Freezing Manipulated Value Output and Pausing Control in APL PIDConL

To freeze the manipulated value (MV) output of an APL PIDConL controller at a constant value during a specific plant operation phase without stopping block execution, use the forced tracking inputs MV_ForOn and MV_Forced. Asserting MV_ForOn = 1 immediately clamps MV to the value of MV_Forced, freezing control output regardless of setpoint or process variable variations. Alternatively, to freeze MV at its current position, wire MV to tracking input MV_Trk and assert MV_TrkOn = 1. Never deactivate the block's EN (Enable) input, as disabling EN halts cyclic execution, creates derivative gain spikes upon re-enabling, and causes severe process bumps.

APL PIDConL Control Modes and Output Overrides

In industrial process automation, continuous PID controllers (such as the APL block PIDConL FB 1874) frequently require pausing during batch holds, catalyst injection pauses, or auxiliary flushing phases. While the plant waits, the control valve must remain immobilized at a fixed opening (e.g. 0%, 50%, or current position) without accumulating integral error ($I$-windup).

Why Deactivating Block Enable (EN) is Operationally Dangerous

Engineers unfamiliar with continuous control principles sometimes attempt to pause controllers by setting EN = 0:

Halts Internal Time Accounting: Setting EN = 0 suppresses execution of the block algorithm entirely. The internal sample time calculation stops, and message blocks (Alarm_8P) fail to dispatch process alarms.
Massive Derivative and Integral Kick: When EN is restored to 1 minutes or hours later, the internal filter evaluates the elapsed time as a single step change. The derivative term ($D$) spikes violently, driving the valve to full open or shut within a single cycle, potentially bursting pipes or blowing relief valves.

Method 1: Clamping Output via Forced Tracking (MV_ForOn / MV_Forced)

To force the valve to an exact, predefined position (e.g., maintaining exactly 25.0% opening during a hold phase):

Assign the desired fixed position to parameter MV_Forced (REAL, e.g. 25.0).
When the freezing condition triggers, assert MV_ForOn = 1.
PIDConL instantly clamps MV = 25.0, halts integral accumulation, and sets feature bit feedback flags.
When MV_ForOn returns to 0, control resumes smoothly according to configured anti-windup rules.

Method 2: Freezing Current Value via Output Tracking (MV_TrkOn / MV_Trk)

To freeze the valve at whatever position it occupied the moment the pause occurred:

Wire the current output PIDConL.MV (or valve position feedback) into tracking input MV_Trk.
Assert MV_TrkOn = 1 upon initiating the pause.
While MV_TrkOn = 1, the controller halts PID calculation and maintains MV = MV_Trk.
When MV_TrkOn transitions back to 0, the controller resumes automatic calculation with zero bump.

Comparison of PIDConL Output Freezing Techniques

The table below contrasts methods for freezing manipulated values in PIDConL:

Freezing MethodTarget ParametersFinal Output ValueBumpless RecoveryOperational Safety RatingForced ValueMV_ForOn = 1, MV_ForcedClamped to fixed setpointYes (Pre-calculated)High (Standard safety override)Tracking ModeMV_TrkOn = 1, MV_TrkFrozen at current position100% BumplessHighest (Standard freeze method)Manual Mode SwitchManModLi = 1, ManManual setpoint valueDependent on operatorModerate (Requires mode change)Integral HoldHold = 1Proportional variations continueBumpless integralModerate (Only freezes I-action)Disabling EN InputEN = 0Freeze (Unsafe)Severe bump / derivative kickProhibited (Violates safety guidelines)

Verification

Verify output freezing and bumpless resumption in CFC:

Open the CFC chart online with PIDConL in automatic mode (AutAct = 1) controlling a live or simulated loop.
Note the active valve output (e.g. MV = 42.5%).
Assert MV_TrkOn = 1; confirm that MV remains frozen at 42.5% despite inducing deliberate changes in SP or PV.
Change process input PV_In; confirm the integral error accumulator does not wind up.
Reset MV_TrkOn = 0; confirm PIDConL smoothly transitions back into closed-loop control without valve surging.

Limitations and Operational Precautions

Observe the following safeguards when manipulating PID outputs:

Feature Bit Verification: Verify Feature.Bit5 (Tracking behavior) to ensure tracking takes precedence over manual commands.
Alarm Suppression: During prolonged forced periods, consider asserting MsgLock = 1 to suppress nuisance control deviation alarms (ER_Alarm).
Anti-Windup Limits: Ensure MV_HiLim and MV_LoLim remain properly clamped to prevent invalid tracking values.

Engineer FAQ

Why is disabling the EN input on a PID block considered dangerous?

Disabling EN suspends cyclic time tracking. When re-enabled, the algorithm evaluates the elapsed time as a single timeslice, producing an enormous derivative spike that slams control valves and destabilizes the process.

What is the difference between MV_Forced and MV_Trk?

MV_Forced forces the output to a specified numeric value regardless of operating mode. MV_Trk tracks a dynamic external signal (such as actual valve feedback) to provide bumpless synchronization.