Control systems can be thought of as having four functions;
Measure, Compare, Compute, and Correct.
These four functions are completed by five elements;
Detector, Transducer, Transmitter, Controller, and Final Control Element.
The measuring function is completed by the detector, transducer and transmitter.
In practical applications these three elements are typically contained in one unit.
A standard example is a Resistance thermometer.
The compare and computer functions are completed within the controller which may
be completed electronically through a Proportional Control, PI Controller, PID Controller,
Bistable, Hysteretic control or Programmable logic controller.
The correct function is completed with a final control element.
The final control element changes an input or output in the control system which affect
the manipulated or controlled variable.
Controller gives control signal to Final Control Element.
In control theory, a controller is a device which monitors and affects the operational
conditions of a given dynamical system.
The operational conditions are typically referred to as output variables of the system
which can be affected by adjusting certain input variables.
Control signal affects the operational conditions of a given controlled dynamical system.
Generally it can be on/off- , analog- or digitally coded signal.
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