Glossary · Automation fundamentals, platforms and components
Step chain
Also known as: Step sequence, Sequencer
German: Schrittkette
In PLC programming, a step chain (German: Schrittkette) is a sequential control structure in which a process is divided into consecutive steps, only defined steps are active at a time, and a transition condition must be fulfilled to move from one step to the next.
- Automation components
In one sentence
A step chain divides a process into steps with transition conditions between them; it is the classic structure for sequential PLC control.
Example
A pick-and-place step chain moves through "gripper down", "close gripper", "gripper up", "move to place" and "open gripper", each step waiting for its sensor.
How it applies
- Engineering: Step chains are implemented in SFC (sequential function chart) or in other languages with step numbers or state machines. Clear rules for step monitoring (timeouts), manual intervention and restart after a stop are part of the design.
- Operation: Showing the active step and the missing transition condition on the HMI helps operators and service staff find out why a machine stopped.
- Documentation: Functional descriptions list the steps with their actions and transition conditions. Use the same step numbers in the description, the program and the HMI messages, so that a message like "Step 40: waiting for gripper closed" can be traced directly.
Step chain vs. state machine
A step chain usually runs forward through a mostly linear sequence. A state machine allows arbitrary transitions between states. In practice the two overlap, and many step chains contain branches and loops.