C135 PCS 7 (PCS7)
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Configuring Independent Internal and External Manipulated Variable Control in APL VlvAnL

Configuring Independent Internal and External Manipulated Variable Control in APL VlvAnL

In the SIMATIC PCS 7 Advanced Process Library (APL) analog valve block VlvAnL (FB1896), the operator selection for Internal versus External Manipulated Variable (MV) is greyed out on the WinCC faceplate when MV_LiOp is locked or when operating mode coupling is enforced. By default, VlvAnL couples MV specification to operating mode (External MV in Auto mode, Internal MV in Manual mode). To allow an operator to independently switch between Internal and External MV regardless of operating mode, set MV_LiOp = 0 (enabling operator control), set OS_Perm.Bit21 = 1 ('Operator can switch to internal MV') and OS_Perm.Bit22 = 1 ('Operator can switch to external MV'), and configure Feature.Bit23 to decouple MV specification from automatic mode.

Table of Contents

Executive Summary

During testing of control valve faceplates in SIMATIC PCS 7 V9.0 SP2, engineers frequently query the operational behavior of the Advanced Process Library (APL) block VlvAnL (FB1896, Analog Control Valve). Operators commonly need the flexibility to decide whether the valve's position setpoint (Manipulated Variable, MV) originates internally from an operator input on the faceplate (MV_Int) or externally from an upstream PID controller or cascade block (MV_Ext), independent of whether the valve is in Automatic or Manual mode. However, in standard configurations, the Internal/External toggle button on the faceplate remains disabled (greyed out), and the block rigidly forces External MV during Auto and Internal MV during Manual. Unlocking independent operator control requires properly configuring the linking input MV_LiOp, permission masks in OS_Perm, and specific mode coupling Feature bits.

Architecture and Fundamental Mechanics

The operational logic governing Manipulated Variable (MV) specification in VlvAnL relies on three interlocking parameters:

1. Linking Parameter Mechanics: MV_LiOp

VlvAnL controls who has authority to select the MV source using MV_LiOp (Manipulated Variable Link/Operator):
- MV_LiOp = 1 (Linked): The MV source is controlled exclusively by CFC logic via inputs MV_IntLi and MV_ExtLi. Operator buttons on the faceplate are permanently disabled.
- MV_LiOp = 0 (Operator): The operator is authorized to select Internal or External MV from the WinCC faceplate, provided operating permissions permit it.

2. Operator Permission Word: OS_Perm

Even when MV_LiOp = 0, the faceplate buttons remain greyed out unless explicitly authorized by OS_Perm:
- OS_Perm.Bit21: '1 = Operator can switch to internal MV'.
- OS_Perm.Bit22: '1 = Operator can switch to external MV'.
- If either bit is 0, the corresponding radio button or selection menu in WinCC Runtime is disabled.

3. Mode Coupling and Feature Bit Dependencies

By default, APL blocks enforce tight coupling between operational mode and setpoint source:
- In Automatic mode (AutModOp), the block assumes supervisory control and automatically forces MV_ExtAct = 1.
- In Manual mode (ManModOp), the block assumes local operator control and forces MV_IntAct = 1.
- To break this coupling and allow External MV during Manual mode or Internal MV during Automatic mode, parameterize Feature.Bit23 ('Evaluation of the manipulated variable specification') and ensure bumpless transfer features (Feature.Bit4 and Feature.Bit5) match plant operating philosophies.

Step-by-Step Implementation Procedure

To enable independent operator selection of Internal and External MV in VlvAnL:

Step 1: Configure MV_LiOp in CFC

  1. Open the CFC chart containing the VlvAnL instance.
  2. Locate the input parameter MV_LiOp.
  3. Set MV_LiOp = 0 (False). Ensure no external logic is connected to MV_LiOp.
  4. Leave MV_IntLi and MV_ExtLi unconnected.

Step 2: Configure Operator Permissions (OS_Perm)

  1. Open the Object Properties of VlvAnL -> Parameters tab.
  2. Locate parameter OS_Perm.
  3. Ensure the following bits are set to 1 (True):
    - Bit 21: Operator can switch to internal MV (1).
    - Bit 22: Operator can switch to external MV (1).
    - Bit 0: Operator can switch to automatic mode (1).
    - Bit 1: Operator can switch to manual mode (1).

Step 3: Configure Feature Bits for Mode Decoupling

  1. Locate parameter Feature on the VlvAnL block.
  2. Configure Feature.Bit23: Set to 1 to enable independent selection of MV specification in the permitted modes.
  3. Configure Feature.Bit4: Set to 1 for bumpless switchover of the manipulated variable when transitioning between internal and external.

Step 4: Compile and Test in WinCC Runtime

  1. Compile and download CFC changes to the S7-400 / CPU 410 Automation Station.
  2. Open the VlvAnL faceplate in WinCC Runtime.
  3. Navigate to the Standard view.

Verification

To confirm independent MV selection:

  1. Manual Mode Internal/External Test:
    - Place the valve in Manual mode.
    - Click the MV specification button. Confirm that both Internal and External options are selectable.
    - Select External: verify that the valve positions according to MV_Ext while remaining in Manual mode.
    - Select Internal: verify that the operator can enter an arbitrary position (MV_Int).

  2. Automatic Mode Test:
    - Switch the valve to Automatic mode.
    - Confirm that the operator retains the ability to toggle or override MV specification if authorized by plant interlocks.

Limitations and Operational Precautions

Operational limitations and precautions:

  1. Process Safety Risks: Allowing operators to switch to Internal MV while the supervisory controller is in Automatic mode can disrupt cascade control loops. Ensure interlocks (Intlock, Protect) take precedence over internal valve positioning.
  2. Bumpless Tracking Alignment: If bumpless transfer is not configured (MV_Trk = 0), switching from External to Internal MV may cause abrupt valve jumps if the operator's last internal setpoint differs from active external flow.
  3. SFC / Batch Control Conflicts: In plants running SIMATIC BATCH, recipes expect automatic control over MV_Ext. Enabling operator override can cause batch phase timeouts.
  4. Commercial Disclaimer: Verify all software libraries, engineering licenses, and system packages with an authorized Siemens distributor or the Siemens Industry Mall.

Engineer FAQ

Why is the Internal/External selection greyed out on the VlvAnL faceplate?

It is greyed out because MV_LiOp is set to 1 (linked to CFC logic), OS_Perm bits 21/22 are 0, or default mode coupling forces external MV in automatic mode.

What is the difference between MV_LiOp and ModLiOp?

ModLiOp controls who commands Automatic vs Manual mode, whereas MV_LiOp controls who commands Internal vs External Manipulated Variable specification.

How do I allow operators to select internal MV?

Set MV_LiOp = 0 and ensure OS_Perm.Bit21 = 1 ('Operator can switch to internal MV').

What happens to the valve position during an external-to-internal switchover?

If Feature.Bit4 is enabled, the switchover is bumpless: the internal setpoint initializes to the current active valve position, preventing sudden flow surges.

Which parameter supplies the external valve setpoint in CFC?

The structured analog input MV_Ext (e.g., connected to the MV output of a PIDConL controller).

Technical References

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Related Tags

  • VlvAnL
  • Analog Valve
  • MV_LiOp
  • OS_Perm
  • APL Setpoint
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