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. Delegation of ControlĪn analogy for understanding cascade control is that of delegation in a work environment. The primary controller may then be left to handle numerous other variables and remote setpoints for other secondary controllers. This allows the secondary controller to be more immediately responsible for fine-tuning setpoint adjustments because of its close proximity to the final control element. An essential requirement of cascaded control is that the secondary process variable be faster-responding (shorter lag and dead times) than the primary 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. 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 archaically, master) essentially “gives orders” to the second controller (called the secondary or archaically, 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. Information from the measuring device (sensor or transmitter) goes to the controller, then to the final control device (solenoid coil, motor drive, or control valve), influencing the process which is sensed again by the measuring device. A simple control system drawn in block diagram form looks like this:
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