First-level controllers are those which actually manipulate the valves or other final control devices to maintain the process variable at a desired setpoint.
Individual controllers — Controllers located either on the control room panel board or in the process area. These operate independently or may receive a setpoint from another controller or computer system in a cascade arrangement.
Direct digital controllers (DDC) — DDC controllers exist as algorithms in the software of a digital computer, and, through the appropriate transducers, continually sample the respective process measurements, compare them with their corresponding setpoints, and manipulate the appropriate valve or other final control device. Reliability of the computer is essential in a DDC system.
Distributed control systems (DCS) — In a DCS installation the controllers and measurement circuits are modularized in small groups (e.g. eight controllers per module) for greater security against failure. The controllers exist in a combination of hardware and software and may be part of a control scheme programmed in a master DCS system. The controllers may also receive setpoints from a separate supervisory computer system. DCS systems support a variety of communication methods, such as a high speed “data highway” serial data transmission concept which can interface to many different computers. Any digital first-level control system must be backed up by a battery powered uninterruptible power supply (UPS) to prevent loss of control of the process during AC power line interruptions.