Position Proportional (POSP)

This information applies to the

CompactLogix

5370,

ControlLogix

5570,

Compact GuardLogix

5370,

GuardLogix

5570,

Compact GuardLogix

5380,

CompactLogix

5380,

CompactLogix

5480,

ControlLogix

5580, and

GuardLogix

5580 controllers.

The Position Proportional (POSP) instruction opens or closes a device by pulsing open or close contacts at a user defined cycle time with a pulse width proportional to the difference between the desired and actual positions.

Available Languages

Ladder Diagram

This instruction is not available in ladder diagram logic.

Function Block

Structured Text

POSP(POSP_tag)

Operands

Function Block

Operand	Type	Format	Description
POSP tag	POSITION_PROP	Structure	POSP structure

Structured Text

Operand	Type	Format	Description
block tag	POSITION_PROP	Structure	POSP structure

See Structured Text Syntax
for more information on the syntax of expressions within structured text.

POSITION_PROP Structure

Input Parameter	Data Type	Description
EnableIn	BOOL	Enable input. If false, the instruction does not execute and outputs are not updated. Default is true.
SP	REAL	Setpoint. This is the desired value for the position. This value must use the same engineering units as Position. Valid = any float Default = 0.0
Position	REAL	Position feedback. This analog input comes from the position feedback from the device. Valid = any float Default = 0.0
OpenedFB	BOOL	Opened feedback. This input signals when the device is fully opened. When true, the open output is not allowed to turn on. Default is false.
ClosedFB	BOOL	Closed feedback. This input signals when the device is fully closed. When true, the close output is not allowed to turn on. Default is false.
PositionEUMax	REAL	Maximum scaled value of Position and SP. Valid = any float Default = 100.0
PositionEUMin	REAL	Minimum scaled value of Position and SP. Valid = any float Default = 0.0
CycleTime	REAL	Period of the output pulse in seconds. A value of zero clears both OpenOut and CloseOut. If this value is invalid, the instruction assumes a value of zero and sets the appropriate bit in Status. Valid = any positive float Default = 0.0
OpenRate	REAL	Open rate of the device in %/second. A value of zero clears OpenOut. If this value is invalid, the instruction assumes a value of zero and sets the appropriate bit in Status. Valid = any positive float Default = 0.0
CloseRate	REAL	Close rate of the device in %/second. A value of zero clears CloseOut. If this value is invalid, the instruction assumes a value of zero and sets the appropriate bit in Status. Valid = any positive float Default = 0.0
MaxOnTime	REAL	Maximum time in seconds that an open or close pulse can be on. If OpenTime or CloseTime is calculated to be larger than this value, they are limited to this value. If this value is invalid, the instruction assumes a value of CycleTime and sets the appropriate bit in Status. Valid = 0.0 to CycleTime Default = CycleTime
MinOnTime	REAL	Minimum time in seconds that an open or close pulse can be on. If OpenTime or CloseTime is calculated to be less than this value, they are set to zero. If this value is invalid, the instruction assumes a value of zero and sets the appropriate bit in Status. Valid = 0.0 to MaxOnTime Default = 0.0
Deadtime	REAL	Additional pulse time in seconds to overcome friction in the device. Deadtime is added to the OpenTime or CloseTime when the device changes direction or is stopped. If this value is invalid, the instruction sets the appropriate bit in Status and uses a value of Deadtime = 0.0. Valid = 0.0 to MaxOnTime Default = 0.0

Output Parameter	Data Type	Description
EnableOut	BOOL	Indicates if instruction is enabled. Cleared to false if PositionPercent overflows.
OpenOut	BOOL	This output is pulsed to open the device.
CloseOut	BOOL	This output is pulsed to close the device.
PositionPercent	REAL	Position feedback is expressed as percent of the Position span.
SPPercent	REAL	Setpoint is expressed as percent of the Position span.
OpenTime	REAL	Pulse time in seconds of OpenOutput for the current cycle.
CloseTime	REAL	Pulse time in seconds of CloseOutput for the current cycle.
Status	DINT	Status of the function block.
InstructFault (Status.0)	BOOL	The instruction detected one of the following execution errors. This is not a minor or major controller error. Check the remaining status bits to determine what occurred.
CycleTimeInv (Status.1)	BOOL	Invalid CycleTime value. The instruction uses zero.
OpenRateInv (Status.2)	BOOL	Invalid OpenRate value. The instruction uses zero.
CloseRateInv (Status.3)	BOOL	Invalid CloseRate value. The instruction uses zero.
MaxOnTimeInv (Status.4)	BOOL	Invalid MaxOnTime value. The instruction uses the CycleTime value.
MinOnTimeInv (Status.5)	BOOL	Invalid MinOnTime value. The instruction uses zero.
DeadtimeInv (Status.6)	BOOL	Invalid Deadtime value. The instruction uses zero.
PositionPctInv (Status.7)	BOOL	The calculated PositionPercent value is out of range.
SPPercentInv (Status.8)	BOOL	The calculated SPPercent value is out of range.
PositionSpanInv (Status.9)	BOOL	PositionEUMax = PositionEUMin.

