Industrial Control Systems (ICS) is a general term encompassing a wide range of control systems, including Supervisory Control and Data Acquisition (SCADA) systems, Distributed Control Systems (DCS), and other smaller control system configurations such as slip-mounted Programmable Logic Controllers ( PLCs ), which are frequently used in industrial sectors and critical infrastructure. ICS are commonly used in industries such as power, water, oil and gas, chemicals, transportation, pharmaceuticals, pulp and paper, food and beverage, as well as discrete manufacturing (e.g., automotive, aerospace, and durable goods) and papermaking.
Control systems are crucial for the operation of critical infrastructure in the United States, which is often highly valued and consists of interconnected and interdependent systems. It is important to note that approximately 90% of the nation's critical infrastructure is privately owned and operated. Federal agencies also operate many of the aforementioned industrial processes; other examples include air traffic control and materials handling (e.g., postal service mail processing). This section provides information on SCADA, DCS, and PLC systems, including typical architectures and components.
Overview of SCADA/DCS and PLC
SCADA systems are highly distributed systems frequently used to control geographically dispersed assets, often spread across thousands of square kilometers, where centralized data acquisition and control are crucial to system operation. They are used in water distribution systems, such as water and wastewater collection systems, oil and gas pipelines, power grids, and rail transport systems.
A SCADA control center provides centralized monitoring and communication networks for field operations, including monitoring alarms and processing status data. Based on information received from remote stations, automated or operator-driven monitoring commands can be pushed to remote station control devices, often referred to as field devices. Field devices control local operations such as opening and closing valves and circuit breakers, collect data from sensor systems, and monitor alarm conditions in the local environment.
DCS is used to control industrial processes such as power generation, oil and gas refining, water and wastewater treatment, as well as chemical, food and automotive production.
DCS is integrated into a control architecture containing supervisory-level controls overseeing multiple integrated subsystems responsible for controlling the details of localized processes. Product and process control is typically achieved by deploying feedback or feedforward control loops, where critical product and/or process conditions are automatically maintained near desired setpoints. To achieve the desired product and/or process tolerances near the specified setpoints, specific programmable logic controllers (PLCs) are used in the field. The proportional, integral, and/or differential settings on the PLCs are adjusted to provide the desired tolerances and a self-correcting rate during process disturbances. DCS is widely used in process-based industries.
PLCs are computer-based solid-state devices used to control industrial equipment and processes. PLCs are control system components used throughout SCADA and DCS systems; they are often key components in smaller control system configurations providing supervisory control for discrete processes such as automotive assembly lines and power plant sootblower control. PLCs are widely used in almost all industrial processes.
Process-based manufacturing typically utilizes two main processes: continuous manufacturing processes. These processes operate continuously, often transitioning between different grades of products. Typical continuous manufacturing processes include fuel or steam streams in power plants, oil in refineries, and distillation in chemical plants. Batch production processes. These processes involve different handling steps for a certain quantity of material. With a defined start and end step, batch processes may have brief steady-state operations in intermediate steps.
Discrete manufacturing typically involves a series of steps on a single piece of equipment to create the final product. Electronic and mechanical component assembly and parts processing are typical examples of this industry. Both process-based and discrete industries use the same types of control systems, sensors, and networks. Some facilities are a hybrid of discrete and process-based manufacturing. While control systems used in distribution and manufacturing are very similar in operation, they differ in some ways. A key difference is that DCS or PLC-controlled subsystems are often located in a relatively narrow, plant-centric area, compared to geographically dispersed SCADA sites. DCS and PLC communication typically uses Local Area Network (LAN) technology, which is generally more reliable and faster than the long-distance communication systems used by SCADA systems.
In practice, SCADA systems are specifically designed to handle long-distance communication challenges such as latency and data loss, which are inherent to the various communication media used. DCS and PLC systems typically employ a greater degree of closed-loop control than SCADA systems because the control of industrial processes is generally more complex than the supervisory control of distribution processes. These differences can be considered only briefly in the scope of this document, which focuses on the integration of information technology (IT) and the security of these systems. SCADA systems, DCS, and PLC systems will be referenced in the remainder of this document.
What are the differences between SCADA/DCS and PLC?
By definition:
DCS is a "distributed control system," while PLC (programmable logic controller) is merely a control "device." The difference lies in the distinction between a "system" and a "device." A system can perform the functions and coordination of any device, while a PLC device only performs the functions required by its own unit.
In terms of main application scenarios:
DCS focuses more on process control (such as chemical, metallurgical, and pharmaceutical industries), mainly on the monitoring and regulation of field parameters, while PLC focuses on the logic control of individual devices/equipment.
So what about SCADA, which is less clearly defined? By definition, SCADA stands for Monitoring, Control, and Data Acquisition System, which looks similar to DCS. Its lower-level computers can be composed of lower-level computers from DCS or devices such as PLCs and RTUs. The upper-level computers often use SCADA software from various manufacturers.
In terms of main application scenarios: SCADA is mainly used for decentralized control points. A single system may cover an area of thousands of kilometers, such as long-distance natural gas pipelines and power dispatching systems.
SCADA is the dispatch management layer, DCS is the plant management layer, and PLC is the field equipment layer.
PLC systems, or programmable controllers, are suitable for measurement and control in industrial settings. They offer strong on-site measurement and control functions, stable performance, high reliability, mature technology, wide application, and reasonable pricing.
DCS systems, or Distributed Control Systems, are large-scale control systems that were at an internationally advanced level in the 1990s. They are suitable for industrial sites with a large number of control points, high control accuracy, and fast control speed. Their characteristics include distributed control and centralized monitoring, networking communication capabilities, strong control functions, reliable operation, easy expansion, convenient configuration, and simple operation and maintenance. However, the systems are expensive.
SCADA systems, or Distributed Data Acquisition and Monitoring systems, are small to medium-sized measurement and control systems. They combine the advantages of PLC systems (strong field measurement and control capabilities) and DCS systems (network communication capabilities), offering a high performance-to-price ratio.
SCADA and DCS are concepts, while PLC is a product; the three are not comparable.
A PLC is a product that can be used to construct SCADA and DCS systems.
DCS evolved from process control, while PLC evolved from relay-logic control systems.
PLC is a device, while DCS and SCADA are systems.
