Manufacturing software is often categorized in a hierarchy with Supervisory Control and Data Acquisition (SCADA) systems at the highest level, Distributed Control Systems (DCS) in the middle level and Programmable Logic Controllers (PLC) at the lowest level. These systems communicate with each other via standardized protocols such as OPC, PLCopen, and Manufacturing Message Specification (MMS) as well as proprietary protocols developed by specific manufacturers.
The lines dividing the levels in the software hierarchy can be
blurred without clear distinction.
SCADA software is typically distributed across very large factories or groups of factories, but it can also be used in smaller factories.
SCADA software is used in distribution systems such as water distribution and
wastewater collection systems, oil and gas pipelines, electrical power grids, and railway transportation systems. A SCADA control center performs centralized
alarm monitoring and status processing. Based on information received from remote stations, automated supervisory commands can be pushed to remote station control devices, which
can be field devices, such as PLC, controlling local operations such as opening and closing valves and breakers, collecting data from sensor systems, and monitoring the local environment for alarm conditions.
An operator can also typically assume manual control through a
software user interface.
DCS software is used to control industrial processes such as electric power generation, oil and gas refineries, water and wastewater treatment, and chemical, food, and automotive production. DCS are integrated as a control architecture containing a supervisory level of control overseeing multiple, integrated subsystems that are responsible for controlling the details of a localized
manufacturing process. Product and process control are usually achieved by deploying control loops whereby process conditions are automatically maintained around a desired set point. To accomplish the desired product and/or process tolerance around a specified set point, specific PLC are employed in the field and proportional, integral,
and differential settings on the PLC are tuned to provide the desired tolerance
and the rate of self-correction during process upsets. DCS are used extensively in process-based
industries.
PLC are digital electronic devices that use a
software programmable memory to store instructions and to implement specific functions such as logic, sequence, timing, counting and arithmetic to control machines and processes. PLC typically control individual machines or manufacturing cells. The controllers inside modern robot systems are often PLC. PLC typically respond quickly to processes occurring within their local
manufacturing cells and respond to higher level commands, such as cycle start or reset cell, from the DCS or SCADA systems. PLC are found on machines and
manufacturing systems such as tire making machines, winding machines, drilling rig top drives, machine tools, process control and assembly lines. Historically software for PLC
have been written in one of the five IEC software languages (structured text, function block diagram, ladder logic, instruction list and sequential function charts).
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