General Overview of SCADA Systems

What is PLC and how is it used?

A Programmable Logic Controller (PLC) is a major component in industrial control systems (ICS).  PLC’s are not geographically dispersed and use local area network (LAN) technology for communication.  They can be connected to input sources such as switches, sensors and relays. Outputs are also connected to it and, depending on how logic is programmed in the central processing unit (CPU), it will perform some action or provide some output, based on the input provided. They are normally used in a feedback control or closed loop systems.   A feedback control system, as the name suggests, utilizes the differences between the input and output feedback as input to the controller to determine the desired output.

Common Field Components

Component/Component Type	Description
Data Historian	This server usually houses a database that allows commands and configurations and feedback to be saved for later analysis.
Servo Drives	Servo motors convert control elements into desired output.  There are two types of servo motors, Alternating Current (AC) and Direct Current (DC). DC servo drives work specifically with servo motors to give commands to the motor and receive feedback from it.
Variable Frequency Drives and AC Drives	AC Drives/Variable Frequency Drives (VFD) work with non-servo AC motors.  They control the speed of the motor by varying the electricity sent to it.
Sensors	Sensor, like the proximity sensor above, produce a signal as a measure of a variable used as a control.  In the example above, it could be used to send a signal when someone or something is within a certain number of previously configured feet. Photo-Eye sensors detect presence (or absence) or distance of an object.
Light-Tower	A field element used to provide indicators of a process or machine state.
Human Machine Interface (HMI)	A terminal used to receive input from or send commands to the system.

Now, the fieldbus, a special type of LAN specifically designed for data acquisition and control of ICS components.  Imagine the PLC as part of a control loop using a fieldbus for communication between the PLC and the other components.  A proximity sensor is used, likely, to detect the presence of person or thing.  When the sensor is tripped it sends a signal to a photo-eye to begin recording and to the light-tower to activate.  An alert is likely sent to the HMI for human intervention (likely to view the video and determine if the presence is friend or foe).  The event is then likely stored in the data historian. An engineering workstation, usually manned by engineers, is used to configure the PLC which connected to the same LAN.  A workstation is likely used to configure proximity limits.

How are PLC’s Used in ICS?

PLC’s are part of Distributed Control Systems and Supervisory Control and Data Acquisition systems.  In DCS, PLC’s are used as field control devices that are coupled with other components and connected to a LAN to provide data and feedback.  In SCADA, PLC’s are used to communicate data to SCADA master stations.  Data is communicated to the PLC’s using sensors. In smaller implementations that require simple, discrete control, PLC’s are the primary controllers.  Discrete control gives operators an indication when a process reaches a certain state.  You may see PLC’s used as primaries in operations that require discrete control, like alerting an operator to the completion of a component.

What is a DCS?

A Distributed Control System (DCS) is a local control system used for facilities in need of continuous monitoring or analog control. It, like the PLC, utilizes LAN’s for communication. It is usually comprised of a supervisory level and multiple substations.  The supervisory level control oversees the substations through distributed controllers that communicate with it over the LAN.  Controllers are configured to communicate or provide feedback when certain measurements are reached.

Description of DCS Applications

Imagine a typical DCS with both supervisory and field controls.You may have:

Component	Description
Machine controller	Consists of drives and motors that are synchronized electronically as opposed to mechanically.
Programmable Logic Controller	Control system with programmable memory area that can store commands for the execution of functions given a certain input or range of inputs.
Single-loop controller	Uses components to handle simple operations.
Process controller	Using actuators and sensors, it processes sensor input and, based on computer programming, determines outputs

All of these components are connected to the supervisory level controller via a local area network (LAN) or as it is sometimes referred, a local control network.  The single-loop controller has sensors and actuators connected directly to it and provide feedback to the supervisory controller whereas, the PLC and Process controllers communicate with their components via a fieldbus (see above for descrition) and the machine controller via LAN.

How is DCS used?

As mentioned previously, DCS’s are used in facilities that need continuous or analog monitoring. DCS’s are designed to be able to be configured for multiple alerts and alarms and present them to multiple operators. For that reason DCS’s lend themselves well to industries such as Chemical plants, Nuclear power plants and power plant systems among others. 

In large scale plants, like chemical plants DCS is crucial to the automation of the functional areas.  Chemical plant functional areas include manufacturing, transport, warehousing/storage and chemical end users.  DCS is an integral part of the manufacturing process.  DCS is used to constantly monitor steam flow, temperatures, pressures and composition of chemical components, to name a few. 

In nuclear power plants DCS provides real-time monitoring of equipment parameters, critical feedback and when parameters are exceeded, controlling equipment based on configured parameters and storing information on plant operations.

In the Energy sector, there are three segments: Electricity, Natural Gas and Petroleum. In the Electricity segment DCS monitors the flow of electricity through transmission and distribution lines.  In the Natural Gas segment DCS integrates gas flow and measurement data with other financial systems for billing and accounting functions.  In the Petroleum sector DCS monitors and transmits pipeline data.