IPID

Process regulator that uses the feedback concept to regulate the process variable according to the difference between its current value and the value of the set point. PID achieves calculation by a method of sampling.
ipid_block
Parameter values for IPID function block:
Parameter
Data Type
Description
PV
REAL
Process variable. This value must be a REAL value. Infinity or Not a Number (NaN) values are not supported by this input.
SP
REAL
Set point. This value must be a REAL value. Infinity or Not a Number (NaN) values are not supported by this input.
Auto
BOOL
Determines whether IPID operates in AUTO or MANUAL mode.
  • TRUE for AUTO mode
  • FALSE for MANUAL mode
XO
REAL
Adjustment value. This value must be a REAL value. Infinity or Not a Number (NaN) values are not supported by this input.
In MANUAL mode, Xout = XO
Kp
REAL
Proportionality constant. This value must be a REAL value. Infinity or Not a Number (NaN) values are not supported by this input.
Ti
TIME
Integral time constant.
Possible values range from 0ms to 23hr59m59s999ms.
Td
TIME
Derivative time constant.
Possible values range from 0ms to 23hr59m59s999ms.
Ts
TIME
Minimum sample period.
If Ts is less than the application scan time, uses the application scan time as the effective sample interval.
The maximum effective sample period is Ts plus the application scan time.
Possible values range from 0ms to 23hr59m59s999ms.
Tdf
TIME
Derivative time constant.
Possible values range from 0ms to 23hr59m59s999ms.
Xmax
REAL
Maximum value of Xout. This value must be a REAL value. Infinity or Not a Number (NaN) values are not supported by this input.
Xmin
REAL
Minimum value of Xout. This value must be a REAL value. Infinity or Not a Number (NaN) values are not supported by this input.
INCstop
BOOL
Increment stop.
DECstop
BOOL
Decrement stop.
Derr
BOOL
Derivative action switch.
TRUE = error
FALSE = PV
Fwd
BOOL
Gain action.
TRUE = forward acting
Xout
REAL
IPID function block output.
Action required on the system to keep the regulation. Xout recalculates when taking each sample.
LIMIThi
BOOL
TRUE if Xout clamps at the maximum value of Xmax.
LIMITlo
BOOL
TRUE if Xout clamps at the minimum value of Xmin.
DQP
REAL
Delta output due to proportional action
DQI
REAL
Delta output due to integral action
DQD
REAL
Delta output due to derivative action
DXout
REAL
Total delta output
TIP: Loss of precision and underflow may occur.
When Xmax < Xmin, the function assumes that the values are reversed and swaps the values. The Reversed error counter increments each time the function has to swap the parameters.
When overflow occurs during execution, the stored error and delta terms set to zero.
In AUTO mode:
  • Xout sets to zero
  • LIMIThi sets to FALSE
  • LIMITlo sets to FALSE
In MANUAL mode:
  • Xout sets to XO clamped to the range Xmin to Xmax
  • LIMIThi sets to XO > Xmax
  • LIMITlo sets to XO < Xmin
AUTO Mode Operation
When a new sample is made and Auto = TRUE, calculates the output Xout:
Xout = Xout + Kp * ( dE[t] + I[t]/Ti + fddD[t] * Td / T)
where
E[t] = SP - PV (error)

dE[t] = E[t] - E(t-1) (delta error)

I[t] = ( E[t] * T ) (integral term)

fddD[t] = ddD[t] filtered, first order lag, time constant Tdf

ddD[t] = dE[t] - dE[t-1] if Derr = true (delta delta error)

ddD[t] = dPV[t-1] - dPV[t] if Derr = false (delta delta PV)

dPV[t] = PV[t] - PV[t-1] (delta PV)
E[t-1] is the stored error value. This is the value during the last sample.
PV[t-1] is the stored process variable. This is the value during the last sample.
T is the elapsed time in milliseconds since the last sample.
The results of calculation are given as outputs:
DQP = Kp * dE[t] (delta proportional term)

DQI = Kp * I[t]/Ti (delta integral term)

DQD = KP * fddD[t] * Td / T (delta derivative term)

DXout = DQP + DQI + DQD (delta total output)
When Ti <10ms, the integral term sets to zero to avoid excessive action.
Using the Fwd input, swap the action between reverse and direct acting. When Fwd = False, a positive Kp gives a rising output on falling PV (reverse action). When Fwd = True, a positive Kp gives a falling output on falling PV (direct action). Setting a negative value to Kp also inverts the action.
After calculating Xout, applies the increment stop (INCstop), decrement stop (DECstop) and the clamp (Xmax and Xmin). When INCstop = TRUE and Xout has increased since the last sample or if DECstop = TRUE and Xout has decreased since the last sample, Xout sets to its previous value. When Xout > Xmax or Xout < Xmin:
  • Xout clamps to the range Xmin to Xmax
  • LIMIThi sets to Xout > Xmax
  • LIMITlo sets to Xout < Xmin
When INCstop, DECstop or the clamp are active, the output Xout stays at its last value. The results of calculation (DQP, DQI, DQD, DXout) continue updating. On initialization, INCstop and DECstop have no effect.
MANUAL Mode Operation
For new samples where Auto = FALSE, the output Xout immediately follows the adjustment value (XO) clamped to the range Xmin to Xmax. LIMIThi sets to XO > Xmax and LIMITlo sets to XO < Xmin. The stored error term sets to (SP - PV) and the stored delta terms set to zero. The increment and decrement stop (INCstop and DECstop) have no effect.
Function Block Diagram Example
(*FBD example with IPID function block*)
ipid_fbdexample
Structured Text Example
IPID(Process1, 25.5, Mode, Adjust, Proportion, IntegralT, DerivativeT, T#20ms, ConstantT, 101.2, 91.8, Increment, Decrement, ActionSwitch, GainAction);

Result:= PID_II.Xout;

High := PID_II.LIMIThi;

Low := PID_II.LIMITlo;

DeltaP := IPID.DQP;

DeltaI := IPID.DQI;

DeltaD := IPID.DQD;

DeltaTotal := IPID.DXout;
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