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Understanding Tracking vs Forced Tracking and Setpoint Modes in PCS 7 APL Controllers

Understanding Tracking vs Forced Tracking and Setpoint Modes in PCS 7 APL Controllers

In the SIMATIC PCS 7 Advanced Process Library (APL), controllers like PIDConL distinguish between Setpoint Modes and Tracking behaviors to ensure safe, bumpless process transitions. Setpoint Modes separate Internal Setpoints (SP_Int, entered locally by operators) from External Setpoints (SP_Ext, supplied by recipes or master controllers). Tracking (Nachführen) is an operational mechanism where internal setpoints or manipulated variables continuously follow an external reference (such as SP tracking PV via SP_TrkPV) to prevent setpoint jumps during mode switches. Forced Tracking (Zwangsnachführen) is an overriding protective interlock (such as MV_Forced) that forcibly clamps the controller output to a safe value regardless of mode or PID error because the downstream actuator is unable or forbidden to receive control commands.

Table of Contents

Executive Summary

Closed-loop feedback control in continuous process plants requires robust handling of mode transfers to prevent dynamic process shocks when switching between manual intervention, cascade control, and emergency interlocks. In the SIMATIC PCS 7 Advanced Process Library (APL), function blocks such as PIDConL, PIDStepL, and MotSpdCL implement sophisticated setpoint and tracking algorithms that are frequently misunderstood. Control engineers often confuse standard Setpoint Tracking (Nachführen) with Forced Output Tracking (Zwangsnachführen), or misconfigure internal versus external setpoint handling. This guide demystifies these control concepts, details their mathematical and logical mechanics, and provides application patterns for bumpless operation.

Architecture and Fundamental Mechanics

Setpoint and Tracking Architecture in APL Controllers

+-------------------------------------------------------------------------+
|                        APL Controller (PIDConL)                         |
+-------------------------------------------------------------------------+
| SETPOINT MANAGEMENT:                                                    |
|                                                                         |
|   [Internal SP (SP_Int)]   <-- Set locally by Operator via Faceplate    |
|   [External SP (SP_Ext)]   <-- Set by High-Level Cascade / Recipe Logic |
|                                                                         |
|   Selection via SP_LiOp & SP_ExtOp:                                     |
|   - SP_LiOp = 0: Operator selects Int/Ext via faceplate                 |
|   - SP_LiOp = 1: Interconnected CFC logic commands Int/Ext              |
+-----------------------------------|-------------------------------------+
                                    |
                                    v
+-------------------------------------------------------------------------+
| TRACKING (Nachführen): Operational Bumpless Transfer                    |
|                                                                         |
|   When SP_TrkPV = 1:                                                    |
|   - While in External SP or Manual mode, SP_Int continuously tracks PV. |
|   - On switching to Internal SP or Auto:                                |
|     Setpoint == Process Value -> ZERO SETPOINT JUMP / BUMPLESS!         |
+-----------------------------------|-------------------------------------+
                                    |
                                    v
+-------------------------------------------------------------------------+
| FORCED TRACKING (Zwangsnachführen): Overriding Protective Clamping      |
|                                                                         |
|   Triggered by: MV_Forced = 1 or Actuator Interlock Active              |
|   - Overrides Manual, Auto, Internal, and External modes!               |
|   - Controller Output (MV) is FORCIBLY CLAMPED to MV_ForcedVal          |
|   - Integrator is held / anti-reset windup engaged                      |
|   - Purpose: Equipment protection (e.g. valve shut on trip)             |
+-------------------------------------------------------------------------+

Comparison of Tracking Mechanisms

  1. Setpoint Tracking (SP_TrkPV):
    - Objective: Eliminates step changes in the control error ($e = SP - PV$) when returning to automatic internal control.
    - Operation: When active, the internal setpoint is continuously updated to equal the live process variable ($SP_{int} := PV$). Upon transitioning to automatic, the controller begins calculating from an initial error of zero.
  2. Manipulated Variable Tracking (MV_Trk):
    - Objective: Allows an external signal (such as actual valve stem position feedback) to steer the controller output during manual or standby states.
    - Operation: When MV_TrkOn = 1, the controller output follows MV_Trk ($MV := MV_{trk}$).
  3. Forced Tracking (Zwangsnachführen):
    - Objective: Highest-priority safety and interlock clamping.
    - Operation: When active via MV_Forced or interlock blocks (Intlock, Protect), normal PID algorithms are bypassed, and MV is immediately held at a fixed safe value (e.g., 0.0% for a fail-closed valve).

