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PID_ET Controller Logic Documentation

Overview

A PID_ET (Proportional–Integral–Derivative with External Tuning) controller is a time-dependent PID control mechanism that allows external control of the execution interval. This is especially useful in applications with non-periodic execution, or when the control interval needs to be synchronized with external events or slower tasks.

This controller reduces human intervention and ensures consistent automatic process regulation using feedback.

Like standard PID, it minimizes the error (e(t)) between the Setpoint (SP) and Process Variable (PV) using:

  • Proportional (P): Correction based on current error.
  • Integral (I): Correction based on accumulated past error.
  • Derivative (D): Correction based on prediction of future error trends.

The PID_ET controller increases timing accuracy in advanced process systems with variable cycles or event-driven calls.

Logic Block Illustration

../../Images//Programing_Manual/Logic_Manual/PID_ET/PID_ET_0.png

../../Images//Programing_Manual/Logic_Manual/PID_ET/PID_ET_1.png

In above picture, we are showing inputs and output of PID-ET block.

PID_ET Function Block – Pins Information

Signal Type Description
INPUT WORD Raw analog signal input from the field.
PV_LO REAL Minimum range value for scaling the process variable (PV).
PV_HI REAL Maximum range value for scaling the process variable (PV).
LOC_SP REAL Local setpoint (operator-entered value).
REM_SP REAL Remote setpoint from another logic block/system.
CAS_SP REAL Cascade setpoint used in cascade control mode.
SP_LO_LM REAL Setpoint low limit.
SP_HI_LM REAL Setpoint high limit.
MVLO REAL Lower limit of PID output (MV).
MVHI REAL Upper limit of PID output (MV).
P REAL Proportional gain.
I REAL Integral time constant.
D REAL Derivative time constant.
MODE BYTE Controller mode: 0 = Manual, 1 = Auto, 2 = Cascade, 3 = Remote.
CTRL_ACTN BIT TRUE = Direct acting; FALSE = Reverse acting.
DIR BIT FALSE = Direct output; TRUE = Reverse output.
SP_TRACK BIT If TRUE, SV tracks PV (e.g., during mode change).
ExtPV_En BIT If TRUE, ExtrnPV overrides internal PV.
ExtrnPV REAL External PV value (if enabled).
FILTER REAL Filtering coefficient for noise reduction on PV.
PVLL REAL Very low alarm limit for PV.
PVLO REAL Low alarm limit for PV.
PVHI REAL High alarm limit for PV.
PVHH REAL Very high alarm limit for PV.
OPLL REAL Very low alarm limit for Output (MV).
OPLO REAL Low alarm limit for Output (MV).
OPHI REAL High alarm limit for Output (MV).
OPHH REAL Very high alarm limit for Output (MV).
HYST REAL Hysteresis value for PV and Output alarms.
ACK BIT Acknowledgement input for alarms.
RTN_ACK_REQD BIT If TRUE, return-to-normal alarms require acknowledgment.
IN_LO REAL Output signal low limit from the program.
IN_HI REAL Output signal high limit from the program.
OP_LO WORD Raw AO output - minimum raw value.
OP_HI WORD Raw AO output - maximum raw value.
PID_OP REAL Manual value applied when in manual mode.
PV REAL Scaled process variable derived from input.
SV REAL Selected setpoint (LOC_SP / REM_SP / CAS_SP).
MV REAL Calculated output from PID (0–100%).
Output WORD Final raw analog output to AO channel.
PV_LL_ALM BIT TRUE when PV < PVLL.
PV_LO_ALM BIT TRUE when PV < PVLO.
PV_HI_ALM BIT TRUE when PV > PVHI.
PV_HH_ALM BIT TRUE when PV > PVHH.
OP_LL_ALM BIT TRUE when Output < OPLL.
OP_LO_ALM BIT TRUE when Output < OPLO.
OP_HI_ALM BIT TRUE when Output > OPHI.
OP_HH_ALM BIT TRUE when Output > OPHH.
WireBreak BIT TRUE indicates analog input wire break.
WireShort BIT TRUE indicates analog input wire short.
Acknowleged BIT TRUE if alarms are acknowledged; resets when ACK is triggered.
TagName STRING(10) Tag identifier for this PID_ET instance.
Desc STRING(12) Description of the function block for SCADA/HMI reference.

Important Notes

  • External Timing (dt) must be provided every execution cycle.
  • Manual Mode overrides controller output with MV_MAN.
  • Reset Input (RESET) clears integrator and derivative memory.
  • BIAS allows feedforward output correction for improved response.
  • All alarm outputs depend on PV limit values and hysteresis.

Alarm Handling:
Each alarm bit output (PV_LL_ALM, PV_LO_ALM, etc.) is activated based on defined PV limits. Alarms can be acknowledged using the ACK input.

Typical PID_ET Workflow

  1. PV is read from sensor (optionally filtered).
  2. Controller compares SP with PV.
  3. Calculates ERROR = SP - PV.
  4. Applies P, I, D control using externally provided dt.
  5. Adds BIAS to generate total MV.
  6. Applies clamping to enforce MV_MIN and MV_MAX.
  7. Output MV is used to control actuator.
  8. In manual mode, MV = MV_MAN.

Faceplate Display Recommendations

  • Display SP, PV, and MV in real time.
  • Show current control mode (Auto/Manual).
  • Allow operator to enter MV_MAN and acknowledge alarms.
  • Display alarm statuses and limits on faceplate.

Use Cases

  • Temperature control with variable scan times.
  • Valve positioning in event-driven control loops.
  • Level control in asynchronously updated systems.
  • Remote PID tuning where you cannot guarantee task timing.

Tip:
Use high-resolution timers or synchronize with a process scheduler to ensure consistent dt. Poorly timed or fluctuating dt values will cause erratic control.