Automating Control Strategy Transitions in SIMATIC PCS 7 SFC Sequences

To automatically switch control strategies (Fahrweisen) upon the completion of a sequence in a SIMATIC PCS 7 SFC Type, assign the target control strategy number to the 'Prepared Control Strategy' input (CS_LL or QCSP) and trigger the start condition of the subsequent sequencer using the completed state of the previous sequencer. Within the SFC Type logic or external CFC chart, evaluate the 'QCS' (active control strategy) and 'READY_TC' (ready for transition) status flags, ensuring that the sequencer terminates cleanly and transitions through the 'Ready to start' or 'Completed' state before the new strategy is applied.

Executive Summary

In SIMATIC PCS 7 batch and continuous recipe automation, process units frequently require sequential operation through multiple distinct phases—such as heating followed immediately by holding, or reaction followed by transfer. In PCS 7 SFC Types, these operating phases are modeled as Control Strategies (CS). When automation engineers attempt to chain control strategies so that Phase B starts automatically upon completion of Phase A, they often encounter transition deadlocks where the sequencer terminates but does not re-trigger. This article details the state machine mechanics of SFC control strategy transitions, the interaction between Prepared Control Strategy (CS_LL/QCSP) and Current Control Strategy (CS/QCS), and the correct CFC/SFC step configurations required for automated sequencing.

Architecture and Fundamental Mechanics

SFC Type Control Strategy Architecture and State Logic:

Control Strategies (CS) in SFC Types:
A Control Strategy defines an autonomous operating recipe or operational behavior for an SFC Type instance (e.g., CS 1 = Clean-in-Place, CS 2 = Production, CS 3 = Pressure Test).
Each SFC Type can contain up to 32 control strategies, each associated with specific sequencers.
Interface Pins for Strategy Handshaking:
CS: Active Control Strategy (INT) reported by the SFC instance.
CS_LL / QCSP: Control Strategy prepared for the next run (Prepared Control Strategy).
CS_LI / CS_EXT: External control strategy request via interconnection.
CS_ACT: Command input to activate the prepared control strategy.
READY_TC: Indicates that the SFC is in a state where a control strategy change is legally permitted by the state manager.
Sequencer Start Conditions and State Transitions:
A sequencer cannot simply restart if the SFC is in the Run state under an active strategy unless the SFC is configured for continuous execution or cyclic restart.
Under standard ISA-88 state logic, when Sequencer 1 completes, the SFC enters the Completed state. To start Sequencer 2 under CS 2, the SFC must either execute a Reset command (returning to Ready to start) or utilize continuous operation where the termination step assigns QCSP := 2 and triggers an internal restart transition.

Step-by-Step Implementation Procedure

Procedure to Configure Automatic Control Strategy Transitions in SFC Types:

Define Control Strategies in the SFC Type Characteristics:
- Open the SFC Type in the SFC Editor.
- In the Characteristics view, expand 'Control Strategies' and create CS 1 (e.g., 'Fill') and CS 2 (e.g., 'Heat').
Assign Sequencers to Control Strategies:
- Under the Sequencer tab, configure Sequencer 1 with Start Condition: RUNNING AND CS = 1.
- Configure Sequencer 2 with Start Condition: RUNNING AND CS = 2.
Configure the Transition Logic between Strategies:
- Method A (Continuous Mode within SFC Type):
- In the final active step of Sequencer 1, add an action: SELF.CS_LL := 2 (pre-select Strategy 2).
- Add an action to assert the strategy change trigger or transition flag.
- Method B (External Interconnection in CFC):
- In the parent CFC chart, evaluate the completion signal of the SFC block: when SFC.QSTATE = Completed and SFC.QCS = 1, trigger a pulse to SFC.CS_LI := 2.
- Connect the pulse to SFC.START_LI := TRUE and SFC.RESET_LI := TRUE with appropriate edge-delay logic.
Enable Operating Mode Interconnections:
- Set AUTOMA_L := TRUE and ENSTART := TRUE on the SFC block instance to allow automatic programmatic starts without operator faceplate confirmation.
Compile and Test:
- Perform a CFC compile (Changes only) and download to the AS.
- Monitor the SFC chart online in the SFC Visualization interface.

Verification

To verify automatic control strategy transitions:
1. In the SFC visualization or faceplate, switch the instance to 'Automatic' mode (AUTOMA_L = 1).
2. Trigger the start of Strategy 1; verify that QCS displays '1' and Sequencer 1 executes active steps.
3. Allow Sequencer 1 to reach its final step. Verify that CS_LL updates to '2'.
4. Confirm that upon completion, the SFC transitions cleanly to Strategy 2 (QCS = 2) without stalling in an intermediate 'Held' or 'Aborted' state.

Limitations and Operational Precautions

Operational precautions:
- Never change CS_LL during active sequence execution if target step interlocks are unfulfilled; doing so can cause transition faults.
- If the SFC Type is managed by SIMATIC BATCH, control strategies are governed exclusively by recipe phases. Do not implement automated internal strategy jumps inside the SFC Type for Batch-controlled equipment phases.
- Avoid zero-cycle transition loops where Strategy 1 immediately re-triggers Strategy 2 without clearing step actions, which can cause OB cycle execution spikes.

Engineer FAQ

Can an SFC Type change its Control Strategy while in the RUN state?

Yes, if the SFC Type sequencer configuration permits 'Changing control strategy during run', or if the active sequencer finishes and hands over execution in continuous mode.

What is the difference between CS and CS_LL?

CS (or QCS) represents the currently active, running control strategy number. CS_LL (or QCSP) represents the prepared control strategy waiting to be executed on the next start or transition.

Why does my SFC hang in 'Completed' instead of starting Strategy 2?

By default, an SFC requires a transition through the 'Reset' command to return to 'Ready to start' unless configured for continuous operation or reset automatically via CFC logic.

How many Control Strategies can a single SFC Type support?

SIMATIC PCS 7 SFC Types support up to 32 distinct control strategies per type.

Can SIMATIC BATCH override internal SFC control strategy switches?

Yes. SIMATIC BATCH allocates equipment phases based on recipe recipes; internal programmatic strategy switches bypass Batch recipe arbitration and should be avoided in recipe-driven units.