Case Study: FB_BatchMixer SFC

A complete sequential batch automation system implemented as a Function Block (FB1) in Siemens TIA Portal using the GRAPH programming language (IEC 61131-3 Sequential Function Chart) — covering step-transition sequencing, sensor-gated transitions, time-based process control, and cyclic reset logic on an S7-1511C-1 PN CPU

Project Overview

FB_BatchMixer [FB1] implements a six-step sequential batch process within a Siemens S7-1511C-1 PN PLC, using the GRAPH programming language. The function block is called from Main [OB1] and manages the complete lifecycle of a mixing batch: from idle standby through tank filling, mixing, dispensing, cleaning, and automatic cyclic reset.

GRAPH was chosen deliberately over LAD or SCL alternatives because batch processes are inherently sequential — each stage must complete before the next begins. Implementing this as a flat LAD network would require manual interlock logic between every stage, producing brittle, hard-to-audit code. GRAPH enforces sequentiality at the language level: a step cannot become active unless its preceding transition is satisfied, eliminating an entire class of interlock errors by design. This is the same architecture used in industrial pharmaceutical, food processing, and chemical manufacturing plants.

Problem

A batch mixing process requires strict ordering of operations: both tanks must fill before mixing begins, mixing must run for a defined duration before dispensing, and the system must clean before restarting. The engineering challenge was therefore:

Sequence Definition

The GRAPH sequence consists of six steps connected by six transitions, forming a single linear sequence with a cyclic return from T6 → S1. The Navigation panel confirms the full chain. Each transition is implemented as a LAD rung embedded within GRAPH.

Step Name Transition Condition Action / Purpose
S1 S_Idle #Start_Button (NO contact) Await operator start signal. System holds here until button is pressed.
S2 S_FillA #TankA_Level_High Open Valve A and fill Tank A. Transition fires on level sensor, not a timer.
S3 S_FillB #TankB_Level_High Open Valve B and fill Tank B. Tank A valve de-energises automatically on step exit (N qualifier).
S4 S_Mix #S_Mix.T > T#60s Enable mixer motor. Duration controlled by GRAPH's native step elapsed time — no external TON required.
S5 S_Dispense #Tank_Empty Open dispense valve and drain. Transition waits on the empty sensor — prevents advancing before drain completes.
S6 S_Clean #Clean_Done → S1 CIP rinse cycle. On assertion, T6 routes the sequence pointer back to S1, completing the cyclic batch loop.

Solution Architecture

Function Block Interface — Inputs (19):
  • Operator commands: Start_Button, OFF_SQ, INIT_SQ, ACK_EF
  • Sensor inputs: Tank_Empty, TankA_Level_High, TankB_Level_High, Clean_Done — all marked HMI-visible (Accessible / Writable / Visible)
  • Mode selectors: SW_AUTO, SW_TAP, SW_TOP, SW_MAN — standard GRAPH operating mode flags for auto, tap, top-of-cycle, and manual modes
  • Sequence navigation: S_PREV, S_NEXT, S_SEL (Int), S_ON, S_OFF, T_PUSH — for manual step-through during commissioning without modifying the FB
Function Block Interface — Outputs (4):
  • S_NO (Int) — current active step number, wired to HMI for live sequence position display ("Step 4 — Mixing" rather than just "Running")
  • Valve_A (Bool) — primary actuator output; HMI-visible and writable for manual override during commissioning
  • S_MORE, S_ACTIVE (Bool) — GRAPH diagnostic outputs indicating sequence presence and activity state, available to the diagnostic buffer
Architecture — Key Design Decisions:
  • GRAPH over LAD. Only the active step's actions are evaluated each scan cycle, reducing CPU load and making the control flow auditable at a glance. A flat LAD implementation would evaluate all six stage rungs continuously, requiring interlock logic to prevent them overlapping.
  • Sensor-gated fill transitions (T2, T3). Level transitions fire on discrete sensor state, not assumed timing. If Tank A fills slower due to supply pressure variation, the sequence waits for actual level confirmation rather than advancing early on a fixed timer.
  • Step-time over external TON (T4). #S_Mix.T only accumulates while S_Mix is active and resets automatically on step exit — no external timer management, no reset logic needed on cyclic re-entry.
  • N qualifier on all step outputs. Non-stored actions de-energise when the step becomes inactive. Each step owns exactly its outputs; nothing bleeds across step boundaries without explicit Set/Reset actions.
  • Cyclic reset via T6 → S1. After S_Clean completes, T6 routes the GRAPH pointer back to S1 without any external reset command. The batch cycle restarts automatically, supporting continuous production without operator intervention between batches.

