Lecture 35 - Cascade Control

Cascade control is a hierarchy of control loops. The secondary (inner or lower-level) control must respond much faster than the primary control. Cascade control has a single final control element (FCE). The secondary control manipulated the FCE. The primary control manipulates the setpoint to the secondary control. In this cascade structure, the system responds faster to potential disturbances than a single controller. The requirement, however, is an additional measurement for the secondary controller.

• Lecture 35 Notes

Homework

1. Course reading for next class: 18.7-18.8 (PPC)
2. Assignment due by the start of Lecture #36: Lab Projects

Discussion Questions

Test the lecture's ideas before or after class - commit to an answer first, then check it with a classmate or an AI tutor.

1. Cascade control adds a second sensor and a second controller but still moves only one valve. What exactly does the inner loop buy, and which class of disturbances does it neutralize before the primary variable ever feels them?
2. The standard rule is that the inner (secondary) loop must be substantially faster than the outer (primary) loop. What goes wrong if it is not - describe the interaction between the two controllers when their speeds are comparable.
3. Why is the inner loop always tuned first, and what mode(s) does the secondary controller usually need - is integral action in the inner loop essential, helpful, or sometimes omitted?
4. A furnace outlet-temperature controller cascades to a fuel-gas flow controller. A supply-pressure swing changes the fuel flow. Trace the disturbance through the cascade and explain why the outlet temperature barely moves - then trace the same upset through a single-loop design.

App: Run single-loop and cascade side by side on the same process in the Feedforward Cascade Control Studio - hit both with the same disturbance at the inner loop and watch the lit P&ID signal path show why the cascade version barely flinches.