Sequential Function Charts, or SFCs, are a standard way to describe step by step control logic in automated warehouse systems. They help engineers organize complex processes such as receiving, scanning, sorting, storing, picking, packing, and shipping. In logistics, an SFC makes it easier to see which machine action happens next and which condition must be true before moving on.
This matters because a clear control sequence improves safety, reliability, and troubleshooting in busy automated facilities.
An SFC is built from steps, transitions, and actions that form a controlled sequence. A step represents a state of the system, such as conveyor running or robotic arm placing a package, while a transition checks conditions such as sensor signals, timers, or barcode matches. In a warehouse, these charts can coordinate conveyors, scanners, robotic arms, AGVs, storage racks, and dock doors so that material flows without collisions or bottlenecks.
SFCs are often implemented in programmable logic controllers, where each step activates specific outputs and waits for verified feedback before advancing.
Key Facts
- An SFC sequence is made from steps, transitions, and actions.
- A transition becomes true when its condition is satisfied, such as Sensor_A = 1 or Timer_Done = 1.
- Cycle time for one item can be estimated as T_cycle = T_receive + T_scan + T_sort + T_pick + T_pack + T_ship.
- Throughput can be estimated as Throughput = items processed / time.
- A safe SFC should include interlocks, fault steps, reset logic, and emergency stop behavior.
- Parallel SFC branches allow multiple warehouse actions to occur at the same time, such as scanning one package while an AGV moves another.
Vocabulary
- Sequential Function Chart
- A graphical control method that represents a process as ordered steps connected by transition conditions.
- Step
- A state in an SFC where one or more actions are active, such as running a conveyor or opening a gate.
- Transition
- A logical condition that must become true before the SFC moves from one step to the next.
- Interlock
- A safety or control condition that prevents an action from occurring unless required conditions are met.
- Programmable Logic Controller
- An industrial computer that reads sensors, runs control logic, and controls machines such as motors, valves, and robots.
Common Mistakes to Avoid
- Skipping transition conditions makes the sequence unclear because the controller must know exactly when to leave each step.
- Treating a step as an instant event is wrong because a step is a state that may remain active until its transition condition is satisfied.
- Ignoring sensor feedback can create unsafe logic because the chart may assume a conveyor, robot, or AGV completed an action when it did not.
- Putting too many actions in one step makes troubleshooting harder because it becomes difficult to identify which machine action caused a fault.
Practice Questions
- 1 A warehouse line has SFC step times of 8 s for receiving, 5 s for scanning, 12 s for sorting, 20 s for picking, and 15 s for packing. What is the total cycle time for one item if the steps occur sequentially?
- 2 An automated dock processes 360 cartons in 2 hours using an SFC-controlled conveyor and scanner system. What is the throughput in cartons per hour and cartons per minute?
- 3 A package reaches a barcode scanner, but the scanner returns an unreadable code. Explain how an SFC should handle this situation using a fault step, transition condition, and operator or automatic recovery action.