In a cascade control system, two or more controllers are cascaded to control one or more process variable. Controllers are said to be "in cascade" when the output (OP.1) of the first or primary controller (master) is used to manipulate the set-point(SP.2) of another or secondary controller (slave). When two or more controllers are cascaded, each will have its own measurement input (PV.1, PV.2) but only the primary controller can have an independent set-point (SP.1) and only the secondary controller has an output (OP.2) to the process. This concept of cascade control is illustrated in the diagram below:
|Block Diagram of a Cascade Control System|
The purpose of cascade control is to achieve greater stability of the primary process variable by regulating a secondary process variable in accordance with the needs of the first. An essential requirement of cascaded control is that the secondary process variable be faster-responding (i.e. less lag time) than the primary process variable.
Cascade control is one of the successful methods for improving the control performance and reducing the maximum deviation and integral error for disturbance responses. Cascade control has been used widely, as it is easy to implement and requires simple calculations.
Benefits of Cascade Control
The principal benefits of a cascade control system are:
(1) The secondary controller corrects disturbances occurring in the secondary
loop before they can affect the primary, or main, variable.
(2) The secondary controller can significantly reduce phase lag in the secondary loop thereby improving the speed or response of the primary loop.
(3) Gain variations in the secondary loop are corrected within that loop.
The secondary loop enables exact manipulation of the flow of mass or energy by the primary controller
Practical Cascade Control Loop:
Consider the cascade control loop shown below:
|A Cascade Control Loop|
As shown in the diagram above, we intend to control the temperature of a process fluid using cascade control. The primary controller, TC, is used to measure the temperature of the process fluid via Temperature transmitter TT, and this is compared with the setpoint value (SP.1) of TC. The secondary controller, FC, measures flow with the aid of flow transmitter, FT, which is then used to keep the fuel flow constant against variables like pressure changes.
The primary controller's output (OP.1) is used to manipulate the Setpoint (SP.2) of the secondary controller (FC) thereby changing the fuel feed rate to compensate for temperature variations of the process fluid. Variations and inconsistencies in the fuel flow rate are corrected solely by the secondary controller: the FC controller.