![]() This outlet temperature is fairly slow to react, as the solid material mass creates a large lag time: The primary process variable is the outlet air exiting the dryer, which should be maintained at a high enough temperature to ensure water will not remain in the upper layers of the solid material. This will lead to the subordinates “lagging” behind the supervisor’s orders, to the detriment of everyone’s satisfaction.Īn example of cascade control applied to a real industrial process is shown here, for a dryer system where heated air is used to evaporate water from a granular solid. 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. ![]() longer-term than the completion time of the tasks given to subordinates). 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. The subordinate takes care of all the little details that would otherwise burden the supervisor if the supervisor had no one to delegate to. 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. shorter lag and dead times) than the primary process variable.Īn analogy for understanding cascade control is that of delegation in a work environment. An essential requirement of cascaded control is that the secondary process variable be faster-responding (i.e. 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. Valve positioners therefore act as “slave” controllers to “master” process controllers controlling pressure, temperature, flow, or some other process variable. 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). ![]() Thus, a cascade control system consists of two feedback control loops, one nested inside the other:Ī 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 first controller (called the primary, or master) essentially “gives orders” to the second controller (called the secondary or slave) via a remote setpoint signal. 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. This flow of information is collectively referred to as a feedback control loop. The controller’s task is to inject the proper amount of negative feedback such that the process variable stabilizes over time. control valve), influencing the process which is sensed again by the measuring device. transmitter) goes to the controller, then to the final control device (e.g. Information from the measuring device (e.g. A simple control system drawn in block diagram form looks like this:
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