Automatic control systems are composed of three components:
Sensor(s), which measure some physical state such as temperature or liquid level.
Responder(s), which may be simple electrical or mechanical systems or complex special purpose digital controllers or general purpose computers.
Actuator(s), which effect a response to the sensor(s) under the command of the responder, for example, by controlling a gas flow to a burner in a heating system or electricity to a motor in a refrigerator or pump.
A regulator such as a thermostat is a typical example of a device studied in automatic control.
All automaticcontrollers use the same general responses, although the internal mechanisms and the definitions given for these responses may be slightly different from one manufacturer to another.
The controller must be able to move the valve, the valve must be able to affect the measurement, and the measurement signal must be reported to the controller.
One way to improve the response of control valves is to use a valve positioner, which acts as a feedback controller to position the valve at the exact position corresponding the controller output signal.
Its purpose is to control the gain of a system in order to maintain some measure of performance over a changing range of real world conditions.
The receiver's gain is automatically adjusted to maintain a constant level of overall visible clutter.
Video copy control schemes such as Macrovision exploit this, inserting signal spikes which will be ignored by most televisions but cause the deck's AGC to overcorrect and corrupt the recording.
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