Project Screenshots

TIA Portal: FB_BatchMixer outputs and sequence start
FB_BatchMixer block interface — Output section (rows 17–21) showing S_NO, Valve_A, S_MORE, S_ACTIVE. Navigation panel shows Sequences (1) with S_Idle active. Canvas shows T1-Trans1 with #Start_Button NO contact rung and S2 / S_FillA below.
TIA Portal: Full input interface listing for FB_BatchMixer
Complete Input interface (rows 1–19). HMI-visible checkboxes confirmed for Start_Button, TankA_Level_High, TankB_Level_High, and Clean_Done. Mode selectors, acknowledgement flag (ACK_EF), and manual navigation inputs (S_PREV, S_NEXT) defined as internal-only.
TIA Portal: Full sequence overview — all six steps
Navigation panel showing the complete six-step chain: S_Idle → T1 → S_FillA → T2 → S_FillB → T3 → S_Mix → T4 → S_Dispense → T5 → S_Clean → T6 → S1. Canvas zoomed to S1 / S_Idle and T1-Trans1, confirming the #Start_Button transition rung.
TIA Portal: S_Mix timed transition T4 and S_Dispense sensor transition T5
Mid-sequence view. T4-Trans4 implements the 60-second mixing hold using CMP > on #S_Mix_T against T#60s. T5-Trans5 gates on #Tank_Empty. Steps S3 (S_FillB), S4 (S_Mix), and S5 (S_Dispense) visible in the sequence column.
TIA Portal: S_Clean and T6 cyclic reset to S1
Final sequence segment. S6 / S_Clean activated after T5 (#Tank_Empty). T6-Trans6 evaluates #Clean_Done and the sequence pointer returns to S1 (arrow at canvas bottom), closing the cyclic batch loop.

Transition Logic — Annotated

# T1 - Trans1: S_Idle → S_FillA
# Condition: Start_Button asserted (NO contact, level-sensitive)
#Start_Button  |--[ ]--|--( )

# T2 - Trans2: S_FillA → S_FillB
# Sensor-gated: waits for actual level confirmation, not a fixed timer
# Prevents advancing early if supply pressure or inlet valve varies
#TankA_Level_High  |--[ ]--|--( )

# T3 - Trans3: S_FillB → S_Mix
# Same sensor-gated pattern as T2
#TankB_Level_High  |--[ ]--|--( )

# T4 - Trans4: S_Mix → S_Dispense
# S_Mix.T is GRAPH's native step elapsed timer
# Only accumulates while S_Mix is active; resets automatically on step exit
# No external TON instance required; no reset logic needed on cyclic re-entry
#S_Mix_T  |--CMP > --|--( )
           Time: T#60s

# T5 - Trans5: S_Dispense → S_Clean
# Drain completion gated on sensor state, not a timer
# Prevents S_Clean from starting before the tank has physically emptied
#Tank_Empty  |--[ ]--|--( )

# T6 - Trans6: S_Clean → S1 (cyclic reset)
# Clean_Done asserted externally by CIP controller or operator
# GRAPH routes sequence pointer back to S1 on satisfaction
# Batch cycle restarts without any external reset command
#Clean_Done  |--[ ]--|--( )  -->  S1

Engineering Decisions Worth Calling Out

Testing & Debugging

The sequence was validated by stepping through each transition manually in SW_MAN mode, verifying that sensor inputs fired transitions correctly and that step outputs de-energised cleanly on exit. Key test cases:

The Watch & Force table was used to monitor S_NO, S_ACTIVE, and the sensor inputs in real time during simulation, confirming that the diagnostic outputs tracked the sequence accurately at each stage.

Challenges Faced

The most significant conceptual challenge was adapting to GRAPH's execution model after working primarily in LAD. In LAD, every rung is evaluated every scan. In GRAPH, only the active step's actions execute — outputs in inactive steps are simply not evaluated. Initially this caused confusion when actuators appeared to behave inconsistently; the fix was understanding that GRAPH manages output scope through step activation, not through continuous coil evaluation.

The second challenge was the mixing timer. The first implementation used an external TON instance with its IN coil gated on S_ACTIVE. This worked on the first cycle but failed on the second — the TON's ET did not reset until IN went low, creating a one-scan window where T4 fired immediately on S_Mix re-entry before the timer had actually started. Replacing with #S_Mix.T eliminated the problem entirely since the step timer has no persistent state across step re-entries.

The third challenge was variable scoping. GRAPH strictly separates Inputs (readable in transitions) from Outputs (writable in step actions). A sensor signal incorrectly declared as an Output produced a compile error when used in a transition rung. Systematically reviewing the interface against GRAPH's variable access rules resolved this and produced a cleaner, more correctly structured block interface.

Development was also impacted by TIA Portal Cloud session timeouts. Unsaved GRAPH edits were lost on two occasions, requiring partial reconstruction of the sequence. This was mitigated by adopting a strict save-after-each-step discipline and capturing screenshots at every milestone.

Key Learning Outcomes

Tools & Technologies

Siemens TIA Portal GRAPH project demonstrating SFC sequence design, step-transition logic, sensor-gated process control, and cyclic batch automation.