How to Stop and Hold an Analog Valve in Motion Using PCS 7 APL VlvAnL and SelA02In

To stop an analog control valve mid-stroke while it is actively traveling toward a setpoint and hold its current physical position, use the Advanced Process Library (APL) selection block SelA02In configured as an edge-triggered position sample-and-hold circuit. Standard interlocks on VlvAnL (FB1896) force the valve to its configured fail-safe position (SafePos) rather than freezing mid-travel. While setting parameter SafePos = 2 (Stop) holds the valve output upon interlock trip, mechanical inertia and continuous feedback updates can cause hunting. By latching the valve's actual position feedback (Pos.Value) on the positive edge of the hold command and feeding it back into the setpoint channel via SelA02In, the valve halts smoothly and resumes travel to the original setpoint once the hold signal clears.

Functional Behavior of VlvAnL Under Interlock and Safety Conditions

In the SIMATIC PCS 7 Advanced Process Library, the VlvAnL block controls analog positioning valves (e.g., modulating control valves with 4–20 mA positioners):
- Normal Operation: In Automatic mode, VlvAnL calculates an analog manipulated output (MV) driven by an external setpoint (SP_Ext), ramping the positioner output until feedback (Pos) matches setpoint.
- Interlock Behavior (Permit, Intlock, Protect): Tripping an interlock forces the valve into the safety position configured by the SafePos parameter:
- SafePos = 0: Valve travels to fully Closed ($0\%$).
- SafePos = 1: Valve travels to fully Open ($100\%$).
- SafePos = 2: Valve Stops and freezes its current command output.
- The Operational Challenge: If an operator or recipe sequence needs to temporarily pause the valve mid-travel without declaring an emergency interlock trip, manipulating SafePos alone is often insufficient.

The Limitation of Direct Feedback Loop-Back (Hunting and Inertia)

A common intuitive approach is using an analog multiplexer (SelA02In) to route live feedback Pos to the valve setpoint whenever the hold condition occurs:
1. Hold signal activates -> Multiplexer switches output setpoint to read live Pos.
2. The Problem: Physical control valves exhibit pneumatic actuator delay and mechanical inertia. At the instant of command, the valve is still moving.
3. In cycle 1, the valve reads feedback $45\%$. The valve continues moving due to momentum.
4. In cycle 2, the feedback advances to $47\%$, writing a new setpoint of $47\%$.
5. The digital positioner interprets the continuously shifting setpoint as an active tracking ramp, continuing motion rather than locking the position.
6. Solution: The circuit must sample the feedback once on the rising edge of the hold trigger and latch that static float value for the entire duration of the hold.

Method 1: Configuring SafePos = 2 (Stop) for Interlock Halting

If the requirement is tied to process safety interlocks (e.g., an upstream pump tripping requires all modulating valves to freeze immediately):
1. Open the CFC chart containing the VlvAnL block.
2. Locate the parameter SafePos.
3. Set SafePos = 2.
4. Connect the interlock condition to parameter Intlock or Protect.
5. Operation: When the interlock de-asserts (FALSE), VlvAnL freezes output parameter MV at its current value, preventing further travel. When the interlock clears, the valve resumes ramping toward the original target.

Method 2: Precision Edge-Triggered Position Latching Using SelA02In

For non-safety operational holds (e.g., recipe pacing or inspection pauses), implement a sample-and-hold circuit using SelA02In (FB1888):

Step 1: Insert SelA02In in the CFC Chart

From the APL library catalog, drag SelA02In into the chart adjacent to VlvAnL.
Connect the output of SelA02In (Out) to the setpoint input of the downstream channel driver or external setpoint input of VlvAnL (SP_Ext).

Step 2: Wire the Feedback Sample-and-Hold Loop

To configure SelA02In as a trigger-latched memory:
- In01 (Input 1): Connect to SelA02In.Out (internal recursive loop to hold previous cycle value).
- In02 (Input 2): Connect to the valve actual position feedback (Pos.Value or VlvAnL.MV_Out.Value).
- Sel_In2 (Select Input 2): Connect to a positive-edge pulse generator (R_TRIG) driven by your Hold Command flag.

