Distributed control system (DCS) Theory FUNDAMENTALS Review

Distributed control system (DCS) Theory FUNDAMENTALS Review

A distributed control system (DCS) refers to a control system usually of a manufacturing system or process or any kind of dynamic system, in which the controller elements are not central in location (like the brain) but are distributed throughout the system with each component sub-system under the control of one or more controllers. The entire system may be networked for communication and monitoring.
Distributed control systems (DCSs) are used in industrial, electrical, computer and civil engineering applications to monitor and control distributed equipment with or without remote human intervention; the nomenclature for the former ‘manual control’ and the latter ‘automated control’.
A DCS typically uses computers (usually custom designed processers) as controllers and use both proprietary interconnections and protocols for communication. Input & output modules form component parts of the DCS. The processor (which is a part of the controller) receives information from input modules and sends information to output modules. The input modules receive information from input instruments in the process (aka field) and output modules transmit to the output instruments in the field. Computer buses or electrical buses connect the processor and modules through multiplexers/demultiplexers. They also connect the distributed controllers with the central controller and finally to the Human-Machine Interface (HMI) or control consoles. See PAS.
DCS is a very broad term that describes solutions across a large variety of industries, including:
* Electrical power grids and electrical generation plants
* Environmental control systems
* Traffic signals
* Water management systems
* Refining and chemical plants
* Pharmaceutical manufacturing
* Sensor Networks
The broad architecture of a solution involves either a direct connection to physical equipment such as switches, pumps and valves or connection via a secondary system such as a SCADA system.
A DCS solution does not require operator intervention for its normal operation, but with the line between SCADA and DCS merging, systems claiming to offer DCS may actually permit operator interaction via a SCADA system.
Distributed Control Systems (DCSs) are dedicated systems used to control manufacturing processes that are continuous or batch-oriented, such as oil refining, petrochemicals, central station power generation, pharmaceuticals, food & beverage manufacturing, cement production, steelmaking, and papermaking. DCSs are connected to sensors and actuators and use setpoint control to control the flow of material through the plant. The most common example is a setpoint control loop consisting of a pressure sensor, controller, and control valve. Pressure or flow measurements are transmitted to the controller, usually through the aid of a signal conditioning Input/Output (I/O) device. When the measured variable reaches a certain point, the controller instructs a valve or actuation device to open or close until the fluidic flow process reaches the desired setpoint. Large oil refineries have many thousands of I/O points and employ very large DCSs. Processes are not limited to fluidic flow through pipes, however, and can also include things like paper machines and their associated variable speed drives and motor control centers, cement kilns, mining operations and ore processing facilities, and many others.
A typical DCS consists of functionally and/or geographically distributed digital controllers capable of executing from 1 to 256 or more regulatory control loops in one control box. The input/output devices (I/O) can be integral with the controller or located remotely via a field network. Today’s controllers have extensive computational capabilities and, in addition to proportional, integral, and derivative (PID) control, can generally perform logic and sequential control.
DCSs may employ one or several workstations and can be configured at the workstation or by an off-line personal computer. Local communication is handled by a control network with transmission over twisted pair, coaxial, or fiber optic cable. A server and/or applications processor may be included in the system for extra computational, data collection, and reporting capability.
[edit] History
The DCS was introduced in 1975. Both Honeywell and Japanese electrical engineering firm Yokogawa introduced their own independently produced DCSs at roughly the same time, with the TDC 2000 and CENTUM systems, respectively. US-based Bristol also introduced their UCS 3000 universal controller in 1975. In 1980, Bailey (now part of ABB) introduced the NETWORK 90 system.