ndustrial Control Networks - An Overview
Industrial control system (ICS) is a general term for several types of control system, including supervisory control and data acquisition (SCADA) systems, distributed control systems (DCSs) and other, less sophisticated system configurations such as skid-mounted Programmable Logic Controllers (PLCs). Such systems are typically used to control the distribution of utilities such as power, gas and water, with control devices (often referred to as field devices) being widely distributed in various remote stations. These field devices control local operations such as the opening and closing of valves and circuit breakers, and collect data from sensors and monitoring systems.
A distributed control system (DCS) is typically used to control processes such as the generation of electrical power, oil and gas refining, water and sewage treatment, and the production of chemicals, food, and motor vehicles. They provide an integrated control architecture that facilitates a supervisory level of control for multiple control sub-systems, each of which is responsible for providing localised process control. Key data about the process is fed back to the controller from sensors, and appropriate control data is fed forward from the controller to the process equipment to ensure that the process continues within set parameters. A DCS fulfills the need for large scale data acquisition and control on a large industrial campus in real time, using high bandwidth and low latency network connections.
Programmable Logic Controllers provide the necessary control logic, timing, and in some instances continuous control within known tolerances to control industrial equipment and processes. They are the control system components used by SCADA and DCS systems, and may be the primary components in smaller systems that control discrete processes such as vehicle assembly lines. PLCs are used extensively in virtually all industrial processes. The PLC evolved because of the perceived need to replace racks of ladder relays, which were time-consuming and difficult to rewire, and not particularly reliable or easy to diagnose when faults occurred.
Supervisory control and data acquisition (SCADA) systems are associated with distribution applications, such as power, gas, and water pipelines, and grew from out of a need to gather remote data through potentially unreliable, low bandwidth and high latency channels. Sites tend to be geographically widely separated, relying on remote telemetry units (RTUs) to send data back to a control center. Some of these remote units had a limited capacity for localised control functions in cases where communication with a control centre was lost. Most modern RTUs have far more autonomy in terms of handling control functions locally.
The differences between these system definitions are blurring as time passes and various technical constraints are overcome. Modern PLC systems can in some cases replace a DCS, while an improved telecommunications infrastructure has enabled SCADA systems to manage closed loop control over long distances. Improvements in processor technology have allowed many DCS products to incorporate subsystems that incorporate the functionality of a PLC. The convergence of the three concepts has led to a new concept – the Process Automation Controller (PAC). Market forces, and time, will determine what impact this has on bringing about a more standardised approach to industrial control.