Definitions and Descriptions

I have had a number of questions and Google searches that are looking for the meaning of a number of terms I use frequently on the site. I know I will oversimplify but this should give a decent overview of how a control system works and what some of its parts do.

Control systems consist of several different sub-systems that act in cooperation to monitor, log and manipulate a physical process. Different vendors have different points of division and sometimes combine sub-systems but ultimately they will include all (or most) of the functions. There is a hierarchy of systems that include sensing and activation systems, data receipt, storage and transfer, presentation, and control.

At the base of the hierarchy are the PLC’s (Programmable Logic Controller) and/or the RTU’s (Remote Telemetry Unit). These devices have an interface to a physical device for either monitoring or activating purposes. Sensors typically convert mechanical or analog data into digital format then store and/or forward it. Actuators receive digital commands and convert them into actions by energizing a solenoid, activating a servo, positioning a synchro, or just turning on a switch. PLC’s act as the system interface to these devices.

PLC’s and RTU’s feed data into historians and operational control systems. The functions of these vary widely depending on requirements of the process or action being either controlled or monitored.

Typically the historian serves as an aggregation point for data from multiple different systems and subsequent actions regarding that data.

The role of this layer can usually be defined in one of three ways. Monitoring, Open Loop Control, and Closed Loop Control.

For the monitoring function the role of the system is to gather and make information from the sensors available for use in various manners. Monitoring is often also a part of open and closed loop controls.

Open loop controls require the action of an external operator to occur.

Closed loop controls are controls that occur without operator intervention.

Each of these control functions requires a slightly different approach to protection and has a different hierarchy of impact priority.