Cascade Control


Cascade control refers to a process control strategy in which a process variable is controlled by manipulating the setpoint of a related process variable. In this particular strategy, the variable of interest is controlled by two control loops working in tandem.

Its loop consists of a primary loop and a secondary loop. These loops might also be referred to as the outer loop and inner loop, respectively. The primary loop provides the secondary loop with a setpoint, or target, for a process related to the primary control objective. The primary loop is sometimes known as the master loop because it provides a setpoint that the secondary loop, or slave loop, must follow.


Structures of Cascade Control
  •             Traditional CascadeBlock Diagram
  •             IMC Cascade Structure
  •             IMC Cascade control with  a simple feedback inner loop.

BEST TYPE OF ARRANGEMENT OR STRUCTURE

With accordance to my research, there is no specific type mentioned to be the best arrangement of all. This is because the arrangements depend on the type of process control.

To determine the best cascade control arrangement, you must identify the most likely disturbances to the system. It is helpful to make a list of these in order of increasing importance. Once this has been done, the designer can review the various cascade control options available and determine which one best meets the overall strategy outlined earlier: to make the inner loop as fast as possible while at the same time receiving the bulk of the important disturbances.

GENERAL GUIDELINES IN TUNING AND SELECTION
                     
The tuning of cascade controllers is the same as the tuning of all feedback controllers, but the loop must be tuned from the inside out. The master controller should be put on manual (i.e., the loop broken), and then the inner loop can be tuned. Once the inner loop is properly tuned, the outer loop can be tuned. This allows the outer loop to “see” the tuned inner loop functioning as part of the total process or as the “all else” that is being controlled by the master controller. If you follow this general inside-first principle when tuning cascade controllers, you should encounter no special problems.

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