Glossary · PLC and control programming
Sequence control
Also known as: Sequential control, Step sequence
German: Ablaufsteuerung
In control engineering, sequence control is a type of control in which a process runs through defined steps one after another, with the transition to the next step depending on conditions such as feedback signals, elapsed time or operator commands.
- PLC programming
In one sentence
Sequence control runs a process through defined steps, moving on when conditions such as feedback signals or time are met.
Example
A washing cycle runs through the steps fill, wash, drain, rinse and spin; each step starts only when the previous one reports completion.
How it applies
- Engineering: Sequence control is typical for machines and batch processes with clear operating steps. It is implemented in Sequential function chart (SFC), as a Finite-state machine (FSM) in structured text, or in vendor-specific sequencers.
- Fault handling: Each step needs defined behavior for faults, timeouts and aborts. Step supervision timers detect a step that takes too long, for example a cylinder that never reaches its end position.
- Operation: Operators need to see which step is active and which condition is missing for the next transition. Manual modes that allow stepping through a sequence help during setup.
- Documentation: A step chart or sequence table (step, actions, transition conditions, supervision times) is the best basis for both the software documentation and the operating manual's description of the machine cycle.
Sequence control vs. logic control
In logic (combinational) control, outputs depend only on current inputs. In sequence control, outputs depend on the current step as well, so the controller has memory of where the process is.