Proportional Integral Derivative (PID)

The PID instruction controls a process variable such as flow, pressure, temperature, or level.

The PID instruction typically receives the process variable (PV) from an analog input module and modulates a control variable output (CV) on an analog output module in order to maintain the process variable at the desired setpoint.

The .EN bit indicates execution status. The .EN bit is set when the EnableIn transitions from false to true. The .EN bit is cleared when the EnableIn becomes false. The PID instruction does not use a .DN bit. The PID instruction executes every scan as long as the EnableIn is true.

Available Languages

Ladder Diagram

FactoryTalk Design Studio

DSL - Ladder Diagram

PID(PID,ProcessVariable,Tieback,ControlVariable,PIDMasterLoop,InHoldBit,InHoldValue);

Operands

There are data conversion rules for mixed data types within an instruction. See Data Conversion.

Ladder Diagram

Operand	Type	Format	Description
PID	PID	structure	PID structure
Process variable	SINT	tag	Value you want to control
	INT
	DINT
	REAL
Tieback	SINT	immediate	(optional)
	INT	tag
	DINT		Output of a hardware hand/auto station which is bypassing the output of the controller. Enter 0 if you don’t want to use this parameter
	REAL
Control variable	SINT	tag	Value which goes to the final control device (valve, damper, etc.)
	INT
	DINT		If you are using the deadband, the Control variable must be REAL or it will be forced to 0 when the error is within the deadband.
	DINT
	REAL
PID master loop	PID	Structure	Optional
			PID tag for the master PID
			If you are performing cascade control and this PID is a slave loop, enter the name of the master PID
			Enter 0 if you do not want to use this parameter
Inhold bit	BOOL	tag	Optional
			Current status of the inhold bit from a 1756 analog
			Output channel to support bumpless restart
Inhold value	SINT	tag	Optional
	INT		Data readback value from a 1756 analog output
	DINT		Channel to support bumpless restart
	REAL		Enter 0 if you don’t want to use this parameter
Setpoint			Display only
Setpoint			Current value of the setpoint
Process variable			Display only
Process variable			Current value of the scaled Process_Variable
Output %			Display only
Output %			Current output percentage value

PID structure

Specify a unique PID structure for each PID instruction.

Mnemonic	Data Type	Description
.CTL	DINT	The .CTL member provides access to the status members (bits) in one, 32-bit word. Bits 07-15 are set by the PID instruction. See .CTL member.
.SP	REAL	setpoint
.KP	REAL	Independent - proportional gain (unitless)
.KP	REAL	Dependent - controller gain (unitless)
.KI	REAL	Independent - integral gain (1/sec)
.KI	REAL	Dependent - reset time (minutes per repeat)
.KD	REAL	Independent - derivative gain (seconds)
.KD	REAL	Dependent - rate time (minutes)
.BIAS	REAL	feedforward or bias %
.MAXS	REAL	maximum engineering unit scaling value
.MINS	REAL	minimum engineering unit scaling value
.DB	REAL	deadband engineering units
.SO	REAL	set
.MAXO	REAL	maximum output limit (% of output)
.MINO	REAL	minimum output limit (% of output)
.UPD	REAL	loop update time (seconds)
.PV	REAL	scaled PV value
.ERR	REAL	scaled error value
.OUT	REAL	output %
.PVH	REAL	process variable high alarm limit
.PVL	REAL	process variable low alarm limit
.DVP	REAL	positive deviation alarm limit
.DVN	REAL	negative deviation alarm limit
.PVDB	REAL	process variable alarm deadband
.DVDB	REAL	deviation alarm deadband
.MAXI	REAL	maximum PV value (unscaled input)
.MINI	REAL	minimum PV value (unscaled input)
.TIE	REAL	tieback value for manual control
.MAXCV	REAL	maximum CV value (corresponding to 100%)
.MINCV	REAL	minimum CV value (corresponding to 0%)
.MINITIE	REAL	minimum tieback value (corresponding to 100%)
.MAXTIE	REAL	maximum tieback value (corresponding to 0%)
.DATA[17]	REAL	The .DATA member stores: .DATA[0] - integral accumulation .DATA[1] - derivative smoothing temporary value .DATA[2] - previous .PV value .DATA[3] - previous .ERR value .DATA[4] - previous valid .SP value .DATA[5] - percent scaling constant .DATA[6] - .PV scaling constant .DATA[7] - derivative scaling constant .DATA[8] - previous .KP value .DATA[9] - previous .KI value .DATA[10] - previous .KD value .DATA[11] - dependend gain .KP .DATA[12] - dependend gain .KI .DATA[13] - dependend gain .KD .DATA[14] - previous .CV value .DATA[15] - .CV descaling constant .DATA[16] - tieback descaling constant

The .CTL member

Bit	Number	Description
.EN	31
.CT	30	cascade type (0=slave; 1=master)
.CL	29	cascade loop (0=no; 1=yes)
.PVT	28	process variable tracking (0=no; 1=yes)
.DOE	27	derivative of (0=PV; 1=error)
.SWM	26	software mode (0=no-auto); 1=yes- sw manual)
.CA	25	control action (0=reverse (SP-PV); 1=direct (PV- SP))
.MO	24	station mode (0=automatic; 1=manual)
.PE	23	PID equation (0=independent; 1=dependent)
.NDF	22	derivative smoothing (0=no; 1=yes)
.NOBC	21	bias calculation (0=no; 1=yes)
.NOZC	20	zero crossing (0=no; 1=for deadband)
.INI	15	PID initialized (0=no; 1=yes)
.SPOR	14	setpoint out of range (0=no; 1=yes)
.OLL	13	CV is below minimum output value (0=no; 1=yes)
.OLH	12	CV is above maximum output value (0=no; 1=yes)
.EWD	11	error is within deadband (0=no; 1=yes)
.DVNA	10	error is alarmed low (0=no; 1=yes)
.DVPA	9	error is alarmed high (0=no; 1=yes)
.PVLA	8	PV is alarmed low (0=no; 1=yes)
.PVHA	7	PV is alarmed high (0=no; 1=yes)

Affects Math Status Flags

Major/Minor Faults

A minor fault will occur if:	Fault Type	Fault Code
UPD ≥ 0	4	35
setpoint out of range	4	36

See Common Attributes for operand-related faults.

Special Instructions

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