In industrial control, what is a sequence controller, and which statement best describes its primary role in time-based on/off operations?

Introduction / Context:
Sequence controllers are common in industrial automation for packaging lines, batch processes, machine cycles, and safety interlocks. Unlike continuous controllers that regulate a variable to a setpoint, a sequence controller executes a predefined series of steps, often actuating devices on or off based on time, counters, or discrete conditions. This question checks whether you can distinguish time/step-driven logic from other control types.

Given Data / Assumptions:

• Applications include start–stop cycles, valve open/close, conveyor advances, and timed dwell periods.
• Implementation may be via PLC ladder logic, function blocks, sequential function charts, or dedicated sequencers.
• Outputs are primarily discrete (on/off), though analog checks can be used as interlocks.

Concept / Approach:
A sequence controller advances through a list of states: Step 1, Step 2, … Each step triggers discrete outputs and waits for a condition (time elapsed, sensor made, counter reached) before proceeding. This differs from PID loops that continuously compute control effort to minimize error. Therefore, the best description highlights on/off actions relative to time or events.

Step-by-Step Solution:

Verification / Alternative check:
PLC manuals describe sequencers/timed state machines that map directly to on/off steps; this validates the choice.

Why Other Options Are Wrong:

Common Pitfalls:
Confusing sequence control (discrete steps) with feedback regulation; mixing the two without clear interlocks can cause unsafe transitions.

Final Answer:
It performs on/off control functions according to a defined time or step sequence