PID

Function Block - PID loop

Inputs

AUTO	BOOL	TRUE=normal mode - FALSE = manual mode
PV	REAL	Process value
SP	REAL	Set point
Xout_Manu	REAL	Output value in manual mode
KP	REAL	Gain
TI	REAL	Integration time
TD	REAL	Derivation time
TS	TIME	Sampling period
XMIN	REAL	Minimum allowed output value
XMAX	REAL	Maximum output value
I_SEL	BOOL	If FALSE, the integrated value is ignored
INT_HOLD	BOOL	If TRUE, the integrated value is frozen
I_ITL_ON	BOOL	If TRUE, the integrated value is reset to I_ITLVAL
I_ITLVAL	REAL	Reset value for integration when I_ITL_ON is TRUE
DEADB_ERR	REAL	Hysteresis on PV. PV will be considered as unchanged if greater than (PVprev - DEADBAND_W) and less that (PRprev + DEADBAND_W)
FFD	REAL	Disturbance value on output

Outputs

Xout	REAL	Output command value
ER	REAL	Last calculated error
Xout_P	REAL	Last calculated proportional value
Xout_I	REAL	Last calculated integrated value
Xout_D	REAL	Last calculated derivated value
Xout_HLM	BOOL	TRUE if the output valie is saturated to XMIN
Xout_LLM	BOOL	TRUE if the output value is saturated to XMAX

Diagram

Remarks

It is important for the stability of the control that the TS sampling period is much bigger than the cycle time.

In LD language, the output rung has the same value as the AUTO input, corresponding to the input rung.

ST Language

(* MyPID is a declared instance of PID function block *)
MyPID (AUTO, PV, SP, XOUT_MANU, KP, TI, TD, TS, XMIN, XMAX,
I_SEL, I_ITL_ON, I_ITLVAL, DEADB_ERR, FFD);
XOUT := MyPID.XOUT;
ER := MyPID.ER;
XOUT_P := MyPID.XOUT_P;
XOUT_I := MyPID.XOUT_I;
XOUT_D := MyPID.XOUT_D;
XOUT_HLM := MyPID.XOUT_HLM;
XOUT_LLM := MyPID.XOUT_LLM;

FBD Language

LD Language

(* ENO has the same state as the input rung *)

IL Language:

(* MyPID is a declared instance of PID function block *)
Op1: CAL MyPID (AUTO, PV, SP, XOUT_MANU, KP, TI, TD, TS,
                XMIN, XMAX, I_SEL, I_ITL_ON, I_ITLVAL,
                DEADB_ERR, FFD)
     LD MyPID.XOUT
     ST XOUT
     LD MyPID.ER
     ST ER
     LD MyPID.XOUT_P
     ST XOUT_P
     LD MyPID.XOUT_I
     ST XOUT_I
     LD MyPID.XOUT_D
     ST XOUT_D
     LD MyPID.XOUT_HLM
     ST XOUT_HLM
     LD MyPID.XOUT_LLM
     ST XOUT_LLM