Explain the operation and purpose of a 'cascade control' system.

A cascade control system is a control strategy that uses two or more control loops in a hierarchical structure to improve the performance of a process control system. The primary purpose is to provide better disturbance rejection and more precise control than can be achieved with a single control loop. In a cascade control system, there is an outer loop, also called the primary or master loop, and an inner loop, also called the secondary or slave loop. The outer loop controls the primary process variable, which is the variable that is ultimately being controlled. The output of the outer loop controller serves as the setpoint for the inner loop controller. The inner loop controls a secondary process variable that directly affects the primary variable and is faster to respond to disturbances. For example, consider controlling the temperature of a liquid in a tank using steam. The outer loop would control the tank temperature by adjusting the setpoint of the inner loop, which controls the steam flow rate. A temperature sensor in the tank provides feedback to the outer loop controller. A flow sensor provides feedback to the inner loop controller. If a disturbance occurs, such as a change in the steam supply pressure, the inner loop will quickly compensate by adjusting the steam valve to maintain the desired steam flow rate. This prevents the disturbance from significantly affecting the tank temperature, which is controlled by the outer loop. The cascade control system is effective because the inner loop isolates the outer loop from disturbances that occur in the steam supply.