalUnsigned
Output_UnitsBACnet Engineering Units
Manipulated_Variable_ReferenceBACnet Object Property Reference
Controlled_Variable_ReferenceBACnet Object Property Reference
Controlled_Variable_ValueReal
Controlled_Variable_UnitsBACnet Engineering Units
Setpoint_ReferenceBACnet Setpoint Reference
SetpointReal
ActionBACnet Action
Proportional_ConstantReal
Proportional_Constant_UnitsBACnet Engineering Units
Integral_ConstantReal
Integral_Constant_UnitsBACnet Engineering Units
Derivative_ConstantReal
Derivative_Constant_UnitsBACnet Engineering Units
BiasReal
Maximum_OutputReal
Minimum_OutputReal
Priority_For_WritingUnsigned
COV_IncrementReal
Time_DelayUnsigned
Notification_ClassUnsigned
Error_LimitReal
Event_EnableBACnet Event Transition Bits
Acked_TransitionsBACnet Event Transition Bits
Notify_TypeBACnet Notify Type
Event_Time_StampsBACnet Array (3) of BACnet Time Stamp
Profile_NameCharacter String

The following example from the standard will present the Loop object structure from an actual building automation application. This example will specifically focus on the air temperature control in a HVAC unit:

Object_Identifier(Loop, Instance 1)
Object_Name“AHU_SAT_LOOP”
Object_TypeLOOP
Present_Value8.3
Description“Supply air temp. PI control”
Status_Flags{FALSE, FALSE, FALSE, FALSE}
Event_StateNORMAL
ReliabilityNO_FAULT_DETECTED
Out_Of_ServiceFALSE
Update_Interval1
Output_UnitsPOUNDS_FORCE_PER_SQUARE_INCH
Manipulated_Variable_Reference((Analog Output, Instance 5),Present_Value)
Controlled_Variable_Reference((Analog Input, Instance 3), Present_Value)
Controlled_Variable_Value56.1
Controlled_Variable_UnitsDEGREES_FAHRENHEIT
Setpoint_Reference((Analog Value, Instance 7), Present_Value)
Setpoint57.0
ActionDIRECT
Proportional_Constant0.5
Proportional_Constant_UnitsPSI_PER_DEGREE_FAHRENHEIT
Integral_Constant0.1
Integral_Constant_UnitsPER-MINUTE
Derivative_Constant0.0
Derivative_Constant_UnitsNO-UNITS
Bias9.0
Maximum_Output15.0
Minimum_Output3.0
Priority_For_Writing10
COV_Increment0.2
Time_Delay3
Notification_Class1
Error_Limit5.0
Event_Enable{TRUE, TRUE, TRUE}
Acked_Transitions{TRUE, TRUE, TRUE}
Notify_TypeALARM
Event_Time_Stamps((23-MAR-95,18:50:21.2),
(*-*-*,*:*:*.*),
(23-MAR-95,19:01:34.0))

Written by: Scott Cosby

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