What is cascade control and when should it be used?

Cascade control uses two controllers - the primary (master) output becomes the setpoint for the secondary (slave). Secondary loop handles disturbances faster before they affect primary variable. Use when: disturbances enter through manipulated variable path (e.g., steam pressure variations), secondary variable responds faster than primary, and intermediate variable can be measured. Example: reactor temperature (primary) cascaded to jacket temperature (secondary). Benefits: faster disturbance rejection, handles manipulated variable non-linearity, improves overall control. Secondary loop must be 3-5x faster than primary.