Cascade control in process automation What does the term “cascade control” specifically mean in the context of feedback control architectures for chemical process units?

Introduction / Context:
Cascade control is a widely used multi-loop strategy in process industries to improve disturbance rejection and reduce the effective dead time seen by the primary controller. Instead of relying on a single loop, cascade introduces a secondary (inner) loop that measures and controls an intermediate variable closely coupled to the actuator or the main disturbance source. This question clarifies what “cascade control” means in practical control-system design.

Given Data / Assumptions:

• Plant is linear enough for standard feedback concepts to apply.
• The primary controlled variable is the ultimate quality or safety variable (e.g., product temperature).
• The secondary variable is faster and nearer to the manipulated element or disturbance (e.g., steam flow or jacket outlet temperature).

Concept / Approach:
Cascade control employs two feedback loops in a hierarchy. The primary controller (outer loop) outputs a setpoint to the secondary controller (inner loop). The inner loop quickly corrects fast disturbances or actuator nonlinearities before they propagate to the primary variable. This architecture often improves bandwidth and phase margin seen by the primary loop, enabling tighter control.

Step-by-Step Solution:

Verification / Alternative check:
Textbook examples (jacketed CSTR, heat exchanger) show significant improvement in disturbance rejection when a fast inner loop (e.g., flow control) is nested inside a slow temperature loop.

Why Other Options Are Wrong:

• Feedforward control: Uses disturbance measurements without feedback; not cascade.
• On–off control: Binary control action; unrelated to cascade structure.
• One single feedback loop: Lacks the hierarchical nesting that defines cascade.

Common Pitfalls:
Assuming any two loops in a plant are “cascade”; true cascade requires that the primary controller's output is the setpoint of the secondary controller and that the inner loop is significantly faster.

Final Answer:
More than one feedback loop arranged hierarchically (primary–secondary).