What is cascade control and what are its benefits?

Cascade control uses two controllers: an inner (secondary) loop controlling a faster variable, and an outer (primary) loop controlling the main variable through the inner loop setpoint. Benefits include: faster disturbance rejection (inner loop catches disturbances before they affect main variable), improved primary control (inner loop linearizes/speeds up inner dynamics), and reduced effect of inner-loop nonlinearities. Common in temperature control (flow is inner loop) and motor control (current is inner loop). Inner loop must be faster than outer loop.