Sequential Function Charts and Program Organization

What Is a Sequential Function Chart?

A Sequential Function Chart (SFC) is one of the five IEC 61131-3 programming languages. Where ladder diagram excels at combinational logic, SFC is purpose-built for processes that move through a defined series of states — exactly the kind of step-by-step motion you see in pick-and-place stations, conveyor indexers, and packaging machines. An SFC breaks a process into steps and transitions. Only one step (or a small set of parallel steps) is active at a time. When the condition on a transition becomes TRUE, the PLC deactivates the current step and activates the next one.

Anatomy of an SFC

Steps are drawn as rectangles, each labeled with a name and number (e.g., S1_Home, S2_Pick). Inside or beside each step you attach actions — output coils, subroutine calls, or function block invocations that execute while that step is active. Actions carry a qualifier: N (non-stored, active only while the step is active), S (set, latches ON), R (reset, latches OFF), D (time-delayed), L (time-limited), and others defined in IEC 61131-3.

Transitions appear as horizontal lines between steps, each with a Boolean condition written beside it. The PLC scans every active step's outgoing transition every scan cycle. As soon as the condition is TRUE, control passes to the next step. Common transition conditions include a sensor input turning ON, a timer done bit (TDN), a counter reaching a preset, or a combination written in ladder or structured text.

An initial step (drawn with a double border) marks where execution begins at program startup or after a reset. A jump or looping connection at the bottom of the last step returns control to the initial step, creating a repeating cycle.

Tracing the Home → Pick → Place Example

Consider a three-step SFC for a simulated pick-and-place station:

1. S1_Home — the arm rests at the home sensor. Action N: energize the home solenoid. Transition: `Pick_CMD AND Home_Sensor`.

2. S2_Pick — the arm moves to the pick position and the gripper closes. Action N: extend pick cylinder, energize gripper. Transition: `Part_Detected AND Gripper_Closed`.

3. S3_Place — the arm moves to the place position and the gripper opens. Action N: retract pick cylinder, move to place position, de-energize gripper. Transition: `Place_Sensor AND Gripper_Open`.

A jump from S3_Place back to S1_Home closes the loop. On your printed worksheet, shade only the rectangle of the currently active step as you watch the simulation. Notice that outputs tied to inactive steps are automatically de-energized when those steps are not active — a key safety advantage over unstructured ladder code where forgotten outputs can remain ON.

Adding a Fourth Step: Inspect

Your modification exercise inserts S4_Inspect between S3_Place and the return to S1_Home. The inspect station uses a vision sensor or a simple part-present sensor to confirm the placed part before the next cycle begins.

• New step S4_Inspect: Action N — activate inspect_light output. Transition: `Inspect_Timer.DN` (a TON timer preset to the required dwell time, typically 2–5 seconds).
• The transition out of S3_Place now points to S4_Inspect instead of directly back to S1_Home.
• The transition out of S4_Inspect points to S1_Home.

After downloading, use online monitoring to watch the step status bits. Most PLC platforms display the active step highlighted in color. Verify that the Inspect step activates immediately after the gripper opens at the place position and that it holds for the expected dwell time before transitioning home. Timing tolerance for this exercise is ±0.5 s of the preset.

Program Organization Units (POUs)

Industrial PLC programs are not one giant block of code. IEC 61131-3 defines Program Organization Units:

• Program (PROG): the top-level POU scheduled by the operating system. Typically written in ladder diagram (LD) and called every scan.
• Function Block (FB): a reusable POU with internal state (memory), such as a TON timer or a PID controller. Multiple instances can run simultaneously with separate data.
• Function (FC): a stateless POU that returns a single value — useful for calculations like unit conversion or scaling.
• Subroutine / routine: on some platforms (e.g., Allen-Bradley Logix), routines within a program serve a similar organizing role.

A two-level architecture for this station might look like: Main LD routine handles safety interlocks and calls the SFC sequence routine; the SFC routine calls a timer subroutine or invokes TON function blocks for timed transitions. Keeping safety logic at the top level ensures it executes every scan regardless of which SFC step is active.

Safety Considerations

Before working on or near the pick-and-place station, apply OSHA 29 CFR 1910.147 lockout/tagout procedures: isolate all energy sources (electrical, pneumatic), apply personal locks, and verify zero energy state before reaching inside the cell. Verify absence of voltage with a calibrated meter following NFPA 70E practices. Machine guarding per OSHA 1910.212 must remain in place during normal operation; safeguarding requirements for the robot cell follow ANSI/RIA R15.06 / ISO 10218. Never rely solely on a software-controlled SFC as the only safeguard — hardware safety relays or a safety PLC must provide independent E-stop and gate-interlock circuits per NFPA 79.