Step-by-Step Implementation Procedure

Step 1: Configure Setpoint Selection Logic in CFC

  1. In SIMATIC Manager, open the CFC chart containing PIDConL.
  2. Determine the setpoint authorization hierarchy:
    - If the operator must choose between local setpoints and external supervisory targets: Set SP_LiOp := 0.
    - If external logic (e.g., batch recipe phase) dictates setpoint switching: Set SP_LiOp := 1 and connect the switching command to SP_ExtLi.
  3. Interconnect the cascade master controller output or recipe tag to SP_Ext.

Step 2: Enable Bumpless Setpoint Tracking (Nachführen)

  1. To prevent process surges when an operator switches from External to Internal setpoint:
  2. Set the input parameter SP_TrkPV := 1 (or configure via Feature bits).
  3. In this mode, while the loop is running on SP_Ext, the internal variable SP_Int continuously mirrors PV.
  4. When the operator toggles from External to Internal, the loop maintains the current operating level smoothly.

Step 3: Implement Forced Manipulated Variable Clamping (Zwangsnachführen)

  1. For unit shutdown, pump trip, or safety interlock conditions:
  2. Connect the trip signal (e.g., Pump_Running = 0) to PIDConL.MV_Forced.
  3. Connect the desired safe actuator position (e.g., 0.0%) to PIDConL.MV_ForcedVal.
  4. When the pump stops, PIDConL immediately forces the control valve closed, suppressing PID integral accumulation.

Step 4: Configure Feature Bits for Tracking Behavior

  1. In the Object Properties of PIDConL, open the Feature word parameter.
  2. Feature Bit 0: Set to 1 to define setpoint behavior during mode changes.
  3. Feature Bit 6: Enable tracking of the manipulated variable in manual mode if position feedback tracking is required.
  4. Compile the CFC chart via Changes Only and download to the AS.

Verification

  1. Bumpless Transfer Test: Run the loop in External Setpoint mode at 60 °C. With SP_TrkPV = 1, switch to Internal mode on the WinCC faceplate. Verify that SP_Int initializes at exactly 60 °C with no jump in controller output.
  2. Forced Tracking Interlock Test: With the loop in Automatic mode controlling at 50%, activate the interlock driving MV_Forced = 1. Verify that MV drops instantly to MV_ForcedVal (0%) and the faceplate displays the forced status banner.
  3. Integral Windup Check: Deactivate the forced tracking interlock. Verify that the controller resumes regulation from the current process value without integral windup overshoot.

Limitations and Operational Precautions

  1. Forced Tracking Overrides Manual: Operators cannot override a forced tracking condition from the faceplate; normal manual control is completely blocked until MV_Forced is de-asserted.
  2. Tracking Lag with Slow Filters: If the input filter time (FiltTi) on PV_In is large, setpoint tracking will track the filtered process value rather than raw field values.

Engineer FAQ

What is the difference between Nachführen and Zwangsnachführen?

Nachführen (Tracking) is an operational feature to achieve smooth, bumpless setpoint transfers; Zwangsnachführen (Forced Tracking) is an overriding protective clamp forcing output to a safe state.

What does SP_TrkPV do?

When SP_TrkPV is active, the internal setpoint continuously follows the live process value (PV) during manual or external mode.

Why is bumpless transfer important in DCS?

Sudden step changes in setpoints cause dramatic valve movements, pipe hammer, product off-spec, and pressure relief trips.

Can operators adjust setpoints during forced tracking?

Operators can edit setpoint values, but the controller output remains clamped to the forced value until the force condition clears.

What block input triggers forced output tracking?

The input parameter MV_Forced (1 = active) forces the output to the value defined at MV_ForcedVal.

What is the difference between SP_Int and SP_Ext?

SP_Int is the local setpoint entered on the faceplate; SP_Ext is an external setpoint provided by supervisory logic or cascade masters.

How do I prevent integral windup when a control valve is closed?

Connect the valve closed limit switch or trip condition to MV_Forced or MV_TrkOn to freeze the PID integral term.

Does MotSpdCL support tracking?

Yes, MotSpdCL supports setpoint tracking and forced speed setpoints for variable frequency drives.

Which APL manual describes controller structures in depth?

Refer to the Siemens application guideline 'Technical Functions for SIMATIC PCS 7 in the Chemical Industry' (Chapter 6.4).

Can tracking be enabled from WinCC?

Yes, faceplates expose options to enable or disable setpoint tracking if permitted by block feature bits.

Technical References

Related Articles

Related Tags

  • PIDConL
  • APL
  • Tracking
  • Forced Tracking
  • Bumpless Transfer
  • Setpoint Modes
  • CFC Control
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