First of all we will see a simple close loop control system. Below figure is simple close loop control system.
Information from the measuring device (e.g. transmitter) goes to the controller, then to the final
control device (e.g. control valve), influencing the process which is sensed again by the measuring
device. The controller’s task is to inject the proper amount of negative feedback such that the
process variable stabilizes over time. This flow of information is collectively referred to as a feedback control loop.
To cascade controllers means to connect the output signal of one controller to the setpoint of
another controller, with each controller sensing a different aspect of the same process. The first
controller (called the primary, or master ) essentially “gives orders” to the second controller (called
the secondary or slave) via a remote setpoint signal.
Thus, a cascade control system consists of two feedback control loops, one nested inside the other
A very common example of cascade control is a valve positioner, which receives a command
signal from a regular process controller, and in turn works to ensure the valve stem position precisely matches that command signal. The control valve’s stem position is the process variable (PV) for the positioner, just as the command signal is the positioner’s setpoint (SP). Valve positioners therefore act as “slave” controllers to “master” process controllers controlling pressure, temperature, flow, or some other process variable.
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 and dead times) than the primary process variable.
An analogy for understanding cascade control is that of delegation in a work environment. If a
supervisor delegates some task to a subordinate, and that subordinate performs the task without
further need of guidance or assistance from the supervisor, the supervisor’s job is made easier. The
subordinate takes care of all the little details that would otherwise burden the supervisor if the
supervisor had no one to delegate to. This analogy also makes it clear why the secondary process
variable must be faster-responding than the primary process variable: the supervisor-subordinate
management structure fails to work if the supervisor does not maintain focus on long-term goals
(i.e. longer-term than the completion time of the tasks given to subordinates). If a supervisor
focuses on achieving goals that are shorter-term than the time required for subordinates to complete their assignments, the supervisor will inevitably call for “course changes” that are too quick for the subordinates to execute. This will lead to the subordinates “lagging” behind the supervisor’s orders, to the detriment of everyone’s satisfaction.