Description

The POSP instruction usually receives the desired position setpoint from a PID instruction output.

Scaling the Position and Setpoint Values

The PositionPercent and SPPercent outputs are updated each time the instruction is executed. If either of these values is out of range (less than 0% or greater than 100%), the appropriate bit in Status is set, but the values are not limited. The instruction uses these formulas to calculate whether the values are in range:

How the POSP Instruction Uses the Internal Cycle Timer

The instruction uses CycleTime to determine how often to recalculate the duration of Open and Close output pulses. An internal timer is maintained and updated by DeltaT. DeltaT is the elapsed time since the instruction last executed. Whenever the internal timer equals or exceeds the programmed CycleTime (cycle time expires) the Open and Close outputs are recalculated.

You can change the CycleTime at any time.

If CycleTime = 0, the internal timer is cleared to 0, OpenOut is cleared to false and CloseOut is cleared to false.

Producing Output Pulses

The following diagram shows the three primary states of the POSP instruction.

Calculating Open and Close Pulse Times

OpenOut is pulsed whenever SP > Position feedback. When this occurs, the instruction sets CloseTime = 0 and the duration for which OpenOut is to be turned on is calculated as:

POSP Caluculating open and close pulse times

If OpenTimen-1 < CycleTime, then add Deadtime to OpenTime.

If OpenTime > MaxOnTime, then limit to MaxOnTime.

If OpenTime < MinOnTime, then set OpenTime = 0.

If any of the following conditions exist, OpenOut is not pulsed and OpenTime = 0.

OpenFB is true or PositionPercent Greater than or equal to

100

CycleTime = 0

OpenRate = 0

SPPercent is invalid

The CloseOut is pulsed whenever SP < Position feedback. When this occurs, the instruction sets OpenTime = 0 and the duration for which CloseOut is to be turned on is calculated as:

If CloseTimen-1 < CycleTime, then add Deadtime to CloseTime.

If CloseTime > MaxOnTime, then limit to MaxOnTime.

If CloseTime < MinOnTime, then set CloseTime to 0.

If any of the following conditions exist, CloseOut will not be pulsed and CloseTime will be cleared to 0.0.

ClosedFB is true or PositionPercent Less than or equal to

CycleTime = 0

CloseRate = 0

SPPercent is invalid

OpenOut and CloseOut will not be pulsed if SPPercent equals PositionPercent. Both OpenTime and CloseTime will be cleared to false.

Affects Math Status Flags

Major/Minor Faults

None specific to this instruction. See Common Attributes
for operand-related faults.

Execution

Function Block

Condition/State	Action Taken
Prescan	EnableIn and EnableOut bits are cleared to false.
Tag.EnableIn is false	EnableIn and EnableOut bits are cleared to false.
Tag.EnableIn is true	EnableIn and EnableOut bits are set to true. The instruction executes.
Instruction first run	N/A
Instruction first scan	OpenTime and CloseTime are cleared to 0.0.
Postscan	EnableIn and EnableOut bits are cleared to false.

Structured Text

Condition/State	Action Taken
Prescan	See Prescan in the Function Block table.
Normal Execution	See Tag.EnableIn is true in the Function Block table.
Postscan	See Postscan in the Function Block table.

Examples

Example 1

In this example, the POSP instruction opens or closes a motor-operated valve based on the CVEU output of the PIDE instruction. The actual valve position is wired into the Position input and optional limit switches, which show if the valve is fully opened or closed, are wired into the OpenedFB and ClosedFB inputs. The OpenOut and CloseOut outputs are wired to the open and close contacts on the motor-operated valve.

Function Block

Structured Text

FlowController.PV := WaterFlowRate;

PIDE(FlowController);

FlowValve.SP := FlowController.CVEU;

FlowValve.Position := FlowValvePosition;

FlowValve.OpenedFB := FlowValveOpened;

FlowValve.ClosedFB := FlowValveClosed;

POSP(FlowValve);

OpenFlowValveContact := FlowValve.OpenOut;

CloseFlowValveContact := FlowValve.CloseOut;

Process Control Instructions

Instruction Set

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