                  ┌───────────────┐
  Normal SP ─────►│ In01          │
                  │   SelA02In    │──────► Valve Setpoint
  Pos.Value ─────►│ In02          │
                  │   Sel_In2     │
  Edge Trigger ──►│   (Trigger)   │
                  └───────────────┘

Step 3: Operational Execution

While Sel_In2 is 0 (FALSE), SelA02In continuously outputs In01 (the recursive latch), holding the static position.
When the Hold Command rises, the one-shot pulse sets Sel_In2 = 1 for exactly one CPU cycle.
SelA02In reads Pos.Value at that microsecond, latches it to Out, and as the pulse clears, locks that exact value into In01.
The valve freezes rigidly at the latched coordinate without hunting.
When the Hold Command is released, switch the multiplexer back to the normal master setpoint.

Verification

To verify that the hold circuit operates smoothly without overshoot:
- In CFC online Test Mode, command the valve from $0\%$ to $100\%$ in Auto mode.
- While the valve is passing $50\%$, pulse the Hold Command.
- Confirm in the CFC chart and WinCC trend:
- The setpoint output freezes immediately at the exact position sampled (e.g., $51.2\%$).
- Actuator travel ceases, and feedback stabilizes at the latched setpoint within 1 to 2 seconds.
- Release the Hold Command: verify that the valve resumes travel smoothly toward the original $100\%$ setpoint.

Limitations and Operational Precautions

Recursive Interconnection Warnings in CFC: Connecting SelA02In.Out back into its own In01 creates an intentional recursive algebraic loop. In the CFC compiler, this generates a minor warning. Ensure the block execution sequence is stable.
Actuator Air Bleed: On pneumatic control valves with single-acting spring-return actuators, holding a valve mid-stroke requires the positioner to maintain balanced pneumatic chamber pressure. If positioner air seals leak, physical drift may occur over long periods.
Do Not Use for Personnel Safety: A software setpoint freeze must never replace a certified safety shut-off valve with a hardwired spring-return fail-closed actuator.

Engineer FAQ

Does VlvAnL have a built-in 'Hold' parameter?

No. VlvAnL provides SafePos (Open, Close, Stop) tied to safety interlocks and force tracking (MV_Forced), but does not feature an independent operational "Pause" input pin. Implementing SelA02In is the standard DCS design pattern.

Can I use Feature bits to modify interlock stop behavior?

Yes. Check Feature.Bit28 and Feature.Bit29 in VlvAnL to configure whether interlocks take precedence over manual mode and whether manual commands can override safety stops.

What is the difference between SelA02In and Mux02?

SelA02In is an Advanced Process Library block that handles compound analog structures (STRUCT: Value + ST), preserving NAMUR signal status. Mux02 is a primitive mathematical block that operates solely on raw floating-point numbers (REAL).

How does the block handle sensor failure during a hold?

If the position transmitter experiences a wire break during hold, Pos.ST drops to 16#00. SelA02In propagates the Bad signal status to VlvAnL, triggering a positioner fault alarm and causing VlvAnL to assert safe fallback.

Technical References

Siemens Industry Online Support: SIMATIC PCS 7 Advanced Process Library (APL) Manual.
SIOS Entry 109742323: PCS 7 Compendium Part A - Configuration Guidelines (Section: Valve Control).
SIOS Entry 109809264: SIMATIC Process Control System PCS 7 Function Manual.
Siemens Application Example: Implementing Advanced Tracking and Holding Circuits in CFC.

Related Tags

VlvAnL
SelA02In
Control Valves
APL
CFC
Sample and Hold
Industrial Automation

How to Stop and Hold an Analog Valve in Motion Using PCS 7 APL VlvAnL and SelA02In