C135 PCS 7 (PCS7)
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Configuring and Dynamically Deactivating Deadbands on APL PIDConL in PCS 7

Configuring and Dynamically Deactivating Deadbands on APL PIDConL in PCS 7

To dynamically deactivate the deadband on the APL continuous controller block 'PIDConL' (FB1874) in SIMATIC PCS 7, set the deadband parameter 'DeadBand' to 0.0 using an interconnected CFC signal or analog multiplexer (such as 'Mux02' or 'SelA02In'). When 'DeadBand' is greater than 0.0, the controller suppresses corrective action as long as the control deviation remains within the threshold range. Deactivating the deadband dynamically is essential when pairing PIDConL with the Control Performance Monitor block 'ConPerMon' (FB1805), as an active deadband artificially masks loop deviations and degrades performance index accuracy.

Table of Contents

Executive Summary

In continuous closed-loop process control, deadbands are applied to PID controllers to prevent actuator chatter and wear on mechanical final control elements (such as pneumatic control valves or dampers) caused by high-frequency measurement noise. However, during automated tuning, loop diagnostic profiling, or tight setpoint tracking, an active deadband impedes control performance and causes diagnostic blocks—specifically the PCS 7 APL Control Performance Monitor (ConPerMon)—to calculate misleading performance metrics. This guide details the mathematical operation of the deadband in PIDConL, methods for dynamic switching in CFC, and interaction rules with ConPerMon.

Architecture and Fundamental Mechanics

Mathematical Mechanics of the PIDConL Deadband and ConPerMon Interaction:

  • Deadband Operation in PIDConL:
  • Error Calculation: $e(t) = SP - PV$ (or $PV - SP$ depending on direct/reverse action).
  • If $|e(t)| \le \text{DeadBand}$, the effective error passed to the P, I, and D algorithms is set to $0.0$.
  • Result: The manipulated variable MV remains completely stationary, preventing unnecessary valve movement.
  • Why Dynamic Deactivation is Needed:
  • During steady-state production, a deadband of $0.5\%$ to $1.0\%$ prolongs valve packing life.
  • During startup, major load disturbances, or loop monitoring with ConPerMon, the deadband introduces phase lag, limit-cycle oscillations, or unrecorded drift.
  • Interaction with ConPerMon (Control Performance Monitor):
  • ConPerMon monitors control loop health by evaluating the statistical distribution of control deviation and manipulated variable oscillations.
  • If a large deadband is active, ConPerMon interprets the lack of corrective action as poor loop response or inadequate tuning, resulting in false 'Poor Performance' warnings.

Step-by-Step Implementation Procedure

Engineering Procedure to Implement Dynamic Deadband Control in CFC:

  1. Instantiate PIDConL and ConPerMon in CFC:
    - Insert PIDConL (FB1874) from the APL library into an OB35 cyclic interrupt chart.
    - Insert ConPerMon (FB1805) and connect PIDConL.PV_Out to ConPerMon.PV and PIDConL.SP_Out to ConPerMon.SP.
  2. Implement Dynamic Deadband Switching Logic:
    - Insert an analog selector block (e.g., SelA02In from the APL library).
    - Connect input In1 := 0.0 (Deadband Deactivated).
    - Connect input In2 := 0.5 (Normal Operational Deadband, e.g., 0.5% of span).
    - Connect a boolean condition (e.g., TuningActive or ConPerMonActive or OperatorDeadbandDisable) to SelA02In.Sel.
    - Interconnect SelA02In.Out directly to PIDConL.DeadBand.
  3. Configure ConPerMon Parameters:
    - In ConPerMon, configure settling time (SettlTime) and sample times matching the process reaction rate.
    - Verify that when ConPerMon is actively recording a benchmark curve, the selector automatically routes 0.0 to PIDConL.DeadBand.
  4. Compile and Test:
    - Compile CFC changes and download to the AS.
    - Monitor PIDConL.DeadBand online to confirm dynamic switching.

Verification

To verify deadband switching and loop behavior:
1. In CFC test mode, force SelA02In.Sel := 1; verify PIDConL.DeadBand reflects 0.5.
2. Introduce a minor process deviation of $0.2\%$; verify that PIDConL.ER (effective error) reads $0.0$ and MV remains frozen.
3. Force SelA02In.Sel := 0 (Deactivate); verify PIDConL.DeadBand immediately switches to 0.0.
4. Verify that PIDConL immediately reacts to the $0.2\%$ deviation and trims MV to restore setpoint alignment.

Limitations and Operational Precautions

Operational precautions:
- Never set an excessively large deadband on integrating or unstable processes (e.g., exothermic reactor temperature or boiler drum level), which can lead to runaway conditions.
- Rapidly toggling the deadband on and off in noisy process environments can induce violent valve chatter; incorporate hysteresis or timer delays on the switching trigger.
- Ensure that DeadBand is parameterized in engineering units or percentage corresponding to the PV range scale (PV_OpScale).

Engineer FAQ

How do I turn off the deadband in PIDConL?

Set the input parameter DeadBand := 0.0.

Why does ConPerMon complain about poor loop performance when a deadband is active?

Because the deadband holds the controller output static despite an existing control deviation, which ConPerMon interprets as sluggish controller response.

What units does the DeadBand parameter use in PIDConL?

The DeadBand parameter uses the same engineering units as the process variable (PV_OpScale).

Can the operator adjust the deadband from the WinCC faceplate?

Yes, by navigating to the 'Parameter' view of the PIDConL faceplate and entering a new deadband value (subject to authorization Level 6).

Does deactivating the deadband affect bumpless transfer?

No, switching DeadBand does not affect bumpless transfer; the integral accumulator adjusts smoothly to the new error value.

Technical References

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Related Tags

  • PIDConL
  • ConPerMon
  • Deadband
  • APL Control
  • Closed-Loop Control
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