Process information, supplied by the transmitters located in the field is send it to the controllers located in small shelters  designed for that purpose or to the central control room.
That process information is used to feed the algorithms that control the process to produce a control signal which is in turn send to the final control element, usually a control valve, solenoid. electric motor or similar device.
Status condition, alarm and process information  are conveniently displayed on terminals located in the central control room.

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Initiator

Process information from the field is sent to the controller by the initiator, this can be a simple switch, a transmitter, a thermocouple or any other element able to provide a signal in response to changes in the process conditions.

Controller

The controller is the device in charge of generating the control signal in response to the information supplied by the initiator. This control signal is sent to the final control element.

Final control element

The final control elements are the control valves, electric motors, solenoids or any other device which status will change after receiving a control signal from the controller in order to modify a process condition.