ABSTRACT
Cascade control is a control strategy that involves a master controller, which acts over a cascade of slave controllers. Two controllers are widely used: one is the master controller, which instructs the second controller, which is the slave controller. However, each controller controls different process variables. The bene›t of such a control strategy over a single-controller system will be understood from the following example. Consider a steam-heated tank where steam žow is manipulated by a temperature controller. However, the temperature of the tank increases when the temperature of the steam coil is increased and vice versa. Hence, the hierarchy of change of temperature is from the steam-coil temperature to the tank temperature, i.e., the change in coil temperature is followed by a change in tank temperature. If the coil temperature slowly rises, the rise of the tank temperature will also be delayed. In order to reduce this delay, the tank temperature controller is connected with another controller that controls the coil temperature. This arrangement is shown in Figure 7.1 where the tank-temperature controller is the master controller, and it senses the tank temperature and sends the output signal as the set point for the coil temperature to the slave controller. The slave controller senses the coil temperature and generates an output signal based on the difference between the output signal of the master controller and the coil temperature and ›nally manipulates the steam žow accordingly. The cascade control system has two controllers and two sensors but one control valve.
